Automated translation. English version for convenience only.
Automated translation. English version for convenience only.
Sonnen Speicher Süd is Austria's largest project currently being implemented to decarbonize district heating supply. An existing basalt quarry in Weitendorf near Wildon is being converted into a seasonal thermal storage facility with a capacity of 1.5 million m³. From 2029, solar heat stored in summer will cover up to 30% of Graz's total district heating demand with renewable energy.
The Weitendorf quarry offers ideal conditions: a naturally dense basalt and marl formation as a storage basin, sufficient space for solar collectors in the immediate vicinity, and an existing district heating pipeline only 4 km away. Using existing infrastructure minimizes interventions and costs.
The main beneficiaries are district heating customers in the greater Graz area – around 30% of Graz's total district heating demand will in future be covered by Sonnen Speicher Süd. The entire region will also benefit from the reduction in quarry operations, regional value creation during construction and operation, and a measurable contribution to Styria's climate targets.
The combination of the storage facility's size, the high share of solar energy and the reuse of an existing quarry is unique worldwide in this form. Larger storage facilities are physically more efficient - the ratio of heat loss to stored energy decreases as storage size increases.
In summer, solar collectors heat the storage water in the flooded quarry to around 90 °C. The heat is stored there until winter. When needed, the stored thermal energy is extracted and fed into the district heating network. The principle is simple - implementing it on this scale is unique worldwide.
The system combines three renewable sources: solar heat from around 28 hectares of gross collector area as the main source, high-temperature heat pumps to increase storage efficiency, and a biomass heating plant for reheating to district heating temperature. The basic principle is "solar first" - biomass is only used when solar energy alone cannot reach the required network temperature. This has been contractually agreed.
The high-temperature heat pumps, with a thermal output of 60 MW, perform a dual function: they use the storage facility as a heat source and at the same time ensure balanced stratification in the storage basin. Deeper, cooler layers are conditioned so that the solar collectors can operate with a colder return flow and therefore higher efficiency. In summer, they also operate in line with electricity prices and contribute to grid stabilization.
A roughly 4 km transport pipeline connects the water heated to district heating level to the existing Mellach-Graz main district heating pipeline. Using existing infrastructure minimizes interventions in the landscape.
Solar heat is generated in summer, while heat demand is highest in winter. A seasonal storage facility bridges exactly this time gap. In addition, the larger the storage facility, the more efficient it is - the ratio of heat loss to stored energy decreases as size increases.
A floating insulation cover on the water surface minimizes upward heat losses. The basalt and marl formation of the quarry provides natural insulation at the sides and bottom.
Once the storage facility is filled with water, the rock formations concerned will no longer be accessible - just as they already are not today. The uniqueness of the Florian layers, relevant at the time, also no longer applies, as several further fossil sites have since been documented (including St. Josef/Fuggaberg and the Retznei quarry).
The marl layer declared a natural monument will remain physically intact - a study by Graz University of Technology confirms this even in the case of direct contact with the stored water. In addition, during construction of the storage facility, fossil-bearing marl occurring outside the protected area is planned to be recovered without damage and stored under optimal climatic conditions at the Römersteinbruch Aflenz. There are also plans to publicly exhibit selected fossils in the quarry's former crusher building. This ensures the fossils' preservation for the long term.
The storage facility lies entirely below groundwater level, embedded in a geologically dense marl formation - so artificial sealing is not required. A cut-off wall in the north ensures that no groundwater reaches the storage cover. Independent hydrogeological expert reports form the basis for the ongoing water law procedure and confirm that there will be no negative impacts on water rights or drinking water supplies in the surrounding area.
The hydrogeological expert reports conclude that negative impacts on water rights or drinking water supplies in the surrounding area are not expected. Groundwater monitoring around the quarry has been ongoing since 2024 and documents the initial state as an undisputed reference baseline.
The quarry has been in active operation for more than 300 years. With Sonnen Speicher Süd, quarrying operations will end - permanently reducing the intervention in the landscape and nature. The individual permitting procedures, including nature conservation and water law, examine all ecological impacts through independent official experts.
Sonnen Speicher Süd replaces a significant share of fossil energy in the district heating network, especially natural gas. The project therefore makes a measurable contribution to the City of Graz's climate goals, which aim for a fully renewable heat supply by 2040.
When active quarry operations end, the previous truck traffic for transporting rock will cease. Biomass deliveries will be added. According to an independent traffic report, the switch will reduce truck traffic on the L603 by at least one third compared with actual quarry operations in recent years.
Operation of Sonnen Speicher will be significantly quieter than the previous quarry operation - no blasting and no heavy extraction traffic. The relevant limits will be reviewed and set as binding requirements as part of the official approval procedures.
During the construction phase, regional jobs and contracts for local companies will be created. Operation will create permanent infrastructure that enhances the site in the long term and replaces active quarry operations.
Yes. The Federal Administrative Court has legally confirmed that an environmental impact assessment is not required. In the ongoing individual permitting procedures - including water law, nature conservation and building law - independent official experts are reviewing all relevant impacts.
The project has received funding commitments from national and European programs. All funding is tied to specific milestones - funds are only paid after implementation has been demonstrated. The funding bodies review the project independently and continuously.
Operation is planned from 2029. The first heat delivery into Graz's district heating network is therefore scheduled for autumn 2029.