Cooling things: Difference between revisions

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When current is passed through a thermocouple, it sets up a temperature difference between the two sides - roughly the Peltier effect (see above for more context).
When current is passed through a thermocouple, it sets up a temperature difference between the two sides - roughly the Peltier effect (see above for more context).


Thermoelectric cooling (TEC) elements, a.k.a. Peltier elements, are many such junctions in series, electrically in series, and physically beside each other, for practical reasons.
Thermoelectric cooling (TEC) elements, a.k.a. Peltier elements,
consist of many such junctions electrically wired up in series, and physically beside each other.




Putting a bunch of voltage will attempt to sustain a specific temperature difference, proportional to the current passed (and it's a mostly [[simple load]]).
Putting a bunch of voltage will attempt to sustain a specific temperature difference,
proportional to the current passed (and it's a mostly A [[simple load]]).




Perhaps the most common use is to try to keep the warm side at room temperature (by adding a chonky heatsink and fan), so that the cold side will be some amount of degrees colder than that. {{comment|...mostly because doing it the other way around, trying to keep the cold side at room temperature, would amount to a an unnecessarily complex heater)}}
Perhaps the most common use is to try to keep the warm side at room temperature (by adding a chonky heatsink and fan), so that the cold side will be some amount of degrees colder than that.


...mostly because doing it the other way around, trying to keep the cold side at room temperature, would amount to a... heater.
An unnecessarily complex one.


The main thing TECs have on them is size and being solid-state, while still being a heat pump,
 
TECs are solid-state, while still being a heat pump,
which means they are more portable and/or safer than e.g. gas based fridges, or refrigeration cycle fridges.
which means they are more portable and/or safer than e.g. gas based fridges, or refrigeration cycle fridges.


It's good enough for beer coolers, mini-fridges, because once those are cold or if they start with cold contents, it's mostly just fighting the little heat that comes through the isolation - and not trying to sustain a 30C difference as fridges may have to.
They are barely good enough for beer coolers, mini-fridges,
because once those are cold or if they start with cold contents,  
then it's mostly just fighting the little heat that comes through the isolation.
 
But it will not manage to sustain a 30C difference, as fridges may have to.




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so there are various more efficient cooling systems both in terms of efficiency and capacity.
so there are various more efficient cooling systems both in terms of efficiency and capacity.


In fact, while the amount of heat moved is proportional to I, the resistive heating is proportional to I<sup>2</sup>, which is part of two issues.
In fact, while the amount of heat moved is proportional to I (the current),
the resistive heating is proportional to I<sup>2</sup>, which is part of two issues.
 
 
For one, the higher current you feed in,
the more of that heat you need to move away comes from the element itself,
rather than the thing you are trying to cool.




One, the ability to move heat relies on the ability to move off heat, which at higher current is mostly that resistive heat.  
TECs are probably most energy-efficient at somewhere between between
: a tenth (for small temperature differences)
: and under half (for larger differences)
... of the max current rating, further depending on other wishes and choices.  


This is a good part of why the most energy-efficient is somewhere between a tenth (for small temperature differences) and under half (for larger differences) of the max current rating, further depending on other wishes and choices.


: It lowers at large currents because you probably can't move the heat off fast enough (and it can't exit the element fast enough), so the net effect amounts to more (resistive) ''heating'' than necessary.  
If you're trying to heat something it becomes more of basic a resistive heater.
:: To the point that too small a heatsink can mean [[thermal runaway]] - basically, it needs to be able to sink the heat generated by the resistive part of the whole. You have a sli
If you're trying to cool something, your real problem becomes moving away heat quickly.
: ...to the point that too small a heatsink on the warm side can lead to [[thermal runaway]]


: It lowers at tiny currents because the movement of heat is smaller than the heating effect.{{verify}}




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In fact, it is easily worth it to run two, each at half current, because in almost any case - the effect will be a bit more than double the heat moved, meaning higher efficiency (though slightly slower movement of said heat).


In fact, it is easily worth it to run two, each at half current, because in almost any case - the effect will be a bit more than double the heat moved, meaning higher efficiency (though slightly slower movement of said heat).




