The efficiency loss in groundwater heat pump systems triggered by thermal recycling

Abstract

Thermal recycling occurring in groundwater heat pump systems is a major cause of efficiency loss in groundwater heat pump systems and its understanding is critical for systems design. However, the response of the systems performance to these phenomena is not easy to predict given that a negative feedback is created, as the rejected heated/cooled water injected back into the aquifer will tend to negatively affect the production well(s) of the doublet(s) starting the negative loop. This work approaches the nonlinearity of the thermal recycling problem for the first time, by making use of distributed numerical models of groundwater flow and heat transport combined with thermodynamic models of geothermal heat pumps. Simulation results showed that thermal interference between production and injection was unavoidable for the scenarios investigated. Different scenarios were found where the impact of thermal recycling was not sustainable during cooling seasons, thus resulting in the inoperability of the heat pump. Thermal recycling was found to be responsible of an increase in operation costs of up to 38.4 k€·year−1, as well as of an environmental impact due to the release of up to 76.2 t eqCO2·year−1 for a 402 kWth installation. The results of this article will allow to improve the understanding of thermal recycling as a nonlinear complex and a transient process responsible for an efficiency loss of groundwater heat pump systems.