1) . . . Novakool site claims its small refrigerators consume about 30 watts . . .
2) . . . runs about 30% of the time on a 70 degree day . . .
3) . . . [let's assume that a] 30 watt Novakool with extra insulation runs about 40% of the time over 24 hours in hot weather. That's about 300 watt-hours per day, or 25 amp-hours . . .
4) . . . let's say a 80-watt panel is able to produce 40 watts during 4 hours on a good day. That's 160 watt-hours or 13 amp-hours, about half of what the fridge will use . . .
5) . . . If a 100 amp-hour battery actually provides about 50 amp-hours, a new and full battery should be able to run the fridge, in theory, for about 2 days . . .
6) . . . Each good solar panel day would extend that by half a day. I get a feeling that things would be far worse in the real world, though . . .
Make any sense?
Excuse my breaking your post down like that, but it makes it easier to refer back to as I answer your question.
1) 30 watts at 12 volts is 2.5 Amps (30 watts/12 volts) . Pretty good for a compressor motor!
2 & 3) 30% * 24 hours * 2.5 Amps = 18.75 Amp hours at 70 degrees; 40% * 24 hours * 2.5 Amps = 24 Amp Hours in hot weather . . . but I think your hot weather estimate is a wee bit optimistic. Even if the refrigerator
box is better insulated the compressor still has to work much harder on a hot day to pump heat from the inside of your 'fridge into the hot air outside. Since you're trying to come up with a worst-case scenario I'd tend to stick with the more pessimistic 50% estimate for a 95 degree day, 60% for 100 degree day I've seen elsewhere. So 60% * 24 hours * 2.5 Amps = 36 Amp hours.
4) Different kinds of solar
panels have different performance curves for different angles of the sun relative to the solar
cells. Monocrystalline panels have the best performance, with a power production curve that produces very meaningful power levels even when the sunlight hits the panel at an angle*. Translated into people-speak this means that 50-watt monocrystaline panel really can make 20-25 amp-hours of 12v electricty on a sunny day.
It's worth mentioning, however, that these numbers assume several things, like 12 hours of sunlight, but during the hottest parts of the year the sun is up longer than 12 hours, so you actually gain a few hours of solar energy production during the part of the year when your 'fridge is likely to work hardest.
The downside of that summer advantage is that you loose out on solar energy production during the winter when the sun is up for less than 12 hours and is always at a steeper angle on the horizon. I'd count myself as lucky to get 7 amp hours of power from my 50 watt panel during the winter.
But if you don't boondock during the winter it's very realistic to think a single 80-amp monocrystaline panel could meet the needs of your 30-watt 'fridge even on a hot day.
5) You have the right idea. using my "worst case" numbers you'd be closer to 1.4 days from a single 100 amp battery.
6) You're right. Your real-world figures will probably be less than what we worked out above. Little things, like campsite tree shadows, can take a big bite out of solar power
output, so I'd plan to have some excess capacity. Two 80-watt monocrystaline panels would do the trick during the regular camping season and give you extra power to run LED lighting
and a few other goodies.
But let's do a reality check: Solar panels cost about $0.50/watt, so an 80 watt solar panel
will run you about $400. Add to that the cost of a solar charge controller and an extra battery and you're spending $500 before you've even bought the 'fridge. Not only will $500 will buy a whole lot of propane
, but after you factor in other maintainance costs for the solar system and compressor 'fridge (your batteries will wear out, your 'fridge compressor will give out long before you start having problems with the ammonium hydride evaporative cooling system in a propane-powered 'fridge), and buying a few dollars of propane every year starts looking a whole lot more attractive.
If you're having problems with your propane 'fridge keeping your groceries cold on a hot day, I'd suggest looking into ways to improve the efficiency of your existing 'fridge's cooling system. If your trailer doesn't have a convectiion flue for your fridge (many don't), consider installing one, or add a small muffin fan (0.28 amps) and a temperature switch that blows air over the coils when outdoor temps crest 90 degrees. These are much more cost effective -- and possibly even more environmentally friendly -- than the compressor 'fridge/solar alternative.
* For the science geeks out there: You'll appreciate the test results I came up with when I was playing with my 50 watt Siemens panel one sunny December day just before Christmas. Testing my panel's output as I rotated it in relationship to the sun I measured a maximum 2.7 amps output with the panel at 90 degrees to the sun. (Remember this is almost winter Solstice, when the Sun's energy is at its lowest.) When I rotated the panel I found it's output was fairly close to (Max Output)Cos(X), where (X) was the incident angle.