Thermal energy recovery system for upgrading waste heat by an absorption heat pump

Abstract

An innovative absorption heat pump (AHP) system is proposed to increase the temperature from waste heat to a level of 80 °C and to produce hot air over 120 °C for drying or simultaneously generating steam of 100-115 °C. Air is heated up directly by heat exchange in the absorber working in the heating mode of a LiBr/H2O AHP system. Steam is sequentially produced by heat exchange, with the absorption solution still maintaining a high temperature. An examination was carried out continuously to evaluate the performance of a bench-scale of the AHP. In the proposed AHP, the temperature of hot air at the outlet of the absorber was typically above 120 °C and, steam up to 115 °C was simultaneously generated by recovering the heat of the hot water at 80 °C. The coefficient of performance, defined by the ratio of heat generated to the power consumed for pumps of fluid flow, exceeded 20. It was also found that the fine particle slurry of LiBr crystals is formed stably in the solution under a supersaturation condition when zeolite powder is suspended. Then an almost saturated concentration is maintained as a result of the dissolution of the crystal, even if the solution is diluted by absorption of water vapor in the absorber. The theoretical analysis based on a heat and mass transfer model predicted that the output power of the AHP improved by almost 100% compared with the conventional solution at concentrations lower than saturation solubility. Measurement of the slurry properties and an experiment on the absorption performance of the slurry were carried out, and the effectiveness of the slurry was confirmed. This chapter reviews a series of works done by the present author.