Novel two-stage adsorption heat pump with high efficiency and energy storage density, powered by non-concentrated sunlight

Abstract

Low-cost solar-powered air-conditioning systems with energy storage have the potential to reduce fossil fuel consumption. Herein, a two-stage adsorption heat pump (AHP) system with high efficiency and high energy storage density, powered by non-concentrated sunlight, is proposed. In this system, solar energy is converted into heat at a selective solar absorber with transparent thermal insulation and is stored as heat of desorption for adsorbent 1. When air conditioning is required, the desorbed adsorbent 1 is used to desorb refrigerant (water) from adsorbent 2, and adsorbent 2 is used to cause the water in the evaporator to evaporate; both the heat of desorption and latent heat of evaporation are then extracted as cooling energy. When the aforementioned cooling operations are performed cyclically, the proposed AHP system can substantially increase the cooling output compared with that of a single-stage AHP even if the amount of adsorbent 2 is small compared with the amount of adsorbent 1. Simulations of solar energy storage during an entire day of sun exposure showed that this system is capable of an energy storage density per unit area of 5.3 kWh m−2 day−1 with a solar-energy-to-air-conditioning-energy conversion efficiency of 70 %. The prototype two-stage AHP was also demonstrated to repeatedly extract both the heat of desorption and the latent heat of evaporation. In outdoor experiments, the desorption of water in zeolite 13X (used as adsorbent 1) under non-concentrated sunlight was achieved.