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For 15 degrees it may peak around a CoP of 2, at ~4V
For 15 degrees it may peak around a CoP of 2, at ~4V


(this actually depends a lot on models - these things have gotten better over time)
(this actually depends a lot on models - these things have gotten a little better over time)




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: an AC may want a large temperature difference at reasonable CoP.  
: an AC may want a large temperature difference at reasonable CoP.  
:: For CoPs that ACs can do, like 3 or 4 (up to maybe 6, with refrigerants that are being phased because they're nasty{{verify}})
:: For CoPs that ACs can do, like 3 or 4 (up to maybe 6, with refrigerants that are being phased because they're nasty{{verify}})
:: TECs can only do a few degrees. And that's direct surface temperature, not exchanged into the air
:: TECs can only do a few degrees if you want good CoP. And that's direct surface temperature, not exchanged into the air
 
 
A single TEC running at a high CoP implies running it a low power, so you would need dozens to even approach the overall amount of heat transfer of even the smallest ACs. (And you don't have tiny ones both because a tiny refrigeration cycle is harder to make efficient, and because you probably need a few hundred Watts of power - a human puts out ~100W when idle, and so does various equipment, and solar influx may do a bunch more even with nicer windows)


And the heatsinking.


And good heat exchange with the air (not so easy for low temperature differences).


And the separation between inside and outside of the room you cool (it's not as easy to split, like a split AC), which basically implies a lot of wall space.
And an idle human puts out maybe 80 watts, so even keeping up with that, a bit of solar influx, and a laptop,  
will take


So you can do it, you may ''even'' do it at ''vaguely'' comparable CoP, but it's a pain to design. In practice, you can be very happy with a CoP of 2.
If you want TECs running at high CoP, (more at low power) you probably need at least dozens before you even vaguely approach the capacity and efficiency of ACs.


So you can do it, you may ''even'' do it at ''vaguely'' comparable CoP, but it's a pain to design.
In practice, you can be very happy with a CoP of 2.






And no, you don't want to stack them.
And no, you don't want to stack them.
Yes, that makes for higher temperature differences
Yes, that makes for higher temperature differences,
 
but consider that each one has to forcibly transfer move the heat the previous one(s) puts out - they're being forced to do the same work ''again''. (a more technically correct view is a little more involved)
can't stack them - intuitively because each one has to forcibly transfer move the heat the previous one(s) puts out - they're being forced to do the same work ''again''. (a more technically correct view is a little more involved)




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* TEC1-12704 is has 127 couples, 4 Amp
* TEC1-12704 is has 127 couples, 4 Amp
* TEC1-03112 is has 31 couples, 12 Amp
* TEC1-03112 is has 31 couples, 12 Amp
{{comment|(If there's a T''number'' at the end, it' made for a higher-than-usual temperature difference (like 100, 150, or 200 ºC, rather than the usual 60ish ºC, but you probably don't want these)}}
{{comment|(If there's a T''number'' at the end, it' made for a higher-than-usual temperature difference (like 100, 150, or 200 C, rather than the usual 60ish C, but you probably don't want these)}}




A lot will function between approx 2V and 18V (see datasheet). Some shops sell them as 12V, but most are pretty inefficient by that point. Look at the curves for the specific model.
A lot will function between approx 2V and 18V (see datasheet).
Some shops sell them as 12V, but most are pretty inefficient by that point.
Look at the curves for the specific model.


The maximum refrigeration power, in Watts, is generally at maximum power, and again, you probably don't want to use them anywhere near that if you care about efficienct use of electricity.
The maximum refrigeration power, in Watts, is generally at maximum power, and again, you probably don't want to use them anywhere near that if you care about efficienct use of electricity.
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and heat-exchanging somehow at both ends.  
and heat-exchanging somehow at both ends.  


Hydronic systems may also have a buffer tank somewhere
Hydronic systems may also have a buffer tank somewhere.
 
 
 
It's sort of like central heating that uses radiators - but now filled with ''cold'' water.




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=On efficiency=
=On efficiency=
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===Windows and windcatchers===
===Convection in houses; Windows and windcatchers===
<!--
<!--
Houses will concentrate heat, by being closed, without much draft, and being made of materials that absorb more than zero heat. Isolation lessens this, but only so much.


