Impact of Surface Area and Porosity on the Cooling Performance of
Evaporative Cooling Devices
by
Trang Luu
S.B., Massachusetts Institute of Technology (2018)
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September 2020
© Massachusetts Institute of Technology 2020. All rights reserved.
Author………………………………………………………………………………………………
Trang Luu
Department of Mechanical Engineering
August 31, 2020
Certified by…………………………………………………………………………………………
Daniel Frey
Professor of Mechanical Engineering
MIT D-Lab Faculty Research Director
Thesis Supervisor
Accepted by………...………………………………………………………………………………
Nicolas Hadjiconstantinou
Chairman, Department Committee on Graduate Theses
2
3
Impact of Surface Area and Porosity on the Cooling Performance of
Evaporative Cooling Devices
by
Trang Luu
Submitted to the Department of Mechanical Engineering
on August 31, 2020, in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
Evaporative cooling devices are low-cost, low-energy solutions for post-harvest storage of fruits
and vegetables on farmlands. Surface area and porosity are two design parameters that affect the
cooling devices’ evaporation rate and cooling performance. Both design parameters lack prior
systematic testing that methodically varies levels of surface area and material porosity to
understand their effects on these devices’ cooling performance (e.g. maximum temperature drop,
duration of high internal relative humidity, cooling efficiency and total cooling). For fruits and
vegetables, storage environments with low temperature and high humidity are critical to reduce
deterioration. In this thesis, ridges were cut into the outer wall of pot-in-pot evaporative cooling
devices at four different interridge distances to vary total available surface area. Sawdust was
added to clay in different ratios to create devices with varying porosity. A new performance metric
of total cooling is also introduced to account for the maximum temperature drop and the total
duration of evaporative cooling. The surface area experiments reveal that adding corrugations on
the surface introduces competing effects between increased surface area for water evaporation and
decreased vapor concentration gradient inside of the corrugations’ troughs; consequently, among
the devices with corrugations, the amount of total surface area does not always correlate with
cooling performance. Between the devices with some surface corrugation and the device without
corrugation, the devices with corrugation do consistently achieve greater temperature drops.
However, the devices with corrugation are unable to maintain temperature drops and high levels
of internal relative humidity for as long as the device without corrugation. The porosity
experiments conclude that the greater the porosity in the device’s outer vessel, the greater the
maximum temperature drop. This is due to the reduced transport resistance during water and
moisture movement to the device’s surface. Higher percentages of porosity lead to faster
evaporation rates which deplete the amount of water inside the devices quicker and explain why
the temperature drops and internal relative humidity of the more porous devices do not last as long
as the temperature drops and internal relative humidity of the less porous devices. This thesis
investigates two design parameters of cooling devices and shows that increasing surface area and
porosity increases maximum temperature drops but decreases both the duration of temperature
drops and high internal relative humidity. Between the two design parameters, increasing porosity
is the more practical and less burdensome solution to improve the overall performance of
evaporative cooling devices for low-resource communities.
Thesis Supervisor: Daniel Frey
Title: Professor of Mechanical Engineering; MIT D-Lab Faculty Research Director
4
Acknowledgements
I wish to express my gratitude and appreciation to Professor Daniel Frey for his continuous support
of my study and research. I would also like to extend my deepest gratitude to Dr. Eric Verploegen
for his guidance, mentorship, and advice throughout the duration of my time at D-Lab. The
evaporative cooling devices used in these experiments were handmade by Jason Pastorello and the
work could not have been done without his pottery expertise. This thesis was conducted inside of
D-Lab, and I want to thank Jack Whipple for always extending a helping hand. The funding for
this work came from the generous support of the National Science Foundation Graduate Research
Fellowships Program.
I would like to thank my great friend, Krithika Swaminathan, who took the time to help me solve
hard problems and better articulate my research. I am grateful to Daniel Kreus for all the late night/
early morning help with setting up my experiments, editing my thesis and for his unwavering
support of my endeavors. I am thankful to Alejandro De La Parte Autrán for his support and for
his help building the initial momentum of this research project. I wish to extend my sincerest
thanks to Danyal Rehman for his insights into the data analysis. For answering all my MATLAB
questions, I want to thank my friends Pierre Walker and Chen Horng. For reading over my thesis
and helping me better express my ideas, I want to thank my friends Tyler Okamoto, Mark Chang,
and Carla Pinzón Gaytán.
Lastly, I would like to thank my parents, my sister, and my brother-in-law who have always been
my foundation in life.
5
Table of Contents
TABLE OF CONTENTS ............................................................................................................. 5
LIST OF FIGURES ...................................................................................................................... 6
LIST OF TABLE .......................................................................................................................... 7
INTRODUCTION......................................................................................................................... 8
EXPERIMENTAL DESIGN ..................................................................................................... 18
2.1. Instrumentation .............................................................................................................. 20 2.2. Experimental Metrics ..................................................................................................... 23 2.3. Experimental Methodology ........................................................................................... 25 2.4. Experiments ................................................................................................................... 27
RESULTS AND DISCUSSION ................................................................................................. 31
3.1. Surface Area Experiment ............................................................................................... 33 3.2. Effects of Porosity .......................................................................................................... 67 3.3. Porosity Mini-Experiments ............................................................................................ 95 3.4. Porosity versus Surface Area Design Parameters .......................................................... 98
CONCLUSION ......................................................................................................................... 101
SUPPLEMENTARY INFORMATION .................................................................................. 105
BIBLIOGRAPHY ..................................................................................................................... 110
6
List of Figures
Figure 1. Regions with Hot Desert Climates Based on Koppen-Geiger (1980-2016) -------------- 10
Figure 2. Population with Access to Electricity in 2016 ------------------------------------------------ 11
Figure 3. Pot-in-Pot Evaporative Cooler ------------------------------------------------------------------ 12
Figure 4. Sensor Placement Inside each Cooling Device ----------------------------------------------- 19
Figure 5. Ambient Sensors Placement -------------------------------------------------------------------- 20
Figure 6. Characterization of Surface Corrugations in Surface Area Experiment ------------------ 28
Figure 7. Evaporative Cooling Devices in Surface Area Experiments ------------------------------- 33
Figure 8. Wet-Bulb Temperature Comparison Between Surface Area Experiment 1 and 2 ------ 35
Figure 9. Internal Temperature Drop for Surface Area Experiment 1 -------------------------------- 37
Figure 10. Internal Temperature Drop for Surface Area Experiment 2 ------------------------------ 38
Figure 11. Mass Transfer Coefficient for Surface Area Experiment 1 ------------------------------- 44
Figure 12. Mass Transfer Coefficient for Surface Area Experiment 2 ------------------------------- 45
Figure 13. Evaporation Surface Temperature Drop for Surface Area Experiment 1 --------------- 48
Figure 14. Evaporation Surface Temperature Drop for Surface Area Experiment 2 --------------- 49
Figure 15. Cooling Efficiency for Surface Area Experiment 1 ---------------------------------------- 52
Figure 16. Cooling Efficiency for Surface Area Experiment 2 ---------------------------------------- 53
Figure 17. Total Cooling for Surface Area Experiment 1 ---------------------------------------------- 57
Figure 18. Total Cooling in Surface Area Experiment 2 ----------------------------------------------- 58
Figure 19. Relative Humidity in Surface Area Experiment 1 ------------------------------------------ 61
Figure 20. Relative Humidity in Surface Area Experiment 2 ------------------------------------------ 62
Figure 21. Evaporative Cooling Devices in the Porosity Experiments ------------------------------- 67
Figure 22. Wet-Bulb Temperature Comparison Between Porosity Experiment 1 and 2 ----------- 68
Figure 23. Internal Temperature Drop for Porosity Experiment 1 ------------------------------------ 72
Figure 24. Internal Temperature Drop for Porosity Experiment 2 ------------------------------------ 73
Figure 25. Evaporation Surface Temperature Drop for Porosity Experiment 1 --------------------- 77
Figure 26. Evaporation Surface Temperature Drop for Porosity Experiment 2 --------------------- 78
Figure 27. Mass Loss Rate and Mass Loss for Porosity Experiment 1 ------------------------------- 79
Figure 28. Mass Loss Rate and Mass Loss for Porosity Experiment 2 ------------------------------- 80
Figure 29. Cooling Efficiency for Porosity Experiment 1 ---------------------------------------------- 83
Figure 30. Cooling Efficiency for Porosity Experiment 2 ---------------------------------------------- 84
Figure 31. Total Cooling for Porosity Experiment 1 ---------------------------------------------------- 86
Figure 32. Total Cooling for Porosity Experiment 2 ---------------------------------------------------- 87
Figure 33. Relative Humidity in Porosity Experiment 1------------------------------------------------ 90
Figure 34. Relative Humidity in Porosity Experiment 2------------------------------------------------ 91
Figure 35. Moisture in Sand Gap for Surface Area Experiment 1 ---------------------------------- 105
Figure 36. Moisture in Sand Gap for Surface Area Experiment 2 ---------------------------------- 106
Figure 37. Mass Loss Rate and Mass Loss for Surface Area Experiment 1 ----------------------- 108
Figure 38. Mass Loss Rate and Mass Loss for Surface Area Experiment 2 ----------------------- 109
7
List of Table
Table 1. Internal Temperature Drop Cooling Performance for Surface Area Experiments ------- 39
Table 2. Evaporation Surface Temperature Drop Cooling Performance for Surface Area
Experiments --------------------------------------------------------------------------------------------- 50
Table 3. Cooling Efficiency Performance for Surface Area Experiments --------------------------- 54
Table 4. Total Cooling Performance for Surface Area Experiments --------------------------------- 59
Table 5. Relative Humidity of the Surface Area Experiments ----------------------------------------- 63
Table 6. Comprehensive Cooling Performance Metrics of Surface Area Experiments ------------ 66
Table 7. Internal Temperature Drop Cooling Performance for Porosity Experiments ------------- 74
Table 8. Evaporation Surface Temperature Drop Cooling Performance for Porosity Experiments
------------------------------------------------------------------------------------------------------------ 81
Table 9. Cooling Efficiency Performance for Porosity Experiments --------------------------------- 84
Table 10. Total Cooling Performance for Porosity Experiments -------------------------------------- 88
Table 11. Relative Humidity of the Porosity Experiments --------------------------------------------- 92
Table 12. Comprehensive Cooling Performance Metrics of Porosity Experiments ---------------- 94
Table 13. Devices in Mini-Experiment Comparing Porosity Due to Different Clay Material
Versus Porosity Created by Sawdust ---------------------------------------------------------------- 96
Table 14. Devices in Mini-Experiment Comparing Porosity Due to Different Clay Material ---- 97
Table 15. Comprehensive Cooling Performance Metrics of Surface Area And Porosity
Experiments ------------------------------------------------------------------------------------------- 100
8
Chapter 1
Introduction
Food loss and waste pose serious challenges for both developed and developing countries’
economic development and food security [1]. The Food and Agriculture Organization of the
United Nations (FAO) reported that 13.8% of all food globally is lost from post-harvest up to but
not including retail in 2019 [2]. At the same time, the number of chronically malnourished people
in the world has been rising since 2014 [3]. There are approximately 60 million more malnourished
people in 2019 than in 2014 globally—a total of nearly 690 million people or 8.9% of the world
population [3]. The COVID-19 pandemic may have added an additional 83 – 132 million people
to the total number of undernourished people in 2020 [3]. The underlying cause of the increased
undernourishment globally stems from deteriorating economic conditions that threaten food
affordability for low-income and vulnerable communities [3]. To increase affordability and
availability of nutritious foods, reducing harvest losses at the production level is an vital first step
[3]. Due to inadequate crop processing technology, lack of labor, and insufficient/unreliable cold
9
storage infrastructure throughout the supply chain, underdeveloped countries primarily suffer from
food loss and waste during the post-harvest storage and processing stage [1,3]. Post-harvest
storages, such as mechanical refrigerators and cold rooms, can reduce how quickly produce perish
but require expensive initial capital investment and reliable electricity to operate. Consequently,
they are inaccessible or unattractive to most farmers in underdeveloped communities [3,4].
Evaporative cooling devices are low-cost alternatives to mechanical refrigerators and cold rooms
that can increase the shelf life of fruits and vegetables by providing cooler and more humid storage
conditions. At high temperatures, the respiration rate, water loss, ethylene production, and
microbial development shorten the shelf life of the produce, and at low relative humidity, produce
tends to wilt, soften, and lose moisture content—all of which decrease the ability of farmers to sell
their produce [4,5]. Communities can use locally available material to manufacture and operate
these cooling devices without using electricity or requiring much training. These cooling devices
use the evaporation of water to create the cooling effect inside; thus, these devices work best in
dry and arid climates that are conducive to water evaporation [6]. Specifically, evaporative cooling
devices are well suited to benefit communities in the Near East and North Africa (NENA) 1 region
due to its desert climate (Figure 1) and limited access to electricity (Figure 2). In 2016, the Food
and Agriculture Organization (FAO) reported that the NENA region alone had 250 kg per capita
each year of food loss and waste; this constitutes up to 50% of all fruits and vegetables produced
in the NENA region each year [7,8]. Reducing the amount of food waste and loss will enable
farmers to have more food available for consumption and sale [9]. Furthermore, reducing food loss
1 Countries include: Algeria, Bahrain, Egypt, Iran (Islamic Republic of), Jordan, Kuwait, Lebanon, Libya,
Mauritania, Morocco, Oman, Qatar, Saudi-Arabia, Tunisia and the United Arab Emirates
10
and waste can decrease the price of food and reduce household expenditures, hence facilitating the
acquisition of other basic necessities related to health, education, and quality of life [3,9].
Figure 1. Regions with Hot Desert Climates based on Koppen-Geiger (1980-2016).
The red identifies the arid and dry climates around the world. These areas provide the
environmental conditions that maximize the cooling performance of evaporative cooling
devices. BWh indicates hot desert climates.2 The NENA region overlaps with the region
of hot desert climates near North Africa.
2 By Beck, H.E., Zimmermann, N. E., McVicar, T. R., Vergopolan, N., Berg, A., & Wood, E. F. -
"Present and future Köppen-Geiger climate classification maps at 1-km resolution". Nature Scientific
Data. DOI:10.1038/sdata.2018.214., CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=74673078
11
Figure 2. Population with Access to Electricity in 2016. The data represents percentage
of people who have electricity, both on-grid and off-grid, for their home.3 Part of the NENA
region overlaps with areas that largely do not have access to electricity.
Evaporative cooling devices come in two primary categories: large evaporative cooling chambers
and household coolers [10,11]. This thesis focuses on the latter because they cost less and require
less labor to construct for the users. The author anticipates that the findings on household coolers
can later be generalized for larger cooling chambers. For household coolers, the three most
common configurations are pot, cabinet, and curtain configurations [11–13]. The evaporative
cooling devices in the pot configuration are the most used. Coolers in the cabinet and curtain
configuration are adaptation of the common pot configuration design; the main difference among
the three lies in where water evaporates from. This thesis explores the cooling performance of
evaporative coolers in the pot configuration. Mohammed Bah Abba, an inventor, from Nigeria
created the most common cooler in the pot configuration, called the Pot-in-Pot Preservation
3 By World Bank – World Development Indicators. 2016. http://data.worldbank.org/data-catalog/world-
development-indicators
12
Cooling System (Zeer Pot) seen in Figure 3 [14]. Abba’s invention consists of an earthenware pot
placed inside a larger pot and has sand filling the gap between the two pots. Users place fruit and
vegetables in the inner pot and cover the whole configuration with a wetted cloth. Inside the inner
pot, evaporative cooling creates a cooled and humidified environment to extend the shelf life of
the stored fruits and vegetables.
