Long-term heat storage opportunities of renewable energy for district heating networks

Abstract

This work compares different thermal heat storage solutions existing in the market, fueled with energy from different renewable energy sources with a focus on integrating it into the district heating grid. The paper bases a case on the municipality of Silkeborg, which has the largest solar thermal panel plant in northern Europe. It has been possible to collect data on the produced energy for the municipality of Silkeborg from different renewable energy sources. We used a theoretical approach to compare with assumed excess power from wind and solar in the DK1 area, with Silkeborg's allocated excess power at 0.01%. This yielded overall efficiencies between η = 0.739 - 0.765 and η = 0.864 - 0.895 for the Silkeborg solar thermal plant. These data have been utilized in the analysis, where the journey from harvesting the renewable energy until it reaches the end consumer, comparing the different solutions. Four different thermal heat storage solutions were compared: Tank Thermal Energy Storage, Pit Energy Storage, Aquifer Thermal Energy Storage, and Borehole Energy Storage (BTES). The analysis showed that the best solution for storing thermal energy for a longer period of time was the BTES out of the four solutions compared, with the lowest heat loss rate of 0.6% and the highest efficiency of up to 89.5%. However, some complications make it difficult to establish a BTES storage solution, since it is very much dependent on the earth's conditions and initial capital.