Houses will concentrate heat, by being closed, without much draft, and being made of materials that absorb more than zero heat.
Isolation lessens the absorption, though not so much other concentration.


Which means that by the time it's evening, the walls in general and the air in some room will be warmer than the evening air.
Which means that throughout the day, the walls in general, and the air in some rooms, will be warmer than the outside air.
Particularly when evening air gets colder in the day.


Windcatchers, well, catch wind that wouldn't otherwise have gone through your house,
In fact, desert areas see some specific designs ''exactly'' because the day/night temperature difference is larger,
and put it through your house.
and you may have a little more space to work with too.  




Like windows. But larger effect, and useful when you have enough space, and/or large day/night temperature differences. Which is why you largely see them around desert areas.


It's not hugely efficient, but it's passive so it's free.
When opening a window on two sides of the house, combined with a little wind,
You can lessen local buildup of heat above ambient, but also that you never go below ambient
creates enough draft that you are replacing the air throughout the day,  
then you are moving out the concetrated heat,  
and replacing it with whatever the outside air is.


Ideally, houses don't trap heat like that,
but where they do, this can matter a few degrees for free.






Windcatchers refers to one of two or three things.


Windcatchers refers to one of two or three things,
which all amount to designing for some draft without active machinery.


One is using present wind to create a draft.
Few are hugely efficient, but it's passive so it's free.
You can lessen local buildup of heat above ambient, but also that you never go below ambient




Another is doing this without any wind. You can basically make a chimney that is directly heated by the sun (sun-facing wall), which heats the air inside so that it slowly moves upwards (is never in the home) and that movement draws in air from whatever other ventilation the house has, usually just outside air through windows and such.
One is using already-present wind to create a draft, as just mentioned.




If you can't count on wind, then putting the windows to the north and south helps a little,
because one heats more than the other, and you get some natural convection.


Some are designed as effective evaporative coolers, basically a the tower design that then passes wind over an underground canal. This is evaporative cooling, and it can cool down air to the temperature of that water.
https://en.wikipedia.org/wiki/Windcatcher


You can also make what amounts to a chimney, that is directly heated by the sun (so on a sun-facing wall), which heats the air inside that chimney so that it slowly moves upwards (and is never in the home), and that air movement draws in air from whatever other ventilation the house has, usually just outside air through windows and such.


-->


===Convection in houses===


<!--
In some circumstances you can do even fancier things.


Any kind of building that can trap air is also likely to build up heat in that air.
If you are very lucky and have an underground canal near you,
this is likely to be somewhat colder.  


If you also have the space for large towers, you can catch wind,
and blow them over that canal. This is basically larger-scale evaporative cooling,
and in theory you can can cool down air to the temperature of that water.


Opening a window helps keep the air inside the same temperature as the air outside,
by keeping it moving.


If you can't count on wind, then putting the windows to the north and south helps a little,
https://en.wikipedia.org/wiki/Windcatcher
because one heats more than the other, and you get some natural convection.
 
 
Taller version of this can amplify this effect some, and also give more shade.
See also windcatchers.
-->
-->


==Device cooling==
==Device cooling==
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-->
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===Spray bottle, misting system, etc.===
===Spray bottle, misting system, etc.===
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More efficient, heat-wise? No.
More efficient, heat-wise? No.


Possibly lasts longer than e.g.  water in a PET bottle, though.
They will probably last longer than e.g.  water in a PET bottle, though,
E.g. the glycol lessens the expansion.{{verify}}
because glycol lessens the expansion{{verify}} and thereby the willingness of water to break whatever it is in, particularly in larger containers.




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'''One-time instant ice packs'''
'''One-time instant ice packs'''


Goes cold when squeezed, E.g. used in first aid kits, sports, particularly where keeping ice is not practical.
The things that go cold when squeezed,  
e.g. used in first aid kits, sports, particularly where keeping ice is not practical.


Often contains
Often contains
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It's not advisable to use them as ice packs afterwards, due to the toxicity{{verify}}
It's not advisable to use them as ice packs afterwards, due to the toxicity{{verify}}




https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p081/chemistry/how-do-cold-packs-work
https://www.sciencebuddies.org/science-fair-projects/project-ideas/Chem_p081/chemistry/how-do-cold-packs-work




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'''What is the first-aid purpose of those ice packs?'''
'''What is the first-aid purpose of ice packs?'''


