Why is it hard to get things cold?

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Why is it hard to get things cold?

Postby tomandlu » Fri Aug 22, 2014 9:58 am UTC

Fridges, air-con, etc. all require energy to work. Largely, afaik, to circulate the fluid responsible for conducting energy away, and for running a fan.

Okay, fine, but then I started thinking, well, we're basically getting rid of energy here (heat) - how come we need an external energy source to run this. Why isn't it possible to make a fridge or an air-con unit self-powering? Is there some fundamental reason, or are we just not that good at engineering yet?
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Re: Why is it hard to get things cold?

Postby eSOANEM » Fri Aug 22, 2014 11:08 am UTC

At thermodynamic scales (which the human-scale world is), things don't like being at different temperatures.

Obviously, energy is conserved, so if we cool the inside of the fridge down, we need to heat something else up with that energy except, of course, heat flows from a hotter to a cooler and wants to undo all that usefulness we did cooling the fridge down.

In order to get a stable temperature difference you need to constantly put work in fighting the fact that entropy wants the inside of the fridge and the room outside to be at the same temperature.

The energy used in fridges and air-cons is mostly to compress the fluid at various points in its cycle. This is the energy we have to spend because of thermodynamics (and Carnot's theorem in particular); all the rest can be reduced by better engineering.
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Re: Why is it hard to get things cold?

Postby tomandlu » Fri Aug 22, 2014 12:27 pm UTC

eSOANEM wrote:At thermodynamic scales (which the human-scale world is), things don't like being at different temperatures.

Obviously, energy is conserved, so if we cool the inside of the fridge down, we need to heat something else up with that energy except, of course, heat flows from a hotter to a cooler and wants to undo all that usefulness we did cooling the fridge down.

In order to get a stable temperature difference you need to constantly put work in fighting the fact that entropy wants the inside of the fridge and the room outside to be at the same temperature.

The energy used in fridges and air-cons is mostly to compress the fluid at various points in its cycle. This is the energy we have to spend because of thermodynamics (and Carnot's theorem in particular); all the rest can be reduced by better engineering.


Yeah, I think I got that part - the 'enemy' is entropy - but at the same time, if you want something reduced from a more energetic state (warm) to a less energetic one (cold), you'd think there would be some way to exploit that. I suppose another way of putting it is, "why isn't the heat put out by an air-con unit being used to run a small steam engine?"
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Re: Why is it hard to get things cold?

Postby jewish_scientist » Fri Aug 22, 2014 1:31 pm UTC

I think I understand what tomandlu is getting at.

A heat engine converts a hot reservoir into a cold reservoir and work.

What would happen if you made a heat engine where the hot reservoir and the cold reservoir are both the air inside the same box? I could be wrong, but I think that a real heat engine would produce work until the air became to cold for it to function. An invincible, but not 100% efficient, heat engine would just keep cooling the air; producing less and less work each cycle.

If for some reason that does not work, then what would happen if you made a heat engine where the air outside a box is the hot reservoir and the air inside that box the cold reservoir? I believe that the temperature of the air inside the box would decrease until it was at thermal equilibrium with the exhaust (assuming that you do not allow pressure to build inside the box).

The purpose of a refrigerator is to turn hot air into cold air, which a heat engine can do. The fact that it also produces work is a bonus.
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Re: Why is it hard to get things cold?

Postby Sizik » Fri Aug 22, 2014 1:40 pm UTC

jewish_scientist wrote:A heat engine converts a hot reservoir into a cold reservoir and work.


You'e got that backwards. A heat engine converts a hot reservoir and a cold reservoir into two warm reservoirs and work. It extracts energy from the difference in the temperatures, moving them closer to each other until they're equal. If you want to increase the the difference in temperatures, you have to input energy, which is what AC/refridgerators do.

jewish_scientist wrote:What would happen if you made a heat engine where the hot reservoir and the cold reservoir are both the air inside the same box?


That's called a Stirling engine.
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Re: Why is it hard to get things cold?

Postby Mokele » Fri Aug 22, 2014 1:48 pm UTC

I think the difference between why it's easy to heat something but hard to cool it is because it's apples and oranges, technologically. We can make a heat engine that uses work to make a thermal difference between two reservoirs, sure, and you can have your "target area" to be heated or cooled on either side just as easily. But if you want to heat something, you can skip the complicated heat-engine crap and just shove a big old resistor across a voltage gradient, resulting in a much more compact and user-friendly system (possibly because the electricity generating stuff is outside the "box" of the house, so you're tapping into an energy flow that would otherwise just go on to the next house).

