District heating and cooling networks
District heating and cooling networks play an important role in the sustainable and emission-free supply of districts, since they enable the integration of renewable heat sources and the thermal energy exchange between buildings.
What is district heating and cooling?
District heating networks consist of a supply and return pipe, which are usually installed in the ground and transport hot water from a heating center (energy hub) to buildings. In heat exchangers, the hot water is used to provide space heating or domestic hot water. The water is cooled down in heat exchangers and flows back to energy hub, where it is heated again. District cooling networks operate in a similar way: Cold water (usually below 10 °C) is transported in a supply pipe to the buildings and used for air conditioning or process cooling. The slightly heated water then flows back to a cooling center or energy hub, where it is again cooled down to the supply temperature. In the energy hub, the water is cooled down by compression chillers or absorption chillers. In recent years, in addition, more and more fifth-generation district heating and cooling networks are planned and installed. These networks have a warm and cold line and can provide both heating and cooling energy.
Which types of district heating networks exists?
Heating networks are often subdivided into different generations: The first heating networks were steam networks (1st generation), which transported hot steam in pipes from an energy hub to the buildings. Nowadays, this type of heat network is not installed anymore, because high heat losses occur in the pipe network due to the high steam temperatures. However, this type of heating network is still in operation in some cities, such as New York City. During the 20th century, more and more heating networks with pressurized water at temperatures of around 100 °C (3rd generation) have been installed. These networks are still in operation today and in some cases are still being constructed. A modern generation of heat networks are 4th generation district heating networks (low-temperature district heating). These operate at lower temperatures of around 70 °C. Lowering the flow temperature enables new heat sources, such as solar thermal, to be integrated and reduces distribution losses. The latest development are so-called 5th generation district heating and cooling networks (5GDHC), which are are also known as anergy networks. They do not have a defined flow and return pipe, but consist only of a warm and cold pipe. The advantage of these heating networks is that they can provide both heat and cold with just one network (2 pipes). Furthermore they enable the use of ambient heat, such as heat from river water, near-surface geothermal energy or waste water. There are also alternative classifications of heating and cooling networks.