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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

Thanks.

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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.

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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.

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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.

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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.

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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

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