How useful it would have been to be able to store the “surplus” heat of recent weeks in a place where it would be available again when the days get colder. The technology is not so far advanced yet, but it soon will be. Starting next year, the revolutionary new high-pressure thermal energy storage facility now under construction in Vienna’s Simmering district will supply district heating to 20,000 households.

A large number of industrial processes produce (waste) heat. Not just "a bit of cosy warmth”, but quantities of heat that make it economically and ecologically worthwhile to recover and store it, and then transfer it to where heat is needed rather than just letting it escape through a smokestack.

Wonderful water

The varied world of “Wien Energie”, Vienna’s energy utility, contains many heat producers, and starting in 2013 many of them will start feeding a new high-pressure thermal energy storage facility. They include not only combined heat and power stations in Simmering, Donaustadt and Leopoldau, but also waste incineration plants, the wood biomass power station in Simmering, and – at a later date – Aspern geothermal plant.

The problem is that heat cannot, in itself, be stored: it requires a storage medium. A medium that absorbs the heat and – ideally – is also suited to transport it to where it can be separated from this medium again to become pure heat. Without digressing too much into technical details, water has proved an ideal medium for this purpose.

Under high pressure

Under enormous pressure, it is heated up to 150 degrees Celsius where the heat is generated before being conveyed through a high-pressure pipeline to a thermal energy storage facility consisting of two storage tanks. A storage capacity of 850 megawatts is sufficient to supply up to 20,000 households with heating and hot water.

In order for this to work, it is necessary to heat up 11,000 cubic metres of water (since it is pressurised, equivalent to over 11 million litres) at a pressure of up to 10 bar. This pressure is necessary not only to acquire and store the heat, but also to cover the distance between the thermal energy storage facility and the end customer, and even differences in height of up to 150 metres.

Size for size

This naturally requires a great deal of space, which is why the two storage tanks, each 45 metres high, are being constructed by 80 to 100 men using 2,000 cubic metres of concrete. The thermal energy storage facility not only makes it possible to use existing heat really economically, but it also has a positive ecological impact. “Every year this will enable us to save exhaust gas containing about 10,000 tonnes of carbon dioxide. This is roughly equivalent to the amount you would save if you decommissioned about 4,200 medium class cars for a whole year,” explains Wolfgang Daschütz, project manage for this ambitious project by “Wien Energie”.

Energy strategy

The two enormous heat storage tanks – each of them eight metres higher than the roof of St. Stephen's Cathedral – are the most striking project in “Wien Energie’s” energy strategy. By increasing its use of wind and solar power, and also geothermal sources, the company intends to obtain half of the energy it produces from renewable sources by the year 2030.

Perhaps technology will be sufficiently advanced by then to allow us to save 5° Celsius for bad weather periods whenever the temperature soars to 37° Celsius. That would be a fine achievement indeed!