Performance analysis of a variable-stage open absorption heat pump combined with a membrane absorber

Abstract

Compared to conventional closed absorption cycles, the open absorption cycle has a greater potential in latent heat recovery from exhausted moist gas since it recovers latent heat by mass transfer instead of heat transfer. Existing open absorption cycles including single-effect and multi-stage cycles have limitations on the waste heat utilization. This paper suggests a novel variable-stage open absorption heat pump cycle which can work on a wide range of temperature and humidity of exhaust gas. A membrane contactor is also suggested to replace traditional direct-contact absorbers to integrate with the new cycle. The membrane contactor has a higher mass and heat transfer efficiency and a greater potential to avoid solution carryover. Thermodynamic model of the new cycle combined with the membrane absorber model is developed. Parametric analysis is carried out to evaluate the performance of the new cycle. Simulation results show that, under the analyzed conditions, the equivalent stage number of the new cycle varies from 4 to 1 while coefficient of performance increases approximately from 1.2 to 2 with the increase of generation temperature and air humidity ratio. It indicates that the new cycle can cover the working temperature or humidity ranges of at least four conventional absorption cycles, including single-stage, 1.5-stage, double-stage, triple-stage absorption cycles. The new cycle works on a much wider temperature and humidity ranges than conventional multi-stage cycles because the pressures of the low-pressure generator and absorber will adjust themselves to adapt to the variations of generation temperature and air humidity ratio. The self-adaption of the pressures of the low-pressure generator and absorber is realized by the control of the solution mass flow rate. During the evaluated working ranges, coefficient of performance of the new cycle increases by 5.9–28.2% compared to conventional multi-stage absorption cycles. Influences of the operating conditions on the absorption rate of the membrane absorber is also conducted and discussed.