This restricts blood circulation, which can numb pain, and limit the amount of bruising.  
This restricts blood circulation, which can numb pain, and limit the amount of bruising.  


-->
-->
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"What is a BTU?"
"What is a BTU?"


A British Thermal Unit (BTU, Btu) was is defined as the heat required to heat a specific mass of water by 1 degree Farenheit (there is an analogous definition for Joules), though there are varying definitions, though they differ less than 0.5%.  
A British Thermal Unit (BTU, Btu) was is defined as the heat required to heat a specific mass of water by 1 degree Farenheit (there is an analogous definition for Joules).
 
Though there are varying definitions, they differ less than 0.5%.


In terms of SI, a BTU equals about 1055 Joules
In terms of SI, a BTU equals about 1055 Joules
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BTU is useful as a measure of capacity.  
BTU is useful as a measure of how much heat we can consistently move.  
For example, in an AC, it works out as an indication of how well it will keep a specific volume of air down.
For example, in an AC, it works out as an indication of how well it will keep a specific volume of air down.


It's imperfect at that (because it's also related to the amount of temperature difference, how well insulated something is, how well ventilated)
It's imperfect at that (because that's also related to the amount of temperature difference, how well insulated something is, how well ventilated), yet it's still great for estimation - you might need 5000 BTU for a small room, 10000 for a larger room, and 50000 for a small house.
 
But it's still useful for estimation - you might need 5000 BTU for a small room, 10000 for a larger room, and 50000 for a small house.


A window AC may be 10000 BTU, a small portable variant may be half that.
A window AC may be 10000 BTU, a small portable variant may be half that.
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It looks like it's just convention.
It looks like it's just convention.
People aren't used to thinking in Joules.


-->
-->


===COP, EER===
===COP, EER===
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===Super!===
===Super!===
<!--
<!--
Terms like '''SuperFrost''' amount to temporarily cooling more.  
 
Terms like '''SuperFrost''' and '''supercool''' amount to temporarily cooling more.  
 
More than would normally be required for a stable temperature.
More than would normally be required for a stable temperature.




Why? Well, for context, freezers are ''slow''. Once they're cold, they don't have to work hard at all,
Why? Well, for context, freezers are ''slow''.
because they're insulated well.   Mostly they're optimized to be efficient at cooling fairly slowly.  
 
Once they're cold, they don't have to work hard at all,
because they're insulated well.
 
And since they will spend most of their operation working lightly,
they are optimized to be efficient at cooling fairly slowly.  
 


Yet if you put in a ''lot'' of new groceries,
the average temperature will spend a few hours warmer than the target.


Yet if you put in a ''lot'' of new groceries, the average temperature will spend a few hours warmer than the target.


With Superfrost ('''Supercool''' seems to be the same idea but for fridges rather than freezers, but these terms are sometimes brand specific so eh), you make it cool harder for maybe a few hours.


With Superfrost ('''Supercool''' seems to be the same idea but for fridges rather than freezers, but these terms are sometimes brand specific so eh),  
...often still without measuring temperature much, so if you do this without a reason,
you make it cool harder for a while.  
you might get it to be temporarily maybe 10 degrees colder than usual.  


Still without measuring temperature much, so if you do this without a reason, you might get it to be maybe 10 degrees colder than usual for a while.


So should you do this before you put the groceries in, or after?
So should you do this before you put the groceries in, or after?
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though also that some things may freeze more than you want.
though also that some things may freeze more than you want.


At the same time or after means less time of other things being warmer,
At the same time or after means less time of other things being warmer.


It doesn't really matter,
unless one of your foods is very fragile in some sense - something that really should stay warm too long (suggesting before), or that really shouldn't freeze harder (suggesting after).






In a freezer the same may apply, but is usually less important because a freezer is often -18C  
In a freezer the same may apply, but is usually less important because a freezer is often -18C to start,
making it unlikely the temperature the rise will be enough for anything to melt.
making it unlikely the temperature the rise will be enough for anything to melt.


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Most freezers switch back to regular cooling automatically.
Most freezers switch back to regular cooling automatically.


