segunda-feira, 28 de setembro de 2009

Putting sun and wind in a bottle*

One of the main criticisms against renewable energy sources is that they cannot be stored. The intermittence of wind, wave or solar energy creates a lot of trouble to network operators and raises doubts when it comes to security of supply. Some people consider them not to be economic because renewable sources need additional conventional power plants for the days that wind does not blow or the sky is cloudy. They claim that you can store oil, coal, gas or uranium and have them handy for the moment you need them but you cannot put sun or wind in a bottle. Or maybe you can?

The Commission has been financing research in two interesting projects that do precisely this. The first one is in the beautiful island of Hierro, the smallest in the Canary archipelago. It has been declared Biosphere Reserve by UNESCO and its island government has taken the brave decision of producing 100% of their energy needs from renewable sources. The main element in reaching this goal is a wind-hydro-energy system that does store renewable energy. How does it work? Five large wind turbines produce electricity that is sent to the network for local consumption. When there is an excess of production (for instance, at night) the electricity is used to desalinize sea-water and to pump that water up to an artificial lake 700 m above the sea level. When the wind does not blow or there is an increase electricity demand, the water is piped down at high pressure and water-turbines generate electricity from it. When the wind blows again, the cycle re-starts with water being pumped up.

This system is expected to be in operation next year and the experience of Hierro is likely to be exported to other islands or other territories of the EU. Currently, the island of Hierro is producing its electricity from a 10 MW fuel-oil power station. When the new system comes into service, 18.700 tonnes of CO2 emissions will be avoided.

The second project aims at storing solar energy and is located near the sunny city of Seville in the South of Spain. In this case, the heat of the sun is concentrated with large mirrors to the top of a 50m high concrete tower, where temperatures are as high as in any thermal power station. Heat makes the water boil, and high temperature steam moves a normal generator. The real revolution of the system is that the sun’s rays are also used to heat salts to the point that they melt and reach very high temperatures. When the sun sets or the clouds cover the sun, these hot salts are the ones that produce the steam that generates the electricity. When the sun rises the process starts over and the salts again store the energy of the sun.

Those are simply two examples. Other scientists consider that hydrogen and fuel-cells could be another good way of storing renewable energy, while some specialists think that electric cars connected to smart grids may be the solution of the future. In any case, I think it is not acceptable to say that we’d better use fossil fuels instead of renewable energies because wind and sun cannot be stored. As a matter of fact, fossil fuels are nothing but stored solar energy, and we’d better to have something else stored in stock, for the moment they run out.

*Andris Piebalgs, Comissário Europeu da Energia

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