Cooling things: Difference between revisions

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Free cooling means "use the fact that it is cold outside to cool inside".  
Free cooling means you do not have to create a temperature difference, you just have to use it.


 
Usually, use the fact that it is cold outside to cool inside.
For example, in cold climates, you can cool industrial processes or datacenters just by ''moving'' water or air around.
Say, in cold climates, you can cool industrial processes or datacenters just by ''moving'' water or air around.




Yes, you could open the window, but more controlled in terms of humidity but also the actual temperature,
Yes, you could open the window, but more controlled in terms of humidity but also the actual temperature,
so this often amounts to heat exchangers to cool an otherwise closed system.
so this often amounts to heat exchangers to cool an otherwise closed system.  




Which still takes some energy, but far less than what amounts to ACs.


If temperature difference are seasonal, you may prefer heat pumps - putting in a little more energy
That said, if temperature difference are seasonal, you may prefer heat pumps. They take a little more energy,
for a lot more over-the-year guarantees, so just mean part of the system becomes more efficient in some seasons.
but they give better guarantees throughout the year -- they just becomes more efficient when it's colder outside.
 
This is no longer ''free'' cooling, but still ery useful.




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* the '''Seebeck effect'''
* the '''Seebeck effect'''
:: a temperature gradient leading to an electric potential
:: designs where a temperature gradient leads to an electric potential
:: e.g. used in thermocouples, often to measure temperature
:: e.g. used in thermocouples, often to measure temperature


* the '''Peltier effect'''
* the '''Peltier effect'''
:: an electric potential leading to a temperature gradient at the junction of two dissimilar metals.
:: designs where an electric potential leads to a temperature gradient (at the junction of two dissimilar metals)
:: e.g. used in Peltier elements
:: e.g. used in Peltier elements




You can see Seebeck and Peltier as basically the same thing in reverse, complemetary.
You can see Seebeck and Peltier as basically the same thing in reverse, so complementary effects.


In terms of things you buy, specific things are optimized for different use of one effect.
In terms of things you buy, specific things are optimized for different use of one effect.
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A peltier element you can buy is a whole bunch of individual peltier-effect junctions in series.
A peltier element that you can buy is a whole bunch of individual peltier-effect junctions in series.


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As in, those boxes no larger than the size of a computer fan,  
For some years there was a fad of selling boxes no larger than the size of a computer fan,  
sitting on your desk?
sitting on your desk.




At best, these are little swamp coolers.  (there ''are'' larger variants of these. You can tell by the fact they need water - and do not have a hose)
At worst, they are basically just fans.


The smaller the volume it's cooling, the better,  
At best, these are small [[swamp coolers]], which you can tell by needing water.
because "personal" effectively means "by itself it probably won't even keep up with your body's natural heat generation".


So no, they will ''not'' cool a room.
At their small size, they probably won't even keep up with your body's natural heat generation,
so they will ''not'' cool a room.


But they may work slightly better than just a fan pointed at you.
Yet they ''may'' work slightly better than just a fan pointed at you.


How they compare to a spray bottle, eh.
How they compare to a spray bottle, eh.
At worst, they are basically just fans.




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because that's going to have a larger effect than the evaporative cooling.  
because that's going to have a larger effect than the evaporative cooling.  


Basically a tiny heat pump between fridge and room, with some required human steps in the middle.
...at which point it is a manually operated heat pump between fridge and your room.


You're just using your fridge to cool you,
You're just using your fridge to cool you,
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And, as with any swamp cooler, it increases the room's humidity, decreasing its own effectiveness and that of your own sweating.


And as it increases the humidity increases, its effectiveness decreases.
Also, if you're combining them with an AC, you're making that AC work a little harder - to remove the humidity.
Potentially to zero, except you probably have ''some'' sort of draft.
 
Also, if you're combining them with an AC, you're making it work a little harder - to remove the humidity.
But only slightly, given how small most of these things are.
But only slightly, given how small most of these things are.


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{{stub}}
{{stub}}


Car ACs are refrigeration cycle designs.
Car ACs are [[refrigeration cycle]] designs.


