Thermoelectric generators in space

What i still don't understand about "space power generation" is why they don't use the sun side and the shadow side of an object, or a designed object itself as a thermal generator.

With a temperature difference of 500F or around 280 C between te sunside and the backside a Stirling type or Peltier type generator would be roaring.
Even a closed loop steam turbine would be no problem.

When your orbit is within the sunlight you will have a continues difference between the back and front.
You can solid state, or rotate the exposure sides.
When you experience shadow periods you can store heat, within molten salt for example.


Wikipedia: Without thermal controls, the temperature of the orbiting Space Station's Sun-facing side would soar to 250 degrees F (121 C), while thermometers on the dark side would plunge to minus 250 degrees F (-157 C).20 mrt. 2001

I googled a little and there seems to be something like this:

A flight-proven capable source of power is the Radioisotope Thermoelectric Generator (RTG)–essentially a nuclear battery that reliably converts heat into electricity. NASA and the Department of Energy (DOE) have developed a new generation of such power systems that could be used for a variety of space missions.

But i don't understand why you need the nuclear .

Other systems use overengineered antenna's like:

www.technology.matthey.com...

But why isn't the whole object an "antenna" ?

Not an expert here, so ther must be some obvious reason i'm missing.

Anyone?

"Even a closed loop steam turbine would be no problem."

I think that could be a problem in space, since steam is water in air and water freezes at 0c.

a reply to: hombero

I don't think you understand "closed loop"

In a closed loop sytem you can easy regulated the temperature at any stage when you have a hot side and a cold side as a given.

RTGs are used for deep space and planetary missions where the sun is too far away or not constantly available to provide practical power.

a reply to: CraftBuilder

Nuclear Tegs i presume you talk about.

In that case, feir enough for out of solar reach objects.

But most man made objects in space operate within the reach of the Sun's heat. And the cold side seams to be everywhere.

The decision to use a RTG on a science mission is not taken lightly and is rarely authorized. The risk of contamination during a failed launch and the expense justifies RTGs only for very specialized science missions.

a reply to: CraftBuilder

Yup, i've read about that. And good so.

Now back to the subject.

Maybe they aren't reliable.

a reply to: BrianFlanders

That seems not a problem to me. As they use the same technique with a nuclear powersource in space already ( see above )
The Nasa stirling engine for example is a quite familiar concept.

Replace the nuclear heat source with solar heat source should not be a great reliability change, As long as you have (periodic ) sunlight.

a reply to: EartOccupant

Vacuum of space is a great thermal insulator, as heat is only transferred by radiation. So to get any useful amounts of power from the temperature differential you would need a lot of radiators, which means a lot of weight.

Solar cells are the better choice, provide much more power per kg.

a reply to: moebius

Hmm the insulation i wondered about indeed. But space explores keep claiming they have to handle the extreme temperature differences. In space suits, or equipment etc.


As seen in the OP with the space station for example.

So if the heat and cold is a problem.. it is also energy at the same time. You can not have one without the other.

Besides, they have to cool a nuclear TEG as well.
And the amount of surface for solar panel make it more vulnerable for small meteors and debree, as well as docking cargo ships.

You know, I'd like the answer to this, too. Other than having no gravity to work with for circulating water back to the hot side, 120c would make decent steam. With some plastic reflectors they might even be able to make a solar concentrator for serious heat.
Heat makes steam, steam runs a generator... cooling fins on the dark side condense the steam back to water.

There are a few technical problems to overcome.
First being size. To get the kind of pressure drop they'd need to run a generator, they'd need to have a LARGE condensation unit- or at least have it far away from the generator. Otherwise it won't have time to condense the steam, and the generator will fail from backpressure. Would be easy to vent to space- but then you're wasting water. Unless they find a way to capture those ice crystals... Hm.

That brings us to the next problem, safety.
Steam pressure can get dangerous fast. if unchecked it turns its container into shrapnel. In space, this is bad. A minor leak
on earth could turn into a jet that screws up your telemetry in space- if the boiler pops, you've got shrapnel with no gravity or air resistance to slow it down.

Third problem would be weight!
I don't think I've ever once seen anything using steam that was built to be light. Water is heavy, too- this contraption would have to be fairly large and fairly robust, so it's going to be heavy. I'm not going into the reasons why that's a problem.

So, steam is out... but there must be other ways to make electricity from heat- especially given cold just across the hall.

originally posted by: EartOccupant
What i still don't understand about "space power generation" is why they don't use the sun side and the shadow side of an object, or a designed object itself as a thermal generator.

With a temperature difference of 500F or around 280 C between te sunside and the backside a Stirling type or Peltier type generator would be roaring.
Even a closed loop steam turbine would be no problem.

When your orbit is within the sunlight you will have a continues difference between the back and front.
You can solid state, or rotate the exposure sides.
When you experience shadow periods you can store heat, within molten salt for example.


Wikipedia: Without thermal controls, the temperature of the orbiting Space Station's Sun-facing side would soar to 250 degrees F (121 C), while thermometers on the dark side would plunge to minus 250 degrees F (-157 C).20 mrt. 2001

I googled a little and there seems to be something like this:

A flight-proven capable source of power is the Radioisotope Thermoelectric Generator (RTG)–essentially a nuclear battery that reliably converts heat into electricity. NASA and the Department of Energy (DOE) have developed a new generation of such power systems that could be used for a variety of space missions.

