Here is the executive summary on marine fridges:
When it comes to marine fridges Eutectic designs couple with the need to charge batteries at least once a day. One runs the propulsion motor (usually without the prop turning) to charge batteries and fridge. The fridge is charged by freezing down a "holding plate" which is a sealed box containing brine (although far more sophisticated eutectic solutions are available). These devices use quite a bit of current when running - more than most boat sized solar panels can provide.
The total lowest power consumption per amount of cooling over 24 hours however is provided by sealed compressor units such as the Danfoss BD-35F. These will require a duty cycle of approx 40% over 24 hours in the tropics - depending on heat load on the box.
The BD35F generally comes with a cooling coil and a fan like a regular fridge, which requires two motors - one for the compressor and one to drive the fan to pass air over the coils. It runs on both 12 and 240 volts, which can be handy.
There is however a variant that requires no cooling fan but rather uses a "keel cooler" which is essentially a metal block attached to the outside of the hull through which the refrigerant passes. If your house does not float on water then a water tank, swimming pool or dam will suffice as long as its really close to the compressor.
This latter variant is the lowest energy consumption fridge you can construct. But it requires 24 hour feeding, albeit at a lower than 50% duty cycle. This may impact on the rest of your energy strategy.
A top loading box with seals which are under positive pressure (think old style ice box door handles) performs best. The rule of thumb last time I did it the sweet spot was 4 inches of foam insulation for a fridge box and 6 inches of foam for a freezer box. Same foam requirement for the lids of course. You need to use closed cell foam so it does not absorb moisture so EPS is generally out. Optimum cold box panels are lined with aerogel but are silly money.
Don;t forget to skin the outside of the foam in alu foil to reflect radiant heat.
Throw away the awful bimetallic strip "thermostats' and get some digital electronic ones with proper temperature sensors. You will save a heap of amps.
Then never run it empty - put in bottles of water to keep it topped up to exclude as much air as possible.
All this stuff is out there if you search.
Be aware however that this will cost more than a nice shiny store bought fridge. Possibly much more.
If you are living on a boat with limited battery capacity and surface area for panels all this is critical. It is less so in an environment where adding panels is an option.
Good luck
Posted Saturday 13 Feb 2016 @ 12:56:26 pm from IP
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