Standing in my swimming trunks on the banks of Germany's Rhine River in the middle of January, I dip my toes in the icy water. I'm not an all-weather swimmer, and the wind is not only cooling the already freezing air, it's also making me question my own resolve.

On my journey to this point, experiencing the cold water had felt relevant to understanding what I have come to explore, which is how it's possible to heat thousands of homes with a cold river.

It now feels like less of a necessity but it's too late to turn back, so I venture into the frigid water. One ginger step at a time. Once submerged, my whole body tingles as I wait for the endorphins I have heard will kick in to actually do so.

But they remain absent, leaving me to feel pain rather than joy and to keep my dip in the wintry waters short. At least now I know exactly how cold the river is.

Looking across to the other side as I dry off and encourage my toes to thaw, it seems unimaginable that 3,500 apartments over there are heated with this same water. Yet they are, via a giant power plant operated by municipal energy supplier MVV.

Replacing coal with river heat

A short time later, Felix Hack, project manager at MVV, shows me around the complex.

"Millions of liters of water flow through the Rhine," Hack explained, adding that it contains enough thermal energy to heat Mannheim several times over — even in the depths of winter. "It's now 6 or 7 degrees Celsius [44 Fahrenheit], so physically there is still enough heat in it."

Mannheim still generates most of its electricity and heat from coal, which is particularly harmful to the climate. Long term though, the city is planning to shift to a mix of geothermal energy and heat from both waste incineration and the river. The river heat pump started operating here in 2023 and is, according to MVV, currently the largest in Germany.

Against a backdrop of smoke billowing from a coal-fired power station chimney, Hack leads me to a sparkling hall with an aluminium facade. Inside, the noise is thunderous.

The heat pump, which is the size of a truck, is running at full speed to pump 800 liters (211 gallons) of water every second.

River heat pumps: How do they work?

The Mannheim river heat pump essentially works like a conventional heat pump that uses heat from the air or the ground. In this case, it's just much bigger and the source is the river.

The cold water turns a rapidly evaporating refrigerant into a gaseous state. The gas expands and an electric pump compresses it, generating heat. The process is comparable to a bicycle pump, where the valve is kept closed though pumping continues. The pump gets hot and the heat generated is used to warm water to 99 degrees Celsius (210 degrees Fahrenheit) before it's fed into the grid.

The river heat pump has a share of just 3% in the district heating network, but Hack said it's only the beginning.

"We want 50,000 households to be supplied with heat from river water," he explained. 

Two additional machines are already being planned. Water heat pumps have also been commissioned or are planned in Berlin, Hamburg, Stuttgart and Rosenheim, in Bavaria.

More supply than demand

Heat in buildings and industry consumes by far the most energy worldwide, and 72% of demand is met with fossil fuels. According to the International Energy Agency, large and small heat pumps combined could prevent as many emissions by 2030 as all the cars in Europe consume per year.

Currently, 15% of buildings in Germany are supplied with district heating. But by 2045, large heat pumps, including those that run on thermal energy from wastewater, industrial waste heat and geothermal energy, could generate some 70% — largely replacing coal, oil and natural gas.

Fabian Ahrendts, from Germany's Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems, sees the potential of these large-scale pumps.

Ahrendts co-authored a study highlighting the high volume of environmental and waste heat available for use by heat pumps in Germany, which showed that supply far outstrips heat demand for buildings and industrial processes.

 

Scandinavian countries have been relying on this technology for some time already. In the Swedish capital, Stockholm, about 90,000 apartments have been supplied with heat from industrial wastewater for years. In the southern Norwegian town of Drammen, more than half of the 103,000 residents heat their homes with thermal energy from the ice-cold fjord.

Are there risks to the river?

But since heat extracted from the Rhine is returned to it colder after the pumping process, this technology is not without implications for the environment. MVV said the small quantities of water currently used in Mannheim mean the river temperature only changes minimally, but Ahrendts cautioned that it could become more difficult if all cities and industrial plants along the Rhine were to use the river heat.

"Fortunately, most of our waters are overheated at the moment due to climate change, so it's beneficial up to a point," said Ahrendts.

Although heat pump technology has been available for years, it has taken some time for large-scale projects to kick off in Germany. Hack said this is connected to the low fossil energy prices of the past.

In Mannheim, the prospect of high CO2 taxes, local climate targets and state subsidies for the construction of the plant were decisive factors in the decision to switch to heat from the river.

Though by the end of my tour, I understand how it works, my toes can still feel the impact of my dip in the Rhine. Despite its thermal power.

Edited by: Tamsin Walker