Renewable Energy 28 (2003) 1617–1628www.elsevier.com/locate/renene

Feasibility study of using agriculture waste asdesiccant for air conditioning system

J. Khedari a,!, R. Rawangkul a, W. Chimchavee b, J. Hirunlabh a,A. Watanasungsuit c

a Building Scientific Research Center, King Mongkut’s University of Technology Thonburi, BangmodRasburana, 91 Pracha U-thit Rd., Thungkru, Bangkok 10140, Thailand

b The University of the Thai Chamber of Commerce, 126/1 Vibhavadee-Rangsit Road, Bangkok 10320,Thailand

c Engineering Management, South-East Asia University, 19/1 Petkasem Road, Nong Khaem,Bangkok 110160, Thailand

Received 8 April 2002; accepted 13 December 2002

Abstract

This research was aimed at investigating the feasibility of using dried agricultural waste asdesiccant for an open cycle air conditioning system. The natural fibers are, therefore, intendedto replace chemical desiccant such as silica gel, molecular sieves etc. The investigation waslimited to Coconut coir (Cocos nucifera) and Durian peels (Durio zibethinus).Experimental results confirmed that dry coconut coir and durian peel can absorb 30 g and

17 g H2O per 100 g dry product, respectively, from air at the average condition of 32°C and75% relative humidity. The optimum airflow rate is about 84 and 98 m3/hr-100 g dry product,respectively. Therefore, the dry coconut coir is more suitable than the dry durian peel.Comparison between the dry coconut coir and silica gel showed that the average adsorption

rate of coconut coir is less than that of silica gel by about 5 g/h-100 g dry product at anairflow rate of 84 m3/h and 60 min operating time. However, it is still an interesting optionto replace silica gel in open cycle air conditioning system, as the decrease of average adsorptionrate is rather small.The other extremely interesting advantage of coconut coir is that during moisture absorption

the heat generated during the process is less important. That means the air leaves the coconutcoir bed at a lower temperature compared to that with a silica gel. Therefore, the saving ofcooling energy is much more important. 2003 Elsevier Science Ltd. All rights reserved.

! Corresponding author. Tel.: +66-2-470-8625; fax: +66-2-427-9062.E-mail address: Joseph.khe@kmutt.ac.th (J. Khedari).

0960-1481/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved.doi:10.1016/S0960-1481(03)00003-X

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Keywords: Coconut coir; Durian peel; Natural fiber; Adsorption; Silica gel

1. Introduction

Among Thailand agricultural wastes, coconut coirs and durian peels represent themajor part [1]. Coconut coir is widely used, particularly in culture, due to its waterabsorption, whereas Durian, and other agriculture waste as well, are disposed tolandfill which cause significant environmental issues. Our research team, the BuildingScience Research Center (BSRC), had initiated various studies on the use of garbageto create added values and build novel knowledge in different aspects mainly inbuilding technologies to saving energy. Those researches have permitted the develop-ment of new low thermal conductivity particleboards [2] and lightweight constructionmaterials, which are composed of a mixture of cement, sand and fiber from coconutcoir or durian peel [3].Located in the hot tropical zone, Thailand relies on air-conditioners to ensure

residents’ thermal comfort. Today, the energy consumed by air conditioners rep-resents 50–70% of the total energy consumption in the commercial and residentialsectors [4]. In addition, due to the release of CFCs or HCFCs, they destroy theenvironment steadily. To overcome these issues, various national and internationalresearch works were devoted to air conditioning systems using desiccant to savingenergy consumption. An interesting review was reported in [5] and new advancesin desiccant technologies were published in [6]. Active adsorption and passive regen-eration of silica gel beds for drying was also conducted in [7]. In general, liquid andsolid chemical humidity-absorbents such as silica gel and zeolite are often used.These research works unveiled the materials’ good capability in this regard (moistureadsorption) but the temperature of the outlet air from the desiccant system was sig-nificantly high, thereby requiring more energy for cooling it for air conditioningsystem. That explains why there is no wide application of this system in Thailandas the expected benefit is rather limited.Recently, an experimental study on the use of kenaf core as a desiccant for moist-

ure adsorption was conducted in [8]. Kenaf core was compared to silica gel to deter-mine its suitability as a packaging desiccant. The study concluded that kenaf couldbe used as a substitute of chemical adsorbents. Another study [9] proposed the useof grains as desiccant for store in dryers.This paper presents an investigation on the feasibility of air humidity absorption

by using dry coconut coirs and dry durian peels towards application in opened-cycleair conditioning.

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Fig. 1. Flow chart of material preparations.

2. Experimental methodology

2.1. Materials preparation

The preparation of Coconut coir and durian peels was performed using commonmethodology following the flow chart given in Fig. 1. The initial moisture contentof products (about 85%) is reduced to 5% in dry basis. Samples of wet and driedcoirs and peels are shown in Fig. 2.

