How much kJ heat can you drain from a person?

[Korto]

OK, odd question time.
Let's say you have a way to instantly drain heat from the top 1cm of a person's body*, from all over the surface. How much heat could you take (or, what temperature could you reduce that layer to) before it:
a) Starts impeding the persons actions
b) Critically impedes the person's actions
c) Renders them unconscious
d) Would kill them.
These effects don't need to be instant, but inevitable. The person is in the situation of being in active combat, therefore having your actions impeded could be important.
The drain happens instantaneously, and the person may then reheat from normal metabolic processes. Then again, it's also possible that they may also have lesser amounts of heat being drained continuously (from 50W up to 1000W), keeping the layer at that reduced temperature (or even dropping).
Externally supplied heat (the sun, the air) has no effect for this.
I did consider having the top 2cm being affected, but then I thought that would go deep enough to directly affect the heart and brain. (Give a whole new meaning to "Brain Freeze")

Stats I've used
Human body at 1.9 m^2 SA and 80kg
Specific heat 3.47 kJ / kg (Engineeringtoolbox.com)
Specific weight 1.08 g / cm

Mass of flesh directly affected : 20.5 kg

*It's actually an alien-tech suit that drains body-heat to power itself, but that's not important.

[LaCroix]

This is tricky, as our power output is coupled to our activity level.

During sleep, the body's energy expenditure is about 65 Calories (kCal) per hour. At rest lying down, it rises to 80 kCal/hr and, when sitting up, to 100 kCal/hr. During light exercise, this jumps to as much as 200 kCal/hr and, during heavy physical work, to 500 kCal/hr.

Using that as upper limits, if there was a total conversion to heat, a human would produce between 75 and 600 Watts, depending on activity level.
Since at sleep, we mostly only retain body temperature, 75 Watts would be good lower limit for an active person.

As soon as you start using more power than the current activity level generates, it will cool the body down. That effect will at first be limited to your defined areas (~2cm deep), but blood circulation will quickly start to even that out, leading to general hypothermia.

In general, loss of 1-2 degrees body heat will cause first symptoms of hypothermia - shivering and loss of fine motor control.
2-4 degrees will result in moderate hypothermia, which apart of violent shivering, will reduce a human to a state not unlike being very drunk - dazed, little motor control, slurring, etc.
5+ degrees loss will render a person unable to function, and will quickly result in loss of consciousness and death.

Remember that massive energy extraction using that system would result in full body frostbite within a very short time span, anyway, even if hypothermia is avoided due to quick application of heat. It would be like pouring liquid nitrogen over a body.

Using your example of drawing 1000W from an highly active person ( assume 50% conversion - 300W of heat production ) for 15 minutes (shielding during a fight, for example) would mean that during that time you've cooled his whole body down by roughly 175 Wh, or 630 kJ.

Doing this would cool the whole body down by about 2 degrees, causing quite severe signs of hypothermia, and I'd bet he'd have severe frostbite, as well.

[Terralthra]

Any device which can convert heat into usable energy beyond the efficiency of a Carnot engine, or when there does not exist a differential of temperature from one body to another, can also be the basis for a perpetual motion machine of the second kind.

[LaCroix]

Good point. So the efficiency would be what - our heat reservours would be body temperature(~310K) and ambient temperature(~ 293K), so we got
1 - 293/310 => roughly 5.5% efficiency for a Carnot engine, if I'm not mixing things up.

So the suit would almost kill you to get 55W of power out of you? Doesn't sound like a good trade to me.

[Terralthra]

Core temperature of 35 C (308 K) will generally involve moderate to severe shivering, numb and blue-gray skin, etc. I really don't think this alien power system is going to be a great method.

[Korto]

In my urge to not muddy up a (medical) science question with too much science fiction hand-wave, I left out one important detail. The suit, in some way, has contact with an effectively infinite heat sink at 0 Kelvin, so the temperature differential is 300 K to 0 K. I was hoping to use watts and joules in a nice simplistic form.
No, there is no explanation how any of this works, in fact the exasperated main protagonist repeatedly refers to the thing as the "magic suit".

This is tricky, as our power output is coupled to our activity level.

Yes, but there's also "make-work" such as shivering.

