5th generation district heating and cooling networks in Europe
A recent generation of district heating and cooling networks is developing in Europe, using low temperature renewable energy heat sources (such as shallow geothermal).
An increasing number of 5th generation district heating and cooling networks (5GDHC) have been built in Europe in the past few years. According to review articles, these systems have been installed mainly in central Europe where the temperate climate conditions facilitate the energy shift between heating and cooling seasons. Future investigation is needed to adapt this technology to warmer and cooler regions of the world.
Definition of 5th Generation district heating and cooling networks (5GDHC) [2-4]
5GDHC networks are decentralised, bidirectional network systems with a direct exchange of warm and cold return flows. They operate at temperatures so close to the ground that they are not suitable for direct heating purpose. There is an exchange of thermal energy between buildings with different needs. The principal grid carries a low temperature flow (up to 20 °C) to substations which upgrade the temperature to the required level.
The low temperature of the carrier medium allows direct exploitation of industrial and urban excess heat and the use of renewable heat sources at low thermal exergy content. The possibility of reversing the operation of customer substations permits the heating and cooling demands of different buildings to be covered simultaneously and with the same pipes.
Compared with previous generations of district heating networks, heat losses hardly occur in 5GDHC since the temperature levels are close to soil temperatures. Heat gains can even be achieved if geothermal systems are used as a heat source. Various low-grade renewable energy sources can be used as heat sources for 5GDHC, such as shallow geothermal, industrial waste flows, conversion waste, waste from cooling processes, sewage, etc.
5GDHC also have disadvantages. For instance, high volume flows are required due to the low small temperature difference between the pipes (3–6 K). Moreover, large pipe diameters are required to keep the pressure loss in the system as low as possible, which leads to higher investment costs.
Case study: 5GDHC with geothermal collector and air-cooled chiller plant in Germany 
In 2020, a large-scale geothermal collector system was built in “Neustadt am Rübenberge”, Germany. The geothermal collector area was installed on a surface of 3000 square metre at a depth of 1.50 m below a rainwater retention basin. The annual heat energy required in the construction area is supplied by the geothermal collector area in combination with an air-cooled chiller plant and the 5GDHC. The 5GDHC supplies about 100 single-family and multi-family houses. Given the size of the project, the network was also planned with central feeding pumps located in an energy centre.
The two heat sources (geothermal collectors and air-cooled chiller plant) are hydraulically connected and therefore enable the shifting of energy loads. Together with the 5GDHC, the consumers are supplied with heating and cooling energy. When the air temperature is higher than the network temperature (for instance in spring when the air temperature exceeds 15 °C), heat can be extracted from the air via the heat transfer surface of the air-cooled chiller plant. During the warmer months, the 5GDHC and the neighbourhood are supplied with heat from the air. No heat is extracted during the summer, which contributes to the natural regeneration of the collector. Meanwhile, the network provides free cooling at a directly usable temperature level. According to a previous study, the optimum temperature for free cooling is around 14 °C.
Other pilot projects have been presented here.
 Interreg North West Europe (NWE) D2Grids project. https://5gdhc.eu/
 Zeh R, Ohlsen B, Philipp D, Bertermann D, Kotz T, Jocić N, Stockinger V. Large-Scale Geothermal Collector Systems for 5th Generation District Heating and Cooling Networks. Sustainability. 2021; 13(11):6035. https://doi.org/10.3390/su13116035
 The 5 principles of 5th generation district heating and cooling. D2Grids. https://www.construction21.org/articles/h/d2grids-the-5-principles-of-5th-generation-district-heating-and-cooling.html
 Buffa, S., Cozzini, M., D’antoni, M., Baratieri, M., & Fedrizzi, R. (2019). 5th generation district heating and cooling systems: A review of existing cases in Europe. Renewable and Sustainable Energy Reviews, 104, 504-522. https://doi.org/10.1016/j.rser.2018.12.059