Submitted by samskiter t3_zsiyxn in askscience

So from my memory of thermodynamics, an ideal heat pump is the carnot cycle. This cycle uses an ideal gas on both the hot and cold sides of the pump. However in the real world we use the refridgeration cycle with an evaporator and a compressor.

I understand that the Carnot cycle is 'ideal' and therefore we can't get to Carnot efficiencies in real life.

But what real life factor means we can't try and use a gas both sides (with a turbine to replace the evaporator? Is it energy density? Cost? Complexity? Do space/military grade heat pumps with high performance requirements do something different?


Edit: just a quick edit to say thanks so much for all the responses so far, it's exactly the sort of detailed science and real world experience I wanted to understand and get a feeling for. I will try and respond to everyone shortly!

Edit2: bonus question and I think some commenters have already hinted at this: flip the question, what would it take / what would it look like to have an all-gas cycle and if money were no object could it outperform a phase change cycle? I'm assuming extremely high pressure nitrogen as the working fluid to achieve a good energy density... Enormous heat exchangers. Could it get closer to Carnot COPs?



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GenericUsername2056 t1_j18b5ce wrote

Phase changes occur at fixed temperatures. When you introduce a saturated liquid in a heat exchanger and extract a saturated vapour from it, your temperature difference between the exchanger and your heat source has remained constant (assuming a sufficiently large source). Same goes for a saturated vapour entering a heat exchanger and leaving as a saturated liquid. This is more efficient than heating a vapour, as its temperature will increase, causing a smaller temperature difference between itself and its heat source/sink and with that a reduced heat flux. The latent heat of vaporisation for e.g. water is quite high, which means you can absorb or reject a lot of heat at a constant temperature.

This is readily apparent from the heat capacity of water vapour and the latent heat of vaporisation of water. The c_p of water vapour is roughly 1.8 or 1.9 kJ/(kg K) at 0 degrees Celsius. This means that adding 1.8 kJ to one kilogram of water vapour will raise its temperature one Kelvin. The latent heat of vaporisation for water at 0 degrees Celsius is about 2500 kJ/kg. Meaning one kilogram of saturated liquid water will absorb 2500 kJ before its temperature will start to rise.

So to answer your question more directly, yes, you can use a heat engine with only a gas as a working fluid, but phase transitions are an excellent way of absorbing or rejecting large amounts of heat quickly. An example of a real-world gas-only heat engine is the Stirling engine, which runs on the Stirling cycle.


WatchManSam t1_j18kdhs wrote

Just to tag along on that last sentence, Stirling engines do have a couple niche use cases today such as cryocooling. Instead of using a temperature differential to create mechanical movement, one can apply mechanical movement to create a temperature differential. In commercial applications these can reach down to 40-50 Kelvin. I just think they're neat.


Mikeynolan t1_j1fsbqj wrote

You can buy an off-the-shelf Stirling engine two-stage cryocooler giving 10-20K. Radio astronomers use them all the time.

You put in about a kiloWatt of power and get about 2W of cooling, so use a good Dewar. They use Helium as the working gas.


whereisthenutella t1_j1961r0 wrote

Technical nitpicking phase change happens at fixed temperature for a given pressure for pure fluids.

We use mixtures as refrigerant fluids for liquefying natural gas for example although not the only way.


paulHarkonen t1_j19vkki wrote

I'm curious, what are you using? The only liquefaction plant I've gotten to visit mentioned they were using gas as the working fluid (it was a peak shaving plant) but I have no idea if that's standard or not.


whereisthenutella t1_j1a6dnr wrote

Regasification I imagine you mean.

Large scale liquefaction will use some kind of proprietary process. The core is a the main cryongenic heat exchanger(s) one or more in series, that is large aluminium multistream exchanger.

The refrigerant is compressed, cooled in the main cryongenic heat exchanger and then expanded and it then becomes the cold stream. The refrigerant is a natural gas mixture adjusted based on the Natural gas composition.

If curious search for LNG MR process, or air products MR process, you are bound to find something.


paulHarkonen t1_j1a7wpz wrote

No, I meant liquefaction. It's a peak shaving facility so they take from their distribution system, liquify for storage in summer and then re-gasify for injection in winter.


whereisthenutella t1_j1a8xxy wrote

Ahh okay I haven't encountered cases of that.

