Abstract
The vapor compression heat pumps are very popular solutions regarding heat supply systems of modern, low energy buildings. It is partly due to the fact that they are treated as a sustainable heat supply. The question arises: Can a vapor compression heat pump be treated as a sustainable heat supply? To answer this question; the exergy model of a heat pump system operation has been proposed. The proposed model has been employed for evaluation of exergy efficiency of an existing heat supply system equipped with two heat pumps installed in an educational building located on the campus of Poznan University of Technology, Poznan, Poland. The analysis shows that the system exergy efficiency decreases with an increase in outdoor temperature and its values are in the range of 10.9% to 42.0%. The primary exergy efficiency, which considers the conversion of fossil fuel into electricity, is on average 3.2 times lower than the system exergy efficiency for the outdoor temperature range of −9 °C to 11 °C. The performed analysis allowed for the identification of a set of solutions that may increase the exergy and primary exergy efficiency of the system. The first solution is to cover a part of the electricity demand by a renewable energy source. The second proposition is to apply a low-temperature emission system for heating. The third idea is to apply a district heating network as the heat supply instead of the heat pump. The conclusion is that the exergy performance of systems with heat pumps is rather poor because they generate low-quality heat from high-quality electricity. The best way to improve the primary exergy efficiency of a heat pump system is to power the system by electricity generated from a renewable energy source.
Highlights
The vapor compression heat pumps are thought to be a sustainable solution for heating
For the outdoor temperatures below 0 ◦ C, despite the fact that the heating power provided by the heat pumps is constant, the total flux of exergy losses increases from 86,755 W for the outdoor temperature of −9 ◦ C to 95,118 W for 0 ◦ C
The useable exergy flux decreases with an increase in the outdoor temperature
Summary
The vapor compression heat pumps are thought to be a sustainable solution for heating. Due to this, they are becoming more and more commonly used in modern, low-energy buildings. Can a vapor compression heat pump be treated as a sustainable heat supply? This study is an attempt to answer the question. The exergy model of a heat pump operation has been proposed. The exergy balance analysis is based on the first and the second law of thermodynamics. This enables both quantitative and qualitative evaluation of energy flows in thermodynamically open systems
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