Performance of solid desiccant-based evaporative cooling system under the climatic zones of India

Transcription

1 *Corresponding author: rediffmail.com Performance of solid desiccant-based evaporative cooling system under the climatic zones of India... H. Parmar 1 * and D.A. Hindoliya 2 1 Department of Mechanical Engineering, Ujjain Engineering College, Ujjain, India; 2 Rajeev Gandhi Technological University, Bhopal, India... Abstract For better use of evaporative cooling techniques in humid climate, a desiccant cooling system (DCS) can be an alternative option in place of a conventional cooling system. This paper presents a theoretical comparative study of the performance of a DCS for four different climatic conditions of India (i.e. hot and dry, warm and humid, moderate and composite climates). From the analysis, it was found that the performance of a DCS is suitable in warm and humid climate. The thermodynamic equations of intermediate states of the desiccant cooling cycle are also presented. Effects of some parameters such as outdoor specific humidity, R/P ratio (regeneration air flow/process air flow) and power required to regenerate the desiccant wheel have been studied. The theoretical minimum R/P ratio has been computed for the maximum coefficient of performance (COP) of a DCS and its value was found to be 0.55 in Mumbai (warm and humid climate) for the 80% effectiveness of ECW. This paper also presents the relationship between COP and regeneration power with the different values of R/P ratios. Keywords: desiccant cooling; evaporative cooling; climatic zones; COP; desiccant wheel Received 11 July 2011; revised 12 September 2011; accepted 29 December INTRODUCTION Desiccant cooling systems (DCSs) are used to dehumidify and cool the air for space cooling in buildings. A desiccant material is a powerful weapon against moisture and humidity. Moisture present in air is a serious threat to product integrity in a variety of industries. The property of the desiccant material is to absorb moisture from air by creating an area of low vapour pressure at the surface of the desiccant. For absorbing the moisture from air, a rotary wheel is used, i.e. desiccant wheel. A desiccant wheel has holes in a honeycomb structure for passing humid air. The honeycomb structure is preferred due to low pressure drop of air in the wheel. Some common adsorbent materials used are LiCl, SiO 2 (silica gel), Al 2 O 3 (activated alumina), LiBr and Zeolite. White et al. [1] compared the performance of a new desiccant wheel composed of zeolite and super adsorbent polymer, with the conventional silica gel desiccant wheel and it was found that the super adsorbent polymer desiccant wheel achieved higher dehumidification with low regeneration temperature. A new kind of composite desiccant material was fabricated by Jia et al. [2], and it was found that the newly developed composite desiccant wheel removes 50% more moisture than the conventional wheel. A desiccant wheel rotates at a very low rpm in comparison with the energy conservation wheel (60 times more slowly than ECW). Fifty per cent of the area of the wheel is utilized by process air and the remaining 50% by regenerative air. India is a country whose climatic conditions differ with latitude locations ranging from moderate to very harsh conditions. On the basis of this, monthly mean data recorded in 233 stations located in the various part of India were divided into six climatic zones by Bansal and Minke [3]. These climatic zones are hot and dry, warm and humid, moderate, cold and cloudy, cold and sunny, and composite. This paper evaluates the effect of ambient conditions on the performance of desiccantaugmented evaporative cooling in different selected cities of four different climatic zones of India. The regions with cold and cloudy and cold and sunny climates have been known as cold regions and there is no requirement of cooling. Subramanyam et al. [4] applied a desiccant wheel for low humidity air conditioning and also explained how the different parameters affect the performance like air flow rate, compressor pumping capacity, speed of the wheel etc. and found an optimum wheel speed of 17.5 rpm for high moisture removal and a maximum COP. Dhar et al. [5] proposed various solid desiccant cycles for hot and humid climate and found that International Journal of Low-Carbon Technologies 2013, 8, # The Author Published by Oxford University Press. All rights reserved. For Permissions, please journals.permissions@oup.com doi: /ijlct/ctr051 Advance Access Publication 29 February

