How high will the kites fly?
Since the power contained in the wind increases with the cube of the wind speed, and the wind speed increases with altitude, it seems it would make sense to fly the kites as high as possible. However, by increasing the length of the tether which connects the kite to the ground, the drag on the system is increased, which negatively effects performance. Also, if the tether angle with respect to the ground is increased above about 30 degrees, the power output is effected significantly. In the end, the operating altitude of the kites will be the result of an optimization problem, taking into account the physics but also the economics of the system.
Based on published results from a number of academic groups and the indicated product specifications of the leading companies in the field, the operational altitude for MW scale systems will likely be between 500 and 1000m. This is an altitude range referred to as the 'upper boundary layer', and represents an altitude where the effects of the earth's surface on the wind speed and consistency begin to have less of an effect. As shown in the figure under "What are the advantages?", the largest relative gains in wind speed are achieved up to 1000m, after which the increase is roughly linear.
There is also a lot of talk about accessing the jet stream or 'tropospheric' winds, which are at an altitude of roughly 10 km. As shown in the figure below taken from [8], these winds contain an enormous amount of power which is estimated at 100 times the global energy demand. However, in order to harness this vast ocean of energy effectively, extremely large devices will be required, likely in the range of tens of MWs. The reason for this is that the tether drag increases with it's diameter and the force carrying ability increases with it's area, or diameter squared. These scaling laws allow very large systems to have proportionately less tether drag and can therefore have longer optimal tether lengths.
