Transient modeling of a high temperature borehole thermal energy storage coupled with a combined heat and power plant

Sammanfattning: Coupling High-Temperature Borehole Thermal Energy Storages (HT-BTES) with existing Combined Heat and Power (CHP) systems is a promising approach to increase energy efficiency of district energy systems through recovery of otherwise wasted heat. This solution is currently being discussed in Sweden by the company Tekniska Verken in Linköping AB, for storing waste heat from their CHP operation in summer in a HT-BTES and to utilize it during peaks in winter. This would increase the flexibility between energy supply and demand in one of their plants. The available supply temperature during charge of the BTES is around 95C. There is, though, still limited experience of HT-BTES operation with just a few installations throughout the world.   The aim of this Master´s thesis has been to evaluate a potential system design configuration for effective extraction and storage of waste heat from the Gärstadverket CHP-plants in connection to a HT-BTES. Data from previous operation of the CHP-plants and an existing TRNSYS model, developed at KTH and Bengt Dahlgren AB based on the well-known DST approach (Duct Ground Heat Storage Model), was used as a starting point to the development of a new, more complete model that includes a heat pump. The heat pump model was developed from manufacturer’s data for a non-standard 50 MW heat pump system using R717 as refrigerant. As an additional objective, design and operational experience of already existing HT-BTES installations has been compiled and analyzed.   The BTES design were simulated with varied number of boreholes and borehole depth. The system was furthermore simulated with two different borehole heat exchangers (BHEs): double U-pipes and coaxial. Based on the results three optimized designs were found: 1 400 boreholes with double U-pipes and a borehole depth of 300 m, 1 300 boreholes with coaxial BHEs and a borehole depth of 300 m, and a design with 1 500 boreholes and 275 m borehole depth – all three designs with a borehole spacing of 5 m and with loops of 3 boreholes connected in series. The three BTES designs showed similar results with a potential to store around 107 GWh/year and to extract around 93 GWh/year with the use of a GSHP. The resulting discharge temperature from the BTES ranges between 40-60C, and up to 70C in the initial discharge period in the tenth simulation year. Further investigation is though needed regarding if there are any coaxial BHE available on the market that can work with the high temperatures in the BTES. Coupling High-Temperature Borehole Thermal Energy Storages (HT-BTES) with existing Combined Heat and Power (CHP) systems is a promising approach to increase energy efficiency of district energy systems through recovery of otherwise wasted heat. This solution is currently being discussed in Sweden by the company Tekniska Verken in Linköping AB, for storing waste heat from their CHP operation in summer in a HT-BTES and to utilize it during peaks in winter. This would increase the flexibility between energy supply and demand in one of their plants. The available supply temperature during charge of the BTES is around 95C. There is, though, still limited experience of HT-BTES operation with just a few installations throughout the world.   The aim of this Master´s thesis has been to evaluate a potential system design configuration for effective extraction and storage of waste heat from the Gärstadverket CHP-plants in connection to a HT-BTES. Data from previous operation of the CHP-plants and an existing TRNSYS model, developed at KTH and Bengt Dahlgren AB based on the well-known DST approach (Duct Ground Heat Storage Model), was used as a starting point to the development of a new, more complete model that includes a heat pump. The heat pump model was developed from manufacturer’s data for a non-standard 50 MW heat pump system using R717 as refrigerant. As an additional objective, design and operational experience of already existing HT-BTES installations has been compiled and analyzed.   The BTES design were simulated with varied number of boreholes and borehole depth. The system was furthermore simulated with two different borehole heat exchangers (BHEs): double U-pipes and coaxial. Based on the results three optimized designs were found: 1 400 boreholes with double U-pipes and a borehole depth of 300 m, 1 300 boreholes with coaxial BHEs and a borehole depth of 300 m, and a design with 1 500 boreholes and 275 m borehole depth – all three designs with a borehole spacing of 5 m and with loops of 3 boreholes connected in series. The three BTES designs showed similar results with a potential to store around 107 GWh/year and to extract around 93 GWh/year with the use of a GSHP. The resulting discharge temperature from the BTES ranges between 40-60C, and up to 70C in the initial discharge period in the tenth simulation year. Further investigation is though needed regarding if there are any coaxial BHE available on the market that can work with the high temperatures in the BTES.