Influence of different regulation modes of compressor speed on the performance of direct-expansion solar-assisted heat pump water heater

A quasi-dynamic mathematical model of a direct-expansion solar-assisted heat pump water heater is presented on the basis of theory and experiments, and the system performance in various regulation modes of the compressor speed are analyzed comparatively. A lumped parameter method is used to establish the models of a compressor and an electronic expansion valve, and a uniform flow distributed parameter method is used to establish the models of an unglazed flat-plate solar collector/evaporator and a winding micro-channel condenser. The mathematical model predicts well the experimental data within a maximum relative error of 10%. Three regulation modes under constant environmental parameters and four regulation modes under variable environmental parameters are proposed, respectively. Numerical calculations as well as experimental studies show that there is little effect of the regulation mode of the compressor speed on the average system coefficient of performance under the same operating and environmental parameters. Under typical working conditions with average compressor speed of about 2900 rpm, when water temperature rise from 28.2 to 61 +/- 1 degrees C, the average system coefficient of performance reaches above 4.6 for different regulation modes of the compressor speed. When the average compressor speed decreases from 2910 to 2650 rpm, the average system coefficient of performance increases by 7.5%. In addition, the acceleration mode of the compressor speed enables to stabilize the evaporating temperature, and the deceleration mode of the compressor speed enables to stabilize the compressor power.