District cooling networks
District cooling networks are a promising and efficient technology to provide cooling energy in districts.
What is district cooling?
District cooling networks are technically similar to district heating networks, but operate at a different temperature level: they transport cold water from a central generation unit (energy hub) to the decentralized consumers via pipe systems laid in the ground consisting of a supply and a return. The advantage of district cooling networks over a decentralized cold supply is that the generation of cold by large plants at a central location is more efficient and economical. The disadvantage is that there are losses, i.e. heat inputs, in the pipe network between the central producer and the decentralized consumer, which reduces the efficiency of the systems. For this reason, the pipe system must generally be insulated, also to prevent condensation.
Difference between district cooling and 5GDHC networks (anergy networks)
5GDHC networks can both provide heat in winter and supply buildings with cold in summer. Technically, the two types of networks are very similar. 5GDHC networks tend to operate at a higher temperature level, usually above 10°C, whereas district cooling networks operate at a temperature level that provides sufficiently low temperatures for the building energy system for all consumers (< 8°C). In 5GDHC networks, heat pumps are installed in the buildings and the primary purpose of the networks is to provide heat. Both network types have in common that high volume flows occur due to the low temperature difference between supply and return, which requires large pipe diameters.
How common are district cooling networks?
Currently, district cooling networks are mostly found only in larger cities. In recent decades, several major European cities have built district cooling networks, such as Barcelona, Amsterdam, Stockholm, Vienna and Paris. Smaller central areas in Berlin are also supplied by a district cooling network. Some universities, such as RWTH Aachen (Germany), also operate their own district cooling network for air conditioning large lecture halls or cooling central computers. The reason why district cooling networks are still not widespread is the high investment costs for setting up the network. Especially if mainly air-conditioning is provided, which has only a few hundred hours of full use per year in many region of Europe, the installation is often not yet economical. Nevertheless, district cooling networks are considered to have great potential in future energy systems, since the demand for cooling will continue to increase in the future, the comfort requirements for room air conditioning are rising, and decentralized air conditioning systems on facades and balconies are becoming less and less accepted for visual and acoustic reasons. Furthermore, the proportion of partially or fully glazed office facades is increasing, and internal heat loads in increasingly mechanized offices are steadily rising. The rising cooling demands mean that district cooling networks are reaching economic viability for more and more areas in cities.
How are district cooling networks supplied?
Central cooling is provided by compression and absorption chillers. Free cooling with the outside air is also possible at certain times. Absorption chillers use heat at a high temperature level (> 100 °C) to generate cold. In summer, heat at a high temperature level can be provided by fossil generators, such as combined heat and power plants, or solar thermal fields. Compression chillers, on the other hand, are electrically powered. Due to the lower investment for compression chillers, they are mostly provided as peak load generators and absorption chillers for base load coverage.
- Rutz et al.: Modulare, erneuerbare Nahwärme- und Kältenetze. Ein Handbuch. WIP Renewable Energies, 2017.