A THERMO-ECONOMIC STUDY OF SOLAR COOLING/HOT WATER SYSTEMS

Abstract

Solar air-conditioning appears to be uneconomical as the collectors tend to remain "idle" for significant operating periods owing to high collector output temperature requirements for absorption cooling. The performance could be improved by incorporating a water heating loop in the basic cooling system so that the collectors could be utilised for hot water production during such "idle" periods. Commercial applications for such hybrid systems exist in installations such as hotels and hospitals where air-conditioning and hot water services art3 required simultaneously. This study considered the effects of design parameters such as collector type, storage tank size and control strategy on the performance of the hybrid systems. These effects are determined by transient system simulation. Although, there exist computer simulation programs such as TRNSYS, WATSUN and others, they do not allow direct simulation of the present hybrid systems. Consequently, a specific computer simulation code was developed to perform the task. The computer model, including the solar collectors, storage tanks and absorption chiller, is described in terms of design parameters. Ambient dry bulb temperatures, solar radiation, cooling and hot water loads are the required inputs. System temperatures, energy flows and coefficient of performance can then be predicted.

Besides the normal thermal performance in terms of solar fraction and first law efficiency of the systems, second law analysis was also carried out in this study. Being a cooling machine, the coefficient of performance (COP) may be compared with the ideal reversible machine to identify component irreversibilities. This would help in the improvement of system configuration, the control strategy and component selection.

Results obtained showed that the hybrid system is superior to the solar cooling only system in terms of thermal as well as economic performance. Effects of the various design parameters for all 3 systems considered (cooling only, hybrid with parallel heat exchanger and hybrid with series heat exchanger) showed that the collector flow rate has a great influence on the performance of the system and low flowrates of ·12 to 24 kg/ h- m2 of collector area yield the best results. Storage tank sizes of greater than 20 kg/m 2 ( kg per sq. m of collector area) do not increase the solar fractions considerably. However, for the case of service hot water storage volume, the solar fraction of the hybrid system with series heat exchanger peaks at 20 kg/m2 before decreasing in performance. On the other hand, the system with the parallel heat exchanger, solar fraction increases up to 40 kg/m 2, and thereafter, improve considerably less for further increases. Economic analysis clearly showed that the solar hybrid system with cooling and hot water improved in solar savings over the solar only cooling system. However, at present fuel prices, solar alternatives are still not competitive with the conventional (electrically-driven, vapor-compression) cooling systems. This will, however, improve if the price of electricity increases or higher tax incentives are given. However, one note of caution is that the use of present worth method of analysis dea1 with a certain amount of assumptions that can only make conclusions tentative with a lot of dependence of the 'quality' and 'reliability' of the assumptions used. Results obtained from the simulation studies clearly show that the hybrid cooling system is economically promising and should, therefore, be looked into as a practical alternative to pure absorption cooling for solar air-conditioning applications.