High-altitude Wind Energy Generation Using Controlled Tethered Wings

Sustainable energy generation is one of the most urgent challenges that mankind faces today. Sources of fossil fuels (oil, gas and coal) presently cover about 80 % of the global primary energy demand.^1,2 This poses significant problems, ranging from increasing costs (due to the increasing energy demand on limited resources), to impact on climate change and pollution, to geopolitical consequences because fossil fuels are being supplied by few producer countries. It is now widely recognised that a key strategy required in the face of these issues is a much more extensive use of renewable energy sources. Among the renewables, wind energy is the most important source for electricity production, apart from hydropower. Wind energy has an average of $30 billion per year of investments and there is currently 20 % yearly growth in installed capacity. Indeed, recent studies have showed that by exploiting just one-fifth of the world land sites that are profitable for wind technology – based on wind towers – the entire world’s energy demand could be supplied.³

Limits of Actual Wind Energy Technology
The share of global electricity generated from wind power is currently only ~1 % worldwide and ~2 % in the Organisation for Economic Co-operation and Development countries. The high growth rates of wind installations are only possible thanks to political measures and incentives, which are not sustainable in the long run. Furthermore, the actual growth rate of global wind power installations and renewable energy plants in general is still not sufficient to meet the ambitious objectives that most major countries have adopted, such as EU 20/20/20.⁴

The main reasons preventing the development of wind power installations, in spite of its huge potential energy generation, are:

• the high cost of the technology per unit of energy produced;
• the limited capacity factor; and
• the low density of generated power per unit area.

A comprehensive overview of the present wind technology is given in Thresher et al.⁵ This review also points out that although no breakthroughs are expected over the next decade or two, there are many evolutionary steps that can cumulatively result in 30–40 % improvements in cost effectiveness.

Wind turbines operate at a maximum height of about 150 m over the ground. This value is difficult to improve on due to structural constraints that have reached their technological limits. On the other hand, wind speed generally increases with height above the ground: at 500–1,000 m, the mean wind power density is about four times that at 50–150 m; at 10,000 m it is 40 times greater.⁶ This suggests that a breakthrough in wind energy generation can be realised by capturing wind power at altitudes higher than can be reached by wind towers.

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