The compression/absorption cycle – influence of some major parameters on COP and a comparison with the compression cycle

Abstract

This article concerns two main studies: a parameter study and a comparison. In the parameter study, the CAHP is focused upon. Its COP sensitivity to changes in the absorber and desorber falling-film tube length, heat exchanger area distribution, and concentration change of the solution in the absorber is studied. The relative distribution between the desorber and absorber is found to have little impact on COP. The area distributed to the solution heat exchanger and the concentration change in the absorber is found to have a large impact on COP when examined separately, but when they are studied together, and with optimized concentration change for each area distribution, the total impact is low. It is shown that the falling-film tubes should be designed to be as long as possible in order to increase the COP. The comparison involves a new procedure for comparing the performance (COP) of a compression/absorption heat pump (CAHP) with that of a compression heat pump (CHP). In the procedure local heat transfer coefficients and pressure drop are taken into account. Further, the comparison is performed for various heating applications and with specified investment level. The heating applications are typically industrial or district heating cases and are chosen to study impact of three different kinds: size of the sink and source temperature change (glide), temperature lift for a given sink and source glide and temperature level for a given sink and source glide. Ammonia/water is used as working fluid in the CAHP and isobutane in the CHP. A relevant industrial design is assumed for the CHP (including an indirect economizer coupling, suction gas heat exchanger, sub-cooler and surface enhancements in evaporator and condenser), which is not the case in previous comparisons of this type. The absorber and desorber in the CAHP are modeled as vertical falling-film tube-and-shell heat exchangers. The main results for the comparison study are: (1) the COP of the CAHP is as good as that of the CHP when the sink and source glides are 10 K; (2) when the glide of the sink and source is increased to 20 K, the CAHP has a 12% better performance than the CHP; and (3) an increased temperature lift and an increased temperature level give the CAHP a relatively worse COP. Some COP-increasing design parameters to be studied further are proposed for the CAHP.