Renewable wastewater energy reduces dependence on natural gas

Quickly available, renewable and politically less controversial energy sources are now in demand - and the technology is already available in Austria: Wastewater for sustainable cooling and heating of buildings.

© Rabmer

The energy transition, in particular the phase-out of natural gas, has taken on a new urgency due to the war in Ukraine and the tense relationship between the West and the East. Quickly available, renewable and politically less controversial energy sources are in demand – the technology is already available in Austria, now it is time to roll it out as quickly as possible. Energy generation from wastewater, for example, in which our partner company Rabmer specializes, is said to have great potential. According to the company, around 14% of the thermal energy required for buildings in Austria can be produced regionally and sustainably.

In the past, the term “sustainable energy” primarily referred to renewable energy sources. With the war in Ukraine, the political component of the term and security of supply are now also gaining in importance. Europe’s dependence on natural gas is suddenly no longer primarily a climate-related problem, but also a supply problem due to the high market share of Russian gas – which covers around 80 percent of gas demand in Austria. The time pressure to find reliable alternatives quickly is increasing.

Fourteen percent of the heating requirements of all buildings in Austria can be covered by energy from wastewater

Three quarters of energy consumption in the building sector in this country is used for heating, cooling and hot water preparation, and almost exclusively CO2-critical energy sources such as oil or gas are currently used for this.

“Rising energy prices and the energy shortage caused by the war in Ukraine show that we urgently need new energy sources that reduce dependency and guarantee security of supply. This also applies to delivery, spare parts and, last but not least, technical expertise – in the event of a crisis, you should no longer be dependent on global supply chains. It must be possible to implement the respective system in the relatively short term, but it must promise long-term, reliable output. In addition, CO2 emissions must also be reduced,” explains Ulrike Rabmer Koller, Managing Partner of the Rabmer Group. “The list of requirements is long, but there are systems that meet all these criteria. One of these is the use of wastewater, a renewable energy source for the sustainable heating and cooling of buildings, with which around 14% of the required heating energy and the increasing demand for cooling energy can be produced regionally and sustainably,” says Rabmer-Koller.

In urban areas in particular, there are optimal conditions for energy from wastewater for renewable heating and cooling of buildings. In Vienna, for example, the average temperature of the wastewater is 16 degrees Celsius all year round and does not fall below eleven degrees even in winter. This high output temperature, which is higher than that of geothermal energy, groundwater or outside air – the usual energy sources for heat pumps – is what makes the use of wastewater so interesting, as the heat pump works much more efficiently and therefore requires little additional energy itself.

Energy from wastewater has been recognized as renewable energy since 2018 and government subsidies have recently become available. “In 2021, energy from wastewater was funded by the Climate and Energy Fund for the first time. This has now also given the technology a turbo boost in Austria. In the last 12 months alone, we have examined over 20 major projects. Many of them could be implemented in the short term,” explains Rabmer-Koller.

One of the country’s newest systems was installed by Rabmer at the new Wien Kanal headquarters in Inzersdorf in 2021. This covers 100 percent of the building’s heating and cooling requirements and delivers up to 450 kilowatts of heating and 500 kilowatts of cooling capacity in full operation. Rabmer will soon be implementing another lighthouse project in Vienna, namely the supply of the new “VIO Plaza” real estate complex at the Meidling Hauptstraße U4 station with a capacity of 1.2 MW of heating and 6 MW of cooling from the canal. Rabmer obtains all the materials needed to build the systems from the EU.

How does energy generation from wastewater work?

Um Abwasser als erneuerbare Energiequelle nutzbar zu machen, werden zunächst Wärmetauscher im öffentlichen Kanal oder bei größeren Anlagen als Bypass außerhalb des Kanals bzw. auch im Kläranlagenabfluss angebracht. The wastewater in the sewer flows around the heat exchangers and heats a separate water circuit, which in turn is connected to heat pumps in the building to be supplied. These heat pumps extract the heat from the water and bring it to the desired temperature level. In winter, a building can be heated economically or hot water can be produced, while in summer the process is reversed to cool the building. In addition, wastewater energy can also be fed into district or local heating networks and district or local cooling networks.

The basic requirement for this type of energy generation is a sewer with a flow rate of at least ten liters per second and a dimension of DN 400 or larger. In addition, the wastewater temperature must be constantly above eight degrees Celsius, the distance to the consumer must not exceed 900 meters, and the heating or cooling load requirement must be at least 50 kW.

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