Ground source heat pumps : potential applications & limitations in Malta

Abstract

The principal aim of this paper was to explore the potential of utilizing geothermal energy for cooling, exploiting the high thermal mass of the indigenous globigerina limestone, so consistent under most of Malta. This paper stems from a larger research project, where testing out was carried out extensively using a GSHP (Ground Source Heat Pump) system during the summer season. The objective was to assess the efficiency of vertical versus horizontal pipe configurations for a given pipe diameter, while varying the water flow rate and the depth of the U-tube pipe. Direct Thermal Response Tests (TRTs) were carried out using two pipe configurations, essentially both of 20m each in length, encased in concrete; however one laid in a horizontal trench, at 1.2m depth, while the vertical run was inserted in a 20m deep borehole, 0.3m in diameter. In both the vertical and horizontal configurations, U-tube pipes (40mm diameter) were connected separately to a hot water source and a variable circulation flow pump, with inlet and outlet temperatures all monitored through a data logger and a computer. The latter were set up in a purposely-built test cell, also interconnecting the two configurations independently. Ground temperatures were recorded at depths of 1.2m, 5m, 10m, 15m and 20m respectively, in tandem with ambient conditions. Preliminary results indicate that the ground temperature at 20m below for globigerina limestone, was relatively stable at around 200C all year round. In the two cases tested, the vertical configuration proved to be more efficient by 20%, with the highest efficiency registered at 20m depth. This augurs well as a pre-cooling for environmental control systems, thus improving the thermal efficiency, also curtailing carbon emissions.