[ExI] Obama Transition Team Examining Space Solar Power

Tom Nowell nebathenemi at yahoo.co.uk
Tue Dec 23 00:31:40 UTC 2008

Kevin H wrote
"But the solar constant is on average 1366 W/square meter.  There's just no
more energy out there from the sun than that at the distance of the Earth (1AU).  That's our upper bound of what we could expect.  That's 732 square m of panels/reflectors to transmit 1GW."

Kevin, I checked David MacKay's work over the weekend - when you analyse earth figures, most renewable electricity and biodiesel schemes produce from 0.5-2W/square m when averaged over a year. Current best performance by German photovoltaics is 5W/sq m. As to drive a current car 50km takes about 40kWh of energy according to MacKay, it take a few hundred square m just to power this - the average Brit's daily travel (let alone the average American's).

 Space-based power generation is colossally more efficient than earth-based renewables, but the killer is the launch cost. Keith said that with rockets it's economically marginal, but with better launch costs (laser launch partway) it is worth it, and when you reach ultra-low cost launch like space elevators it becomes the obvious option. The problem is developing the better launch systems costs money, which isn't justified by the current size of the space launch market, which is restricted because of high launch costs.....

Until we break out of the current vicious circle by someone saying "we need to develop a major launch capability to do X" then solar power satellites are marginal. If people are convinced solar power satellites are the way ahead, then developing new launch systems looks massively worthwhile. Addressing these connected problems has been a major focus of space activists like Keith. If the people trying to lobby the Obama team are successful, maybe we can get a programme set up to cut through the problem by funding the whole thing through, or introducing a huge financial incentive for power satellites together with tax credits for developing cheaper launch systems. (Massive govt tender or market-driven solution, whichever floats your ideological boat or whichever looks more likely to get people employed during the current economic hard times). 

Keith - for comparison, what's the amount of power generated per square metre of rectenna? From the figures I've seen on the net, a 5GW power sat is expected to have the energy picked up by a 10 x 14km rectenna, but I don't know if it's a perfect ellipse or not. Anyway, the rectenna uses land far more efficiently than most earth renewables, with much less need for load balancing. Also, the land the rectenna is over can be used to grow crops on or build hydro reservoirs for, even if people don't want to live under it. Therefore, from a land efficiency viewpoint, space solar power is actually a winner for population-dense or land-poor nations, and still a good idea all round.



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