Experimental and numerical studies on the performance of horizontal U-type and spiral-coil-type ground heat exchangers considering economic aspects

Abstract

The benefits of the use of horizontal U-type and spiral-coil-type ground heat exchangers (GHE) were investigated and compared based on heat transfer performance and economic analysis. The feasibility of a novel backfill material (controlled low-strength material) for a horizontal ground heat exchanger was also assessed. A numerical analysis model was developed to simulate the heat transfer in a horizontal ground source heat exchanger and calculate the annual heat exchange using 100 case studies with different GHE types, backfill materials, fluid velocities, and installation depths. A thermal response test was conducted using a spiral-coil GHE backfilled with sand and a controlled low-strength material to validate the numerical model. The economic feasibility of each case was investigated using analytical tools including the annual heat exchange, payback period, and internal rate of return. The results indicate that, although a spiral-coil-type GHE has 30% higher total initial installation costs, its high annual heat exchange (twice that of U-type) results in a shorter payback period (12.7 years compared with 8.4 years for U-type) and a higher internal rate of return (12.0% compared with 6.9% for U-type). The use of a controlled low-strength material leads to a slight increase in the initial investment; however, as expected, its high heat exchange capacity shows better long-term economic performance than a ground heat exchanger backfilled with normal soil.