2 Performance of solid desiccant-based evaporative cooling system under the climatic zones of India among ventilation, recirculation and the Dunkle cycle, the Dunkle cycle is better for a wide range of outdoor conditions. Ruivo et al. [6] investigated a new pair of independent effectiveness parameters of the desiccant wheel, derived the correlations for their estimation and performed an easy and quick predicting effectiveness method of the desiccant wheel. Numerical simulation was conducted for the desiccant wheel by Ge et al. [7] and two key operating parameters of the desiccant wheel have been examined for the range of regeneration temperatures from 60 to 1508C. Optimization of these two parameters was conducted based on the wheel s performance evaluated by means of its moisture removal capacity (MRC). 2 DESCRIPTION OF DCS A solid DCS consists of a desiccant wheel, ECW and direct evaporative coolers (DECs). Return air from conditioned space Figure 1. Desiccant cooling system. is mixed with the outdoor air to decrease the temperature of process air. A desiccant wheel absorbs moisture from air due to pressure difference between partial pressure of moisture in the air and desiccant materials. Temperature of air is increased after dehumidification. The dry and high temperature air is sensibly cooled by a heat exchanger. Sensibly cooled air is cooled by a DEC and is supplied in the room. Regeneration of the desiccant material can be made by supplying hot air from any waste heat source or solar energy. A desiccant-augmented evaporative cooling system employing ventilation and recirculation cycle is shown in Figure 1, the moisture (latent load) present in the process air is removed by a desiccant wheel A, which has a honeycomb structure for passing humid air. The desiccant material is impregnated on the inner surface of the honeycomb structure. When humid air is passed through the desiccant wheel, it absorbs moisture and increases the temperature of the air leaving from the desiccant wheel. Then the temperature of the dried process air is reduced by a sensible cooler B (e.g. heat exchanger/ecw/enthalpy wheel). This sensibly cooled air passed through a DEC D to obtain the desired thermal comfort condition. The desiccant wheel is regenerated by the application of heat to release the moisture, which is discharged to the outdoors. The heat for regeneration can be supplied from any source such as solar energy, waste heat, natural gas and off-peak electricity. Waste heat from a microgenerator was used for regeneration of the desiccant wheel by Angrisani et al. [8]. The schematic arrangements of a DCS are shown in Figure 1 and the corresponding processes are represented in Figure 2. Figure 2. Thermodynamic processes of the DCS on the psychrometric chart. International Journal of Low-Carbon Technologies 2013, 8,

3 H. Parmar and D.A. Hindoliya 3 METHODOLOGY For theoretical analysis of the desiccant cooling cycle, the following assumptions have been made. (1) The reactivation temperature of the silica gel desiccant wheel is in the range of C. (2) The effectiveness of DEC is assumed to be 85% (most of the coolers are having effectiveness in the range of 85 95%). (3) The effectiveness of the ECW is assumed to be 80% (most of the ECWs are having effectiveness in the range of 6 80%). (4) Fifty per cent return air is mixed with process air and the remaining fifty per cent return air is mixed with regeneration air for utilizing low-temperature air from the conditioned space. (5) The ratio of regeneration air flow to process air flow (R/P) is assumed to be between 0.55 and 1. Weather data have been obtained from the weather data and design condition for India [9]. The following mathematical equations [10] have been used for computing temperatures at different intermediate points in the desiccant cycle. Temperature, after mixing of return air and outdoor air, is computed as follows T 1 ¼ x T 0 þð1 xþt R1 ð1þ where T 1 9 is the intermediate temperature, x the per cent of outdoor air to process (dimensionless) and T R1 the temperature of return air from the conditioned space. Leaving temperature (T 2 ) and moisture conditions from the desiccant wheel can be estimated by the selection of curve of 858C regeneration temperature in a typical performance curve of a silica gel desiccant wheel shown in Figures 3 and 4. T 3 ¼ T 2 1 cw ðt 2 T 6 Þ where e cw is the effectiveness of ECW (dimensionless). T 4 ¼ T 3 1 DEC ðt 3 T 3w Þ T 6 ¼ T 5 1 DEC ðt 5 T 5w Þ T 7 ¼ ðt 2 T 3 Þ þ T 6 R=P T 8 ¼ 808C for reactivation of the silica gel desiccant wheel. Heat input to the generator to regenerate the desiccant wheel is given by Q R ¼ðT 3 T 7 Þm p c p T 3 ¼ T 8 T 2 T 1 R=P ð2þ ð3þ ð4þ ð5þ ð6þ ð7þ Figure 3. Variation between the inlet and outlet moisture of a desiccant wheel. Figure 4. Variation between the inlet moistures and outlet temperatures of a desiccant wheel. The COP of the system is defined as follows: COP ¼ Cooling effect Regenerattion heat 4 RESULTS AND DISCUSSION For the analysis, four cities have been selected from the Indian climatic zone (i.e. Jodhpur, Mumbai, Bangalore and New Delhi). For standard (assumed) room conditions, supply temperatures and specific humidity have been computed and presented in Table International Journal of Low-Carbon Technologies 2013, 8, 52 57

