Optimising Geothermal Energy Storage in Harsh Climate Conditions

Abstract

A novel hybrid geothermal energy storage was modelled, built and started to monitor in Central Finland. Large amount of waste heat is produced by compressors in the industrial process. Waste heat is collected and transformed to the optimized and monitored borehole thermal energy storage (BTES) which is located under oat field. Solar heat is also used to charge the heat storage at summer time. Primary utilizer of waste and stored heat is a local swimming bath near the factory. Ground temperatures during the operation will be monitored continuously. Hence it is possible to optimize BTES charging and utilization even during operation. The BTES was planned and optimized using Comsol Multiphysics modelling software. Undisturbed average ground temperature is approximately 5 °C on the area. Ground is charged at the fluid temperature of 70 °C and utilized until temperature will drop to 20 °C. Charging time is 24 hours for 150 days followed by 120 days of energy utilization time. The BTES is monitored by temperature sensors located in the borehole heat exchangers and optic measuring cable which is installed to the top and the bottom of the topmost insulation layer. As a result the most optimal BTES solution was considered to be hexagonal shape energy well field which has 61 energy wells to the depth of 45 meters. The distance between boreholes is approximately 2.5 m. The BTES consist four energy well circles which will be charged and discharged according to the current ground and waste heat fluid temperatures. All wells are equipped with 40 mm borehole heat exchanger pipes with thickened wall. Due to cold climatic conditions insulation layer is needed for the uppermost part of the BTES. This configuration provides that the temperature may rise approximately to 67 ° in the middle of BTES and it is possible to utilize instantaneous power of 150 kW. The BTES efficiency will increase every year and will achieve the level of 60% during ten years of operation. Due to the short operating time only very preliminary field data is available. According to the field measurement the subsurface temperature evolution is close to the values predicted by the simulations on the middle part of the BTES but on the peripheral area measured temperature is lower than simulated. Our simulation and very preliminary temperature measurements reveals that it is possible to plan and built efficient BTES on the harsh climatic condition. The knowledge of local thermogeological factors is essential. Climatological requirements has to be noticed when planning, construct and operate the system. More information from temperature measurements and temperature sensor development work is needed in the future. It is also important to report different international underground energy storage projects to increase the knowledge of geothermal storage possibilities and hence increase the heat recycling to reduce greenhouse gas emissions.