In recent years, hydropower has slipped a little under the radar of the public energy debate. Between the photovoltaic boom and the wind power debate, it almost seemed like the forgotten granddaddy of renewable energies: a bit long in the tooth, solid, but with little potential for excitement. But perhaps this quiet energy producer is about to make a comeback. In fact, hydropower is more prominent in the German government's coalition agreement than it has been for a long time. There is talk of wanting to “leverage existing potential in small and large hydropower and pumped storage power plants”. This is real progress, as the three previous governments had simply ignored the issue of hydropower. There were also reasons for this. “In fact, the potential for generating electricity from hydropower in Germany has been almost completely exhausted,” says Dr. Anna Billerbeck from the Fraunhofer Institute for Systems and Innovation Research. “There is still a small amount of potential, but 70 percent of this lies in the modernization of existing plants.” Large new projects would also be ecologically sensitive: New dams and weirs worsen the condition of watercourses – and that is legally taboo.

Source of thermal energy

But now another possibility is coming into focus: rivers and lakes as a source of thermal energy, i.e. river heat pumps. And this technology, it turns out, has a lot to offer. “The thermal heat potential from flowing waters in Germany is considerable,” emphasizes Prof. Dr. Gerhard Haimerl from Biberach University of Applied Sciences. This is particularly interesting for large cities. River heat pumps could particularly shine in the area of low-temperature heat – i.e. anything below 100 degrees Celsius. After all, this accounts for over two thirds of heating requirements. In principle, a river heat pump works like any other heat pump – except that it does not draw its energy from the air or the ground, but directly from a river or lake. River water is extracted via an inlet structure and passed through a heat exchanger, where its thermal energy is transferred to a refrigerant. This evaporates, is compressed in a compressor and thus brought to a higher temperature. The heat obtained in this way can then be fed into a heating system – such as a district heating network or a building. The refrigerant then cools down, liquefies and the cycle starts all over again. The used river water is returned only a few degrees colder.

The technology is not new. In Zurich, a system in the town hall has been heating the offices with river water since 1937. There is also a modern example in Mannheim, where a heat pump supplies 3,500 households with climate-friendly heat from the Rhine. The advantages are obvious: “Watercourses are available everywhere – over 400,000 kilometers in Germany,” explains Haimerl. In addition, water is clearly superior to air as an energy source: “Water is 800 times more dense than air. This means it can store and supply significantly more heat.” Another plus point: the temperatures in rivers are more stable in winter and usually higher than air temperatures – ideal for operation during the heating period.

Prof. Boris Lehmann from TU Darmstadt also sees great potential: “Rivers run through our landscapes like a natural district heating network. Many metropolitan regions are located directly next to them.” And: compared to the often hyped air heat, river heat pumps are thermodynamically much more efficient.

But what does this mean in concrete terms for the energy transition? Fraunhofer researcher Billerbeck has calculated: “For 2050, we have a technically usable potential of around 45 terawatt hours – through river heat pumps alone.” A study by TU Braunschweig even comes up with an “ecologically usable river heat potential of 860 to 900 TWh per year”.

What if you combined this with existing hydropower? “The combination makes particular sense,” says Haimerl. “Hydropower plants use the river flow anyway and already have infrastructure such as intake and diversion structures, screens or fish migration aids.” And where water is already being extracted, there is no need to reinvent the approval process for heat utilization. This results in what Haimerl calls “water-heat power plants” – plants that produce electricity and heat at the same time. The electricity generated from hydropower drives the heat pump directly on site. This saves on grid fees and reduces losses. At the same time, the profitability of small hydropower plants is increased, making them more attractive to investors.

“Our rivers are often too warm”

How big do such plants need to be? “Even small hydropower plants with an electrical output of four to ten kW can generate one to ten MW of heat,” says Haimerl. A plant with an electricity output of 100 kW could therefore supply heat for a small town. The problem: the distance between the river and the heat demand. “The decisive factor for economic efficiency is that the heat can be used nearby,” says Haimerl. Inner-city hydropower plants are therefore particularly interesting. This can already be seen in practice in cities such as Rosenheim: the heat pump there uses water from a small tributary.

Ecological aspects must also be taken into account when planning such plants. Prof. Florian Leese from the University of Duisburg-Essen urges caution: “Temperature changes influence the development cycles of many organisms in the water. A drop in temperature of two degrees can mean that insects hatch later or fish have smaller spawning sizes.” Nevertheless, Leese also emphasizes the advantages. “Our rivers are often too warm today – due to industrial discharges, sewage treatment plants, a lack of shade or climate change. Targeted cooling can increase oxygen solubility and thus improve aquatic ecology.”

So how realistic is it that river heat pumps will be used throughout Germany? Joachim Ferstl and Simon Koderer from the Research Center for Energy Economics (FfE) are cautiously optimistic: “In Bavaria, around 20 percent of municipalities could cover their heating requirements all year round using aquathermics.” And even if this is not possible everywhere, the technology offers an excellent supplement to other heat sources.

Perhaps hydropower is about to enter a new chapter. One in which, together with river heat pumps, it could become the centerpiece of a sustainable energy supply. “Basically, everyone involved agrees that we don't want to degrade our waters with hydrothermal use,” emphasizes Dr. Karsten Rinke, Head of the Department of Lake Research, Water Resources and Environment at the Helmholtz Centre for Environmental Research (UFZ). “But the exact design has not yet been defined.” This uncertainty is leading to a backlog of decisions, “as the authorities concerned do not have a framework of recommendations for the permits.” These are currently being worked on by the Working Group of the Federal States on Water Issues (LAWA).

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