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, | 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. | 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. | |||
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. | ||
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. | |||
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. | |||
: | If you're trying to heat something it becomes more of basic a resistive heater. | ||
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]] | |||
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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). | |||
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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 | ||
And | 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 | |||
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) | |||
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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 | {{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. | ||
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--> | --> | ||
=On efficiency= | =On efficiency= | ||
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===Windows and windcatchers=== | ===Convection in houses; Windows and windcatchers=== | ||
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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 | 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. | |||
In fact, desert areas see some specific designs ''exactly'' because the day/night temperature difference is larger, | |||
and | and you may have a little more space to work with too. | ||
When opening a window on two sides of the house, combined with a little wind, | |||
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, | |||
which all amount to designing for some draft without active machinery. | |||
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 | |||
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. | |||
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. | |||
In some circumstances you can do even fancier things. | |||
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. | |||
https://en.wikipedia.org/wiki/Windcatcher | |||
--> | --> | ||
==Device cooling== | ==Device cooling== | ||
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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. | ||
They will probably last longer than e.g. water in a PET bottle, though, | |||
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''' | ||
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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In freezers, and in fridges with freezer compartments, | In freezers, and in fridges with freezer compartments, | ||
that means any moisture in the air will | 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 | 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. | 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. | |||
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. | 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. | |||
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. | |||
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. | |||
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''. | |||
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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''' | ||
You may wish to let it warm up completely. | You may wish to let it warm up completely. | ||
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<!-- | <!-- | ||
Yes | 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 | 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 | 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, | 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). | ||
--> | --> |
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.