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I am IT engineer not a refrigeration expert, pardon my basic question.

Would a NH3 or other absorption cycle work with a generator temperature of say 30C, and condenser temperature of 10C by maintaining correct pressure level.

Most of the technical papers I read on the internet, talk about requirement of 80C+ temperature for generator and 25-30C for condenser. This would make sense if one has access to hot water source like geo thermal, or any other free heat source. But if I have access to low temperature cool water of 10C, and relatively stable heat source of 25C+ (say atmospheric temperature), can a absorption refrigeration system be designed to harvest that energy. One of the main objective is to have very low electrical energy input, so compression system may not work out. The second objective is to have a -3C or lower evaporator temperature to make salty ice and store it, and use it when the cold water source is not available. If at all its possible, what major issues to keep in mind.

I guess even the standard off the shelf absorption systems can be made to use 30C heat, by first passing it through a heat pump to increase that temperature to 80C+. But that additional stage also need lot of electricity.

Any suggestion to effectively harvest this low temperature source would be of great help.

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  • $\begingroup$ Your temperature range in not practical $\endgroup$
    – Fennekin
    Apr 29, 2016 at 6:23

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Basically to vaporise the NH3 in generator and pressurise it, you need to add heat that will be greater than the latent heat of ammonia due to fact that water is present it will take some part of heat.

To vaporise and seperate water and NH3 at lower temperature you will need to add more amount of heat (red line) with respect to heat you will add to higher temperature (blue line).

further more the temperature range you have provided is not practical. I mean the heat added to the aqua ammonia by just atmosphere would be insufficient to vaporise the refrigerant. let alone it to pressurise.

to add heat to it you need to give it some external heat.

Note:- the diagram below is for ammonia refrigerant I have drawn basic diagram on the left. I didn't find the diagram for aqua ammonia strong solution. Actual curve of the solution may vary

enter image description here

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  • $\begingroup$ When you say atmospheric heat itself is not sufficient to vaporize ammonia, what is the reason for it ? Ammonia boiling point is at -33.34C at 1ATM right. $\endgroup$
    – RNR
    Apr 29, 2016 at 12:19
  • $\begingroup$ Let say I have two ammonia-water solutions A) One at 10C with a concentration of 30% NH3 B) Another is at 40% NH3. If both these are connected via a tube. If I apply heat to B and bring it up to 30C. What will happen to ammonia concentration ? I think, ammonia concentration in B will reduce to 30%, and A will become 40% solution. As ammonia solubility in water differs by temperature. If I apply the same logic and cool the migrating gas to 10C in condenser and then pass it via expansion value, would that not produce a colder than 10C temperature ? $\endgroup$
    – RNR
    Apr 29, 2016 at 12:22
  • $\begingroup$ Its aqua ammonia which is strong mixture of water and NH3 $\endgroup$
    – Fennekin
    Apr 30, 2016 at 2:47
  • $\begingroup$ reference :absorptionchiller4u.blogspot.in/2013/02/… $\endgroup$
    – Fennekin
    Apr 30, 2016 at 2:48
  • $\begingroup$ also to mention that you are correct about change in ammonia solubility in water. it depends on the temperature. (with some quick math assuming ammonia separates) Ideally it should pressurise by 1.26997 times (pressure before heating). however in actual practice it would be even lower. and the cooling effect would not satisfy your needs. $\endgroup$
    – Fennekin
    Apr 30, 2016 at 3:03

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