22 Sep 2021
by IEA DHC

Stadtwerk Lehen in Salzburg, Austria

New low-energy apartments, a kindergarten and a student dormitory were built on an area of approx. 43,000 m² in a new city Quarter in Salzburg/Lehen. It is named ‘Stadtwerk Lehen’, is an energy-saving construction and is the core project of Salzburg’s contribution to the EU project ‘Green Solar Cities’ in the Concerto programme. It entails heat supply for low-temperature network with solar system and storage resulting in a win-win.

The heat is stored from a total of 2,047 m² (gross collector area) of thermal collectors on the roofs in the central 200 m³ storage. Here, a heat pump uses the temperature difference in the buffer storage and additionally cools the lower volume of the storage. The heat pump increases the yield of the solar system by approximately 15%. The heat is distributed to the users from the storage. The annual average supply temperature is 65 °C and the return temperature ranges from 35 to 45 °C. An additional 20.2 kWp PV system on the roof supplies electricity for general systems (ventilation, heat pump, underground car park lighting), a further 30 kWp were installed by the utility Salzburg AG in the immediate vicinity.

In each apartment, there is a substation where the domestic hot water is produced locally as required (hygienically the best solution) and the heat requirement for heating and domestic hot water is counted. A LED visualisation on the storage informs all passers-by how much thermal energy and how much electricity is currently produced by the sun and how much of the demand has been supplied in the last 24 hours (Figure 80). Due to the low-energy construction of the buildings, the total energy requirement was approx. 1.5 GWh/a, approx. 30% lower compared to the usual construction standard. Furthermore, 68% of the primary energy and 76% carbon dioxide can be saved compared to a conventional oil-heated residential complex.

After commissioning in November 2011 an energy monitoring started with special emphasis on informing residents. Stadtwerk Lehen was selected as a test area for smart meters and interested residents can obtain information on energy consumption online on an ongoing basis. This should have a positive influence on user behaviour and the satisfaction of the residents. Monitoring supports the operator (Salzburg AG) in system optimisation and makes it transparent for the user that a high proportion of renewable energy is used in this project.

Figure 80_CS5.png

The monitoring shows, that especially due to the low network temperatures and the heat pump the solar yield is higher than calculated. But the energy demand of the buildings is much higher than calculated as well, due to higher room temperatures than designed for.