On the PBS show "Nova" tonight...

On Apr 25, 2007, at 14:53 UTC, bobunf wrote:

> The idea of "beaming" power to Earth is completely ridiculous.

Why? Some very smart people who are experts in the field have studied
it and found it to be not ridiculous. What do you know that they don't
know?

> 1. Power in sunlight at Earth orbit: 1300 Watts/m2

About 8 times the power density on the Earth's surface.

> 2. Solar Cell efficiency: 15% (I know, 30% have been run in the
> lab, so just to be ridiculously optimistic, let it be) 30%
> 3. Conversion of electricity to microwave, optimistically: 50%

50-60% has been demonstrated, I believe.

> 4. Losses in transmission through the atmosphere, including from
> pollutants, targeting problems, and anomalous events such as dust
> storms, icing, desert varnish, flocks of birds and other phenomena:
> 20%

No, this would be pretty close to zero. There isn't much which
interferes with a microwave beam, except metal, you don't see metal
grids floating about in the atmosphere too often.

> 5. At present there are no devices to convert the microwave
> radiation directly back into useful amounts of electrical power.
> Currently the best method would be to heat water with the microwaves,
> and run the steam through a turbine with efficiency at,
> optimistically: 40%

Wow, what a whopper! Up till this point I thought you knew something
about the subject. In reality, this is the easiest step: a simple
metal grid (plus a few diodes) converts microwave energy directly to
electricity with extremely high efficiency -- 90% can be achieved, and
that's the end-to-end efficiency (including transmission), not just the
rectenna, which itself has an efficiency close to 100%.

You can demonstrate the efficient conversion of microwaves into
electricity by putting, say, a fork in your microwave, turning it on,
and watching for sparks. (Sparks form of course because the
electricity has noplace to go short of jumping to ground; in a rectenna
the voltage would be drawn off as a steady current.)

> So, 1300 x .30 x .5 x .8 x .4 = 62.4 watts per square meter of
> photovoltaic cellsat the very most, probably, really more like 20.

Experts estimate 75 to 100 W per m^2 of PV in space. I'm actually
surprised you were as close as you were. :)

> On the surface of the Earth one would eliminate steps 3 and 5.
> Figure the diurnal cycle would cut production by 50%.

It's more like 66% even under ideal conditions, and there are darn few
places on Earth with ideal conditions.

> This would leave 1300 x .5 x .8 x .3 = 156 watts per square meter.

Hah! No. Terrestrial solar power is in the neighborhood of 15 W/m^2.
And then you have the (massive) problem of power storage, since your
supply is very intermittent.

> In short, double to triple the amount of electricity and you don't
> even have to put the stuff in orbit, which, of course, would be
> incredibly more expensive.

If that were true, SSP would be ridiculous. But it's not true, and SSP
is not ridiculous. Here's a good reference which, with a bit of
effort, you can find online or at your local library:

Hoffert et al., "Advanced technology paths to global climate stability:
energy for a greenhouse planet." Science 298: 981-987 (Nov. 2002).

Best,
- Joe

Joe Strout -- joe@...