Dynamic simulation and exergy analysis of adsorption chiller powered by low-grade waste heat from a fuel-cell system: Effect of multibed configuration and time constant

Abstract

Energy usage for space cooling and air conditioning has been rapidly growing in recent years and is expected to increase further owing to global warming and climate change. Adsorption chillers can provide an eco-friendly solution to space cooling because they utilize low-grade thermal energy such as waste hot water produced by the primary power-generation unit. In this study, dynamic analysis of a fuel-cell-system-driven adsorption refrigeration system was performed to expedite the optimal multibed configuration and its operating conditions. The scope of the study was extended to investigate maximizing the waste heat utilization achieved via exergy analysis of multibed configurations. The adsorption-to-desorption time ratio was integrated into the physical configuration of the adsorption chiller, and a new variable was introduced as a “time constant” that controls the cycle time and, under ideal conditions, equates the heating requirements of different bed configurations. Dynamic simulations for different bed configurations were conducted to evaluate the performance, specific cooling capacity, and unutilized exergy in the outlet waste stream. Under the operating conditions of 25 °C and 80 °C cold water and hot water temperatures, respectively, the five-bed configuration with a time ratio of 0.25 proved to be optimal, resulting in increases of 45.8%, 10.2%, and 15.1% in the specific cooling capacity, performance, and exergy efficiency, respectively, compared with those of the conventional two-bed configuration. The four-bed configuration yields very similar results, but with a slightly lower specific cooling capacity.