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SUMMARYThe design of the energy storage serving solar-powered LiBr-water absorption air-conditioning systems to meet 24-h a day operation is an important parameter that determines the performance of the system and optimizes its components. Introducing hybrid storage designs can improve system performance, reduce collector size, lower storage tank capacity, and hence make 24-h a day operating solar-powered LiBr-water absorption air-conditioning systems more feasible. Moreover, hybrid storage designs will ensure uninterrupted operation in case of maintenance for one of the storage tanks. This paper investigates four hybrid storage designs (heat and refrigerant storage, cold and refrigerant stfsorage, heat and cold storage, and heat, cold, and refrigerant storage) for a continuous operation (24 h a day) of solar-powered LiBr-water absorption air-conditioning. The study includes system description, features, and selection analysis for a constant cooling load of 5 kW in a typical family house in Dhahran City, Saudi Arabia. The results indicate that the hybrid cold and refrigerant storage system is the most suitable design for 24-h a day supply of the cooling effect.

Purpose -The paper seeks to focus on obtaining the transient torque required to rotate the inner cylinder in open ended vertical concentric annuli for a fluid of Pr ¼ 0.7 in the laminar natural convection flow regime over a wide range of the controlling parameter Gr 2 /Ta. The inner wall is heated and subjected to an impulsive rotation while the outer one is stationary and maintained adiabatic. Design/methodology/approach -The governing transient boundary-layer equations are numerically solved using an iterative linearized finite-difference scheme. Findings -The transient induced flow rate and absorbed heat for different annulus heights are presented. High rotational speed (i.e. low values of Gr 2 /Ta) increases the flow rate and heat absorbed in short annuli. However, for considerably tall annuli, Gr 2 /Ta has slight effect on the flow and heat absorbed. The steady-state time is tangibly influenced by Gr 2 /Ta in considerably short annuli and very slightly affected for considerably tall annuli. Practical implications -The investigated problem can simulate the start-up period of naturally cooled small vertical electric motors. Originality/value -The paper presents results not available in the literature for the effect of Gr 2 /Ta on the developing velocities, pressure, flow-rate induced, absorbed heat by fluid and required torque in vertical concentric annuli with impulsively rotated inner walls under the transient free-convection heat transfer mode.