I like a little cream with my morning coffee so I had to figure out a way to keep things cool on Serenade, our 30' sailboat. She has a nice big insulated icebox, but I find that maintaining ice supplies to be somewhat of a hassle. We were spending about five bucks for 2 blocks of ice every time it needed to be replenished, and I figured this would justify the cost of buying some kind of refrigeration unit.

If you're going offshore or sailing through areas with limited sources of supply, you probably need to consider a robust compressor based cold plate system in order to support a large volume of refrigeration and freezing capacity. In our case, most of the time we are only a day or two away from civilization, so I just needed something compact and efficient to keep dairy products and meats cooled for a few days, and I don't need to freeze anything for long term storage.

I discovered a type of solid state electronic technology based on something called the Peltier Effect that fitted the bill perfectly. In 1834, a scientist named Jean Peltier discovered that passing an electric current through two dissimilar metals would cause a temperature change and thus cooling to take place (it was really the opposite principle of the thermocouple that creates voltage when heated). This results in a small heat pump, later referred to as a thermo-electric cooler (TEC). A TEC can also provide heat to keep food warm if the current is reversed (switchable on many units).

This technology has now been refined to the point where it can be applied practically to compact refrigeration systems. Except for a small fan, they have no moving parts, no compressor noise, no environmentally damaging refrigerant gas and they provide reasonably efficient cooling at a very reasonable price (under a $100 for a small one). They run on 12v DC, but many models will also run on standard AC dock power as well. They are also very compact, an important factor if you have limited on-board space.

I installed a unit on our boat (5 liters of internal space) which consumes around 30 watts and my solar power system has no problem running it continuously. I am considring upsizing to the 15 liter model (pictured above with open door) that has 3 times the internal volume and consumes only 50 watts (max). It is only 440 mm (17 inches) high and will fit into a very compact space.

If you really need portable freezing capability, check out the Coleman Stirling Power Cooler-freezer. It uses high efficiency Stirling cycle engine technology to provide high performance cooling and freezing. It runs on 12v and only consumes 48 watts. 

The Stirling cycle cooler is a free piston, linear motor driven device, with internal running surfaces supported by gas bearing - there is no contact wear. It is capable of continuous modulation and high efficiencies down to very low lifts. This  engineering-speak means that it adopts easily to cooling needs and keeps performing with high efficiency even at low demand.

The performance of the Stirling cycle cooling motor is many times more efficient than conventional compressor systems. According to Sunpower, the developers of the Stirling cycle machine, the Stirling cycle compressor is a "drop-in" replacement for conventional compressors in domestic and commercial refrigerators, air conditioners and heat pumps. It has the added advantage of running on 12v solar systems.