Watercourses in Germany reach average annual temperatures of eight to ten degrees Celsius – and the trend is rising. This heat can be used as an energy source, for example to heat residential buildings. Heating accounts for around half of Germany's energy consumption. The share of renewable energies here is significantly lower than in the electricity sector. Over 70 percent of heating systems are powered by fossil fuels such as gas and oil. Heat pumps are considered a climate-friendly alternative – but they currently account for only around six percent of the market.

The principle behind heat pumps is simple: thermal energy is extracted from the environment and converted into usable heating energy with the aid of a refrigerant and a cycle. The refrigerant absorbs the (heat) energy and evaporates. This vapor is compressed in a compressor using electricity, causing the temperature of the vapor to rise. This heat is “tapped” and transferred to the water in a heating network, causing the refrigerant to become liquid again. Then the cycle starts all over again.

Aquathermal energy: harnessing heat from rivers

“The heat stored in bodies of water is already being harnessed today with the help of aquathermal energy systems,” says Jessika Gappisch from the Hydraulic Engineering and Hydraulics Group in the Department of Civil and Environmental Engineering at TU Darmstadt. Unlike conventional heat pumps, which use ambient air, aquathermal systems work with river water, among other things. These systems extract heat from the water, use it to heat water for heating purposes, and feed it into individual households or a supply network.

The temperature provided varies depending on requirements and the size of the system. Large-scale systems can heat water to up to 100 degrees Celsius and feed it into a district heating network. In smaller neighborhoods, so-called cold local heating networks are used, in which the water temperature is around 20 degrees Celsius. The river heat is then distributed to the neighborhood via a small heating network and only heated to the desired temperature in the individual houses by heat pumps. It can also make sense to supply heat to individual locations such as mill buildings.

Electricity is required to operate aquathermal systems. Although this is not cheap in Germany, the use of aquathermal systems can still be worthwhile. Heat pumps can generate three to five times as much heat energy from one kilowatt hour of electricity. The higher the temperature of the heat source, the more efficient the heat pump. Since water temperatures are higher than air temperatures during the cold season, river water pumps are particularly cost-effective when heating demand is at its highest.

Permits and laws: What is still holding back river heat?

The biggest difficulty in using river heat is not so much the technical implementation or economic efficiency, says Gappisch, but rather the planning and approval process. “The lack of empirical data leads to uncertainty,” she says.

The research project “Aquathermie-Viewer Deutschland” (Aquathermal Viewer Germany), in which Gappisch is involved and which will run until early 2027, aims to close this gap. The project, funded by the German Federal Environmental Foundation, records existing plants, maps them, and makes the data publicly available. It is currently unclear how many aquathermal plants there are in Germany. “As part of our research project, 75 percent of German licensing authorities have responded so far, reporting around 100 plants,” reports Gappisch.

Germany's largest river heat pump is currently located in Mannheim. It supplies 20 megawatts of thermal energy and is expected to be increased to 150 megawatts in the future – enough to supply several thousand households. Similar projects with comparable output are being developed in Cologne (150 megawatts in Niehler Hafen) and Hamburg (230 megawatts in Energiepark Tiefstack). Further details on the existing plants are expected in the course of the research project.

Gappisch sees another challenge in the ecological legal framework. Aquathermal plants extract water from the river, remove heat from it, and return it slightly cooled. Or they extract heat from the water using heat exchangers that are installed directly in the water. “While there are legal requirements for the discharge of heated water, such as cooling water from power plants, there are no clear regulations for cooling the water,” she says. Technical questions remain unanswered: What are reasonable limit values? What is the effect of a temperature plume? And how can the escape of refrigerant be reliably prevented? The scientist sees a need for further research in this area. Temperature changes in the water should remain moderate and be based on natural fluctuations. If implemented correctly, aquathermal energy can even contribute to ecological relief – by specifically removing excess heat from overheated waters as a result of climate change.

“Our project aims to raise awareness,” emphasizes Gappisch. The resulting database will serve as a knowledge base for authorities, planning offices, and energy suppliers, laying the groundwork for future approval processes and thus contributing to the further spread of river heat.