Sewers and bodies of water have become particularly interesting for energy researchers. Although temperatures in sewers, rivers, and lakes are low, especially in winter, modern heat pumps, which have made leaps in development in recent years, can be used to heat buildings or generate industrial steam. The magic word: aquathermal energy.

Every person produces large amounts of wastewater every day: when showering, draining pasta, flushing the toilet. Everything comes together in the sewer and has an average temperature of 25 degrees Celsius. On its way through the underground to the sewage treatment plant, everything cools down, but maintains an average temperature of 15 degrees Celsius in the insulating soil – slightly less in winter, slightly more in summer. That is warm enough to heat with in winter and cold enough to cool with in summer.

According to a 2017 study by the consulting firm Enervis, the energy from the sewer system in Germany has the potential to cover around 14 percent of the total heating requirements of buildings. There are basically three ways to tap into the heat: directly in the building, in the sewer, or at the end in the sewage treatment plant.

The idea is not new. The first attempts in Germany date back to the 1920s. Recently, around 100 larger plants have been built in Europe. In France, for example, the Élysée Palace, seat of the French president, and the National Assembly building are heated with heat from the Paris sewer system, among other sources.

The largest German plant, with a capacity of 2.1 megawatts, is expected to one day supply two-thirds of the heating for the Neckarpark development area in Stuttgart. However, this is not a breakthrough for the innovative technology.

To date, Germany still covers more than 80 percent of its heating and industrial process energy needs with fossil fuels. “Until two years ago, oil and gas were simply too cheap to warrant a change in thinking,” says Volker Stockinger, professor of energy-efficient construction and building technology at Nuremberg Tech.

But the war in Ukraine has triggered a change: “Especially in municipalities, which plan for the long term, the realization has taken hold that the heating sector must also be converted to renewable energies as soon as possible.”

Stockinger knows what he is talking about. In Bamberg, he is leading the accompanying research for the heat supply of the new Lagarde inner-city district, whose

base load is to be covered by a combination of geothermal energy and wastewater heat. The energy obtained is collected at the central station, where it is fed into a cold local heating network and distributed to the heat pumps installed decentrally in the buildings.

Cold local heating networks are a new variant of conventional heating networks that operate at low water temperatures close to or even below the ambient temperature. This minimizes heat loss and allows renewable heat from the sea, rivers, lakes, groundwater, soil, or ambient air to be integrated, as well as previously unused waste heat from commerce and industry.

In order to heat with the comparatively cold water, the temperature is raised to the required level at the end using a heat pump. Cold heating networks are considered a key technology due to their flexibility and low operating temperatures.

The pipes of the cold heating network on the Bamberg campus are laid without insulation, explains Volker Stockinger: During the heating period, the operating temperature is below the temperature of the surrounding soil: “The network then acts like a collector that extracts heat from the ground.” It can also be used to cool buildings during the summer months.

Wastewater is “one of the sleeping giants of the heat transition,” emphasizes Stockinger. “All the technology needed for this is already available.” Heat exchangers can be installed in both new and existing sewers. However, this requires sufficient wastewater flow: a guide published by the German Environmental Foundation estimates that a minimum of 15 liters per second is needed on days without rain to ensure technically and economically efficient operation.

The 2005 paper requires a minimum diameter of 80 centimeters for the installation of the heat exchanger. In addition, the sewer must be easily accessible – for example, by installing an access hatch for cleaning work.

Nevertheless, there are still a number of issues to be clarified. For example, the question of how many heat exchangers a sewer network can accommodate. It is clear that even if a lot of heat is extracted from the wastewater, its temperature quickly rises again due to the inflow and the soil. However, according to Stockinger, it is not known how long the recovery distance between two heat exchangers needs to be.

Rivers and lakes also contain a lot of energy. To use this for heating, energy sheet piles are used, among other things, which are driven into the ground on the shore or in a harbor basin as fastenings and extract additional heat from the water. The sheet pile wall and heat pump are connected by a closed circuit that brings the whole system up to the required heating temperature. This is worthwhile even in winter, when the river is six degrees Celsius and the lake water is eight degrees Celsius. This concept is currently being implemented just outside Leipzig. The operators expect to be able to cover a total annual heat demand of 400 megawatt hours.

