Interpretation of rock mass thermal conductivity at the design stage of heat pump installation and its impact on system efficiency (COP)

Abstract

The recognition of geological and thermal conditions of the rock mass in the case of designing a vertical borehole heat exchanger as the ground source for heat pump installations is a key issue affecting the efficiency of the heating/cooling system operation. This is especially important for large-sized buildings with a high demand for thermal power, which affects into the size of the ground source installation. The aim of the article is to indicate the difference in the obtained results concerning thermal calculations at the design stage of the brine/water heat pump installation with the vertical heat exchanger, in relation to the theoretical values of the rock mass thermal conductivity and the real (measured) values obtained during the thermal response test (TRT). For this purpose, calculations of thermal efficiency from one meter of the current rock mass were made, with particular emphasis on the change in the value of the thermal conductivity coefficient in the tested drilling profiles. Correspondingly, heat pump coefficients of performance (COP) were calculated, which allowed to analyze the influence of the over/undersizing phenomenon of the ground source on the technical parameters of the heat pump's operation and the economic effect of the investment.

Keywords

• Heat pumps
• Thermal conductivity
• Thermal response test