Figure 3. Pot-in-Pot Evaporative Cooler. The Zeer Pot (pot configuration) consists of an
outer and inner clay vessel with wetted sand to fill the gap and a wet cloth to cover the top.
The porous vessels allow water to travel from the sand gap to the outer and inner surfaces
to evaporate. As the water evaporates from the lid and the surfaces, the internal temperature
decreases and the internal relative humidity increases.
Evaporative cooling performance is limited by geographical constraints because the cooling
fundamentally relies on water evaporation. Environment conditions such as relative humidity,
vapor pressure, air movement, and temperature affect rate of water evaporation and thereby the
cooling performance. Relative humidity is the percentage of water vapor in the air. It relates the
13
partial pressure of water vapor to the saturated water vapor pressure at the same temperature. For
water to evaporate, the vapor pressure at the liquid-air interface needs to be higher than the
surrounding air to create a concentration difference that drive the evaporation process [15]. As
water evaporates into vapor, the relative humidity immediately around the evaporative cooling
device gradually increases, reducing the driving force for further evaporation of water. With no
natural or forced air movement around the evaporative cooling devices to replace the saturated air
with drier air, the vapor pressure gradient would continue to decrease and the rate of evaporation
would continue to slow until it stops completely. There are two temperature measurements that
are relevant to evaporative cooling performance: dry-bulb temperature and wet-bulb temperature.
Dry-bulb temperature is the air temperature measured by ordinary thermometers. Most people
commonly refer to this as air temperature. Higher ambient air dry-bulb temperatures lead to more
sensible heat that water inside evaporative cooling devices can use to convert to latent heat for
evaporation [6,15]. Wet-bulb temperature measures air temperature at 100% relative humidity and
marks the lowest temperature obtainable via evaporative cooling. To measured it, drape a wet cloth
over a thermometer. The wet bulb temperature is lower than the dry bulb temperature, and the
difference between them represents the maximum decrease in temperature achievable using
evaporative cooling devices [4]. It is important to note that even at peak performance, evaporative
cooling devices cannot prevent rapid spoilage for commodities that will perish if not kept
consistently below 20 °C (such as medications, meat, and animal products) or products that need
low humidity to prevent mold growth (grains, coffee, and cereals) [10]. Evaporative cooling
devices best increase the shelf life and slow the physiological deterioration of fruits and vegetables
by providing cooler and more humid storage conditions to reduce wilting, water loss, and over-
ripening [4].
14
Water evaporation from porous materials such as clay pots occurs in two stages [16–18]. In the
first stage, water evaporates from the wetted surface directly into the atmosphere, and in the second
stage, when there is disruption in the capillary water flow to the surface, moisture evaporates
within the porous medium and then vapor diffuses to the surface [16–18]. The main two factors
that influence the two stages of evaporation are respectively, the ambient conditions that affect
water vapor exchange with air at the surface and the topology of pore spaces inside the porous clay
pot [16,17]. For evaporative cooling devices, surface area and porosity are two design parameters
that influence the devices’ rate of evaporation and can be optimized to improve cooling
performance. Increasing the amount of surface area for water evaporation will increase the rate of
evaporation [6,13,19]. Scaling the overall evaporative cooling device up in size will provide more
surface area over which evaporation can occur, which in theory should provide more cooling, but
in practice, increasing the overall size of the cooling device also increases the internal volume the
cooling device needs to cool. Thus, to study the effects of how just the surface area influence
cooling performance, the internal volume of the cooling device must remain constant. Gustafsson
et al. conducted a study on the effects of hanging an evaporative cooling device to allow for
evaporation from the bottom of the pot, thereby exploring the effects of increasing the surface area
over which evaporation can occur without increasing the internal volume [20]. Gustafsson et al.
found that the hanging pot reached a 64% temperature decrease relative to the its wet bulb
temperature, 9% greater than the device placed on the ground and attributed the result to extra
exposure to airflow from hanging the pot [20]. The result of the previously mentioned study shows
that increasing surface area for evaporation lead to better cooling performance, but the design is
difficult to implement practically on farmlands. Gustafsson et al. incorporated forced convection
with wind velocity of 3-3.5 m/s into the experiment [20]. The introduction of forced convection
15
requires electricity that not all farmers can access. Furthermore, the weight of evaporative coolers
makes them impractical to hang off the ground. Another approach to studying how increasing the
surface area without increasing the internal volume affect the cooling performance involves
constructing evaporative cooling devices with ridges on the outer vessel. The added surface
corrugations increase the surface area of the outer vessel without significantly changing the overall
volume of the device. This thesis will compare the cooling performance of cooling devices with
different total amount of available surface area for evaporation.
It is common knowledge that the outer vessels of evaporative cooling devices should have enough
porosity to allow water to pass through to the surface from the sand gap to evaporate. To date, no
literature is available detailing the effect of varying the porosity level of the inner and outer
material of evaporative cooling devices on the cooling performance of the devices. There have
been studies on the effects of varying levels of porosity on water evaporation from porous samples.
Unno et al. compared the evaporation behavior of a single water droplet on top of porous and non-
porous bodies of epoxy resin and found that evaporation rate increased by 1.2 times when the
droplet was on the surface of a porous body [21]. Aboufoul et al. found that pore space topology
influence water evaporation from porous asphalt [16]. The experiments showed that relatively
large pores weaken capillary forces and shortened the stage-1 evaporation, but increased
continuous air void connection and evaporation rates during stage-2 evaporation [16]. The
experiments conducted by Aboufoul et al. is more similar to the experiments in this thesis than the
experiments by Unno et al. but still differ in that the evaporation was only allowed out the top
surface [16]. Regarding evaporative cooling devices, there have been studies using different
materials for the inner vessel of the cooling device such as clay (porous) versus plastic and metal
(non-porous). Salaudeen et al. compared the clay pot-in-pot configuration with a tin pot-in-pot
16
configuration in Nigeria and discovered that the tin pot-in-pot did achieve between 2 °C – 3.5 °C
greater temperature drop and 10% - 11% higher relative humidity than the clay pot-in-pot
throughout the nine days of experiment [22]. The tin-in-pot showed a greater temperature drops
during the cooler morning and night, than during the hotter afternoon because as Salaudeen et al.
suggests, tin serves as a good conductor for heat transfer from the environment. Nevertheless,
Salaudeen et al. ruled out using tin as a practical material due to tin’s chemical reaction with the
stored fruits which increased the ripening process [22]. Verploegen et al. conducted a similar
experiment that compared the performance of a clay pot in clay pot cooler with a plastic pot in
clay pot cooler and a metal pot in clay pot cooler [23]. Once again, the metal container in the clay
pot achieved the greatest average temperature decrease at 2.2 °C and 57% cooling efficiency
followed by the plastic container in clay pot at 1.8 °C with 47% cooling efficiency and then the
clay pot-in-pot at 1.7 °C with a 43% cooling efficiency. Verploegen et al. noted that because the
experiments with plastic and metal inner pots did have greater temperature drop and because they
are easier to clean in case of fungal growth than porous material, they make viable alternatives to
using clay internal pot [23]. The two previously mentioned studies explored the effects of using
inner vessels made of material with different conductivity and indirectly investigated the effects
of porosity at the extreme end. These studies differ from the experiments in this thesis in that they
did not vary the porosity of the outer vessels. In the work presented here, the porosity of the clay
body was varied by mixing sawdust into the clay to alter its porosity after firing. With
consideration to practicality, using sawdust to increase the porosity level in the evaporative cooling
device does not require additional electricity usage or cost. Sawdust can be locally sourced in most
places for little or no additional cost to clay pot makers and farmers.
17
To measure the cooling in the surface area and porosity experiments, this thesis uses common
metrics of performance which include temperature drop from ambient conditions, relative
humidity inside the device, and cooling efficiency. In addition to the common metrics of
performance, the author has developed a new metric to find the total amount of cooling by
computing the total area in between the device’s internal temperature curve and the ambient
temperature curve. This performance metric allows readers to compare cooling devices that have
large temperature drops at the beginning of the experiment but quickly dry out to cooling devices
that maintain a steady level of cooling for a longer time period. The thesis also includes practical
time metrics such as the time taken for the internal temperature drop to return to zero and the
internal relative humidity to measure below 80%. The time metrics are analyzed in relationship to
the amount of water added and the relative humidity threshold of 80% was chosen to better
compare the devices’ performance. All the performance metrics are evaluated in a series of four
experiments that systematically vary surface area and porosity. The author hypothesizes that as
surface area and porosity increase, the maximum temperature drops of the evaporation cooling
devices also increase. This work will enable practical improvement to the design, functionality,
and evaluation of evaporation cooling devices in rural, arid environments for low-resource
communities.
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Chapter 2
Experimental Design
In order to systematically evaluate the effects of surface area and porosity on the cooling
performance of household evaporative cooling devices, 3 temperature and relative humidity (joint)
sensors, 3 moisture sensors, and 1 load cell are used per device. An additional 4 sensors are placed
in the ambient environment around the experiments. Water bottles are used as thermal loads inside
the cooling devices. Figure 4 provides the location of the sensors on each cooling device and
Figure 5 shows the location of the ambient sensors and load cells. For the surface area
experiments, there are 4 cooling devices to represent the levels of variation, and in the porosity
experiment, there are 3. Each level of variation is evaluated using common measuring points such
as temperature and relative humidity difference from ambient and cooling efficiency. In addition
to these common metrics, this thesis also includes time metrics: when the temperature differences
19
return to zero, when relative humidity inside the device measures below 80%, and a newly
developed method to quantity the total cooling the device has over the course of the experiment.
Figure 4. Sensor Placement Inside Each Cooling Device. The white squares represent
temperature and humidity (joint) sensors. They are placed inside the middle of the cooling
device, inside the lid, and strapped on the outside in the middle of the cooling device. The
triangles represent moisture sensors that are placed in the sand of the cooling device, in the
lid, and strapped on the outside in the middle of the cooling device. Water bottles surround
the temperature sensor inside the middle of the pot. The entire cooler sits on top of a load
cell.
20
Figure 5. Ambient Sensors Placement. Four ambient sensors measure the ambient
environment temperature and humidity around the evaporative cooling devices.
2.1. Instrumentation
Two microcontrollers control the sensors measurements: the microcontroller onboard SenSen
sensor data collection units (http://www.sensen.co/) and the ATmega2560 on board the Arduino
Mega 2560. Arduino and Processing software read and record the measured data. Data from the
SenSen sensor data is collected every 300 seconds and data from the Arduino sensors is collected
every 52-53 seconds. The sensors on the SenSen data collection unit are used to supplement the
sensors measuring ambient temperature and relative humidity controlled by the Arduino Mega.
The SenSen data collection unit is a standalone data collection box. It has 2 temperature and
relative humidity sensors and 2 moisture sensors.
21
Temperature and Humidity Sensors
DHT22 sensors from Adafruit measure the temperature inside the middle of the evaporative
cooling device, inside the lid of the device, and outside the device (Figure 5). Four sensors
measure the ambient conditions of the experiments. The sensors are rated to measure
temperature from -40 to 80 °C with ± 0.5 °C accuracy, and measure relative humidity from 0-
99.9% with ± 2-5% accuracy.
One BME280 sensor is used as an ambient sensor on the Sensen data collection unit. It
measures temperature with ±1.0 °C accuracy and humidity with ±3% accuracy.
Before the start of every experiment, all 22 temperature and humidity sensors are bundled
together and placed inside a closed box for calibration purposes. The calibration period for
each experiment lasts between 14 – 21 hours. During that time frame, all the temperature and
relative humidity sensors should have the same measurements. As part of the calibration
process, a calibration shift factor is calculated for each of the 22 temperature sensors. Each
experiment yields a separate calibration shift factor which is then applied to the respective
experiment. The calibration shift factors range from -0.32 to 0.62 °C for temperature and from
-2.75% to 3.91% for relative humidity. The shift factor values are close to the error range of
the sensors.
Relative humidity values that read 99.9% do not have shift factors applied to them to avoid
situations where the data reads constant humidity at 79.3% for example. The relative humidity
reading realistically will only stay a constant value at saturation. Relative humidity values
greater than 99.9% after the calibration shift factors are applied are set to 99.9% because
relative humidity is a ratio and anything above 100% is not physically feasible.
22
Moisture sensor
SparkFun Soil Moisture Sensors are used to measure the moisture level in the sand, in the top,
and outside each device. The sensors output an integer between 0 (dry) and 880 (wet) and their
magnitudes indicate the level of wetness. The moisture sensors are not calibrated and are used
to determine wetness and dryness. In this thesis, their units are assigned as Moisture Units.
Load cell
The load cell is from JiuWu (Model number: YZC-1B) and has a maximum capacity at 50kg.
It has a combined error of ± 0.015kg. The combined error specifies the hysteresis of the load
cell. The temperature effect on sensitivity is 8 x 10-4 kg/°C and its creep is 0.015 kg/ 30
minutes. The experiment is conducted indoors and the temperature changes through the course
of the experiments is less than 10 °C so the error due to temperature changes is minimal.
However, the creep tendency of the load cell does add errors to the mass measurements because
of how long the experiments lasted. The first surface area experiment lasts 306 hours. The first
porosity experiment lasts 331 hours. The second surface area and porosity experiment last 613
hours. The creep tendency introduces error into the load cell data that all of the devices
experience but the maximum and minimum required offsets to reset the load cells back to zero
at the start of each experiment is one to two orders of magnitude lower than the amount of
water added. Additionally, no conclusion is drawn from load cell measurements near the end
of the experiments. The load cells are tested for accuracy and precision prior to their usage.
To conduct the accuracy test, known weights (5 lbs., 10 lbs., 25 lbs.) are used to verify the load
cell reading of individual weights and combinations of weights. Verifying the accuracy of the
combinations of weights is important because the weights could not be exactly the labeled
weight. The precision test involves incrementally adding and removing water balloons of
23
known weights. Before starting each experiment in the study, each of the six load cells were
tared to read 0 kg.
2.2. Experimental Metrics
The metrics for quantifying cooling performance include:
Relative humidity [%]
Relative humidity is the percentage of water vapor in the air. It relates the partial pressure
of water vapor to the saturated water vapor pressure at the same temperature.
Wet-bulb temperature difference from ambient temperature [ºC]
Wet-bulb temperature measures air temperature at 100% relative humidity and marks the
lowest temperature obtainable via evaporative cooling. To calculate the wet bulb
temperature, the following formula was used [24]:
𝑇𝑤𝑒𝑡−𝑏𝑢𝑙𝑏 = 𝑇𝑑𝑟𝑦−𝑏𝑢𝑙𝑏 ∗ 𝑎𝑟𝑐𝑡𝑎𝑛 [0.151977 ∗ (𝑅𝐻 + 8.313659)12]
+ 𝑎𝑟𝑐𝑡𝑎𝑛(𝑇𝑑𝑟𝑦−𝑏𝑢𝑙𝑏 + 𝑅𝐻) – 𝑎𝑟𝑐𝑡𝑎𝑛(𝑅𝐻 – 1.676331)
+ 0.00391838 ∗ (𝑅𝐻)32 ∗ 𝑎𝑟𝑐𝑡𝑎𝑛(0.023101 ∗ 𝑅𝐻) – 4.686035
[Eq. 1]
where,
𝑇𝑤𝑒𝑡−𝑏𝑢𝑙𝑏 is the wet-bulb temperature in [ºC]
𝑇𝑑𝑟𝑦−𝑏𝑢𝑙𝑏 is the air temperature given by a thermometer not exposed to direct
sunlight, measured in [ºC].
24
𝑅𝐻 is the relative humidity of the air around the pot, measured in [%].
