How do heating networks work?

Heating networks are closed cycles. The water used as a transport medium is first pumped to the recipient and then returned via a separate return pipe. The cables are typically made of plastic or metal and are well insulated to minimize the loss of energy. In the vast majority of cases, the pipes are laid underground.

The water in the heating network is typically transported at a temperature between 80 and 130 degrees Celsius. To protect the pipes from corrosion, decalcified and often additionally desalinated water is used. Depending on the size of the network, the water passes through pumping stations along the way, which ensure that transport volume and pressure remain stable.

Structurally, a distinction is usually made in district heating networks between the main transport routes and the end pipes. The terminations often form separable subnets that are fed from the main lines. This is particularly useful with regard to maintenance work. For example, in the event of local defects or necessary construction work, only small parts of the network have to be switched off.

Conventional district heating networks are fed by one or more large heat sources. Key sources include:

• Combined heat and power plants
• Incinerators
• Industrial plants

The heat generated in such systems is transferred via a heat exchanger to the water circulating in the circuit of the heating network. In conventional heating networks, the energy sources typically have to be very potent in order for the feed-in to be worthwhile. The background is that the flow temperature in the network is comparatively high.

In the meantime, however, more and more modern heating networks are being created in which even minor heat sources can be tapped. In particular, so-called cold heating networks can be fed with heat energy in a much more decentralised way.

With a view to climate and cost efficiency, the question also arises as to how high the energy losses are in a heating network. Typically, it is assumed that about eleven to 14 percent of the thermal energy is lost on the way to the end consumer. Most of it is lost in the last section of the network, i.e. after the warm water has left the main transport routes.

How efficient the specific heat transfer is therefore depends not least on how the infrastructure of the grid is structured.

By comparison, conventional heating systems based on combustion today usually have a local efficiency of 90 to 95 percent. However, this does not take into account losses that occur during the provision of the fuel.

In properties connected to the district heating network, so-called district heating transfer stations will be installed. With the help of a heat exchanger, heat energy is transferred from the district heating network to the local heating circuit. The water cooled in the heating network as a result is returned via a separate return pipe.

The transfer station replaces the conventional heating system in buildings equipped with it. Due to the fact that no combustion takes place in the local systems, district heating systems require much less maintenance than other systems. There is also no need for the chimney sweep to inspect the chimney.

The heat exchanger is also equipped with a heat meter. This measures how much energy has been taken from the heating network at the transfer station. This makes it possible to bill the heating energy used.

The so-called anergy network (cold district heating) or local or district cooling network is a special variant of the network concept. Instead of hot water with a temperature of 80 to 130 degrees Celsius, a "cold" heating network transports water at ambient temperature (10 to 30 degrees Celsius). This has the advantage that significantly more different heat sources can be used to heat the water to be transported.

However, heat pumps must be installed in the buildings to be heated. The heat pump then uses the anergy network as an energy source to bring the heating system in the building to the required temperature. Compared to heat pumps, which draw their energy from the earth, groundwater or ambient air, such systems work more efficiently. In addition, their installation is much easier.

Unlike "hot" district heating networks, such anergy networks can also be used as a cooling network at the same time, for example to supply cold stores or office buildings with cooling.

In principle, heating networks are of course only as climate-friendly as the energy source used to provide the heat. However, heating networks have the enormous advantage that heat from different sources can be used in them. This includes in particular waste heat that would otherwise be lost.

For example, waste heat from waste incineration plants, fossil fuel power plants or large industrial plants can be put to good use. But more unconventional sources can also be tapped for heating networks. In the London borough of Islington, for example, the thermal energy bound in wastewater is made usable for the heating network with the help of heat exchangers.

Another important option is the integration of so-called seasonal energy or heat storage systems into a district heating network. One of the major challenges in the transition to renewable energies is the unsteady availability of wind and solar energy. By storing energy in summer and releasing it as district heating in winter, this can be solved, at least in part.

Nothing changes for consumers connected to the grid when new heat sources are tapped. In the best case, this will allow a large number of end customers to be converted to climate-friendly heating energy at the same time.

Hoval: Your partner for district heating