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AMMONIA – CARBON ADSORPTION CYCLE RESEARCH AT THE

UNIVERSITY OF WARWICK

A.M. Rivero-Pacho1, R.E. Critoph

1, S.J. Metcalf

1, M. van der Pal

2

1 University of Warwick, Coventry, CV4 7AL, United Kingdom,

2 ECN, Petten, 1755 ZG, The Netherlands,

ABSTRACT

Active carbon-ammonia cycles have been developed from the

1980’s in the USA and are still a major research interest at

the University of Warwick, where systems have been built

for car air conditioning, solar refrigeration and gas-fired heat

pumps. The basic cycles are introduced and a brief

description of the historical development is presented. The

paper then presents work to date on domestic gas fired heat

pumps with preliminary results from the latest system under

test, and describes plans and the prospects for future

products. The possibility of using carbon-ammonia in thermal

transformers for industrial use is modelled theoretically and

the prospects discussed and compared with using ammonia

and chemical salts.

INTRODUCTION

The most basic adsorption refrigeration cycle is

shown in Fig. 1 where two linked vessels are presented.

The vessel on the left hand side acts as the generator,

containing the solid adsorbent and adsorbed refrigerant,

whilst the vessel on the right hand side acts as a

receiver, condenser and evaporator during the cycle,

containing refrigerant gas. In the Stage (a) of the cycle

the system is at low pressure and ambient temperature.

The adsorbent contains a high concentration of

adsorbed refrigerant whilst the right hand vessel

contains refrigerant gas. When the vessel on the left is

heated, the adsorbed refrigerant gets driven out and the

pressure of the system increases.

In Stage (b) the pressure of the system is high

enough so that the refrigerant gas starts to condense in

the right hand vessel whilst rejecting heat. After that

the generator is cooled back to the initial low

temperature readsorbing the refrigerant in the adsorbent

material which creates a drop in the pressure of the

system. When the pressure drops, the liquid refrigerant

boils and produces the cooling effect, absorbing heat.

In Stage (c) the system returns back to the initial state

and the cycle is completed. The cycle provides

heating/cooling discontinuously for approximately half

of the cycle time. If two or more generators are

operated out of phase the heating/cooling could be

produced continuously.

When continuous heating/cooling is produced the

right hand vessel mentioned above is usually split into

two separate components: a condenser and evaporator.

Each generator of the system is connected to the

condenser and to the evaporator via separate automatic

check valves as shown in Fig. 2a). Once the refrigerant

is desorbed from the generator, it is condensed in the

condenser and it passes through an expansion valve to

reduce its pressure. After that the refrigerant evaporates

in the evaporator and finally it can be adsorbed in the

generator completing a cycle. The ideal adsorption

cycle assumes that the condensation and evaporation

occur at constant pressures and that the void volume in

the bed is zero so that the pressurisation and

depressurisation are isosteric processes.

Fig. 1 – Simple adsorption cycle schematic (Metcalf et al., 2012)

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Fig. 2 – a) Refrigerant circuit for one sorption generator b) p-T-x (Clapeyron) diagram for a single adsorption cycle

Figure 2b) presents the adsorption cycle and it can

be described in four stages:

Stage 1 à 2 – isosteric pressurisation: The

adsorbent in the bed is heated and its pressure rises

from evaporating to condensing pressure.

Stage 2 à 3 – isobaric desorption: The adsorbent

continues to be heated and starts desorbing refrigerant

at the condensing pressure while the refrigerant

condenses in the condenser.

Stage 3 à 4 – isosteric depressurisation: The

adsorbent in the bed is cooled and its pressure

decreases from condensing to evaporating pressure.

Stage 4 à 1 – isobaric desorption: The adsorbent

continues to be cooled and starts adsorbing refrigerant

at the evaporating pressure while the refrigerant

evaporates in the evaporator.

