Our Eutectic Cold Plate Fridge

No, we won't have a dorm fridge in our boat. 

Why?  It's all about power and reliability.  CAI tells me that power on a boat costs about $4/kW-hr.  That's more than 30X what you pay at home which, on average in the USA, is a mere 12 cents/kW-hr.    Based on that, wouldn't you want to conserve?

But we will have a fridge. So instead of a dorm fridge, we are using an "Isotherm 3701" that employs something called a eutectic cold plate and will be installed in a HIGHLY insulated box.  This system is like having a big chunk of ice on the side wall of a camping cooler.  The hallmark of the Isotherm 3701 is that the cold plate will freeze colder than ice.

It can do this because it is eutectic.  Eutectic means it uses two fluids that, when mixed homogeneously, freeze at a temperature lower than either of the two fluids alone.  The freezing point of our cold plate will be 18 degrees colder than ice. Pretty cool.  OK, yes, pun intended.  :-)

As I said our "cooler" box will be highly insulated. It will have 4" thick walls. Let's get our head around that.  Imagine the typical camping cooler.  It's about 36" x 17" x 17" and holds about 3 cubic feet, like a dorm fridge.  If you changed the walls of this cooler to 4"thick, the storage area drops down 65% to only 1 cubic foot.  So the insulation comes at a cost of space but it preserves our precious energy. We aren't sure the final storage volume of our fridge yet but I am expecting it will be about 2.7 cubic feet. 

Our Isotherm 3701 Refrigeration System
(big honkin' insulated box not shown)

So together, the extremely low freezing point and the huge amount of insulation will retain sufficiently cool temps to chill our food for hours after a charge.  Maybe even a whole day.  And this is good because we want to avoid running the compressor because it is such a power hog.  If we are lucky we will be able to run the compressor only when we actually have excess power.

Wait, excess power? Yes.  We have excess power when we are running our engine. And this brings me to another hallmark of the Isotherm 3701.  It actually recognizes when we are running our engine and will automatically cycle on to charge the cold plate at that time.  We might be running the engine to charge our batteries once a day, so this should work well.

If it didn't work this way, and the fridge cycled on and off all day long, it could eat up to 50% of our budgeted battery bank every day.  If we only chill our cold plate when the engine is running, and if we do do this daily, then our batteries never have to power the fridge.  Very very cool. (I like puns).

Here are some stats:

Typically power draw from a boat fridge:	50 to 150 amp-hrs/day
Capacity of a typical car battery:	50-70 amp-hrs
Capacity of a 100W 3ft x 3ft solar panel:	36 amp-hrs  (sunny day)
Capacity on AtLast:	~300 amp-hrs*

*(Actually 600 amp-hrs, but we shouldn't use more than 300 before recharging in order to prolong our battery life.)

A final note.  Our system will be water cooled.  It will dump the heat to the sea water through a hole in our hull.  Ugh.  Another hole in the boat.  I never like holes in the boat but they are a necessary evil.

There are at least two reasons to dump the compressor heat to the water.  In the tropics, when it's 90 degrees, we will avoid heating the cabin with hot air from an air-cooled compressor. 

The other reason is that it increases the efficiency of compressor.  It doesn't use fans and heat is transferred much easier from fluid to fluid than from fluid to air.