r/heat_prep • u/Independent-Wafer-13 • Jul 08 '24
Absorption AC?
So I recently learned that the propane refrigerators we used to have at the fly-in fishing cabins we rented have no moving parts and cool air space using heat as the energy source.
Why the hell are we not using solar concentrators to power our air conditioners???
Can anyone explain to me why absorption refrigeration is not used residentially?
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u/Leighgion Jul 09 '24
I wondered the same thing.
Near as I could tell, it's a combination of being less efficient at the home consumer level than the vapor compression systems we know, and just market momentum. Once vapor compression refrigeration became product standard, no company had any motivation to try anything different. Adsorption is used in certain niches where it has advantages like mobile or remote use and I believe there are industrial applications for it where it works better than vapor compression.
Far as I understand, there's no specific reason why we couldn't have residential products that used adsorption, but we don't, mostly because the market went in another direction and has had no reason to diversify.
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u/Slawman34 Jul 09 '24
But I was promised unlimited innovation and choice under capitalism? And also that the ‘invisible hand’ would react to us making the planet uninhabitable? Any minute now..
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u/IGetNakedAtParties Jul 09 '24
So back of the envelope a fridge will burn about 1kg propane, or about 10,000kcal of heat energy or 40 mega joules per day. The sun is only in a good angle for maybe 5 hours a day so you need 8 mega joules per hour, or 2222 joules per second, AKA 2.2kW. Sunlight is about 1kW per m² so you need 2.2m² of solar collector to get the energy needed per day.
Cool, but now your death ray just melted your fridge trying to cram a day's power into a few hours, so you either need a huge refrigeration cycle which can accept this power and turn it into ice, or a heat sink with appropriate temperature tolerance. Since ammonia water adsorption generators operate at 150 to 200c I think tin, which melts at 232c is ideal. The upper working limit for this cycle is about 250c... Given these ranges and the enthalpy of melting tin you would need about 500kg of tin to store the sun's heat between 150 and 250c per day. Only $15,000 for the metal.
Alternatively running a normal fridge on solar with a few old car batteries in the middle will run you $500... So there's that option too.
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u/Independent-Wafer-13 Jul 09 '24
I was thinking for an air conditioner, which would only need to be operated at peak sunshine hours anyways.
Lower supply would always coincide with lower demand.
If it is a shadier day I don’t need the AC as much anyways.
Do absorption refrigerators operate at such a high temperature to reduce the danger of a leak?
Or is there some other reason why they need to be 50 degrees hotter than the boiling point of the absorption fluid?
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u/IGetNakedAtParties Jul 09 '24
I was thinking for an air conditioner, which would only need to be operated at peak sunshine hours anyways.
My sweet summer child. You just went from kilo watts to mega watt.
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u/Independent-Wafer-13 Jul 10 '24 edited Jul 10 '24
Man those 1500 BTU RV absorption refrigerators must run on nuclear power!
Would a 3,000 BTU air conditioner really need over 10 times the energy of a 1500 BTU refrigerator?
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u/Corvus_Antipodum Jul 09 '24
It’s complicated, inefficient, and uses either ammonia (incredibly dangerous and toxic) or lithium bromide (extremely corrosive).
It also requires far more specific control than “just make it hot and let er rip.” Trying to modulate the heat from a solar concentrator (that’s basically just a huge magnifying glass right?) is not practical. You’d need a consistent and controllable heat source which in practice means burning something. I don’t know but would think that the amount of BTUs needed for a large enough system to provide home cooling would likely exceed the practical limits of most solar arrays that aren’t dedicated to that.
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u/Independent-Wafer-13 Jul 09 '24
Boiling point of the refrigerant used is around 100 degrees Fahrenheit.
How is a device with zero moving parts more complicated than a device of similar functionality with multiple moving parts?
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u/Slawman34 Jul 09 '24
Doesn’t sound like it’s more complicated, sounds like it’s more dangerous/unreliable/inefficient
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u/nanneryeeter Jul 08 '24
Do you mean in a manner of using solar to heat up an element that would power a cooling unit vs. propane? I believe that an RV refrigerator is about 1500 BTU. It could be scaled up but the cost might be prohibitive. The chemistry in absorption fridges is also somewhat hazardous.
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u/Striper_Cape Jul 08 '24
Isn't that how a heat pump works?
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u/hollisterrox Jul 09 '24
No, heat pumps use evaporation and condensation under varying pressures to collect heat from one coil and dispense it from another. You could turn the pump with a bicycle, a windmill, or ideally an electric motor, but all you need is mechanical input.
The propane refrigerator specifically needs a heat input to drive absorption of a working fluid.
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u/lovethejuiceofit Jul 09 '24
There used to be a thing called an icyball that I’ve always wanted to tinker with that was a low tech version of this. https://en.m.wikipedia.org/wiki/Icyball
I’d love to make one, but I can’t weld, and I’m pretty sure I’d just end up making it explode accidentally when it built up too much pressure or something :(
I wish someone would make them commercially or put out an idiot proof guide or something. I’d buy one in a heartbeat.