Energy efficiency

Using waste heat to heat homes

11 January 2024

In all manner of situations, ranging from aluminium waste recycling to data centres and the pasteurisation of milk on dairy farms, heat gets generated which mostly goes to waste. And yet, according to calculations performed by the Fraunhofer Institute, the waste heat generated just by Germany’s energy-intensive industries could be used to heat between half a million and two million homes. In this article we present five interesting approaches to using waste heat.


Energy from the sewers

Nothing refreshes you in the morning like jumping in the shower, and nothing is more relaxing than enjoying a hot bath last thing at night. But once the hot water has done its job of rinsing or soaking our body, it simply goes down the drain and enters the sewage system. And yet, wastewater contains a great deal of energy: In the winter, it averages twelve to 15 degrees Celsius; in the summer this figure climbs to 17 to 20 degrees. Making it a regenerative heat source which could be used all year round to heat buildings in the winter or keep them cool in the summer. Exactly how this can work is demonstrated by a housing complex in Bamberg. The new dwellings which are being built on the site of a former barracks will derive their energy from their own wastewater. To this end, heat exchanging mats have been laid in the sewer which runs from the building. These stainless-steel mats contain a mix of fresh water and brine which can absorb the heat from the wastewater. This heat is then fed into the heat pumps which serve the dwellings, where its temperature is stoked further, mainly by a roof-mounted solar installation: to 60 degrees for showers, baths and kitchen use and 30 degrees for use in underfloor heating. According to its own calculations, Bamberg’s municipal utility produces some 2,300 megawatt hours of heat per year, saving the equivalent of 230,000 litres of heating oil. This would be more than enough to supply the average single-family house in Germany with heat and hot water for over 100 years.

In Berlin, schools, office buildings, a swimming pool, a DIY centre and a Swedish furniture store are already in part being supplied with heat from the sewage system. These are shortly to be joined by the Haus der Statistik, a former East German statistics centre, in the city’s Alexanderplatz. The idea here is to use large-scale heat pumps to supply a building complex extending over 110,000 square metres with heat energy from shower and washing-up water. As in Bamberg, the electricity to run the pumps will be generated by roof-mounted solar panels. In this way, some 720 tonnes of CO2 will be saved per annum in comparison to heating with natural gas. According to the calculations of the Berliner Wasserbetriebe, Berlin’s water company, energy from the sewage system could cover up to five percent of the capital’s heating needs. Although this might sound pretty modest on first hearing, it would amount to over 99,000 households – in other words, enough to provide nearly the entire population of Mainz with heat energy.


Waste heat from mine water

The last anthracite colliery in Germany closed for good in 2018. But the rainwater which flows into the abandoned shafts still needs to be pumped out. Until now, the mine water from the Camphausen colliery in the town of Quierschied in the Saarland has simply been pumped into a nearby river. But the idea is now to use this water to provide the town of Sulzbach with environmentally friendly heat. To this end, a large-scale heat pump will stoke the mine water, which naturally reaches a temperature of 36 degrees, up to 90 degrees before it is fed into the district heating grid.  From 2025, this will supply 750 households with heat and hot water, saving 6,300 tonnes’ worth of CO2 emissions each year. According to energy company Steag, this is equivalent to what a medium-sized car would emit over a distance of 44.5 million kilometres.



Underground carparks as a heat source

(Waste) heat from below ground can also be derived from an unexpected source: Researchers from the Martin-Luther-Universität Halle-Wittenberg (MLU) have established that the waste heat generated by cars in urban underground carparks serves to heat the groundwater. But the problem here is that this increase in temperature runs the risk of changing the balance of the species of micro-organism to be found there – in other words, reducing the quality of the groundwater from which we derive most of our drinking water. However, the researchers have come up with a possible solution: The heated groundwater could, they say, be pumped up to the surface as a geothermal heat source and, in combination with heat pumps, be used to supply heat energy. Once cooled, it would then go back underground, in a win-win situation for water quality and environmentally friendly heat provision. In Berlin, the researchers calculate, underground carparks feed enough energy into the groundwater to heat 14,500 households.


Using milk to warm our homes

The DMK Group processes a billion kilogrammes of unpasteurised milk into cheese, butter and milk powder in its dairy in the town of Edewecht in Lower Saxony each year. This is an energy-intensive business. The milk is heated up and immediately cooled down again. The dairy has previously sourced its heat from steam from its own power station. But the biggest dairy cooperative in Germany has now switched over from steam to hot water. Combined with smart waste heat usage, DMK is by its own reckoning saving 24 gigawatt hours of natural gas and 6,000 tonnes of CO2 per annum. The Schwarzwaldmilch company intends likewise to put an end to the wastage of the waste heat generated in its dairy in Freiburg. Once a new heating station is complete, the waste heat generated in production will go on to supply the districts of Haslach and Vauban with environmentally friendly heat, thereby avoiding the emission of 5,000 tonnes of CO2 per annum.


Heat on wheels

Using waste heat in district heating makes sense especially in densely populated areas and where the source of the waste heat is not too far away from its users. In rural areas, it’s unusual to find this kind of combination. Which is why Bavarian company Swilar Eetech is transporting the waste heat from biogas or waste incineration plants by road to consumers. The heat used in the Leichtalbad swimming pool in the upper Bavarian town of Kaufering has been transported to the site by lorry since 2019. It is stored in a seven-metre container filled with a special saline solution. This solution is first charged up with the waste heat from a biogas plant in Weil, some ten kilometres away. Just like an outsize hand warmer, the saline solution is converted into a solid at the place the heat is used, where it slowly releases the stored heat, which is then routed via a heat exchanger to supply the warm water for the pool. This allows the operators almost entirely to do without the gas heating on which they used to rely, saving some 300 tonnes of CO2 per annum.

This system is not designed for family homes but generally for buildings and districts which use a lot of energy. These heat containers are accordingly being used to heat another swimming pool, a retirement home and a residential district with waste heat from biogas plants. A school complex near Hanover and a swimming pool in Switzerland are getting their container heat from the waste heat generated by waste incineration plants. According to Swilar Eetech, using mobile waste heat at sites between two and 15 kilometres from its source is particularly economical. In other words, wherever a local heating grid is no longer efficient, and it isn’t too expensive to transport the heat by lorry.