Zero CO2 – From Vision to Reality

Zero CO2 – From Vision to Reality
District heating and cooling using lake water: a CO2-neutral energy concept in Bregenz
For some time now, Generali Real Estate (GRE) Austria has been consistently pursuing the goal of operating its sites in a CO2-neutral manner. Much has already been achieved to date, ranging from industry-leading real-time monitoring of energy consumption and the exclusive use of green electricity to the systematic expansion of photovoltaic systems across its portfolio. An initiative by Stadtwerke Bregenz came at just the right time; as a pioneer in Austria, the company offers a renewable district heating supply using water from Lake Constance. The provincial capital of Bregenz is already connected to this system via its indoor swimming pool and festival theatre. Now Generali is following suit with its existing properties on Quellenstraße in Bregenz – thereby opening yet another new chapter.

Bregenz Municipal Utilities as the driving force
The impetus for the technically innovative solution – utilising the thermal delta, i.e. the temperature difference, of lake water for a large-scale heat pump system – came from Bregenz Municipal Utilities. Whilst searching for potential target customers, a particularly suitable property was also found within the GRE portfolio: a mixed-use building complex dating from the 1990s with approximately 4,300 m² of lettable space and a central heating and cooling system that was due for a technical overhaul. In close collaboration and following a project planning phase that began in spring 2025, the conversion work has now commenced. The conversion of the building complex from the current gas supply to sustainable lake water district heating and cooling is expected to be completed by mid-2027.

Using Lake Constance as a solar energy storage reservoir
But how does a lake water heat pump actually work? Broadly speaking: by utilising the temperature difference between the upper and deeper water layers of Lake Constance. Each season, the temperatures in the upper water layers rise from around 6 degrees in winter to 20–25 degrees in summer. The solar energy stored in the surface water in this way is harnessed via heat exchangers by feeding it into an external circuit. The cooled lake water is then ‘returned’ to the lake at a greater depth (schematic diagram). In exactly the opposite way, the temperature difference between the lake water layers is utilised for cooling, i.e. also via heat exchangers.

In purely mathematical terms, given the vast surface area of Lake Constance (approximately 500 km²), it would indeed be possible to supply all residents with heat – and the temperature of the lake would drop by a mere 0.2 degrees.

In practice, however, the cooling effect of the lake water is not significant enough for the use of lake water to be considered not only fully renewable but also ecologically neutral. This is also demonstrated by experience in Switzerland, where the same technology has been used reliably and successfully in Lake Geneva, Lake Zurich and Lake Lucerne, in some cases for decades.

Photovoltaic system for own use to achieve zero CO₂ emissions
For the GRE properties on Quellenstraße, however, the connection to the CO₂-neutral lake water district heating system is only one part of a comprehensive project. As part of the energy-efficiency refurbishment, on-site PV systems are also being installed, which use their own heat pumps to bring the preheated water up to the final operating temperature. In addition, much of the extensive cooling and ventilation technology is being completely replaced and brought fully up to the latest technical standards – right down to smart meters.

Wolfgang Hufnagl, construction project manager at GRE: “At Quellenstraße, we have achieved an ideal combination of cost-effectiveness, durability and environmental sustainability, which makes everyone involved truly proud of what we have achieved together.” GRE Asset Manager Bernhard Mauritz: “For me, Quellenstraße is a showcase project demonstrating how, through sustainable optimisation of our existing portfolio, we can simultaneously create value for ourselves and – through low operating costs and green energy – added value for our tenants.”

Generali Real Estate S.p.A.
Austrian Branch
Lugeck 1-2
1010 Vienna / Austria
T +43 1 535 01 01