Simulation of a Double-Effect Absorber Cooling System Operating at Elevated Vapor Recompression Levels

Abstract

Applications of aqueous lithium bromide (LiBr/water) single-effect absorption chillers are often limited by their inherently low COP. Multiple-effect absorption cycles, which take advantage of higher temperature heat sources to achieve a higher COP, are more expensive and are limited by corrosion effects, which accelerate significantly above 160°C (320°F). Double-effect chillers are not competitive with comparable electric chillers under current market conditions due to their inherent first-cost premium and oversized footprint. Consequently, research to further improve double-effect absorption chillers, resulting in a lower cost per unit of cooling output, is investigated in this study. Simulation studies of a low differential pressure, vapor recompression absorber (VRA), double- effect absorption cycle utilizing aqueous lithium bromide reported in the literature confirm approximately a 7% efficiency (i.e., COP) and a similar capacity improvement over a conventional double-effect cycle operating at comparable temperatures and pressures. The improvement has been attributed to resulting increased refrigerant mass flow rates and a higher solution temperature entering the high stage generator. The VRA cycle is an alternative to a triple-effect absorption cycle. Substituting a compressor for the fan in the VRA cycle results in further COP and capacity improvements, which makes such a cycle more attractive than both conventional double-effect and proposed triple-effect aqueous lithium bromide absorption chillers. In this paper, a reverse series flow, double-effect absorption system employing a VRA unit operating over a wide range of internal pressure differences is investigated. The performance is compared to a conventional double-effect cycle. Since the VRA requires an electric motor, the impact of gas and electric utility rates on the operating costs of double-effect with the VRA unit cycles and conventional double-effect absorption chillers also are compared.