-->
-->


===On frost===
===On frost===
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In freezers, and in fridges with freezer compartments,
In freezers, and in fridges with freezer compartments,
that means any moisture in the air will, over time, condense and freeze on those noticeably-cooler elements.  
that means any moisture in the air will condense and freeze on those noticeably-cooler elements.  
That is what [https://en.wikipedia.org/wiki/Frost frost] ''is''.
That is what [https://en.wikipedia.org/wiki/Frost frost] ''is''.




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Fridges (without freezers) aim for ''just'' above freezing point (4° C, 40° F), but the ''evaporator'' (the coldest part inside, because it's the thing that draws out heat) is often still a little colder.
Fridges (without freezers) aim for ''just'' above freezing point (4° C, 40° F), but the ''evaporator'' (the coldest part inside, because it's the thing that draws out heat) is often still a little colder.


Bottom line, when there is ''anything'' below freezing, and you add air, you add moisture that will eventually become frost ''somewhere''.
Bottom line, when there is ''anything'' below freezing, and you add air, you add moisture that will eventually become frost ''somewhere''.


And over time, there will be a lot.
Frost buildup is both inconvenient and makes the fridge less efficient,
so there is a slew of names (''Self-defrosting''', '''auto-defrost''', ''NoFrost'', '''Low Frost''', '''Smart Frost''', and more)
that refer to designs that try to manage this somehow.
Some designs may try to take moisture out.
But that only postpones the problem.


Frost build up is both inconvenient and makes the fridge less efficient,
so there is a slew of names (''Self-defrosting''', '''auto-defrost''', ''NoFrost'', '''Low Frost''', '''Smart Frost''', and more) that refer to designs that try to manage this somehow.


This usually means ensuring frost happens in one main place, where we can deal with it.
It more usually means ensuring frost happens in one main place, where we can deal with it more easily.




Say, '''Self-defrosting''' basically means there is a heater that fairly directly warms up the evaporator every now and then. Say, every day, for twenty minutes.
Say, '''Self-defrosting''' basically means there is a heater that fairly directly warms up the evaporator every now and then.  
Say, every day, for twenty minutes.




Yes, this means the temperature inside the fridge would fluctuates a little.
Yes, this means the temperature inside the fridge would fluctuates a little.


But it means the evaporator should never be overwhelmed with ice,
But it also means the evaporator should never be overwhelmed with ice,
so you never have to deal with defrosting it, or having it defrost at unpredictable times.
so you never have to deal with defrosting it, or having it defrost at unpredictable times.


 
Without this, the evaporator would slowly be insulated and become less and less effective.
 
Yes, self-defrosting takes more energy than not doing so in the short run -- though a freezer that is 'never' defrosted becomes less efficient, and oveer enough time this tips over to become less efficient (scale of a year or so).
 
Exactly where such a feature lies between convenient no-brainer and most efficient possible is hard to know.






Yes, self-defrosting takes more energy than not doing so -- in the short run.


Other designs may try to take moisture out - often in the same basic way, but not necessarily on the main evaporaor{{verify}}.
Again, enough ice on the evaporator would make it inefficient over time.
After a year or so (order of magnitude), chances are it's doing badly.




Exactly where such defrosting a feature lies between 'convenient no-brainer' and 'where is the most efficient choice possible' is hard to know.


fridges and freezers, a.k.a. ;;;"auto-defrost''', have a heater on the evaporator that is occaionally turned on for a short while, to ensure the evaporator never builds up frost.


Because this may happen frequently (sometimes once a day), and because you want the evaporator to be dry before freezing again (meaning this may take ten minutes), this take a little more energy to operate.  
Because this may happen frequently (sometimes once a day), and because you want the evaporator to be dry before freezing again (meaning this may take ten minutes), this take a little more energy to operate.  
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An even cheaper solution may be that fridges just ''just switch off for a while''.  
 
Independent of other features with "frost" in the name, some fridges will sometimes decide to defrost in the sense of ''just switching off for a while''.
 
 
 




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Low Frost sometimes refers to a specific variant,
Low Frost sometimes refers to a specific variant,
and sometimes to the fact that No Frost is usually a lie.
and sometimes to the fact that No Frost is usually a lie.




