Heat pump at Therme Wien, photo: Wien Energie/Max Kropitz

Heat pump at Therme Wien, photo: Wien Energie/Max Kropitz

ThermaFLEX demo project: Heat from the thermal baths
Use of waste heat in Vienna’s district heating network

Over 440,000 households and 7,800 business customers in Vienna are currently connected to the district heating network. In the future, Wien Energie would like to supply 56 % of all Vienna’s households with district heating. By 2040, all heat generated should be climate neutral. This can only be accomplished by making use of new sources of heat. In addition to tapping into deep geothermal energy, more heat should be generated in future using large heat pumps and local use of waste heat like at Therme Wien.

The integration of local waste heat from various sources can make an important contribution to the decarbonisation of heating networks. In future, Wien Energie would like to utilise the thermal water from Therme Wien to supply heat to roughly 1,900 households in Vienna. Waste heat energy from the thermal water, which has previously been discharged into the canal unutilised, will become an environmentally friendly source of heat for
Vienna’s Oberlaa district. This will mean a saving of about 2,600 tonnes of CO2 per year.

Innovative concept

After the thermal water has been used internally by Therme Wien, the remaining heat present in the water (at a temperature level of about 30 °C) will serve as a new heat source for Vienna’s district heating network. For this purpose, Wien Energie installed two identical water-cooled compact heat pumps in the technical room at the thermal baths. These are designed to feed about 2.2 MW of heat into the local district heating network year-round. As part of the ThermaFLEX lead project, the energy system at Therme Wien was analysed with extensive investigation of specific aspects of the heat source, such as availability, temperature level and chemical properties.
Due to fluctuations in temperature and mass flow of the heat source, hydraulic switches were integrated into the concept on the cold side of the large heat pumps. Plate heat exchangers made of titanium were used due to the corrosive properties of the thermal water. The heat pumps are designed for a maximum output temperature of 82 °C. An additional power-to-heat plant with 375 kWth thermal output raises the temperature to 90 °C when the outside temperature is below -5 °C.
The project serves as an example for similar configurations to make use of new sources of waste heat. System monitoring will also be carried out over the remaining term of the ThermaFLEX project to generate valuable insights for future projects.
Output: around 2 megawatts
Annual heat production: 11 gigawatt-hours of heat
Environmentally friendly district heating for roughly 1,900 households in Oberlaa
CO2 savings: 2,600 tonnes per year
Investment: EUR 3 million



  • Photo: Therme Wien
    Photo: Therme Wien