Submitted by samskiter t3_zsiyxn in askscience
seven_tech t1_j18cb0w wrote
Because phase changes using compressor/condensor/evaporator refrigerator systems are, in the real world (non-ideal), very efficient ways of transferring huge amounts of heat from one place to another, for low amounts of work. Phase changing liquid to gas enables it to absorb large amounts of heat, that's pumped out from the heat exchanger. It then fully evaporates to gas, expelling some heat, before being compressed and condensed to pure liquid and the heat of this change also dumped out by a heat exchanger and fan and the cycle starts again. Liquids transfer heat better in the heat exchangers than gases due to molecular density and surface area effects.
Also we've spent the better part of 150 years making heat pumps on the premise of electric motors running compressors for changing phases of gas and liquid, making those motors extremely efficient. We can input up to 3 times less electric energy for the same transfer of 'heat' energy in a very efficient heat pump.
TL;DR- Phase changes (liquid-gas-liquid) in the real world, with compression and evaporation, is much more efficient in work input terms, than using just gas.
incredibolox t1_j18gg7f wrote
Ah right so the latent heat of fusion or freezing isn't lost because both are used to cancel each other out and deliver more energy exchange
exbm t1_j18uzsj wrote
The way this was explained in HVAC/refrigeration school was a typical refrigerant cycle deals with two phases of matter liquid and gas. When a material changes phase from liquid to gas it requires extra energy to complete the change. The molecule will absorb that energy from the surrounding area. This is called latent heat (heat is energy). This lack of energy makes everything cold. Because cold is really the absence of energy.
The reason you compress the gas on the high side is because in a gas temperature and pressure correspond. Increasing the pressure of the gas increases it's temperature. By increasing the temperature to higher than ambient air you allow the latent heat to to flow into the ambient air. Heat/energy flows from hot to cold. Like water flows downhill. Once this latent heat has been absorbed into the ambient air the gas will phase change back to a liquid. Now you can slowly let the liquid back in the lowside of the cycle for it to be evaporated.
Source: AS in environmental control technology
zebediah49 t1_j18x0wm wrote
Note that this only works because of vapor pressure differences.
It is necessary that the compressor lower the pressure on the cold side such that its boiling point is below the cold reservoir temperature, and raise the pressure on the hot side such that that boiling point is above the hot reservoir temperature.
St1cks t1_j190ivz wrote
The compressor doesn't cause the lower pressure side on its own. We use a type of metering device to achieve a flash off within the suction line. Ideally a 25/75 mix of liquid and gas should be present at the start of the evaporator coil. Metering devices can be fixed or adjusting, which generally use a sensing bulb attached at the inlet of the evaporator.
zebediah49 t1_j191who wrote
This is true; you need some type of pressure drop device.
I give disproportionate credit to the compressor, due to it being the part that does the Work.
exbm t1_j19rjrr wrote
You need to use a compressor some types of systems use external heat but all systems need a flow restricting device
zebediah49 t1_j1a0qqf wrote
Strictly speaking many external-heat systems don't. They use an absorption refrigeration cycle, and while the partial pressure of the refrigerant components changes around, the total pressure of the system is approximately constant.
bob0979 t1_j199pol wrote
This latent heat for the phase change is also energy that doesn't require as much temperature difference, allowing more efficient energy extraction because some of the energy being transferred isn't received 'in temperature' but in physical state change. The Temps don't equalize as quickly because some of this energy is stored elsewhere than temperature. The whole system just gets to be a higher energy process by including the buffer of state changes.
samskiter OP t1_j18nnn7 wrote
So this is maybe a little bit of the crux of my question. A Carnot cycle is isentropic and I believe that phase changing would introduce randomness and therefore reduce theoretical efficiency limits.
