Experimental and numerical investigations on the thermal performance of a horizontal spiral-coil ground heat exchanger

Abstract

Horizontal spiral coil ground heat exchanger (HSGHE) has been used in ground source heat pump due to its efficiency and low cost. The influence laws of different factors on thermal performance of HSGHE are vital for its optimal design and operation. In this paper, a small scale test device of HSGHE was constructed to investigate thermal performance of HSGHE with different inlet temperatures, spiral pitches, surface wind speeds, and coil styles. The results indicate that the soil temperature interference distance is about 0.4 m for various inlet temperatures under the test conditions. At the same time, reducing spiral pitch can increase total HER, but it also decreases the HER per unit length of pipe. As for surface wind speed, the heat transfer of HSGHE can be enhanced by increasing surface wind speed, and the shallower the burial depth, the better the heat transfer enhancement effect. As far as heat transfer performance is concerned, the spiral coil is superior to slinky coil, but the slinky coil is higher than spiral coil in terms of soil thermal interference. From angle of improving heat exchange efficiency, it is advisable to increase wind speed and inlet fluid temperature, reduce the pitch and use spiral coil. A 3-D numerical model has been developed by COMSOL soft to further explore the influences of operation modes, soil types and coil diameters on thermal performance of HSGHE. The simulation results indicate that the soil temperature can be restored to a certain extent by using intermittent operation mode. The longer the interval time, the better the soil temperature resume and the more obvious the improvement of thermal performance. For the effects of soil type, the sandstone has the best heat transfer effect, followed by sand, and the clay is the worst. Nevertheless, in term of reducing the soil thermal interference, clay is the most advantageous. Additionally, increasing the coil diameter can improve total HER of HSGHE slightly, but it will also cause the reduction of the HER per unit length. Therefore, the coil diameter should be set reasonably according to the specific conditions in the engineering design.