At least in Europe there are some underground storage facilities, basically stored in underground caverns (wells) for that very purpose.


paulHarkonen t1_j1aa45p wrote

They use depleted wells for long term storage of compressed gas but some places also use LNG for on system peak shaving. It sounds from your comment like it's reasonably common to use some processed natural gas (presumably with all the heavies extracted) as the refrigerant feeding the cryogenic heat exchangers.



NeverPlayF6 t1_j19m26y wrote

> Phase changes occur at fixed temperatures.

This is a great explanation... but 1 minor note I'd like to add- phase changes (at a given pressure) for many PCMs occur over a range of temperatures. And there is also the phenomenon of hysteresis where melting occurs at 1 temp range while solidification occurs at a lower range. So a material's state at a given temp near the MP can be dependent on what state it started in.


bigflamingtaco t1_j19rx39 wrote

To remain in a gas phase, we also need to run at lower pressures, which decreases density of the gas, further decreasing the amount of heat you can absorb.

To compensate, you will have to move a LOT of gas, and your radiators have to be a lot bigger. You end up with a system that consumes more power and takes up more space to achieve the same temperature change in the medium you are trying to heat or cool.


hkeyplay16 t1_j1a6wc5 wrote

In addition to this, I remember my thermo 1 instructor in college mentioning that it's extremely inefficient to pump gas in a closed system as compared to pumping the same substance in liquid state.


AkagamiBarto t1_j1a0vso wrote

>Phase changes occur at fixed temperatures.

This is true, but the main point is that they occurr both at fixed temperature and pressure.

You can have an isotherm with a gas, but it won't be at the same pressure at all (pressure will change continously), which is a pain to deal with.


naut t1_j1c38kw wrote

There was a project I saw in Alaska I believe that used low temp geothermal to drive a centrifugal compressor backwards to generate electricity. It was for a lodge that used a ton of fuel oil in the winter to heat and make electricity that now used lower temp underground heat. I found it again here


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.


[deleted] t1_j18hcby wrote



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.


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.


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.

  1. 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.


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.


[deleted] t1_j18oltx wrote



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.


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


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.


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.


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_j1b6s2k wrote

It will vary between models and setups, but yes, that seems like a reasonable number to expect, broadly speaking.


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.


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


Jamie1897 t1_j18h4np wrote

We do in fact have refrigeration systems that use gases as the working fluid. They utilized the Joule-Thomson effect (most gases cool upon expansion) and were called "Bell-Coleman machines" or "air cycle refrigeration machines". And they do exactly what you suggest, use a turboexpander as an energy recovery device rather than an expansion valve, the throttling losses in a gas system being too deleterious on performance to ignore. Even with this, the refrigeration COP was rarely greater than 1. This method is still used in air liquefaction devices in air separation plants, and for aircraft air conditioning where compressed air is readily available by bleeding from jet engine compressors. The low efficiency and the large circulating volume of gas needed per unit of refrigeration put them at a distinct disadvantage compared to mechanical vapor compression refrigeration. Here is a guy who built an air-cycle air conditioner powered by a vacuum cleaner and with a turbocharger as the turboexpander.


samskiter OP t1_j18nw7y wrote

Awesome, bonus points for the Vidya. Will certainly watch later!


mypoorlifechoices t1_j19un41 wrote

An important side note, his "lift" (the difference between his hot and cold side) was like 17 degrees F... A higher pressure air compressor will do more, but by carefully selecting your refrigerant to have a boiling point that changes rapidly with pressure, a heat pump can easily have a lift of 70 degrees, which is needed to heat a house in the winter, while still having manageable pressure ranges.

Tangentially, to convert this open loop system he has set up from cooling to hearing you'd have to move the radiator from outside to inside. To convert a heat pump, you just need one valve at the compressor that reverses the input and output, pretty much.


General_Urist t1_j1acjxa wrote

Thanks for the run-down, and thanks for the video link: A good explanation and a fun piece of amateur engineering besides!

Why's this cycle still used for air liquefaction? No suitable material for a vapor-compression cycle that reaches such low temperatures?


