Previously unused waste heat from the Werdhölzli sewage treatment plant will supply around 30,000 households with renewable, local heating and cooling on completion. The Altstetten and Höngg energy network is the largest of its kind in Switzerland and an important milestone on the road to a fossil-free heating supply. In addition, it demonstrates how an enormous supply area can make the transition to renewable energy sources within a few years.
 
The energy planning of the city of Zurich envisages better use of renewable energy sources in the future. In February 2019, this was shown to also be in line with what the population wants. With 88 per cent support, the city of Zurich’s voters said yes to the  property loan of CHF 128 million for the Altstetten and Höngg energy network. A good year and a half later, heat was already flowing into the first connected households.

The energy network as a success concept

The urban areas of Altstetten and Höngg offer ideal conditions for an energy network. They have a high density of heat consumption and are situated close to a source of heat, the Werdhölzli sewage treatment plant. Good experiences with our Schlieren heating and cooling network have already shown that using waste heat from purified waste water does work. The Schlieren energy network saves around 11,700 tonnes of CO₂ emissions per year compared to heating oil (average value 2021/2022). The commissioning of the Altstetten and Höngg energy network allows us to utilise the previously untapped heat potential of the Werdhölzli sewage treatment plant for the Schlieren network.

How does the energy network work?

The heart of the new energy network is the site of the Werdhölzli sewage treatment plant. The  primary energy source, available all year round, is the waste heat from the treated waste water of the city of Zurich. This has a temperature of between 11°C in winter and around 25°C in summer. Zurich’s disposal and recycling service treats up to 80 million cubic metres of waste water each year (around half the volume of Lake Greifensee). Before the treated water flows into the Limmat river, it passes through a waste heat recovery facility, where heat is taken from it. In the energy centre, heat pumps bring the temperature up to the level for useful heat.
 
Another heat provider is the sludge recycling plant, which is also located on the site. The waste heat generated in the course of incinerating the sewage sludge is a valuable energy source. The network uses both the direct surplus heat from the sludge recycling plant (around 2 MW of power) and the condensation heat from the exhaust gases (2.5 MW of power). Because the sludge recycling plant has temperatures of 70 to 80°C, this can be fed directly into the system without needing to go via a heat pump.
 
In addition, we direct the waste heat generated from ice production in the new Swiss Life Arena of the ZSC Lions into the Anergy grid. And from summer 2022 the energy centre of the Swiss Life Arena will produce cooling for connected properties that will be used to cool offices and commercial spaces. The cooling process is achieved (as with heat production) through highly efficient heat pumps.

Where and when can a connection be made?

All property owners within the network area have the option of connecting their property to the network. Around half of all potential customers in the Höngg and Altstetten North area have already concluded a contract with us. Connection is voluntary, but offers benefits for homeowners: in future, they will heat their homes in an environmentally friendly manner and using local energy, and thus already meet the requirements of future energy laws. Implementation will take place in the Höngg Central energy network areas from 2023 to 2032 and in Altstetten Ost from 2024 to 2032. The implementation in Altstetten West will be carried out by Energie 360 °AG. You can find detailed information about the energy network here (in German).

Subsidies for collaborative projects

If you switch your heating to a district heating connection, you can apply for subsidies. The application must be submitted no more than two years before the measures are taken and must be submitted before the conversion of the in-house heating system. You can find further information and submit an application here (in German).