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Improvement of existing ground source heat pump system by incorporating a compressor frequency converter

At a time of increasing electricity consumption and environmental pollution, high efficiency and environmental friendliness have placed heat pumps high on the list of priorities of local and regional energy strategies. Additional growth in the market share of heat pump systems is possible only with further advances in technology. The control subsystem is of great importance in the development of technology. One important aspect of regulation is maintaining the adequate refrigerant flow rate to precisely match the required heating/cooling capacity. This is possible by controlling the rotational speed of the compressor. Therefore, this paper investigates the impact of compressor frequency control on the reduction in energy required for heat pump operation. The inverter driven heat pump is compared to the base case of a single-stage heat pump. An experimental study was performed on an existing ground source heat pump with a 130 m deep borehole heat exchanger at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, with single-stage system operation. The parameters of the system were monitored by heat and power meters and temperature and pressure sensors. The frequency converter was subsequently added, and the measurements were repeated. The obtained results were compared with the base case for temperature hysteresis of the storage tank set at 3, 6, and 9°C. The results of the research showed that advanced control and the increase in the storage temperature hysteresis prolongs the compressor working interval. An increase in evaporation temperature was also recorded, which has a positive effect on the coefficient of performance of the heat pump. In addition to increasing the COP by 6%, the frequency control reduces the stress during switching the compressor on, thus extending its service life.

Inverter driven heat pump, retrofit, geothermal heat pump, cycling losses, control strategy, variable speed drive

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