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

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 c hanges in the absorber and desorber falling-film tube length, heat exchange r area distribution, and concentration change of the solution in the absorb er 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 ha ve a large impact on COP when examined separately, but when they are studie d together, and with optimized concentration change for each area distribut ion, the total impact is low. It is shown that the falling-film tubes shoul d 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 heal pump (CAHP) with that of a compression he at primp (CHP). In the procedure local heat transfer coefficients and press ure drop are taken into account. Further, the comparison is performed for v arious heating applications and with specified investment level. The heatin g applications are typically industrial or district heating cases and are c hosen to study impact of three different kinds: size of the sink and source temperature change (glide), temperature lift for a given sink and source g lide 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 i n 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 comp arison study are: (1) the COP of the CAHP is as good as that of the CHP whe n the sink and source glides are 10 K; (2) when the glide of the sink and s ource 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 lev el give the CAHP a relatively worse COP. Some COP-increasing design paramet ers to be studied further are proposed for the CAMP. (C) 1999 Elsevier Scie nce Ltd and IIR. All rights reserved.