Refrigeration off the grid
Energy Efficient Refrigeration
by Scott Middlekauff
I used to be among the hordes of off-the-grid homeowners in search of an affordable method of keeping my food cold. My ice chest was affordable, but little else. My tiny propane fridge cost $800, required frequent trips to town for fuel, cost over $200 per year for propane, and it was depressing to be buying fossil fuels. I longed for a Sunfrost, which is efficient, but costs more than my car. Finally, my regular upright fridge tripled my total energy usage. The defrost cycle alone in this an “energy star” fridge used 450 watt/hours per day. 450 watt/hours just to heat my fridge! To top it off, every time I opened it all the cold air spilled out onto my feet. Mine used over double what it was rated, using about 2400 watt/hours per day.
At long last, I think I have discovered the cheapest and most energy efficient refrigeration system. My method costs a bit over $300, and uses about 350 watt/hours of electricity per day. This could run on the equivalent of one 75 watt solar panel.
Basically, I just hooked up a regular chest freezer to a cheap appliance timer.
First, I will assume that you already have an electrical system with an inverter. I don’t know what size inverter that you need to account for the surge, but the compressor uses about 100 watts once it’s running. I would guess that a 600 watt inverter would be plenty of capacity for the surge.
I bought a regular 8.8 cubic foot chest freezer. You could get a bigger one, but I wouldn’t recommend any smaller, because the relative energy efficiency is a bit lower. Then I got a “heavy duty” plug-in appliance timer for $13, a couple of large tupperware containers, and a piece of stiff mesh metal screening (a piece of plywood with holes drilled in it would work fine). A digital thermometer is optional.
The timer needs to be a type with at least three on settings and three off. I chose to set my timer to cycle on for one hour at 9am, one hour at noon, and one hour and a half at 3 pm. The reason for this time schedule is to have the compressor running during times when the sun would normally be shining, so that there will be no strain on the batteries. I also chose to spread out the “on” times as much as possible, so that the fridge doesn’t actually get frozen in the middle of the day, or warm up too much during the night. The compressor cools off the fridge pretty fast, so you wouldn’t want it to be on for much longer than one and a half hours all at once, or else some foods will get a bit frozen.
The thermometer (optional)is to check the temperature in the fridge during the first couple of weeks and adjust the total number of on hours on the timer to get the exact temperature that you want. I feel personally happy with 36-40 degrees. Below 34 risks freezing. At mid 40’s food spoils too fast for me. Based on your personal use, you may need to have your compressor running more, or less, than 3 1/2 hours per day. Our ambient temperature is around 75 degrees.
The stiff metal screening and the tupperware containers have two functions; thermal mass to keep the fridge cool all night, and to raise the bottom of the freezer box so that it is more convenient for people to use. You see, the bottom of the freezer is two different levels; there is a step where the compressor is housed. The result is that most of the freezer box is over 30 inches deep. This is fine for occasional access of typical freezer users, but too awkward for daily usage. So, what I did was fill two large tupperware containers with water (for thermal mass), and place them in the low part of the freezer. To even out the surface so that the entire freezer bottom is at the same level, I placed the stiff metal screening to create a shelf, on top of the two tupperwares. The result is a fridge of about 7 cubic feet, with about 70 pounds of water at the bottom.
If you need the entire 8.8 cubic feet of space, and you want to devise other ways of utilizing the bottom part of the fridge, try it out and let me know how it goes. I would forget about the food way down there, but with properly marked stacked containers, an orderly person could surely have success.
When you first start up the fridge, you will have to leave it on for several extra hours at first, in order to cool off the 70 pounds of water. That’s all. Your fridge is ready to go.
Warning: Don’t insulate a chest freezer. Unlike most refrigerators, the cooling and heat dissipating coils are located all over the walls of the freezer box, so if you add insulation, you will prevent the compressor from getting rid of the heat. The compressor and the food inside will heat up. When the freezer is running, you can verify this by putting your hand on the outside of the freezer; it’s quite warm. An exception on our freezer is the lid. There are no heating or cooling coils on the door. So, actually, I could glue some foam sheeting to the lid with beneficial results. My friend Ann keeps a blanket on top of her freezer.
By the way, this system can be used as a freezer instead of a fridge. I am using one of each. I have been very impressed with the energy efficiency as a fridge, but only moderately impressed with the results as a freezer. The only difference is that I set the timer for the freezer to turn it “on” at 8am, “off” at 6pm and then “on” again around midnight for an hour. Normally, freezers keep a temperature of about 5 degrees. I keep it running for a total of 11 hours per day, and this keeps the temperature at about 15 degrees. With this temperature, very sweet things like frozen fruit are sometimes a bit soft, though ice cubes maintain their integrity. I accurately monitored the temperature inside my freezer by leaving a jar of sand inside.
I use a timer for my freezer for two reasons: First, freezers normally keep a temperature of about 5 degrees, and I found that about 15 degrees keeps food frozen enough. Second, this way, the compressor only runs during sunlight hours, instead of responding to the thermostat. The total energy usage for our freezer is about 1100 watt/hours per day, which is a pretty big chunk of energy on a solar system.
Based on my (incomplete) research, the only other truly energy efficient refrigeration available seems to be the Sunfrost. Unfortunately, I have heard bad things about their plywood construction, tendency for coolant leaks, poor customer service, and super high prices ($2000+). The Conserve chest freezer is probably great, but their stand-up fridge is an energy hog and costs over $900. I couldn’t find anything else really enticing. So there it is.
Addendum, three years later:
Where I live, the high humidity causes many electronic items to cease working in short order. For example, in four years we have been through four printers, half dozen telephones, over a dozen flashlights (high quality brands), and countless other plugs, fittings, light sockets, and adaptors. We have also thrown away five of the “heavy duty” appliance timers used on our 3 freezer/fridges. They are rated for 1,500 watts, and I’ve measured the running power consumption at under 100 watts. I have no way of measuring the surge, though I guess it is somewhere around 300-500 watts. In any case, after a year of trouble-free operation, these timers sometimes fail to turn the fridge on and sometimes fail to turn it off. My guess is that the contact points on the switch are becoming corroded, though that leaves me confused about the failure to turn it off. I’m curious if other people have had this experience.
In any case, we recently purchased some digital water heater timers. They need to be hard wired, rather than just plugged in. I wired male and female plugs onto ours. These timers are rated at 30 amps, and they cost around $70. Hopefully, this will solve the problem.