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Ran D. St. Clair · 08-27-2009, 08:36 PM

An electric ice box? Ridiculous!

All of this set me on the path of at least looking into some very efficient 12V refrigerators. Heck, I would even consider a very efficient AC powered refrigerator. The loss of efficiency through the inverter isn’t that bad.

In broad general terms anything that runs off of 12V DC is going to require less power than the same thing that runs off of AC because the inverter that converts DC to AC has a limited efficiency, around 90%. An exception to this rule would be a 12V DC appliance that has its own inverter built in, like some microwaves. In that case you are trading a large, hopefully efficient inverter, for a small inverter that may or may not be very efficient. You would have to look up the specifications on the individual appliance.

There are 12V coolers using Peltier effect, meaning solid state thermocouple type devices that act as a heat pump. They work fine for keeping cool stuff cool, but they don’t have a lot of heat transfer capacity, meaning they won’t cool things down very quickly, and they also use a fair amount of precious electrical power. I have a cooler of this sort that draws less than 5A at 12V, but it takes forever to cool anything down. I want a real refrigerator with a real freezer section that can reliably keep frozen things frozen and make ice cubes as well.

I found some very efficient 12V refrigerators made by Sun Frost. The Sun Frost RF12 is a 12 cubic foot model that draws somewhere between 1A and 2A on average, weighs 230lb, and will cost me almost $2,800 with shipping taxes and all the rest. These are made for stationary use, so it is not clear to me how well they would hold up in an RV. They also offer a 19 cubic foot model that would draw between 2.58A to 5A, weigh 320lb, and cost me about $800 more. By contrast a modern and relatively efficient Frigidare 20 cubic foot model is rated at .52A at 110VAC, or an equivalent average of 5.3A at 12VDC, assuming it was powered through an inverter.

I think I can live with 12 cubic feet. $2,800 is a lot of money for a small fridge but there’s nothing about this project that’s cheap. A small fridge like that would best be mounted in a cabinet of some sort, so I might as well design it with some shock absorbing materials to take some of the sting out of bouncing down the road, better insulate it, and keep it from sliding around. The radiator coils in the back would need good air flow but I think I can manage that. Yes, the heat it generates would dump into my living quarters but it’s tiny by comparison to what the roof top AC unit can pump. The fridge creates about 24W of heat energy, or about 82 BTU/hour, and the AC is rated at 9200 BTU/hour, so the fridge accounts for less than 1% of its capacity.

At 2A DC compared to 80A for my battery charger, I would have to run my generator a minimum of 36 minutes per day to support it. I think I can live with that.

The following are some useful web sites I found:

http://www.sunfrost.com/refrigerator_models.html

http://www.firemountainsolar.com/refrigerators.html

To be continued…

08-27-2009, 08:36 PM	#56
Ran D. St. Clair Senior Member Join Date: Apr 2009 Posts: 212	An electric ice box? Ridiculous! All of this set me on the path of at least looking into some very efficient 12V refrigerators. Heck, I would even consider a very efficient AC powered refrigerator. The loss of efficiency through the inverter isn’t that bad. In broad general terms anything that runs off of 12V DC is going to require less power than the same thing that runs off of AC because the inverter that converts DC to AC has a limited efficiency, around 90%. An exception to this rule would be a 12V DC appliance that has its own inverter built in, like some microwaves. In that case you are trading a large, hopefully efficient inverter, for a small inverter that may or may not be very efficient. You would have to look up the specifications on the individual appliance. There are 12V coolers using Peltier effect, meaning solid state thermocouple type devices that act as a heat pump. They work fine for keeping cool stuff cool, but they don’t have a lot of heat transfer capacity, meaning they won’t cool things down very quickly, and they also use a fair amount of precious electrical power. I have a cooler of this sort that draws less than 5A at 12V, but it takes forever to cool anything down. I want a real refrigerator with a real freezer section that can reliably keep frozen things frozen and make ice cubes as well. I found some very efficient 12V refrigerators made by Sun Frost. The Sun Frost RF12 is a 12 cubic foot model that draws somewhere between 1A and 2A on average, weighs 230lb, and will cost me almost $2,800 with shipping taxes and all the rest. These are made for stationary use, so it is not clear to me how well they would hold up in an RV. They also offer a 19 cubic foot model that would draw between 2.58A to 5A, weigh 320lb, and cost me about $800 more. By contrast a modern and relatively efficient Frigidare 20 cubic foot model is rated at .52A at 110VAC, or an equivalent average of 5.3A at 12VDC, assuming it was powered through an inverter. I think I can live with 12 cubic feet. $2,800 is a lot of money for a small fridge but there’s nothing about this project that’s cheap. A small fridge like that would best be mounted in a cabinet of some sort, so I might as well design it with some shock absorbing materials to take some of the sting out of bouncing down the road, better insulate it, and keep it from sliding around. The radiator coils in the back would need good air flow but I think I can manage that. Yes, the heat it generates would dump into my living quarters but it’s tiny by comparison to what the roof top AC unit can pump. The fridge creates about 24W of heat energy, or about 82 BTU/hour, and the AC is rated at 9200 BTU/hour, so the fridge accounts for less than 1% of its capacity. At 2A DC compared to 80A for my battery charger, I would have to run my generator a minimum of 36 minutes per day to support it. I think I can live with that. The following are some useful web sites I found: http://www.sunfrost.com/refrigerator_models.html http://www.firemountainsolar.com/refrigerators.html To be continued…