Measurement and analysis of the thermal properties of the ground for ground heat exchanger applications in Cyprus

Abstract

Geothermal energy is a renewable resource and is attracting increasing attention for heating but also for cooling of domestic and commercial buildings in warm climates. Successful utilization of geothermal energy requires knowledge of the geothermal properties of the ground. In Cyprus only very limited research has been carried out to-date on the use of ground source heat pumps, and information is needed to enable engineers to size correctly Ground Heat Exchangers (GHE) for Ground Source Heat Pump applications. To address this, the main objective of the research presented in this thesis was to investigate the thermal properties of the ground at a number of locations in Cyprus and use the results to develop data and easy to use tools to enable engineers and researchers to evaluate the potential and design Ground Heat Exchangers (GHEs) for specific locations and thermal loads. The research involved an extended geological sampling on the island and measurements of the thermal properties of 148 ground samples in the laboratory in their dry and water saturated states. Thermal conductivity values for dry samples were found to be in the range between 0.4 and 4.2 W m–1 K–1, thermal diffusivity values between 0.3 and 1.910–6 m2 s–1 and specific heat capacity between 0.5 and 1.5 J K–1 kg–1. Results also showed thermal conductivity and thermal diffusivity to increase with water content for most of the ground samples investigated. To understand and visualize all measured data, Geographic Information System (GIS) software was used to generate maps of ground density, thermal conductivity, and thermal diffusivity. From the maps, the Troodos Ophiolite terrane which dominates the central part of Cyprus, was found to offer the best thermal properties for the utilization of geothermal energy on the Island. Geothermal modeling was carried out to investigate the effect of (a) summer and winter mode of operation, (b) ground temperature variation with depth to consider the effects of daily and seasonal ambient temperature variations on ground temperature, (c) borehole radius, (d) borehole grout properties, (e) U-tube diameter, (f) U-tube leg and distance from the centre of the borehole, and (g) ground water level and flow velocity, on the performance of GHEs. For the prediction of the heat injection rate of a GHE, a tool was developed with the use of FlexPDE software (PDE Solutions Inc). The tool considers GHE characteristics, the installation area and ground properties and groundwater flow. Twenty-two boreholes located in Nicosia were simulated to determine their geothermal performance. GIS software was employed to develop, for the first time, maps that provide information on the geothermal properties of the ground in Cyprus per meter depth to enable easy evaluation of the suitability of the ground for the installation of GHEs. All geothermal maps compiled in the framework of this research, are now available online, in a web application at https://amccy.maps.arcgis.com/apps/ImageryViewer/index.html?appid=d81a63acc03c4c35a80c65e8c1689c77 to facilitate easy accessibility by engineers working in the GHE design and installation field, for the use of engineers and designers of GHEs and ground source heat pump systems.