During sleep, the body's energy expenditure is about 65 Calories (kCal) per hour. At rest lying down, it rises to 80 kCal/hr and, when sitting up, to 100 kCal/hr. During light exercise, this jumps to as much as 200 kCal/hr and, during heavy physical work, to 500 kCal/hr.

Using that as upper limits, if there was a total conversion to heat, a human would produce between 75 and 600 Watts, depending on activity level.
Since at sleep, we mostly only retain body temperature, 75 Watts would be good lower limit for an active person.

There's differing figures. I also looked at Engineering Toolbox. It has (all in watts):
Light effort at 70 to 175 (I called it 100),
Moderate 175 to 260 (called it 200)
Heavy 260 to 420 (300)
Very heavy 420 to 700 (500)
Extreme 700+ (called it 1000, which may, or may not, have been over the top)
However, I did then misuse it, because it seems to be TOTAL watts produced, not just that responsible for heating. A percentage would be heat, and a percentage work. But what percent? (segue to...)

I forgot about that "Living in Space" thing. Also from that is this bit

While working, 45 percent of the heat is lost by normal heat transfer methods and 44 percent through perspiration. The remaining 11 percent goes toward the actual accomplishment of physical work. Calculations based on these numbers indicate that the human being is approximately 11 percent efficient or about as efficient as an internal combustion gasoline engine" pg 50

Which gives a percentage with some justification behind how much energy out of the wattages above to attribute to heat and how much to work.

As soon as you start using more power than the current activity level generates, it will cool the body down. That effect will at first be limited to your defined areas (~2cm deep), but blood circulation will quickly start to even that out, leading to general hypothermia.

It's true that due to the instant nature of it, the body wont have time to clamp down on blood flow to the surface, which will cause more core loss of temperature than would normally be expected. I suppose I could assume blood flow spreads the heat loss quickly, and therefore work simply off core temperature, which I'll do if there's no better option, but really there should be an increased ability to chill the surface before the core becomes seriously affected.

Remember that massive energy extraction using that system would result in full body frostbite within a very short time span,...
Doing this would cool the whole body down by about 2 degrees, causing quite severe signs of hypothermia, and I'd bet he'd have severe frostbite, as well.

According to what I've read, frostbite only occurs when flesh freezes, and the ice crystals rupture the cells, therefore at a localised temperature of less than 0C. Reducing the temperature of the entire body by a few degrees might kill them, but it wont give them frostbite.

Core temperature of 35 C (308 K) will generally involve moderate to severe shivering, numb and blue-gray skin, etc. I really don't think this alien power system is going to be a great method.

Assuming my sums are right, if the entire body was reduced by 2 degrees, that would yield over 550 kJ of energy, which at a draw of 1000W would be 9 minutes, not accounting for heat being replaced by metabolic processes. 1000W is supposed to be at the upper end of power drain, and only viable for short periods (due to the user collapsing, from cold, exertion, or both)

[LaCroix]

I just realized that a Carnot engine using 0K as cold reservoir will always work at 100% efficiency, as 0 divided by anything will always be 0. - Is that right or am I missing something?

You are also forgetting that pulling out enough heat to reduce core temperature to 35 degrees within 10-15 minutes, you will have practically shock-frosted his skin. You can be quite sure that the person won't be able to fight after a mere handful of minutes. The experience would be very much akin to falling into a frozen lake. (Which would cool you down by a similar rate.)

[Korto]

I just realized that a Carnot engine using 0K as cold reservoir will always work at 100% efficiency, as 0 divided by anything will always be 0. - Is that right or am I missing something?

Yep, that's right. It's a bit of a cheat, but it keeps it simple, it makes sure a decent amount of power is there, and I'm the fucking author anyway.

You are also forgetting that pulling out enough heat to reduce core temperature to 35 degrees within 10-15 minutes, you will have practically shock-frosted his skin. You can be quite sure that the person won't be able to fight after a mere handful of minutes. The experience would be very much akin to falling into a frozen lake. (Which would cool you down by a similar rate.)

