District heating networks
District heating networks are an important technology for the decarbonization of heat supply, since they enable the integration of renewable heat sources and the thermal coupling of buildings in district energy systems.
What is district heating?
District heating networks are used to transport heat from a central heat generation plant (energy hub) to consumers. Two water-carrying pipelines are laid between heat generation and buildings: A flow pipe and a return pipe. In the flow pipe, hot water is pumped from the heating center to the buildings, which use the hot water in the heat exchanger to provide space heating or domestic hot water. In the heat exchanger, the water in the flow line cools down by around 10-30 Kelvin and is then transported via the return line back to the heating center, where it is heated again to the flow temperature. Conventional heating centers have fossil heat generators such as gas boilers or gas-fired combined heat and power units. To decarbonize heat generation, however, regenerative heat sources are increasingly being used, such as solar thermal energy or waste heat from industrial processes, which would otherwise be released unused into the environment.
Classification of heating networks
District heating networks are often divided into 5 different generations: The first generation was built from the end of the 19th century and was operated hot steam. An example of this type of heating network is the district heating network of New York City, which is still in operation. Nowadays, mostly 3rd generation or 4th generation district heating networks are built. 3rd generation district heating networks use pressurized hot water of around 100 °C in the supply line. However, high water temperatures lead to high heat losses, especially in summer when little heat is consumed. In 4th generation district heating networks, flow temperatures are around 70 °C. This enables the use of regenerative heat generators such as solar thermal energy, geothermal energy or waste heat from industrial processes and reduces heat losses in the distribution network. The latest development are so-called 5th generation district heating and cooling networks (5GDHC), which are also known as anergy networks. With the help of 5GDHC networks, it is possible to provide heating and cooling with a single network and to use waste heat from buildings with cooling demands to heat buildings with heat demands, thus partially balancing out heating and cooling demands in the districts.