SolKaN2.0

Up to now, a mixture of water and antifreeze has been used in so-called cold district heating networks to distribute the heat from central heat sources or a thermal energy storage with outlet temperatures below the freezing point of water to the individual consumers. However, the use of a mixture of water and antifreeze as heat transfer medium in the cold district heating network has both, economic and ecological disadvantages compared to the use of water as heat transfer medium.

The aim of the research project ‘SolKaN2.0’ is therefore to develop a new or second generation of solar cold district heating systems in which water is used as the heat transfer medium in the cold district heating network instead of a mixture of water and antifreeze, as it is the case today. This can reduce investment costs by more than 20 % and operating costs by around 10 %.

However, the use of water as a heat transfer medium results in several technical challenges for which adequate solutions must be developed. Particularly the hydraulic integration of a thermal energy store and the solar thermal air-brine-collectors used as the central heat source, which utilize solar radiation and ambient heat.

In the SolKaN2.0 concept an ice store is used as a thermal energy store. This is primarily characterized by a relatively high effectively usable specific heat capacity in the relevant temperature range due to the phase change of water. In order to utilize this advantage, the ice store must be discharged via an additional centrally positioned heat pump, which operates in the temperature range from approx. - 10 °C to + 10 °C. The use of this central heat pump ensures that the temperature level in the cold local heating network is always above 0 °C and that water can therefore be used as the heat transfer medium. The cold district heating network then serves as a heat source for the decentral heat pumps installed in the individual buildings, which raise the heat to a usable temperature level. The aim is to maintain or increase the system efficiency despite the use of an additional central heat pump. To ensure this, the efficiency optimization of the central heat pump for nominal and partial load conditions is of crucial importance. The aim is to use natural refrigerants as the working fluid in the heat pump. The investigation, including the development, realization and testing of the central heat pump, is a focus of the ‘SolKaN2.0’ project.

Other main aspects are the development of a control strategy for the overall system including the central and decentral heat pumps, the implementation and testing of the newly developed SolKaN2.0 concept in a real-scale experimental system and the ecological and economic evaluation of the SolKaN2.0 concept.