Effects of the application of Demand-Side Management to Ground-Source Heat Pumps

The current approach to abate greenhouse gas emissions of heating systems is the use of full electric systems through the use of heat pumps. Ground source heat pumps (GSHP) are applied for this purpose, using borehole heat exchangers (BHE). At the same time, the increased use of electricity as the unique energy carrier for most uses and the increased exploitation of non-controllable renewable energy sources stresses the power grid. Demand-Side Management (DSM) may reduce the mismatch between energy availability and energy demand. Applying this to GSHPs implies a modification of the scheduling of heat transfer in BHEs, moving up part of the daily operation period. This may cause either an increase or decrease in the performance of the system. Indeed, the temperature field modifies during the heat exchange. This requires a progressive decrease (or increase, in case of heat rejection through the BHE) in the fluid temperature. The time shifting of the use periods may provide a pause in the heat transfer that induces a relaxation of the temperature field, allowing for a recovery of the performance. Yet, the actual effect and its relevance must be investigated. A system consisting of a borehole field and a heat pump at ENEA Casaccia research centre was used to assess the performance of BHE coupled with criteria of DSM. The system is equipped with a Distributed Temperature Sensing system (DTS) which allows to measure the ground temperature over the depth of the wells and thermocouples on the pipes measuring the thermal power exchanged by the working fluid. The experimental apparatus allows to investigate the influence of both the DSM scheduling parameters and the natural resource: ground temperatures, thermophysical properties and groundwater flow rates are evaluated. Tests are realized following a test matrix based on two DSM parameters: the duration of the operation period that is moved up and the delay between the early operation and the normal operation. The parameters monitored with the experimental apparatus are the exchanged thermal power per unit length of BHE, the energy exchanged with the ground and the evolution of the thermal field during the operations of the system. The aim of the research is to figure out the relevance of the effect of DSM scheduling on the performance of the BHE according to the different possible operating conditions. The results show that long-term thermal energy potential is mainly determined by ground temperatures and thermophysical properties. Short-term performances are verified to be influenced by scheduling parameters with an improvement of heat exchange performance due to DSM strategies, limited to short delays between early and normal operation.