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'''Manual defrost'''
'''Manual defrost'''


'''Defrosting'''
 


You may wish to let it warm up completely.  
You may wish to let it warm up completely.  
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<!--
<!--
Yes, but modern freezers tend to have a feature that regularly defrosts the cooling element with a heater.  
Yes.
 
But modern freezers tend to have a feature that regularly defrosts the cooling element with a heater - see the previous section.


This means that there may be ice everywhere, it won't affect cooling much until it clogs up airflow.
This may means that there may be ice in the compartment,  
it may not affect cooling much until it starts clogging up airflow to where the actual cooler is.




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====In defrosting a fridge, where does the water go?====
====In defrosting a fridge, where does the water go?====
<!--
<!--
The same place that the small but fairly continually generated condensation water goes: out the hole in the bottom, onto a pan over the compressor, which due to being slightly warm will be slowly evaporated.
The same place that the small but fairly continually generated condensation water goes: out the hole in the bottom, and classically onto a pan over the compressor, which due to being slightly warm will be evaporated somewhat soon.


In theory, extreme amounts of frost can make that overflow. Due to positioning this should nor be an electrical risk, but during manual defrosts you may still care to catch most of the water (use towels), to not have a pool of water everywhere, including there where you can't see it.
In theory, defrosting large amounts of frost can make that overflow.
Due to positioning this should nor be an electrical risk,
but during manual defrosts you may still care to catch water (use towels),  
to not have a pool of water everywhere, including there where you can't see it.


Note that this hole is small, and sometimes clogs with food. Declog is with anything pokey (there's nothing much you can damage here), though there often is a plastic doohickey for it (that you may have thrown away not knowing what it is).
Note that this hole is small, and sometimes clogs with food. Declog is with anything pokey (there's nothing much you can damage here), though there often is a plastic doohickey for it (that you may have thrown away not knowing what it is).
-->
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Latest revision as of 15:04, 30 June 2024

Physical mechanics of cooling

Passive cooling

Passive cooling tends to mean 'what happens with no moving parts'.

...so whatever amount of conduction, radiation, and/or convection would happen anyway.


Sometimes includes adding a fan, to add to the convection.

You're stirring the air better than just convection would, so heat transfer goes a faster than if warm air just sits around - but the difference is rarely much -- convection always does this at least a little when there is temperature difference (if you're in gravity; this is about density differences).

And you could argue that's technically active cooling (because you're adding work, so using energy), but intuitively it feels like it hardly qualifies.



On the technical side

This tends to mean

  • conduction - a good conductor spreading heat throughout
if any cooling happens, conduction's spreading brings the whole down
  • radiation - thermal radiation means movement of charges in materials (anything above 0 K) is radiated as EM at the surface
(black-body radiation can be seen as a "thermal radiation's real-world math becomes easier if we make some assumptions like that it's not really interacting in other ways")
  • convection - fluid flow, in this context often
air,
flow caused by heat changing temperatures and densities
that flow assisting better heat interchange with that fluid, because warmer air moving up tends to draws in colder air from the sides (which technically is an effect that needs gravity)

In practice there's more than one of these happening, but often one that counts for most exchange.





Evaporative cooling

Refrigeration cycle

Refrigerants

Heat pumps

Free cooling

Dessicant based systems

Peltier effect and related effects

Theory

Peltier device behaviour and uses

Transport

On efficiency

Using the effects - practice and products

When passive cooling works

Convection in houses; Windows and windcatchers

Device cooling

Passive device cooling

Heatsinks

Heat pipes and vapor chambers

Personal cooling

Fans / Why wind helps even when you're not sweating

Spray bottle, misting system, etc.

Some small tricks

Wrapped towel

Ice

Swamp coolers

Mini dehumidifier

"Personal ACs"

Air conditioning

One-hose and two-hose portable air conditioners

Car AC

Is a flipped AC a heater?

"Eco coolers"

Passive effects

Windows

Parts of designs

Heat exchanger

Unsorted

Cold and hot drinks

Ice packs

BTU

COP, EER

Adding some more considerations: SCOP, EER2, SEER, SEER2, CEER
Comparison?

More on fridges and freezers

Super!

On frost

Does a lot of ice make a fridge/freezer less efficient?

My freezer causes freezer burn on food, what do I do?

In defrosting a fridge, where does the water go?

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