Experimental investigation of frost on an air-source heat pump evaporator

Abstract

The application of heat pumps for buildings heating systems in the cold or transitional season of the year is increasingly common not only in Lithuania but also in other European Union countries. Due to the growing popularity of air to air or air to water heat pumps in the building sector, the problem of evaporator heat exchanger freezing is also increasing in importance. As the outdoor temperature drops, the heat pump's efficiency – coefficient of performance for heating (COP) decreases. At high relative humidity there is also a risk of the evaporator heat exchanger to start freezing. This increases the energy consumption of the system, has a negative effect on heat exchange. Given the need for experimental and numerical research, the aim of this work is to analyse and present the characteristics of air source heat pump (ASHP) in the presence of the risk of freezing and to qualitatively assess the COP reduction due to frost formation. This paper investigates the operation of an ASHP in the cold season, observes the freezing of the evaporator as environmental conditions change. An experimental stand was constructed for this research; the necessary measuring devices and a camera were installed for visual observation of the experiment and assessment of frost formation. The experiments were carried out during the transitional - cold season of the year. It was found that frost formation on the evaporator started when the outdoor temperature was <3.5 °C and the relative humidity reached 88%. The defrost cycle took an average of 5 minutes. The impact of evaporator freezing on the operation and efficiency of the system - COP - has been assessed.