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Water as a year-round heat source
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 consider this method to be?
The vacuum liquid ice principle used here makes it possible to tap surface waters as a year-round heat source, regardless of temperature. Initial findings from the short-term test show high efficiency; particularly at low water temperatures, heat can be supplied just as reliably and predictably as with a conventional heat pump process. Operating systems are typically used to cover the summer base load, whereas high heating loads during the cold season pose the greatest challenge to achieving the energy transition in the heating sector. The AQVA-HEAT concept can be an important building block in this regard.
Can you briefly explain the principle behind it?
River water is pumped to the test facility on the grounds of Stadtwerke Zittau GmbH using a submersible pump installed in the suction strainer. The water then enters a vacuum chamber that is nearly airless. The pressure inside corresponds to atmospheric pressure at an altitude of approximately 28,000 to 35,000 meters. This process takes advantage of water’s unique physical properties—under vacuum conditions, it can exist simultaneously as a liquid, vapor, and ice. A specially developed compressor draws vapor from the water reservoir, compresses it, and then condenses it, during which the vapor transfers heat to a downstream water circuit or to a heat pump. The cooled river water, partly in the form of a water-ice mixture, is then discharged back into the river.
The use of heat pumps and the behavior of water under subatmospheric conditions have already been tested. What makes this project so special?
What makes this concept unique is that the vacuum liquid ice process is being used for the first time without additives in an open-loop system on a body of water. In addition to the high efficiency in tapping the heat source (water bodies), the system can be used year-round, as latent heat in the form of ice can be utilized in addition to sensible cooling. Efficiency can be further increased through the parallel use of cold.
What significance would this method of heat generation have for a former coal-mining region that has relied on district heating from lignite-fired power plants for decades?
This technology can help gradually transform existing district heating infrastructure into green, renewable networks. In addition to transforming the heat generation and distribution infrastructure, a regionally organized consortium aims to drive the creation of new value chains and contribute to structural change in our region.
How is the collaboration with the Institute for Air Conditioning and Refrigeration Technology in Dresden shaping up?
From the very beginning, we have jointly developed the collaborative project concept. The close collaboration with Mr. Steffan and his colleagues in Dresden is now bearing fruit for the first time and gives us confidence that we can further develop this technology and ultimately provide a market-ready, comprehensive system for heat generation based on water heat.
Does the colder water returned to the Mandau River have any impact on the river?
To address this, colleagues from BTU Cottbus-Senftenberg, led by Dr. Lessmann, conducted aquatic ecological monitoring. In addition to recording data series on temperature distribution and oxygen concentration before and after the river water was reintroduced, substrate samples were placed in the Mandau River and examined for possible changes after they had been colonized by organisms. Initial analyses show that the technology has a minimally invasive impact on the water body.
To turn this idea into a technology of the future, the next project phase is expected to begin next year. What are your plans?
Next year, the second phase of the AQVA-HEAT project will begin. During this phase, the entire process will be developed, procured, and installed at the sites in Zittau and Weißwasser. This investment initiative will be followed by a two- to three-year long-term trial of the entire system, including ecological monitoring of the water bodies. At the Weißwasser site, the municipal utility’s administrative building will be supplied with heat from a nearby pond. At the same time, in Zittau, the feed-in of heat into the municipal district heating network will be tested, thereby contributing to future heat supply based on aquatic 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 is also set to be strengthened through cooperation with component and system providers active both regionally and throughout Germany.
What are your hopes for the future?
For the future, I hope that we, as researchers, can actively work on new solutions to address the upcoming challenges of ensuring a secure and resource-efficient energy supply. With the AQVA-HEAT system, part of the required heat can be provided in the future using water heat from renewable sources.
The AQVA-HEAT project, a collaboration between Zittau/Görlitz University of Applied Sciences, the Dresden-based air-conditioning and refrigeration technicians, Brandenburg University of Technology Cottbus-Senftenberg, as well as the Zittau and Weißwasser municipal utilities and the Görlitz district, launched in March 2021 after several years of preparation and will enter its second and third project phases in 2023.
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