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solar-powered liquid desiccant

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SOLAR Powered Liquid DesiccantCooling SystemTARUN B PATELME ENERGY ENGINEERING140190739009GUIDED BY Dr K V ModiGEC VALSAD1

Conventional cooling technologies employing harmful refriger-ants usually need more energy and lead peak loads which result in negative effect on the environment . As the energy and environmen-tal issues have to be dealt with globally, developing and promoting environmentally benign sustainable systems emerge as an urgent need. Solar assisted desiccant cooling is one such encouraging sys-tem, given the fact that solar energy is abundant renewable source to meet the cooling load requirements . The idea of a liquid desiccant evaporative cooling system is to combine liquid desiccant dehumidification with an evaporative cooling system in order to advance the overall system performance and utilize solar energy as a clean, renewable energy resource. Invsuch a hybrid system, the desiccant dehumidification system is motivated to eliminate the latent load, while IEC system to provide the sensible heat load [1]. Further, as only low-grade energy is required for the regeneration process of the desiccant dehumid-ification system, solar energy as a renewable energy resource can handle running liquid desiccant air conditioning system .

A promising solar cooling method is through the use of a liquid desiccant system, where humidity is absorbed directly from the process air by direct contact with the desiccant. The desiccant is then regenerated, again in direct contact with an external air stream, by solar heat at relatively low temperatures.The liquid desiccant system has many potential advantages over other solar air conditioning systems and can provide a promising alternative to absorption or to solid desiccant systems.

The number of main components is reduced by one by transferring condensation of the refrigerant from a condenser to the environment. (2)Capital-intensive pressure-sealed units are avoided as the whole system operates at atmospheric pressure. (3) The amount of refrigerant (water) evaporated in the regenerator does not require an evaporator elsewhere in the system,thus providing greater flexibility. (4) Efficient utilization of very low heat source temperatures is possible.

ContentPaper reviewedWork/System DescriptionPerformance/ResultConclusion


Paper ReviewedK. Gommed et al, Experimental investigation of a liquid desiccant system for solar cooling and dehumidification, Solar Energy 81 (2007) 131138.

Rajat Subhra Das et al, Investigations on solar energy driven liquid desiccant cooling systems for tropical climates, Australian Solar Energy Council (2012) 978-0-646-90071-13

Review 1 K. Gommed et al, constructed a solar-driven liquid desiccant system for cooling, dehumidification and air conditioning and tested the concept, identify problems, carried out preliminary design optimization and measure performance. The system is capable of using as its source of power low-grade solar heat from low-cost flat plate collectors and has the potential to provide both cooling and dehumidification in variable ratios, as required by the load.


The system is capable of using as its source of power low-grade solarheat from low-cost flat plate collectors and has the potential to provide both cooling and dehumidification in variable ratios, as required by the load.While many of its characteristics are similar to those in earlier liquid desiccantsystems, it possesses several advanced features such as automatic controls and the ability to store coolingcapacity in regenerated desiccant.The significance of this work lies in the potential to provide solar-powered cooling,dehumidification and air conditioning for residential and commercial applications.

Work description 1Air-condition a group of offices in Haifa, Israel.Outside summer conditions (typical for design) are 300 C and 70% relative humidity. Room design conditions have been selected at 240 C and 50% relative humidity.The total cooling capacity required is 7.2 kW, with a grand sensible heat factor (GSHF) of 0.62.The total supply air circulation needed is 0.4 kg/s (720 cfm).The desired conditions of the supply air are 14.70 C and 86% relative humidity.The total latent heat load of 2.75 kW, the solar energy demand was estimated to be 4.77 kW. the solar collector area was selected at 20 m2.Solution storage in the amount of 120 l of LiCl solution at 43% concentration and a 1000 l hot water tank added to the system.


The system under consideration is designed to air-condition a group of offices on the top floor of the Energy Research Center building at the Technion Israel Institute of Technology (Haifa, Israel).The total floor area of the conditioned space is 35 m2. The walls and roof are made of 8 in. (20 cm) prefabricated low-weight concrete, not insulated, with cement plaster. Each office serves two people and their computers; hence total occupancy is six persons.33 north latitude, it has the typical climate of Mediterranean cities. Outsidesummer conditions (typical for design) are 30 C and 70% relative humidity. Room design conditions have beenselected at 24 C and 50% relative humidity.Solution storage in the amount of 120 l of LiCl (lithium chloride-salt)solution at 43% concentration anda 1000 l hot water tank added to the system make it possible to operate for a total of 4 h with no insolation a typicalsituation in the summer during the morning hours.

