and integrate high shares of variable renewable energy into the district heating by providing high flexibility to the electricity system. According to the review by Lund et al. those systems have to have the following abilities: "1. Ability to supply low-temperature district heating for space heating and domestic hot water (DHW) to existing buildings, energy-renovated existing buildings and new low-energy buildings." "2. Ability to distribute heat in networks with low grid losses." "3. Ability to recycle heat from low-temperature sources and integrate renewable heat sources such as solar and geothermal heat." "4. Ability to be an integrated part of smart energy systems (i.e. integrated smart electricity, gas, fluid and thermal grids) including being an integrated part of 4th Generation District Cooling systems." "5. Ability to ensure suitable planning, cost and motivation structures in relation to the operation as well as to strategic investments related to the transformation into future sustainable energy systems". Compared to the previous generations the temperature levels have been reduced to increase the energy efficiency of the system, with supply side temperatures of 70 °C and lower. Potential heat sources are waste heat from industry, CHP plants burning waste, biomass power plants, geothermal and solar thermal energy (central solar heating), large scale heat pumps, waste heat from cooling purposes and data centers and other sustainable energy sources. With those energy sources and large scale thermal energy storage, including seasonal thermal energy storage, fourth generation district heating systems are expected to provide flexibility for balancing wind and solar power generation, for example by using heat pumps to integrate surplus electric power as heat when there is much wind energy or providing electricity from biomass plants when back-up power is needed. Therefore, large scale heat pumps are regarded as a key technology for smart energy systems with high shares of renewable energy up to 100% and advanced fourth generation district heating systems. Fifth generation/cold district heating A fifth generation district heating and cooling network (5GDHC), also called cold district heating, distributes heat at near ambient ground temperature: this in principle minimizes heat losses to the ground and reduces the need for extensive insulation. Each building on the network uses a heat pump in its own plant room to extract heat from the ambient circuit when it needs heat, and uses the same heat pump in reverse to reject heat when it needs cooling. In periods of simultaneous cooling and