In this communication, performance solar air dehumidifier has been studied for Kota, Rajasthan. The experimental set up was built in laboratory at UCE RTU Kota. The parametric study has been carried out in the year of 2014-15, in which desiccant inlet temperature, air inlet temperature, air flow rate, desiccant flow rate are studied. The experimental results expressed in terms of graphical method and found that by increasing regeneration temperature of desiccant improves the moisture absorption capacity of air, which decreases the cooling load of air conditioner. This concept is working on solar energy and it is found a feasible solution to reduce the cooling load.
Refrigeration is a process of removing heat from a substance under controlled conditions. Air conditioning means maintaining the internal atmospheric conditions for Human Comfort. Mainly two cycles are generally used for Refrigeration and Air Conditioning processes, these are _ Vapour Compression Refrigeration (VCR) Cycle and Vapour Absorption Refrigeration (VAR) Cycle. In Vapour Compression Cycle, a working substance, refrigerant is used for heat transfer between Evaporator and Condenser. In evaporator, the refrigerant absorbs its latent heat from chamber which is to be cooled. It is required to change the condition of refrigerant for the refrigeration process _ and with the help of latent heat we change the condition of refrigerant. Condenser is used for taking latent heat of refrigerant that was gained from evaporator and reduce the temperature of the refrigerant.
In Vapour Absorption Cycle, we replace the compressor by a absorber, a pump, a generator and pressure reducing valves. The function of these components and compressor is same. Compressor is used as mechanical energy in order to change the condition of refrigerant in Vapour Compression Cycle. In Vapour Absorption Cycle, we use heat energy to change the condition of refrigerant. At small scale, we use evaporative coolers for the space cooling. These coolers use water as a refrigerant, which is used for cooling of the air, and cold air is transferred to the space which is to be conditioned. The droplets of water sometimes go with the air and increase the humidity of air and this is not desirable.
In Vapour Compression System, it is required that air is to be cooled below its dew point temperature for dehumidification. This is the main disadvantage of the Vapour Compression System because, the cooling of air below its dew point temperature for dehumidification results the temperature of air very low to required temperature. The combination of Conventional Air Conditioning System, which operates on closed cycle using electricity with a desiccant system can controls both the temperature and humidity within human comfort range.
In desiccant system, we use desiccant for the dehumidification of the air for human comfort. The dehumidification is achieved by passing the air over the solid desiccant or through the spray of liquid desiccant. The desiccant absorbs the moisture from the air and makes it dry. Thus, the desiccant quantity is increased and this excess quantity of air is removed by the regeneration process _ and is done by the help of Solar Energy. Solar collector is used for providing the heat energy for regeneration process. The pressure drop in Solid desiccant is high as compared to that of liquid desiccant. Silica Gel is a good example of solid desiccant. In solid desiccant system, the air is passed over the desiccant wheel where it is dried. The desiccant wheel absorbs moisture from the air and becomes wet. This absorbed moisture is removed by other fan. This system requires large wheel surface area, this results the desiccant wheel bulky. Thus, this system is expensive. Liquid desiccant have many advantages over the solid desiccant mainly the pressure drop is low as compared to that of solid desiccant. The regeneration temperature required for liquid desiccant is lower as compared to that of solid desiccant. We can do pre-cooling and pre-heating of desiccant in liquid desiccant system in the heat exchanger. Types of liquid desiccant are Calcium Chloride, Lithium Chloride, Lithium Bromide, etc.
Lithium Chloride works with low pressure and requires large surface area of heat exchanger. This results the system expensive. Calcium Chloride has many advantages over the Lithium Chloride Desiccant. It is inexpensive, non-toxic, non-flammable, etc. Mostly it doesn’t freeze easily and has strong affinity of water. The Solar Air Dehumidifier (Liquid Desiccant) Consists of a absorber and a regenerator. In absorber the outside air is passes through a spray of cold desiccant where it is dried and a weak solution is generated. This weak solution is pumped to the regenerator through the indirect-contact counter-flow heat exchanger with the help of pump. In regenerator, the weak solution of desiccant is changed into strong solution by removing excess amount of moisture by the Solar Energy and Air.
