HSZG scientists are researching green district heating with partners, extracting thermal energy from rivers and lakes.
Generating sustainable heat for tomorrow. How can this have anything to do with small mountains of snow? In Zittau and Weißwasser, scientists are researching green district heating by extracting heat from rivers and lakes and returning the cooled water, sometimes as a mixture of water and ice. "The water-ice mixture does not have to be returned to the water, but can also run freely down the embankment. As ice is lighter than water, it floats up so that snow accumulations form through separation," explains Thomas Gubsch.
And it is precisely these that have adorned the banks of the Mandau in recent weeks. In a hall provided there by Stadtwerke Zittau GmbH, research was carried out diligently until the end of November as part of the AQVA-HEAT project. Thomas Gubsch is a researcher at the Institute of Process Engineering, Process Automation and Metrology (IPM ) and is working on a joint project with the Institute of Air Handling and Refrigeration from Dresden and the BTU Cottbus-Senftenberg to develop and test a vacuum liquid ice generator and examine its ecological impact on the water.
The aim is to efficiently generate and use heat from surface water all year round. This heat is to be supplied to district heating networks and neighborhoods, for example.
A very special system from the Dresden refrigeration engineers is available for this purpose. Using a vacuum chamber, water is brought to boiling point at temperatures around freezing point, the resulting energy-rich water vapor is extracted and then condensed again. The heat is then supplied to a central large heat pump or to several decentralized heat pumps and raised to the required temperature level.
What exactly is so innovative about this project? And what future plans do the researchers have? In this interview, Thomas Gubsch gives an overview of the process and talks about the prospects for the next phase of the project.
Mr. Gubsch, surface water from rivers and lakes as an alternative heat source. How efficient do you think this method is?
The vacuum liquid ice principle used enables surface water to be tapped as a year-round heat source regardless of the temperature. Initial findings from the short-term test show a high level of efficiency; in contrast to a conventional heat pump process, heat can be provided just as predictably and reliably, especially at low water temperatures. Systems in operation are mostly used to cover the summer base load, whereas high heating loads in the cold season represent the major challenge to achieving the energy transition in the heating sector. The AQVA-HEAT concept can be an important building block here.
Can you briefly explain the principle behind it?
The river water is pumped to the test facility on the premises of Stadtwerke Zittau GmbH using a submersible pump installed in the suction basket. The water then enters an almost airless vacuum chamber. The prevailing pressure in this chamber corresponds to air pressure at an altitude of approx. 28,000 to 35,000 meters. The special physical properties of water are used here - under vacuum conditions, it can exist simultaneously as a liquid, vapor and ice. A specially developed compressor draws in steam from the water supply, compresses it and then condenses it, whereby the steam transfers the heat to a downstream water circuit or heat pump. The cooled river water, partly as a mixture of water and ice, is then fed back into the river.
The use of heat pumps and the behavior of water under sub-atmospheric conditions has already been tested. What makes the project so special?
The special thing about this concept is that the vacuum liquid ice process is being used for the first time additive-free via an open circuit on a body of water. In addition to the high efficiency in tapping the heat source (water), the system can be used all year round, as latent heat in the form of ice can also be used in addition to sensitive cooling. The efficiency can be further increased through the parallel use of cooling.
What significance would this type of heat generation have for a former coal-mining region that has been supplied with district heating from lignite-fired power plants for decades?
The technology can help to gradually convert existing district heating structures into green, renewable networks. In addition to the transformation of the heat generation and distribution structure, the creation of new value chains is to be driven forward by a regional interest group and a contribution made to structural change in our region.
How is the collaboration with the Institute of Air Handling and Refrigeration in Dresden structured?
We developed the joint project idea together right from the start. The close cooperation with Mr. Steffan and his colleagues in Dresden is now bearing fruit for the first time and gives us confidence that we will be able to further develop this technology and ultimately provide a marketable overall system for heat generation based on water heat.
Does the colder water returned to the Mandau have an impact on the river?
Dr. Lessmann's colleagues at BTU Cottbus-Senftenberg carried out an aquatic ecology monitoring study to find out. In addition to recording series of measurements on temperature distribution and oxygen concentration before and after the reintroduction of river water, growth carriers were laid out in the Mandau and examined for possible changes after colonization. Initial evaluations show the minimally invasive influence of the technology on the water.
The next project phase is expected to start next year in order to turn the idea into a future technology. What are your plans?
The second project phase of the AQVA-HEAT project will start next year. The entire process is to be developed, procured and installed at the Zittau and Weißwasser sites. This investment measure will be followed by two to three years of long-term testing of the entire system, including water ecology monitoring. At the Weißwasser site, the municipal utility's administration building is to be supplied with heat from a nearby pond. At the same time, the supply of heat to the municipal district heating network in Zittau will be tested, thus contributing to the future heat supply based on water heat. In addition to the existing partners, the accompanying research will also be supported by colleagues from the newly founded Fraunhofer IEG. The growing network will also be strengthened through cooperation with regional and national component and system providers.
What are your hopes for the future?
For the future, I hope that we as researchers can actively work on new solutions to overcome the upcoming challenges of a secure and resource-saving energy supply. With the AQVA-HEAT system, part of the required heat can be provided from renewable sources based on water heat in the future.
The AQVA-HEAT project of the Zittau/Görlitz University of Applied Sciences, the ventilation and refrigeration engineers from Dresden, the Brandenburg University of Technology Cottbus-Senftenberg, the Zittau and Weißwasser municipal utilities and the district of Görlitz started in March 2021 after several years of preparation and will enter the second and third project phases in 2023.