Development of a Novel Hybrid Storage System for Aqua-Ammonia Solar Absorption Refrigeration Cycle

Abstract

This research work proposes the development of a novel hybrid storage absorption refrigeration (HSAR) cycle using aqua-ammonia mixture, capable of providing continuous refrigeration and is powered by solar energy. The cycle integrates a cold storage tank with two aqua ammonia storage tanks and one ammonia storage tank to accommodate a 24-hour uninterrupted daily cooling load. During the daytime, solar energy is used to provide the refrigeration that suffices both the daily cooling load and the production of both ammonia and ice for the night cooling load. The cold storage tank together with the ammonia and aqua-ammonia tanks share the night cooling load. The proposed absorption cycle reduces the size of the storage tanks by 50 % or even more for the same cooling capacity compared to the currently available storage designs. The proposed cycle continues the refrigeration even if one of the storage tanks ceases to operate or need maintenance. The study also explores the impact of unsteady solar intensity and ambient conditions on the operation and performance parameters of an aqua-ammonia absorption chiller of 5kW cooling power. It presents an unsteady analysis for five representative days of summer and winter in Dhahran, Saudi Arabia. The unsteady analysis gives the variation of the absorption-chiller parameters every one hour. Capacity of these tanks and the required solar collectors area are computed for 24-hour operation per day at 5kW cooling power. The results show that, for the given cooling load in Dhahran, the coefficient of performance is better in winter than in summer (0.68 as compared to 0.39) but the required solar collector field area is more in winter than in summer. This research work applies energy and exergy analysis for comparing a solar-powered absorption refrigeration system with different storages. These storages include refrigerant, cold/ice and a combination of refrigerant and cold/ice storage systems (hybrid storage) to ensure meeting the night time cooling demand. The results indicated a slight drop in coefficient of performance (COP) and exergetic coefficient of performance (ECOP) in HSAR system compared to that of the refrigerant storage system while the cold storage system achieves the least COP/ECOP. An exergo-economic analysis is implemented on HSAR cycle to compare the components based on the costs of initial capital investment and the costs of irreversibilities. The components of the refrigeration cycle are also evaluated and compared using exergo-economic variables such as the relative cost difference, exergy destruction cost rate and exergo-economic factor. The effect of generator temperature, condenser temperature and evaporator temperature on the exergetic efficiency of the system is also studied. Finally, this research work presents a quasi-steady exergy and exergo-economic analysis for a representative summer day of Dhahran region. This study can be further used in the optimization of design variables of the studied refrigeration cycle.