But cold-shock upon falling into freezing water is survivable. In fact, it sounds as if it wouldn't kill you, except for the reflex of inhaling a lung-full of ice water. Remaining in the water, over the next ten minutes you lose fine (and not-so-fine) motor control, and over the next hour after that (of remaining in the water) you lose consciousness.

Just spent half an hour trying to find if anyone had ever put on the net the rate of heat loss of a person in freezing water. Couldn't find anything, so I tried to patch something together. I am showing my working here, so you'll know where these numbers came from.
An old wind-chill formulae is WCI=(10 x sqrt{V} - V + 10.5) x (33-Ta)
Where
WCI = Wind Chill Index in kiloCalories per m^2 per hour (fucking non-SI units... grumble...)
V= wind velocity (assume 0, as it's not a factor)
Ta = Temperature of Air in Celcius

Using that, the heat loss in air at 0C would be 346.5 kCal / m^2 / hr
As Area = 1.9 m^2 then heat loss = 0.765 kilowatts (in air) (converting calories to joules)

There's two rates I found for the multiple difference in heat loss between water and air. The heat loss is 70 times faster if you're moving about, 20 times faster if you're not (and probably curled up in a ball). Obviously, for this, the 70 times is more appropriate
Heat loss (in water) = 53.6 kW,
As we're talking about an instantaneous loss here, I'll whistle casually, wait till everyone's looking the other way, and then change "53.6 kWatts" to "53.6 kJoules", a loss of which would lower the temperature of the top 1cm of skin by 3/4 of a degree.

So, it looks like an instantaneous drain of 53.6 kJ is quite survivable, particularly with training and acclimatisation.

What's that? 53.6 kJ isn't 550 kJ ? Ummm... no, it's not, is it...
I can say that losing 550 kJ from the top 1cm of the body would result in a drop of nearly 8 degrees in that layer. So it wont actually freeze it, but it'll certainly feel like it.
I can try to calculate how long you would have to be in water to lose that amount of heat. This time I'd use the x20 multiple, as the person in the water probably huddles after the initial shock.
Time (s) = 550 kJ / (0.765 kW x 20) = 36 s
Therefore, losing 550 kJ is akin to being in freezing water for 36 seconds . No, I don't think that's right, but I don't know how to fix it. It can't be being upset by my ignoring the body trying to create heat over time, as that has a negligible effect in ice water. To indicate just how wrong it is, the entire body will reach 0C throughout in 11 minutes, using this method. No wonder he loses fine motor control.
grumble... stupid bloody thing... grumble...

Hmmm... what I'm not compensating for is the body taking steps to conserve heat, which would slow the heat loss. Could that be the problem? Possibly. The outer flesh allowed to go to zero, while the heat is kept in tight. Can't think how to compensate for it, though, and even if I did, losing 550kJ over 10 minutes is not the same as losing 550 kJ in less than a second. Even if the end temperature is the same, the difference in shock to the system is incredible.
I'm open to ideas, otherwise I'm just going to make shit up.

[LaCroix]

I think the best thing would be to go with Newtons law of cooling, since you are basically sucking that heat out of the body, somehow.

T(t) = T_surroundings + (T_0 -- T_surroundings) * e^(-k * t)
Where:
T(t) = temperature of the object at time t
T_surroundings = temperature of the ambient environment
T_0 = the temperature of the object at t = 0
k = numerical constant that depends on the object and the environment

You could now use your calculated temperature loss over time for various settings to calculate k for that power drain.
This would allow you to come up with how long a person would be able to endure wearing the suit at various settings before a certain body heat loss occurs.
(Of course, you would need to factor in the amont of heat a person produces and reduce the heat removed by the suit by that wattage...)

[Zeropoint]

Unless you're also using "magic" technology to drain heat from the whole 1 cm layer simultaneously, you've also got issues with heat transfer within the layer of flesh. To have a heat flux that high will require a temperature differential across the layer that will put one side at tissue-destroying temperatures. For an example of what I'm talking about, put a 1 cm thick steak in a REALLY hot frying pan, and check 1) how long it takes for the top side to start getting hot, and 2) what the bottom side looks like at that point.

[Terralthra]

Zeropoint, the whole thing is already "magic" tech. We currently know of no way to "drain heat" and turn it into other useful forms of energy that don't cost energy to use themselves. Like I said, the idea of something that can turn a single reservoir of heat into mechanical work is fundamentally a perpetual motion machine.