This is actually why I don't have fire salamanders. I can set up a 150F basking spot for a desert monitor lizard no problem, just plug in a giant ass spotlight bulb (or two or three). But to maintain a salamander tank at 55F in an 80F house, I need refridgeration systems or AC or whatever, which is way bulkier and more expensive.
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Re: Why is it hard to get things cold?

Postby jewish_scientist » Fri Aug 22, 2014 3:03 pm UTC

Hyperphysics says that a heat engine's input is a hot reservoir and the output is work and a cold reservoir. Is the diagram at the bottom of this page an oversimplified?
http://hyperphysics.phy-astr.gsu.edu/hb ... ng.html#c1

Although the Stirling Engines are really cool, I was thinking of something that did not have a heating element. My engine would take in air, produce work and exhaust colder air. Then the cold air goes into the engine as an input. The engine takes the cold air, produces work and exhausts even colder air. It would just continue in this cycle. However, as the air got colder, there would be less energy to extract from it. Each cycle would produce less and less work. At some point a real engine would stop working and an invincible engine would produce ~0 work.
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Re: Why is it hard to get things cold?

Postby thoughtfully » Fri Aug 22, 2014 3:34 pm UTC

Part of the problem seems to be the distinction between making a hot reservior by toppling over a big pile (generally chemical) of potential energy, versus the lack of convenient access to cold reservoirs. But this is just a recapitulation of thermodynamics. Systems at a high potential tend to get rid of the excess potential as soon as it's feasible to do so. But systems at low potential tend to stay there.
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Re: Why is it hard to get things cold?

Postby firechicago » Fri Aug 22, 2014 5:01 pm UTC

jewish_scientist wrote:Hyperphysics says that a heat engine's input is a hot reservoir and the output is work and a cold reservoir. Is the diagram at the bottom of this page an oversimplified?
http://hyperphysics.phy-astr.gsu.edu/hb ... ng.html#c1

It doesn't output a cold reservoir, it takes heat from a hot reservoir and outputs it into a cold reservoir.

The fundamental insight here is that heat flows from hot to cold, and if you want to move heat from a cold system to a warmer system, you need to do work. You can't extract net energy by moving heat out of a cold system into a warmer one any more than you can extract net energy by moving water from a low reservoir into a higher one.

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Re: Why is it hard to get things cold?

Postby Sizik » Fri Aug 22, 2014 5:07 pm UTC

jewish_scientist wrote:Hyperphysics says that a heat engine's input is a hot reservoir and the output is work and a cold reservoir. Is the diagram at the bottom of this page an oversimplified?
http://hyperphysics.phy-astr.gsu.edu/hb ... ng.html#c1


The engine takes energy from a hot reservoir and uses part of it to do work, but is constrained by the second law of thermodynamics to exhaust part of the energy to a cold reservoir.
Image


The cold reservoir is where the heat from the hot reservoir is transferred to. It isn't created or cooled down by the heat engine.

Edit: Those arrows aren't just a black-boxing of the heat engine, they represent actual energy flow. Notice how it's pointing out of the hot, and into the cold.
Also, it's not explicitly stated but QH = W + QC.
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Re: Why is it hard to get things cold?

Postby jewish_scientist » Fri Aug 22, 2014 11:00 pm UTC

O.k., I think I know where I made my mistake.

This is what I thought. The hot and cold reservoirs contained hot and cold gasses. The heat engine takes some gas from the hot reservoir. The amount of energy in this gas is Qh. The heat engine produces work. the amount of work done is W. The gas is then sent into the cold reservoir. The amount of energy in the cold gas is Qc. The net result is that there is less hot gas in the hot reservoir, more cold gas in the cold reservoir and some work was done.

This is what the diagram really means. Energy is taken from the hot reservoir, cooling it. The amount of energy taken is Qh. Some of this energy is converted to work. The amount of work done is W. The rest of the energy is sent to the cold reservoir, heating it. The amount of energy sent is Qc. The net result is that the hot and cold reservoirs are closer to the same temperature and some work was done.
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Re: Why is it hard to get things cold?

Postby lgw » Sat Aug 23, 2014 1:04 am UTC

Mokele wrote:I think the difference between why it's easy to heat something but hard to cool it is because it's apples and oranges, technologically. We can make a heat engine that uses work to make a thermal difference between two reservoirs, sure, and you can have your "target area" to be heated or cooled on either side just as easily. But if you want to heat something, you can skip the complicated heat-engine crap and just shove a big old resistor across a voltage gradient, resulting in a much more compact and user-friendly system (possibly because the electricity generating stuff is outside the "box" of the house, so you're tapping into an energy flow that would otherwise just go on to the next house).

