Field Test and Numerical Simulation for Evaluating an Effect of Ground Surface Coverage on Slinky-Coil Type Horizontal Ground Heat Exchanger

Abstract

The installation cost of Slinky-coil type Horizontal Ground Heat Exchanger (HGHE) is lower than vertical ground heat exchangers because of using a highly versatile power shovel for excavating soil. However, the heat exchange behavior of HGHEs installed in shallow depth is unstable due to the change of weather conditions such as ambient temperature and solar radiation. Besides, the effect of weather conditions on the heat exchange behavior of HGHE is varied by the type of ground surface coverage. In this research, in order to verify experimentally the effect of the ground surface coverage on the HGHEs, a long-term monitoring of ground temperature and Thermal Response Tests (TRTs) were conducted in Akita city, Japan in 2017 using three Slinky-coil type HGHEs under lawn, soil and asphalt surfaces. The monitoring showed that the ground temperature behavior under the lawn was most stable among the all. In addition, the TRTs showed that the temperature increase per unit heat exchange rate of the lawn was the lowest among the all, indicating the high heat exchange efficiency in cooling period. Therefore, the field tests clarified that the ground surface coverage affects ground temperature behavior and heat exchange efficiency. After the interpretation of field test results, numerical models of HGHEs considering the characteristics of ground surface coverage were developed for long-term cooling and heating simulations and sensitivity studies. The numerical model was verified based on history matching with the TRT results. The long-term cooling and heating simulations were then carried out for evaluating the operation efficiency of ground source heat pump (GSHP) system. As the result, the calculated COP (Coefficient of Performance) of GSHP system in case of the lawn showed about 1.1 times as large as the COP of the asphalt. Thus, the type of ground surface coverage is considered to be the important factor for optimum design of the GSHP system. Finally, using the validated models, the sensitivity studies were performed by changing the solar reflectance and evaporation efficiency, etc., as representative parameters of ground surface coverage to estimate the correlation between heat exchange rate and characteristic of ground surface coverage.