Ground heat exchangers in geothermal anomaly zones: a case study

In this paper vertical ground heat exchangers in geothermal anomaly zone are investigated. Ground Coupled Heat Pumps (GCHP) have been increasingly used in the last years around Europe; therefore companies producing heat pumps, drilling wells and boreholes are paying more and more attention in this field. Temperatures into the ground usually vary from 7°C to 20°C, depending on outdoor mean annual air temperature. An interesting and promising field of application is represented by geothermal areas, where the temperatures in the shallow underground can reach 30°C to 85°C, since the borehole ground heat exchangers can be reduced in terms of depth or number due to the favourable high temperatures. In these contexts the ground can be used only as a source for heating and not for injecting heating during summer due to the high temperatures which might be reached. The aim of this paper is the evaluation of the possibility to use the direct coupling between the building heating system and the borehole heat exchangers. In North of Italy there are some places where thermal anomaly condition of the ground is present; the ground temperature is about 70-85°C, therefore the mean temperature over a depth of 100 m below the ground can be around 30-35°C instead of usual values which are about 13-15°C. In the present study an energy analysis of a case study of a residential building has been carried out by means of the simulation tool TRNSYS, coupled with the CaRM model (acronym of “Capacity Resistance Model”), developed by authors, which is able to consider in detail the thermal behaviour of the ground heat exchangers. Several thermal plant solutions have been compared to evaluate the best solution in terms of energy consumption.