GenericUsername2056 t1_j18q7dc wrote
Instead of 'randomness' it's probably clearer to think of entropy as the amount energy unavailable to perform useful work. So pressure losses and frictional losses for instance consist of energy we cannot use to generate electricity. This is also why entropy must always remain equal or increase.
seven_tech t1_j18u41q wrote
An ideal Carnot cycle is Isentropic. The real world isn't ideal. Entropy is increased in the real world because of loss of energy through friction, intra-molecular force in gas and other similar processes.
And again, liquid is a much better transfer medium for heat than gas inherently.
samskiter OP t1_j193400 wrote
Yea that seems to be the conclusion, but was wondering about the why. Like why we don't try and get as close to the carnot cycle as possible. The replies with numbers helped a lot with getting a sense / intuition for the scales and sizes that make the liquid so much better
seven_tech t1_j1anx4r wrote
That's kind of like asking why don't we try and get to the theoretical efficiency of an internal combustion engine - what does it solve? Liquid to gas refrigeration systems are inherently more efficient, so it doesn't serve any real purpose to try. Same as we know we can't keep using petrol, so there's no longer a point to try in ICEs.
samskiter OP t1_j1cnzgp wrote
It's more like saying why we use 4 Vs 2 stroke.
seven_tech t1_j1coweo wrote
Mmm, kind of? 2 stroke has its place though. It's reliable, cheap to build and easy to maintain vs 4 stroke. Which makes it perfect for simple engines like lawn mowers and yard trimmers.
Gas-gas refrigeration doesn't really have any advantages over liquid-gas. So we don't really use it since perfecting the gas-liquid version. They used to use air refrigeration cycles. But not really anymore because it's so bulky due to the sheer amount of space/gas needed.
zebediah49 t1_j18wosy wrote
No, you can have an isentropic phase change cycle, such as the Rankine cycle.
bl1eveucanfly t1_j19redl wrote
Your question has a fundemental misunderstanding of entropy, specifically in a thermodynamic sense.
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GenericUsername2056 t1_j18i7th wrote
>Mmm, not fusion. Fusion is a specific physical process that only occurs in stars and H bombs (so far). And not freezing. That's liquid to solid.
The latent heat of fusion is the amount of energy required for a substance to transition between its solid and its liquid state. Their terminology is correct.
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seven_tech t1_j18j4up wrote
I've literally never heard it called that. Enthalpy of fusion, yes. But not latent heat of fusion. Though it does appear it can be called that. Still referring to a process that doesn't happen in commercial refrigerators (solid-liquid or vice versa).
At uni here, latent heat refers to the amount of heat a substance can absorb before changing state. If you are talking about the heat absorbtion of a process (like state change) that's the 'heat of' or 'enthalpy of', not latent heat of. Not sure if maybe it's a difference of country thing.
GenericUsername2056 t1_j18jqxb wrote
'Latent' comes from latin 'lying hidden', i.e. heat which does not result in a change in temperature, as opposed to sensible heat. 'Latent heat of fusion/(de)sublimation/melting/vaporisation' etc. are widely used terms. Just type in 'latent heat of fusion' in Google scholar to see for yourself.
The person you responded to made a mistake in the type of latent heat relevant here, but not in their use of 'fusion' to refer to a specific type of latent heat.
seven_tech t1_j18l0vk wrote
Again, widely used where? We don't use them here in Australia. We use simply heat of or, more correctly and usually, enthalpy of. Latent heat is a property of a material to me (specific latent heat). We don't use latent heat to describe a process, because it's confusing vs the material property called specific latent heat. That's how we were taught in High school and Uni. In fact I remember a lecture our year 12 physics teacher gave about not using latent heat to describe a process, because of the confusion with the material property, so use enthalpy.
Also, we no longer use fusion to refer to melting. That's an old terminology that's being replaced as fusion has very specific meaning now in physics/chemistry since we discovered the process in the early 20th century. Fusion meaning melting was coined well before this.
whyisthesky t1_j18lsv9 wrote
Latent heat of fusion is widely used in the UK and US at university physics level
seven_tech t1_j18mlkb wrote
It's not in Australian university physics. It would be enthalpy of desublimation crossed conversations-enthalpy of melting. I did high school physics and 2 years of physics at uni. I never once heard it called latent heat of or fusion.
