Effects and Opportunities ofSeasonally Reversing GeothermalFlow Direction

Abstract

Heating and cooling accounts for nearly half of the total energy consumption in the European Union,making geothermal systems a vital component for the energy transition. However, their operational lifetime is limited. Injected cold water forms a cold thermal front that spreads outwards. When this front reaches the production well, thermal breakthrough occurs and production temperatures drop. This report investigates whether seasonally reversing the flow direction by injecting hot water during periods of low heat demand can extend the reservoir’s lifetime or allow for reduced well spacing. This is important because it can optimize the use of geothermal systems in the future.To do this, annual energy demand was studied to find when heat demand is low. The available solar power in this period was then calculated for three different scenario’s to determine what reversed flow rate can be delivered. After establishing an analytical model and baseline simulation of the Delft geothermal reservoir, the three scenario’s were tested and the new breakthrough times computed. The effects of reservoir thickness and well-spacing were then studied.It was found that with the solar power currently available on the TU Delft campus, the lifetime of the geothermal reservoir can be extended by 7 years from the original 21 years. With a limited expansion of solar power on campus, this extension could be 11 years and if the system is run at maximum capacity,21 years is possible. It was found that if the lifetime were to be kept constant, the well spacing could be reduced from the original 1100 metres down to between 934.8 and 789.2 metres depending on the scenario.It was concluded that seasonally reversing the flow direction offers several opportunities to optimize geothermal systems. Either by extending the lifetime of the geothermal system or by allowing closer wel spacing to minimize costs whilst keeping a reasonable reservoir lifetime.