Experimental studies have been carried out on liquid-desiccant air conditioner by Lowenstein et al., in which Lithium Chloride and water solution was used as desiccant. It was found that, the minimum pressure drop was obtained for liquid-desiccant conditioner compared to other two conditioners and the temperature of air delivered was lower for the same conditioner. Madhukeswaran and Parkash experimentally investigated the effect of different coatings on performance of flat plate solar collector. It was found that, the maximum temperature was obtained for black chrome coating compared to other two coatings and the thermal efficiency of collector was highest for the same coating. They optimized the tilt angle of the flat plate collector. Anmi me tal. studied the Liquid Desiccant Dehumidifier with cooling capacity using compression heat pump system, in which Lithium Chloride was selected as refrigerant. It was found that, the water condensation rate increases with increasing desiccant flow rate, air inlet humidity ratio and desiccant inlet concentration. It changes very little with air inlet temperature and desiccant inlet temperature. Experimental studies have been carried out on Evaporative Air Coolers coupled with Solar Water Heater by Alosaimy, in which Calcium Chloride and water solution was used as desiccant. It was found that, desiccant minimum temperature was proportional to the humidity potential between the indoor and outdoor conditions (temperature and humidity). The experimental results show that, Calcium Chloride solution with 30% concentration can be regenerated up to 48% using solar energy. Bakhtiar et al. experimentally studied liquiddesiccant air dehumidifier, in which Lithium Chloride was used as refrigerant. It was found that, the higher air velocity obtained faster air dehumidification and the higher desiccant flow obtained larger effectiveness but effectiveness was slowly come down after some time of their experiment.
Kishore and Dilip experimentally analyzed the liquid desiccant dehumidifier, in which Calcium Chloride was used as refrigerant. It was found that, as the regeneration temperature was increased, moisture absorbing capacity of air was increased and dehumidification rate in the absorber was increased. It was found that moisture removal rate was increased with increasing in regeneration. The whole study is clarified that the relative humidity have not been controlled with solar regeneration system by any researcher and this factor is very important for comfort cooling of buildings.
In this communication we experimentally studied the effect of various parameters that affect the relative humidity of air for cooling applications. In this manner we analyzed the Solar Air Dehumidifier experimentally with the solar energy, used for regeneration process, and compare the outcomes of the results with the standard results.
Classification of Dehumidification Systems
Conventional Dehumidification System
In conventional dehumidification system, the air is cooled below its dew point temperature for removal the moisture from the air. The air is dehumidified by cooling and condensation. The moisture removal rate depends upon the temperature of cooling coil. Lower the temperature, the air will be drier. This system consists of evaporator, condenser, compressor and expansion valve.
Desiccant Dehumidification System
The working principle of desiccant dehumidification system is different from the Conventional Systems. In the desiccant dehumidification system, the moisture is absorbed with the help of desiccant. When the vapour pressure is low at the surface of the surface of the desiccant, they attract the moisture. At that time the vapor pressure exerted by molecules of water is higher, so the water molecules enter into the desiccant & thus air becomes dry (this is known as dehumidification process). In the regeneration process, the moisture is removed to the air when the vapour pressure of desiccant is higher than the air. This system also improves the quality of conditioned air because we can use fresh air instead of used air as in conventional air conditioning.
Solid Desiccant Dehumidifier
In this system, the air is passing over the desiccant wheel and the wheel adsorbs the moisture from the air. After adsorption the wheels becomes wet and the adsorbed moisture is removed by the help of other fan as shown in Fig. 1.
Liquid Desiccant Dehumidifier
Working of the liquid desiccant dehumidifier is almost same to the Solid Desiccant Dehumidifier which extracts moisture from the air and rejects to the atmosphere with the help of regenerator. Outside humid air or return air is supplied to the dehumidifier where it loses its moisture to the cool concentrated desiccant solution and it becomes hot & dry, which is supplied to the conditioned space. Then this diluted or weak desiccant is heated up with heater (low grade heat/waste heat can be used) & sprayed in the regenerator. Where its concentration increases (it becomes strong solution) after giving its water to the air. And this air is then exhausted to the atmosphere.
Advantage of Liquid Desiccants over Solid Desiccants
-Pressure drop is higher in Solid Desiccant System as compared to the Liquid Desiccant System.
-The desiccant wheel has large surface and is bulky too.
-Liquid desiccant can be heated up to dry off moisture; significant heat may remain in hot and dried solution. In the case of solid desiccant system, it is hard to recover this heat.
Material And Methods
Problem Description
For good air conditioning the air should not be more humid. Dry air is required for corrosion protection at military storages, electronic protection, condensation prevention, ice rinks, injection mouldings, surface preparations & coatings, comfort cooling in buildings. Our objective is to minimize the power consumption and make the air dehumidified for human comfort. We can use solar energy instead of electrical energy for regeneration of desiccant. The heat energy for regeneration is given by the Flat Plate Solar Collector. Calcium Chloride is used as a desiccant. It is the cheapest and best choice for desiccant system. Other advantages are that it has strong affinity for water and it doesn’t freeze easily unlike the Magnesium Chloride brine.
