Solar energy is a renewable and ozone-friendly energy source. Solar cooling is the most attractive subject for many engineers and scientists who work on solar energy applications. Most of the research and development efforts have been carried out using an absorption cooling system. This system is usually a preferable alternative, since it uses thermal energy collected from the sun without the need to convert this energy into mechanical energy as required by the vapor-compression system. Besides, the absorption cooling system utilizes thermal energy at a lower temperature (i.e. in the range 80-110°C) than that used by the vapor-compression system.
Research and development studies on solar ARSs using different combinations of refrigerants and absorbents as working fluids have been done. These ARSs have good potential where solar energy is available as low-grade thermal energy at a temperature level of 100°C and above.
The principle of operation of a solar-powered absorption cooling system is the same as that of the absorption cooling system shown in Figure 3.57, except for the heat source to the generator. In Figure 3.57, we presented a solar absorption cooling system using an R-22 (refrigerant)-DMETEG (absorbent) combination as a working fluid (Dincer et al., 1996). Its operation can be briefly explained as follows. In the absorber, the DMETEG absorbs the R22 at the low pressure and absorber temperature supplied by circulating water, and hence a strong solution occurs (2). This strong solution from the absorber enters a solution pump, which raises its pressure and delivers the solution into the generator through the heat exchanger (3-6). The generator, which is heated by a solar hot water system, raises the temperature of the strong solution, causing the R-22 to separate from it. The remaining weak solution flows down to the expansion valve through the heat exchanger and is throttled into the absorber for further cooling as it picks up a new charge of the R22 vapor, becoming a strong solution (6-2) again. The hot R-22 vapor from the generator passes to the condenser and is released to the liquid phase (8-9). The liquid R-22 enters the second heat exchanger and loses some heat to the cool R-22 vapor. The pressure of the liquid R-22 drops significantly in the throttling valve before it enters the evaporator. The cycle is completed when the desired cooling load is achieved in the evaporator (10–12). Cool R-22 vapor obtained from the evaporator enters the absorber while the weak solution comes to the absorber continuously. The R-22 vapor is absorbed here (12–1). This absorption activity lowers the pressure in the absorber, causing the vapor to be taken off from the evaporator. When the vapor goes into liquid solution, it releases both its latent heat and a heat of dilution. This energy release has to be continuously dissipated by the cooling water.
Solar-operated ARSs have so far achieved limited commercial viability because of their high cost/benefit ratios. The main factor which is responsible for this drawback is the low COP associated with these systems, which generally operate on conventional thermodynamic cycles with common working fluids. It is essential to investigate the possibility of using alternative working fluids operating in new thermodynamic cycles. Also, development of more efficient, less expensive solar collectors will be a continuing need for solar energy to reach its full potential.