Fig. 2. Wet and dry coconut coir and Durian peel. (a) Coconut coir; (b) Durian peel.

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Fig. 3. The experimental setup.

The performance of coconut coir and durian peel will be compared to that ofcommercial spherical silica gel grains (2–5 mm diameter) of a bulk density of 670kg/m3 and initial moisture content less than or equal to 5% like natural fiber speci-mens.

2.2. Experimental setup

An experimental setup was built schematically as shown in Fig. 3. Its design canallow us to adjust the air dry bulb temperature and relative humidity.For the natural desiccant, two beds were considered, Figs. 4 and 5. The first (Fig.

Fig. 4. Dimensions of the tube bed for the lab-scale investigation.

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Fig. 5. (a) Dimensions of the rectangular bed, (b-c) Position of temperature and humidity measurementat the inlet and outlet of the cabinet.

4) was a small PVC tube bed 8 cm in diameter. It can contain 100 grams of drynatural desiccant within 0.35 m length. When the silica gel is used (100 grams) thecorresponding bed length is only 0.05 m. This bed was mainly used to study therelationship between water adsorption and air velocity for comparison purposebetween the different products.The other desiccant box (Fig. 5) can contain 4 kg. It was divided into two parallel

beds of the following dimensions: 0.58 × 0.58 × 0.58 m. The beds are 0.12 m distancefrom each other. The air flows perpendicular to the beds. This box was used to assesthe adsorption performance of natural desiccants with respect to practical applicationin building air conditioning.

2.3. Testing methodology

Two series of tests were undertaken. In the first, the air conditions were set equalto Thailand average summer conditions, namely 75% relative humidity and 32°CD.B.T. (dry bulb temperature) and twelve tests were performed by varying the airflowrate and desiccant. One hundred grams of product were used in each test. Itshould be pointed out that in each test we ran the experimental set up 30 min beforepacking the desiccant. This is to ensure that the air condition in all tests are sensiblyequal. Also for this series, two operating time were considered, namely 3 and 9 h.For the second series, 4 kg of product was used and eighteen tests were performed

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Table 1aConditions of experiment for the first series (100 grams of product)

Ex. no Material Air flow rate (m3/h- Operating time (h)100gdp)

1–5 Coconut coir 115/100/84/66/58 36–9 Durian peel 120/100/90/84 310 Coconut coir 84 911 Durian peel 84 912 Silica gel 84 9

Table 1bConditions of experiment for the second serie (4 kg of natural desiccant and 13 h operating time(Experiments No. 13–30))

Material % RH Air flow rate (m3/h-100gdp)

Coconut coir/Durian peel 68 1.54Coconut coir/Durian peel 68 1.10Coconut coir/Durian peel 68 0.62Coconut coir/Durian peel 75 1.54Coconut coir/Durian peel 75 1.10Coconut coir/Durian peel 75 0.62Coconut coir/Durian peel 92 1.54Coconut coir/Durian peel 92 1.10Coconut coir/Durian peel 92 0.62

for different air flowrates and relative humidity. The temperature was maintained at32°C. Table 1 summarise all test conditions.The inlet and outlet air properties (relative humidity and dry bulb temperature and

velocity) of desiccant beds are measured and displayed in 1 min intervals using Testo454 recorders. The accuracy of measurements is ±0.1%RH, ±0.01°C, and ±0.05 m/s,respectively. The weight of specimens is measured using digital balances (ToriusModel BP 610 and 3100). The reading error is ±0.1 and ±1 g, respectively.

3. Experimental results and discussion

3.1. Relationship between water adsorption and air velocity

Fig. 6 shows the effect of the air flow rate, or the air velocity, on the amount ofwater adsorbed by coconut coirs and durian peels. It can be seen that under conditionsused here, 100 g of dry coconut coirs at 84 m3/h of air flow rate (or 4.2 m/s) canabsorb the maximum amount of water of about 30 g. The amount of water adsorbedby Durian is less important.

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Fig. 6. Relationship between the absorbed amount of water and air flow rate (average air condition,32°C, 75% RH, operating time 3 h).

Fig. 7 shows the water gain over time for the different adsorbents. It can be seenthat coconut coir and silica gel behaved more or less similarly. High water adsorptionrate was observed during the first hour. Next, water is absorbed slightly and equilib-rium was attained after approximately 2 h. Silica gel absorbed 35 g of water, anincrease of 5 g compared to coconut coir. While durian peel absorbed water quasilinearly but at a very low rate. It should also be pointed out that no mold was visuallyobserved in the samples.

Fig. 7. Water adsorption curve for the different absorbents (air flow rate 84 m3/h-100g, 32°C and75%RH).