Also, type fusion into Google. Which page number do you have to go to before you find it being referred to as the process of liquid to solid? It may have been used as a standard reference to that process. It isn't anymore, because fusion (the atomic process) was discovered.
craigiest t1_j18otq8 wrote
Just because you haven’t experienced something or it isn’t a thing right around you doesn’t mean it isn’t common anywhere else. Jeez.
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whyisthesky t1_j18nksj wrote
I’ll see your 2 years at uni studying physics and raise you 4 years doing a masters in physics in the UK where the term is common.
It is a somewhat antiquated term, but it’s not alone in that in physics.
seven_tech t1_j18ozp2 wrote
Again, not saying it isn't used anywhere. It isn't used here anymore. Because it's no longer specific.
We use enthalpy instead of latent heat, and we use desublimation instead of fusion. Because latent heat is also a material property and fusion is an atomic process.
kagamiseki t1_j18ya45 wrote
"fusion" is not "latent heat of fusion"
That's like saying hot dog isn't a valid term that means an American style sausage in a bun, because look what comes up on Google when you search "dog".
Of course if you search a different term, or only part of a term, you will get a different result.
seven_tech t1_j1aozpx wrote
Mmm, I get what you're saying, but I don't agree.
- Fusion is a word to describes a process, in both cases. It's not a noun made up of multiple words like hot dog. And 2) Science terms in the 21st century are coined to be unambiguous. That's the point of defining something in science.
If we want to get technical, it's nuclear fusion. But that still leaves the idea you could be discussing 'nuclear' melting, which is again, ambiguous. Science doesn't like ambiguity.
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GenericUsername2056 t1_j18lwsl wrote
>Again, widely used where?
Internationally. The exact same term is used for instance by Y. Cengel in his textbook Thermodynamics: An Engineering Approach, which is a very popular book on engineering thermodynamics for university-level courses on this topic. This terminology continues to be used to this day by a plethora of researchers. If you don't believe me, again, just search for the term 'latent heat of fusion' on Google Scholar. This is an odd hill to want to die on.
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GenericUsername2056 t1_j18ov2h wrote
>(which, if you Google and read a bit, will confirm that calling desublimation, fusion is an old phrase that is being replaced).
>if you suddenly called desublimation, fusion. We've not used fusion to refer to desublimation since the 70s.
Now I know for sure you don't know what you're talking about because desublimation is the phase transition from a vapour directly to a solid, not from a solid to a liquid. I was listing several types of latent heats earlier, not synonyms as you must've erroneously assumed.
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Calvert4096 t1_j19fxxj wrote
People are jumping on your back a bit, but I never really liked the term myself exactly for the confusion reason you say.
If someone says "fusion" with no context or qualification I think nuclear fusion.
That said, "latent heat of fusion" doesn't have any ambiguity for me since high school, nor does it seem to for most of the English-speaking world. If they don't teach that in Australia... I guess they set you up to waste your time on conversations like this one.
seven_tech t1_j1asa41 wrote
Haha, thanks. Yes, this was my whole point. I was never taught 'latent heat of fusion'. Nor were my colleagues. So we never had that ambiguity. Hence why I started the argument.
But hey, it's the internet. You'll get dragged for calling water wet...
LionSuneater t1_j18yo9l wrote
Latent heat of fusion is a pretty common term in the US. I use it in our physics department and nobody bats an eye.
seven_tech t1_j1apyhr wrote
Yes, I've had this argument several times.
We don't use that term in Australia. Because it's ambiguous. Fusion (more specifically nuclear fusion) is a specific physical process and its use in science is replacing fusion as in 'melting', which is a term dating back several hundred years. So we use melting now, because it's unambiguous otherwise.