[Korto]

Did'ja see that!? Did'ja see that!? Some completely random person broke in, held me at gunpoint, typed a whole lot of weird shit into my computer, and then ran away again! Really, really fast! I tried to catch him, but, you know, he was really fast, and he had a gun, and... he was really fast... and...
You're really not buying this, are ya?
I don't know, it was late, and I was trying to find some way of doing it that didn't involve me having to answer how thick the "wall distance" was for the heat equation, when there isn't a wall distance, it's two surfaces in direct contact, but you've got to have a wall distance or the equation blows up.
I found this problem acknowledged here, where basically their solution is to assume a hopefully suitable wall thickness, ie make shit up.
LaCroix, your equation doesn't use wall thickness, which is good, but is k the standard thermal conductivities?
And why was I even doing this again, when it's completely tangential to the actual problem of what's the medical effects of removing X amount of heat from the top 1cm of flesh?

Yeah, Zeropoint, just call it magic. The main character in the story does. He's given up trying to work it out. The heat is reasonably evenly taken from the entire depth. If I can have force fields, I can have this too. (Actually, force fields might be related to how it's done, since Simon_Jester pointed out they would have a cooling effect)

Terralthra, I'm not getting you calling it a perpetual motion machine, though. I'm a bit sensitive on that, because I had to put a fair bit of thought into some of the super-tech to avoid them being perpetual motion devices (particularly the bloody teleport drive). I picture this as being analogous to a thermocouple, with a hot and cold end. Energy is taken from the hot end to the cold end, and work is done by that. The fact that the cold end isn't, apparently, in our universe, doesn't change that energy is indeed being used. If the objection is to the perfect 100% efficiency, well, that's only a simplification. Maybe it's 99.9%. Maybe it's 98%. It really doesn't matter.
Actually, how do you know it's perfectly efficient? Maybe it takes twice the amount it usefully uses, and the other half is dumped as waste heat, here or "elsewhere". I'm only looking at how much energy it's taking right now, not how much it usefully uses. Not even I know how efficient it is yet.

The point of this thread was for to work out some reasonably grounded limitations on this magic-tech, so that when I was writing, I could check back over what the thing can do to make sure it was consistent. The limitation is meant to be the person wearing it.
Therefore, it's a medical science problem, and if I keep on talking about the pseudo-physics the mods will kick me over to SF.

[Terralthra]

Terralthra, I'm not getting you calling it a perpetual motion machine, though. I'm a bit sensitive on that, because I had to put a fair bit of thought into some of the super-tech to avoid them being perpetual motion devices (particularly the bloody teleport drive). I picture this as being analogous to a thermocouple, with a hot and cold end. Energy is taken from the hot end to the cold end, and work is done by that. The fact that the cold end isn't, apparently, in our universe, doesn't change that energy is indeed being used. If the objection is to the perfect 100% efficiency, well, that's only a simplification. Maybe it's 99.9%. Maybe it's 98%. It really doesn't matter.
Actually, how do you know it's perfectly efficient? Maybe it takes twice the amount it usefully uses, and the other half is dumped as waste heat, here or "elsewhere". I'm only looking at how much energy it's taking right now, not how much it usefully uses. Not even I know how efficient it is yet.

The point of this thread was for to work out some reasonably grounded limitations on this magic-tech, so that when I was writing, I could check back over what the thing can do to make sure it was consistent. The limitation is meant to be the person wearing it.
Therefore, it's a medical science problem, and if I keep on talking about the pseudo-physics the mods will kick me over to SF.

"It's using a cold reservoir, that cold reservoir is just in another universe" is nonsensical. If it can be interacted with something in our universe, by definition, it's in our universe, since the universe is defined as everything that exists, and if it can be interacted with by some x in our universe, it exists, by any practical definition of "exist" or "interact."

Whether it operates efficiently doesn't particularly matter. If it runs at 50% efficiency, and outputs the rest as waste heat, it can then take that waste heat as input energy to run the machine some more, which will generate some more waste heat, which can be used as input to run the machine some more...that's why it's a perpetual motion machine!