The COP (Coefficient of Performance) of the basic

adsorption cycle presented above is quite low. In order

to increase it, heat regeneration techniques that allow

efficient heat transfer from one adsorption generator to

another should be used. Multiple bed cycles are

a simple but way of achieving heat recovery between

the generators. In addition a good heat transfer design

in the generators is needed to achieve a good

performance.

At the University of Warwick heat driven

adsorption cycles are chosen to be developed over

mechanically driven vapour compression system as

they use primary energy sources more efficiently and

can use energy from renewable sources such as solar

collectors or waste heat from other sources. Adsorption

systems are preferred over absorption ones because of

their low operational cost and maintenance, higher

reliability, simple operations and no crystallisation,

corrosion or chemical disposal issues.

REFRIGERATION

EU-TOPMACS project. Introduction The developed cooling system in this project was

designed to simulate a mobile air conditioning system

(MACS) for a Class C passenger vehicle (such as

a Ford Focus or Fiat Bravo) with a 1.9 l turbo diesel

engine. This choice was very challenging due to the

high engine efficiency and therefore low waste heat

availability.

The engine coolant was to be used to provide the heat

input to the cooling system at a temperature of 90 °C

and a nominal flow rate of 24 l/min. The cooling power

required was determined to be 2 kW and, although it is

highly variable during the driving cycle, the nominal

heat input available is 5 kW – thereby necessitating a

nominal COP 0.4.

Generator design and sorption material The sorption generator used was a nickel-brazed

stainless steel design with 29 layers of active carbon

adsorbent each 4 mm thick. By incorporating the

carbon adsorbent in thin layers, conduction path length

through the material is reduced and the area for fluid

heat transfer is increased which enables rapid

temperature cycling and thus a high SCP (Specific

Cooling Power). The separating stainless steel plates

are constructed from chemically etched shims with

0.5 mm square water flow channels on a 1 mm pitch.

These channels give a high heat transfer coefficient and

a large heat transfer area, further improving heat

transfer performance. The square design ensures equal

flow path lengths in every channel and therefore even

heating and cooling of the adsorbent. The internal

pressure (up to 20 bar when condensing at 50°C) is

withheld by the stainless steel shims which act as

supporting webs to the outer wall, which only needs to

be 3 mm thick despite being straight. The open end of

the front face as shown in Figure 3a) is used to insert

and remove the carbon in order that a range of

adsorbents can be tested. Once the generator is filled

with carbon water manifolds and pressure flanges are

put into place prior to testing.

The sorption material used in the generators is

active carbon type SRD1352/3, especially good for

refrigeration applications, based on coconut shell

precursor manufactured by Chemviron Carbon Ltd.

1 kg of grains of carbon were mechanically compressed

between the shims of each generator reaching a bulk

density of 435 kg/m3 and yield a thermal conductivity

of 0.42 W/(mK). The heat transfer coefficient between

the water channels and the shims was 4150 W/K and

between the shims and the carbon 420 W/K. The

overall heat transfer coefficient of each generator was

380 W/K (Tamainot-Telto et al., 2009).

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Fig. 3 – a) Plate heat exchanger generator design, b) Schematic diagram of laboratory MACS

System description A schematic diagram of the system is shown in

Figure 3b). The engine coolant is alternately passed

through the two generator beds in order to heat them.

A second pumped coolant loop is used to recover heat

between the two beds. An air to water heat exchanger

placed in front of the vehicle radiator (labelled

adsorption heat exchanger) is used to cool the

generator beds to ambient temperature.

An interconnecting pipe with a valve is also

incorporated which enables the ammonia side of the

two generators to be connected for mass recovery

purposes. In this process, the heated high-pressure bed

is connected to the cooled low-pressure bed and

ammonia is transferred from the high pressure to the

low. This increases the concentration change in the

adsorbent during the cycle, thereby increasing SCP and

COP. Check valves are used to control the flow of

ammonia between the generators and the condenser

and evaporator, which are as per a conventional

system. One key difference however is the use of an

indirect evaporator with an intermediate chilled water

glycol loop – this prevents leakage of toxic ammonia

into the cabin, which could occur with a direct

evaporator. For the purposes of controlling both the

condensing and evaporating temperatures during the

experimental tests, an indirect condenser

(ammonia/water) and a temperature controlled bath

linked to the evaporator (ammonia/water) are utilised.

