The integration of pit storage in Borlänge’s district heating network

A majority of today’s heating in Sweden is done by district heating. The district heating of the future should follow the energy transition, where sustainable fuels play a key role. The district heating of the future should also be flexible and able to meet heating demands in a cost-effective way. Pit Thermal Energy Storage, PTES, is a storage method that can be implemented in district heating grids and help in this transformation. Heat storage makes cost optimization possible by replacing expensive production with cheaper stored heat. From a sustainability view, heat storage can also make it possible to replace unsustainable fuels. This study investigates installation of a PTES in the main district heating grid of Borlänge, both fitting placement in relation to the grid as well as an economic analysis. The fuels used for district heating in Borlänge is mainly made up of waste and biofuels, with biofuels expected to rise in price the coming years. The report is limited to only analysing PTES with electric heating, where the storage is used to replace biofuel burning production during favourable conditions. The results of the study show that heating of PTES with electric heating is not economic at the current prices. Over the last years the price of biofuel has been increasing quickly. From year 2022 to 2024 the price of wood chips in Sweden increased with 76 %. A continued increase of biofuels of 60 % could cause integration of a PTES to lead to savings of around three million SEK per year. The payback period (without discount rate) of the PTES is at least 37 years. With an 80% increase of biofuel prices the payback period is 20 years, which is the expected lifetime of the PTES (about 20 years). These results are based on tests with 30 MW charge and discharge power. Considering the discount rate, the payback period is significantly longer than the lifetime of the PTES. The result is based on the district heating grids production values for 2023 and 2024, as well as the electricity price for the same period. The result is based on a literature study, simulations, and calculations. In the report calculations with different assumptions for biofuels prices is performed. Simulations show that the maximum charge power and discharge power is limited to 5 MW -10 MW with current infrastructure. This is because the placement of the storage causes the pressure differences to be the limiting factor. This power limits the usage of the storage as well as its financial viability. Higher discharge power is preferrable because it increases the number of charge cycles. A storage with higher discharge power also gives the grid more resilience during maintenance for other production facilities. The costs/savings is dependent on how the electricity prices as well as the biofuel prices will look like in the future. It is possible to speculate how these prices will change, but one conclusion is that a PTES in combination with electric heating is economically risky at the moment.