If it doesn't produce waste heat (which is in essence fuel), then it violates the second law by decreasing entropy in an isolated system.

[Korto]

"It's using a cold reservoir, that cold reservoir is just in another universe" is nonsensical. If it can be interacted with something in our universe, by definition, it's in our universe, since the universe is defined as everything that exists, and if it can be interacted with by some x in our universe, it exists, by any practical definition of "exist" or "interact."

Oh, for Christ's sake, give it a rest. It's a medical question, not philosophical physics. Who give's a fuck if the "Other Place" is considered this universe or another by pedants? There's more than one single definition of the word 'Universe' anyway.

Whether it operates efficiently doesn't particularly matter. If it runs at 50% efficiency, and outputs the rest as waste heat, it can then take that waste heat as input energy to run the machine some more, which will generate some more waste heat, which can be used as input to run the machine some more...that's why it's a perpetual motion machine!

If it doesn't produce waste heat (which is in essence fuel), then it violates the second law by decreasing entropy in an isolated system.

So, if you had a tall column of water, with a hole in the bottom, and you used the flow of water through that hole to run a machine, you've got a perpetual motion machine because the water still exists even after it's run out the bottom? Even though it would take work to recover it?

[Terralthra]

"It's using a cold reservoir, that cold reservoir is just in another universe" is nonsensical. If it can be interacted with something in our universe, by definition, it's in our universe, since the universe is defined as everything that exists, and if it can be interacted with by some x in our universe, it exists, by any practical definition of "exist" or "interact."

Oh, for Christ's sake, give it a rest. It's a medical question, not philosophical physics. Who give's a fuck if the "Other Place" is considered this universe or another by pedants? There's more than one single definition of the word 'Universe' anyway.

So, it's not 100% magic tech, it just ducts heat to another universe to generate work. Uh-huh. Whatever you say.

Whether it operates efficiently doesn't particularly matter. If it runs at 50% efficiency, and outputs the rest as waste heat, it can then take that waste heat as input energy to run the machine some more, which will generate some more waste heat, which can be used as input to run the machine some more...that's why it's a perpetual motion machine!

If it doesn't produce waste heat (which is in essence fuel), then it violates the second law by decreasing entropy in an isolated system.

So, if you had a tall column of water, with a hole in the bottom, and you used the flow of water through that hole to run a machine, you've got a perpetual motion machine because the water still exists even after it's run out the bottom? Even though it would take work to recover it?

No, because the water isn't the fuel, the gravitational potential energy of the water is the fuel, and once it's at the bottom of the column, the GPE is gone. This is really simple thermodynamics.

GPE is not heat. The second law of thermodynamics doesn't say ""No process is possible whose sole result is the absorption of Gravitational Potential Energy from a reservoir and the conversion of this GPE into work". It does say that about heat.

[Korto]

So, it's not 100% magic tech, it just ducts heat to another universe to generate work. Uh-huh. Whatever you say.

Yes, you're right. I've been claiming all along that this technology is completely feasible with our current knowledge, you know, like where I said

Yeah, Zeropoint, just call it magic. The main character in the story does. He's given up trying to work it out.

and where I called it

pseudo-physics

Now, it's true that I wouldn't say the technology is completely impossible. I would say as far as we know it's completely impossible.

. . . . . . . . . . . .

The second law of thermodynamics doesn't say ""No process is possible whose sole result is the absorption of Gravitational Potential Energy from a reservoir and the conversion of this GPE into work". It does say that about heat.

Do you have a different 2nd Law than me? The 2nd Law (according to Wikipedia)

The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibrium—the state of maximum entropy.

Well, we've already established it's NOT AN ISOLATED SYSTEM, since it has a connection to "some other place".

Informal descriptions
The second law can be stated in various succinct ways, including:
It is impossible to produce work in the surroundings using a cyclic process connected to a single heat reservoir (Kelvin, 1851).

It's not bloody cyclic. The energy goes into an "infinite sink", and doesn't return.

It is impossible to carry out a cyclic process using an engine connected to two heat reservoirs that will have as its only effect the transfer of a quantity of heat from the low-temperature reservoir to the high-temperature reservoir (Clausius, 1854).

