Relationship between heat exchange efficiency and ground surface coverage in Slinky-coil type horizontal ground heat exchanger

• Published in: Journal of the Geothermal Research Society of Japan Vol. 42; no. 1; pp. 15 - 27
• Main Authors: FUJII, Hikari, TSUYA, Shunsuke, KOSUKEGAWA, Hiroyuki
• Format: Journal Article
• Language: English; Japanese
• Published: Tokyo THE GEOTHERMAL RESEARCH SOCIETY OF JAPAN 2020; Japan Science and Technology Agency
• Subjects: Asphalt; Coils; Efficiency; Field tests; Foreign exchange rates; ground heat exchanger; ground surface coverage; Heat exchange; Heat exchangers; Mathematical analysis; Moisture content; Numerical models; numerical simulation; Slinky-coil; Soil moisture; Thermal conductivity; Thermal response; thermal response test
• ISSN: 0388-6735; 1883-5775
• DOI: 10.11367/grsj.42.15

The heat exchange efficiency of horizontal ground heat exchangers is affected by ground surface coverages. However, the effect of ground surface coverages on heat exchange efficiency had not been verified experimentally. In this study, field tests were carried out using three sets of Slinky-coil type horizontal ground heat exchangers in Akita city, Japan. The ground surface coverages on the three horizontal ground heat exchangers were asphalt, soil and lawn. The long-term monitoring of ground temperature at a depth of 1m under each ground surface coverage was conducted for observing the effect on behavior of ground temperature. The measured temperature showed that the variation of behavior of ground temperature of the lawn was smallest among other coverages. Then, short-term and long-term thermal response tests were performed, which showed that the temperature increase per unit heat exchange rate of the lawn was lowest in the results of all tests. This result can be explained by the higher moisture content of the ground under the lawn than other coverages and the thermal conductivity was enhanced. A numerical model of horizontal ground heat exchanger considering ground surface coverages was developed using FEFLOW ver.7.1 to evaluate the effect of ground surface coverage on the operation efficiency of GSHP system. As a result, the COP (Coefficient of Performance) of GSHP system in the case of the lawn became 1.1 times higher than that of the asphalt. Finally, sensitivity studies were carried out to evaluate the effect of ground surface coverage on the heat exchange rate for optimizing the land surface area be installed ground heat exchanger.