Control strategy calibration of a direct-expansion solar-assisted heat pump

As known, the performance of a direct-expansion solar-assisted heat pump (DX-SAHP) is affected by environmental and operational parameters significantly, and the variable capacity system has distinct advantages. The calibration of the control strategy will be very useful for the thermal performance improvement of DX-SAHP systems. Based on the test rig which is used to supply domestic hot water, the original variable capacity control strategy has been calibrated with the quasi-dynamic mathematical model under different conditions. The calibration results show that the optimum range of the degree of superheat is 7-12 degrees C. For winter and spring/ autumn conditions, the optimum range of operation duration are 360-420 min and 300-360 min, and the corresponding optimum range of average compressor speed are 3750-4500 r & sdot;min- 1 and 2750-3750 r & sdot;min- 1, respectively. Experiments prove that the actual operation parameters can be maintained at the optimum range well and the average coefficient of performance (COPm) during the whole heating process has been improved further. For winter and spring/autumn, the average operation duration and compressor speed are 395 min and 3970 r & sdot;min- 1, 345 min and 3066 r & sdot;min- 1, respectively. Moreover, the values of COPm could reach 3.44 and 4.67, respectively. Even at solar radiation intensity of 577.6 W & sdot;m- 2 with ambient temperature of -9.1 degrees C, the COPm could reach 2.38.