4 ms·

The danger isn't from "overflow" radiation when the microwave beam is on target. The danger is when the microwave is off-target by a couple hundred feet (teens

by finnh 13y ago

The danger isn't from "overflow" radiation when the microwave beam is on target. The danger is when the microwave is off-target by a couple hundred feet (teensy fractions of a degree, considering the distances involved) and suddenly your concentrated 1GW beam is pointing at an apartment building. That beam is WAY more powerful than the sun per unit area. If not, we'd just use the sun.

skorgu 13y ago
> That beam is WAY more powerful than the sun per unit area. If not, we'd just use the sun. Not necessarily. Rectennas can get ~80% conversion efficiency, PV panels you can actually get in bulk are closer to 30. So you can have a beam about as strong as the sun and get ~triple the power out the other end by using microwaves instead of light.
- danielweber 13y ago
  When you express what SPS is doing as "making solar 3 times more land-efficient" it looks kind of crazy. Maybe dump the billions of dollars that would be needed for launching just one of these into squeezing another percentage point out of all new solar cells.
worldsayshi 13y ago
Just use multiple source satellites for each destination collector. Each being harmless, together they are powerful enough for a decent output.
lutusp 13y ago
> suddenly your concentrated 1GW beam is pointing at an apartment building. Wait ... have you calculated the antenna size required to produce a beam able to pick out an apartment building, at microwave wavelengths, from orbital heights? Consider the distances and the available microwave frequencies, and you'll see that you cannot get a beam narrow enough to resolve an apartment building from orbital heights, with any reasonable dish size. Remember that, at sufficient distances, all antennas become cosine emitters with no ability to concentrate their energy in a specific location. This is true for lasers, x-ray emitters and it's certainly true for microwave emitters. My point is that a practical system such as we're discussing must have a large receiving array to accommodate the beam size at ground level. It's not going to be anything resembling a death ray.
InclinedPlane 13y ago
"That beam is WAY more powerful than the sun per unit area. If not, we'd just use the sun." A common misconception, but untrue. What matters isn't power per area but cost per kilowatt-hour. To collect solar power on Earth you need a lot of expensive components (photovoltaic cells), plus you probably need to track the sun, and even so you'll have huge losses due to night-time, weather, winter sun angle, and so on. In comparison, a rectenna array is just a bunch of little bent pieces of wire. It's almost trivially inexpensive to build, and in some cases it can live side by side with other land uses. Even if the power density is only a few hundred watts m^2 (compared to over a kilowatt/m^2 from the sun at peak power) that power can be converted with upwards of 80% efficiency (compared to much less than 50% efficiency) and the full amount of that power is available 24/7 through every season. The hypothetical 1kw/m^2 at noon on a summer's day actually rounds out to only a tiny fraction of that over the entire year. Meanwhile, a rectenna converting 100 W/m^2 of RF power can end up producing the same average power as a solar installation, except that power is always available and the cost of the ground component is a teeny, tiny fraction of the ground solar installation. That still leaves the cost of the space component, of course, which is decidedly non-trivial. But a big advantage is that the space component can have a very, very long service life and can also serve various markets (especially remote regions where power delivery is difficult) which can provide the up-front high-profit business to pay off its cost.
- finnh 13y ago
  Thanks for the really informative post - i appreciate it.

by finnh 13y ago

skorgu 13y ago

> That beam is WAY more powerful than the sun per unit area. If not, we'd just use the sun. Not necessarily. Rectennas can get ~80% conversion efficiency, PV panels you can actually get in bulk are closer to 30. So you can have a beam about as strong as the sun and get ~triple the power out the other end by using microwaves instead of light.

danielweber 13y ago
When you express what SPS is doing as "making solar 3 times more land-efficient" it looks kind of crazy. Maybe dump the billions of dollars that would be needed for launching just one of these into squeezing another percentage point out of all new solar cells.

worldsayshi 13y ago

Just use multiple source satellites for each destination collector. Each being harmless, together they are powerful enough for a decent output.

lutusp 13y ago

> suddenly your concentrated 1GW beam is pointing at an apartment building. Wait ... have you calculated the antenna size required to produce a beam able to pick out an apartment building, at microwave wavelengths, from orbital heights? Consider the distances and the available microwave frequencies, and you'll see that you cannot get a beam narrow enough to resolve an apartment building from orbital heights, with any reasonable dish size. Remember that, at sufficient distances, all antennas become cosine emitters with no ability to concentrate their energy in a specific location. This is true for lasers, x-ray emitters and it's certainly true for microwave emitters. My point is that a practical system such as we're discussing must have a large receiving array to accommodate the beam size at ground level. It's not going to be anything resembling a death ray.

InclinedPlane 13y ago

"That beam is WAY more powerful than the sun per unit area. If not, we'd just use the sun." A common misconception, but untrue. What matters isn't power per area but cost per kilowatt-hour. To collect solar power on Earth you need a lot of expensive components (photovoltaic cells), plus you probably need to track the sun, and even so you'll have huge losses due to night-time, weather, winter sun angle, and so on. In comparison, a rectenna array is just a bunch of little bent pieces of wire. It's almost trivially inexpensive to build, and in some cases it can live side by side with other land uses. Even if the power density is only a few hundred watts m^2 (compared to over a kilowatt/m^2 from the sun at peak power) that power can be converted with upwards of 80% efficiency (compared to much less than 50% efficiency) and the full amount of that power is available 24/7 through every season. The hypothetical 1kw/m^2 at noon on a summer's day actually rounds out to only a tiny fraction of that over the entire year. Meanwhile, a rectenna converting 100 W/m^2 of RF power can end up producing the same average power as a solar installation, except that power is always available and the cost of the ground component is a teeny, tiny fraction of the ground solar installation. That still leaves the cost of the space component, of course, which is decidedly non-trivial. But a big advantage is that the space component can have a very, very long service life and can also serve various markets (especially remote regions where power delivery is difficult) which can provide the up-front high-profit business to pay off its cost.

finnh 13y ago
Thanks for the really informative post - i appreciate it.