Cooling Efficiency [%]
Cooling efficiency is the percentage of maximum temperature drop achieved by the device.
A hundred percent indicates that the temperature inside the evaporative cooling device has
dropped to the wet-bulb temperature. It is calculated using the following equation: [25].
𝜀 = 𝑇𝑎,𝑑𝑏−𝑇𝑝,𝑑𝑏
𝑇𝑎,𝑑𝑏−𝑇𝑎,𝑤𝑏 𝑥 100 [Eq. 2]
where,
𝜀 is the cooling efficiency, measured in [%]
𝑇𝑎,𝑑𝑏 is the ambient air dry-bulb temperature, measured in [℃]
𝑇𝑝,𝑑𝑏 is the internal pot air dry-bulb temperature, measured in [℃]
𝑇𝑎,𝑤𝑏 is the ambient air wet-bulb temperature, measured in [℃]
Total Cooling [°C x Hour].
The total cooling metric allows for better comparison between cooling devices that have
large temperature drops at the beginning of the experiment but quickly dry out versus
cooling devices that maintain a steady level of cooling for a longer time period. The total
cooling is calculated by computing the total area in between the device’s temperature curve
and the ambient temperature curve. Its units are [°C x Hour].
Time metrics [Hour]
o The time it takes to achieve the maximum temperature drop.
o The time it takes for the internal relative humidity to drop below 80%.
25
2.3. Experimental Methodology
Before the start of every experiment, all 22 of the temperature and humidity sensors are gathered
into an enclosed container for calibration purposes. The sand used in the previous experiments are
mixed among 3 large buckets and left overnight to dry. Jute sacks are used as the lids of the
evaporative cooling devices because of how widely available they are in Sub-Saharan and East
Africa. The jute sacks require overnight soaking to fully absorb water.
All moisture sensors are tested to ensure that none has deteriorated, and all the load cells are tared
to 0 before each experiment. Before loading the sand into each evaporative device, samples are
taken from each of the 3 sand buckets to ensure that the sand density from each bucket is within
0.05 grams/cm3 of each other. Sand from each bucket is used to fill each cooling device. This mixes
sand even more to further ensure the same sand consistency across all devices. The inner vessel
sits concentrically inside the outer vessel with its opening plane at maximum 1 cm below the
opening plane of the outer vessel. Sand fills the gap between the inner and outer vessel.
Next, the jute sacks that were left to soak overnight are taken out of water and hanged to eliminate
excess water. The jute sack should not be dripping with water when placed on top of the
evaporative cooling device. While the jute sack hangs, the next step is to add 8 oz. water bottles
inside the inner vessels to serve as thermal mass. Each device is packed with as many water bottles
as possible without crossing above the inner vessel’s opening plane. Adding the water bottles
before placing the sensors allow the water bottles to act as a point of reference for sensor
placement. In the first experiment of the surface area and porosity experiments, the temperature
sensor inside the middle of the inner vessel of each device is freely placed above and below one
layer of water bottles. In the second surface area and porosity experiments, the same temperature
26
sensor in the middle of the inner vessel is tied to a water bottle to reduce temperature differences
due to sensor placement. The temperature sensor on the outer vessel of each device is strapped in
place with rubber bands, and the sensor to measure the lid (the jute sack) is set aside because the
lid has not been added yet. One moisture sensor is placed in the sand of the evaporative devices.
Another moisture sensor is wrapped in paper towel and strapped to the outside of each device’s
outer vessel. The moisture sensor measuring the lid is also set aside to be added later. Lastly, the
four ambient sensors are placed in the same location for all of the experiments (Figure 5).
After all of the sensors are placed, each damp jute sack is measured and the weight of the heaviest
jute sack is noted. Water is then added to each of the other jute sacks so that all the jute sacks have
the same amount of water. The re-wetted damp jute sack is not yet placed on top of the evaporative
cooling device until water has been added to device’s sand gap. Because cooling begins the
moment water evaporates from the device, adding water to the sand gap and jute sack is always
the last steps. Each device in the experiments has the same sand to water ratio to account for the
differences among all the devices’ volumes. The variations in volumes are due to the nature of
handmade clay pots. Because the volume of each device is not exactly the same, the absolute
amount of water added varies among the devices. After water is added to a device’s sand gap, its
jute sack is added. To cover the top of the devices, each jute sack is folded in such a way that
ensured complete coverage of each outer vessel’s opening. The sensors measuring the temperature
and relative humidity and moisture of the lids are now added. The sensor for temperature and
relative humidity is placed in one of the jute sack folds and the moisture sensor is clipped in the
middle of the jute sack.
The experiments start after all the cooling devices have been watered and all the jute sacks are
placed on top. No changes are made to the set up for the duration of the experiment. The
27
experiments end when the moisture sensor in the sand returns to zero. The second experiment in
both the porosity and surface area experiments ran longer to further observe the devices’ cooling
performance.
2.4. Experiments
Surface Area Experiment
The evaporative cooling devices used in the experiments exploring the effects of surface area
have corrugations on their outer vessel. The corrugations are created with ridges. Figure 6
shows the characterization of the corrugations. The surface area experiments compare four
devices with different amount of surface area available for water evaporation on their outer
vessels. Adding the ridges increases the surface area over which water can evaporate, and the
smaller the interridge distance, the more surface area is added to the outer vessel. The author
hypothesizes that adding more surface area over which water can evaporate will increase the
rate of evaporation and thereby the amount of cooling.
28
Figure 6. Characterization of Surface Corrugations in Surface Area Experiment. The
ridges on the outer vessels add more surface area to the devices without increasing the
volume. The devices are labeled based on the ratio between the devices’ surface area with
corrugation and their surface area without corrugation to account for the differences in
volume among the devices.
29
The surface area experiment is done twice to check for repeatability. Between the first and
second surface area experiment, the devices are shifted to different positions to ensure that the
cooling performance is not a result of the device’s location. The first experiment ran for 306
hours and the second experiment ran for 613 hours.
Porosity Experiment
The evaporative cooling devices in the experiments investigating the effects of porosity are all
made from terracotta clay. To achieve the incremental percentage of porosity among the outer
vessels, sawdust is added in different clay to sand ratio. Adding sawdust to wet clay changes
the final outer vessel’s porosity because during firing stages, the added sawdust burns out,
leaving cavities. The more sawdust mixed into the wet clay, the more cavities the final vessels
have and the higher the porosity. The sawdust added went through a 60-mesh sieve. The
percentages of porosity tested are 3.14% (no saw dust added), 7.20% (40 clay: 1 sawdust), and
11.22% (20 clay: 1 sawdust). To calculate the percentage of porosity, the following steps are
taken:
1. Weigh the dry samples of each pot
2. Put the samples in water and boil them for about 15 minutes
3. Take the sample out of water individually and shake off any excess water. The sample
is ready to be weighed when there is no water dripping from it. Do not use a cloth to
dry the excess water because the cloth may soak up water from inside the sample.
4. Weigh the wet sample and dry the scale between each sample.
5. Porosity is calculated by
30
𝑃𝑜𝑟𝑜𝑠𝑖𝑡𝑦 = 𝑊𝑒𝑡 𝑊𝑒𝑖𝑔ℎ𝑡−𝐷𝑟𝑦 𝑊𝑒𝑖𝑔ℎ𝑡
𝐷𝑟𝑦 𝑊𝑒𝑖𝑔ℎ𝑡 𝑥 100 [Eq. 3]
The effects of using terracotta with different percentages of porosity are explored in two
different experiments. In the first experiment, all the internal storage vessels are made up of
the 3.14% porosity terracotta clay, while only the outer vessels vary in porosity. In second
experiment, the porosity percentages of the inner and outer vessels are the same. The first
experiment ran for 331 hours and the second experiment ran for 613 hours. The author
hypothesizes that adding more porosity will increase the rate of evaporation and the maximum
temperature drop.
31
Chapter 3
Results and Discussion
Two main competing processes affect the cooling performance of evaporative cooling devices: the
heat transfer out of the device due to water evaporation and the heat transfer into the device via
convection and radiation from ambient environment. In evaporation, mass transfer and heat
transfer are tied together. The direction of the heat and mass transfer depends on the partial vapor
pressure and the temperature difference between the surface of the devices and the ambient air
[26]. The evaporation flux from the surface of the devices depends on the capillary flow of water
through the device to the surface of the clay walls and on the water vapor diffusion across the
boundary layer formed at the surface [27–30]. As the water in the devices depletes, the network of
liquid filled pores in the clay walls becomes disconnected and the evaporation flux becomes
dependent on the rate of vapor diffusion from within the sand gap and the clay wall to the outer
surface [16–18].
32
For all the devices, four distinct zones are observed: Zone 1—Cooling Transient, Zone 2—Cooling
Steady State, Zone 3—Drying Transient, Zone 4—Drying Steady State. Each evaporative cooling
device enters and exits each zone at a different time. For clarity, only the general divide of the
zones is displayed on the figures. The divide is determined by observing the changes in the internal
temperature profiles. It is important to note that because all the devices were conducted at room
temperature, the temperature drop from ambient is not as large as it would have been in hotter
environment.
In Zone 1, the cooling effect due to evaporation is in a transient state. The water added in the sand
gap is still diffusing into the sand and through the clay wall of the device via capillary action [27–
29]. Water can evaporate from three different surfaces in each device: the internal storage vessel’s
surface, the outer vessel’s surface, and the lid’s surface. In Zone 2, the majority of the clay wall
has been wetted and the device reaches its maximum cooling performance. The cooling effect
remains steady but not constant because of the daily temperature fluctuation in the test
environment. In Zone 3, the device enters another transient state. Here, the evaporative cooling
effect decreases as there is less water to evaporate. As the network of liquid filled pores inside the
device becomes disconnected, the capillary actions that supplied the outer vessel’s surface with
water decrease [31]. This causes the internal temperature drop from ambient and the internal
relative humidity to also begin decreasing. In Zone 4, the device is another dynamic steady state.
The internal temperature and the relative humidity have not fully equilibrated with the ambient
conditions. As the outer vessel’s surface dries out, the vapor concentration gradient between the
surface and the environment continues to decrease, and a vapor pressure gradient develops inside
the cooling devices’ sand gap and clay wall [31]. The latter gradient causes evaporation to occur
inside the sand gap and clay wall and drives diffusion towards the outer surface [31]. The cooling
33
effect in Zone 4 is smaller and slower because the evaporation rate is lower. As more time passes,
the internal temperature and relative humidity eventually do return to ambient conditions, signaling
the end of Zone 4.
3.1. Surface Area Experiment
The evaporative cooling devices used in the two Surface Area experiments are shown in Figure
7. Within each experiment, the same sand to water ratio is the same. The differences in the devices’
dimensions from being individually handmade account for the differences in total absolute amount
of water added to each device. The devices are labeled based on the ratio between the device’s
total surface area with added corrugation and the theoretical total surface area of the same device
without corrugation.
Figure 7. Evaporative Cooling Devices in Surface Area Experiments. The corrugated
surface on the evaporative cooling devices increases the surface area over which water can
evaporate without increasing the volume internally. Each device is labeled based on the
ratio between the device’s surface area with corrugation and the device’s surface area
without corrugation. The color of the devices matches the color on the plots.
34
Wet-bulb Temperature
Figure 8 compares the wet-bulb temperature drop from ambient of the two surface area
experiments. The two experiments are conducted in the exact same indoor space, yet their wet-
bulb temperature profiles differ. As a result, the cooling performance of individual devices cannot
be compared across experiments. However, general trends, similarities, and differences that repeat,
despite the wet-bulb temperature difference, provide insight into the cooling device’s performance.
For the first experiment, the general four zones are: 0 – 15 hours, 15 – 90 hours, 90 – 230 hours,
and 230 hours onward. For the second experiment, the general four zones are 0 – 60 hours, 60 –
230 hours, 230 – 310 hours, and 310 hours onward. The general four zones approximate each
device’s four zones. The lengths of the zones differ between the first and second surface area
experiment because of ambient conditions. The oscillations seen in the data are caused by daily
temperature fluctuation in the testing environment. The missing data between 50-61 hours in the
first experiment corresponds to a temporary halt of the data recording software due to an automated
computer restart.
35
Figure 8. Wet-Bulb Temperature Comparison Between Surface Area Experiment 1
and 2. The wet-bulb temperature is the lowest temperature achievable through evaporative
cooling. The difference in wet-bulb temperature between the first and second experiment
is large enough to affect the cooling performance of the devices. This difference prevents
the devices from being compared individually across experiments, but the general trends,
repeated similarities and differences are noted and analyzed.
36
Internal Temperature
Figure 9 and Figure 10 show the internal temperature drop for the two surface area experiments.
Table 1 summarizes key performance metrics of the devices in the two experiments. In Zone 1—
Cooling Transient, the added water wets the entirety of each cooling device as it disperses through
the sand and clay walls. While some water settles down at the bottom of the cooling device, a large
majority of the water binds to the sand, keeping the sand gap and adjacent clay surfaces wet.
Moisture data of the sand gap is provided in the Supplementary Information S.1. The temperature
drops of all the devices in Zone 1 are similar to each other because all the devices’ evaporation
rate is limited by the same ambient conditions that determine how quickly the saturated air near
the surface of the devices can be replaced with drier air [18,28]. The effect of surface area on the
cooling performance is not seen until Zone 2 when the majority of the outer surfaces have been
wetted and the cooling effect is at its maximum. The temperature drop profiles of the devices begin
deviating from each other in Zone 2 because the added corrugations change the dynamics of vapor
transport on the surface, which then affects water evaporation rate. All the cooling devices with
added surface corrugation have greater internal temperature drops than the device without
corrugation. The corrugation adds more surface area that allows for more water evaporation to
occur. The net mass losses and mass loss rates of both surface area experiments reflect that the
water evaporation rate is higher for the devices with surface corrugation. This information is
included in the Supplementary Information S.2.
Among the devices with some surface corrugation, the addition of more corrugation does not
always result in a greater temperature drop. In the first surface area experiment, the 2.59x Device
has the greatest temperature drop, followed by the 1.52x Device instead of the expected 1.77x
Device. In the second experiment, the 1.77x Device has the greatest temperature drop followed by
37
the 2.59x Device and the 1.52x Device. The inconsistency in performance between the two
experiments likely results from the competing effects introduced by the added surface corrugation.
Figure 9. Internal Temperature Drop for Surface Area Experiment 1. The lines
indicating the four zones approximate when each device enters and exits each zone. In
Zone 2, the devices with the added corrugation have greater temperature drops than the
device without. Among just the devices with corrugated surface, the temperature drop does
not always increase with more corrugation. This suggests that using corrugation to increase
surface area introduces other competing factors that affect the cooling performance.
38
Figure 10. Internal Temperature Drop for Surface Area Experiment 2. The
temperature drop of the devices with corrugated surface is greater than the temperature
drop of the device without corrugation. This is also observed in the first experiment. The
device that has the greatest temperature drop differs between the two experiments. This
further suggests that corrugated surfaces increase surface area but also introduce other
competing effects that influence cooling performance.
