Feasibility study of using agriculture waste as desiccant for air conditioning system

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1 Renewable Energy 28 (2003) Feasibility study of using agriculture waste as desiccant 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, Bangmod Rasburana, 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 , Thailand Received 8 April 2002; accepted 13 December 2002 Abstract This research was aimed at investigating the feasibility of using dried agricultural waste as desiccant for an open cycle air conditioning system. The natural fibers are, therefore, intended to replace chemical desiccant such as silica gel, molecular sieves etc. The investigation was limited 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 H 2 O per 100 g dry product, respectively, from air at the average condition of 32 C and 75% relative humidity. The optimum airflow rate is about 84 and 98 m 3 /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 an airflow rate of 84 m 3 /h and 60 min operating time. However, it is still an interesting option to replace silica gel in open cycle air conditioning system, as the decrease of average adsorption rate 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 coconut coir bed at a lower temperature compared to that with a silica gel. Therefore, the saving of cooling energy is much more important Elsevier Science Ltd. All rights reserved. Corresponding author. Tel.: ; fax: address: Joseph.khe@kmutt.ac.th (J. Khedari) /03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi: /s (03)00003-x

2 1618 J. Khedari et al. / Renewable Energy 28 (2003) Keywords: Coconut coir; Durian peel; Natural fiber; Adsorption; Silica gel 1. Introduction Among Thailand agricultural wastes, coconut coirs and durian peels represent the major part [1]. Coconut coir is widely used, particularly in culture, due to its water absorption, whereas Durian, and other agriculture waste as well, are disposed to landfill which cause significant environmental issues. Our research team, the Building Science Research Center (BSRC), had initiated various studies on the use of garbage to create added values and build novel knowledge in different aspects mainly in building technologies to saving energy. Those researches have permitted the development of new low thermal conductivity particleboards [2] and lightweight construction materials, which are composed of a mixture of cement, sand and fiber from coconut coir 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 represents 50 70% of the total energy consumption in the commercial and residential sectors [4]. In addition, due to the release of CFCs or HCFCs, they destroy the environment steadily. To overcome these issues, various national and international research works were devoted to air conditioning systems using desiccant to saving energy consumption. An interesting review was reported in [5] and new advances in desiccant technologies were published in [6]. Active adsorption and passive regeneration of silica gel beds for drying was also conducted in [7]. In general, liquid and solid chemical humidity-absorbents such as silica gel and zeolite are often used. These research works unveiled the materials good capability in this regard (moisture adsorption) but the temperature of the outlet air from the desiccant system was significantly high, thereby requiring more energy for cooling it for air conditioning system. That explains why there is no wide application of this system in Thailand as the expected benefit is rather limited. Recently, an experimental study on the use of kenaf core as a desiccant for moisture adsorption was conducted in [8]. Kenaf core was compared to silica gel to determine its suitability as a packaging desiccant. The study concluded that kenaf could be used as a substitute of chemical adsorbents. Another study [9] proposed the use of 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-cycle air conditioning.

3 J. Khedari et al. / Renewable Energy 28 (2003) 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 common methodology following the flow chart given in Fig. 1. The initial moisture content of products (about 85%) is reduced to 5% in dry basis. Samples of wet and dried coirs and peels are shown in Fig. 2. Fig. 2. Wet and dry coconut coir and Durian peel. (a) Coconut coir; (b) Durian peel.

4 1620 J. Khedari et al. / Renewable Energy 28 (2003) Fig. 3. The experimental setup. The performance of coconut coir and durian peel will be compared to that of commercial spherical silica gel grains (2 5 mm diameter) of a bulk density of 670 kg/m 3 and initial moisture content less than or equal to 5% like natural fiber specimens Experimental setup An experimental setup was built schematically as shown in Fig. 3. Its design can allow 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.

5 J. Khedari et al. / Renewable Energy 28 (2003) Fig. 5. (a) Dimensions of the rectangular bed, (b-c) Position of temperature and humidity measurement at the inlet and outlet of the cabinet. 4) was a small PVC tube bed 8 cm in diameter. It can contain 100 grams of dry natural desiccant within 0.35 m length. When the silica gel is used (100 grams) the corresponding bed length is only 0.05 m. This bed was mainly used to study the relationship between water adsorption and air velocity for comparison purpose between the different products. The other desiccant box (Fig. 5) can contain 4 kg. It was divided into two parallel beds of the following dimensions: m. The beds are 0.12 m distance from each other. The air flows perpendicular to the beds. This box was used to asses the adsorption performance of natural desiccants with respect to practical application in building air conditioning Testing methodology Two series of tests were undertaken. In the first, the air conditions were set equal to Thailand average summer conditions, namely 75% relative humidity and 32 C D.B.T. (dry bulb temperature) and twelve tests were performed by varying the air flowrate and desiccant. One hundred grams of product were used in each test. It should be pointed out that in each test we ran the experimental set up 30 min before packing the desiccant. This is to ensure that the air condition in all tests are sensibly equal. 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

6 1622 J. Khedari et al. / Renewable Energy 28 (2003) Table 1a Conditions of experiment for the first series (100 grams of product) Ex. no Material Air flow rate (m 3 /h- Operating time (h) 100g dp ) 1 5 Coconut coir 115/100/84/66/ Durian peel 120/100/90/ Coconut coir Durian peel Silica gel 84 9 Table 1b Conditions of experiment for the second serie (4 kg of natural desiccant and 13 h operating time (Experiments No )) Material % RH Air flow rate (m 3 /h-100g dp ) Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel Coconut coir/durian peel for different air flowrates and relative humidity. The temperature was maintained at 32 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 Testo 454 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 (Torius Model 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 of water adsorbed by coconut coirs and durian peels. It can be seen that under conditions used here, 100 g of dry coconut coirs at 84 m 3 /h of air flow rate (or 4.2 m/s) can absorb the maximum amount of water of about 30 g. The amount of water adsorbed by Durian is less important.

