A primary safety mechanism for space-based solar power would be to make the beams wide
, so that any point in the beam isn't receiving more power than you would from, say, standing in direct sunlight. This makes them quite passively safe. The main problem with this is that the receiving rectennas have to be quite large to receive any significant power if you're limiting power per area. They're not necessarily horribly obtrusive (something like a grid of wires above the ground; you could even have plants and wildlife living underneath them, given that the intensity of the microwaves isn't high enough to be harmful), but you need to have a big open space you aren't using for anything else. This might be a problem in, say, Japan, which has little enough space already.
A second safety method mentioned in the Wikipedia article uses a method that basically makes it impossible for the microwave beam to be focused anywhere that doesn't have a "pilot" beam transmitting to the station. If the pilot beam goes away, the power beam defocuses (presumably to levels that are again below the level of direct sunlight).
The last problem that needs to be dealt with is launch costs. If we want to launch this stuff from Earth, for it to be economically feasible, space launch costs had better come way down. The one good thing here is that part of the reason why space launch is so expensive is there isn't a reliable demand for enormous quantities of it; space-based solar power might provide enough demand to really get some economies of scale going. Remember that an airliner may cost several hundred million dollars, and need to produce say a 10-15% annual return on investment for people to be interested in fronting the money for one, but it carries hundreds of people every day, so you can spread the costs out quite a bit.
Another problem is that space has lots of nasty radiation and such in it; the solar panels degrade a lot faster out there than they do down here. You'd have to keep replacing these things. That might be useful for ensuring a constant supply of high-quality low-cost space launch, come to think of it . . .
The primary advantage
of space-based solar over ground-based solar is that it's reliable, 24/7 power. You can't get that from ground-based solar, and storing electricity, while possible, is annoyingly difficult and expensive. There's a reason the electrical grid is currently mostly designed to produce electricity exactly when it's needed rather than storing it for later.
edited 8th Dec '11 6:19:20 PM by Shinziril