[ExI] Obama Transition Team Examining Space Solar Power

hkhenson hkhenson at rogers.com
Wed Dec 24 05:00:17 UTC 2008

At 05:31 PM 12/22/2008, Tom wrote:

>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.

Close enough.  It's a circle projected on (typically) 45 deg north 
latitude earth surface.  The old designs used 230 W/m^2 (or 23 
mW/cm^2) in the center of the rectenna.  The power fall off is close 
enough to gaussian.  Eric Drexler proposed using different 
distribution called a "top hat" for the power beam.  This power level 
was selected partly for safety reasons, you get more power to your 
head at this frequency using your cell phone.  At some power level 
(10kW/m^2?), you cook ducks on the way through the beam.  At some 
(higher?) level the path through the ionosphere shorts out.

The area is close enough to 100 square km.  So the average power is 
5,000,000,000/100 x 1000 x1000 or 50 W/m^2.  A lot of this area is 
just to get the microwaves down to extremely low levels at the 
fence.  Eric's "top hat" power distribution puts a more constant 
level into a smaller area.

>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.

Right.  There are large advantages to being able to put power sources 
close to the load.


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