A small ammonia heat pump using linear compressor

Abstract

The electrification of space heating and hot water via heat pumps is a widely pursued policy aim but field trials demonstrate poorer seasonal performance than anticipated. Linear compressors without crank mechanism can be oil-free. The potential for capacity modulation by adjusting the compressor stroke offers significant advantages over traditional methods including on-off control and variable frequency given that heat pumps operate at part load most of time. R717 (ammonia) has superior latent heat which offers a higher heating capacity compared to R134a, making it a very promising refrigerant for small heat pumps but there is no compatible compressor due to corrosivity. This paper introduces a novel small ammonia heat pump using an oil-free linear compressor technology that is compatible with R717. A numerical model was established to predict the heat pump baseline performance. The outcomes show a linear relation for specific power input, heating capacity, and Coefficient of Performance (COP) against ambient temperature. The system can achieve COP of 2 when ambient temperature is 0 °C. With linear compressor, there will be no on-off cycling for heat pumps so that the seasonal COP will be significantly higher than on-off control and variable frequency drive systems.