Would Stirling engines be good in space?

Question

My knowledge of Stirling engines is that they work via a heat difference and are extremely heavy when used on a larger scale. This makes the use of them on vehicles a strange choice when there are many other lighter engines. Still, with this information it seems that space vehicles would find the Stirling engine of great help, as space I believe is mostly devoid of heat and weight is mostly not an isend.

Could I I use them having the cold side in the vacuum of space and the warm in the ship or is there a more viable option?

Answer 1

Stirling Engines, like all heat engines, generate work by tapping the transfer of heat energy from a hot source to a cold source. For most combustion-driven engines, the hot/cold dichotomy is created via the combustion of some fuel. This generates a point source of heat, and the cold source is just everywhere else. The Stirling Engine doesn't necessarily need a combustion source, but it does need heat, and it does need cold. This is where the troubles will exist for an engine in space.

Space lacks very much heat, but it would be a stretch to call it cold. In a nutshell, this is because there is no medium to carry any heat energy. As a result, if we had a Sterling Engine in space we could heat one side of the machine to create the hot potential, but we would have to rely completely on radiative cooling to create the cold potential. This is not to say it wouldn't work, mind you, just that we're working with a more limited set of possibilities.

From a practical perspective, we could probably mount a low-differential Stirling on the outer skin of a ship or station. The heat of the station is already radiating away, so the Sterling would effectively recapture some of that radiated heat energy and turn it into mechanical energy. The question is why. Why do we want to do this?

Mechanical energy aboard a spacecraft will have little use, but not zero use. If an electrical generator was being powered by the engine, it would essentially be turning the wasted heat energy of the station back into usable electrical energy. That's not nothing, but it isn't exactly much. The efficiency of this process will be low, and the total output will also be very low. Overall it might seem desirable to save some of the wasted heat energy, but there is also the total weight of the engine itself, which will be considerable. Getting the engine into space will expend massive amounts of energy, and it would probably not be worth it, unfortunately.

I can't recommend the use of a Stirling Engine in space, but it would probably work if you wanted it to.

Answer 2

The question is will stirling engines be good in space ? Answer is Yes, maybe not the best option but it will work. I have experimented with stirling engines and designed a workable engine myself,the biggest obstacle is designing a cold cylinder part with the best (Blue aluminum) metal possible on earth. Problem one, keeping the cold cylinder cold in space for as long period as possible. If mounted on a structure for example the ISS, the cold cylinder part can be mounted outside the structure and the hot cylinder(Preferably high grade Copper)inside the station frame for optimal performance.

Answer 3

If you're operating near a sun ("near" being a relative term, but inside 2 AU is a quick rough estimate), you will probably need to have good heat management to deal with all the energy hitting the ship's skin from the sun. Even if you're far away from the sun, you will still need some heat management to deal with the heat generated by your ship's engines, equipment, and personnel. In either case, you have to deal with moving heat around.

Stirling engines generate power from the movement of heat from one place to another. They do so with higher efficiency and using lower gradients than many traditional engines. You can also put power into a Stirling engine to move heat against a gradient.

So, would a Stirling engine interact well with a heat management system? Unlikely but it's plausible.

If I wanted a pseudo-realistic version of this, I would go for something like an outer hull layer with many radiative/absorbtive baffles. That lets you pull heat in or radiate it out as efficiently as possible (and it might look cool. Ships could have fur, or moss, or whatever other motif). Inside that, wrap the ship in a layer of Stirling pumps, all in battery. You'll get something like a big Peltier-effect blanket. It would take a little handwaving to say why this is better than just using Peltier elements, but that's just an engineering problem ("the shared gas pool does better with load-balancing for hot spots; the mechanical system is more resilient to cosmic rays; the medium provides additional shielding; Peltier belts tend to short out in big chunks; etc).

So, that gives you an element of your ship which protects from heat, provides warming and cooling, and generates some power (maybe) near anything that pumps heat into your system. If stealth is important to you, this might provide that too (pump all the heat out of the side of the ship facing the sensor). Functionally, you have something that can degrade without breaking, can leak and be fixed with chewing-gum-style patches, makes noises as needed, is very important to the safe operation of the ship, and isn't immediately fatal on failure. Also, it can run on whatever unobtanium you want as its gas medium.

Answer 4

Stirling engines are interesting because they are the most efficient engines possible, which is what attracts the attention of inventors of all stripes. While they are capable of coming closest to the theoretical maximums of the Carnot cycle, in the real world they are very complex devices, and tend to operate best in a very limited ranges of outputs (as a caveat, you can design the device to match the particular set of inputs and outputs you want. Some Stirling engines can operate with the temperature of your hand as the input, but are very constrained and limited in power output).

In terms of space, your Stirling engine can operate with the hot end at the maximum temperature your materials can handle, while the cold end can theoretically be radiating to an infinite heat sink at 3K, which implies that you can have a very high level of efficiency. Once again, the actual mechanics are quite involved, and you are not going to have a mechanically simple device.