The heat goes from the higher (300K) to the lower (0K)

If thermodynamic work is to be done at a finite rate, free energy must be expended. (Stoner, 2000)[32]

And it is certainly expended, in fact, that was what this thread was about. How much could be expended without killing the generator.


I don't see any conflict.

[Terralthra]

I don't think you understand what "cyclic" means, in the thermodynamic context. In this case, a thermodynamic cycle is one in which work is done, and then the working system returns to its initial state. For example, in an internal combustion engine, the piston moves as the fuel combusts to do work, then returns to its initial state by rotation of a camshaft, to be moved again by the next cycle of the engine. Similarly, in your infinite magitech heat engine, a "cycle" would be that it drains some amount of heat energy "to some other place" in order to do some amount of mechanical work (say, lifting the armored suit arm); after that work is done, the suit arm falls back to its original position. Cycle complete. Work expended.

Now, you want to say that it's not a single-reservoir heat pump, because it's actually pumping the heat "to some other place" having a temperature of 0 degrees Kelvin, and extracting work in the process. If you have sufficient energy to open a thermodynamically-permeable wormhole to "some other place", then you have more energy than you could possibly gain by draining the heat energy of a human body. Oh, and you don't kill the wearer either. More importantly, even if you could somehow open a wormhole from the suit to the "some other place" that's 0 Kelvin (without expending more energy than a human body contains), why wouldn't you just open TWO wormholes: one to the place that's zero Kelvin, one to the center of a star. Then you have more energy than you could ever possibly need in a suit of armor.

[LaCroix]

LaCroix, your equation doesn't use wall thickness, which is good, but is k the standard thermal conductivities?
And why was I even doing this again, when it's completely tangential to the actual problem of what's the medical effects of removing X amount of heat from the top 1cm of flesh?

K is the thermal conductivity between the human body and the medium. Which is a problem in your case, since we don't have that constant for your suit, yet. You would need to solve for K first, with a set of known values for your suit (and probably for more than one setting).

It may seem tangential, but it's the best we can come up with for a system that operates on magic. It's the standard formula used to determine time of death in a cooling body. So it's basically modelling the same mechanics your suit uses, only with a drain against ambient temperature instead of absolute zero. Because of the suit is isolating you against the environment and draining you against a 0K source, I think you can substiture T_surround by 0K, which means you would get a single constant times t as formula for heat loss using your suit. It removes the need for magic in the heat removal, and instead invokes magic in the sense of "the suit's magitech material's temperature is actually 0K, but it can insulate the wearer somehow, except for when it needs energy". Like a startrek forcefield, with a frequency depending on energy need, exposing the wearer to 0K for fractions of a moment, just long enough to not cause contact damage, but drawing heat that way.

If you set an arbitrary in-universe time for when a wearer would suffer frostbite, you can use it to calculate K for that event, and use it accordingly.

[Korto]

I don't think you understand what "cyclic" means, in the thermodynamic context.

From what I just read on thermodynamic cycles, it requires the system to return to the same state, which seems to include energy, and my device doesn't. Energy is consumed.
And this is the point I vacate the field, on the grounds that you might just know what you're talking about. I will say that when I hear "cyclic" in connection with perpetual motion, I associate it with one of those ridiculous devices that as part of their operation return the energy to the high ground, like this:

If my thing is a perpetual motion by definition, this one doesn't offend me as the user does have to pay for its operation. It doesn't give free energy, or even run forever.

The aliens could make more sensible use of their technology, and maybe they do, but the humans don't know about it. These are the suits given to a human splinter group, and the way the suits work may say more about the psychology and motives of these (unseen) aliens then anything else (believe me, in-story there are groups of very smart people troubling over the very same thing). Or there may be limitations to the material that we don't know about. All that the humans know for sure is that it drains heat from the person wearing it, and it does stuff, but the stuff it does never (in entropy terms) reduces entropy by more than the increase in entropy indicated by the amount of energy drained.

When you say "Now, you want to say that it's not a single-reservoir heat pump..." are you here offering advice as to how it should work? I will admit that what I actually want is to say as little about how it works as possible, and let readers come to their own theories. Easier that way, and I can't be wrong.