39
Table 1. Internal Temperature Drop Cooling Performance for Surface Area
Experiments. The maximum temperature drops and the time they occur is listed for all
devices in both experiments. The device with the maximum temperature drop is
highlighted. Temperature drop difference of greater than 0.5 ℃ are outside the sensor error
and are more significant. The total amount of water added and the time when the
temperature drops first return to zero are also included. The temperature drop of the 1x
Device in Experiment 2 does not return to zero within the experiment timespan. The time
it takes the devices in the second experiment to reach their max temperature drop is almost
2-3 times longer than in the first experiment because of differences in ambient conditions.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Max
Internal
Temperature
Drop
[°𝐂]
Time Max
Internal
Temperature
Drop
[Hour]
Time
Temperature
Drop
Returns to
Zero
[Hour]
1 1x Device 2.84 5.72 14 218
1 1.52x Device 3.43 6.56 63 216
1 1.77x Device 3.03 5.35 62 123
1 2.59x Device 3.13 7.24 62 213
2 1x Device 3.06 4.28 41 Never
2 1.52x Device 3.32 5.81 135 360
2 1.77x Device 3.06 5.42 207 360
2 2.59x Device 3.13 5.68 207 360
Gao et al. and Haghighi et al. both conducted experiments on the effects of evaporation from wavy
surfaces [28,32]. Gao et al. studied the effects of evaporation from a bed of sand with a single
“wave” in natural convection [28]. The experiments conducted by Gao et al. found that a steeper
“wave” caused lower vapor concentration gradient at the surface and led to lower evaporation rates
[28]. Haghighi et al. conducted experiments on evaporation from wavy sand surfaces in turbulent
airflow and also found that evaporation flux is suppressed in the trough of the “wave” and that the
evaporation rates depend on the ratio of height to distance between each “wave” due to the
competing effects between increased evaporating surfaces and reduced vapor concentration
gradient [32]. There are differences between the experiments on the wavy surfaces and the surface
area experiments conducted in this thesis that were taken into consideration before extrapolating
40
the studies’ findings. Together, the differences prevent a direct extrapolation for some of the
studies’ findings, but the insight that wavy geometry reduces vapor concentration gradient in the
troughs of the corrugation can explain the competing effects seen in the surface area experiment.
The differences include the water diffusion direction, the shape and scale of the corrugation, the
evaporation surface material, the presence of heat transfer due to radiation, and the presence of
controlled airflow. In the surface area experiments of this thesis, the capillary gradient required for
water diffusion to the outer evaporation surface is smaller because water does not have to act
against gravity to reach the evaporation surfaces. The effect of gravity is mostly seen when the
device is running low on water because there is more vertical distance between the water and the
evaporation surface [28,32]. The extrapolation of the findings from the study is restricted to when
the cooling rate is constant near the beginning. The corrugations on the devices in this thesis are
not sinusoidal; they are better approximated as square waves. Nevertheless, the wavy surface still
serves as a good approximation of evaporation behavior over a wavy surface and as a reference
for the corrugations. Although the scale of the corrugations in the studies is larger, the ratio of the
wave’s height to width in Gao et al. experiments and ratios of corrugated surface area to
corresponding flat-surface area in the Haghighi et al. experiments are respectively comparable to
the corrugation in this thesis. In the surface area experiments, water evaporates from the surface
of clay pots which is more rigid and less permeable than sand. This means that in the surface area
experiments of this thesis, water has more resistance to get to the surface, but once water is on the
surface, how the corrugation affects water evaporation is comparable to the effects seen in the
studies by Gao et al. and Haghighi et al. The presence of radiation increases the temperature of the
device which increases evaporation rate but does not completely negate the effect of the
corrugation on the vapor concentration gradient. The presence of controlled airflow in the Gao et
41
al. and Haghighi et al. experiments simplified the dynamics because air always flowed in one
direction. This does not completely describe the devices in the surface area experiments of this
thesis because the devices are exposed to air flow from random directions. Haghighi et al. did
include data on airflow that are perpendicular and parallel to the corrugation that better aid in
understanding the airflow dynamics of the devices [32].
To further understand the competing effects due to corrugation, Figure 11 and Figure 12 present
the mass transfer coefficient of the devices in the surface area experiments. The mass transfer
coefficient quantifies how much water vapor has diffused from near the device’s surface to the
environment, normalized by the area and concentration gradient. The higher the mass transfer
coefficient, the more unsaturated air is replenished near the surface of the device to allow for more
evaporation to occur. It is calculated as follows [18,26,31]:
ℎ𝑚 =𝐸𝑒𝑣𝑎𝑝
𝑀𝑤𝑅
(𝑃𝑠𝑎𝑡,𝑠𝑢𝑟𝑓𝑎𝑐𝑒
𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒𝑅𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒−
𝑃𝑠𝑎𝑡,𝑎𝑚𝑏𝑇𝑎𝑚𝑏
𝑅𝐻𝑎𝑚𝑏) [Eq. 4]
where,
ℎ𝑚 is the mass transfer coefficient, measured in [𝑚
𝑠]
𝐸𝑒𝑣𝑎𝑝 is the evaporation flux, measured in [𝑘𝑔
𝑚2𝑠]
𝑀𝑤 is the molar mass of water, 0.018 [𝑘𝑔
𝑚𝑜𝑙]
𝑃𝑠𝑎𝑡,𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the saturated vapor pressure of air at the surface, measured in [Pa]
𝑇𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the temperature of air at the surface, measured in [K]
𝑅𝐻𝑠𝑢𝑟𝑓𝑎𝑐𝑒 is the relative humidity of air at the surface, measured in [%]
𝑃𝑠𝑎𝑡,𝑎𝑚𝑏 is the saturated vapor pressure of ambient air, measured in [Pa]
𝑇𝑎𝑚𝑏 is the temperature of ambient air, measured in [K]
42
𝑅𝐻𝑎𝑚𝑏 is the relative humidity of ambient air, measured in [%]
𝑅 is the ideal gas constant, 8.314 [𝐽
𝑚𝑜𝑙 𝐾]
Buck’s equation is used to find the saturated vapor pressure at the surface and in the environment
[33]:
𝑃𝑠𝑎𝑡(𝑇) = (100)6.1121𝑒((18.678− 𝑇
234.5)(
𝑇
257.14+𝑇))
for liquid water T > 0 ºC [Eq. 5]
where,
𝑃𝑠𝑎𝑡(𝑇) is the saturated vapor pressure, measured in [Pa]
𝑇 is the temperature at the specific location, measured in [ºC]
The mass transfer coefficient of both experiments is steadiest in Zone 2 because that is when the
evaporation flux is steadiest. The negative and increased values of the mass transfer coefficient in
Zone 3 and 4 are mathematical results that occur when the concentration of water vapor in the
ambient air is greater than or close to the concentration in the air near the surface. This mostly
occurs when the devices have started to deplete their water sources.
Adding more surface area over which water can evaporate, in isolation, will increase the
evaporation rate, but in practice, the geometry of the corrugation causes a competing effect that
reduces vapor concentration gradient inside the trough of the corrugation; this creates a bottleneck
in the water evaporation processes [34,35].
In the first experiment, the mass transfer coefficient of the devices varies inversely with the
increased surface area. In the second experiment, the same pattern is observed but to a lesser
degree. This inconsistency in performance of the surface corrugation devices is likely due to the
43
patterns of airflow around the devices given the openness of the experiment’s location. Haghighi
et al. observed that the airflow dynamics in parallel to the troughs resemble that of over a flat
surface [32]. Because the devices in the surface area experiments of this thesis are exposed to
varied airflow with no set direction, the changes in airflow, combined with the varying amount of
irregularities in the added corrugation that come from the fact that the devices are handmade,
explain the inconsistent results across the two experiments. The surface area experiments show
that adding some corrugation to flat surfaces consistently resulted in greater temperature drops,
but reducing the interridge distance to add more corrugation also reduced the vapor concentration
gradient from the surface to the ambient environment which slows evaporation. This suggests that
the parameters of the corrugations could be optimized to maximize cooling performance.
44
Figure 11. Mass Transfer Coefficient for Surface Area Experiment 1. The mass
transfer coefficient is how fast saturated air is replenished with drier air near the surface of
the device. A lower mass transfer coefficient reflects a low evaporation flux because the
concentration of lingering saturated air at the device’s surface decreases water evaporation.
Adding corrugation to the devices causes the mass transfer coefficient to decrease because
the geometry reduces the vapor concentration at the surface [32]. Zone 2 has the steadiest
mass transfer coefficient because the evaporative cooling rates are steadiest there. Near the
end of the experiments, the large increases and negative values of the mass transfer
coefficient come mathematically from the decrease in concentration gradient between the
ambient air and the air near the surface as the devices have less water.
45
Figure 12. Mass Transfer Coefficient for Surface Area Experiment 2. The mass
transfer coefficient profiles in experiment 2 have the same inverse relationship with
increased surface corrugation but not as distinctly due to differences in airflow ambient
conditions. The negative values and large increased in value near the end are caused as the
depleting water source decreases the concentration gradient. The increased in values near
the beginning results from ambient conditions.
In Zone 3—Drying Transient, the temperature drops of all the devices with surface corrugations
begin to decrease as they have less remaining water to evaporate; consequently, their surfaces
begin to dry. Their rates of evaporation are limited by the reduced partial vapor pressure gradient
inside the troughs of the corrugation. In Zone 3, the effect of having a greater absolute amount of
water added has the most influence on the device’s cooling performance. Each device has a
different amount of water added but maintains the same sand to water ratio to account for the
differences in dimensions of each device. The more water a given device has, the longer it is
46
expected to maintain evaporative cooling. Nonetheless, the devices with the added corrugated
surfaces have a higher absolute amount of water added than the device without corrugation, yet
their temperature drops start to decrease sooner than the device with no added corrugated surface.
This occurs because devices with corrugated surfaces have faster rates of evaporation, which leads
to a faster depletion of total water reserve. In the first experiment, the temperature drop profile of
the 1.77x Device deviates from the other devices’ profiles, measuring temperatures greater than
ambient temperature for an initial time period before returning to similar temperature drop values
as the other devices. When the temperature drop profile of the 1.77x Device measures above zero,
it still captures the same undulation as the other devices; however, the data is offset, indicating
sensor hardware error. Table 1 summaries key performance metrics.
Entering Zone 4—Drying Steady State, all the devices continue to approach a temperature drop of
zero as the remaining water is still evaporating. The thermal mass of the devices also helps
maintain the coolness. The cooling effect in Zone 4 is not as large as in the other zones, but it lasts
longer because the evaporation rate is low. The evaporation rate is now limited by how quickly
water vapor inside the sand gap and clay wall can diffuse to the surface [18,28]. In Zone 2, the
evaporation rate was limited by how quickly water vapor can diffuse away from the outer surface
through air, which is faster than the water vapor transport dynamics through the device. In
experiment 2, the runtime is twice as long, and near the end of Zone 4, the temperature drops of
all the devices approach zero.
47
Temperature on Evaporation Surface
Figure 13 and Figure 14 display the temperature on the evaporation surface of each device as a
function of time. The temperature drop profiles on the evaporation surfaces best reflect the
temperature changes due to water evaporation. The greater the temperature drop, the more
evaporative cooling that occurred. In both experiments, the temperature drops of the devices with
surface corrugation are greater than the temperature drop of the 1x Device. This further supports
that adding surface corrugations leads to more evaporation and evaporative cooling. Table 2
displays key performance metrics of the temperature drop on the evaporation surface. Among the
devices with added corrugation, the temperature drop performances of the individual devices are
not consistent between experiment 1 and experiment 2. The inconsistency is attributed to the
competing factors that arise from increasing available surface area. Adding corrugations will
increase the total surface area for more water evaporation but will decrease the vapor pressure
gradient inside the troughs.
48
Figure 13. Evaporation Surface Temperature Drop for Surface Area Experiment 1.
The temperature on the evaporation surface reflects the amount of heat transfer due to
evaporation. The devices with the surface corrugation all have greater temperature drop
than the device without. This signifies there is more evaporative cooling that occurs on the
surface of the devices with the added corrugation. Among the devices with added
corrugation, the devices with more surface area added record greater temperature drops.
49
Figure 14. Evaporation Surface Temperature Drop for Surface Area Experiment 2.
The temperature drop profiles on the evaporation surface in experiment 2 have less
differences among them than in experiment 1 because of the ambient conditions. In Zone
2, the 1x Device has a smaller temperature drop than the devices with added surface
corrugation. This is also observed in experiment 1. Among the devices with added
corrugation, the 1.52x Device that had the lowest temperature drop in experiment 1 now
has the greatest temperature drop. The inconsistency in performance of the devices with
surface corrugation is because of the competing effect between increasing surface area for
evaporation and decreasing the mass transfer coefficient.
50
Table 2. Evaporation Surface Temperature Drop Cooling Performance for Surface
Area Experiments. The maximum temperature drop on the surface and the time it occurs
are presented. The greatest temperature drop is highlighted. The time the devices in the
second experiment take to reach the maximum temperature drop is almost 2-3 times as
long as in the first experiments due to ambient conditions.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Max Surface
Temperature
Drop
[°𝐂]
Time Max
Surface
Temperature
Drop
[Hour]
1 1x Device 2.84 4.24 12
1 1.52x Device 3.43 4.91 62
1 1.77x Device 3.03 5.39 62
1 2.59x Device 3.13 5.49 62
2 1x Device 3.06 3.00 41
2 1.52x Device 3.32 4.27 135
2 1.77x Device 3.06 4.07 207
2 2.59x Device 3.13 4.28 207
51
Cooling Efficiency
Figure 15 and Figure 16 illustrate the cooling efficiency profiles of the different devices. For both
experiments, the greatest efficiency occurs during Zone 2—Cooling Steady State—and aligns with
when the cooling devices have their lowest temperature drop. In Zone 3, the cooling efficiency of
all the devices except for the 1x Device’s drops as the internal temperature drop decreases. In
Zones 3 and 4, the 1x Device records the greatest cooling efficiency because it has the greatest
temperature drop. The 1x Device still has water for evaporative cooling due to its slower rate of
evaporation. The cooling efficiency profiles of the first and second experiments share similar
trends and observation as show in Table 3. The maximum cooling efficiency of the devices occur
before their maximum internal temperature drops in experiment 1 but after their maximum internal
temperature drops in experiment 2. The difference in performance is due to the differences in
ambient conditions. The cooling efficiency captures what percentage of maximum temperature
drop (wet-bulb temperature drop from ambient) the devices achieved so it is not unexpected that
the time the maximum cooling efficiency occurs does not align with when the maximum internal
temperature drop occurs.
52
Figure 15. Cooling Efficiency for Surface Area Experiment 1. The maximum cooling
efficiency occurs in Zone 2 and before the maximum internal temperature drop. The 1x
Device has the lowest cooling efficiency of all the devices in Zone 2 but near the end of
Zone 3, it has a highest cooling efficiency. The 1x Device’s slower evaporation rate enables
it to still have water to evaporate. The 1.77x Device has a negative cooling efficiency
because it recorded internal temperature above ambient temperature due to sensor hardware
error as previously discussed.
53
Figure 16. Cooling Efficiency for Surface Area Experiment 2. The maximum cooling
efficiency in experiment 2 occurs in Zone 2. The devices’ maximum cooling efficiency
occurs after their maximum internal temperature drops. The devices with the surface
corrugation have the maximum cooling efficiency overall. In Zone 3 and 4, the 1x Device
has a greater cooling efficiency because it still has water to evaporate due to its slower
evaporation rate.
