Seasonal Low Temperature Borehole Thermal Energy Storage - Utilizing excess heat for district heating in Gothenburg.

Abstract

Occasionally during summer the heat load in the district heating (DH) network of the Gothenburg region exceeds the heat demand of the customer, i.e. there is an excess of heat in the DH-network. Nowadays, this excess heat is cooled against a river or in a cooling tower at a waste incineration plant. The aim of this study is to store this excess heat in the ground instead of cooling it against a river and use this excess heat during occasions when the heating demand is higher, during winter. This excess heat will be stored in a borehole thermal energy storage (BTES). The studied BTES will be connected to existing heat pumps at Ryaverket owned by Göteborg Energi AB. The conditions, regarding temperature and volumetric ow rate, of the heat source of the heat pumps, treated sewage water, will be improved with the aid of a BTES. When these previously mentioned conditions of the sewage water are improved more heat can be generated by the heat pumps. An increased heat generation by the heat pumps will replace heat generated by other, more expensive, heat generation units. To evaluate the economic profitability of such a system the net present value has been calculated. Designs for BTES, storing 50GWh and 25GWh of waste heat, are found with the aid of a software named GLHEpro and the investment cost for these designs are calculated. To reach economic profitability for this project the savings made by this new system,when heat generated by the heat pumps is increased, should meet the extent of the investment cost. To be able to calculate the savings, a software named Martes is used. The investment cost of a BTES is ten times larger than the savings ever will be in the most probable scenario regarding the investment cost of a BTES. Economic profitability is only reached if the investment cost of the BTES is in the minimum price range, if subsidies to cover 40% of the investment cost from Horizon2020 is gained, if an interest rate of 5% is used for economic calculations and when the availability of the heat pumps is increased by 15%. Economic profitability can also be gained for a scenario when the prices for the investment cost are in the minimum price range and an interest rate of 0% is used for economic calculations. It seems rather unlikely to gain all these privileges for this case study to become economically profitable.