BigWiggly1 t1_j18lp9z wrote

There are a few reasons the refrigeration cycle is particularly useful to us, and they do in fact relate to energy density and cost as you guessed. Another factor is operating range.

A major reason is that latent heat of vaporizations are orders of magnitude higher than sensible heat capacities. By using phase changes, we can move much more energy using much less fluid.

For example, imagine we're using steam cool a hot process. The heat capacity of steam (water vapor) around 100-200 C is about 1.9 kJ/kgC.

So in order to remove 1 MW of energy (1000 kJ/s), we would need about 10.5 kg/s of steam if we could get a 50 C temperature drop in our steam. 10.5 kg/s is a lot of steam. In my line of work we measure steam in 1000's of lbs/hr. 10.5kg/s is 83 klb/hr or 83,000 lbs per hour. Nothing to scoff at.

Alternatively, if we could use a steam condensing heat exchanger, we could accomplish the same energy transfer with 0.44 kg/s. That's 24x less steam. That means smaller pipes, smaller pumps and compressors moving the fluid around, smaller valves, etc. Those costs balloon quickly.

Handily, it also outputs condensate water instead of steam, which is easier to handle and pump than steam. It can be pumped right back to the boiler feedwater system.

A second reason we use refrigerants is because they offer convenient phase change temperatures. Water vapor is useless for space cooling, because it condenses at 100C. We'd have to use liquid water for cooling instead, which wouldnt be able to use the carnot cycle.

For space cooling with the carnot cycle, we'd need to use a compound that's a gas at usable room temperatures and high pressures like nitrogen. The problem is that we'd need to use a LOT of it to get any meaningful energy transfer out of the system. We'd need large high pressure storage cylinders, high pressure piping, large heat exchangers. We're talking 100's of thousands of dollars of equipment per house.

Refrigerants are easily contained, the equipment to pump them around is cheap since we don't need much per house. It keeps costs manageable.


samskiter OP t1_j18od2r wrote

Excellent answer, thanks so much. I'm certainly curious in what the gas-obly system would cost/look like if we made it and whether it could outperform phase-chabgw systems. (If we spent those 100s of thousands as you said!)


sorenabergard t1_j18rn23 wrote

A very practical reason that is implied by this explanation but not quite explicit is related to the quantity of fluid. Because the heat of evaporation is so high, you can achieve your cooling effect with less fluid (as the previous poster said). This means way smaller pipes, which are much cheaper to install, and smaller heat exchange surface areas, reducing equipment size.


kilotesla t1_j1aic57 wrote

To expand on this point:

> Water vapor is useless for space cooling, because it condenses at 100 C.

That's at atmospheric pressure. Part of the concept of how a refrigeration cycle works is to change the pressure so as to change the boiling point. So then the question becomes, why not lower the pressure an lower the boiling point, and use water as the working fluid for an air conditioner? A problem with that is the low pressure needed (circa 0.01 atmosphere) would mean the gas would be very low density, and you'd need to flow and compress a very large volume of it to move significant heat. (The other problem is that you couldn't go below 0 C without the water freezing.)


samskiter OP t1_j1cq9k9 wrote

Nice! So your system could dynamically flex the boiling point to match the desired working range (within reason)?


kilotesla t1_j1da7rf wrote

It could within some range. It would be best as a heat pump for heating from moderately warm temperatures up to high temperatures, perhaps 40° C up to 100° C, for example.

Note that that's also how a conventional heat pump works, with an HFC refrigerant. Let's say it's operating between 10° C and 40° C. The evaporator pressure will be set up so that the boiling point of the HFC is around 10° C, and the condenser pressure will be high enough to make the boiling point there 40° C.


QtPlatypus t1_j1bb7my wrote

>Water vapor is useless for space cooling, because it condenses at 100C. We'd have to use liquid water for cooling instead, which wouldnt be able to use the carnot cycle.

