Performance evaluation of refrigerant mixtures in heat pumps.

摘要

The phase-out of the currently used refrigerants during the next decade requires fast and accurate methods to evaluate possible alternatives for the existing refrigerants. This thesis investigates possible replacement refrigerants for R22 and new methods for their evaluation are developed. The replacements are binary zeotropic mixtures of the following hydrofluorocarbons (HFCs): R23, R32, R125, R134a, R143a and R152a.; A computer simulation study yielded that two refrigerant mixtures, R32/R134a and R32/R152a, were found to perform better than R22 with respect to the Coefficient of Performance (COP) and volumetric capacity for certain composition ranges using counterflow heat exchangers. When the two binary mixtures are compared then the R32/R152a mixture shows up to 15% lower volumetric capacities at up to 5% higher COPs over the R32/R134a mixture.; The tests were conducted at four typical heat pump operating conditions (two heating and two cooling conditions.) The capacities were kept constant for each separate operating condition independent of the working fluid. The performance differences between the mixtures were not measured to be as significant as predicted in the simulations. The implementation of a liquid line to suction line heat exchanger in the heat pump system proved to be advantageous for both mixtures. For the R32/R134a the internal heat exchange is more advantageous than for the R32/R152a mixture.; The overall conductances for the mixtures were obtained to be 0% to 22% greater than those of the tests with R22. A heat pump system using the here proposed mixtures would most likely not need larger heat exchangers than a R22 system.; Two graphical concepts were developed that allow for a useful presentation of the entropy generation in heat exchangers which is indicative for the encountered performance advantages.; The newly investigated research approach of using a simulation model to scan a large number of replacement candidates prior to the testing of only the best candidates proved to be an accurate method. The results of this study give confidence that this time saving combination of simulating and testing is a very powerful engineering tool.