Experimental Investigation of Frost Formation Influence on an Air Source Heat Pump Evaporator

Tomas Kropas,Juozas Bielskus,Giedrė Streckienė

doi:10.3390/en14185737

Abstract

The application of heat pumps in the heating systems of buildings in the cold or transitional season is becoming an increasingly common practice not only in Lithuania but in other countries as well. Due to the growing popularity of air-to-air or air-to-water heat pumps in the building sector, the problem of the evaporator heat exchanger freezing is also becoming more and more relevant. As the outdoor temperature drops, so does the heat pump’s coefficient of performance (COP) for heating. The freezing of the evaporator surface increases the energy consumption of the system, has a negative effect on heat exchange, distorts the normal operating cycle of the heat pump and the energy is wasted for defrosting processes. This article describes the experimental investigation of an air-to-water heat pump, presents the results obtained during the experiments and their interfaces. The experiments were carried out during the transitional/cold season. 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 defrosting cycle took an average of 5 min. The impact of the evaporator freezing on the operation and COP of the heat pump was assessed.

Highlights

The levels of human activity lead to an increase in global warming and greenhouse gas emissions
With the environmental conditions changing throughout the year, the efficiency of systems with integrated air source heat pumps (ASHP) is volatile: their coefficient of performance (COP) is significantly higher in the transitional season than in the cold season
During the period of November 11–12, the average ambient air temperature was 4.02 ◦C, the average relative humidity was recorded as 85.75%

Summary

Introduction

The levels of human activity lead to an increase in global warming and greenhouse gas emissions. Air-to-water and air-to-air heat pumps are becoming an increasingly popular source of heat, providing buildings with both heat and cool. Systems that integrate heat pumps in the heat sector are promoted at the level of the national policy and the European Union. The active spread of such systems is noticeable in newly built, energy-efficient [2] or nearly zero-energy buildings [3]. The development of such buildings is encouraged by national legislation. With the environmental conditions changing throughout the year, the efficiency of systems with integrated air source heat pumps (ASHP) is volatile: their coefficient of performance (COP) is significantly higher in the transitional season than in the cold season

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Conclusion