Experimental results and analysis Tests were carried out under the normal European

summer conditions, condenser temperature of 32°C and

evaporating temperature of 20°C. With an operating

driving temperature of 80°C and 1.3 m3/h flow on each

coolant (source and sink) and with mass recovery, the

system met the cooling power target: 1.3 kW vs.

1.2 kW target. However the COP was below the target:

0.23 vs. 0.52. The cooling power achieved corresponds

to a power density (the cooling power per unit volume)

of 93 W/l based on generator volume and 62 W/l based

on the total system volume. The specific cooling power

(the cooling power per unit mass of carbon) SCP is

about 0.650 kW/kg. With increased driving

temperature up to 90°C, the cooling power increased to

1.6 kW, exceeding the 1.2 kW target by 33%. The COP

was 0.22, which is close to the target value of 0.24. The

decreased COP obtained with higher driving

temperature is due to the fact that the cycle time was

not optimised for each condition. The power density

was 114 W/l based on generator volume and 77 W/l

based on total system volume. The SCP reaches

0.8 kW/kg. The effect of coolant flow rates through the

generator from 0.46 to 1.25 m3/h over the range tested

is minimal. The system should therefore not be

significantly affected by the variation in the coolant

water flow rate from the engine during the driving

cycle. The current preliminary performance could be

improved when operating the system with both mass

and heat recovery and with an optimised control

strategy (cycle time).

Quadcon refrigerated container with solar

adsorption refrigeration system

Introduction The aim of this project was to develop and built

a stand-alone solar powered adsorption refrigeration

device. The refrigerator would be integrated with

a standard US Army QuadCon insulated container with

energy requirement based primarily on solar thermal

and solar photovoltaic power. In order to achieve this,

the development of a high adsorbent capacity, high-

density and high thermal conductance carbon along

with a compact plate heat-exchanger could lead to

a truly grid-free/fuel-free refrigeration system. The

goal of the project was to achieve in a QuadCon

a consistent 5ºC air temperature during both day and

night time operations and during conditions which

necessitated door openings during the day.

Generator design and sorption material The generators used in this project were very

similar to the ones used in the EU-TOPMACS project

but in this case the number of carbon slots was

decreased to 10 and their thickness increased to 12 mm

as a carbon with higher thermal properties was used.

The sorption material used in the generators is

a polymer mix of active carbon SDS3 based,

manufactured by Entegris Inc. (previously ATMI Inc.).

2 kg of this carbon mixture were compressed into

smooth-sided monolith blocks and placed between the

generator shims using a high thermal conductivity

epoxy coating to improve heat transfer between carbon

and shims. The bulk density of the carbon mixture was

895 kg/m3 and yielded a thermal conductivity of

2.04 W/(mK) (Carruthers, 2011).

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Fig. 4 – a) Solar Thermal Panels at QuadCon Adsorption Refrigeration System, b) System schematics

System description A schematic diagram and photograph of the

system is shown in Figure 4b). The system consists of

two beds and has mass and heat recovery, very similar

to EU-TOPMACS project. The evaporator is immersed

in an ice bank of nearly 200 l within the QuadCon.

Experimental results and analysis While the machine was tested (March in Arizona,

US) the average solar radiation was 650 W/m2

(peaking at 750 – 800 W/m2), which meant that the

target water (heat transfer fluid) temperatures of 140ºC

needed to yield a good COP was reached for at least

3 h of the 8 h of sun contact. Low cooling powers of

around 400 W were registered during testing. The COP

reached between 0.3 and 0.5 corresponding to cooling

cycles between 250 and 600 s. An average cycle had

180 s of heat/cooling period, 10 s mass recovery and

20 s heat recovery. If the solar radiation was very high

the heat recovery was eliminated to ensure a good

temperature control of the generators.