To give you an idea of the potentials of Stirling engines, a man named Darryl Phillips designed a Stirling engine that would capable of powering a light aircraft. Sadly, the article "Why Aviation Needs the Stirling Engine" does not seem to be on the net anymore, but you can see the potential for a lightweight and powerful engine, which can be adapted as a generator. (For those of you who are interested, the engine Phillips described seems to be a variation of the "Ringbom" Stirling engine design, where the displacer is moved by the movement of the gasses and power piston rather than a mechanical throw or crank).

So using a Strling cycle engine in space as an electrical generator using heat from a nuclear reactor or rocket engine seems to be possible, if the output is great enough to overcome the weight and mechanics complexity penalties.

Answer 5

Scratch using the heat radiating for the station having the "hot" plate directly in the sun would be much more efficient considering the skin of the ISS can reach temps of 250 F° which is almost definitely more energy then any other source. The largest problems I can see is removing the energy from other side to produce the thermal difference. The second being the weight. There is a reason spacecraft use almost exclusively solar panels and RTGs ( such as a few mars rovers.

Answer 6

Stirling engines are heat engines, they require a temeprature difference (for example betwwen something heated by the sun and a large radiator in the space ships' shadow).

At large scales, a solar concentrator + heat engine + generator could be a more efficient way to turn sunlight into electricity. Efficient here meaning useful energy harvested per capture surface, not per weight.

This would effectivly be a solar concentration plant with the added benefit that in space the sun always shines and there are never clouds (It should be noted that solar concentration plants have a bigger problem with clouds than PV cells do, as PV cells can use diffuse light, concentration plants can't. Even so, at large scale solar concetration plants are considered economical compared to PV cells). In space, it could be in some regards easier to build any heat engine as you have plenty of the best thermal insulator around - vaccuum! So a solar concentration plant powering a heat engine supplying a space station could be plausible. Solar concentrating Stirling engines (with the hot side at the focus of a parabolic antenna) where even a commercial thing, briefly.

In any heat engine, the efficiency is higher the higher the heat difference between the working fluid at it's coldest and hottest (CArnot efficiency). The closest contender with a stirling engine would IMO be a steam engine (Rankine cycle). On earth, steam engines achieve far higher efficency then Stirling engines because of difficulties inherent in the design, all the heat has to pass the walls of the engine twice (in and out) as conductive or radiative transfer, meaning the availible temperature difference can't be exploited fully. The achievable efficiency depends also greatly on the regenerator.

In space setup wit hsolar concentration, things look a bit different: The heat transfer into the engine is the same regardless of type (Stirling or Rankine), waste heat also has to be shed via radiators (here steam engines could do with smaller ones, all else beeing equal). Stirling engines don't run with or on liquids, while with a steam engine you somehow have to make sure the condensed working fluid makes it's way to the boiler feed pump without entrained bubbles.

So if you want to fit a stirling engine into your story, you can do it, the biggest constraint is of course weight and here the regenerator of the engine is a problem so I would look for semi plausible but SF-nal solutions with microlattices 3D-printed from new materials with a very high thermal capacity.

Answer 7

There are people working on using them in radioisotope thermoelectric generators (devices used in space to generate electrical energy from radioactive decay of isotopes, usually Plutonium-238), such as the ones in the Curiosity probe and the New Horizons mission.

This is actually a lot more efficient than a traditional RTG that uses a thermocouple to generate energy. Thermocouples work by having wires of different metals connected at different locations in the spacecraft, and these generate electrical energy from the difference in temperature. This is very inefficient because a lot of heat will conduct to the other side normally instead of being transferred as electrical energy. The reason why this is done is because the thermocouple is very reliable and does not break easily due to its simplicity and lack of moving parts.

Stirling engines, on the other hand, can be very efficient, but are more complicated due to the moving parts and extra pieces that can be broken. These moving parts, while they are insignificant for anything on Earth that can be accessed and repaired by a human and do not have to withstand the high g-forces of a rocket launch (often 5 g or more can be experienced for prolonged periods of time). This extra complexity and chance of things going wrong, and the added mass of the stirling engine, are the main reasons why they haven't been used before, as well as the fact that most spacecraft do not need that extra efficiency. If we build any spacecraft that need that extra efficiency or if we get to the point where we are building things in space and we are powering human habitats that way (we might need it to power human colonies on or around gas giants, for example), then Stirling engines would start to look like a very good idea.

Answer 8

Stirling engines are nice, as a toy. You can put them on your hot tea cup and watch it move up and down.

If you want something to use the temperature difference between inside and outside a spaceship, you should take a look at NiTiNol engines:

https://en.wikipedia.org/wiki/Nickel_titanium

https://www.youtube.com/watch?v=8-dCIkJAjyM

NiTiNol is a memory alloy with two working points. You can generate energy from a temperature delta of 2 Kelvin, if done right.

Still, they aren't what I'd call efficient, but much better than Stirling engines.