LaCroix : Yes, I suppose flickering on and off could be the way it works. As I said, I don't want to go into it too closely. It's tangential, though, because my real question is "How much heat can be taken from the surface flesh of a human before it affects them in differing ways?", not "How do I calculate the amount of heat being taken?" The amount of heat that gets taken will be the amount I say, I just want to know what the effect would be on the person of the amount I say.

[Terralthra]

Energy can be consumed in a thermodynamic cycle. Indeed, they could not work if energy were not consumed. For example, the Otto cycle, the diesel cycle, the Brayton cycle, etc. Those are all "power cycles", in which heat energy is used to produce mechanical work by transferring from a greater heat reservoir to a lower heat reservoir. In the case of an internal combustion engine, the heat energy is the combusted fuel/air mixture exerting pressure (ideal gas law: pressure = temperature, assuming equal volume and gaseous mass).

The other type of thermodynamic cycle is a heat pump cycle, in which mechanical work is expended to move heat energy (generally a poorly defined quantity, but you can think of it as "temperature") from a low-heat reservoir to a high-heat reservoir. The most common one you'd be familiar with is a refrigerator: it expends energy (electricity, generally) exploiting phase changes: by compressing a refrigerant, heat is generated outside the intended cool space (this is why the back of your fridge is typically warm), condenses, then circulated into the cooled space and allowed to rapidly expand, causing it to cool rapidly and transferring heat energy from the cooled space into the refrigerant. This refrigerent then circles back to the condenser. Cycle complete. Energy expended.

The key thing to understand about thermodynamics are the two laws, typically stated (rather abstractly) as 1) energy can neither be created nor destroyed, and 2) a closed system tends towards higher entropy. These are formulated best for laypeople as 1) you can't win and 2) you can't even break even. The perpetual motion machines you are thinking of are referred to as "perpetual motion machines of the first kind", as they violate the first law: they purport to generate energy from nothing. The second type are the more relevant to this example: they are closed systems which decrease entropy.

What is implied by "a closed system tends toward higher entropy" is that any change of energy from one form to another results in inefficiency, typically in the form of waste heat. Maybe best illustrated by the refrigerator example: Suppose we have a perfect refrigerent: there is no measurable friction in the transfer system. Suppose further that we have a perfect condenser and a perfect compressor, that have no frictional loss of input energy to waste heat etc. In this case, we still don't win, because there's still input energy to condense the refrigerant. This is where we get to the second type of perpetual motion machine. Say we have some device that uses heat energy to create mechanical work. We start our machine with a kickstart. It condenses the refrigerant, which outputs heat. We store this heat in a convenient location, then start the refrigeration cycle. When the refrigerant comes back to the condenser, we use the heat->work device to condense it again. The waste heat (inefficiency) now powers the device and we now have an infinifridge. Or, to use your example, we connect your power device to a refrigerator, it cools the fridge (by dumping heat into the nether), and generates power, which we then use to heat the fridge up some (doesn't matter how; a coil heater would work just fine), which we then cool some more, which generates more power...See the problem?

When you say "increase in entropy indicated by the amount of energy drained," you have it backwards. Cooling the person inside the suit is a net loss in entropy. Cooling something, that is a device which transfers energy from a person to another thing to which it otherwise would not transfer, is an entropy decreasing act, hence implies that work is being done to transfer the energy. In this case, the "other thing" is the "0 K reservoir in another universe". The immediate question, thermodynamically, is that if this is to generate power, in the form of a heat engine, that implies that the heat energy would transfer anyway, and we're tapping the existing transfer. But we're not, we're creating a connection which otherwise would not exist. If we're cooling something and generating power in the process, there must somewhere else be an energy expenditure greater than the power gained by the heat engine, or it's a violation of the second law. The simple question is, if you already have a power source greater than the energy gained by heat transfer (as is necessary under the known laws of physics), why use it to transfer heat from a place that needs it (the user's body) and generate less energy in the process, instead of simply using the power source to work the suit?

