but relatively low-cost in-ground insulated reservoirs or borehole systems. The expected heat loss at the 203,000m³ insulated pond in Vojens is about 8%. With European countries such as Germany and Denmark moving to very high levels (80% and 100% respectively by 2050) of renewable energy for all energy uses there will be increasing periods of excess production of renewable electrical energy. Heat pumps can take advantage of this surplus of cheap electricity to store heat for later use. Such coupling of the electricity sector with the heating sector (Power-to-X) is regarded as a key factor for energy systems with high shares of renewable energy. Heat distribution After generation, the heat is distributed to the customer via a network of insulated pipes. District heating systems consist of feed and return lines. Usually the pipes are installed underground but there are also systems with overground pipes. The DH system's start-up and shut downs, as well as fluctuations on heat demand and ambient temperature, induce thermal and mechanical cycling on the pipes due to the thermal expansion. The axial expansion of the pipes is partially counteracted by frictional forces acting between the ground and the casing, with the shear stresses transferred through the PU foam bond. Therefore, the use of pre-insulated pipes has simplyfied the laying methods, employing cold laying instead of expansion facilities like compensators or U-bends, being so more cost effective. Pre-insulated pipes sandwich assembly composed of a steel heat service pipe, an insulating layer (polyurethane foam) and a polyethylene (PE) casing, which are bonded by the insulating material. While polyurethane has outstanding mechanical and thermal properties, the high toxicity of the diisocyanates required for its manufacturing has caused a restriction on their use. This has triggered research on alternative insulating foam fitting the application, which include polyethylene terephthalate (PET) and polybutylene (PB-1). Within the system heat storage units may be installed to even out peak load demands. The common medium used for heat distribution is water or superheated water, but steam is also used. The advantage of steam is that in addition to heating purposes it can be used in industrial processes due to its higher temperature. The disadvantage of steam is a higher heat loss due to the high temperature. Also, the thermal efficiency of cogeneration plants is significantly lower if the cooling medium is high-temperature steam, reducing electric power generation. Heat transfer oils are generally not used for