I was wondering why use expensive solar cells to power vehicles in earth orbit etc why not use a generator powered by a gas turbine? Massive swings in temp of sunlight and darkness are available in the vacuum of space by simply transferring from the dark side of earth to light side by orbiting the earth. Maybe the same effect could be had by simply rotating the generator unit while in orbit. How difficult could it be to design such a unit? In orbit there is a light/dark cycle w/one cycle vaporizing the working fluid to gas to turn the turbine that generates electricity then the other cycle turns the hot vapor back into fluid as the craft enters the dark side of earth. Or instead of using the dark and light side of an orbiting system the unit could be designed to rotate. Turning the craft* would expose one tank to the heat of the sun while the other (cold) tank receives the working vaporized gas every revolution. I see no reason this would not work and why it would not be cheaper and far more powerful than solar cells per dollar spent. reva
If you mean steam-based turbines, it's an interesting idea but who knows how quickly the steam would condense back into water or how you would get it to flow to the bottom of the receptacle again without gravity.
Yes that might be a problem! Maybe another type of working fluid could be found? Also I guess a big factor would be how radical the temp swings are might determine if the concept is feasible. It's so simple I am sure someone has thought of it. reva
There could be heaps of substitutes for water. That would not be an issue. I think there would be two issues, one being how fast can heat radiate from the radiator (as mentioned above) and other is the mass of the thing. If that is heavier than solar cells then it may be cheaper just to put the solar cells in orbit.
I would imagine that from a materials point of view that the constant heating and cooling of a gas/liquid and the tubes and turbines with moving parts would pose more possible malfunction risks than simply having electrical components. The temp differences in space vary by several hundred degrees celsius........at least that it what it is on the moon with no atmosphere. Also, you would literally have one change of phase per orbit so I don't know how much power you would actually get out of it. Its not constant over a period of time. As soon as it gets hot everything is gas..........and then 12 hours later it turns to liquid again when it gets cold.........then another 12 hours. I just don't see that working.
A gas turbine in free fall [in orbit you are falling ... following an orbit, but you are falling] would mean a certain quantity of motion to a system in absence of gravity [almost null sum of forces, to be accurate]. The capsule would rotate ... This could also allow [but the turbine should be big and the orbital module a wide cylinder] to generate a counter rotation of the external sectors ... with even a low artificial internal gravity. But this is at the limit of science fiction: to do this with efficiency you would need a big steam turbine [a locomotive in orbit! And solar hit couldn't generate all that power using a steam system.]. Going back to little systems, to exploit the thermal exchange in orbit is a possibility, but the efficiency of this system would be quite limited]. Using inertial systems to avoid undesired rotations you should consume substantially the energy you generate to make the turbine rotate [so it would be useless!]. If you leave the capsule rotate, you could get some energy from your gas turbine, but how much? Too few, I'm afraid. The sun issues a great amount of energy, but while solar cells can collect it on a 2D surface, with a high efficiency, a steam based system would have to transmit this energy to a volume of water to make it boil [a 3D volume], just this would reduce a lot the efficiency of the system, since with the same surface you should collect and transfer well more energy just to start the process of production of vapor [while solar cells are already supplying energy to the capsule]. The problem is that while a side of the capsule is exposed to solar light, the other side is in the shadow. In absence of atmosphere this means an enormous differential of temperature, so that to make your water boil you will need more time. A solution would be to use solar cells with tubes in them [tubes where the water flows], but again the absence of atmosphere wouldn't grant the conservation of the heat in any part there is shadow.
Closed cycle turbines require a lot of cooling to work. And it's really, really hard to cool things in the vacuum of space. You would have to build a heat-sink of massive size (and massive mass) to get it to work. Not to mention a host of mirrors of equally massive size (and massive mass) to provide the heat. And with launch costs at $5000/kg, the price would quickly become exorbitant. So for a given amount of collector area, PV gives you 20% efficiency, and a good closed-cycle turbine might give you 40% efficiency. But the closed cycle turbine requires a massive heat-sink that PV does not, and it requires the turbine itself, which PV does not. So it weighs a lot more, probably three times more, to get you twice as much electricity. Which means yes, it does cost more than PV, because launch costs are far greater than manufacturing costs. Obviously, phase-change working fluids will not work in space because of zero-gravity problems: the working fluid must be gaseous at all times. But that's not really a big deal. It's the cooling that's a big deal.