This is actually why I don't have fire salamanders. I can set up a 150F basking spot for a desert monitor lizard no problem, just plug in a giant ass spotlight bulb (or two or three). But to maintain a salamander tank at 55F in an 80F house, I need refridgeration systems or AC or whatever, which is way bulkier and more expensive.


Resistive heating is far less efficient than a heat pump for your utility dollar. It seems like resistive heating would have to be 100% efficient, doesn't it? But that's only true if the heater is 100% resistive (which would, in fact, be cheap to operate). In order to do any work in heating, you have to send lots of voltage back to the power company. Also, I'd be just as happy not knowing what a giant ass-spotlight bulb is.
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Re: Why is it hard to get things cold?

Postby Goemon » Sat Aug 23, 2014 4:33 am UTC

Here's a simple method for converting heat to work:

Say you have a sealed piston full of air, and it requires 300 joules of work for you to fully compress the piston. Once it's compressed, you could get some work out of the piston by making it push on something as it expands when you release it, but the max work you could get out is 300 joules. The piston is just storing energy, like a spring.

Now suppose you shove the piston into a 200C oven (approx 500 kelvin vs 300 kelvin at room temp). When the air inside the piston is hot, it takes something like 500 joules to compress, and you could likewise extract 500 joules from it as it expands.

So here's the strategy for getting work out of heat: (1) expend 300 J to compress the piston while it's at room temp (2) stick in the oven and let it heat up (3) use the piston to do work by letting it expand, extracting 500 J worth of energy (4) take the piston out of the oven and let it cool back down to room temp; REPEAT

If you have a 500 kelvin hot reservoir and a 300 kelvin cold reservoir, then for each 500 J of heat energy extracted from the hot side, you can get a net work output of 200 Joules, and the other 300 J ends up as waste heat transferred to the cold side. Turns out this is a general thermodynamic law, and you can't do any better than this no matter what kind of process you use.

If your hot reservoir is 300K and your cold reservoir is 300K, (e.g. both ends of your machine are connected to the inside of the same box) then you're just compressing and releasing the piston at the same temp, and your net work output is zero.
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Re: Why is it hard to get things cold?

Postby eSOANEM » Sun Aug 24, 2014 12:28 am UTC

lgw wrote:
Mokele wrote:I think the difference between why it's easy to heat something but hard to cool it is because it's apples and oranges, technologically. We can make a heat engine that uses work to make a thermal difference between two reservoirs, sure, and you can have your "target area" to be heated or cooled on either side just as easily. But if you want to heat something, you can skip the complicated heat-engine crap and just shove a big old resistor across a voltage gradient, resulting in a much more compact and user-friendly system (possibly because the electricity generating stuff is outside the "box" of the house, so you're tapping into an energy flow that would otherwise just go on to the next house).

This is actually why I don't have fire salamanders. I can set up a 150F basking spot for a desert monitor lizard no problem, just plug in a giant ass spotlight bulb (or two or three). But to maintain a salamander tank at 55F in an 80F house, I need refridgeration systems or AC or whatever, which is way bulkier and more expensive.


Resistive heating is far less efficient than a heat pump for your utility dollar. It seems like resistive heating would have to be 100% efficient, doesn't it? But that's only true if the heater is 100% resistive (which would, in fact, be cheap to operate). In order to do any work in heating, you have to send lots of voltage back to the power company. Also, I'd be just as happy not knowing what a giant ass-spotlight bulb is.


The main thing is that heat pumps have an efficiency greater than 1 (although it tends towards 1 as the hot reservoir gets hot) in that the heat going into the hot reservoir is more than the work done (because it also draws in heat from the cold reservoir).

Using radiators not heat pumps for heating houses is really silly and, hopefully, we'll eventually realise this and switch.
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Re: Why is it hard to get things cold?

Postby billy joule » Mon Aug 25, 2014 2:46 am UTC

eSOANEM wrote:
Using radiators not heat pumps for heating houses is really silly and, hopefully, we'll eventually realise this and switch.


Well that depends where the heat for the radiators comes from.
Using gas heating directly is better than using a heat pump powered by a gas power plant.

Using waste heat from a local power plant or CHP is even better:

http://en.wikipedia.org/wiki/Micro_comb ... _and_power

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Re: Why is it hard to get things cold?

Postby Qaanol » Mon Aug 25, 2014 3:02 pm UTC

It’s not always hard to get things cold—some guy named Daniel just mixed ammonium chloride with ice-water and it got pretty cold.

If you’ve ever made ice cream the old-fashion way, that uses the same trick but with salt.
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