Fig. 5. Variation of temperature & solar irradiation with time (1500 rpm).Figure optionsDownload full-size imageDownload as PowerPoint slide
The temperature of refrigerant at the compressor outlet for a fixed compressor speed depends on the inlet compressor refrigerant temperature, which comes from two evaporators: the solar collector and the cooling coil in the desalination chamber. When solar irradiation is low, the total distillate production is low, and hence there is less heat recycled at the cooling coil which condenses distillate vapor. As a result, the collector outlet temperature has a bigger influence on the compressor outlet temperature than the cooling coil. When solar irradiation is high and a there is a higher rate of distillate production, a relatively larger amount of heat is recycled and this QVDOPh process is repeated, making the compressor outlet temperature less prone to collector outlet temperature fluctuations.
The collector efficiency is a function of fluid temperature in the collector, the ambient temperature and solar irradiation. Fig. 6 shows the fluctuations in solar irradiation causes similar variations in the solar collector efficiency, which is typically high with values over 0.9 and sometimes even crosses 1. The reason is the useful solar energy output, Qu, as by equation (1) by Hottel–Whillier Qu=AcFR[S−UL(Ti−Ta)]Qu=AcFR[S−UL(Ti−Ta)]where, Ti is the inlet fluid temperature to the collector. Evaporator collector has an inlet temperature which is less than ambient temperature, the loss term UL(Ti − Ta) becomes a gain term in this case, as observed in Fig. 6. In other words, ambient air becomes a heat source for evaporator collector. Therefore, it is capable of attaining a higher efficiency.