Science-Fiction Solar Power

Orbiting Solar Power Stations

In 1945, Arthur C. Clarke predicted the existence of geostationary manmade satellites. These now exist, and are by far the greatest success of the space program.

There would be a huge advantage in having a solar power collector in such an orbit. It is in constant sunlight for as long as it is not in the Earth's shadow. That's about 22 hours of sunlight per day.

But How Do You Collect the Power?

Clarke also imagined the possibility of geostationary cables, extending from the equator to the satellites, and suspended by the centrifugal force of an additional mass beyond the satellite. The thing is entirely mathematically possible, given a strong enough material for the cable, 36,000 km long at sea level. Graphite fibres are almost strong enough, Buckminsterfullerene filaments probably are. But highly conductive cables as long as that are yet another problem.
Or perhaps we could have the power station satellite simultaneously align its mirror to the sun, and beam its power to earth by some laser mechanism.

The trouble with that, apart from the engineering difficulties, is that a worthwhile solar power satellite is collecting a gigawatt of power. If you can aim a gigawatt beam of energy accurately at some target on Earth, you've built a Death Ray device!

Photosynthesis

Coppiced willow, or possibly some grasses, can give an annual harvest pf polysaccharides like cellulose and lignin for a heat rate of ideally a Gigawatt-year per year from an area of 410 square miles. Note that both sugar cane and corn (maize) are grasses, and that grasses are hungry for nitrogenous fertilizer. Willow is fairly frugal, if it gets enough water.

There is no chance that ethanol from such a harvest can do better than two thirds of that much energy.

We have to sacrifice at least a third of the carbon, to feed the yeast.
C₆H₁₂O₆ = 2CO₂+2C₂H₅OH
And the bio-reactors would have to re-fix the nitrogen.

Possibly, photosynthesis from such an area is limited by the atmospheric concentration of carbon dioxide, or by the concentration of some nutrient, so conceivably algae grown in an enclosure fed with fertilisers and an atmosphere of higher CO₂ concentration could do better.

But the level of engineering and bioengineering required would be quite prodigious. It would include the genetic engineering of super-algae adapted to make use of such conditions. The environmental impact, in the event of an escape of the super-algae required, is difficult to estimate or even predict.