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I was thinking about how hot it was outside, when a thought came to me. There are propane refrigerators, so why not replace the propane as the heat source with solar heat and then change it from a refrigerator to an AC unit? The only power you might need is for the fan to push the air around.

This seems so obvious, you only need an AC to run when it is hot out and the chemistry is a closed loop reaction of ammonia, hydrogen and water so it never needs refilling. However, if it leaks, it's (mainly) non toxic and green, so why isn't it done?

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It is a lot more efficient to convert the solar to electricity and run a decent heat pump for AC and normal refrigeration. Absorption refrigeration is expensive, bulky, not terribly versatile, and has poor thermal efficiency.

The Coefficient of Performance of a large single stage absorption chiller is about 0.7.

The conversion efficiency of solar electric vs solar thermal is about 10%. Coefficient of performance of a heat pump unit in real world operation is about 11. So 1.1/0.7 electric conversion is about 60% better without trying very hard. And the unit is much more versatile.

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  • $\begingroup$ Wow. I would have thought that the conversion efficiency of light to electricity would have been less than using thermal directly. Too bad. $\endgroup$
    – Adrian
    Commented Jul 20, 2021 at 2:55
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This was my senior team project in engineering school, in 1974. Here were our findings:

The power needed to run a small absorption refrigerator can be supplied by a solar collector of about 32 square feet, but the temperature required to boil the ammonia out of the water requires a concentrating collector (not a flat plate)- and hence requires an aiming mechanism for the optics.

Next, since the sun don't shine in the night time, you need some means of storing the cold overnight until the sun comes up again. Water frozen into ice is the cheapest solution, so the reefer makes ice while the sun shines and stays cold overnight on that ice.

But now you have to manage the melt water- catch it as it melts and return it to the ice maker in preparation for the next day's ice making.

By the way, since the energy required to run the reefer costs nothing, you can afford to compromise on the coefficients of performance and still have a viable operating model for the device. This is why ammonia absorption air conditioning is commonly used as a topping cycle on the outlet stream of a big boiler operation where there's lots of waste heat on hand.

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  • $\begingroup$ Otherwise known as a Servel refrigerator about 80 years ago ; no electricity, no compressor , just cool. At that time they used a LPG burner for heat, but any heat will do. As noted , not much capacity relative to size. I looked for a car AC using exhaust heat. But the ammonia absorption section must be level and not moved. $\endgroup$
    – blacksmith37
    Commented Jul 22, 2021 at 21:16
  • $\begingroup$ yep yep, agree. -NN $\endgroup$
    – niels nielsen
    Commented Jul 23, 2021 at 3:57
  • $\begingroup$ Re, "now you have to manage the melt water- catch it as it melts and return it to the ice maker in preparation for the next day's ice making." Why not simply keep the water trapped in bottles in the "freezer" compartment, and provide a path for heat to "leak" at a controlled rate from the "refrigerator" compartment to the freezer? When the machine is running, water will freeze, but the freezer temperature will not go much below 0C so long as some liquid H2O remains. When not running, water will melt, but the "freezer" temp will not go much above 0C so long as some solid H2O remains. $\endgroup$
    – Solomon Slow
    Commented Jun 21, 2022 at 20:31

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