Solar Air Conditioning System

Mr. S. M. Kale (smkale@coe.sveri.ac.in), Assistant Professor, Department of Mechanical Engineering, SVERI’s College of Engineering, Pandharpur

1. Introduction

As presently, the energy shortage emerges as an threat to the vastly growing world, coupled with an issues to environment posed by the conventional refrigerants, the need  arises to come up with the new-fangled energy-saving as well as environmental-friendly air-conditioning systems has became more essence than ever before. The solid-desiccant dehumidification systems driven by solar energy sources can satisfactorily meet those needs; As desiccants are chemicals with great affinity towards moisture content in the air. Therefore, desiccants can be used as supplement to conventional vapor compression systems in order to remove the latent heat load. Desiccants are efficient in handling latent heat load (i.e., reducing the humidity), but the evaporator in the vapor compression system is efficient in handling the sensible cooling loads (i.e., lowering the air temperature). Desiccant air-conditioning system by using solar energy is a clean technology which can be used to condition the internal environment of buildings and operates without the use of harmful refrigerants as well as It consumes very little electrical energy, and for regeneration process it allows the use of solar energy and waste energy.

2. Desiccant Cooling

Desiccant cooling consists of dehumidifying the incoming air stream by forcing it through a desiccant material (solid or liquid) and then drying the air to the desired indoor temperature. To make the system working continually, water vapour adsorbed/absorbed must be driven out of the desiccant material (regeneration) so that it can be dried enough to adsorb water vapour in the next cycle. This is done by heating the material desiccant to its temperature of regeneration which is dependent upon the nature of the desiccant. A desiccant cooling system, therefore, comprises principally three component, namely the regeneration heat source, the dehumidifier (desiccant material), and the cooling unit. The efficiency of desiccant system depends strongly on the Sensible heat Ratio [5].

2.1 Principal of Desiccant Cooling:

The desiccants are a natural or synthetic substance which absorb or adsorbs water vapor present in air due the difference of water vapor pressure between the surrounding air and the desiccant surface. lithium chloride, triethylene glycol, silica gels, aluminum silicates (zeolites or molecular sieves), aluminium oxides, lithium bromide solution and lithium chloride solution with water, etc are the some of the commonly used desiccant materials. Liquid desiccant are having lower regeneration temperature and flexibility in utilization, lower pressure drop on air side while solid desiccant are compact, less subject to corrosion and carry over.

Figure 1. Principle of Desiccant Cooling System[6]

The desiccant materials are used in various arrangements. Solid desiccants are packed to form a sort of adsorbent beds exposed to the incoming air stream which takes up its moisture. As the beds are periodically moving in the direction of the regeneration air stream and then returning to the process air stream. In case of liquid desiccants, liquid desiccants are often sprayed into air streams or wetted on the contact surfaces to absorb water vapor from the incoming air which latterly like the solid desiccants, regenerated in a regenerator where water vapors previously absorbed is evaporated out from it by heating.

Figure 1 depicts the principle of desiccant cooling system in which process air is passed over a desiccant dehumidifier in which desiccants absorb the moisture from air. The same air is then cooled in a cooling unit up to desired temperature & then it is supplied to the room which is to be conditioned. The heat supplied from the regenerative heat source is utilized for flash out the moisture absorbed by the desiccant material.

3. Description of solid desiccant-based evaporative cooling Air Conditioning system

The schematic of the solid desiccant-based evaporative cooling Air Conditioning system is shown in Fig 2. It consist of two inlet fans, Desiccant wheel with silica gel as a desiccant material, motor for driving the desiccant wheel, Indirect heat exchanger & Direct heat exchanger.

Figure 2. solid desiccant-based evaporative cooling Air Conditioning system

Ambient outdoor air enters in to the supply (process) air duct through fan 1. This air passes through a desiccant wheel which is rotating continuously with a constant speed with the help of motor. As desiccant wheel contains silica gel, it absorbs moistures from the supply-air (process air) hence the air coming out of the desiccant wheel is hot & dry. This increase in temperature is due to the heat of absorption. As this process is continuous, hence for the purpose of regeneration of desiccant material we are supplying the ambient outdoor air through fan 2 in to another duct (regeneration air duct). This air passes over direct heat exchanger through which hot water heated through solar energy is passed as a another fluid. Hence ambient air takes heat from hot water in direct heat exchanger & same air is then passed through another half part of desiccant wheel. As desiccant wheel is continuously rotating hence the desiccant material present in the desiccant wheel regenerate when it comes in contact with hot air. Because of this process, desiccant material is continuously able to absorb the moisture particles available in the ambient supply air. The hot and dry supply-air coming out of desiccant wheel is then passed through indirect heat exchanger in which water is used for evaporation purpose. Hence hot & dry air transfers much of its heat to the evaporation of water sprayed in indirect heat exchanger & at the outlet of indirect heat exchanger we get cold air which can be supplied to the room which is to be cool through ducts.

The same system can be represented on psychrometric chart as

Figure 3. Thermodynamic processes of Air Conditioning system on psychrometric Chart

Various operating parameter are affecting on the performance of the system. Those are  i) Temperature of regenerative air, ii) volume flow rate of process air, iii)Speed of desiccant wheel, iv) Desiccant material present in to the desiccant wheel v) humidity ratio at the inlet of process air vi) Quantity of Desiccant material present in to the desiccant wheel etc. Ali Alahmer [8] did study on effect of various operating parameter on performance on desiccant cooling system & came to the conclusion that;

(i) higher regeneration air temperature leads to an improvement of dehumidification and reduction of the thermal coefficient of performance, (ii) an increase in the process inlet volume flow rate causes a reduction in the process removed moisture; (iii) the greater Regenerative air flow/process air flow ratio (R/P ratio) is required to remove more moisture, while a lower R/P ratio implies the lower power consumption for regeneration; (iv) silica gel desiccant is preferred when greater moisture removal is required at high inlet relative humidity, while molecular sieve desiccant is preferred when low dew point is required at low inlet relative humidity.

4. Advantages & Disadvantages of Solar Air Conditioning system over Conventional Air Conditioning System:

4.1 Advantages:

 (1) Consumption of very small electrical energy:  As the growing use of air conditioners in homes and offices around the world is demanding more & more electrical energy which is becoming the main of the concern hence by using desiccant based air conditioning system using solar energy require less electric supply as it uses solar energy, waste heat or natural gas

 (2) Elimination or reduction of the use of ozone depleting CFCs :As this system produces air conditioning without using any of the CFC refrigerant hence it helps for Elimination or reduction of cause for ozone depleting.

(3) Reduction of Green House Gas Emission: As over 50 % of green house gas emission is caused mainly due to Heating, Air Conditioning & Hot Water, hence by using desiccant based air condition system the percentage of green house gases can be reduced.  

(4) Improvement in indoor air quality: improvement in indoor air quality is likely to occur because of the normally high ventilation and fresh air flow rates employed. Also, desiccant systems have the capability of removing airborne pollutants.

4.2 Disadvantages:

            The only disadvantage of this system is, if the relative humidity of ambient air  is too high then desiccant cooling becomes impracticable simply because of the regeneration temperatures required are excessive.

5. Conclusions

Desiccant cooling is a simple technology which can reduce the use of electricity consumption, Ozone depletion, amount of green house effect & operating cost in comparison with the conventional system. silica gel desiccant material is preferred when greater moisture removal is required at high inlet relative humidity, while molecular sieve desiccant material is preferred when low dew point is required at low inlet relative humidity. Desiccant cooling system can be under humid climatic conditions in buildings to provide the cooling needs. It is being observed that various parameters affect on the performance of system.

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