A common science fiction trope is that the heat can be radiated out into space with a laser - I’ve seen it in a few books by “hard” SciFi authors. Not sure on the mechanism that converts the heat to a laser, but is such a concept physically possible?
Normally excessive waste heat in a system can be put to good use, but I am assuming with space/weight/complexity considerations in space the hardware to transfer the heat into a useable form is not available
A laser seems like a bad idea since you need a low-entropy power source to drive it. I don’t think that would really work.
However, what you can do is dump all of your waste heat into a small area, and then get rid of it in a beam via parabolic reflector.
Suppose, for instance, you have a battery-powered cooling system for your ship. The battery doesn’t inherently generate any waste heat, but the refrigeration system does. However, there’s no limit to how hot you can make the hot side, as long as you’re willing to sacrifice efficiency.
So you could make your system dump all its heat into a little 1-meter square at thousands of degrees. It’ll radiate very effectively due to the Stefan-Boltzmann law, which says that power transfer is proportional to the fourth power of temperature. You then use a reflector to put it into a beam shape that you aim away from the bad guys that are tracking you.
The efficiency of a cooling system goes way down as the temperature differential increases, so the system wouldn’t work for long, but it would work for a while.
To put the heat to some useful purpose you have to have a cold sink, and when the cold sink reaches equilibrium with the heat source you can’t do any more.
I do remember in “Sundiver” by David Brin the ship exploring the sun had a refrigeration laser. But I’m not sure if such a system is really thermodynamically possible. However, he’s the guy with the PhD, not me.
The idea of getting greater thermal efficiency and emissivity by using a “laser radiator” is in using the waste heat to pump up a laser to very high temperature. However, high energy lasers have a fundamentally inefficient throughput (the amount and rate at which energy can be conveyed through the system) even when starting from a narrow spectral source, and I really don’t know any straightforward way of converting waste heat at a moderate temperature into a free electron plasma, much less any other lasing medium. If you had some magical laser that could convert thermal energy radiating at any temperature into a lasing medium then I suppose you could use it as a mechanism to increase exhaust efficacy, but that’s about as science fantasy as antigravity. A laser could be used to cool an object it was directed at at a specific wavelength by pumping molecules on the surface to a specific intermediate excited state, but that would require the laser and its power source to be external to the spacecraft.
Putting “excessive waste heat in a system to good use” (e.g. performing useful work) requires access to a suitable low temperature reservoir. In terrestrial environments we have plenty of low temperature reservoirs in the form of rivers, lakes, and seas made of a nearly ideal type of matter for high density convective heat transfer (i.e. water). In the case of a spacecraft, although the background temperature of space is low, the rate is limited by heat rejection via the mechanism of radiation, which requires large outward facing surfaces to radiate energy away versus flowing cold water through a tightly weaving tube or closely packed fins. And if your system of heat rejection is not able to keep up with the production rate of the source of excess heat, temperature will continue to build monotomically until you experience functional or material failure of the system (e.g. the spaceship becomes so hot that it stops working or even starts to melt or evaporate).
Stranger
A refrigeration laser would fundamentally be the same principle as the parabolic reflector at the hot end of your refrigerator, and just as consistent with the laws of thermodynamics (given that you have some power source, and are willing to accept that you’ll be dumping a lot of extra energy from that power source, too). I have no idea how easy the engineering would be, for either system, in practice, but there’s no law against either.
Two problems; one is that batteries generate thermal energy by their very nature, and two is that you can’t “dump waste heat into a small area” as if it is a physical thing that can be mechanically manipulated. Heat will flow from a high temperature system to a low temperature system, so the only way your waste heat will go to any area is if the temperature is lower than the operating temperature of the battery, or in the case of a heat pump/refrigeration system, the output temperature of that system.
The commonly espoused idea that refrigeration or heat pump cycles can move “heat” around without limit is just not correct. Such systems can move more heat energy than the energy put in them do perform the work (a measure referred to as efficacy) but they will have both thermodynamic and material limits depending on the operating temperature and functional/melting points of the materials used in the system.
Stranger
I dunno. The radiator has a few advantages over the laser in terms of entropy:
As far as I know, actually, it’s possible to beam EM radiation with perfect efficiency–that is to say, it’s like mechanical or electrical power (which can approach 100% transfer efficiency) as compared to heat (which must abide by the Carnot limit). But that means the input to our laser is a Carnot-limited process. We can’t just power the laser with generic waste heat; that violates the 2LOT. We need a heat engine of some kind, which is by necessity Carnot limited.
A blackbody radiator doesn’t have this problem. The efficiency still isn’t great, but the process in transferring the energy is exactly 100% efficient, so you don’t have the issue that running the device itself generates waste heat.
Practical, real-world batteries–yes. But that isn’t a law of nature, just a characteristic of current battery technology.
If you wish, our “battery” can be a flywheel with superconducting electrical generator. Even today, we can build a system like this with many “nines” of efficiency.
This isn’t like a heat engine, where physical law prevents us from exceeding certain efficiency values.
The heat radiates into space, and does so quite efficiently if the radiator is hot enough. Carnot gives a limit as to what power is required to pump heat from one place to another.
The context here is a hard sci-fi story story, where the rules are usually that we’re allowed to posit “unobtanium” as long as it doesn’t violate known physical law. A 99.999% efficient battery is ok, as is a material that melts at 10,000 K. A heat pump that exceeds the Carnot limit isn’t.
Strictly speaking, all physical processes have a maximum efficiency imposed by the laws of thermodynamics. It’s just that, for a lot of them, the effective temperature is so high that we need never worry about it.
Dr. Strangelove, the points you mention are good arguments for the thermal-source-and-reflector being the more practical system. But I still maintain that both are possible.
Where is the thermodynamic efficiency loss in power transmission through a mechanical shaft? How about electrical transmission through a superconductor? As far as I know, these processes can be precisely 100% efficient.
Now, you have a point in that for some processes, there is a semi-hidden temperature that imposes an efficiency limit. For instance, solar cells have a hard upper bound on efficiency based on the surface temperature of the Sun. That’s higher than anyone really cares about, so photovoltaic inefficiency comes from other places. But I would not say that all physical processes follow the same reasoning. In particular, quantum-mechanical processes like superconductivity and superfluidity should be perfect due to the nature of quantization.
But if the laser has zero entropy, then how is the system as a whole getting rid of it? It’s not enough to just emit energy; we have to emit entropy as well. Because otherwise we have to store it internally, and that necessarily implies greater temperature (since making things hotter allows for more entropy). And that defeats the nature of the thought experiment (if we’re just going for something very short term, we could just forget the beam and store the heat in an insulated container).
I posit that a heat pump, powered by a high-efficiency battery and emitting waste heat though a radiator, is at least sciencifically plausible. But I see no way to use heat to drive a laser, at least not in a way where you aren’t always better off just getting rid of the laser.
Found this link last night, and learned 1.) it is not possible and 2.) ZapperZ is a douchenozzle.