RESEARCH ARTICLE

Vijay MITTAL, B. Kant KHAN

Experimental investigation on desiccant air-conditioningsystem in India

© Higher Education Press and Springer-Verlag 2009

Abstract An experimental investigation in India waspresented to evaluate the performance and energy savingcapacity of a desiccant air-conditioning system composedof a silica gel bed, a split type air-conditioner (1.0 tonrefrigeration) installed in a room with a volume of 86.4 m3,air ducts and a blower. The experiment was made in such away that the percentages of return air, outdoor air andindoor air mixed with the air leaving the desiccant anddesiccant bed thickness could be adjusted. Tests wereconducted on several days with relatively similar ambientconditions. Under the test conditions in this experiment, a7 cm bed thickness is recommended with a maximumadsorption rate of 403 g/h. The optimum percentages of airratios were as follows: 10% of outdoor air, 10% of returnair (mixed together at the desiccant bed inlet) and 80% ofindoor air mixed with the dry air leaving the desiccant. Thecorresponding electricity saving was about 19%. Asexpected, simple economic analysis indicates that thedesiccant air-conditioning is not viable for smaller coolingcapacities.

Keywords air ratios, ambient conditions, desiccant air-conditioning, silica gel

1 Introduction

The desiccant air conditioning cycle is an open heat drivencycle, which can be used both to cool and dehumidify air.Being a heat driven cycle, desiccant air conditioningaffords an opportunity to utilize the heat obtained from thesun. It can therefore be coupled to solar collectors toproduce an air conditioning system. Beggs and Warwicker[1] have shown that desiccant air conditioning is best

applied to installations where the bulk of the sensible airconditioning is performed by a water-based system, suchas a chilled ceiling. In such applications, desiccant airconditioning should be used to treat the incomingventilation air. Beggs and Halliday [2] have demonstratedthe potential opportunity for harnessing solar energy todrive desiccant systems in northern Europe and the UK.These studies yielded good results, and demonstrated thatfor much of the air conditioning season most of theregeneration heat required could be provided by solarenergy. Henning et al. [3] have explored that the use of theheat produced by solar thermal collectors is an interestingoption for thermal driven desiccant air-conditioning.Meckler [4] has also found that the use of desiccant pre-conditioning of supply air can improve the humiditycontrol capabilities of overall refrigeration and airconditioning systems and reduce energy costs. Further-more, incorporation of desiccant preconditioning into suchsystems allows a higher percentage of fresh air in thesupply stream. Collier et al. [5], Albers et al. [6], Griffiths[7], and Peng and Howell [8] have found that with recentadvances in desiccant dehumidification, liquid sorbentequipment are becoming more attractive applications forair-conditioning. Jurinak et al. [9] have compared theperformance between desiccants and vapour compressionsystems for residential applications and the cost andenvironmental impact of using solar energy for regenera-tion. Kinsara et al. [10] have proposed and evaluated adesiccant-based air-conditioning system. Ani et al. [11]have tested various packing heights of the absorbercomponent to determine the optimal performance of thecombined unit consisting of vapour compression unit, aliquid desiccant system, and a flat solar hot water collector.Stevens et al. [12] have developed a computationallyefficient model for packed-bed, liquid-desiccant heat andmass exchangers. Factor and Grossman [13] have studied apacked column air-liquid contactor in application to airdehumidification and regeneration in solar air conditioningwith liquid desiccants. Hirunlabh et al. [14] haveconducted an experimental analysis to investigate the

Received March 5, 2009; accepted May 22, 2009

Vijay MITTAL (✉), B. Kant KHANThe Mechanical Engineering Department, BRCM College of Engineer-ing and Technology, Bahal-127028, Bhiwani, IndiaE-mail: vmittal1231@rediffmail.com

Front. Energy Power Eng. China 2010, 4(2): 161–165DOI 10.1007/s11708-009-0070-5

performance and energy saving of the well-knowndesiccant air-conditioning system in Thailand. Kabeel[15] has proposed and studied the solar powered airconditioning system using liquid desiccants in a honey-comb desiccant rotary wheel that contain porous materialin a solar air heater used for regeneration purpose. Smith etal. [16] have developed a mathematical model of a solar-assisted desiccant air conditioner and simulated itsperformance in a residential building. Worek and Moon[17] have studied the performance of a desiccant,integrated, hybrid, vapor-compression cooling systemnumerically. Areemit and Sakamoto [18] have reportedresults of a passive room-dehumidifying system using anexisting attic space as a chamber and wood as a desiccantmaterial. Nia et al. [19] have presented a model of adesiccant wheel for dehumidifying the ventilation air of anair-conditioning system.In this work the experiments are conducted in order to

investigate the assessment of the performance of desiccantair-conditioning under Indian conditions. A simpleeconomical analysis is also conducted to predict thepayback and internal rate of return of the system.