Rosenheim is doing things differently. In this Bavarian town, three large heat pumps are supplied with water from the nearby Mühlbach stream, feeding the energy they generate into the 180-kilometer municipal district heating network. The cooled water is returned to the Mühlbach stream. The three large heat pumps, each with a heating capacity of 1.5 megawatts, contribute around one-tenth of the city's district heating requirements.

The Munich-based Research Center for Energy Economics recently estimated the potential for heat generation from rivers in Bavaria. The study found that around one-fifth of all municipalities, especially those located along major rivers, could cover most of their heating requirements in this way.

However, using rivers or lakes as a heat source also has an impact on the ecology. It is therefore necessary to clarify in advance how cooling affects the flora and fauna of the water body.

In rivers, there are hardly any negative consequences to fear if heat extraction is limited, says Jessika Gappisch from the Chair of Hydraulic Engineering and Hydraulics at TU Darmstadt: “Since the water is constantly being renewed and mixed, the overall temperature balance of the river is hardly disturbed.” The situation is different in standing waters. There, the water stratifies into temperature zones due to differences in density. In summer, the surface is heated by the sun and thus becomes lighter. The transition to cool and heavy deep water is called the thermocline, where the temperature drops sharply. In the deep zone, the water is only four degrees Celsius. The thermocline separates the deep layer from the surface – there is no exchange of oxygen or nutrients. In autumn, the surface cools down, becomes heavier and sinks. This leads to what is known as autumn circulation: oxygen-rich water from above mixes with the nutrient-rich and oxygen-poor deep water. In winter, the upper layer is around zero degrees. The deep layer below is a uniform four degrees.

At the end of winter, spring circulation occurs and the two layers mix again.

Depending on the depth at which the water cooled by the heat pumps is discharged and its temperature, the stratification in the lake is disturbed to a greater or lesser extent. This could have an impact on the underwater flora and fauna. However, a study commissioned by the Innovation Region Central Germany states that temperature changes of less than one degree are harmless as long as the water still contains at least six milligrams of oxygen per liter.

According to Jessika Gappisch, a researcher at Darmstadt University of Technology, the heat extraction by human installations actually tends to relieve the pressure on the water bodies: “A slight cooling counteracts climate change.” In fact, rivers and lakes in Germany have warmed significantly over the past 30 years, which has worsened living conditions, writes the Federal Environment Agency in its 2023 monitoring report on Germany's adaptation strategy to climate change. This is because the warmer water becomes, the less oxygen dissolves in it.

Accordingly, heat recovery from rivers and lakes benefits both nature and civilization. This is especially true since water reacts slowly to temperature changes and is much warmer than the air in winter when houses need to be heated. In addition, water has a high energy density.

Its specific heat capacity—i.e., its ability to store or release energy—is 4.2 kilojoules per degree of heating or cooling per kilogram, compared to only one kilojoule for air. “That's why a heat pump extracts four times more thermal energy from one kilogram of water than from one kilogram of air,” explains Jessika Gappisch.

The difference in density between the two substances, i.e., the ratio of mass to volume, must also be taken into account: while one kilogram of water has a volume of one liter, air has a volume of 1,000 liters. This is why water heat pumps are many times more efficient than the conventional air heat pumps that dominate the market in Germany. The same applies to wastewater heat pumps.

Compared to geothermal energy, aquathermal energy has the added advantage of requiring far less investment, as there is no need for expensive drilling work. Waste heat recovery from bodies of water or canals has a high power density and requires little space. Gappisch suggests that systems on rivers and lakes could often be integrated into existing structures: “For example, former mill canals or hydroelectric power plant sites could be used, as well as the cooling water pipes of decommissioned fossil fuel power plants.” This minimizes the impact on nature and protects rivers, lakes, and their shore areas. With aquathermal energy, which is already established in Scandinavia, Switzerland, and the Netherlands, the heat transition in Germany could also receive a powerful boost. “You use heat that is already available,” says the scientist. And it's free, too.