You could argue, and many people would still agree, gay means happy. Yet you also wouldn't be unsurprised if people thought they were homosexual if you said 'I thought he was very gay' and many young people would never have heard gay used in any other context. Language changes and it's ambiguous. And science when speaking of fusion, doesn't like ambiguity.
Lookheswearingabelt t1_j190ikq wrote
Latent heat of fusion (solid to liquid) and latent heat of evaporation (liquid to gas) are common terms here in Canada as well
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user31415926535 t1_j18ie2a wrote
>heat of fusion
"fusion" just means "melting" in this case, changing state from solid to liquid.
seven_tech t1_j18jtxf wrote
Yes, I've never heard it called that, though the internet says it's a thing. At Uni here, it's called enthalpy of phase transition (or enthalpy of melting). Fusion is only the literal atomic process of fusing 2 atoms. I have a feeling heat of fusion is a very antiquated version that's being replaced, because fusion has such a specific chemical meaning. Sublimation, evaporation, melting and condensation are the only phase change phrases we have ever referred to at Uni.
But also, still not relevant here. Almost all commercial (and most industrial) refrigeration uses gas and liquid, not solid and liquid.
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Bunslow t1_j19h5gu wrote
> We can input up to 3 times less electric energy for the same transfer of 'heat' energy in a very efficient heat pump.
[edited] how close do residential electric [heat pump] heating systems reach this number? in other words, how much of a waste is it to heat my place via use of [resistive] stove/oven rather than the central electrical [heat pump] heating?
NeverPlayF6 t1_j19nlsu wrote
If your residential electric heating is resistive heating, then there isn't much difference between a resistive stove (the ones with the heating elements) and a heater, if there is a difference at all. In real terms, though- heating your whole house from a single point is probably less efficient due to a lack of air flow and distribution of that heat.
Bunslow t1_j19yxij wrote
well by default i assumed it wasn't resistive, but it could be for all i know. but i assumed that since heat pumps are more efficient that it would be a heat pump
SufferingIdiots t1_j1a10db wrote
Do you have an outdoor compressor/heat exchanger? If not it may just be an electric furnace that heats with resistive elements.
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NeverPlayF6 t1_j1dh202 wrote
A lot of residential buildings have resistive heating. Baseboard radiators, cable ceiling, regular forced air heating are all potentially resistive heating.
Regarding how close residential heat pumps can get to 3x the efficiency of resistive heating- that's about where they are right now. Depending on the temperature at the exchanger, a bit better than 3x is not uncommon. But they become less efficient outside of optimal working temps.
GenericUsername2056 t1_j19r3cj wrote
It's 100% efficient, that is all electricity is converted into heat, eventually. So a 1 kW oven running at maximum capacity will consume some 1 kW of electricity to produce the same amount of heat. So it generates heat from electricity. A heat pump on the other hand merely 'pumps heat' using electricity. This means at certain operating conditions (this is dependent on e.g. the outside and inside temperatures) it will use 1 kW of electricity to move 3 kW of heat from the cold outside into your warm home. This gives it a Coefficient Of Performance (COP) of 3 kW/1 kW = 3 at those operating conditions.
Bunslow t1_j19z1mz wrote
right, but do residential heat pumps actually reach 300%, or do they only actually reach 250% or 200% or whatever and 300% is only possible with industrial heat pumps?
SufferingIdiots t1_j1a1nrq wrote
This be will dependent on the temperature differential, the refrigerant and your specific equipment.
