Is it possible to use thermal pumping for power generation in space? [closed]

Question

There's this one component that exists in real life that uses the thermoelectric effect to pump heat, basically, by creating a temperature differential using voltage. Is it possible to use this in power generation, to pull the heat out of the air and use it to generate electricity through a Stirling engine or some other heat-to-power method? The main hangup probably would be the actual energy generated (does the pumping take more energy than what the generator provides?). I'm especially interested in how this could be used for space travel, as it could make massive radiators unnecessary and generate energy from what used to be waste.

Answer 1

No, you cannot do that

Your question boils down to: can you recycle waste heat into useful energy?

No.

Why?

Because all heat-to-exergy reactions generate (more) waste heat. You have to take exergy and put into the heat pump to get it to make the waste heat hotter, and then you have added more waste heat.

And now you are saying...

"I am not using energy from the generators, I am only using waste heat"

That will not work because then it was not waste heat, it was heat you could use for exergy generation. And that generation will result in waste heat.

"Okay, convince me"

Think of it this way: any conversion of energy from one form to another — especially when we are talking about any kind of heat engine — leads to losses, that is to say: waste heat. You cannot get away from that. It is like exchanging money, where there is a more-than-zero percent commission taken each time, you will(!) lose some with each exchange.

In short: every conversion means loss; means more waste heat.

Another way of looking at it is that you cannot unblend warm and cold water after you have stirred them together. All you are left with is something luke-warm, and that cannot be used to extract useful work.

So, concerning this statement:

"I'm especially interested in how this could be used for space travel, as it could make massive radiators unnecessary and generate energy from what used to be waste."

No, there will be waste heat, and you will need to get rid of it through radiators. You cannot harvest that, unless luke-warm water/air/whatever is what you want.

"But energy cannot be destroyed, only converted, right?"

True, but once it becomes waste heat, it cannot be converted again. You cannot destroy energy, but you can(!) destroy exergy, and exergy is what you need to do any useful with it.

"But..."

No, no but:s... "every spontaneous reaction increases entropy", you will not get away from that. You cannot make a closed system where waste heat is circulated back to exergy.

Answer 2

You could reduce the size of the massive radiators.

It could quite an engineering challenge to make it worthwhile, but you can at least theoretically use smaller radiators by using a heat-pump to concentrate the heat to be rejected by heating up the external radiators for a higher rate of radiative heat loss - which is proportional to the 4th power of the absolute temperature.

You would still have to eject additional heating coming from the energy being used by the heat pump, but since a heat pump can have a coefficient of performance (COP) > 1 (heat transferred to the higher temperature medium compared to the heat added by the heat pump) an actual reduction in the radiator size would be possible.

It would certainly not be a large reduction, as actual heat-pumps COP is never drastically larger than 1. And you sure as anything could not run your system based on the waste heat in your space station/ship.

In this house we obey the laws of thermodynamics! -- Homer Simpson

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Based on a few questions/comments I decided to add some more detail.

Do existing satellites use heat pumps? I don't know of any, but would be glad for a reference. This was in reference to a comment that has been deleted - but the discussion is useful.

Because of the square cube law, the larger space vehicle with active systems, the higher the percentage of heat loss will have to be augmented via external radiators (not just the natural heat loss via the walls of the vehicle). Of course, the more energy the intensive internal components are, the greater the need for active cooling.

Fortunately, since the International Space Station is the biggest man-made thing in space I was able to find detail. I read Boeing's paper on the ISS cooling system. This includes block diagrams of the thermal loops, other diagrams, and considerable explanatory text. None of this indicates using an active heat pump to raise the temperature of the cooling loop fluid in order to reduce the radiator size. In fact, the total power load for the system is specified at 275 watts each for the 2 pumping systems yet nothing was mentioned for power for the heat pump which would certainly be larger. This was stated in the non-sense units of watts per orbit (about 93 minutes) - I'm just assuming the paper intended that these were average rates over an orbital cycle.

Also interesting was that each of the 4 radiators was capable of radiating away 14 kw of heat. These radiators are actually the solar arrays. The cooling system serves a dual purpose of cooling both the station and the solar panels.

I don't think any no-heat pumps exist. Therefor the cooling loop is rejecting heat at something in the vicinity of room temperature. It may be somewhat warmer because the internal heat piping mentioned collecting the heat via heat exchangers located near heat sources such as laboratory equipment.

Since in the case of the ISS, solar panels are used to power the stations means 2 things from a design viewpoint. You don't have a large fraction of waste from power generation ending up inside the station in the first place and you already have a convenient set of radiators for heat rejection.

I'm sure the ISS engineers wisely concluded that no heat pumps were needed. A different vehicle might have more motivation to try to reduce the size of the external radiators.

What COP can you expect?

Actually, all you need to calculate the maximum theoretical COP for heat pump is the absolute temperature of the hot and cold sources. This is also known as the Carnot COP

Carnot COP = Thot/(Thot-Tcold)

What does this mean for a house in the winter? Let's assume 22C/295K/72F inside, and 0C/273K/32F outside? Carnot COP = 293 / (293 - 273) = 13.32

But as a practical matter, the internal air discharge temperature must be warmer, typically around 32C/90F/305K

Carnot COP = 10.1

Add in the fact the the external discharge temperature must be a little cooler than ambient, and that the real world pumps, motors, pumping losses and other imperfections must occur, real world heat pumps max. out at a little more than 8 - but this is for comparatively mild winter temperatures. Drop the ambient temperature to -18C/255K/0F and

Carnot COP = 305 / (305 - 255) = 6.1 or at best a little more 4 in reality.

