Aircon at Anchor

[StuM]

Originally Posted by OzePete

Here is something to think about if it's only a small cool sleeping area you require: Store energy as ice! Thermal energy, then use this energy at night to cool your cabin.
Here is how its done:
Water as it phase changes (freezes) 'stores' 80 watts per litre, add another 10 watts for specific storage and each litre can provide 90 watts of cooling. So lets say a water tank of 80 litres is well insulated and say two refrigeration evaporator coils are located within and each is coupled to a 3.5cc codensing unit. A total of 6300 watts (about 20000BTU) can be stored. (90% ice)

Nope Watts don't cool anything. It takes energy and that's Watt hours.

(And to be pedantic, water releases energy as it freezes, it doesn't store it. It absorbs energy as it melts.)

Also your numbers are a bit out.

The latent heat of fusion is 334kJ/kg. In addition, the specific heat of water is 4.2 kJ/kg/K, so to melt 1 litre of ice at 0°C and raise it to say 25°C would take about 440 kJ which is about 122 Watt hours (or 10Ah @ 12V) or 417 BTU.

If you freeze 70 litres in your insulated tank, that would store about 8.5 kWh or 29,000 BTU.

Of course, the problem is generating the energy to freeze the water in the first place, if your refrigeration system has a Coefficient of Performance of around 3, it will still take about 3kWh (250Ah @ 12V) to do the job.

Or to put it another way 250Ah of energy use for about te same cooling as 2.5 hours running of a 12,000BTU/h aircon. That's a lot of Amp hours!

[Ip485]

Interesting solution, but I have to say for all that weight and cost why wouldnt you install a Genset (possibly not on a boat of this size). Properly installed they are reasonably quiet and I see no reason why you wouldnt run the Genset for a few hours to pull the cabin temperature right down. I have seen installations with an auto start on the Genset when the A/C triggers a power demand which is clever so you can set a temperature and have the Genset kick in as required rather than run it all night. Sounds to me like a better solution.

[OzePete]

Originally Posted by Ip485

Interesting solution, but I have to say for all that weight and cost why wouldnt you install a Genset (possibly not on a boat of this size). Properly installed they are reasonably quiet and I see no reason why you wouldnt run the Genset for a few hours to pull the cabin temperature right down. I have seen installations with an auto start on the Genset when the A/C triggers a power demand which is clever so you can set a temperature and have the Genset kick in as required rather than run it all night. Sounds to me like a better solution.

Hi IP, yes I agree with you, a gen set would be my choice also. My suggestion was aimed at those who object to generators and would be less than half the weight that batteries would be if to provide the equivalent cooling and a whole lot safer.

Cheers OzePete Ozefridge | 12 Volt Refrigeration Systems