Stratospheric solar array

This is something that has been brewing in my mind for a while that seems to have enormous potential if the details can be worked out.

The basic concept is to have a vast constellation of hydrogen blimps floating in the stratosphere collecting the sun's energy.

Advantages of being in the stratosphere

• In permanent sunshine
• Above most of the weather
• Out of the way of air traffic and wildlife
• The hours of sunshine are slightly increases due to the altitude
• Less attenuation of sunlight due to being above 2/3 of the atmosphere
• Do not take up any land

If the array is far out in the ocean then there is no shadow being cast where people live and no-one will be able to see it. Even where there is shadow it will be no different to shadow from cloud except that it won't be continuous as it will be fragmented due to the spacing of the blimps.

Parabolic solar recievers

Photovoltaic cells are still very expensive, especially ones with high efficiencies (>20%). It may be better to use parabolic reflectors aiming the light onto a thermal recievers. These have been developed at Sandia National Laboratories in the US, the EU's SolAir project and by companies such as Stirling Energy Systems and Infinia.

There might be a number of ways of achieving this:

• Large blimps having rows of parabolic dishes + stirling engines along the top surface. It may be possible to make very lightweight parabolic reflectors using aluminized mylar sheets strung in panels across a lightweight frame or perhaps an inflatable spheroid with one half being a transparent material and the other half being reflective on the inside surface. Blimps would have to have counterweights underneath to compensate for the weight on the top surface - propellor units for the blimp might be enough.

• Blimps simply carrying parabolic mirrors underneath which point at a separate recieving station perhaps held by a much larger blimp which receives energy from many reflectors. Blimps would be in herds comprising of many reflector carriers and a mothership with the reciever.

• Inflatable spheroids mentioned in the first option could blimps themselves rather than being carried by one. Weights with adjustable positioning are hung underneath to adjust the angle of the reflective surface inside the envelope so as to point in the right direction. Blimps could simply be relectors pointing at a receiving blimp or each blimp has stirling engine at focal point. If there are significant aerodynamic issues of having an angled flattened spheroid, the blimp could be spherical with a parabolic reflector contained within.

Station keeping

• Could be tethered to ground with ultrafine, ultra strong fibre. Probably not desirable as get in the way of traffic and vulnerable to storms
• Use GPS and divert some of the received energy to power ducted propellers or other kind of thruster
• Stay in same general region by varying altitude to take advantage of different wind directions at different altitudes similar to the way hot air balloons can have some control of their direction of travel.
• Tug blimps with lateral booms gently pulling a number of carriers

Energy transfer

• From sky to sea
  • Depends on configuration of collectors. Cables connecting each blimp reciving direct solar energy, then mothership blimps carrying high voltage cable down to floating ground station.
  • Or use principle from solar power satellite and generate relatively low energy density microwave beam directed at a floating rectenna
• and from sea to land
  • Undersea cable back to land
  • Or use energy to split seawater (salty and good for electrolysis) and gather the hydrogen. Hydrogen then either highly compressed, absorbed in a metal hydride or liquefied is transported by tanker or pipeline to land to help power hydrogen economy.

Potential issues

• Energy cost of station keeping
• Aiming the mirrors