Stratospheric solar array

This is a concept 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.

Concept sketch

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 extended due to the altitude
• Less attenuation of sunlight due to being above 2/3 of the atmosphere
• Do not take up any land space

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. Also 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.

Stirling Energy Systems state that using their system "a solar farm 100 miles by 100 miles could satisfy 100% of the America’s annual electrical needs".

Blimp configuration

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.

View looking skyward of one potential 'herd' configuration which consists of multiple 'families'. Each family consists of many reflector blimps shining at a common thermal receiver, the mother, which generates power from a stirling engine or other energy conversion process. Lightweight electrical cables connect all the mothers and transfer power to a matriarch. Matriarch is very large and either carries the a high voltage cable to the sea surface or contains the equipment to beam the energy to a rectenna floating on the sea below.

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 receiving 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

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
• Cable or microwave beam hazard depending on method of final energy transfer. Cable could be made radar reflective. Beam could be of low enough energy density to be harmless to passing people and creatures. Beam can also turn off autonomously based on radar if necessary.

Other thoughts

• If sea-based recieving station generates hydrogen as fuel, it could of course also be used for filling the blimp envelopes.

• Envelopes of blimps could be biopolymer films grown in moulds, using nutrients in sea or perhaps CO₂ and water to create hydrocarbon raw material. This also might mean that blimps are biodegradable.

• The 'cost' and complexity may make this concept not seem economically viable today, however if the whole scheme was designed to employ closed-loop automation principles so once it has been commissioned it could use the resources of the sea (and perhaps the sea-bed) to create what it needs to run continuously, with little or no human intervention, and if the design of the energy plant (and the autonomous maintenance systems) is done by people using open collaborative design then that might mean a project of this scale become possible. Feasibility fundamentally is a matter of material, energy and intelligence in situations like this - not our current notions of economics.