water cooled peltier cool box conversion

[ShinyShoe]

5 litres - doesn't accomodate 2 x 500ml cans ... ;-)

 

[TLouth7]

As the article says, it is a CONTINUOUS drain of an amp or so. I can understand why - to simplify, a Peltier effect device pumps heat from the cold box via a highly heat conductive path to a hotter environment. As soon as the current goes off, that highly effective conductive path simply allows heat to travel back from the hotter environment to the cold box.

This isn't quite true as the peltier device itself has a pretty low thermal conductivity; if it didn't you would have a lot of back-flow of heat from the hot to the cold side when it was on, reducing performance.

In my experience of specifying cooling systems peltier devices have a number of shortcomings:
- they are only efficient at a specific operating point so you have to design the system very carefully
- their performance drops off badly as ▲T increases, so you can't get very far below ambient
- because of the above you need really good heatsinks with small ▲T
- off the shelf systems including thermostatic control are surprisingly expensive
- as with any heat pump it is more important to get heat out of the hot side than into the cold side

Having said all that, you could easily build a system that would hold a steady state coldbox temperature 10°C below ambient at a COP of ~1.5 (1.5W heat moved out of the coldbox for every 1W electricity)

For example for a heat flux of 6W you could use two of these peltiers coupled with one of these heatsinks on each side. Wire the peltiers in series and put 8V across them and you would see a current draw of 0.5A (=4W) and a ▲T of 10°C (20°C across the peltier - 10°C across the heatsinks).

 

[vyv_cox]

MY idea was to use my 100 gallon fresh water tank ( which has a large surface area on the underwater hull) as the heat sink ( no holes in hull). I will appreciate comments on using the fresh water tank.

This works very well. My fridge is once-through seawater cooled but using the fresh water tank was an option when it was built, just difficult to arrange on our boat. My son Owen, who built it, has used freshwater tank cooling several times, which avoids the fouling that occurs with seawater. When we are hauled out ashore I use a bucket as a reservoir, about 6 litres total, water drops out of the fitting on the transom into the bucket, then a hose takes it back to the skin fitting below the galley. In ambients up to 30C and more it will run constantly like this without ever getting even luke-warm.