Because the efficiency of heat pumps is strongly dependent upon the temperature difference, you will never achieve a high temperature rise needed to increase the exhaust temperature to reduce radiator size by a significant fraction. Especially when you consider that you would also need to exhaust the heat from operating the heat pump also.

Homer Simpson is rarely a good source of wisdom, but he got this one right.

Answer 3

What you have in mind is possible, and already used in space application.

You should give a look at the thermoelectric effect.

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple.

The Seebeck effect (German pronunciation: [ˈzeːbɛk]) is the electromotive force (emf) that develops across two points of an electrically conducting material when there is a temperature difference between them. The emf is called the Seebeck emf (or thermo/thermal/thermoelectric emf). The ratio between the emf and temperature difference is the Seebeck coefficient. A thermocouple measures the difference in potential across a hot and cold end for two dissimilar materials. This potential difference is proportional to the temperature difference between the hot and cold ends.

Thermoelectric generators are like a thermocouple/thermopile but instead draw some current from the generated voltage in order to extract power from heat differentials. They are optimized differently from thermocouples, using high quality thermoelectric materials in a thermopile arrangement, to maximize the extracted power. Though not particularly efficient, these generators have the advantage of not having any moving parts.

When the hot side is produced by a decaying radioactive element and the cold side is given by space, you have your space thermoelectric generator.

With the Peltier effect, you get the inverse:

When an electric current is passed through a circuit of a thermocouple, heat is generated at one junction and absorbed at the other junction. This is known as the Peltier effect: the presence of heating or cooling at an electrified junction of two different conductors.

A typical Peltier heat pump involves multiple junctions in series, through which a current is driven. Some of the junctions lose heat due to the Peltier effect, while others gain heat. Thermoelectric heat pumps exploit this phenomenon, as do thermoelectric cooling devices found in refrigerators.

Answer 4

The thermoelectric effect does not really turn heat into electricity. It turns the flow of heat into electricity. This is used on spacecraft, just look at RTGs on vessels like voyager or new horizons. Unfortunately, when compared to mechanical heat engines like brayton power converters, it is very ineficient.

Answer 5

Any time you have warm end and cold end near each other, you can get some useful energy. The catch is, to make it effective you want as big temperature differential as possible. That's sad reality of thermodynamics, no amount of cleverness is going to help.

The warm end is the space station's waste heat, you can't increase the temperature of that as you presumably don't want to boil yourself alive.

The cold end temp can go down to background temperature of space, in practice it's tens of kelvins. However the lower the temperature, the bigger the radiator you need because, clearly, lower temperature radiator radiates less heat per area. You most likely end up with some bad compromise with the radiator still too big and the recovered energy not being worth it.

Answer 6

The component you're referring to is called a thermoelectric generator (TEG), which directly converts temperature differences into electrical voltage through the Seebeck effect. While TEGs can, in principle, be used alongside heat engines like the Stirling engine to generate power from heat differentials, their efficiency is a significant concern. In practice, the energy required to create a substantial temperature differential with a TEG often exceeds the energy produced by the subsequent conversion of heat to electricity, especially in environments with minimal temperature differences like open space. However, in space travel, TEGs have found a niche application in radioisotope thermoelectric generators (RTGs), where they convert heat from decaying radioactive materials into electricity, providing power for spacecraft over extended periods. This method doesn't eliminate the need for radiators but does offer a reliable power generation method from what is essentially waste heat. The idea of using TEGs to eliminate radiators and generate additional power is intriguing but would require significant advances in thermoelectric material efficiency to become viable.

Answer 7

Can it produce Power: Yes

Will it help you cool your ship: No

Drawing power from the flow of any sort of system whether it be wind, water, steam, or heat, is all about applying a resistance on that flow. A very simplified way of describing the thermoelectric effect is that you redirect some of the flow of energy from the inside of your ship to the outside back into your ship's electrical systems. So instead of expelling your waste energy as heat, you will retain it.

All of that retained energy will eventually have to be re-released somewhere inside of your ship where it will again become thermal energy.

Can Something Better Than Radiators Exist?

Radiators are the only practical mechanism for cooling a ship at this time, but if you want to improve the cooling of a ship beyond modern tech, the most plausible near-future tech would be to use metamaterials. The key here is that the laws of Thermodynamics apply to entropy, not to heat. There are known edge cases where heat and energy can move in unexpected ways. The reason the rules of Thermodynamics are based on Entropy instead of heat is that Entropy is a probabilistic model, but heat flow at the subatomic scale is a discrete action. With the right manipulations at the subatomic scale, we can manipulate all sorts of weird thing about how heat flows. We just do not yet have materials that can make practical use of these manipulations for real world applications.

Because of this, it could be possible to create things like quantum synchronized solids that take advantage of being a zero entropy system to transfer waste heat directly from your reactor room to your radiators at the speed of sound allowing most of the heat to go around the inhabited parts of your vessel and to heat your radiators up to the same temperature they would be if they were making direct contact with the reactor: Liquid Helium has already been used to prove that this can happen in principle, but like with superconductors, these materials are hard to make work at useful temperatures. That said, there have been huge strides in recent years with creating quantum synchronized solids for quantum computing processors, so there is plenty of reason to believe it is possible.

Or you could perhaps use some variation of the Casimir Effect. The Casimir Effect does not just let you make negative energy, it lets you make negative entropy as well. If you were to use the Casimir Effect to make a negative entropy virtual axis, then the same mechanics being experimented with right now by DARPA for exploiting vacuum energy for propulsion could also be applied to the movement of heat as well.

Then there is Maxwell's Demon Gate Theory which basically states that entropy is irrelevant if you can make a "smart" material that only allows energy to pass through in one direction... how one would make such a material is unknown to modern science, but still not beyond the realm of possibility.