4 Performance of solid desiccant-based evaporative cooling system under the climatic zones of India Table 1. Supply temperature of a DCS in different climatic zones of India. No. Name of cities and climates Outdoor air DBT WBT Specific humidity (g/kg of dry air) Room supply air DBT WBT Specific humidity (g/kg of dry air) 1 Jodhpur (hot and dry) 2 Mumbai (warm and humid) 3 Bangalore (moderate) 4 New Delhi (composite) Table 2. COP and regeneration heat (Qr) for different R/P ratios. R/P ratio Jodhpur Mumbai Bangalore New Delhi COP Qr COP Qr COP Qr COP Qr With the help of the desiccant wheel, the maximum percentage (i.e %) of specific humidity has been decreased in Mumbai and is minimum in Bangalore (i.e. 4.7%). From the supply condition, it was observed that a DCS is more suitable in the warm and humid climate of India. The COP and the heat (Qr) required to regenerate the desiccant wheel at different R/P ratios have been computed and a comparison is presented in Table 2. The performance of the DCS at different R/P ratios has been evaluated for different places and it was observed that the COP of the system at 0.55 R/P ratios is highest in Mumbai, i.e for 80% effectiveness of the ECW. The COP is highly sensitive with regard to the R/P ratio and it decreases with an increase in the R/P ratio. The variation of the COP with the R/P ratio is presented in Figure 5. Further increases in the COP could also be achieved by employing staged regeneration, as suggested by Waugman et al. [11]. The theoretical minimum R/P ratio has been computed for better performance of the desiccant wheel. The optimum value of the R/P ratio is influenced by the effectiveness of the ECW. At 0.55 R/P ratios, the COP of the DCS has been obtained maximum in Mumbai for 80% effectiveness of the ECW. In Jodhpur, the DCS is not suitable because the percentage of Figure 5. Variation of the COP with the R/P ratio. Figure 6. Variations of the COP and regeneration power with the specific humidity. reduction in humidity is much lower. The COP of the system is also highly influenced by the outdoor absolute humidity; it increases with an increase in the outdoor specific humidity and the power required for regeneration of the desiccant wheel decreases with an increase in the specific humidity. The specific humidity in Mumbai is more in comparison with the other selected cities; then the performance of the DCS was found to be better with a high COP and less regeneration power. The International Journal of Low-Carbon Technologies 2013, 8,

5 H. Parmar and D.A. Hindoliya Regeneration power is also influenced by the R/P ratio and it increases with an increase in the R/P ratio. Figure 8 shows the variation of power required to regenerate the desiccant wheel for different climatic conditions of India. From the analysis, it was found that with an increase in the R/P ratio, the regeneration power also increased. Figure 7. Variations of regeneration heat with specific humidity. Figure 8. Variation of regeneration power with the R/P ratio. variations of the COP and regeneration heat with the specific humidity are presented in Figures 6 and 7. In Bangalore (moderate climate), the COP of the system has been found low with high regeneration power. 5 CONCLUSION The performance of the DCS has been analysed for climatic conditions prevailing in India. Some parameters such as regeneration heat, COP, R/P relationship (Regeneration air flow/ process air flow) and intermediate temperatures in the desiccant cooling cycle have been considered for this study. For the analysis, four Indian cities from different climatic zones have been selected. The computed supply air condition shows that the system is able to provide human thermal comfort in the warm and humid climate of India. It was found that the R/P ratio and the outdoor specific humidity are more influential parameters on the performance of the DCS. The performance of the system at 0.55 R/P ratios has been found to have the maximum COP (4.98) in Mumbai (warm and humid climate) with less regeneration power (0.4 kw) for 80% effectiveness of the ECW. In Mumbai, the percentage reduction in specific humidity was found to be 21.39%. In general, it was observed that with an increasing R/P ratio, the COP of a DCS decreases and the regeneration heat requirement increases. Hence, it is required to operate the system with an optimum value of the R/P ratio which can be obtained by the parametric analysis in the zone of operation. REFERENCES [1] White SD, Goldsworthy M, Reece R, Spillmann T, Gorur A, Lee DY. Characterization of desiccant wheels with alternative materials at low regeneration temperatures. Int J Refriger 2011;34: [2] Jia CX, Dai YJ, Wu JY, et al. Experimental comparison of two honeycombed desiccant wheels fabricated with silica gel and composite desiccant material. Energy Convers Manage 2006;47: [3] Bansal NK, Minke G. Passive Air Conditioning. Kernforsch- ungsanlage Julich GmbH Zentralbibiliothek, 1988; Julich, 19, 104. [4] Subramanyam N, Maiya MP, Murthy SS. Application of desiccant wheel to control humidity in air conditioning system. ApplThermal Eng 2004;24: [5] Dhar PL, Kaushik SC, Jain S. Thermodynamic analysis of desiccant-augmented evaporative cooling cycles for Indian conditions. ASHRAE Trans 1995;101: [6] Ruivo CR, Costa JJ, Figueiredo AR, Kodama A. Effectiveness parameters for the prediction of the global performance of desiccant wheels an assessment based on experimental data. Renew Energy 2012;38: [7] Ge TS, Li Y, Wang RZ, et al. A review of the mathematical models for predicting rotary desiccant wheel. Renew Sustain Energy Rev 2008;12: International Journal of Low-Carbon Technologies 2013, 8, 52 57

6 Performance of solid desiccant-based evaporative cooling system under the climatic zones of India [8] Angrisani G, Capozzoli A, Minichiello F, et al. Desiccant wheel regenerated by thermal energy from a microcogenerator: experimental assessment of the performances. Appl Energy 2011;88: [9] Weather data and design condition for India. Indian Society of Heating Refrigeration and air Conditioning Engineers and American Society of Heating Refrigeration and Air Conditioning Engineers (Indian Chapter), 1996;17 8. [10] Camargo JR, Godoy E, Ebinuma CD. An evaporative and desiccant cooling system for air conditioning in humid climates. J Brazil Soc Mech Sci Eng 1995;27:1 16. [11] Waugman DG, Kini A, Kettleborough CF. A review of desiccant cooling system. Trans ASME J Energy Resour Technol 1993;115: 1 8. International Journal of Low-Carbon Technologies 2013, 8,