RENEWABLE ENERGY Flying power plants By Dr Peter Harrop, IDTechEx The possibility of flying power plants that can export energy to the earth is intriguing. S olar powered aircraft is old news. From Solar Impulse supporting one person day and night with nothing but photovoltaics on the wings to the high altitude unmanned aircraft being developed by Boeing to stay aloft in the upper atmosphere for five years on nothing but sunshine, it is all very exciting. However, if we benchmark what is happening with other electric vehicles such as autonomous underwater vehicles (AUV), then multiple energy harvesting should be the order of the day, even with aircraft. Indeed, some Electroflyer electric aircraft already work the motor backwards in thermals to charge the battery. This idea should be developed further and combined with the familiar solar power to improve the continuity of energy supply and the amount, even exporting energy to earth. After all, humble electric buses are about to add regenerative shock absorbers to the regenerative braking and the occasional solar panel in their energy harvesting toolkit. Enter IFO-Energy Unlimited in Hungary, creator of an untethered, autonomous flying wind power plant, which describes it as follows. There are at least three well known problems of conventional wind energy production: firstly there are fluctuations and the unsteady nature of surface winds and these fluctuations in wind energy production put stress on the power system so capacity reserves (surplus power plants) are necessary to ensure system reliability. The company points out that another problem of wind power plants is their low efficiency. A wind turbine will generate around 20% to 30% of its maximum rated capacity depending on the location of the plant, which means that if we want to utilise wind power in large volumes we have to build wind power plants as far as the eye can see. The third common problem is that wind power suffers from a lack of energy density: that is, not only are large numbers of wind generators (and thus large land areas) required to produce useful amounts of electricity but huge dimensions too. Therefore, the idea of harnessing high altitude winds blowing at 10,000 meters altitude seems to be promising. Wind towers or tethered floating devices are not the only way to harvest energy from the atmosphere. Untethered flying units are also capable of harvesting wind energy. In order to do away with tethering, the device would need to use the updrafts and dynamic soaring utilising the energy in wind gradients commonly used by soaring birds. The storage of energy would need to be considered and according to one of the possibilities, an air liquefaction machine is set in motion that lets the produced liquid air be the Wing 7 at airborne wind power company Makani Power’s test site in Alameda, California, December 2011. Turbines on the wing generate energy during flight. Excess electricity powers a panel of 1,000 watt lights, but future prototypes will send power down the tether into the grid. (Image source: Makani Power, A. Dunlap, 2011). 62 ESI AFRICA ISSUE 3 2013