It is believed that due to the low solar radiation and

due to the lower than expected heat transfer properties

in the generators the system did not achieve the target

level of refrigeration.

HEAT PUMPING: 2-BED HEAT DRIVEN HEAT

PUMP

Introduction The heat pump developed in this project was

intended to be used in a domestic environment (space

heating), replacing a gas condensing boiler. For space

heating of a typical family home in the UK, a three

bedroom semi-detached house, which is required to be

maintained at an internal temperature of 18°C, the heat

pump should supply a heating power of 7 kW (Metcalf,

2009). The 2-bed machine would be driven by the heat

supplied by a gas burner and used pressurised water as

heat transfer fluid. The water used in the heating

system of the house is passed through the ammonia

condenser and then through the cooler (fluid to fluid

heat exchanger) where it increases its temperature.

Generators design and sorption material

The type of heat exchanger used in this project was

shell and tube. The core of generators was made of

stainless steel 304 and their tubes had an outer diameter

of 1.2 mm and an inner diameter of 0.8 mm. The end

plates of the generators had a diameter of 144.5 mm

and 1777 tubes that were nickel brazed together

creating the core of the generator as it can be observed

in Figure 5a). After the core was filled with 3 kg of

a mixture of carbon grains and powder, it was slid

inside the two generator shells. Finally, two flanges

were attached at the ends of the shell completing the

generator assembly.

The generator is effectively a thermally driven

compressor and is the most critical part of the design.

Originally, with the old shell design (one shell and two

end flanges), it was discovered that the generator

flanges contained 10 kg of stainless steel which

reduced significantly the COP. In order to reduce the

thermal mass of the generator shell new domed end

flanges that reduce the mass of steel of the generator

from 10 kg to 2 kg were designed and manufactured

and will be soon tested in the machine. A comparison

of both shell versions can be observed in Figure 5b)

and the new shell version of the generators already

installed in the heat pump can be seen in Figure 5c).

The sorption material used in the generators is

active carbon type 208C, especially good for heat

pumping applications, based on coconut shell precursor

manufactured by Chemviron Carbon Ltd. 3 kg of

a mixture 2/3 grains and 1/3 powder of carbon was

vibrated in the shell and tube heat exchanger reaching

a bulk density of 640 kg/m3 and yield a thermal

conductivity of 0.3 W/(mK). The heat transfer

coefficient between the water channels and the shims

was 5500 W/K and between the shims and the carbon

1300 W/K. The overall heat transfer coefficient of each

generator was 1000 W/K (Rivero Pacho, 2014).

So

lar

coll

ecto

rs a

nd

ho

t o

il p

um

p

Sorption

Generator

G1

Sorption

Generator

G2

Evaporator / ice store

Coole

rC

onden

ser

Ammonia

Solar

collector oil

loop

Adsorber Oil

Cooling loop

Ambient air

Intercooling

loop

Check valve

Cold space

FAN

Two-way valve Solenoid valve

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Fig. 5 – a) Generator core, b) Comparison of old and new version of generator shell,

c) Generators installed in the machine

System description

The heat pump prototype was constructed to test the

performance of the generators along with the carbon

mixture and to validate the computational model

developed. The water schematics and the ammonia

pipework of the heat pump are shown in Figure 6a), b).

The components used in the prototype were:

· Generators: as previously described.

· Condenser and cooler: plate heat exchangers.

· Evaporators: Even though the heat pump was

design to be air sourced, during the testing the

type of evaporator used was flooded

evaporator (water source) since it was more

convenient and controllable.

· Receiver: cylindrical stainless steel vessel

made by the University of Warwick

workshops.

