| Title | Ten Years of Thermal Response Tests with Heating Cables |
|---|---|
| Authors | Jasmin RAYMOND, Maria Isabel VÉLEZ MARQUEZ, Daniela BLESSENT, Louis LAMARCHE, Louis GOSSELIN, Jean ROULEAU, Mikael PHILIPPE, René THERRIEN, Michel MALO |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | heat pump, ground heat exchanger, borehole heat exchanger, thermal conductivity |
| Abstract | The use of heating cables was proposed at the World Geothermal Congress in 2010 to conduct thermal response tests in ground heat exchangers without water circulating in the pipes of the exchangers. Heating water at a constant rate at surface during a test in the field can be difficult due to varying temperature at the surface, the need to insulate the unit components, the establishment of a significant temperature difference between the pipe inlet and outlet, potential pipe leaks, air trapped in the circulating water, and parasitic heat losses. Circulating the water in a ground heat exchanger during a test additionally brings analytical challenges because the heat injection rate along the borehole likely varies with depth. Ten years of research have therefore been conducted since this original development, providing an alternative thermal response test method to avoid problems due to water circulation. The objective of the research was to evaluate the in situ subsurface thermal conductivity to design ground-coupled heat pump systems with a simplified field method using a low power source, typically less than 1 to 2 kW. Thermal response tests with a heating cable assembly have been conducted in various types of ground heat exchangers using three main cable configurations: 1) continuous heating cable in a 30 m deep heat exchanger installed for a direct expansion heat pump, 2) ten short 1.2 m length heating cable sections deployed in 100 to 140 m deep heat exchangers and 3), more recently, a continuous heating cable in a 100 m deep heat exchanger. This last test with a long and continuous heating cable was successfully conducted with a heat injection rate of less than 10 W m 1 to minimize the power requirement and has been possible due to the improvement of submersible data loggers, having a temperature accuracy and resolution of ±0.1 and ±0.05 °C, respectively. Field tests that combined a heating cable and fiber optic distributed temperature sensing were also conducted to better understand heat transfer mechanisms during such tests. A method to infer groundwater flow direction and magnitude during the test with temperature sensors surrounding the heating cable has additionally been proposed. This paper reviews recent developments for thermal response tests with heating cables and highlights novelty in this field. |