Though cooling is just the other side of warming. Steam is a great way to heat a room.


martinborgen t1_j18eb3l wrote

Apart from what others have said, the phase change can be seen as a nifty way to realize the carnot cycle. The phase changes are isotermic, and expansion valve is a simple way to get an almost isentrpoic expansion. Just the compressor to work on.


samskiter OP t1_j18efbn wrote

Interesting. The carnot cycle would extract energy during the expansion. Is there a way to do that with the evaporator? Why don't we do it?


saywherefore t1_j18gmh3 wrote

You absolutely could implement a turbine rather than an orifice to drop the pressure of the fluid in a phase change cycle, and in doing so you would recover some energy that is otherwise lost. However the added complexity and maintenance overhead is not worthwhile in many real world situations such as domestic refrigerators.

Thermodynamic efficiency is only one of several competing criteria. Another commenter already mentioned that the size of heat exchangers is massively lower in a system with liquid than a purely gas heat cycle. This is far more important than you might think, given the packaging constraints of say a domestic refrigerator.


TackoFell t1_j18gahn wrote

Expanders are essentially compressors run in reverse (in fact can be made modified from such) and recover some energy. They aren’t used in many applications because the work recovered doesn’t make up for the cost and potential maintenance needs etc


cheses t1_j18i5bp wrote

Most of the time when you expand via a valve in a refrigerant cycle you expand into the two phase region from a subcooled state. Expanding a liquid does not give us a lot of energy back. With co2 as a refrigerant there are ideas to use an expander to raise the efficiency of the cycle. But that's a special case as co2 has a flat saturation curve where we have a high vapor content while expanding. Also, expanding while having liquid content in the fluid can damage most expanders.


TackoFell t1_j19l3md wrote

Also the comparatively very large pressure difference in a CO2 system helps! Say 1600 psi down to 600 psi, rather than say 600 down to 200. (Still may have flash gas…)


RelativisticTowel t1_j1b44mn wrote

In many domestic refrigerators we do recycle a bit of heat. The external areas around the door gaskets are colder, and prone to a buildup of condensation if you don't heat them up somehow to match the rest of the exterior. We can and on occasion do put electric heaters there, but the best solution when possible is to simply route the starting section of the condenser (where the gas temperature is pretty high) through those areas. Bad move from a heat pump efficiency perspective since some of that heat leaks back into the cold side, but good for the efficiency of the appliance as a whole.


LMF5000 t1_j18sxl0 wrote

Aircraft engineer here. I'd like to point out a counterexample. The "air cycle machines" that create cool air in aircraft air conditioning systems run on compressed air bled off from the engines. They don't involve phase changes, the air stays gaseouos the whole time.


dombar1 t1_j18r1eh wrote

To answer your bonus question:

While not technically a gas (but also kind of a gas), Supercritical CO2 is being used as a heat exchange fluid that is very efficient. Here is an example

When using turbines to extract power from any gas cycle, you can approach Carnot cycle efficiency by continuously adding more stages of turbines and/or recuperators to manage waste heat. The design of such systems goes as far as the cost of approaching the Carnot cycle matches the benefit.

For gas turbines (or steam turbines) you’ll notice that ground based systems can get quite large to maximize efficiency, where aircraft engines will attempt to minimize the size to save weight. It’s all an engineering balance of cost vs benefit.


sharfpang t1_j18segu wrote

Largely, the matter of size, total volume of refrigerant.

Liquid is incompressible so you can't really run Carnot cycle on liquid alone. Gas alone, due to low density, has very lousy specific heat - can't transfer much heat per unit of volume. So, for refrigeration on gas alone you'd need lots and lots of gas volume to circulate quite fast, making your freezer unreasonably big.

By only keeping a relatively small part of the circuit filled with gas, you assure a decent amount of slowly moving, efficient heat-absorbing coolant (cold liquid evaporating in the evaporator) and heat-expelling coolant (gas compressed into liquid cooling down in the radiator), while only a small part of the circuit carries large amounts of gas, fast, between the evaporator, through compressor, into the radiator, providing the pressure transition required by the Carnot cycle.


pilotavery t1_j19a0dx wrote

Because the phase change from liquid to gas actually absorbs quite a lot of energy. It's called latent heat of evaporation. It's the reason why when you pour methanol or alcohol on your hands, they feel cold as it evaporates.


