District heating (DH) systems are one of the most energy efficient heating systems in urban environments, with proven reliability within many decades already. District heating systems are identified as key systems to achieve the de-carbonization of heating energy in European Cities.
Renewable and waste heat sources are foreseen at the same time as decarbonized heat sources and the way to guarantee competitive energy costs with limited influence of fossil fuel supply price volatility. To achieve this, a transition is needed in DHs, comprising not only measures to improve overall performance (temperature level reductions, improvement of substations, etc.), but to guarantee system viability as a whole in a context of reduced heat loads with the transition to Near Zero Energy Buildings (NZEB).
A joint initiative of 14 industrial companies and research institutes across from various countries in Europe (RELaTED) deploys a decentralized Ultra-Low Temperature (ULT) district heating network concept, which allows for the incorporation of low-grade heat sources with minimal constraints, larger shares of renewable energy sources (RES) and distributed heat sources. ULT DH reduces operational costs due to fewer heat losses, better energy performance of heat generation plants and extensive use of de-carbonized energy sources at low marginal costs.
In the transition towards NZEB and Plus Energy Houses (PEH), RELaTED allows for a prosumer scheme, where positive buildings deliver energy to the grid.
Limitations of current district heating networks
DH systems were designed many decades ago. In most cases, they are designed and operated to distribute heat at about 80 °C to consumers. Their capacity to reduce operational temperatures is related to radiator capacity to deliver sufficient heat to meet comfortable temperatures in buildings and to allow for the safe preparation of Domestic Hot Water (DHW) preparation. DHW limits potential temperature reductions due to the need to avoid legionella-related issues. Depending on specific national regulations, storage temperatures in the range of 55-75 ºC are prescribed.
Overall RELaTED concept
RELaTED pursues the development of DH networks with service temperature levels as low as 40-50 ºC. In many alternatives, traditional DHW preparation methods are substituted by “innovative methods”. In these concepts, mains water is primarily heated by the DH, and then complemented by electric heaters/boosters up to the required temperature levels. In more advanced alternatives, heat pumps are used for such purposes.
In RELaTED every single building is converted into an energy node, where socalled triple function substations (3FS) allow for bi-directional heat exchange between the building and the network, with the additional functionality of grid injection of excess local solar heat. In fact, adaptations are made to Building Integrated Solar Thermal (BIST) systems to adapt them to Low Temperature (BILTST), with reduced local storage, as the connection to the DH makes it redundant.
Additionally, District-heating connected Reversible Heat Pump systems (DHRHP) allow for recovery of exhaust heat from cooling applications (e.g. air conditioning, ventilation, etc.).
Ultra-Low Temperature District Heating
Even before the consideration of further technological improvements, Ultra-Low Temperature (ULT) temperature levels substantially improve the performance of heat production systems. Furthermore, ULT allows for the integration of virtually any waste heat source from industry, sewage, etc.
RELaTED builds a top of the existing trend for integration of large solar thermal plants systems in DH networks, some of them comprising large seasonal storage systems. RELaTED incorporates large ST plants, but also provides the framework for the integration of BIST into the main ULT DH concept.
With lower fluid temperature when compared with regular BIST integration levels, performance levels are expected to rise by 20%, due to lower heat loses. An additional 80% rise is calculated when avoiding local storage due to direct DH connection. The RELaTED ULT network acting as a perfect heat sink avoids storage stagnation situations, thus allowing for larger ST performance levels.
DHRHP systems allow for the de-coupling of temperature levels in DH network and building level HVAC systems. With the DH as heat source, stable temperatures at 35-40 ºC ensure stable COP levels of 6-7 for the DHRHP allyear-round. These units provide an economic way for the preparation of DHW, while at the same time allowing for the connection of buildings with higher temperatures in their HVAC design (i.e. older buildings).
The RELaTED concept, when implemented with a substantial share of RES provides a robust framework to ensure the economic viability of DH networks, in the context of the transition of the building stock to NZEB along the following decades.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 768567.