54
Table 3. Cooling Efficiency Performance for Surface Area Experiments. The max
cooling efficiency and the time the efficiency occurs is presented with the maximum
efficiency highlighted.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Max
Efficiency
[%]
Time Max
Efficiency
[Hour]
1 1x Device 2.84 52.39 13
1 1.52x Device 3.43 58.50 40
1 1.77x Device 3.03 45.59 39
1 2.59x Device 3.13 65.28 40
2 1x Device 3.06 40.95 184
2 1.52x Device 3.32 69.98 182
2 1.77x Device 3.06 61.22 182
2 2.59x Device 3.13 66.10 182
55
Total Cooling
The total cooling performance metric accounts for the duration of the evaporative cooling to better
compare the devices with slower but longer rates of evaporation, i.e. the 1x Device to the other
devices. The 1x Device has a smaller temperature drop overall but maintains evaporative cooling
for a longer time period than the others. Figure 17 and Figure 18 show the total cooling as a
function of time for the two experiments. In both experiments, the total cooling profiles of the
devices with surface corrugations have two distinct slopes determined by what factors limit their
evaporative cooling. In Zone 2 of both experiments, the entirety of the devices’ surface becomes
wet and the evaporation rate is limited by how quickly vapor saturated air can be replaced with
drier air; temperature, relative humidity, and air flow controls this stage [18,28]. As the devices
lose more water, and the surfaces of the devices begin to dry out, the evaporation rate is now
limited how quickly water vapor inside the sand gap and clay wall can diffuse to the surface; the
porosity, permeability, and tortuosity of the sand and clay wall dictate the evaporation rate in this
stage [18,28]. The slope at the beginning of the experiment is steeper than later in the experiment
because water vapor on the surface can diffuse into the ambient air quicker than it can diffuse
through the sand gap and clay wall. How quickly each device with corrugated surface reaches the
transition point where the slope changes depends on when the liquid filled cavities near the outer
surface become disconnected because capillary forces that brought water to the outer surface to
evaporate stop [31]. The 1x Device reaches its transition point much later than the devices with
corrugated surfaces even though it has the least amount of absolute water added. The 1x Device
maintains its cooling effect for longer than the others because its rate of evaporation was smaller
in the previous zones; it still has more water left to evaporate and sustain capillary forces to draw
water to its outer surface.
56
The total cooling of the devices in the first experiment reveals that the 1.52x Device has a lower
total cooling amount until Zone 3 when it surpassed the 2.59x Device. The crossover occurs
because the 2.59x Device receives 0.26 kg less total amount of water than the 1.52x Device and
has a higher rate of evaporation at the beginning of the experiment. The 1x Device receive 0.59 kg
less than the 1.52x Device but at the end of Zone 4, its total cooling is comparable to 2.59x and
1.52x Device. Table 4 presents the total cooling achieved by the end of the experiments. The 1x
Device total cooling performance presents an interesting practical opportunity for users in water
scarce, hot, and arid environment. The device does not provide as great of a temperature drop but
maintains it cooling effects for longer with less water added.
The 1.77x Device exhibits unexpected result that is also reflected in Figure 9, the internal
temperature drop figure. The performance of the 1.77x Device is attributed to sensor error so it is
not included in the analysis. The second experiment lasts twice as long as the first experiment to
further observe the devices’ cooling performance. In the second experiment, the total cooling of
1x Device surpasses the total cooling of the 1.77x Device near the end of the experiment.
57
Figure 17. Total Cooling for Surface Area Experiment 1. The total cooling performance
metrics accounts for how long evaporative cooling lasts. The 1x Device has a smaller
temperature drop than the other devices but by the end of the experiment, its total cooling
is comparable to the others. The 1.77x Device total cooling profile reflects the sensor
hardware error.
58
Figure 18. Total Cooling in Surface Area Experiment 2. The total cooling of the devices
with the surface corrugation share similar profiles. The point where the profile changes
slope at the end of Zone 2 aligns with when the devices run low on water. The 1x Device
has the least amount of water added but is able to maintain cooling effect and achieve a
comparable total cooling amount at the end of the second experiment. Its low evaporation
rate allows it to still have water to evaporate when the other devices have already
evaporated the majority of their water supply. The 1x Device surpasses the total cooling of
the 1.77x Device near the end of the experiment.
59
Table 4. Total Cooling Performance for Surface Area Experiments. The total cooling
at the end of the experiments is provided. The 2.59x Device in experiment 1 has the greatest
total cooling until near the middle of Zone 3 when it is surpassed by the 1.52x Device. The
total cooling amount of the 1.77x Device in the first experiment is omitted because of the
sensor hardware error. In experiment 2, the 1x Device surpasses the 1.77x Device near the
end of the experiment.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Total
Cooling
[°𝐂 × 𝐇𝐨𝐮𝐫]
Time Total
Cooling
[Hour]
1 1x Device 2.84 811 306
1 1.52x Device 3.43 923 306
1 1.77x Device 3.03 - 306
1 2.59x Device 3.13 890 306
2 1x Device 3.06 1097 613
2 1.52x Device 3.32 1205 613
2 1.77x Device 3.06 1059 613
2 2.59x Device 3.13 1152 613
60
Relative Humidity
At the start of the experiment, the added water begins evaporating into the internal storage vessel
of the device and out into the environment. Figure 19 and Figure 20 display the relative humidity
inside the internal storage chamber, on the surface, and in the environment around the device. The
relative humidity inside the device increases faster than the relative humidity on the surface
because the inside is covered by a lid. When air in the inner vessels reaches saturation, more water
begins to evaporate from the outer surface because the saturated air inside the internal storage
vessel has less capacity to hold more water vapor. The relative humidity and the partial pressure
of water vapor increases around the vicinity of the device as more water evaporates from the
outside surface. The difference between the partial pressure of water vapor at the surface and in
ambient environment creates a concentration gradient that moves the water vapor away from the
device to allow for more evaporation.
In experiment 1, the relative humidity measurements of the 1.52x Device and the 2.59x Device
suggest that the measurement sensors have gotten wet inside the device. It is unrealistic for any
devices to measure 99.9% relative humidity in Zone 4 as they all have porous lids that cannot
contain water vapor for over 230 hours. The 99.9% relative humidity measurements inside of the
1.77x Device is a result of the sensor hardware error. It is also unrealistic for the relative humidity
inside the 2.59x Device to increase in Zone 4. The internal relative humidity data for the devices
with surface corrugation is thus omitted from analysis.
In experiment 2, the devices with corrugation all drop below 80% relative humidity before the 1x
Device. This is attributed to the differences in surface area. The devices with surface corrugation
evaporate water faster than the 1x Device so that by Zone 4, these devices run low on water even
though they all have greater absolute amounts of water added. Among the devices with surface
61
corrugation, adding more surface area increases how long the internal relative humidity lasted.
This is because added corrugation can decrease the concentration gradient at the surface which can
slow evaporation. Table 5 summaries the key relative humidity metrics inside and on the
evaporative cooling surface.
Figure 19. Relative Humidity in Surface Area Experiment 1. The relative humidity
levels inside and on the evaporative surface of the device are greater than the ambient
environment relative humidity because of water evaporation. The internal relative humidity
is greater than on the surface because of the wetted lid. The relative humidity inside and
on the surface decreases over time because the devices run out of water to evaporate. The
2.59x Device dips below 80% relative humidity first because its higher levels of
evaporation means that it has less water to evaporate. The saturated relative humidity of
the 1.52x and 1.77x Device suggests that the measurement sensors may have encounter
water inside the device that wetted the measurement sensor because it is unlikely for any
cooling device with a porous top to maintained saturated levels of relative humidity until
Zone 4.
62
Figure 20. Relative Humidity in Surface Area Experiment 2. The relative humidity
levels inside and on the surface of the device is higher than the ambient relative humidity.
This is also seen in Experiment 1. The difference between the relative humidity on the
surface and in the ambient is smaller in experiment 2 than in experiment 1. The effect of
this difference is reflected in the other performance metrics as well. The internal relative
humidity of the 1x Device remain above 80% for longer than all devices with surface
corrugation. This is due to how the 1x Device has a smaller evaporation rate than the others
so it is able to maintain evaporation until the end of Zone 4.
63
Table 5. Relative Humidity of the Surface Area Experiments. The relative humidity
(RH) at the end of the experiment inside the device is provided along with the time when
the relative humidity first measured below 80%. No data is presented from the devices with
corrugations in the first experiment on the time the relative humidity measures below 80%
and on the final measurement of the internal relative humidity measurement at the end of
the experiment because the data does not realistic represent the relative humidity inside
those devices.
Experiment
Number Devices
Amount
of Water
Added
[kg]
Time <80%
Internal RH
[Hour]
Internal RH
at the End
[%]
1 1x Device 2.84 222 63.27
1 1.52x Device 3.43 - -
1 1.77x Device 3.03 - -
1 2.59x Device 3.13 - -
2 1x Device 3.06 485 46.98
2 1.52x Device 3.32 303 28.07
2 1.77x Device 3.06 311 32.73
2 2.59x Device 3.13 335 31.41
The devices in the surface area experiments have shown that adding corrugations results in greater
rates of evaporation, higher max cooling efficiencies, and greater temperature drops internally
compared to a device that does not have corrugation. However, adding more corrugation does not
consistently lead to better cooling performance. The corrugation geometry introduces competing
effects between the increased available surface area for water evaporation and lowered
concentration gradient for mass transport of saturated air inside the troughs of the corrugation
[28,32]. When the concentration gradient is reduced the diffusion in the boundary layer becomes
the bottle neck in the evaporation process.
The inconsistency in cooling performance between experiments among the devices with
corrugations is due to differences in temperature, relative humidity, and airflow in the ambient
environments. For the devices in the surface area experiments, airflow particularly has a large
64
influence because the direction of the airflow over the corrugations impacts the concentration
gradient in the troughs [32]. When the air flows perpendicular to the corrugation, it induces an
inverse relationship between the amount of added surface area and the mass transfer coefficient;
when the air flows parallel to trough, it has the same airflow patterns as flowing over a flat-surface
[32]. Since the devices in the surface area experiments are exposed air flow with no set direction,
the cooling performance is varied depending on ambient conditions. The corrugations on the
devices itself have irregularities in their dimensions that also contribute to the inconsistent cooling
performance.
Between the two experiments, the devices with some corrugation consistently have a greater
temperature drop internally than the device without corrugation; however, more corrugation does
not always lead to better cooling performance. This suggests that the parameters of the corrugation
can be optimized for better cooling performance. The length, width, orientation and entrance angle
of the corrugation can impact the cooling performance. Table 6 shows a comprehensive table of
all the performance metrics.
The relative humidity inside the device without corrugation lasts the longest with the least absolute
amount of water added. Its evaporation rate is slower than the devices with corrugation so it is able
sustain its water supply until near the end of each experiments. The device without corrugation has
the smallest temperature drop but overall, its total cooling performance is comparable to the other
devices. For users in water scarce area, the device without corrugation may be more appropriate.
For users with more access to water, adding more water at the end of Zone 2 will enable the devices
to sustain its maximum temperature drop and high internal relative humidity for longer.
In general, adding corrugations to the outer vessels increases maximum temperature drop, but there
are practical tradeoffs for users to consider. To create uniform surface corrugations on clay pots,
65
additional equipment such as corrugation cutting tools and pottery wheels are required. The users
will need access to more skilled potter to create the corrugations as well. Adding corrugations will
increase the total weight and the total amount of time required to make a single pot. The
corrugations are more susceptible to being chipped so they necessitate additional care in handling
and transportation. The added weight and additional required care further burdens clay pot
vendors. A survey of clay pot and container vendors in Burkina Faso found that only a third of
vendors would deliver clay pots to their customer because of how heavy and fragile clay pots are
[23]. Depending on the final geometry of the added corrugation and the availability of skill labor,
evaporative cooling devices with added corrugations may not be practical or accessible.
66
Tab
le
6.
Co
mp
reh
ensi
ve
Co
oli
ng
Per
form
an
ce
Met
rics
of
Su
rface
A
rea
Exp
erim
ents
. T
he
max
imum
v
alu
e o
f ea
ch
per
form
ance
met
ric
is h
ighli
gh
ted. F
or
the
tem
per
ature
met
rics
, any v
alue
that
is
wit
hin
the
sen
sor
erro
r o
f ±
0.5
°C
of
the
max
imum
val
ue
is a
lso
hig
hli
gh
ted
. T
he
"nev
er"
term
is
appli
cable
only
wit
hin
the
tim
espan
of
the
resp
ecti
ve
exp
erim
ent.
Th
e co
oli
ng
per
form
ance
met
rics
th
at a
re m
ost
pra
ctic
al f
or
use
rs a
re t
he
inte
rnal
tem
per
ature
dro
p,
the
tim
e b
efo
re t
he
inte
rnal
tem
per
ature
dro
p r
etu
rns
to z
ero
and
ho
w l
ong h
igh l
evel
s of
inte
rnal
rel
ativ
e hum
idit
y l
ast.
In b
oth
exp
erim
ents
, th
e dev
ices
wit
h s
urf
ace
corr
ug
atio
n h
ave
gre
ater
inte
rnal
tem
per
ature
dro
ps
bu
t did
not
sust
ain t
hei
r over
all
evap
ora
tive
cooli
ng
fo
r as
lo
ng
as
the
dev
ice
wit
ho
ut
surf
ace
corr
ug
atio
n.
The
dev
ice
wit
hout
corr
ugat
ion a
lso m
ainta
ins
rela
tive
hum
idit
y a
bo
ve
80%
fo
r lo
ng
er t
han
the
dev
ices
wit
h c
orr
ug
atio
n.
Th
ere
is a
tra
de-o
ff b
etw
een h
avin
g g
reat
er i
nte
rnal
tem
per
atu
re d
rop
an
d l
on
ger
per
iod
s of
inte
rnal
cooli
ng
an
d h
igh
lev
els
of
inte
rnal
rel
ativ
e hum
idit
y.
For
use
rs i
n w
ater
-sca
rce
area
, th
e dev
ice
wit
ho
ut
corr
ug
atio
n m
ay b
e m
ore
appro
pri
ate.
Inte
rna
l
RH
at
the
En
d
[%]
63
.27
- - -
46
.98
28
.07
32
.73
31
.41
Tim
e
<8
0%
Inte
rna
l
RH
[Ho
ur]
22
2
- - -
48
5
30
3
31
1
33
5
Tim
e
To
tal
Co
oli
ng
[Ho
ur]
30
6
30
6
30
6
30
6
61
3
61
3
61
3
61
3
To
tal
Co
oli
ng
[°C
× H
ou
r]
81
1
92
3
-
89
0
10
97
12
05
10
59
11
52
Tim
e M
ax
Eff
icie
ncy
[Hou
r]
13
40
39
40
18
4
18
2
18
2
18
2
Max
Eff
icie
ncy
[%]
52.3
9
58.5
45.5
9
65.2
8
40.9
5
69.9
8
61.2
2
66.1
0
Tim
e
Max
Su
rface
Tem
p
Dro
p
[Hou
r]
12
62
62
62
41
135
207
207
Max
Su
rface
Tem
p
Dro
p
[°C
]
4.2
4
4.9
1
5.3
9
5.4
9
3
4.2
7
4.0
7
4.2
8
Tim
e
Tem
p
Dro
p
Ret
urn
s
to Z
ero
[Hou
r]
218
216
123
213
Nev
er
360
360
360
Tim
e
Max
Inte
rnal
Tem
p
Dro
p
[Hou
r]
14
63
62
62
41
135
207
207
Ma
x
Inte
rnal
Tem
p
Dro
p
[°C
]
5.7
2
6.5
6
5.3
5
7.2
4
4.2
8
5.8
1
5.4
2
5.6
8
Am
ou
nt
of
Wa
ter
Ad
ded
[kg
]
2.8
4
3.4
3
3.0
3
3.1
3
3.0
6
3.3
2
3.0
6
3.1
3
Dev
ices
1x
Dev
ice
1.5
2x
Dev
ice
1.7
7x
Dev
ice
2.5
9x
Dev
ice
1x
Dev
ice
1.5
2x
Dev
ice
1.7
7x
Dev
ice
2.5
9x
Dev
ice
Exp
Nu
mb
er
1
1
1
1
2
2
2
2
67
3.2. Effects of Porosity
Figure 21 summarizes the devices used in the two porosity experiments. In the first experiment,
all the inner storage vessels are the same material. In the second experiment, the inner and outer
vessels of each device share the same level of porosity. Only the 7.20% porosity and the 11.22%
porosity devices use sawdust to form their porosity. All three of the devices share the same
terracotta base clay.
