Berlin-based wind energy developer NTS and experts from the Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart are now working on a project to harness the power of winds at altitudes of up to 500m.
Joachim Montnacher, an engineer at the IPA, said: ‘The kites fly at a height of 300–500m, perfectly positioned to be caught by strong winds. Cables, about 700m in length, tether the kites to vehicles and pull them around a circuit on rails.
‘A generator then converts the kinetic energy of the vehicles into electricity. The control and measuring technology is positioned on the vehicles.’
Compared with conventional wind farm technology that relies on rotors, this technology is said to offer a wide range of advantages: at a height of 100m wind speeds are around 15m/sec while at 500m they exceed 20m/sec.
‘The energy yield of a kite far exceeds that of a wind turbine, whose rotor tips turn at a maximum height of 200m. Doubling the wind speed results in eight times the energy,’ said Montnacher. ‘Depending on wind conditions, eight kites with a combined surface area of up to 300m² can equate to 20 conventional 1MW wind turbines.’
According to Fraunhofer, figures for the past year show that at a height of 10m there is only an approximately 35 per cent chance of wind speeds reaching 5m/sec, but at 500m that likelihood goes up to 70 per cent.
This makes any number of new low-land sites viable for the production of wind energy. Another advantage is that it costs less to build a system that, among other things, does not require towers.
High-altitude wind farm
NTS will design the kites and construct the high-altitude wind farm, and the researchers from the IPA will develop the control and measuring technology, which includes the cable winching mechanism and cable store.
One of the jobs of the control unit is to transmit the measuring signals to the cable control and kite regulation mechanisms. A horizontal and vertical angle sensor located in each cable line and a force sensor within the cable distributor guarantee precise control of the kite’s movements as it follows either a figure-of-eight or sine-wave flight path up above.
These flight manoeuvres are claimed to generate a high pulling power of up to 10kN — meaning that a 20m² kite has the capacity to pull one ton. Each vehicle is pulled by a different flight system.
At a test site in Mecklenburg-West Pomerania, IPA researchers and NTS sent a remotely controlled kite on its maiden voyage along a 400m-long straight track.
The team now want to reconfigure the test track making it into a loop and computers will eventually be used to achieve fully automatic control of the kites.
Guido Lütsch, managing director of NTS, said: ‘According to our simulations, we could use an NTS track running a total of 24 kites to generate 120GWh/year. To put this into perspective, a 2MW wind turbine produces around 4GWh/year.
‘So an NTS system could replace 30 2MW turbines and supply power to around 30,000 homes.’
After successful test flights on the demonstration track, the project partners are confident that their computer simulations will hold up in reality. The first investors are already on board.