A hard fact that many wind energy enthusiasts struggle to accept is that not everywhere is suitable for wind power. The energy in the wind varies widely around the world and if the wind is not strong enough to be a nuisance most of the time then it is just not economical to harvest it.
Before making a big investment in wind energy it is vital to measure the wind at the exact location of the prospective project. In my Wind Distribution page I explain how to analyse the wind speed data that should be collected with an anemometer.
Another tough fact to face is that the ability to resist storms is critically important. There is no point installing something that gets destroyed in the first really strong gust. Any wind energy conversion system (WECS) needs to be big to harvest a large volume of wind. That size means that it experiences huge forces when the wind occasionally gets really frisky. In the turbine industry it has been well established that harvesting the power from winds faster than 11 to 15m/s is just not economical. The generator starts to cost too much, as does the associated electronics. The turbine must start to protect itself by the time the winds reach 10 to 12m/s. Most big turbines are completely stopped when the wind exceeds 25m/s. Such winds do not last long and it just not worth the cost of trying to operate in such conditions. Kites will be able to operate over a wider range of wind speeds as explained below.
The third important consideration is how the wind increases in speed with altitude. The surface of the ground forces the wind near it to a stop and this creates wind shear. Trees, buildings and smaller objects all create resistance to wind movement so the affect of the shear rises higher if these obstacles are present. A wind turbine should be mounted on a tower at least twice the height of nearby obstacles. If there is a high structure in the area then the turbine should be at a distance of 10 times its height away from it.
Wind shear means that the wind speed continues to rise to an altitude of about 10 000m. Harvesting the wind at such altitudes is very unlikely to ever be economical. However, the importance of going as high as possible deserves considerable thought because the increase in power available is considerable. With a conventional turbine on a mono-pole tower the mass of tower increases 8 fold every time the height is doubled. Although mono-poles are preferred because they are generally considered to look better, they very soon become too expensive to make significantly tall. Stayed towers cost less for a given height but even they are strictly limited. Using a kite on a tether allows us to reach much greater altitudes for much less cost.
In fact using kites brings a huge number of advantages. Take a drinking straw (or small stick) and imagine that it is a wind turbine tower. Feel how much force it takes to make it buckle at the bottom. Now feel how much force it takes to break it by pulling the 2 ends apart. The difference is several thousand fold. With a turbine it is not just the tower that must resist bending moments that cause buckling. The blades must also be extremely stiff. With a kite nearly everything flying in the air can be under tension. This means the mass of those wind harvesting parts are thousands of times lighter than the equivalent parts of a turbine. Even if significantly more expensive materials are needed, this huge mass saving means a huge cost saving must also be possible.
Kites will definitely be able to harvest the more powerful winds at higher altitudes for a lower price so the field is growing rapidly. It is generally called airborne wind energy (AWE). A legitimate question is why AWE has been so slow to emerge. The main problem is that it is very difficult to stop the kite tethers getting tied in a knot. Human operators are presently required to make sure the kite stays where it should while the wind gusts and circles in every direction. As we learn to automate the technology the need for human supervision will decrease and the costs will come down. At Visventis we are working on a manually controlled kite. Once we have it operating well under human control we will start working on the automation of the various processes.
Another huge advantage AWE has is that it can make use of a much wider range of wind conditions. The wind blows more steadily at high altitude and it in many locations the wind up there is fast enough for economical harvesting even when the wind at turbine altitudes is far too weak. More importantly, because the kites can be quickly changed it is possible to have a quiver of them for different wind conditions. In light winds a large light kite is flown. As the wind increases the kites are swapped for smaller tougher versions. The generator and the other expensive electrical parts are therefore working harder for longer, bringing the price of AWE down even more.Global warming
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