Diagram of liquid desiccant

work in three different modesmanual mode used for testing individual components of the systemtwo modes are automatic, selected by the userOne automatic mode is for full operation of the system (FOP) and the second is for regeneration only (REG). 6

The liquid desiccant system is designed to serve as an open-cycle absorption system that can operate with low gradesolar heat.The system consists of six major components -an air dehumidifier or absorber, a solution regenerator or desorber, two water-to-solution heat exchangers, a solution-to-solution heat exchanger, and an air-to-air heat exchanger.Air flow is represented by thick solid lines, solution flow by thin solid lines and water flow by thin dashed lines.

The dehumidifier (absorber) consists of a packed tower and operates in an adiabatic mode. Solution is pumped from the absorber pool at the bottom of the tower into the plate heat exchanger (state 7), where it is cooled by water from a cooling tower. The solution leaving the heatexchanger (state 8) then proceeds to the distributor at the top of the packing, from where it trickles down in counter- flow to the air stream and collects in the pool. Ambient air at state 13 entering the bottom of the absorber packedsection is brought into contact with a concentrated absorbent solution entering the unit at state 8. Water vapor isremoved from the air stream by being absorbed into the solution stream. The dehumidified warm air leaving the absorber passes through the blower and leaves the system toward the air-conditioned space at state 14. The blower controls the flow of air, while raising its temperature slightly. A controlled solution stream is transferred from the absorber pool to the regenerator, as shown (state 11). The return (pumped) stream from the regenerator (10) goes directly into the absorber pool.

As evident, the regenerator (desorber) device is very similar to the dehumidifier, and so are the flow system andassociated components. The solution is heated in the liquid-to-liquid heat exchanger by solar-heated water(states 12). Ambient air is pre-heated in the air-to-air rotary heat exchanger (H.X.) by recovering heat from theexhaust air leaving the desorber. After pre-heating, the air stream (state 15) enters the desorber where it serves tore-concentrate the solution. The exhaust air leaves the desorber, passing through the blower, then pre-heats the entering air stream and is rejected to the environment. The solution-to-solution heat exchanger facilitates pre-heatingof the weak solution leaving the dehumidifier (states 11 12) and recovers heat from the hot strong solution leaving the regenerator (states 910).

In the overall setup, the liquid desiccant system is connected in a flow arrangement allowing storage of concentratedsolution and a capability to work in three different modes. The first is a manual mode used for testing individualcomponents of the system. The other two modes are automatic, as may be selected by the user. One automaticmode is for full operation of the system (FOP) and the second is for regeneration only (REG). In the automaticFOP mode, all system components operate, including the solution storage circuit. In this case, the absorbersolution pump may supply the dehumidifier with solution from both the absorber pool and from the solution storagetank, in parallel. Thus, dehumidification can continue independent of regeneration. If the solar collectors cannotsupply water at sufficiently high temperature, or if the concentration of the solution in the storage tank and/orthe regenerator pool rises above a set limit, the regeneration side of the system will shut down for a certain time.In the REG mode, only the regenerator (desorber) side of the system operates. The system shuts down automatically when the concentration of the solution in the storage tankreaches a certain high value or when the temperature of the hot water drops below a certain limit. At the end ofdays of high insolation, when a large amount of solar heat has been collected and stored in hot water, the usercan set the system to operate in the automatic REG mode before leaving the site.


7COP1 is a strictly thermal COP, not including parasitic losses; the denominator contains the thermal energy supplied, in this case from the solar collectors.COP2 includes in the denominator the sum of (solar)heat input and parasitic losses.COP3 is the same as COP2, but the parasitic losses in the denominator are converted to their equivalent heat value, assuming power plant electricity generation at 40% efficiency.COP4 is the ratio of latent heat removed from the process air to the electric power equivalent of the input energy, consisting of (solar) heat and parasitic power.

Fig. 4 illustrates the variation of the absolute air humidity as a function of time during a typical day (21 August2003): ambient air (Win), desorber outlet air (Wout1) and absorber outlet air (Wout2). Note that the absorber outletair humidity is also that of the supply air to the conditioned space. As evident, the outside air humidity has remainedapproximately constant during the whole day, at about16 g/kg, with a slight increase toward the evening. Theabsorber outlet humidity was equal to that of the outside air when operation began at about 10:00, and was reduced to about 8 g/kg within 20 min. The machine was able to keep this humidity steady throughout the day. The desorber outlet air humidity, which is the control parameter,

Conclusion 1The system functioned well, with 16 kW (average) dehumidification capacity.The data analysis indicates a thermal COP of about 0.8, with parasitic losses on the order of 10%.The COP calculations performed on the monitoring data have yielded satisfactory results, particularly with regard to the thermal COP.8

Review 2Das et al presented a study about solar thermal driven liquid desiccant air conditioning system for tropical climates like Delhi. The system consisted of a dehumidifier, regenerator, indirect evaporative cooler, several heat exchanger (solution-solution, air-water, and solution-water) and solar collectors. Lithium Chloride (LiCl) solution as desiccant.The performance of the overall system is presented in terms of its cooling capacity, moisture removal rate and COP.


