Tallinn University of Technology

“One’s garbage is another’s treasure” certainly sounds like a circular economy catchphrase. However, recycling materials instead of disposing of them is currently of high priority, especially in light of limited resources and climate change. TalTech’s junior researcher Kertu Lepiksaar and engineer Siim Poom describe how the principles of circular economy can be applied in energy. 

ringmajandus energia energeetika kokkuhoid

One of the sustainable solutions we could use is energy cascades, which help to increase the energy efficiency of the district heating system and offer heating for more and more buildings. Anna Volkova, head of the research group for smart district heating systems and integrated assessment analysis of greenhouse gases emissions and professor at TalTech, explains in more detail how it works.

Recycling the same water

Energy cascades are a technical solution which connects district heating networks with different temperature levels. For example, the supply of district heating networks is usually 80-100 °C, depending on the size of the network and the temperature requirements of the building. The temperature of the backflow is 50-60 °C in winter and 40-50 °C in the summer period.

In the case of a conventional district heating network not utilising energy cascades, the backflow goes back to the boiler house or cogeneration plant to be heated again and sent on another round. Newer buildings with better energy efficiency which use more modern heating solutions, such as under-floor heating, can use heat transfer mediums with much lower temperatures. For example, a supply temperature of 55-60 °C would suit them well. If there are many such energy-efficient buildings with low-temperature heating systems, they can form an entire area of low-temperature district heating. These developments allow for the use of the energy cascade solution, where a low-temperature district heating area consisting of newer buildings uses the heat of a larger district heating network’s backflow. Simply put, water which has provided its heat to a large district heating network and is at around 55 °C is fed though a special connection into a smaller, low-temperature district heating system, supplying heat to buildings in this area as well, before being directed back to the boiler house to go on another round.

Advantages and disadvantages

Professor Anna Volkova and her team have researched this technical solution in depth and the results have been published in the prestigious scientific journal Energy in an article entitled Energy cascade connection of a low-temperature district heating network to the return line of a high-temperature district heating network. The article describes various solutions for using energy cascades.

The backflow temperature of a large district heating network is not always high enough to supply it directly to a smaller network. To this end, various solutions were analysed in articles, such as shunt connections and mixing units as well as heat pumps. The pros and cons of all solutions are explained in detail and their suitability and usability in networks have been assessed. In addition, the team analysed the economic efficiency of the proposed solutions and highlighted other useful aspects besides actual energy savings. For example, the efficiency of the boiler house and cogeneration plant increases slightly when the backflow temperature of district heating decreases.

The article attracted great interest

The article is of significant benefit to local district heating operators, developers and other entrepreneurs, such as Utilitas, Tallinn’s Old City Harbour, Bekker Port, KPMG and others. The article also piqued the interest of foreign research organisations, such as EuroHeat & Power and International Energy Agency District Heating and Cooling. In the beginning of 2022, the authors of the article were awarded Best Research Article of the Year by TalTech, and its results were used to write an application for a research project on the same topic. An article on the same topic entitled Cascade sub-low temperature district heating networks in existing district heating systems, published in the journal Smart Energy, provides a thorough overview of the examples and best practices regarding energy cascade use. It highlights factors that hinder or facilitate the implementation of energy cascades, which are based on a survey conducted among several energy companies and experts. It also introduces various technical aspects related to the use of energy cascades.

There’s more to be used!

Low-temperature district heating is a priority for Estonia and the development of the district heating sector, as it reduces losses in the network and therefore also the consumption of primary energy. Low temperatures in the network provide an opportunity to use various sources of waste heat in addition to burning fuels for energy, and thus reduce the consumption of primary energy even more.

For example, it is possible to use the waste heat of the industry and, in order to obtain additional heat, various natural heat sources, such as bodies of water and waste water. Using energy cascades to gradually connect small, low-temperature district heating networks to large networks would mean taking small, but definite, steps towards a modern and energy-efficient district heating system.