The 38-year-old is an engineer at the responsible utility Rheinenergie. He says: “If we are serious about achieving the energy transition, there is no way around it.”

According to the German Energy Agency, three quarters of buildings across Germany use gas and oil to heat rooms and water. This accounts for almost 40 percent of German CO₂ emissions. In order for Germany to become climate-neutral by 2045, as the federal government decided last year, the country must not only switch to wind and solar power, but heating must also become fossil-free.

By 2030, local authorities should therefore obtain 30 percent of their heat from renewables or unavoidable waste heat from industry, and by 2040 the target is 80 percent. Great hopes are pinned on heat pumps. These appliances are already the most commonly used systems in new buildings. However, the larger versions that could supply municipal district heating networks are still few and far between.

Yet rivers in particular are considered to be rich sources of energy for the systems. They run through the landscape as a kind of heat belt, and most large cities are located on their banks. The principle is simple: the pumps extract heat from the river water and feed this energy into a district heating network. According to a study by TU Braun- schweig, up to 94 percent of Germany's demand in the so-called low-temperature range, i.e. mainly for heating and hot water, could be covered by the temperatures from the waters. More than half of the 80 large cities studied could even be supplied with heat entirely with the help of rivers. And with far less electricity than the conventional small pumps in households need to draw heat from the air.

The water in the rivers is warmer than the air on most winter days; the systems require less energy to reach the desired temperature in the pipes.

As tempting as it sounds, the large-scale variants have so far been an absolute niche product. Germany's first flow heat pump has been supplying just 3,500 households in Mannheim for almost two years. There are also more than a hundred other systems. However, they are mostly tiny, often only at a single mill, for a handful of houses. In other countries, however, especially in Scandinavia, the systems have long been an integral part of heating networks. If you talk to economists and ecologists about the reasons for this here, it's all about permits and costs, acceptance – and cheap Russian gas. Even engineer Felk makes it clear: “There are many difficulties.” Heat pumps that draw their energy from water work in a similar way to conventional household appliances. They divert some of the water, filter it and feed it into the system. A refrigerant captures the energy stored in it, and the water itself returns to the river slightly cooled. The heated refrigerant evaporates. This vapor is compressed, whereby it heats up and transfers its energy to the water in the district heating network. The refrigerant cools down and becomes liquid again. The process starts all over again. This allows the water in the district heating network to be heated to more than 100 degrees. Even if the river is only five degrees. The idea is not actually as “absolutely new” as Felk explains. Only the dimensions are. Switzerland has been relying on energy from the river for almost 90 years. In 1938, Zurich became one of the first cities to heat its town hall not with coal, but with the Limmat, which flows beneath it. In Germany, however, the potential of the Rhine and Ruhr, Spree and Saale rivers is only now being discovered.

MANY FLOWING WATERS WOULD BE SUITABLE

Jessika Gappisch from TU Darmstadt and her colleagues are investigating just how big this is. According to the environmental engineer, it was actually known early on in Germany that it would work. “There was just no need.” The conventional systems for gas and oil were established and imports from Russia were “unbeatably cheap”. She says: “The investments and the risk were simply too high for a long time.” The 31-year-old now knows from her research: In principle, all flowing waters are suitable for the pumps, including the narrower ones and even streams. “However, planning and construction are expensive at first,” she says. Streams where there is a lot of water flowing, as warm and clean as possible and with high water levels, are therefore particularly suitable. The filters are less likely to clog, the risk of freezing in winter is lower and the yield is simply greater in relation to the investment. Gappisch adds that the systems become even more practical under one condition: If they work together with a so-called cold local heating network. According to an estimate by RWTH Aachen University, there are already more than 50 such networks.

Similar to normal district heating networks, water transports the stored energy from the source to households and other consumers through a network of pipes. However, it is not 90 or even more than 100 degrees hot, but only around 20 degrees. In the buildings themselves, conventional heat pumps raise the temperature to the desired comfort level. The advantage of this variant is that the pumps work most efficiently when the temperatures between the source and the medium to be heated – between the water in the river and that in the pipes – do not differ too much. In addition, unlike conventional networks, such systems can not only heat houses. They can also cool them. On hot summer days in particular, the water can remove the heat from buildings in a much more environmentally friendly way than air conditioning systems, which actively blow cold air into the room.

What are the “many difficulties” with the Cologne plant? Project manager Felk says: “Technically, we are now ready to go.” The refrigerant, for example, had been a headache for a long time, especially for these dimensions. Although the agents used in the past were often particularly easy to use, they were extremely harmful to the environment and climate or highly explosive and flammable. Their choice fell on ammonia. In the event of an accident, this would also be toxic in large quantities. However, numerous safety systems are designed to prevent it from being released into the environment. The electricity that powers the plant was not so easy either, he explains. “The pumps need as much as a small town.”

However, they were lucky: the Niehl power plant, to which the large heat pumps will belong, already had an extra-high voltage connection to feed surplus electricity into the grid. They want to make use of this in future with additional transformers and switchgear. Their plan is to rely on gas when electricity prices are high and on the pumps when they are low. A combination, they hope, that will keep heating prices stable or even lower them in the long term. Other experts are more skeptical.

Among them Dietrich Schmidt, an expert in urban heating systems at the Fraunhofer Institute for Energy Economics and Energy System Technology. He says: “The fact that electricity is so expensive and often not available in sufficient quantities is a real problem.” Compared to “natural gas, which is still cheap”, this makes operating the systems expensive and the risks high. Nevertheless, he also admits: “There aren't that many fossil-free alternatives.” Conventional heat pumps are even more electricity-intensive, geothermal energy is often even more expensive and not available everywhere, and biomass is not efficient enough. He is therefore hoping for the lower electricity prices for industry that are currently being discussed – and public funding.

This is an excerpt from the original article that appeared in Welt am Sonntag.