Liquid desiccants are preferred over the solid ones due to the possibilities of simultaneous cooling during dehumidification, integration of solar collector with regenerator, easy storage of regenerated desiccant during non-sunshine hours.

Work description 2


A small capacity solar liquid desiccant cooling system as shown in Fig. 1 has been developed in the laboratory. The system is 100% fresh air unit consisting of a dehumidifier or absorber (b), a regenerator or desorber (i), an indirect evaporative cooler (d), a c ooling tower (f), two water-solution heat exchangers (g, j), and one solution to solution heat exchanger (h).Hot and humid ambient air (a) enters the unit and passes through a dehumidifier (b).In the dehumidifier (also called an absorber), concentrated liquid desiccant absorbs water vapor from the air.The air and the liquid channels are separated by semipermeable micro porous membranes.Only the water vapour passes through the membranes into the liquid desiccant when the vapour pressure of the air exceeds that of the liquid desiccant. Indirect contact between desiccant and air stream in the dehumidifier eliminates carryover of theliquid desiccant into the supply air.The dehumidified air then flows through a fin-tube air-water heat exchanger (c),where it is sensibly cooled. The air is further cooled in an indirect evaporative cooler (d) and then supplied to the room via a fan (e).In the indirect evaporative cooler, part of the dehumidified process air is recirculated and comes in direct contact with water to produce cooling. The process air does not come in direct contact with water and is thus, sensiblycooled, while the recirculated air gets humidified and is exhausted to the atmosphere. Onlythe sensibly cooled dehumidified air from the process channels is delivered to the room.

Moisture-laden dilute desiccant from the dehumidifier is preheated in the solution solution heat exchanger (h) by recovering heat from the hot concentrated desiccant leaving the regenerator and stored in a tank (o). The preheated dilute desiccant is then pumped to the plate heat exchanger (j) where it is further heated by hot water. Water is heated in solar collectors (k) (Evacuated tube Collectors, Heat Pipe Collectors) and stored in a hot water storage tank (l).

The hot dilute desiccant flows through the regenerator (i), where it releases water vapor to the air stream owing to the vapor pressure difference. The concentrated desiccant from the regenerator is stored in a tank (n) and then precooled in the solution-solution heat exchanger (h). The solution is then pumped to the plate heat exchanger (g) where it is cooled by water from the cooling tower. Cooling increases the vapor pressure difference between air and concentrated desiccant,resulting in higher dehumidification.

Results 2


1-Air having higher specific humidity exerts greater partial water vapor pressure. Thus, with increasing specific humidity, the partial vapor pressure difference between air and desiccant increases, triggering higher absorption of water vapor by liquid desiccant. Therefore, the change in specific humidity and moisture removal rate increase with the increase in inlet air specific humidity (fig. 2). Specific humidity change of around 2 to 8 g/kg and moisture removal rate of 0.2 to 1.6 g/sec is attained.

2 and 3 -The effect of ambient conditions on the dehumidification and enthalpy effectiveness of the dehumidifier is presented in figs. 3 and 4.The DBT of fresh air does not have significant effect on the dehumidification and the enthalpy effectiveness. With the increase in inlet specific humidity, both actual and maximum possible change in humidity increases. However, former increases at a higher rate than the latter. Thus the dehumidification effectiveness rises with higher inlet specific humidity. Similar is the case for enthalpy effectiveness. It is seen that the effectiveness values mostly lie between 30 to 60%. The dehumidification and enthalpy effectiveness seems to be limited by the additional mass transfer resistance due to the indirect contact between air and desiccant in the dehumidifier.

4-An increase in L/G ratio facilitates proper interaction between fresh air and liquiddesiccant in the dehumidifier, which improves the absorption of water. As a result there is higher specific humidity change and hence better dehumidification effectiveness is achieved (fig. 5). The scatter in fig. 5 is due to differences in the outdoor conditions and uncertainty inmeasurements.

5 and 6 -The variations in the capacity and the thermal COP of the system with ambient DBT and w of the air are shown in figs. 6 and 7. There is not much change with the change in DBT. However, with the higher specific humidity the potential for moisture transfer from air into the desiccant increases, which further augments the enthalpy difference between inlet and supply points. Thus, there is a gradual rise in capacity and COP of the system with increasing ambient specific humidity. The capacity of the system lies between 2.5 to 5.5 kW, while COP varies between 0.4 to 0.8.

Conclusion 2Maximum specific humidity change and moisture removal rate of 8 g/kg and 1.6 g/s is achieved.The capacity of the system is found to vary between 2.5 to 5.5 kW and COP lies between 0.4 to 0.8, at different ambient conditions.Both the capacity and COP of the system increase with rising specific humidity of ambient air. There is also a slight increase in the dehumidification and enthalpy effectiveness of the dehumidifier with increasing inlet air specific humidity.