Figure 21. Evaporative Cooling Devices in the Porosity Experiments. The increased
levels of porosity allow for higher levels of water permeability and greater rates of
evaporation. The devices are labeled based on their porosity level and are colored to match
the plots.
68
Wet-bulb Temperature
Figure 22 compares the wet-bulb temperature of the two porosity experiments. The wet-bulb
temperature profile of the second porosity experiment is the same as the temperature profile of the
second surface area experiment because those experiments were conducted in parallel. Similarly,
to the surface area experiments, the differences between the temperature profiles prevent direct
comparison of the cooling devices across experiments. The devices have a greater cooling potential
in the first experiment due to a lower wet-bulb temperature profile.
Figure 22. Wet-bulb Temperature Comparison Between Porosity Experiment 1 and
2. The wet-bulb temperature represents the greatest cooling achievable through evaporative
cooling. The difference in wet-bulb temperature between the first and second experiment
is large enough to prevent a direct comparison across experiments, but the general trends,
repeated similarities and differences are noted and analyzed.
69
Internal Temperature
The internal temperature drop increases as the porosity in the clay wall increases as shown Figure
23 and Figure 24. The four zones depicted on the figures are also an approximation determined
by observation of changes in the internal temperature drop profile. Each device enters and exits
each zone at a separate time based on its internal properties.
In the previous section on surface area experiments, all devices have the same composition; as a
result, all devices share similar temperature drops at the beginning of the experiments. Their
evaporation rates are limited by how quickly the saturated air near the surface of the devices can
be replaced with drier air [18,28]. However, in the porosity experiments, the devices with more
porosity in their clay wall have greater vapor concentration because their clay walls have more
cavities to hold water. A greater vapor concentration at the surface leads to a greater concentration
gradient, which in turn drives the evaporation rate faster. In Zone 1 of the first porosity experiment,
the two devices with greater porosity have greater temperature drops than the device with lower
porosity due to the differences in vapor concentration at the surface of each device.
The devices in the first experiments all have internal vessels at 3.14% porosity. In the second
experiment, the porosity levels of the internal vessels match the porosity levels of the outer vessel.
For the 7.20% porosity and 11.22% porosity devices, the increase in porosity levels of the internal
vessels decrease the concentration gradient in these devices’ sand gap between experiment 1 and
2. The decreased concentration gradient inside the sand gap, in combination with the lowered
cooling potential as evident by the wet-bulb temperature profiles in Figure 22, explain why near
the beginning of Zone 1 of the second porosity experiment, the temperature drops of the three
devices are similar to each other. The temperature drop profiles of the three devices separate as the
added water wets more of the outer surfaces. The temperature drop profile of the 11.22% porosity
70
device deviates from the other profiles first because its higher percentage of porosity in its outer
vessel allows for greater capillary flow to wet its outer vessel’s surface quicker. The temperature
drop profile of the 7.20% porosity device separates from the 3.14% porosity later in the middle of
Zone 2.
In Zone 2, the temperature drop profiles reach a steady cooling rate. The oscillation occurs because
of the daily fluctuation of temperature in the indoor testing space. The difference in temperature
drop in Zone 2 across devices is due to the influence of porosity on water transport resistance. As
more water evaporates from the surface, the devices with more porosity are able to rewet their
surface quicker because there is more voids near the surface to hold water [16]. As expected, the
device with the highest porosity (11.22% porosity device) achieves the greatest temperature drop.
It was able to achieve the greatest temperature drop even with 0.45 kg less water than the 7.20%
porosity device. The device with the lowest porosity (the 3.14% porosity device) has the lowest
temperature drop but its cooling effect lasts the longest in both experiments. In experiment 1, Zone
2 of the 3.14% porosity device lasts until the end of the experiment, and in experiment 2, Zone 2
of the 3.14% porosity device lasts until around the 300-hour mark. It sustains evaporative cooling
for longer than the other devices because its lower porosity slows its evaporation rate. As a result
of the slower rates, the 3.14% porosity device has more water left by the end of the experiment
even though it has the least absolute amount of water added.
The maximum temperature drops in the second experiment of the 7.20% porosity and the 11.22%
porosity devices are not as large as their drops in experiment 1 due to lower cooling potential and
differences in their internal vessels’ material porosity between experiment 1 and 2. In experiment
2, the internal vessels of the 7.20% porosity and the 11.22% porosity have greater percentages of
porosity than in experiment 1. The greater percentage of porosity allows for water to evaporate
71
into the inner storage vessel more easily and reduce the concentration gradient inside of the sand
gap of the devices and the overall evaporation rate that influences the internal temperature drops.
The temperature drops in the devices with higher levels of porosity begin decreasing due to
depleted levels of water in Zone 3. The water inside the devices with more porosity experience
less transport resistances across the clay wall. As the devices dry, the evaporation rates slow due
to reduced water availability near the evaporation surface and disruption to the continuous network
of liquid-filled pores inside the clay walls [17]. The moisture content in the device is greatest near
the bottom because of gravity and close to the internal vessel because the water near the surface
evaporates first. The evaporation rate of the 11.22% porosity device decreases first because the
11.22% porosity device has the least remaining water due to its greater rate of evaporation in the
previous zones and because it has a lower absolute amount of water added than the 7.20% porosity
device. Table 7 summarizes the cooling performance of the devices in both experiments.
In Zone 4, the temperature drops of the two devices with more added sawdust gradually approach
zero. They have not reached zero yet because there is still moisture inside the device that is
evaporating and the device’s thermal mass provides cooling inertia [31]. Although the devices
have not completely dried out, their evaporation rates are lower than in the previous zones.
72
Figure 23. Internal Temperature Drop for Porosity Experiment 1. The temperature
drop increases as the porosity of the device’s outer clay wall increases. The effect of the
added porosity is seen immediately in Zone 1 when the temperature drops of all the devices
deviate from each other. The greatest temperature drop occurs in Zone 2 when the
evaporative cooling is the steadiest. The device with the lowest porosity maintains its
evaporative cooling for the longest while the two devices with sawdust begin drying out in
Zone 3 as evident by the temperature drop decrease. Zone 2 of 3.14% porosity device lasts
until around the 300-hour mark. Near the end of the experiment, the temperature drop of
the devices with sawdust begin decreasing to zero while the 3.14% porosity device without
sawdust continues its constant evaporative cooling rate. It still has water to evaporate due
to its lower evaporative rate throughout the entire experiment.
73
Figure 24. Internal Temperature Drop for Porosity Experiment 2. The internal
temperature drops of the devices with higher porosity are greater than the temperature
drops of the devices with lower porosity throughout of Zone 1 and Zone 2. In Zone 3, the
temperature drops of the higher porosity devices decrease as water depletes until Zone 4
where their evaporative cooling is still present but at a lower rate. The second experiment
lasts for twice as long as the first experiment to observe the dynamics of the devices. At
the end of the second experiment, all devices share a similar temperature drop and just
started to reach zero.
74
Table 7. Internal Temperature Drop Cooling Performance for Porosity Experiments.
The maximum temperature drop and the time it occurred is listed for each device in both
experiments. The total amount of water added along with the time when the temperature
drop first returns to zero is also included. The maximum temperature drop is highlighted.
The devices with the lowest amount of porosity never had their temperature drop return to
zero. In both experiments, the devices with the highest level of porosity achieved the
greatest temperature drop.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Max
Temperature
Drop
[℃]
Time Max
Temperature
Drop
[Hour]
Time
Temperature
Drop
Returns to
Zero
[Hour]
1 3.14% Porosity 2.91 4.41 280 Never
1 7.20% Porosity 3.85 8.43 89 218
1 11.22% Porosity 3.39 9.45 62 219
2 3.14% Porosity 3.06 4.28 41 Never
2 7.20% Porosity 3.73 5.60 135 360
2 11.22% Porosity 3.61 7.67 109 360
75
Temperature on Evaporation Surface
The temperature drop measured on the evaporation surface is a direct indication of evaporative
cooling. A greater temperature drop indicates a greater evaporation rate. Figure 25 and Figure 26
display the temperature drop profile on the evaporation surface for the devices in each experiments.
The devices that have higher levels of porosity achieve greater temperature drops than the device
with lower porosity, as expected, in both experiments. In the first experiment, between just the
devices with sawdust, it is unexpected that the 7.20% porosity device has the same temperature
drop on the surface as the 11.22% porosity device in Zones 1 and 2 when their internal temperature
drops do not share the same overlap. The discrepancy does not occur in experiment 2 even though
there is an overlap between the 3.14% porosity and the 7.20% porosity devices near the beginning.
The overlap in experiment 2 is reflected in the internal temperature drop measurements. A possible
explanation for the discrepancy seen in experiment 1 could be the presence of a temperature
gradient inside the devices that then affects temperature sensor readings depending on their
placement. The sensors were generally placed in the same area with respect to the water bottles
added inside as thermal mass, but variation could have occurred. To cross-validate the cooling
performance of the devices in experiment 1, Figure 27 displays the mass loss and the evaporation
rate. The evaporation rates of the 7.20% porosity and the 11.22% porosity devices are remarkably
similar, which indicates that their internal temperature drop should have recorded similar values.
Figure 28 shows the mass loss and evaporation rate of the second porosity experiment. The
11.22% porosity device has a greater evaporation rate than the 7.20% porosity device. The 3.14%
porosity and the 7.20% porosity devices share an overlap in evaporation rate that is reflected in
both the temperature drop profile on the surface and inside the device. In both experiments, the
76
evaporation rates of the two sawdust devices are higher than the evaporation rate of the device
without sawdust.
Zones 3 and 4 of both experiments’ evaporation surface temperature drop profiles share similar
trends with the internal temperature drop profiles. The devices with sawdust have the biggest
temperature drop decrease in Zone 3 because they had the greatest evaporation rates in Zone 2.
The temperature drop of the devices with sawdust experience a more gradual decrease for the
duration of Zone 4. The temperature drop of the 3.14% porosity device did not return to a
temperature drop of zero in the first experiment. In the second experiment that ran for 613 hours,
the temperature drop on the surface of the 3.14% porosity device does approach zero.
The inconsistency in when the devices reach their greatest temperature drop between the two
experiments is attributed to ambient conditions and the differences in internal vessel’s porosity.
The 3.14% porosity device is the same in both experiments, but the 7.20% porosity and the 11.22%
porosity devices have more porous internal vessels in the second experiment. The two devices with
sawdust (7.20% porosity and 11.22% porosity devices) took more time before reaching their
maximum temperature drop in the second experiment because the devices in the second
experiment have less transport resistance in the inner vessels’ material. In the first porosity
experiment, the inner vessels of the 7.20% porosity and the 11.22% porosity devices have higher
transport resistance that led to the development of a larger gradient on the surface of each device’s
inner vessel. The concentration gradient in the sand gap drives more water to the outer surface,
which has higher porosity and lower transport resistance. Table 8 shows the temperature drop on
the evaporation surface for both experiments.
77
Figure 25. Evaporation Surface Temperature Drop for Porosity Experiment 1. The
temperature on the evaporation surface reflects the amount of heat transfer due to
evaporation. The devices with higher levels of porosity both record greater temperature
drop than the device with lower porosity which reflects the influence of porosity on
evaporation. Between just the devices with sawdust added, it is unexpected to see that the
7.20% porosity device has the same temperature drop as the 11.22% porosity device when
its internal temperature drop is lower. This is attributed to a sensor placement difference.
78
Figure 26. Evaporation Surface Temperature Drop for Porosity Experiment 2. The
temperature drop profiles on the surface of the devices in the second porosity experiment
reflect the same trends as the internal temperature drop profiles. The temperature drop
increases when porosity increases. The overlap of the 3.14% porosity and the 7.20% at the
beginning is also reflected in the internal temperature drop. This overlap is caused by a
combination of internal structure and ambient conditions that resulted in the two devices
sharing similar cooling performance.
79
Figure 27. Mass Loss Rate and Mass Loss for Porosity Experiment 1. A curve (dotted
line) was fitted to the mass loss of each cooling device. In the legend, the normalized root
mean square error is given for each curve fit. The derivative of the curve fit is the
evaporation rate because water evaporation is the only mass transfer that occurs. The
evaporation rate of the 7.20% porosity and the 11.22% porosity device are very similar and
further confirms that their internal temperature difference is a function of sensor placement.
Both the devices with higher porosity clay walls records greater rates of evaporation than
the 3.14% porosity device.
80
Figure 28. Mass Loss Rate and Mass Loss for Porosity Experiment 2. A curve was
fitted to the mass loss of each cooling device. In the legend, the normalized root mean
square error is given for each curve fit. The derivative of the curve fit is the mass loss rate
and synonymous with the evaporation rate because water evaporation is the only mass
transfer that occurs. The 3.14% porosity and the 7.20% porosity share an overlap of
evaporation rate at the start of Zone 2 that is reflected in the internal temperature drop and
the evaporative surface temperature drop.
81
Table 8. Evaporation Surface Temperature Drop Cooling Performance for Porosity
Experiments. The maximum temperature drops on the surface and the time they occur are
presented. The greatest temperature drop is highlighted. The devices with the greatest
porosity have the greatest temperature drop. It takes the devices with sawdust longer to
reach their maximum temperature drop in the second porosity experiment than in the first
because of differences in ambient conditions and internal structures that slow the rates of
evaporation.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Max
Surface
Temperature
Drop
[℃]
Time Max
Surface
Temperature
Drop
[Hour]
1 3.14% Porosity 2.91 3.46 280
1 7.20% Porosity 3.85 7.30 39
1 11.22% Porosity 3.39 7.47 62
2 3.14% Porosity 3.06 3.00 41
2 7.20% Porosity 3.73 4.74 135
2 11.22% Porosity 3.61 6.42 108
82
Cooling Efficiency
The cooling efficiency of the devices in both experiments increases with higher levels of porosity.
For the devices with sawdust, their greatest cooling efficiencies occur in Zone 2, after which their
efficiency begins decreasing in Zone 3 as their surfaces dry. Although the cooling efficiency of
the 3.14% porosity device is low, it is the most stable and lasts the longest due to its slower
evaporation rate. In the second experiment, the longer experimental run time shows how the
cooling efficiencies of the devices gradually decrease to zero as less water is present for
evaporative cooling to occur. The negative cooling efficiencies at the end of the experiment occur
when the temperature inside the devices measure hotter than the ambient temperature.
Table 9 provides the maximum cooling efficiency values and the time they occur. The cooling
efficiency of the devices in the second experiment is generally lower than in the first experiment
because of the differences in the porosity of the internal storage vessel between the two
experiments and ambient conditions. The time that the maximum cooling efficiency and the
maximum internal temperature drop occur do not always align because the cooling efficiency is
what percentage of the maximum total cooling (wet-bulb temperature) the device captured. In the
first experiment, the maximum cooling efficiency occurs at the same time as the greatest maximum
temperature drop for the 3.14% porosity and the 7.20% porosity devices. The maximum cooling
efficiency for the 11.22% porosity device occurs sooner than its greatest temperature drop by 24
hours. In the second experiment, the 11.22% porosity device is the only device where the
maximum cooling efficiency occurs at the same time as its maximum temperature drop. The 3.14%
porosity device’s maximum cooling efficiency occurs 143 hours after its maximum temperature
drop and the 7.20% porosity device’s maximum cooling efficiency occurs 47 hours after its
maximum temperature drop.
