Ground source heat pumps: observations from United Kingdom ground thermal response tests

The United Kingdom is experiencing a period of rapid growth in the use of ground source heat pump systems. Most installations in the United Kingdom use vertical ‘borehole’ heat-exchanger arrays, the design of which depends on four parameters: formation thermal conductivity, formation heat capacity, heat-exchanger resistance and heat-exchanger grout material heat capacity. Conventionally, two of these parameters (conductivity and resistance) are obtained from a thermal response test carried out on a trial heat exchanger at the site of interest by fitting thermal response data to classical line-source heat conduction theory. This test method gives no information on the heat capacities of the formation and grout material and requires an assumption about the former to enable the heat-exchanger resistance parameter to be extracted. In this work, a new method is developed for extracting all four parameters using a trust-region search algorithm in conjunction with a detailed numerical model of the test heat exchanger. Results give excellent agreement between the fitted-model predictions of heat-exchanger outlet water temperature and measured outlet water temperature for 13 test cases. A further advantage of the method developed here is that it can be used with data sets that contain disturbances and discontinuities. Practical applications: Most of the ground source heat pump installations in the United Kingdom use vertical ‘borehole’ heat-exchanger arrays. The design of these arrays requires information about the rock formation thermal conductivity and volume specific heat capacity and the borehole heat-exchanger thermal resistance and grout material volume specific heat capacity. These design parameters are usually obtained from a thermal response test carried out on a trial heat exchanger at the site of interest. In this work, thermal response test results from 13 UK sites are presented and a new method for obtaining the four design parameters is developed and proposed.