In article , rickman writes:
On 8/16/2015 11:08 AM, bilou wrote:
There is also the regularly reapearing projects of transmitting
power from a geostationnary solar panel to ground by microwave.
I have great difficulties to understand how it could be efficient.

I'm lost. What is the problem? If you are worried about the losses in
transmitting power to the ground, I think you are missing the point.
The only thing that matters is the amount of power delivered, vs. the
cost of the system. In other words, you are comparing apples to
oranges. You may have losses in the transmission of power, but all
power systems have losses. The important part is the system, not one
isolated portion of it. The point of the orbital PV cells is to greatly
increase their power output. As long as those gains can make up for
other system losses *and* the increased cost of the system, the system
costs less per watt.

I don't know what to say about that comment.

Yes, electric power generation has its inefficiencies, although I
understand that at this point it is possible to get 50% and even
somewhat better with combined cycle gas turbine generation. And
there are a lot of what seem to be poorly documented losses in
long distance power distribution. But no one says to ignore the
losses.

I do understand the idea of measuring the entire system and not
individual pieces, and the fact that power received by an orbital
array but not delivered to the target is unimportant.

But everything in orbit is incredibly expensive. First, you have
to get it there. Then you have to have built it to be so reliable
that it will not break down. Except that it will. And then you
have to be prepared to spend more money to send humans up to repair
it.

In that sense, I guess, efficiency is the least of the designer's
worries. And all those other problems, plus the safety issue
mentioned later in your post, are the reason it's never been done.

Still, I would be curious as to whether that 50% rule - if it is
accurate at all (does a laser beam hitting a black body have to
reflect back 50%?) - applies to a microwave downlink.

George