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Fig. 8. Comparison between the adsorption rates of coconut coir and silica gel.

Fig. 8 compared the absorption rate of coconut coirs and silica gel for an airvelocity of 4.2 m/s. The average corresponding adsorption rates per 100 grams pro-duct is 35.45 and 40.60 g/h, respectively. When the air flowrate decreased (0.05 m/s)the average adsorption rate of coconut coir decreased considerably. Therefore, highair flowrate is recommended.Table 2 summarizes the testing results of moisture adsorption using 4 kg natural

desiccants for various air relative humidity and flowrate. Obviously, the higher theair moisture and flowrate, the higher the amount of water absorbed by the desiccant.

Table 2Experimental results of air moisture absorbed by coconut coir and durian peel (Tests No. 13–30)

Average air Coconut coir Durianrelativehumidity %

Weight of water adsorbed (g) Weight of water adsorbed (g)

Air velocity/air flow rate (m/s)/(m3/hr-100g Air velocity/air flow rate (m/s)/(m3/hr-100gdry product) dry product)

0.052/1.54 0.037/1.10 0.020/0.62 0.052/1.54 0.037/1.10 0.0200.62

68 363.01 263.22 174.75 424.21 404.23 357.4774 590.78 441.56 312.43 520.22 518.01 454.5392 1149.67 753.6 643.84 929.03 888.08 790.30

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3.2. Feasibility analysis of using natural desiccants in air conditioning

As mentioned in the introduction, the main problem encountered with chemicaladsorbents for air conditioning systems is that the temperature of air leaving thedesiccant is quite high. Fig. 9 shows the temperature difference between air at inletand outlet of the bed for the two natural desiccants and silica gel as a function ofadsorption rate. It should be pointed out this adsorption rate is a relative index,calculated in percentage, used to indicate how much moisture has been absorbed(removed from the air) by the adsorbent during a time interval (1 min). Data wereextracted from tests No. 10–12 at 5, 30, 60, 80, 100 and 120 min. It can be seenthat higher temperature difference is observed with silica gel and durian comparedto coconut coir. The higher the adsorption rate, the higher the temperature difference.The outlet temperature of silica gel was higher because of silica gel’s surface charac-teristics and pore structures. Also, as silica gel and durian peels have higher densitythan coconut coir, adsorbing water from the air at the intersurface is more difficult.Consequently, the heat released during the adsorption is higher than that of coconutcoir. That explains why the outlet bed temperature of the coconut coirs is coolerthan that of silica gel and durian peel at the same absorption percentage. In addition,it is found that durian peel’s water adsorption is too small which limits its use inpractical application. Further analysis is recommended to clearly understand the pro-cess involved and to give a more accurate explanation.A plot of the dehumidification process on a psychrometric chart is shown in Fig.

10. It can be seen that for the same amount of absorbed water the enthalpy of airleaving the coconut coir bed is smaller than that of the silica gel bed. That meansless cooling energy is required which is extremely interesting.

Fig. 9. Comparison between bed temperature difference and the absorption rate for different adsorbents(Data from tests No. 10–12).

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Fig. 10. Dehumidification process on psychrometric chart (2 kg of desiccant, operating time 60 min, airflow rate: 84 m3/h-100 g).

3.3. Regeneration of coconut coir

Towards practical application, regeneration of desiccant should be considered. Tothis end, a test was conducted using 100 grams of coconut coir at about 24% moisturecontent in dry basis. The ambient air used for regeneration was heated to temperaturevarying between 48–57°C. The air flowrate was 11 m3/h. It was found that regener-ation of coconut coir is feasible at a relatively short time (37 min) giving us goodsatisfaction and indicating promising potential for application (Fig. 11).

4. Conclusion

Initial investigations conducted in this paper confirmed that agriculture waste couldbe used as a desiccant instead of chemical desiccants such as silica gel. Dry coconut

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Fig. 11. Regeneration curve of coconut coir (100 grams) (average air condition: 11 m3/h, 29% RH).

coirs present the best performance compared to Durian peels. The temperature ofthe air leaving the coconut coir bed is lower than that of silica gel, which is extremelyinteresting for air conditioning systems as less cooling energy will be required. Inaddition, using dry peels can give additional value as they are agricultural wastesand help preserving our environment and promoting natural products. Before generalapplication of the proposed new concept, further studies should be conducted whichinclude the life cycle of the coconut coir (adsorption-regeneration), the quality ofair leaving the bed and full testing of the system such as open cycle air condition-ing system.

Acknowledgements

The authors would like to thank the Thailand Research Fund (TRF), The NationalResearch Council of Thailand (NRCT) and the Ministry of University Affairs (MUA)for providing partial financial support to this study.

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