Feasibility of underground seasonal storage of solar heat in Finland

The global challenge of climate change is urging for renewable energy sources to reduce greenhouse gas emissions. One of the applications of renewable energy is the solar district heating plant, which uses solar heat for space heating on a community scale. Seasonal storage of solar heat is crucial in solar district heating plants as a solution to the mismatch between the energy supply and demand. This is especially important in countries located in high latitudes with high levels of solar insolation and low heating demand in the summer, and low sun insolation and high heating demand in the winter time. This study investigates the selection of the most feasible method for seasonal storage of solar heat at high latitudes. The aim is to identify the key aspects of method selection and design of underground solar heat storage. Practices of underground thermal energy storage in Finland and other countries with similar ground conditions are reviewed. Five heat storage methods are evaluated based on their efficiency, cost, construction method, and suitability for typical ground conditions in Finland. The available methods of seasonal heat storage are compared, and the most promising method for solar heat storage in Finland is proposed. In addition, the potential provided by a combination of two or more methods is examined. The borehole storage method is proposed for storing solar heat in Finland. The combined method with a rock pit used for short-term heat storage and boreholes for long term storage is also an attractive and technically feasible option for Finnish ground conditions.