But i don't understand why you need the nuclear .

Other systems use overengineered antenna's like:

www.technology.matthey.com...

But why isn't the whole object an "antenna" ?

Not an expert here, so ther must be some obvious reason i'm missing.

Anyone?

I'm a spacecraft designer(semi-retired). For the last three years I worked at Planet Labs, helping to design their next generation Earth-monitoring small spacecraft. Before that, I worked at NASA, designing Lunar, Mars, and deep space missions.

Your basic question is "why can't you use a solar dynamic converter to make electricity, since there is a temperature difference between the hot side and the cold side of a spacecraft?"

The answer is, you could, but it wouldn't necessarily be better than using solar cells. Most spacecraft that are intended to operate at approximately Earth distance from the Sun are usually designed to have a hot side (pointing to the Sun) and a cold side (pointing to deep space). So heat naturally flows from the hot side to the cold side. Solar cell panels are designed to make use of this temperature gradient the same way that a fluid dynamic system would; they just do it with no moving parts. The optimum efficiency of typical space-rated solar cells is around 30% when they are operating at their optimum temperature. As the cell temperature increases, their quantum efficiency decreases; they are little thermodynamic systems, by themselves. So, the normal practice is to point the front of the solar array as directly to the Sun as possible and either allow the back side to point directly to dark space, or heat sink it to a cold structure.

If you were to use the same amount of collection area to provide electrical power by using a thermal dynamic converter of some kind, you would have to have a solar concentrator of some kind, heat exchangers for the working fluids, insulated pipes from the concentrator to the expander, an expander of some kind connected to a generator, heat rejection radiators, and probably some kind of pump. For normal sized spacecraft, that amount of complexity usually weighs more and costs more than simply using solid state solar cells.

Another factor is that any dynamic system that is pumping fluids around and has rotating or reciprocating machinery imposes vibrations and torques on the spacecraft structure. If your spacecraft is intended to make precision observations with telescopes, etc. this can seriously complicate the pointing and control problem. Also, a system with no moving parts is almost always more reliable than one with moving parts.

The best solution along these lines for normal sized spacecraft that I have seen is the Advanced Stirling Radioisotope Generator (ASRG). It was originally designed to use the decay heat from Plutonium 238 to power a little reciprocating Stirling engine connected directly to a reciprocating alternator. A Stirling engine has a hot side and a cold side, so it is easy to place the hot side in contact with any heat source and point the cold side to space. They were intended to be operated in opposing pairs, so that the vibrations would mostly cancel each other. A pair would put out about 100 W, or so. They were also designed with very high reliability, redundant electronics so that they could expect a 10 year lifetime, or so. The motivation for designing this kind of thing in the first place was to extend the supply of Plutonium. The standard Radioisotope Thermoelectric Generator (RTG) has a conversion efficiency (thermal to electric) of about 5%. The ASRG has a conversion efficiency of around 26%, so it can use 1/5 as much Plutonium and still supply the same electrical power. But, the thermodynamic efficiency of the ASRG is still not as good as aerospace grade solar cells, so there is no advantage to using one of these converters when you are in near Earth space.

When you get to outer planet missions, solar powered missions (of any kind) become pretty infeasible and you have to go to nuclear power. For example, Saturn is 10 AU from the Sun, so the solar power available is 1% as intense as it is at Earth.

a reply to: 1947boomer

Thank you for your extensive answer.

I did not think of this one, but it makes sence : "imposes vibrations and torques on the spacecraft structure"

I was however under the impression however that stirling generators are up to 50% efficient.

Anyway thankf for your input!

originally posted by: EartOccupant
a reply to: moebius

Hmm the insulation i wondered about indeed. But space explores keep claiming they have to handle the extreme temperature differences. In space suits, or equipment etc.


As seen in the OP with the space station for example.

So if the heat and cold is a problem.. it is also energy at the same time. You can not have one without the other.

Besides, they have to cool a nuclear TEG as well.
And the amount of surface for solar panel make it more vulnerable for small meteors and debree, as well as docking cargo ships.

Temperature only tells you the average energy of a medium. What actually matters is the total energy aka heat. There is very little medium in a vacuum thus the great insulation. So those "extreme temperature differences" in space are actually not much of an issue.

To get any power you need a temperature difference (hot and cold). That is how any heat engine operates.

RTG do need radiators. In fact the bigger the radiator (better cooling) the higher their efficiency would be. But RTGs are not used for their efficiency (about 5%) in the first place.

The surface area for your TEG radiators would exceed the comparable solar cell area by a large factor. So it would be even more vulnerable.

originally posted by: EartOccupant
a reply to: 1947boomer

I was however under the impression however that stirling generators are up to 50% efficient.

You said it--"UP TO 50% efficient".

That means it will never exceed that number. The actual efficiency of a Stirling engine depends on a lot of parameters: the temperature difference between the hot and cold side, the working fluid, and the amount of internal energy dissipation due to friction of internal moving parts, sneak heat path losses, etc. The ASRG units are small--about the size of a thermos bottle. Small heat engines tend to have higher relative internal friction losses and higher surface area to volume ratios and therefore lower efficiencies. The ASRG units were tested extensively over many years, so the 26% value is as measured.