83
Figure 29. Cooling Efficiency for Porosity Experiment 1. The cooling efficiency of the
devices show that the devices with more porosity have greater cooling efficiency until
around Zone 3 when water in the devices with higher porosity runs low. The cooling
efficiency of the 11.22% porosity device decreases fastest because it has the fastest
evaporation rate. It also has less total amount of water added than the 7.20% porosity
device. The 3.14% porosity device has the most consistent cooling efficiency that lasted
until the end of the experiment due to its slower evaporation rate.
84
Figure 30. Cooling Efficiency for Porosity Experiment 2. The cooling efficiency of the
devices in the second porosity experiment shows that the devices with more porosity have
greater cooling efficiency. The second experiment ran for almost twice as long as the first
and reveals how long the cooling effect in Zone 4 lasts as compared to Zone 1-3.
Table 9. Cooling Efficiency Performance for Porosity Experiments. The max cooling
efficiency and the time the efficiency occurred is presented with the maximum efficiency
highlighted.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Max
Efficiency
[%]
Time Max
Efficiency
[Hour]
1 3.14% Porosity 2.91 37.24 280
1 7.20% Porosity 3.85 59.36 89
1 11.22% Porosity 3.39 66.61 38
2 3.14% Porosity 3.06 40.95 184
2 7.20% Porosity 3.73 54.63 182
2 11.22% Porosity 3.61 69.07 109
85
Total Cooling
The total cooling metric takes into account the maximum temperature drop as well as the length
of evaporative cooling process. It is expected that the devices with a higher absolute amount of
water added will have greater total cooling but that is not always the case. Figure 31 and Figure
32 show the total cooling in the first and second experiment.
In the first experiment, the total cooling of the 7.20% porosity device and the 11.22% porosity
device should be more similar, and their differences may be a result of sensor placement as
previously discussed. Nonetheless, 11.22% porosity device has a lower total amount of water
added and still achieves total cooling comparable to the 7.20% porosity device. Both the devices
with greater percentages of porosity achieve greater total cooling than the device with lower
porosity percentage. In second experiment, the 11.22% porosity device achieves the greatest total
cooling even though it has a lower absolute amount of water added than the 7.20% porosity device.
This is an important finding for users in water scarce area.
Each total cooling profiles has two distinctive slopes. The slope changes based on what factors
limit the devices’ evaporation rates. In fact, the evaporation rate figures (Figure 27 and Figure
28) have similar profiles to the total cooling plots. In Zone 2, the added water wets the entirety of
the devices’ surface, and the devices’ evaporation rates are limited by the water vapor diffusion
away from the surface. As the surface dries and the network of liquid filled pores is disrupted, the
evaporation rate is limited by how quickly water vapor or moisture can diffuse through the device
to the evaporation surface. The slope at the beginning is steeper than the slope at the end because
water vapor diffuses through air faster than it does through solids. Near the end of the second
experiment, the gap in the total cooling profiles between the 7.20% porosity and the 11.22%
porosity increases because water vapor can diffuse through more porous materials with less
86
transport resistance. In experiment 1, the total cooling of the 3.14% porosity device is a linear line
because its lower evaporation rate sustains constant cooling until the end of the experiment. The
second experiment allows more time for all devices to dry, and near the end of Zone 4, the total
cooling profile slope of the 3.14% porosity device decreases. Table 10 provides the total cooling
amount at the end of the experiment.
Figure 31. Total Cooling for Porosity Experiment 1. The total cooling performance
metrics accounts for how long evaporative cooling lasts in addition to maximum
temperature drop for each device. The two devices with the highest porosity records higher
total cooling than the 3.14% porosity. Between just the sawdust devices, the higher total
cooling amount of the 11.22% porosity device could be a factor of sensor placement as
previous discussed. The total cooling profile of the two devices with sawdust have two
clear slopes that the total cooling profile of 3.14% porosity device lacks. The 3.14%
porosity device is still evaporating water as fast as the air around its surface allows. With
more time, the 3.14% porosity device will also exhibit the same plateauing behavior as
seen in the second experiment as it runs out of water.
87
Figure 32. Total Cooling for Porosity Experiment 2. The 11.22% porosity device clearly
achieves higher total cooling than the other lower porosity devices. It also received a lower
absolute amount of water added than the 7.20% porosity device. The longer experimental
run time of the second experiment shows how the 3.14% porosity device just began
plateauing near the end. The point where the slope changes for the devices coincides with
when the evaporative cooling is no longer limited by the ambient conditions but by the
internal structures of each device. The more porous the device is, the easier it is for water
vapor to diffuse through the device and the greater the evaporative cooling effects. That
explains why the 11.22% porosity device reach its critical point before both of the other
porosity device but is still able to maintain a greater total cooling amount.
88
Table 10. Total Cooling Performance for Porosity Experiments. The total cooling at
the end of the experiments is provided. The devices with the greater amount of porosity
achieve the greater total cooling even if their internal temperature drop near the end of the
experiment is lower than the internal temperature drop of the 3.14% porosity device.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Total
Cooling
[°𝑪 × 𝑯𝒐𝒖𝒓]
Time Total
Cooling
[Hour]
1 3.14% Porosity 2.91 776.50 331
1 7.20% Porosity 3.85 1018.71 331
1 11.22% Porosity 3.39 1065.61 331
2 3.14% Porosity 3.06 1096.73 613
2 7.20% Porosity 3.73 1160.62 613
2 11.22% Porosity 3.61 1411.73 613
89
Relative Humidity
Figure 33 and Figure 34 show the relative humidity inside and on the evaporation surface of each
device during the two experiments. A higher relative humidity on the surface signifies greater
evaporation activity. A higher relative humidity inside the device is desirable for practical purposes
of storing fruits and vegetables. In both experiments, the relative humidity on the surface of the
devices with added sawdust is higher than the relative humidity on the surface of 3.14% porosity
device. The devices with more porosity allow water to flow to the evaporation surface with less
transport resistance which result in a higher evaporation rate and higher relative humidity
measurements on the surface. As more water evaporates from the devices with higher levels of
porosity, near the end of the experiment, there is less water available to evaporate into the internal
storage vessels to maintain high levels of relative humidity. This explains why the internal relative
humidity of the 3.14% porosity device lasts the longest.
Between the first and second porosity experiment, the difference in porosity of the internal storage
vessels changes the internal relative humidity profiles. In experiment 1, the 7.20% porosity and
the 11.22% porosity device show similar patterns and values in their internal relative humidity
profiles, particularly as the humidity drops below 80%. In the second experiment, there is less
overlap because the higher levels of porosity in the internal storage vessel of the 11.22% device
causes its relative humidity to return to ambient faster than the 7.20% porosity device. Table 11
summarizes the relative humidity inside and on the surface of the devices.
90
Figure 33. Relative Humidity in Porosity Experiment 1. Added water in the device can
evaporate into the internal vessel or out into the environment. The relative humidity inside
the internal vessel is higher than on the relative humidity on the surface because there is a
dampened top that covers the internal vessel. There is also water that evaporates into the
internal vessel from the dampened top. The relative humidity inside the 3.14% porosity
device never dips below 80% even though it has the least amount of added water. Its lower
porosity levels allow it to sustain high levels of relative humidity for longer. The 3.14%
device also has a lower steady evaporation rate as evident by its relative humidity
measurements on its surface. The devices with higher porosity have higher levels of
relative humidity on the surface. These devices do not sustain high levels of relative
humidity for as long as the lower porosity device.
91
Figure 34. Relative Humidity in Porosity Experiment 2. The relative humidity of the
second experiment shows different trends than in the first experiment. The difference in
porosity of the internal storage vessels causes the internal relative humidity of 11.22%
porosity device to return to ambient quicker than the relative humidity of the 3.14% and
the 7.20% porosity device. At the beginning of the experiment, the difference between the
relative humidity on the surface and in the ambient environment is smaller than in
experiment 1. This difference is due to different ambient conditions. Both experiments
show that the relative humidity on the surface is greater for the device with more porosity
and that the relative humidity inside the 3.14% device lasts the longest.
92
Table 11. Relative Humidity of the Porosity Experiments. The relative humidity (RH)
at the end of the experiment inside and on the surface is provided along with the time when
the relative humidity first measured below 80%. The device that has the longest time above
80% is highlighted. The 3.14% porosity device never recorded internal relative humidity
levels under 80%.
Experiment
Number Devices
Amount of
Water
Added
[kg]
Time <80%
Internal RH
[Hour]
Internal
RH at the
End
[%]
1 3.14% Porosity 2.91 Never 89.40
1 7.20% Porosity 3.85 243 50.21
1 11.22% Porosity 3.39 241 40.79
2 3.14% Porosity 3.06 485 46.98
2 7.20% Porosity 3.73 364 31.47
2 11.22% Porosity 3.61 364 25.96
The porosity experiments show that the evaporative cooling devices with higher levels of porosity
in their outer vessels achieve greater maximum temperature drop, higher cooling efficiency, and
larger overall total cooling. As the porosity increases, the evaporation rate also increases because
the water moves more readily through the clay outer wall. As the devices dry, having a larger
porosity is still advantageous because the water vapor can diffuse through the clay wall more
easily. The trade-off of having more porosity includes the internal relative humidity not lasting as
long because there is less water left in the devices to evaporate into the internal chamber. For users
with more access to water, adding water near the end of Zone 2 will allow the devices to sustain
their maximum temperature drops and high levels of internal relative humidity for longer.
The evaporative cooling devices in the porosity experiments mix sawdust in clay to create
additional pores, but any organic material that can burn away when the clay pots undergo firing
will serve the purpose. A 60-mesh sieve was used on the sawdust before mixing it into clay. Adding
any organic matter to the clay also has the benefit of producing uniformly burnt pots and reducing
93
fuel and power consumption because the additives double as fuel during firing [36]. Adding
sawdust also reduces the total weight of the devices [36].
Besides the organic pore-formers, no other material is needed to increase porosity in clay pots.
There is a factor of time required for experimenting on what amount of sawdust can be added to
what amount of the specific local clay but no other higher skill level is required. Overall, increasing
cooling performance by increasing porosity with sawdust presents a practical opportunity to
improve the cooling devices. A comprehensive table of all the performance metrics is included in
Table 12.
94
Tab
le 1
2. C
om
pre
hen
siv
e C
ooli
ng P
erfo
rman
ce M
etri
cs o
f P
oro
sity
Exp
erim
ents
. T
he
max
imum
val
ue
of
each
per
form
ance
met
ric
is h
igh
lig
hte
d.
Fo
r th
e te
mper
ature
met
rics
, an
y v
alue
that
is
wit
hin
the
senso
r er
ror
of
±0
.5 °
C o
f th
e m
axim
um
val
ue
is
also
hig
hli
gh
ted
. T
he
"nev
er"
term
is
appli
cable
only
wit
hin
the
tim
espan
of
the
resp
ecti
ve
exper
imen
t. T
he
coo
lin
g p
erfo
rman
ce
met
rics
th
at a
re m
ost
pra
ctic
al f
or
use
rs a
re t
he
inte
rnal
tem
per
ature
dro
p,
the
tim
e bef
ore
the
inte
rnal
tem
per
atu
re d
rop
ret
urn
s to
zero
and
ho
w lo
ng
hig
h lev
els
of
inte
rnal
rel
ativ
e hum
idit
y las
t. I
n b
oth
exper
imen
ts, t
he
dev
ices
wit
h h
igh
er p
erce
nta
ge
of
poro
sity
hav
e gre
ater
in
tern
al t
emp
erat
ure
dro
ps
but
do n
ot
sust
ain t
hei
r over
all
evap
ora
tive
cooli
ng
for
as l
on
g a
s th
e d
evic
e w
ith
lo
wer
per
cen
tag
e of
poro
sity
. T
he
dev
ice
low
er p
oro
sity
per
centa
ge
also
mai
nta
ins
rela
tive
hum
idit
y a
bo
ve
80%
for
lon
ger
. A
s w
ith
th
e
dev
ices
in
the
surf
ace
area
ex
per
imen
ts,
ther
e is
a t
rade-o
ff b
etw
een h
avin
g g
reat
er i
nte
rnal
tem
per
atu
re d
rop
an
d l
ong
er p
erio
ds
of
inte
rnal
coo
lin
g a
nd
hig
h l
evel
s of
rela
tive
hum
idit
y. F
or
use
rs i
n w
ater
-sca
rce
area
, th
e d
evic
e w
ith l
ess
po
rosi
ty m
ay
be
mo
re
appro
pri
ate.
In
tern
al
RH
at
the
En
d
[%]
89
.4
50
.21
40
.79
46
.98
31
.47
25
.96
Tim
e
<8
0%
Inte
rna
l
RH
[Ho
ur]
Nev
er
24
3
24
1
48
5
36
4
36
4
Tim
e
To
tal
Co
oli
ng
[Ho
ur]
33
1
33
1
33
1
61
3
61
3
61
3
To
tal
Co
oli
ng
[°C
× H
ou
r]
77
6.5
10
18
.71
10
65
.61
10
96
.73
116
0.6
2
14
11
.73
Tim
e M
ax
Eff
icie
ncy
[Hou
r]
28
0
89
38
18
4
18
2
10
9
Max
Eff
icie
ncy
[%]
37.2
4
59.3
6
66.6
1
40.9
5
54.6
3
69.0
7
Tim
e
Max
Su
rface
Tem
p
Dro
p
[Hou
r]
280
39
62
41
135
108
Max
Su
rface
Tem
p
Dro
p
[°C
]
3.4
6
7.3
0
7.4
7
3
4.7
4
6.4
2
Tim
e
Tem
p
Dro
p
Ret
urn
s
to Z
ero
[Hou
r]
Nev
er
218
219
Nev
er
360
360
Tim
e
Max
Inte
rnal
Tem
p
Dro
p
[Hou
r]
280
89
62
41
135
109
Ma
x
Inte
rnal
Tem
p
Dro
p
[°C
]
4.4
1
8.4
3
9.4
5
4.2
8
5.6
0
7.6
7
Am
ou
nt
of
Wa
ter
Ad
ded
[kg
]
2.9
1
3.8
5
3.3
9
3.0
6
3.7
3
3.6
1
Dev
ices
3.1
4%
Po
rosi
ty
7.2
0%
Po
rosi
ty
11
.22
%
Po
rosi
ty
3.1
4%
Po
rosi
ty
7.2
0%
Po
rosi
ty
11
.22
%
Po
rosi
ty
Exp
Nu
mb
er
1
1
1
2
2
2
95
3.3. Porosity Mini-Experiments
Two mini-experiments are conducted to add more context to how porosity affects cooling
performance. These two experiments are not conducted in a systematic framework as they only
represent 2 levels of comparison instead of the 3 or 4 used in the main experiments; nonetheless,
they still provide points of references. All the devices were conducted in parallel and share the
same sand to water ratio. The first mini-experiment compares the cooling effect among devices
with different levels of porosity created by sawdust and a device that has a different percentage of
porosity because it is made from different clay material. The second mini-experiment explores
how porosity from different clay materials affect cooling performance.
Porosity due to Different Clay Material versus Porosity due to Sawdust
The devices’ key metrics and properties are in Table 13. Temperature on the surface is the metric
used to compare the devices because it is the more representative of the cooling performance. The
devices in porosity experiment 1 are compared with a device that has inner and outer vessels made
from a different clay material, EM 210; the clay porosity is 6.60%. The surface temperature and
internal relative humidity trends in this mini experiment are the same as in the two main porosity
experiments. The maximum surface temperature drop varies with porosity, and the time the
internal relative humidity drops below 80% inversely varies with porosity.
96
Table 13. Devices in Mini-Experiment Comparing Porosity due to Different Clay
Material versus Porosity Created by Sawdust. The devices with more porosity record
greater temperature drop on the surface and shorter time span of high internal relative
humidity. The added 6.60% porosity device fits into the trend found in the first porosity
experiment.
