C. Montagud

Borehole modelling: a comparison between a steady-state model and a novel dynamic model in a real ON/OFF GSHP operation

The correct design and optimization of complex energy systems requires the ability to reproduce the dynamic thermal behavior of each system component. In ground source heat pump (GSHP) systems, modelling the borehole heat exchangers (BHE) dynamic response is especially relevant in the development of control strategies for energy optimization purposes. Over the last years, several models have been developed but most of them are based on steady- state approaches, which makes them unsuitable for short-term simulation purposes. In fact, in order to accurately predict the evolution of the fluid temperatures due to the ON/OFF cycles of the heat pump, it is essential to correctly characterize the dynamic response of BHE for very short time periods. The aim of the present paper is to compare the performance of an analytical steady-state model, available in TRNSYS environment (Type 557), with a novel short-term dynamic model. The new dynamic model is based on the thermal-network approach coupled with a vertical discretization of the borehole which takes into account both the advection due to the fluid circulating along the U-tube, and the heat transfer in the borehole and in the ground. These two approaches were compared against experimental data collected from a real GSHP system installed at the Universitat Politecnica de Valencia. The analysis was performed comparing the outlet temperature profiles predicted by both models during daily standard ON/OFF operating conditions, both in heating and cooling mode, and the between both approaches were highlighted. Finally, the obtained results have been discussed focusing on the potential impact that the differences found in the prediction of the temperature evolution could have in design and optimization of GSHP systems.

Seasonal performance assessment of a dual source heat pump system for heating, cooling and domestic hot water production

In order to contribute to a global CO2 emissions reduction in 2020, the increase in the use of highly efficient heat pumps for heating, cooling and domestic hot water production in buildings is very recommendable. In this direction, Ground Source Heat Pump (GSHP) systems are generally recognized as one of the most energy-efficient compared to air source heat pump systems. However, this strongly depends on the temperature evolution of the air and the ground during the year, which also depends on the geographical location of the system. Therefore, an optimal system from the energy point of view apparently would be the one that is able to switch from one source to the other in order to operate the heat pump with the highest efficiency. In this context, a new Dual Source Heat Pump (DSHP) unit for heating, cooling and production of domestic hot water, was developed and manufactured in the framework of a H2020 European project called GEOT€CH (Geothermal Technology for €conomic Cooling and Heating). This paper presents the assessment of such a DSHP in different representative locations around Europe. For this purpose, an integrated system model in TRNSYS was developed to assess its performance for different thermal demand profiles obtained for the same building under different conditions.