Solar district heating
District heating networks with large-scale solar thermal systems are a promising and proven concept for the decarbonization of heat supply. Large seasonal heat storages enable the temporal shift surplus heat supply from summer to winter.
What is solar district heating?
Solar district heating networks use large areas with solar thermal collectors as a heat source. The concept is also known as solar district heating (SDH). The technology of solar district heating networks has been proven for years and it can make an important contribution to decarbone the heat supply. Unlike environmental heat, such as from rivers, near-surface geothermal, or low-temperature waste heat sources, solar thermal systems can provide high temperatures of 90 °C and above. This enables the decarbonization of existing urban district heating networks without the need to lower flow temperatures or use large-scale heat pumps.
Seasonal heat supply
A decisive disadvantage of solar district heating networks is the pronounced seasonality of heat generation. In winter, the yield of solar thermal collectors is significantly reduced, but the heat demand for heating is highest. In summer, on the other hand, a large heat supply is matched by a small demand (for domestic hot water). If solar thermal energy has only a small share of the heat supply of the district heating network, this is not a problem. Particularly in summer, it is even possible in this case to switch off fossil heat sources (combined heat and power plants or boilers) and to cover the heat supply for domestic hot water via the solar thermal field completely. However, if the solar thermal field represents a large share of the heat supply of the district heating network, storages must be installed to increase the utilization of the solar thermal collectors.
Seasonal heat storage in solar district heating networks
To shift the heat supply from summer to winter, seasonal heat storages are increasingly being planned. These are water-filled large basins with volumes of 100,000 m³ and more, also called pit thermal energy storage. In summer, excess heat is used to warm the water. In winter, the water basin is cooled down and the heat is used to heat the district. In addition, a heat pump can be installed to discharge the water in the basin to temperatures below the supply temperature of the district heating networks. Due to the small surface-to-volume ratio, storage losses are low even for seasonal storage. A pioneering role in this technology plays Denmark, where solar thermal fields have been planned and successfully operated for years in numerous projects.