Porosity
Formation
Devices
(Outer
Porosity)
Inner
Porosity
[%]
Amount of
Water
Added
[kg]
Max
Surface
Temperature
Drop
[℃]
Time Max
Surface
Temperature
Drop
[Hour]
Time
<80%
Internal
RH
[Hour]
Sawdust 3.14% Porosity 3.14% 2.91 3.46 280 Never
EM 210 6.60% Porosity 6.60% 3.40 6.63 35 249
Sawdust 7.20% Porosity 3.14% 3.85 7.30 39 243
Sawdust 11.22% Porosity 3.14% 3.39 7.47 62 241
97
Porosity due to Different Clay Material
Two clay bodies of different outer material porosity, T 417 and EM 106 are compared as shown
in Table 14. Their porosity levels are 3.50% and 4.08% respectively without adding any sawdust.
The 4.08% porosity device’s cooling performance supports the findings that increasing porosity in
clay material will lead to greater temperature drop. Its internal relative humidity did not stay above
80% relative humidity for as long as the internal relative humidity inside the 3.50% porosity
device; however, that is expected based on the trends seen in the porosity experiments with sawdust
pots. This suggests that porosity due to using different clay materials or due to adding sawdust will
reflect the same trends.
Table 14. Devices in Mini-Experiment Comparing Porosity due to Different Clay
Material. Between the two devices, the 4.08% porosity device (EM 106) records greater
temperature drop on its evaporative cooling surface. The 3.50% porosity device (T 417)
dips below 80% relative humidity before the 4.08% but it hovers around that point until
the end of the experiment while the 4.08% humidity continue to decrease.
Porosity
Formation
Devices
(Outer
Porosity)
Inner
Porosity
[%]
Amount
of Water
Added
[kg]
Max
Surface
Temperature
Drop
[℃]
Time Max
Surface
Temperature
Drop
[Hour]
Time
<80%
Internal
RH
[Hour]
T 417 3.50%
Porosity 3.50% 3.14 3.08 254 55
EM 106 4.08%
Porosity 4.08% 2.49 6.01 14 189
98
3.4. Porosity versus Surface Area Design Parameters
The second experiments of the surface area and porosity experiments are conducted at the same
time, so the individual devices with added surface corrugation and sawdust can be directly
compared. Table 15 details the cooling performance of all the devices in the second experiment
of the surface area and porosity experiments. The 1x Device and the 3.14% porosity device are the
same device. The 7.20% porosity and the 11.22% porosity devices have an average of 0.5 kg of
water more than the average amount of water added to the devices with surface corrugation. The
additional amount of water added explains why the devices with added sawdust have internal
relative humidity measurements above 80% for an average of 47 hours longer than the devices
with added corrugation.
The 11.22% porosity device achieves internal temperature drop approximately 2°C greater than
the average temperature drops of all the devices with surface corrugations and reaches its
maximum temperature drop faster due to its higher levels of porosity. As a tradeoff for its fast
evaporation rate, the temperature drop of the 11.22% porosity device returns to zero within the
same 360-hour mark as all the devices with the surface corrugation even though it has more water.
The 7.20% porosity device has similar temperature drops within the sensor error range as all the
devices with surface corrugation and its internal temperature drop also returns to zero within the
same 360-hour mark even though it has the most amount of water added at 3.73 kg. The 7.20%
porosity device also has the lowest maximum efficiency among all the devices.
The performance metrics of the devices suggest that a high enough porosity percentage is
necessary to achieve greater evaporative cooling than only adding surface corrugation. The devices
with surface corrugation also sustain evaporative cooling for as long as devices with sawdust with
99
less water, making them a more attractive option for users in water scarce areas. For users not
constrained by availability of water, watering the devices on a cycle will further sustain the
maximum temperature drop and high internal relative humidity of the devices, making the 11.22%
porosity device an even more attractive option because it has the greatest maximum temperature
drop.
Another factor for users to consider when choosing between the two design parameters is that
adding surface corrugations require more skill labor and equipment than adding sawdust to clay;
therefore, while the devices with corrugation can sustain evaporative cooling for as long as the
devices with sawdust with a lower amount of water added, constructing corrugation may not be
accessible and too burdensome for some users.
100
Tab
le 1
5. C
om
pre
hen
siv
e C
oo
lin
g P
erfo
rman
ce M
etri
cs o
f S
urfa
ce A
rea a
nd
Poro
sity
Ex
per
imen
ts. T
he
max
imu
m v
alue
of
each
per
form
ance
met
ric
is h
ighli
ghte
d.
For
the
tem
per
ature
met
rics
, an
y v
alue
that
is
wit
hin
th
e se
nso
r er
ror
of
±0
.5 °
C o
f th
e
max
imu
m v
alu
e is
als
o h
igh
lighte
d.
The
"nev
er"
term
is
appli
cable
only
wit
hin
the
tim
esp
an o
f th
e re
spec
tiv
e ex
per
imen
t. T
he
cooli
ng
per
form
ance
of
all
the
dev
ices
indic
ates
that
a h
igh e
nough p
erce
nta
ge
of
poro
sity
is
nec
essa
ry t
o a
chie
ve
inte
rnal
tem
per
ature
gre
ater
th
an t
he
tem
per
ature
dro
ps
of
the
dev
ices
wit
h s
urf
ace
corr
ugat
ion.
The
per
form
ance
met
rics
als
o s
ho
w t
hat
the
dev
ices
wit
h s
urf
ace
corr
ug
atio
n c
an s
ust
ain e
vap
ora
tive
cooli
ng f
or
as long a
s th
e dev
ices
wit
h s
awd
ust
wit
h les
s w
ater
add
ed.
The
dev
ices
wit
h s
awd
ust
hav
e g
reat
er tota
l co
oli
ng a
nd a
re a
ble
to s
ust
ain inte
rnal
rel
ativ
e h
um
idit
y a
bo
ve
80%
fo
r lo
ng
er b
ecau
se
they
hav
e m
ore
add
ed w
ater
.
Inte
rna
l
RH
at
the
En
d
[%]
46
.98
28
.07
32
.73
31
.41
46
.98
31
.47
25
.96
Tim
e
<8
0%
Inte
rna
l
RH
[Ho
ur]
48
5
30
3
31
1
33
5
48
5
36
4
36
4
Tim
e
To
tal
Co
oli
ng
[Ho
ur]
61
3
61
3
61
3
61
3
61
3
61
3
61
3
To
tal
Co
oli
ng
[°C
× H
ou
r]
10
97
12
05
10
59
11
52
10
96
.73
11
60
.62
14
11
.73
Tim
e M
ax
Eff
icie
ncy
[Hou
r]
184
182
182
182
184
182
109
Max
Eff
icie
ncy
[%]
40.9
5
69.9
8
61.2
2
66.1
40.9
5
54.6
3
69.0
7
Tim
e
Max
Su
rface
Tem
p
Dro
p
[Hou
r]
41
135
207
20
7
41
135
108
Max
Su
rface
Tem
p
Dro
p
[°C
]
3
4.2
7
4.0
7
4.2
8
3
4.7
4
6.4
2
Tim
e
Tem
p
Dro
p
Ret
urn
s
to Z
ero
[Hou
r]
Nev
er
360
360
360
Nev
er
360
360
Tim
e
Max
Inte
rnal
Tem
p
Dro
p
[Hou
r]
41
135
207
207
41
135
109
Ma
x
Inte
rnal
Tem
p
Dro
p
[°C
]
4.2
8
5.8
1
5.4
2
5.6
8
4.2
8
5.6
7.6
7
Am
ou
nt
of
Wa
ter
Ad
ded
[kg
]
3.0
6
3.3
2
3.0
6
3.1
3
3.0
6
3.7
3
3.6
1
Dev
ices
1x
Dev
ice
1.5
2x
Dev
ice
1.7
7x
Dev
ice
2.5
9x
Dev
ice
3.1
4%
Po
rosi
ty
7.2
0%
Po
rosi
ty
11
.22
%
Po
rosi
ty
Exp
Nu
mb
er
2
2
2
2
2
2
2
101
Chapter 4
Conclusion
This thesis investigated the effects of surface area and porosity on the cooling performance of
household evaporative cooling devices. In the surface area experiments, corrugation was cut into
the walls of cooling devices to create more surface area over which water can evaporate without
altering the internal volume. To systematically vary the additional surface area, the distance
between the ridges was increased by 0.25 cm starting from 0.25 cm and up to 0.75 cm, inclusive.
With less distance between ridges, more surface area was added to the device. For the porosity
experiments, sawdust was mixed into wet clay at different clay to sawdust ratios to create
incremental percentages of porosity at 3.14% (no sawdust added), 7.20% (40 clay: 1 sawdust), and
11.22% (20 clay: 1 sawdust). Adding sawdust to wet clay created cavities and increased porosity
because the sawdust burned away during the firing stages of making the clay pots.
102
The work conducted in the surface area experiment showed that adding surface fluctuation
increases the rate of evaporation and maximum temperature drops when compared to devices
without surface fluctuation. However, adding more corrugation did not consistently lead to greater
temperature drops. The geometry of corrugation introduced competing effects between increased
available surface area for water evaporation and decreased vapor pressure concentration inside the
troughs. The device without corrugation had the smallest temperature drop but the steadiest and
longest-lasting cooling effects. Although the device without corrugation had less total water added
at the beginning of the experiment, it was able to maintain its internal relative humidity for longer
than the devices with corrugation. For users in water-scarce areas, the device without corrugation
could be a better choice given the constraints.
The work done on the porosity experiment concluded that increasing porosity in the outer vessel
leads to greater temperature drop and higher cooling efficiency—even with a lower amount of
water added. The increased level of porosity in the device’s clay wall decreased the internal
transport resistance for water and moisture to transfer to the evaporation surface, which then led
to increased vapor pressure concentration on the surface to facilitate evaporation. The tradeoffs
with having higher porosity in the outer vessels include shorter periods of evaporative cooling and
shorter periods of high levels of relative humidity inside the devices.
To further the investigation on the cooling effect of surface area and porosity the author
recommends conducting additional tests in a closed chamber where temperature, relative humidity,
and airflow could be controlled. The devices’ dimensions should be more uniform so that the same
amount of sand and water can be added to each device. The temperature and relative humidity
sensors should have greater accuracy and the load cell should have lower creep error percentage.
More temperature sensors should be added inside the cooling chamber and the placement of the
103
sensors should be uniform across all devices. To further investigate surface area effects,
experiments that focus on the optimization of the corrugation’s length, width, height, and angle
will allow for greater cooling performance and enable the creation of a model that could predicts
response given different parameter values. For porosity, the experiments in this thesis have shown
the advantages of adding porosity but the practicality of implementing the study findings remain
unknown. Experiments that focus on investigating the maximum amount of sawdust that can be
added without compromising the integrity of the structure or the usability of the pot, i.e. the pot
fragility, and identifying what other locally available organic material can be used as pore-formers
will further the practical implementation of the findings in this thesis.
Both increasing surface area and porosity can improve cooling performance of evaporative cooling
devices. When deciding between which design parameter to implement, the important factors to
consider include: the cooling performance achievable, the water use efficacy, and the practicability
of design parameter implementation. The device with the highest percentage of porosity (11.22%
porosity device) achieved a temperature drop of ~2°C greater than the average temperature drops
of all the devices with added corrugations. The 7.20% porosity device achieve similar temperature
drop within the sensor error range of the devices with surface corrugations. The difference in
cooling performance of the two porosity devices in comparison to the devices with corrugations
suggests that a minimum porosity percentage is required for cooling devices to achieve greater
cooling performance than the devices with surface corrugation. The devices with corrugations had
less water added but were still able to sustain their total evaporative cooling for as long as the
devices with added sawdust. This is an important finding for users in water scarce areas. For users
with less water constraints, continuously adding water near the end of Zone 2 will allow the devices
to sustain their maximum temperature drops and high levels of internal relative humidity for
104
longer, making the devices that have greater maximum internal temperature drop more attractive.
Of the three factors to consider, the practicability of implementation carries the most weight
because it determines accessibility. Increasing porosity presents a more practical and less
burdensome solution to increasing cooling performance as compared to adding surface
corrugation. Adding corrugation to the outer clay pots requires additional equipment and skill
levels while adding sawdust to create more porosity only require a single time investment to
pinpoint ideal sawdust to clay ratio. Devices with sawdust are lighter, require less fuel to burn and
burn more uniformly [36]. Sawdust can be locally sourced in most places and any fine organic
material that can burn out during firing can also be used in place of sawdust. The cooling
performance and practicality of devices with sawdust present a viable opportunity to improve
household evaporative cooling devices. For future researches, the author hypothesizes that
increasing both surface area and porosity in individual evaporative cooling devices can lead to
greater cooling performance.
In this thesis, the effects of surface area and porosity on cooling performance were systematically
investigated and evaluated using multiple metrics. A new metric measuring total cooling was
developed to account for the length of the evaporative cooling as well as the maximum temperature
drop. The work done in this thesis furthered the understanding of how the design parameters of
surface area and porosity impacted cooling performance. The study’s finding can serve as the
foundation for future experimentation, optimization, and design of those design parameters to
improve performance of evaporative cooling devices.
105
Supplementary Information
Supplementary Information S.1. Moisture Sensor Data in the Sand Gap.
Figure 35. Moisture in Sand Gap for Surface Area Experiment 1. The moisture sensor
in the sand gap is located near the top. The sand gap of all devices stays wet until near the
end of Zone 2. The device without added surface area maintains its moisture for the longest
time because of its lower evaporation rate. The devices with more surface corrugation dry
faster due to their higher rates of evaporation. The absolute amount of water added to the
sand gap also influence how long moisture lasts.
106
Figure 36. Moisture in Sand Gap for Surface Area Experiment 2. The moisture sensor
data in experiment 2 display hardware sensor malfunction. There is a lot more noise in the
data starting in Zone 2 and continuing to Zone 3 for all the sensors. Most notably, the sand
gap moisture sensor for the 1.52x Device records high level of moisture in Zone 3 that is
not realistic.
107
Supplementary Information S.2. Mass Loss and Mass Loss Rate for Surface Area
Experiments
Figure 37 and Figure 38 display the mass loss and the mass loss rate of the first and second surface
area experiment. The mass loss data for each device is fitted with a polynomial curve of the 5th
order, and the normalized root mean square error is provided in the legend. The derivative of the
mass loss data curve fit is the mass loss rate. In the evaporative cooling devices, the only mass
transfer that occurs is water evaporation, so the mass loss rate is the water evaporation rate. This
temperature drop in Zone 4 was maintained for the longest time over the course of the experiment.
108
Figure 37. Mass Loss Rate and Mass Loss for Surface Area Experiment 1. A curve
was fitted to the mass loss of each cooling device. In the legend, the normalized root mean
square error is given for each curve fit. The derivative of the curve fit is the mass loss rate.
Since water evaporation is the only mass transfer that occurs, the mass loss rate is the rate
of water evaporation. In Zone 2, where the maximum temperature drop happened, the
devices with added surface area all have greater rates of water evaporation as compared to
the device without corrugation.
109
Figure 38. Mass Loss Rate and Mass Loss for Surface Area Experiment 2. The same
curve fitting technique from experiment 1 is used to experiment 2. The rate of water
evaporation of the devices with added surface area also are greater than those of the device
without added surface area. A difference between experiment 1 and experiment 2 is how
the 1.52x Device records a greater rate of evaporation than the 2.59x Device. The greater
rate of evaporation also corresponds with the 1.52x Device’s greater temperature drop. This
difference in performance is due to the competing effects introduced by the corrugation.
110
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