Large solar thermal plant in Mürzzuschlag, photo: SOLID Solar Energy Systems GmbH

Large solar thermal plant in Mürzzuschlag, photo: SOLID Solar Energy Systems GmbH

Flagship project for more flexible heating networks

District heating systems are an environmentally friendly and convenient way to provide energy for heating and hot water. They also play a central role in Austria’s energy supply. Currently, 27 % of Austrian households are heated with district heating, and this figure is predicted to increase further. The total pipe length of the district heating networks of Austria’s heating supply companies increased from 4,100 km in 2010 to roughly 5,600 km in 2020. A network length of 6,500 km is forecast for 2030.1

To quickly increase the share of renewable energy in the supply of district heating from the current level of roughly 50 %, it is necessary to make use of various local and sustainable heat sources. Alongside biomass, other new heat sources to be used include waste heat, deep geothermal energy, large heat pumps, solar thermal energy and power-to-heat. The flagship project ThermaFLEX, led by AEE INTEC in cooperation with an interdisciplinary team of 28 project partners2, is working on exactly this. The project focuses on developing and demonstrating how heating networks can be made more flexible and efficient in order to cope with not using fossil fuels in the future. Demonstration projects with scientific support are being implemented at various locations in Austria for this purpose.

Higher flexibility in the district heating sector

District heating networks are currently supplied centrally by just a few generation plants. The switch to renewable, locally available energy sources will require a more distributed approach with many generating facilities and higher system complexity. To ensure a reliable energy supply, the fluctuating availability of energy from renewable sources must be integrated into the system as a whole, and the fluctuations must be balanced out. This requires flexible options and an intelligent interplay of technical and non-technical elements. Topics of great importance here include energy storage systems, the coupling of different energy sectors, intelligent regulation and control concepts as well as the inclusion of users and relevant stakeholders. Integrated planning, implementation and operational management processes are required in parallel with these efforts, such as new approaches to spatial energy planning or life-cycle analysis.

ThermaFLEX supports the transformation process

As part of the flagship project, technical, non-technical and systemic measures to make heating networks more flexible are considered in combination and implemented in demonstration projects. The focus was placed on 10 locations in small, medium and large district heating supply regions across Styria, Salzburg and Vienna, which all make use of a wide range of different measures, heat sources and flexibility approaches. The project results show that large-scale technical implementations are possible even within a relatively short time. The entire process from problem identification, concept development and detailed planning to implementation, data monitoring and optimisation efforts was accompanied by detailed scientific analysis. Holistic system analysis and life-cycle analysis techniques that take the entire value chain into consideration were used to evaluate the results. Roll-out scenarios were developed in order to demonstrate the potential for scalability and widespread implementation. Many new insights were gained in the process, which can be put to use in all areas of the district heating supply sector. Important future research topics include long-term storage systems, the phasing out of large gas-powered combined heat and power plants and further digitalisation.

ThermaFLEX Projekte; Karte: Waldhör KG
ThermaFLEX demo projects; Chart: Waldhör KG

1 High-temperature heat pump at Wien-Spittelau
Use of waste heat from flue gas condensation at the waste incineration plant as a heat source for a high-temperature heat pump (16 MWth)

2 Renewable heating and cooling from waste water – Wien Kanal
Energetic use of waste water for heating and cooling, with an innovative heat exchanger and heat pump system

3 Use of waste heat by Therme Wien

4 Heat from waste water at Wien-Liesing
Use of waste water directly from the sewer as a heat source for a 2 MWth heat pump and feeding into the secondary district heating network of Vienna

5 Modernisation concept and heat pump integration in Saalfelden

6 Use of industrial waste heat in Hallein and the Salzburg eco-energy park
Concept development for absorption heat pumps (HP) at industrial scale

7 Large solar thermal plant in Salzburg (Big Solar Salzburg)
Concept for a large solar thermal installation (roughly 30,000 m² of collector area), a large water-based thermal storage system (roughly 20,000 m³) and the integration of an absorption heat pump (roughly 14 MWth)

8 Large solar thermal plant in Mürzzuschlag
Integration of a roughly 5,000 m² solar thermal installation into the heating network in combination with a 180 m³ storage system

9 Virtual heating plant Gleisdorf
Coupling of district heating with the municipal waste water treatment plant and use of a new control concept (virtual heating plant)

10 100% renewable district heating for Leibnitz
Use of fluctuating waste heat (maximum of 4 MWth) from a industrial plant and bidirectional coupling of two district heating networks

Photo: Salzburg AG

„Salzburg AG is committed to decarbonisation and ensuring that our district heating plants are ready for the future – aspects that are now receiving increased attention. The questions investigated within the ThermaFLEX project are key factors for achieving this goal. The concepts and know-how from ThermaFLEX will allow us to transfer the results to our district heating systems in order to guarantee a safe, sustainable and cost-efficient heat supply for our private and industrial customers.“
Thomas Herbst, Head of District Heating and Energy Systems
Salzburg AG für Energie, Verkehr und Telekommunikation

Projekt partners:
AEE INTEC (project management), AIT Austrian Institute of Technology GmbH, BEST – Bioenergy and Sustainable Technologies GmbH, FH Joanneum Gesellschaft mbH, JOANNEUM RESEARCH Forschungsgesellschaft mbH, SIR Salzburger Institute for Regional Planning and Housing, StadtLABOR – Innovationen für urbane Lebensqualität GmbH, TU Wien/Energy Economics Group, TU Graz/Institute of Thermal Engineering
Energy suppliers & infrastructure operators:
Abwasserverband Gleisdorfer Becken, Energie Steiermark AG, Feistritzwerke-STEWEAG-GmbH, Haselbacher Nahwärme, Salzburg AG für Energie, Verkehr und Telekommunikation, Stadtwerke Gleisdorf GmbH, Wien Energie GmbH
Know-how and technology providers:
ALOIS HASELBACHER GmbH Haustechnik, ENAS Energietechnik und Anlagenbau GmbH, FRIGOPOL Kälteanlagen GmbH, Green Tech Cluster Styria GmbH, GREENoneTEC Solarindustrie GmbH, Horn Consult, Pink GmbH, Rabmer GreenTech GmbH, ROTREAT Abwasserreinigung GmbH, Schneid Gesellschaft m.b.H., STM Schweißtechnik Meitz e.U., SOLID Solar Energy Systems GmbH

The lead project ThermaFLEX is being carried out under the “Green Energy Lab” research initiative as part of the innovation offensive “Vorzeigeregion Energie”.


  • Absorption heat pump in Hallein, photo: Climate and Energy Fund/Krobath
    Absorption heat pump in Hallein, photo: Climate and Energy Fund/Krobath
  • Biomass heating plant in Saalfelden, photo: Climate and Energy Fund/Krobath
    Biomass heating plant in Saalfelden, photo: Climate and Energy Fund/Krobath