Motivation and Objectives

In Austria, around 40 % of final energy consumption is attributed to the heating and cooling sector, including process heat in industry (energie.gv.at). This makes it the largest contributor to total energy consumption—ahead of the transport sector and other applications. Heat usage is particularly dominant in this context. Currently, 39 % of the energy used for heating and cooling comes from renewable sources. This means that over 60 % is still covered by fossil fuels. This dependency highlights the central role of the heating transition in ensuring the success of the overall energy transition.

Especially in urban areas, numerous locally available renewable heat sources such as ambient air, solar energy, groundwater, or waste heat are accessible. However, many of these sources are location-dependent or only usable to a limited extent. Ambient air and solar energy are almost universally available but have so far been only marginally considered in heating concepts.

The goal of the Multi-WP project was to optimise multivalent heat pump systems—consisting of air and brine heat pumps, geothermal probes, photovoltaics, and seasonal storage—in terms of efficiency, flexibility, and load shifting. By intelligently combining these technologies, locally available non-fossil energy sources can be utilised more efficiently, and the annual performance of air heat pumps can be significantly improved. The project contributes to reducing greenhouse gas emissions, strengthening domestic technology development, and increasing independence from energy imports.

Methodology and Tools

A central element of the system design was the rapid assessment of geothermal potential at each site. For this, the Geothermal Atlas of GeoSphere Austria was used—a free online tool that calculates the energy potential of a geothermal probe field over 20 years based on location data, operating mode, and optional nominal capacity. It automatically provides energy flow diagrams, temperature forecasts, and reports, and was used in the project to evaluate various scenarios and estimate drilling requirements relative to available space.

Detailed simulations were carried out using the enhanced tool PYGsim, based on the pygfunction module. It enables dynamic coupling of heat sources and sinks, considering regeneration effects. Real weather data (2016–2022) from GeoSphere Austria and hourly heating and cooling profiles were used. The profiles are based on annual energy demand and temperature-dependent functions, adjusted for building inertia. Simulations run over 20 years, with the multi-year profile repeated cyclically.

Influencing Factors and System Components

The performance of geothermal probes is influenced by several factors:

• Open space and probe spacing: The larger the available area and the smaller the spacing (5 to 10 m), the higher the potential.
• Drilling depth: Depths of 120 to 150 m are common in Vienna; deeper probes yield more energy.
• Operating mode: Regeneration through space cooling reduces space requirements and improves efficiency.
• Heat Transfer Medium: Water-antifreeze mixtures (typically water-glycol) are standard. Pure water is environmentally friendly and has better performance, but it requires larger probe fields to prevent the risk of freezing. Pure water can be used when there is high waste heat utilization. Water-glycol mixtures are standard; water-ethanol mixtures are an environmentally friendly alternative with low viscosity and high specific heat capacity.
• Subsoil: Temperature, thermal conductivity, and groundwater significantly affect efficiency.
• Probe geometry: Double-U-tube probes with thermally enhanced grouting material are state of the art.
• Specific extraction rate: Below 30 W/m, probes are not economically viable.

Heat pump characteristics were provided by project partner Ochsner Process Energy Systems (OPES). The simulation dynamically calculates heating output, power consumption, and Coefficient of Performance (COP) depending on source temperature. Unlike the Geothermal Atlas, (Quelle: GeoSphere Austria) which uses simplified operating functions, PYGsim allows modelling of complex regeneration strategies—such as using waste heat in winter, as implemented in the Perchtoldsdorf case study with an ice rink. Such strategies can reduce drilling needs by up to 20%.

Probe sizing followed the ÖWAV guideline 207. The goal was to maintain probe inlet temperatures between -3 °C (heating) and +30 °C (cooling/regeneration) during operation. In cases with high waste heat utilisation, frost-free operation was possible, allowing the use of pure water or low antifreeze concentrations.

Results and Conclusions

The six case studies in the Multi-WP project show that all examined concepts are technically feasible. Notable successes include the integration of geothermal probes in densely built areas (e.g. Abelegasse, Natural History Museum Vienna) and the use of various waste heat sources—from an ice rink (Perchtoldsdorf), a data center (Hohe Warte), or combined air and brine heat pumps (Aslangasse). In all cases, CO₂ emissions during operation were significantly reduced, and long-term operating costs were lower compared to conventional heating systems. Investment costs vary by location but remain economically acceptable.

The further development of PYGsim enabled realistic modelling of system dynamics and regeneration effects. The Geothermal Atlas proved to be an efficient tool for initial assessments. The developed concepts are particularly suitable for existing buildings without district heating connections, which are common in many cities. The results provide a solid decision-making basis for planning offices, developers, energy suppliers, and municipalities.

Outlook

The project results are being disseminated through publications, training sessions, and professional events and are already being incorporated into further projects. The developed concepts and tools—especially the simulation techniques—are continuously being refined. Given the growing demand for decentralised, renewable energy solutions in the building sector, the results make a concrete contribution to achieving climate goals and transforming the energy system in line with the "City of the Future" programme.

Project management

Austrian Energy Agency (AEA)

Project or cooperation partners

• Ochsner Process Energy Systems GmbH (OPES)
• Geological Survey of Austria (GBA)

Dr. Franz Zach
Mariahilfer Str. 136
A-1150 Wien
Tel.: +43(1) 5861524-106
E-Mail: franz.zach@energyagency.at
Web: www.energyagency.at