· Expansion valves: electronic controlled

especially design for ammonia.

· Check valves: poppet type check valves later

modified and changed for a ball and conical

seat design.

· Gas burner / Heater: even though the heat

pump was design to be heat-driven by gas,

during the testing the hot water was supplied

by an electric heater since it was more

convenient and controllable.

· Water pumps: fitted in the generators water

circuit, in the load circuit and in the heat

recovery circuit.

· Expansion vessel: used to pressurise the water

circuit as it operated at high temperatures (120

to 170°C).

Fig. 6 – a) Water and b) Ammonia pipework circuits

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Experimental results and analysis

The machine, with the old version of the generators,

was tested with a driving temperature of 150°C and

with an evaporating temperature of 5 °C (temperatures

for normal winter conditions in the UK vary between

0 – 7°C). The machine was tested for two different

cases of delivery temperature:

· Underfloor heating: 36°C flow delivery– 26°C

return.

· Low temperature radiators: 50°C flow

delivery– 40°C return.

The results of the machine testing at the different

delivery temperatures can be seen in Figure 7a). When

compared to the simulation prediction it was observed

that the experimental performances were lower than the

model prediction. The experiment circled, was taken as

an example to be analysed. The COP obtained during

testing was 1.3 and as it was said before, it was lower

than the modelled predicted one that was 1.38. If in the

model, 10 kg of stainless steel (shell flanges and end

plates) were added to the thermal mass of the generator

the model predicted a COP of 1.29, very similar to the

one obtained during testing. This indicated that the

thermal mass of the generator was too high and a new

shell design was needed in order to increase the

performance of the machine. As described in the

previous section, ‘3.2 Generators design and sorption

material’, new domed end flanges and shells that

weight only 2 kg were design and manufactured and

according to the simulation model they would help

increase the COP of the machine from 1.29 to 1.35. At

the time of writing the generators with their new shells

are being installed in the machine in order to be tested

and matched against the model.

In Figure 7b) the experimental heating powers of

the condenser and cooler of the circled case in Figure

7a) are plotted and compared with the prediction of the

correspondent model and taking into account the 10 kg

of extra stainless steel for the old shell design. It is

possible to observe that the experiment and the

simulation are very similar.

Fig. 7 – a) COP vs Heating Power for underfloor heating and low temperature radiators (circled example case), b)

Heating Power vs time of the circled underfloor heating case

THERMAL TRANSFORMERS

Introduction

Where thermally-driven heat pumps pump heat

from low temperature to a medium temperature level

using the work potential of heat flowing from high to

medium temperature, thermal transformers use the

work potential of heat flowing from a medium

temperature level to a low temperature level, allowing

upgrade of heat from medium temperature to high

temperature. Figure 8 gives a schematic representation

of a thermal transformer based on the chemisorption of

ammonia by two (different) salts. This batch process

consists of a ‘charge’ or regeneration phase in which

the ammonia is desorbed from the high temperature

salt (HTS) and adsorbed by the low temperature salt

(LTS). This is achieved by cooling the LTS to ambient

temperature, where the equilibrium pressure of the

reaction between ammonia and LTS is lower than that

of ammonia and HTS at a medium temperature level.

At the end of this phase all ammonia is adsorbed to the

LTS and no ammonia remains in the HTS. By heating

the LTS with ammonia to the medium temperature

level, the equilibrium pressure of the sorption reaction

rises until it exceeds the equilibrium pressure of

reaction of ammonia with the HTS. As ammonia starts

reacting with the HTS, heat is released causing the

HTS to rise in temperature. Once the desired, high

temperature is reached, the remaining heat of reaction

can be extracted and used.

Thermal transformers can be applied when

sufficient heat at medium temperature is available and

high temperature heat is required. This typically occurs

in industrial processes where waste heat is released at

temperatures of up to 150°C (Spoelstra, 2002) where

simultaneously there is a demand for medium pressure

steam (180 - 200°C).