Experimental Setup
An experimental setup is developed in department of mechanical engineering, Rajasthan Technical University Kota under project work. A schematic diagram of experimental setup is shown in Figure 1. Where, two identical towers are used as direct contact type heat exchanger for regeneration and absorber. The material of tower is fibre reinforced plastic and height of tower is 120cm. Packing material is used as polypropylene intalox saddle for a height of 30cm. At the bottom collection tank is provided for each heat exchanger (tower) which is used as storage of desiccant. Two centrifugal pumps are used for pumping the desiccant, having a maximum discharge of 1000litre/hour. Desiccant is sprayed in the towers with the help of nozzle with very fine droplets. For circulation of desiccant, PVC pipes are used. Demister pads are used for elimination of desiccant which is carry over through air stream. Glass wool is used as insulation material. The specifications of measuring devices are given in the above Table 1.
Procedure: In the absorber cooled desiccant is sprayed on the packing and the moisture is absorbed from the incoming air stream. Form the absorber the desiccant is transfer to the collection tank through the counter flow indirect contact type heat exchanger after that it is collected in storage tank and pumped to the regenerator with the help of pump. In the regenerator, the excess moisture is removed by the air and heat energy. For the heat energy we use solar energy instead of mechanical energy. The flat plate solar collector is used for converting solar energy into thermal energy. It consists of six copper tubes of 1inch diameter for flow of fluid and copper plate as absorbing plate. The dimension of Solar Collector is 3x1x1 feet. At steady state conditions we measured different measurements from measuring devices. Thermometer is used for measuring temperature; humidity is measured using the hygrometer.
For measurement of concentration of desiccant, we take samples of desiccant of 100ml. At that temperature its density is calculated with the help of weighting machine and the concentration is calculated using the correlation developed by R Manuel. After taking the readings for absorber, we take the readings for regenerator. On the basis of these readings, moisture removal rate and moisture absorption rate for absorber and regenerator is calculated respectively. The effect of different parameters namely air inlet temperature, desiccant inlet temperature are studied.
Results and Discussion
The objective of the study is to experimentally analyze the effect of various performance parameters of Solar Desiccant System. In this experiment, Calcium Chloride is used as desiccant which is non toxic, non flammable and easily available in market. The parameters are studied as follows: Desiccant inlet temperature, Air inlet temperature, Air and desiccant flow rate.
Effect of regeneration temperature on humidity reduction
The figure shows the effect of the air regenerator temperature on the humidification on air in the regenerator. The moisture absorbing capacity of the air increases as the air regeneration temperature increases. As we decrease the air mass flow rate decreases, humidification rate increases by keeping desiccant flow constant.
Effect of Regeneration Temperature on Temperature Increase
The influence of regeneration temperature on the air exit temperature is given in the Figures 5 shown below. As we increase the regeneration temperature, more heat is utilized to regenerate the solution and resulting, the exit temperature also increases. The moisture absorbing capacity increases as the air regenerator temperature increases. By decreasing air flow rate keeping desiccant flow constant, the moisture absorbing capacity of air decreases.
Effect of Desiccant Inlet Temperature
Figure 6 shows the influence of desiccant inlet temperature on the moisture removal capacity in absorber with different air mass flow rates. It is also shows the relationship between the air flow rate and the moisture removal rate. For analyze the effect of desiccant inlet temperature on dehumidification process, we kept the desiccant flow rate constant. As the desiccant inlet temperature increase in absorber, the moisture removal capacity decreases. As shown in Fig 6 the moisture removal increases as the air flow rate increases.
Effect of Humidity Ratio with Time with Desiccant Flow Variations
The experimental results show that higher air velocity will obtain the faster air dehumidification, but it has low effectiveness. Refer Figure 7.
Conclusion
It is seen from the above discussions, the moisture removal rate increases with the regeneration of desiccant. The moisture removal decreases when the inlet temperature of desiccant in absorber increases. The maximum temperature for the regeneration is obtained for black chrome coating in solar plate. These results are compared with the existing standard results and found that they are approximately same. The water condensation rate doesn’t change much with the air inlet temperature and desiccant temperature. It almost remains constant. The water condensation rate increases with increasing desiccant inlet concentration.
The use of solar energy reduces the cost of operation of the system. The demand of the energy is increasing day by day and it is more sense to use solar driven systems which are very economical as compared to the conventionally electrically driven systems. In solar driven dehumidifier we use solar energy for the regeneration of the desiccant instead of electrical energy and Calcium Chloride is used as desiccant. Resulting, this system can be used as Air Conditioning systems and it is not expensive.
