Performance evaluation of a dual-speed heat pump

An air-to-air heat pump with a two-speed compressor was installed in a single-family residence near Knoxville, Tennessee, and was operated in a weekly alternating schedule of dual and single-speed modes over a full heating and cooling season to permit performance evaluation of the two modes. The residence and the heat pump were extensively instrtunented in order to monitor performance of the unit along with its degradation from steady-state performance for both modes of operation.

For the heating season, the heat pump had a performance factor of 2.17 in the dual-speed mode and a performance factor of 2.11 in the single-speed mode. For the cooling season, the heat pump had performance factors of 2.27 and 2.09 for the dual and single-speed modes respectively. Annual performance factors were 2.20 and 2.10 for the dual and single-speed modes respectively.

Also, an annual performance factor (APF) model created by Oak Ridge National Laboratory for residential air-source heat pumps was used to generate computer simulations of the field performance. Heating season performance factor (HSPF) values were 2.00 for the single-speed mode and 2.05-2.13, depending upon the switch-over temperature used, for the dual-speed mode. The cooling season performance factor (CSPF) values were 1.85 for the single-speed mode and 2.23 for the dual-speed mode. The switch-over from low to high speed compressor for the dual-speed mode during the cooling season was determined by whether or not the system could meet the cooling load or not. If the heat pump system could not meet this cooling load in low speed compressor it would shut off and come back on in high speed compresssor operation. Annual performance factor values were 1.9A for the single-speed mode and 2.12-2.17, depending upon the switch-over temperature used for heating, for the dual-speed mode.

Based on the findings of the field analysis, it is believed that the switch-over control method from low to high speed compressor operation during the heating season that was used on this unit was not very effective at offering the potential enhancements of the dual-speed mode over the single-speed mode. It is recommended that this area be addressed in future studies. Also, the fact that the dual-speed mode always had a higher sensible heat ratio than the single-speed mode during the cooling season should be investigated in future studies in regard to its effect on the comfort level of the indoor air space.