As anyone who has flown a kite knows, it is easy to use the high winds to keep it flying steadily, as long as you can get it high in the air. Now scientists are trying to harness this property to generate electricity by placing large “kites” more than 200 metres above the ground.
The higher you go, the more energetic you are
Winds close to the ground tend to slow down or even change direction because of friction with trees, hills, buildings, etc. As height increases, the average wind speed increases. High-altitude wind energy is more abundant and stable than current near-surface wind energy. Over the past few decades, humans have proposed many ideas for harnessing high-altitude wind energy, such as installing turbines on aircraft. At present, a more feasible method is to use a computer to control the shape of the “kite”.
There are two ways the kite wind system can generate electricity. The first, flying in the air, generates electricity on the ground – one end of the cable is attached to the “kite” and the other end is wrapped around the winch. As the “kite” flies in the wind, it pulls on the cable and turns the winch on the ground to generate electricity. The kite is then closed, reducing its lift, and the cable pulls the kite back. Repeat the process, and you generate electricity periodically. The second, air-powered, air-powered “kite” is designed to be a rigid structure similar to an aircraft wing that supports small wind turbines. When the kite flies, the wind drives a turbine, and the resulting electricity is sent down a cable with a metal core to the ground.
Compared with traditional windmills, “kite” power generation has great advantages in terms of material cost. Building a windmill requires a lot of stone, earth and steel to “lift” the wheel at the right height. In kite-powered systems, these structures are replaced by much smaller ground stations and cables. One study found that a 50-megawatt “kite” system would consume only 913 tonnes of materials over its 20-year lifetime, compared with 2,868 tonnes for a windmill. There is no doubt that “kite” power generation is greener and cheaper.
Kite power is especially important for offshore wind power. In the current field of offshore wind, when the water is too deep to build a windmill base, wind turbines need to be loaded onto huge, barge-like structures that float the turbines, making them expensive. The “kite” power generation system is small in size and light in weight, and the requirements for floating structure are relatively loose.
However, these advantages come at the cost of technological complexity. The economic benefits of “kite” power generation can only be realized based on long-term operation and less manpower input, which undoubtedly puts forward a test in the field of computer automatic control research. Remember, the kite doesn’t just float around randomly as it generates electricity: It maximizes lift when the face of the kite is perpendicular to the direction of the air flow, making the power generation most efficient. This flexible maneuverability needs to be ensured by the computer constantly adjusting the “kite”. Control of the rigid “kite” is achieved by adjusting steering components such as flaps and rudder, just like in an airplane; Control of the flexible “kite” is achieved by adjusting the length of the steering line, similar to a parachute.
Current state-of-the-art “kite-powered” systems are able to fly for days on end, guided by on-board and ground-based computers, when the wind remains steady. But to scale up, kites must be able to reliably cope with sudden weather changes, such as strong winds or showers. Automatic take-off and landing functions are also necessary in extremely bad weather. Scientists say that a short period of smooth operation is not enough, and they are working to make the operating life of the device years or even decades, and that there is still a long way to go.
There are still many technical difficulties
In fact, in addition to the previously mentioned control of aircraft, wind and thunderstorms and other extreme weather equipment safety and operational stability, high altitude wind energy in the application of engineering challenges are many. For example, the manufacture of durable cables, the loss of cable transmission, the operation capability of equipment in high altitude and low temperature environment, life, and the deicing capability of aircraft (including cables), etc. Determining the closest distance between the two devices (in case the cable gets tangled) is a particular headache. This makes sense — the higher you fly, the longer the tether, the further apart the equipment you need, and the greater the scale of the ground system required. There is no agreement on how to define this distance.
As a new technology, engineering problems and social impacts always accompany each other. Traditional wind farms are often subject to complaints from nearby residents, such as mechanical noise and space occlusion, which can affect their living experience. So, can we ensure that the impact of the technology on the living environment is within acceptable limits? Unfortunately, although the kites are not as big as windmills, the noise generated by their movements is not negligible, according to published studies.