Capacity control of heat pumps (Full version)

In Sweden the current trend is to size heat pumps much closer to full coverage of the heating demand than has been the case so far. The driver for this development is both economic and legislative (increasing electricity price, possible power rates and limits in the building code regarding installed electric power for heating). This means that heat pumps will operate at part load almost all the time and that power utilization will be very low. This is particularly the case for new, low-energy buildings and has raised the interest in variable-capacity heat pumps. General aspects of capacity control Capacity control, primarily by means of Variable-Speed Drive (VSD) motors, is commonly used to improve the efficiency of operation of systems for heating, cooling and ventilation. However, before going into the specific application of heat pumps it may be pertinent to look at some of the generic aspects.

Background

Equipment for capacity control by means of frequency controlled motors for fans, pumps, compressors and other types of motor driven HVAC equipment appeared on the market already in the 1970s for pumps and fans and in the early 1990s for compressors in Japanese residential air conditioners. In those days the technology was considered to be very upmarket and consumers in my own country Sweden would pay up to 700 EUR over the price of a single-speed residential air-to-air heat pump (at that time a high-quality inverter which I bought for laboratory purposes only cost 200-300 EUR). Nowadays this technology is standard and the additional cost quite small. An important driver for the break-through of variable-speed drives in Japan was the availability of surplus capacity from a given size of compressor. It was rather common to have the air-conditioner turned off when no one was at home and to use the surplus capacity to quickly bring the room temperature down. With frequency control this can be achieved by feeding a 50-60 Hz motor with 100-120 Hz. This possibility is still of interest but perhaps not the main driver in the Swedish market. Instead the focus is on saving energy.

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