The layout will be different, and e.g. the compressor is driven by the engine via a belt,  
The layout will be different, and e.g. the compressor is probably driven by the engine via a belt,  
and the receiver/dryer is sort of new, ...but overall it's the same high and low side,  
and the receiver/dryer is sort of new,
expansion valve, put one radiator outside, blow air over the other inside.
...but overall it's the same:
a high and low pressure side,
an expansion valve,
one radiator outside,
circulate air inside the car over the inside radiator.


(the receiver/dryer seems to a receiver in that it's an expansion tank for refigerant, and a dryer of moisture that got in during service, protecting the expansion valve {{verify}})
It doesn't cool quite as quickly, but that's generally not an issue.




It's common{{verify}} that there is a heat exchanger in the middle
Notes:
so that only a smallish part of the system has refrigerant and the rest of the system
* It seems common{{verify}} that there is an added heat exchanger
is circulating cold water, letting you cool more things using basic components (no need to make them impossible to leak refigerant), and more user serviceability (you can refill the radiator yourself).
: so that only a smallish part of the system has refrigerant and the rest of the system is circulating cold water,
 
: letting you cool more things using simpler components (no need to make them more robust to avoid leaking refigerant),
It doesn't cool quite as quickly, but that's generally not an issue.
: and more user serviceability (you can refill the radiator yourself).
 


* (the receiver/dryer seems to a receiver in that it's an expansion tank for refigerant, and a dryer of moisture that got in during service, protecting the expansion valve {{verify}})


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Since we're making one side colder and the other side warmer, can you flip it around?


In various countries, ACs are designed that way.
It will do what you think, yes, but there are reasons it not be will be efficient,
 
or or isn't the safest thing to do.
 
And yes, this setup is more efficient than a basic resisive heater.
 
But not as efficient as a variation of this that is ''designed'' to do this.
 


When they are not, you could still mount them in reverse, but
It is not guaranteed that its CoP is still >1 like this - but if it is, then this setup is more efficient than a basic resisive heater.  
* it's not going to be nearly as efficient (if its CoP is still >1 like this, then it is still better than your basic resistive heater).
* there may be reasons it will not work like that, or isn't the safest thing to do.


For various reasons.


Function: Most ACs them will stop moving heat if the cold side is below 15C or so.
Little details like that ACs deal with cold-side condensation by vaporizing it.  That doesn't make much sense when that's on the outside.


Efficiency: An 'hot outside, cool inside' AC is generally not going to push for more than 15 C of difference.  
Most ACs will stop moving much heat if the cold side is below 15C or so,
" When it's freezing outside, you need more than that.
and an 'hot outside, cool inside' AC is generally not going to push for more than 15 C of difference.  
When it's freezing outside, you need more temperature difference than that.


Efficiency/safety: This also carries into the size of the condenser and evaporator.
The size of condenser and evaporator matters (Would this lead to pressure issues?{{verify}})
: Would this lead to pressure issues?


Efficiency: Various designs deal with cold-side condensation by vaporizing it.  That doesn't make much sense when that's outside.


Point is, it will work, but be rather less efficient than a system ''designed'' to do this.


These and other potential issues are designed out of heat pump systems.
Which we tend to call a heat pump.
 


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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.


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===COP, EER===
===COP, EER===
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===Super!===
===Super!===
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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.


Yet if you put in a ''lot'' of new groceries, the average temperature will spend a few hours warmer than the target.
And since they will spend most of their operation working lightly,  
they are optimized to be efficient at cooling fairly slowly.  




With Superfrost ('''Supercool''' seems to be the same idea but for fridges rather than freezers, but these terms are sometimes brand specific so eh),  
Yet if you put in a ''lot'' of new groceries,
you make it cool harder for a while.  
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.
 
...often still without measuring temperature much, so if you do this without a reason,
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.


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===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 build up is both inconvenient and makes the fridge less efficient,  
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.


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


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.


Other designs may try to take moisture out - often in the same basic way, but not necessarily on the main evaporaor{{verify}}.
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.




Revision as of 13:30, 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

Windows and windcatchers

Convection in houses

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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