The first attempt by the Energy Research Centre of

the Netherlands (ECN) to build a thermal transformer

consisted of a shell and tube type reactor in which salt

1,00

1,05

1,10

1,15

1,20

1,25

1,30

1,35

5 6 7 8 9 10 11

CO

P (

hea

tin

g)

Heating Power (kW)

Underfloor heatingLow temperature radiators

0

10

20

30

40

50

60

0 120 240 360 480

Hea

tin

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ow

er (

kW

)

Time (s)

Experiment

CondenserExperiment Cooler

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was deposited without any support material or matrix.

Due to the poor thermal conductivity of the salts as

well as the agglomeration of salt after several cycles of

expansion and shrinking, the salt became a compact,

solid block that was no longer capable of adsorbing

ammonia. Later concepts used nickel foam and

aluminium foam to enhance heat transfer and to

stabilize the salt matrix. Although the results improved,

there were still issues with salt escaping from its matrix

and also for full scale CAPEX were too high and

sensible heat losses significant. This led to the

conclusion that the performance of the salt-matrix

should be studied in more detail before constructing

another reactor or thermal transformer system. This

was done in a setup where a single tube with salt in

a matrix could be repeatedly exposed to any desired

ammonia pressure and temperature (van der Pal, 2013).

By measuring heat fluxes and mass flows, the

performance of the salt in the matrix could be

monitored over many cycles. A matrix of ENG

(Expanded Natural Graphite) and salt proved to be the

most promising combination with stable performance

with MgCl2, CaCl2 and LiCl in ENG-matrices over

1000+ sorption cycles.

At the University of Warwick, in collaboration with

ECN, the ENG-salt matrix option has been further

developed. In the INTERACT project, part of H2020

Marie Curie programme, research is done to achieve

a sorbent reactor with a power density of 1 MW/m3. It

is built by SpiraxSarco and consists of 10 tubes and has

a length of 1200 mm. The inner diameter of the tubes is

1 inch and contains the ENG with a porosity of 90%

which has subsequently been filled with CaCl2 using

a salt solution, yielding a CaCl2 to salt ratio of

approximately 2:1. Pressurised water flows around the

outside of the tubes to provide/extract heat to/from the

sorbent material. Figure 9a) shows the reactor design

made by SpiraxSarco.

Fig. 8 - Schematic representation of a chemisorption-based thermal transformer cycle using two salts

Fig. 9 – a) Schematic drawing of reactor for a 10 kW chemisorption-based thermal transformer, b) Model

calculation of the advancement of the sorption reaction CaCl2.(4-8)NH3 in CaCl2-ENG matrix as a function of time at

five different distances from the tube-wall

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Initial model calculations, that include heat transfer

and kinetics, show rapid adsorption and desorption of

NH3 from CaCl2 for driving temperatures of 10 K as

can be seen in Figure 9b). The kinetic parameters and

thermal resistance and conductivity used in this

simulation have been validated in separate

experiments. Future experiments will focus on

validating the performance in the reactor and the

translation of these results into an economical design of

a heat-driven heat transformer for industrial

application.

REFERENCES

Carruthers, J. D., 2011, Development of a QuadCon

refrigerated container with first generation

prototype solar adsorption refrigeration system,

Technical report NATICK/TR-11/016.

Metcalf, S. J., 2009, Compact, efficient carbon-

ammonia adsorption heat pump, Ph.D. Thesis,

University of Warwick, UK.

Metcalf, S. J., Critoph, R. E., Tamainot-Telto, Z., 2012,

Optimal cycle selection in carbon-ammonia

adsorption cycles, International journal of

refrigeration, 35, pp. 571-580.

Rivero Pacho, A. M., 2014, Thermodynamic and heat

transfer analysis of a carbon – ammonia adsorption

heat pump, Ph.D. Thesis, University of Warwick,

UK.