An analogous example might be, say, "well, isn't this like putting an ice cube in a drink? you expend minimal work to pick up the ice cube and plop it in the drink, and you get way more cooling than the lifting of the ice cube." Yes, you do, but the ice cube doesn't come from nowhere: you had to cool something into ice to get a cube of it, and you expended more energy cooling it. It's a similar thought experiment, in a way. Somehow, someway, there's energy being expended to get the heat of a human body into a place it wouldn't otherwise go, and it simply must be more energy than the power generated by your inverted heat engine, or thermodynamics falls apart.

[LaCroix]

I just want to know what the effect would be on the person of the amount I say.

There is no direct way to calculate that. You could only approximate by calculating the amount of energy removed, and the resulting core temperature.

Or:

Pick the "virtual" ambient temperature you think people would be exposed to when the suit is active, and use the "water time chart"


Use these symptoms


Use these treatments

[Purple]

Random question. Is there any reason why you would only turn this on during combat and thus drain the user into a frozen ice crystal as opposed to running it on very low power (say a 1 or 0.5 degrees drop in body temperature) all the time and using that to charge batteries? Also, if you have such an efficient heat sink why not turn it outward instead of inward? Turn the system away from the user and toward the environment and just use atmospheric heat to power your suit.

[Zeropoint]

Also, if you have such an efficient heat sink why not turn it outward instead of inward? Turn the system away from the user and toward the environment and just use atmospheric heat to power your suit.

Hmm, that IS the big question that we've all been overlooking.

If we want to make the thermodynamics of the thing a little more believable, we can imagine that the suit has a really long pair of hoses that go to a very large radiator array floating in intergalactic space, providing a heat sink which is neither infinite in capacity nor at zero Kelvin, but has a very large capacity at maybe two Kelvin. Of course, there's another question that we've been overlooking:

What dramatic purpose does this system and its limitations serve? How do the characteristics of this system impact the emotions and choices of the characters? It sounds to me like what you're going for is something like the following: the system has a limited use time, using it puts physical and mental strain on the user, and attempting to use it for too long could incapacitate or kill the user . . . but conveniently, if the hero pulls through via willpower and toughness, they're likely to come out with no lasting harm. As a result, the characters have to carefully conserve their limited operation time, until the climax where they risk their lives by pushing it too far.

Is that anything like what you were thinking? I am not a mind reader, so I've probably got it mostly wrong. Sorry!

However, if that is the general effect you're going for, you might be able to achieve the same result in a less game-breaking way by simply reversing the problem: instead of chilling the user, have the suit get hot when it runs. It wouldn't have the "spooky voodoo alien tech" aspect, but it's both physically realistic and something most readers could relate to.

[Korto]

Terralthra - I accept you're probably correct, however, if I was to use what I'm thinking it will be like, to use an analogy, when you've got two huge bodies of water at different levels, and all that's separating them is a rocky ridge. If you can just drill through that ridge, you can tap the movement of the water for effectively forever. In this case, two separate realities (using a multiple universe theory) are the two bodies of water. Just remember it's science fiction.
Although, it is only pure speculation that that's how it works, and possibly completely wrong. It may be only a myth. Maybe the heat is converted to neutrinos (in which case, it would have been detected in a lab somewhere. They may be keeping it secret); maybe it involves the new field of physics, Speculum physics, that's used in interstellar travel.

LaCroix - Thanks for the chart. I'm copying it. I've decided (assuming I keep going this way) to just assume the whole body weight for working out how much heat I can take, for the main symptoms.

Purple - Human's didn't design this thing, and those who did aren't communicative. There may be reasons it can't be done, or there may be reasons it wont be done. The suits do not produce electricity (although something could probably be set up), they have no storage ability, and affect electronics (when encapsulated by the suit) badly.
There are speculations about the aliens reasons, "Sincere desire to help" gets less votes than "Research Experiment on Lab Rats" and "Playing Silly Buggers"

There may be ways to exploit the abilities of my toys I haven't thought of. That's why I stick them up here for "Beta testing".

Zeropoint - Yeah, largely that. It can do this and this, but you can push it, and even heroically sacrifice yourself.
I will consider heating, but it's approaching midnight and I'm getting tired so that's the most I can say.

Finally I have started a thread in the SF forum, describing more about the suits, and the people that use them. You are all cordially invited over. Just press the Magic Button.