Lakes and large rivers have huge heat potential which can be tapped into without posing any harm to the environment. This is demonstrated by studies conducted at the Eawag. The water research institute of the ETH Domain advises authorities on using energy as cleanly and as sustainably as possible.

Alfred Wüest, Professor Professor at EPFL and specialist in Aquatic Physics at Eawag. (Photo: Kellenberger Kaminski Photographie)

Zurich town hall was heated with water from the Limmat as far back as 1938. Nowadays, four lake water networks around the Lake Zurich basin deliver heating and cooling to the surrounding buildings; Lake Geneva is a heat source for heating and cooling the campuses of EPFL and the University of Lausanne, as well as the UN building in Geneva. However, they are just isolated examples. "The technology has long since existed, but even though it has become much more efficient in the meantime, it is still under-utilised," explains Alfred Wüest, Professor at EPFL and specialist in Aquatic Physics at Eawag.

The principle is the same as the use of terrestrial heat. A heat pump draws energy from the environment in order to heat a building which is connected to it. While there is limited terrestrial heat potential in areas which are densely populated, there is an inexhaustible reserve of heat on the very doorsteps of the residents of many Swiss cities thanks to the lakes and major rivers. In addition, cooling lake or river water can be fed directly through the buildings in summer. Free-cooling enables temperatures to stay pleasant without having to use air-conditioning systems, which are heavy energy consumers. The Swiss National Supercomputing Centre (CSCS) is one of the world's most energy-efficient computer centres thanks to the 6° cold water drawn from the depths of Lake Lugano.

Alfred Wüest explains that "it only causes a slight shift in environmental conditions, but nothing new is generated." "Therefore, there are no environmental concerns about using lake water."

Alfred Wüest and his team have conducted detailed studies into the physical and ecological effects of using lake water. Eawag researchers have based a scenario for Lake Constance on the assumption that there are a million residents connected to the lake's water networks. "Even then, intervention is extremely small compared to global warming," the expert points out, in summary.

No adverse effect on ecology

While the extraction of heating energy in the winter to heat buildings may even help to counteract climate change, bodies of water should not be heated any further by waste heat from cooling in the summer. Therefore, experts recommend that warmed water be returned into Lake Constance at a depth of greater than 20 metres. In contrast to the surface and to the depth range, water temperature changes quickly the further down one goes into this so-called thermocline. Calculations show that even with intensive use of heat, there is only a slight increase in the thermocline in Lake Constance. Alfred Wüest explains that "it only causes a slight shift in environmental conditions, but nothing new is generated." "Therefore, there are no environmental concerns about using lake water."

Alfred Wüest, Professor Professor at EPFL and specialist in Aquatic Physics at Eawag. (Photo: Kellenberger Kaminski Photographie)

The studies conducted by Eawag prompted the International Commission for the Protection of Lake Constance to relax the guidelines on the use of the lake's water. A new housing development with 165 apartments in Romanshorn is now set to be heated with water from the lake. The amazing thing is that even a frozen lake in the depths of winter still supplies environmentally friendly heating energy. For instance, in St. Moritz at an altitude of 1800 metres. Since 2007, the Badrutt’s Palace Hotel and a number of houses have been heated with a heat pump which cools water from Lake St. Moritz from four to one degree Celsius.

One of the largest projects is planned in Lucerne. Drawing water from Lake Lucerne, the Energie Wasser Luzern (EWL) energy company is aiming to supply 100 GWh of heating energy and 23 GWh of cooling energy every year. 40,000 people could benefit from this. The overall potential is many times higher and is far in excess of the realistic demand, as Alfred Wüest ascertained in calculations for the Lake Lucerne Supervisory Commission. Construction work is due to get under way on the first phase in Lucerne in 2018. According to EWL, the overall project will entail an investment of around 95 million francs.

"While the investment costs are high, it will be cost-efficient to run after that," explains Alfred Wüest. He estimates that heating with thermal energy from the lake is currently about twice as expensive as burning cheap crude oil. The expensive cost of tapping into the system is the reason why lakes and rivers are still not widely tapped as a source of clean and sustainable heating energy.