7 J. Khedari et al. / Renewable Energy 28 (2003) 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 seen that coconut coir and silica gel behaved more or less similarly. High water adsorption rate was observed during the first hour. Next, water is absorbed slightly and equilibrium was attained after approximately 2 h. Silica gel absorbed 35 g of water, an increase of 5 g compared to coconut coir. While durian peel absorbed water quasi linearly but at a very low rate. It should also be pointed out that no mold was visually observed in the samples. Fig. 7. Water adsorption curve for the different absorbents (air flow rate 84 m 3 /h-100g, 32 C and 75%RH).

8 1624 J. Khedari et al. / Renewable Energy 28 (2003) 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 air velocity of 4.2 m/s. The average corresponding adsorption rates per 100 grams product is and g/h, respectively. When the air flowrate decreased (0.05 m/s) the average adsorption rate of coconut coir decreased considerably. Therefore, high air 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 the air moisture and flowrate, the higher the amount of water absorbed by the desiccant. Table 2 Experimental results of air moisture absorbed by coconut coir and durian peel (Tests No ) Average air Coconut coir Durian relative humidity % Weight of water adsorbed (g) Weight of water adsorbed (g) Air velocity/air flow rate (m/s)/(m 3 /hr-100g Air velocity/air flow rate (m/s)/(m 3 /hr-100g dry product) dry product) 0.052/ / / / /

9 J. Khedari et al. / Renewable Energy 28 (2003) Feasibility analysis of using natural desiccants in air conditioning As mentioned in the introduction, the main problem encountered with chemical adsorbents for air conditioning systems is that the temperature of air leaving the desiccant is quite high. Fig. 9 shows the temperature difference between air at inlet and outlet of the bed for the two natural desiccants and silica gel as a function of adsorption 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 were extracted from tests No at 5, 30, 60, 80, 100 and 120 min. It can be seen that higher temperature difference is observed with silica gel and durian compared to 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 characteristics and pore structures. Also, as silica gel and durian peels have higher density than 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 coconut coir. That explains why the outlet bed temperature of the coconut coirs is cooler than 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 in practical application. Further analysis is recommended to clearly understand the process 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 air leaving the coconut coir bed is smaller than that of the silica gel bed. That means less 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 1626 J. Khedari et al. / Renewable Energy 28 (2003) Fig. 10. Dehumidification process on psychrometric chart (2 kg of desiccant, operating time 60 min, air flow rate: 84 m 3 /h-100 g) Regeneration of coconut coir Towards practical application, regeneration of desiccant should be considered. To this end, a test was conducted using 100 grams of coconut coir at about 24% moisture content in dry basis. The ambient air used for regeneration was heated to temperature varying between C. The air flowrate was 11 m 3 /h. It was found that regeneration of coconut coir is feasible at a relatively short time (37 min) giving us good satisfaction and indicating promising potential for application (Fig. 11). 4. Conclusion Initial investigations conducted in this paper confirmed that agriculture waste could be used as a desiccant instead of chemical desiccants such as silica gel. Dry coconut

11 J. Khedari et al. / Renewable Energy 28 (2003) Fig. 11. Regeneration curve of coconut coir (100 grams) (average air condition: 11 m 3 /h, 29% RH). coirs present the best performance compared to Durian peels. The temperature of the air leaving the coconut coir bed is lower than that of silica gel, which is extremely interesting for air conditioning systems as less cooling energy will be required. In addition, using dry peels can give additional value as they are agricultural wastes and help preserving our environment and promoting natural products. Before general application of the proposed new concept, further studies should be conducted which include the life cycle of the coconut coir (adsorption-regeneration), the quality of air leaving the bed and full testing of the system such as open cycle air conditioning system. Acknowledgements The authors would like to thank the Thailand Research Fund (TRF), The National Research Council of Thailand (NRCT) and the Ministry of University Affairs (MUA) for providing partial financial support to this study. References [1] Ministry of Agriculture and Co-op, 2000, Statistic of Planting of Fruit Trees, Data Processing Sub- Division, Department of Agricultural Extension, p. 30. [2] Khedari J, Charoenvai S, Hirunlabh J. New insulating particleboards from durian peel and coconut coir. Journal of Building and Environment 2003;38: [3] Khedari J, Suttisonk B, Hirunlabh J. New lightweight composite construction materials with low thermal conductivity. CECO Journal 2000;804:1 6. [4] Energy consumption in Thailand, from the website of the national energy policy office:

12 1628 J. Khedari et al. / Renewable Energy 28 (2003) [5] Waugaman DG, Kini A, Kettleborough CF. A review of desiccant cooling system. Energy Resources Technology Journal 1993;115:1 8. [6] Novosel D. Advances in desiccant technologies. Energy Engineering Journal 1996;93(1):7 19. [7] Chindaruksa S, Hirunlabh J, Khedari J, Daguenet M. Active adsorption-passive regeneration design of silica gel beds for drying system, Revue Internationale D Heliotechnique-No Vol. 23, Prientempes 2001; [8] Williams JB. Evaluation of kenaf core as a desiccant. Department of Animal and Dairy Science Box 9815 Mississippi State, MS 39762, from the World Wide Web: [9] Ziegler Th, Richter I-G. Storage of solar drying potential using grain as the desiccant: simulation results. Drying Journal 1998;B: August