Sequential coupled simulation of a dual source heat pump and shallow geothermal reservoir

The numerical simulation is an important tool for the assessment of exploiting geothermal energy. It can be used in shallow geothermal applications to evaluate the different production scenarios and the sustainability of the system (geothermal reservoir and heat pump) on long term. Moreover, in shallow geothermal projects, to simulate the real behaviour of the system, the load profiles of the end user and variations of the working mode of the heat pump should be taken into account. The present work introduces and describes a coupled numerical model, in which a dedicated Matlab® script has been realized to allow a sequential coupled simulation of a shallow geothermal reservoir exploited with a dual source heat pump. A mathematical model of a dual source heat pump that can work with the ground or the air as source/sink has been developed in Matlab® environment. Each component of the heat pump has been modelled considering the equations that govern the physical phenomena. The dynamic numerical simulator FEFLOW® has been used to simulate the behaviour of the geothermal reservoir, subjected to heat extraction/reinjection by a closed loop vertical heat exchangers field. This methodological approach is useful to evaluate the performance of the coupled system on the long term, and it is important for understanding the advantages and limits of the dual source heat pump in assuring the sustainability over time when heat is exchanged with the ground, avoiding the depletion of geothermal resources. The mathematical models have been validated with experimental data from a geothermal plant located in Tribano (Padova, IT). This is one of the four pilot sites realized within the framework of the H2020 GEOTeCH Project. It consists of eight coaxial borehole heat exchangers 30 m deep, connected to the 16 kW dual source heat pump prototype realized by HIREF S.p.A. The geothermal heat pump system has been working, and monitored, since October 2017 and it provides heating and air conditioning to an office area. Experimental results have been used to verify the new coupled model, and although the preliminary results are encouraging, further study and work are necessary to make it robust and stable for future routine work.