>At 8°C, the coefficient of performance (COP) of air-source heat pumps typically ranges from between 2.0 and 5.4. This means that, for units with a COP of 5, 5 kilowatt hours (kWh) of heat are transferred for every kWh of electricity supplied to the heat pump. As the outdoor air temperature drops, COPs are lower, as the heat pump must work across a greater temperature difference between the indoor and outdoor space. At –8°C, COPs can range from 1.1 to 3.7
Bunslow t1_j1ajws0 wrote
hmm, so if it's -10C or -20C outside, and inside i want it at the usual 21 or 22C, then my cop might drop as low as 2? for residential purposes
bluesam3 t1_j1b6pb7 wrote
It's very climate-dependent - the colder the outside is, the less efficient air-source heat pumps tend to be (partly due to inherent reasons, and partly due to having to do work to defrost the outside unit) - if you're somewhere with relatively mild winters, COPs above 3.0 are very achievable with domestic units. If you live somewhere with extremely cold winters, it's much less achievable.
quintus_horatius t1_j19w0bu wrote
> in other words, how much of a waste is it to heat my place via use of stove/oven rather than the central electrical heating?
Since the rest of the conversation is about heat pumps, do you mean a central heat pump when you say "central electrical heating"?
Resistive electrical heating, as others have stated, is 100% efficient: every joule of electricity is used to produce heat.
A heat pump, however, can move a lot more joules of energy than it consumes. The term is "coefficient of performance" rather than efficiency, but you can think of it the same way. Most heat pumps have a CoP of 3 (or more), which means they're effectively 300% efficient - they move three times more heat than the electricity they consume, or three times more efficient than resistive heating.
OP was wrong, btw. Heat pumps are available with CoPs of 4.0.
Bunslow t1_j19zc1y wrote
well i dont know what mine is, but i'd always assumed it was a heat pump exactly since it is indeed 2-4x more efficient than simply dumping a whole bunch of power into heat thru a resistor. but it could be resistive heating for all i know.
and if there are indeed heat pumps with 400% heating efficiency (or CoP or whatever we want to call it), then probably 300% is a very achievable number for even "merely" residential purposes, one would assume?
quintus_horatius t1_j1a6369 wrote
You only have a few options for heat, and only one that can exceed 100% efficiency - a heat pump.
A typical heat pump exceeds 2.5, a good heat pump exceeds 3.0, and a fantastic heat pump approaches 4.0.
The latter generally show up in highly specialized applications like geothermal, where you can tailor your working fluid to a narrow, predictable temperature range.
Bunslow t1_j1ajqfp wrote
cool, so 2.5-3 is totally achievable for residential/end consumer purposes. is that what residential air conditioners achieve as well?
seven_tech t1_j1assin wrote
Mine almost does.
It uses a maximum of 1.9kW of electricity to move up to 5.2kW of heat in best circumstances. That's a CoP of 2.74. And it's definitely not as efficient as they get.
Bunslow t1_j1bfiyx wrote
excellent, thanks for the info
samskiter OP t1_j1co3h0 wrote
There are residential heat pumps with SCOP (seasonal cop) of 5.
bluesam3 t1_j1b6eed wrote
Yes, 3.0 is a very achievable number for a residential heat pump in a mild-ish climate.
Chagrinnish t1_j1avx5z wrote
Here's a Goodman brand heat pump spec sheet (see p21) with COP numbers vs. ambient air temperature. They're giving a COP of 1.2 to 1.5 (120% to 150%) at -10F. It's going to be pretty cold before you'll want to use any resistive heat. The more important factor is that it can't put out as much heat (MBh in the chart) so it might not keep up.
Edit: Looking at price of Propane, Natural Gas, and electricity (in Iowa prices) you need a COP of 1.9 or 2.2, respectively, for the heat pump to be more cost effective. So that translates to the heat pump being more cost effiective around 5F and above vs propane or 15F vs natural gas. Unfortunately it's -6F right now :)
softdetail t1_j1bcp21 wrote
True but it's only that cold for a small portion of the season, so on average, you are still saving money
Bunslow t1_j1bfexu wrote
well i dont exactly have any way to burn fuel around here, so all i got are resistors or heat pumps. lol. im in IL, so not that far away. 0F and falling to the same -9F low. apparently we have the same low temperature from st louis to winnipeg, it's a massive blast of fairly homogenous air
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