24 June 2014
The thermal output of an air source heat pump (ASHP) with constant speed
compressor increases at increasing ambient air temperature while the heating demand of the building decreases. Consequently, the thermal output of the heat pump is much higher than the actual demand for a large part of the year. Speed controlled compressors (VSD) offer the
possibility to adapt the thermal capacity of the heat pump to the current load with the advantages of continuous operation, lower start and stop losses and higher efficiency.
At low ambient air temperatures and especially with heating systems that require high flow temperatures ASHP are operated with high pressure ratios and therefore achieve relatively
poor efficiencies. Additionally, the heating capacity decreases and the compressor discharge temperature increases to very high values at low evaporation temperatures. In order to
improve ASHP systems enhanced vapor injection (EVI) systems are one possibility. By means of a vapor injection into the compressor at an intermediate pressure the compressor
discharge temperature can be reduced and both the heating capacity and the COP of the heat pump can be increased at low evaporation temperatures. In this work a heat pump system comprising a speed controlled compressor and an EVI cycle
(CEV) is analyzed by means of detailed measurements in the laboratory. The measured data is used to parameterize and validate a detailed simulation model of the heat pump. Using this model, dynamic system simulations are performed to analyze the seasonal performance factor (SPF) of the heat pump system in comparison to a standard heat pump (CSD).