How wastewater can be turned into environmentally friendly heating

BErlin, Frankfurt, Hamburg – almost every city sits on a cozy, warm treasure. But it is usually not upscale yet. Because what canals catch underground because it is undesirable above ground could help to heat the city when it is cold or to cool it when it is hot. Environmentally friendly, of course. Sewage and rain flow through the sewer system at a relatively constant temperature. In winter it is still 12 to 15 degrees. It can be a bit warmer in the living room, but the thermal energy contained in this moderate 15 degrees is too valuable to be dumped in the next river. More and more private and municipal energy providers are realizing this.

In the capital, people have had experience for a number of years in harnessing the heat from wastewater. In summer, however, the most powerful system to date started its service on the environmentally friendly heating transition in, or better below, Koppenstrasse near Berlin’s Ostbahnhof. It can generate 600 kilowatts of so-called extraction power. That is enough to provide around half of what the connected 50,000 square meter office building needs, as operator E.ON says. Although the supplier cannot yet determine any real consumption data, it is assumed that there will be at least 20 kWh of cooling and 50 kWh of heating per square meter and year.

Berlin could install 300 megawatts

For this purpose, a 200-meter-long, tailor-made stainless steel heat exchanger is located in the bottom of the canal. The material is corrosion-free and so stable that someone can walk over it to maintain the systems or to clean their surface. The wastewater flows over the plates and releases two to three Kelvin to the closed circulating liquid circuit below. From here on, the water is around 14 degrees, a heat pump takes over the rest. It compresses the refrigerant it contains, which evaporates due to the heat provided. Means: It’s getting hotter. The heat pump first needs its own energy, here it is supplied by a gas-powered combined heat and power plant, it could just as well be electricity. Anyone who spends one kilowatt on the heat pump will get four to five kilowatts of heating energy in the end.

So can’t the entire sewer system quickly become a thermal power station? It’s not that simple, says Hakan Kurc, who heads projects for the use of heat from the city’s wastewater at Berliner Wasserbetriebe. In a potential analysis, he calculated that around 300 megawatts could be installed in the urban area in this way. That is not little, around seven percent of the heating energy required. But the fact that it is no longer is due to the sewer system itself. In order for the technology to be efficient and thus economical, a regular and powerful sewage flow is required. Of the almost 10,000 kilometers of Berlin’s sewer network, however, some only serve as an alternative line, while others are simply too small. The diameter may just be enough for the technology, but not for the technician who wants to lay it. And because the wastewater behind each heat exchanger is colder than before it, there is always a need for a gap between the systems so that the investment costs, which can quickly reach 150,000 euros, pay off. The steel pressurized water pipes that bring the collected wastewater from the center to the sewage treatment plants are particularly suitable. The heat exchanger then does not sit in the duct, but rather surrounds the pipe on the outside.



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