2 System description and experimentalprocedure

Experiments were conducted using a split type AC (1.0 tonrefrigeration) to cool a room of 86.4 m3. Commercial silicagel (4–7 mm diameter) was used in a desiccant bed. Thedesiccant bed was made of stainless steel of 4 mm thickwith an area of 0.36 m2 (0.6 m�0.6 m). The bed thick-nesses of 5 and 7 cm were considered, which can contain 7and 13 kg of silica gel. A 1 hp blower was used to blow airthrough the desiccant bed.The return air (RA) and fresh air (FA) were mixed before

entering the desiccant bed. This mixture of return air andfresh air (RA and FA) leaving the desiccant bed was calleddry air (DA). DA and indoor air (IA) were mixed beforeentering the evaporator of the air-conditioner. Initially,indoor air and return air (IA and RA) were at the sameconditions. Different ratios of air composition consideredare given in Table 1. The airflow rate through theevaporator was maintained at 1500 m3/h. The temperatureof the air at the inlet and outlet of the bed and the desiccantbed were measured at six points in order to calculate an

average bed temperature. The RH and temperature at theinlet and outlet of the desiccant bed were also measuredwith the help of a sling psycrometer.An electrical meter was used to record the electrical

consumption of the AC in kilowatt-hour. The experimentswere performed from 9∶00 AM to 4∶00 PM and data wererecorded at 15 min intervals. In order to ensure that thecooling load remains constant, results of only those dayswere taken which have relatively similar ambient condi-tions. Moreover, the room location was chosen in such away that it minimized the effect of ambient conditions.The research was conducted at the Department of

Mechanical Engineering, BRCM College of Engineering& Technology, Bahal. Bahal is a small town in the districtof Bhiwani on the western fringe of Haryana borderingRajasthan. However, in the study of solar energy, the nameof place does not convey anything, so it is mandatory tospecify its location with the help of latitude and longitude.The latitude and longitude of Bahal are 29°N & 76°28′ E,respectively. The room is located at the first floor of thecollege building.

3 Results and discussion

3.1 Effect of silica gel bed thickness on bed temperature

The effect of thickness of silica gel bed was studied inorder to find out the optimized value of silica gel bedthickness. Initially, only ambient air was circulated throughthe bed and experiments were conducted. Figures 1 and 2depict the variation of temperatures of air and silica gel bedwith a bed thickness of 5 and 7 cm, respectively, on anhourly basis. It can be seen that the temperature of the bedand the temperature of the air leaving the bed were high atthe beginning. This is due to the fact that during theadsorption process of moisture by silica gel, a certainamount of heat is released which depends on the amount ofadsorbed moisture. The more the moisture adsorbed, thehigher the heat released. This phenomenon takes place atthe beginning of the adsorption process as silica gel is dryand, therefore, possesses higher adsorption ability. There-fore, it can be concluded that the air which is available atthe beginning of adsorption should not be used in airconditioning. After that the silica gel moisture contentincreases, which decreases the rate of moisture removaland exit air temperature as well.

Table 1 Different ratios of air composition entering desiccant bed (DB) and entering evaporator

case entering DB entering evaporator

percentage of FA/(m3$h–1)

percentage of RA/(m3$h–1)

percentage of DA/(m3$h–1)

percentage of IA/(m3$h–1)

A 50 50 100 900

B 150 150 300 700

C 250 250 500 500

D 100 — 500 500

162 Front. Energy Power Eng. China 2010, 4(2): 161–165

3.2 Effect of silica gel bed thickness on adsorption rate

Figure 3 depicts the effect of silica gel thickness onadsorption rate.It can be seen from Fig. 3 that by increasing the bed

thickness, the average adsorption rate increases from 330to 573 kg/h. Figure 4 depicts the variation of the pressuredrop between the inlet and outlet with mass the flow rate ofthe bed. It is clear that the increase of pressure drop is verysmall compared with the increase of the adsorption rate.The discussion in this section can be concluded with therecommendation of 7 cm bed, which will now be used inthe subsequent section.