E_M_E_T t1_j19t4mf wrote

Lots of detailed answers but the main gist is that the amount of energy that goes into changing 1g of a liquid to a gas (when it's already at its boiling point) is orders of magnitude greater than the amount of energy it takes to heat 1g of that liquid to its boiling point. So making use of a phase change means you don't need to use as much refrigerant. Less space taken up, less cost of expensive refrigerant, etc.


series_hybrid t1_j19wav4 wrote

An all-gas cycle would be physically larger. Phase-change allows a system to be much more compact for a given movement of BTU's.

Stirling engines used in reverse for cryocooling show that a gas-only cycle can get cold enough to run a refrigeration system.


Sprinklypoo t1_j18nllj wrote

really simply because all of the heat is contained in that phase change.

For instance, water takes about 100 times the energy to change phase (steam or ice) as it does to raise or lower 1 degree. Refrigerants are similar. You can focus on a specific temperature and provide a fairly solid state process.


Onetap1 t1_j18xhui wrote

>really simply because all of the heat is contained in that phase change.

What she/he said.

It takes more energy to evaporate a pound of water than it does to melt a pound of steel.

The phase change is where most of the heat transfer happens.


Arkanor t1_j1agp2i wrote

The Brayton cycle exists (gas to gas) but requires a much higher temperature range for decent efficiency. Phase change allows for significant energy transfer in a small temperature range making it much more efficient for applications outside a power plant.


a_cute_epic_axis t1_j18p97i wrote

> But what real life factor means we can't try and use a gas both sides

You can, and some systems do used a pumped refrigerant economizer, where it just pushes the refrigerant around to move heat around without any phase change or any compression at all.

That said, the amount of energy that you use for a phase change is WAY more than you get with simple heating and cooling.


samskiter OP t1_j18pvem wrote

Would that bit be more of a great engine than a heat pump as you are not working against a heat gradient?


a_cute_epic_axis t1_j18r45c wrote

In real world AC applications, if the outside air is colder (or substantially colder), then you run pumps, not compressors, and push the fluid around to move heat energy from inside to the cold outside. You could do this with water or glycol instead, but refrigerant is way more efficient. You could also just push air between the outside and inside, but then you have humidity issues and potentially pollution/contamination issues, which a refrigerant exchange avoids. If the outside air is warmer than the inside air, you switch to compressors and use the phase change to both overcome to inverse heat gradient (which you could not do at all without some amount of compression), and also because the energy exchanged in phase change is super efficient.

While on paper you might be able to keep everything a gas but achieve some comparably insane pressure to get from say below 50F to WAY above 150F on the inside and outside respectively, it's way easier to use a gas/liquid phase change.

You also have other benefits with gas/liquid... you can have an accumulator or a receiver that holds liquid and effectively reduces the refrigerant charge if there is too much refrigerant for the current operating conditions... and you can flood the refrigerant back into the outdoor condenser to effectively reduce its capacity if you are operating in very cold outdoor conditions but still need DX without freezing your inside coil (or don't have the option for pumped refrigerant economizers).


doctorcrimson t1_j1971ne wrote

Without phase change the only movement through the coolant lines would be convection, with hot gas sitting on the top and cold gas at the bottom. You want the coolant to cycle in the direction from compressor intake to compressor output.


speederaser t1_j1bd4w4 wrote

My company makes the world's smallest "practical" refrigerator. I say practical because you can indeed make smaller refrigerators that use thermoelectrics or other methods, but they are not useful because they are too slow to cool or don't work if the room gets slightly warm or slightly humid. For those reasons we use a phase change refrigerant like just about every other refrigerator on the planet, but the smallest one in the world.

This is the application of all the other comments in the thread. For practical reasons, like size, energy density... us engineers usually end up choosing phase change refrigerants.


TerpenesByMS t1_j1c6xhn wrote

4 Big Reasons: 1. Mainly compactness, as liquids ate much more dense than gases. 2. Also importantly, the enthalpy of evaporation. This adds a significant boost to the heat energy that can be moved by a given volume of refrigerant. 3. Further, liquids have higher thermal conductivity than gases under most conditions, especially low temperature ones. 4. Liquid evap temperature is fixed for a given system, which is convenient for most refrigeration apps like freezing water, or chilling without freezing. A gas-only system, on the other hand, would need precise flow control and sensor feedback to maintain temperature regardless of load.