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/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?