3.3 Performance analysis of desiccant system

The performance analysis of the system is limited to themost significant parameters, viz., temperature, moisture

ratio and enthalpy of air entering the evaporator. Experi-ments were conducted for various combinations of FA &RA entering the desiccant bed and DA & IA entering theevaporator. For simplicity of presentation, only the resultsof the most significant cases as given in Table 1, viz., A, B,C, D, are presented and discussed. Figures 5 and 6 depictthe hourly variation of mixed air temperature and moistureratio before entering the evaporator respectively, whileFig. 7 depicts the hourly variation of enthalpy beforeentering the evaporator. Low temperature and highmoisture ratio are observed with case A, which is due tothe high amount of indoor air recirculated (900 m3/h). Theopposite was observed with case B, which is obvious dueto the increase in the amount of fresh air admitted in thecycle. For case C, air temperature and moisture ratio arehigh, as great amount of ambient air is introduced to thesystem and less indoor air is recirculated to the mixing boxbefore the evaporator. With case D, the moisture ratio andtemperature vary between case A and case C in a differentway than the other cases.In fact, as no return air was used, only ambient air is

circulated through the silica gel bed. Consequently, the

Fig. 1 Hourly variation of bed temperature and air temperaturefor the bed thickness of 5 cm, air flow rate – 750 m3/h

Fig. 2 Hourly variation of bed temperature and air temperaturefor the bed thickness of 7 cm, air flow rate – 750 m3/h

Fig. 3 Hourly variation of absorption rate for the bed thickness of5 and 7 cm

Fig. 4 Variation of pressure drop between inlet and outlet ofdesiccant bed for the bed thickness of 5 and 7 cm

Vijay MITTAL et al. Desiccant air-conditioning system in India 163

adsorption process of air moisture is affected by thevariation of ambient air conditions, which explains the

irregularity of the moisture profile. The correspondingaverage air enthalpy is quite high (60 kJ/kg) as shown inFig. 7, whereas those of cases A and B are significantlysmaller.

3.4 Economic analysis

Figure 8 depicts daily electrical consumption of air-conditioning system for different test conditions. Themaximum energy savings, obtained with case B, is about19%. Therefore, the savings on the electrical consumptionof the air-conditioning system using silica gel desiccant iswell recognized under Bahal conditions.

After finding out the savings in terms of electricity aneconomic analysis was also done to determine the meritsand shortcomings of the proposed system. The mainobjectives are to check the payback period and internal rateof return (IRR) of the system. Based on cost estimates ofthe desiccant system and using the electricity savings of19%, calculations were executed and the results ofeconomic analysis proved that the desiccant air systemwas not a viable option for the small air-conditioningsystem.

4 Conclusion

An experimental investigation using a split type air-conditioning of 1.0 ton refrigeration capacity confirms thatdesiccant air-conditioning following the concept proposedin this paper is a feasible option to be used in India. Variousoperating conditions are considered by varying the ratiosof indoor air, return air and fresh air. The return air andfresh air are mixed before entering the silica gel bed. TheDA leaving the bed is mixed with the recirculated indoorair at the evaporator inlet. Compared with common AC usewhere indoor air is recycled totally, the electricity savings

Fig. 6 Hourly variation of mixed air moisture ratio entering theevaporator

Fig. 7 Hourly variation of mixed air enthalpy entering theevaporator

Fig. 8 Daily electrical consumption of air-conditioning systemfor different test conditions

Fig. 5 Hourly variation of mixed air temperature entering theevaporator

164 Front. Energy Power Eng. China 2010, 4(2): 161–165

is about 19%. The optimum operating air ratios are 70% ofindoor air mixed to 30% DA of which half is outdoor air.A simple economic analysis shows that for a small

cooling capacity (split type unit), this system is not asuitable option as the extra cost is quite significantcompared with the savings.

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