Cooling things: Difference between revisions
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Free cooling means | 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. | |||
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. | |||
That said, if temperature difference are seasonal, you may prefer heat pumps. They take a little more energy, | |||
but they give better guarantees throughout the year -- they just becomes more efficient when it's colder outside. | |||
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* the '''Seebeck effect''' | * the '''Seebeck effect''' | ||
:: a temperature gradient | :: 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 | :: 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, | 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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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 | At worst, they are basically just fans. | ||
At best, these are small [[swamp coolers]], which you can tell by needing water. | |||
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. | |||
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. | ||
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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. | ||
...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. | |||
Also, if you're combining them with an AC, you're making that AC work a little harder - to remove the humidity. | |||
Also, if you're combining them with an AC, you're making | |||
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, | ...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. | |||
It doesn't cool quite as quickly, but that's generally not an issue. | |||
Notes: | |||
* It seems common{{verify}} that there is an added heat exchanger | |||
: 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), | |||
: 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? | |||
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. | |||
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. | |||
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. | |||
Most ACs will stop moving much heat if the cold side is below 15C or so, | |||
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. | |||
The size of condenser and evaporator matters (Would this lead to pressure issues?{{verify}}) | |||
Point is, it will work, but be rather less efficient than a system ''designed'' to do this. | |||
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) | 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 | 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 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. | ||
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=== | ||
Revision as of 13:17, 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.