Project Description

The world’s first high pressure heat storage tank

Wien Energie put the world’s first high-pressure and high-temperature heat storage tank into service in 2013. The heat storage tank in Vienna’s Simmering district makes it possible for heating to be used when it is needed, regardless of when it was actually produced. It covers the annual heating requirements of around 20,000 households and saves around 11,000 tonnes of CO2 every year by optimising production and storage. Total investment amounted to EUR 20 million.

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A picture says more than 1,000 words

See for yourself how our heat storage tank was built.

Project details

A heat storage tank was built in just 14 months for Vienna’s large and complex high-pressure and high-temperature district heating network. The storage tank uses and bolsters the production of renewable heat from the Simmering biomass power plant. Heat producers such as our thermal waste treatment plants and cogeneration plants are also integrated here.

Put simply, the heat storage tank on the premises of the Simmering power plant is nothing more than a gigantic thermos flask. At 45 metres, the two storage tanks are significantly taller than a twelve-storey building and subjected to five and a half times the pressure of a pressure cooker. Both containers are always filled to the brim with hot (150°C) and ‘cold’ (70°C) water. The high pressure is needed so that the water does not evaporate when above 100°C.

The pressure and other physical forces create a dividing layer between the two quantities of water at different temperatures, which means that they never mix. If energy is to be stored, cold water is drained and topped up with hot water. If the heat is needed in the network, hot water is removed and replaced with supplies from the district heating system.

The difference in height between the 1,200 km-long district heating network in Vienna is as much as 150 metres. For this reason, hot water for heating and hot water is transported throughout Vienna under high pressure and at temperatures between 95°C and 150°C. Our heat storage tank was therefore built as a pressure storage tank with a water temperature of up to 150°C. The requirement for it to be a pressure storage tank comes from the topography as Vienna’s district heating network is under a pressure of about 15 bar, meaning that we are even able to supply higher parts of the city with district heating.

Instead of having to produce additional heat during peak consumption periods, the heat generated ‘in-house’, such as in the thermal waste processing plants, is stored directly so that it can be used at a later date.

This thermal storage plant will allow the timing of heat production and heat consumption to be decoupled, thereby making it possible to minimise the use of peak-load boilers during periods of high demand. The system makes a major contribution to optimising the use of the highly efficient cogeneration plants and thermal waste treatment plants in conjunction with renewable energy. CO2 emissions are reduced and daily peaks for district cooling during the summer are handled.

Further reading

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Power-to-heat plant

The power-to-heat plant resembles an oversized kettle that converts excess electricity into environmentally-friendly heat. The plant has a total capacity of 20 MW and supplies 20,000 households with district heating, thereby playing a key role in maintaining stability in the power grid.


Large-scale heat pump

Utilising existing waste heat is an essential part of being able to supply heating in an even cleaner way. Simmering is where we run the most powerful large-scale heat pump in Central Europe, supplying around 25,000 households in Vienna with environmentally-friendly district heating, thereby saving 40,000 tonnes of CO2 every year.