Denmark to store energy in underground balloons
How do you balance the fluctuating supply of wind power in a country with no hydroelectric dams? With a giant water balloon, of course, covered with dirt.
For 40 years, Denmark has been world-leading in energy production from wind. In this flat country, wind is abundant, and today about 20 percent of the country’s electricity production comes from this fluctuating resource. Without any control over when such a large portion of the country’s electricity is produced, matching production with supply can be hard. So far, this has mainly been solved by exporting excess electricity, and importing when needed. But as the share of wind energy grows, Denmark needs a way to balance the supply within its own borders.
In other Nordic countries, variation in electricity production can easily be balanced with the large amount of stored energy in mountain lakes. Dams can be closed whenever demand is low or production from other energy sources is high. Some places, water can even be pumped back up into the lakes. But being so flat, Denmark has to look for other solutions.
A big balloon
One promising invention comes from a 77-year old Danish engineer. The idea is in many ways the same as with hydroelectric dams – using elevation to store energy. The lack of height in Denmark can be compensated by elevating a heavier material. A 20 hectare “balloon” (about 500x500m) is covered with 25 meters of sand. Every time wind turbines produce too much electricity, the energy is used to pump seawater into the balloon, lifting the sand up to 7 meters. The weight of the sand pressurises the water to the equivalent of a 50-metre waterfall before forcing it out through the turbine.
In this way, the balloon functions as a gigantic battery for wind energy. But while a battery of that capacity would use an immense amount of natural resources and chemicals, the balloon uses a plastic membrane and the same pump and turbine as in a hydroelectric dam. The most expensive part of the installation is actually moving all the sand.
Simple, but efficient
So far, 5x5m and 50x50m tests have given results much better than anticipated, with an efficiency of 97 percent. The next step is to build a 200x200m pilot in a clay pit, and using the local clay to cover the balloon. Calculations show that a full scale prototype could have an efficiency of 99.5 percent – as good as any hydroelectric dam . It will be able to store 200 MWh of energy, enough to power a Danish town of 50 000 inhabitants for about eight hours. Installment is estimated to cost 27 million Euro.
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