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.
Jim
Commented on March 3rd, 2006
You might try something I’ve read about recently, using a plug-in thermostat to turn the compressor off and on depending on the temp you have it set for. These are commonly used by wine lovers who want to convert an old fridge into a wine cellar of sorts. The article I read about this in claimed it was more efficient than a Sunfrost! I think the thermostat cost something like $50.00, not a bad expense when you compare it to the price of a purpose-made off grid fridge.
George
Commented on August 18th, 2006
I have been using a similar setup for several years with an upright freezer and an external thermostat. This all runs on a solar electric system. The on/off cycles are very minimal at most 15 minutes an hour with the temp set for 36 degrees. We keep it this cool to extend the life of foods. The main problem with using a freezer as a fridge is the condensation that developes. This requires us to wipe down the base of the interior once every few days. This freezer has been owned for years and this created a new life for it! I’m going to try the containers of water idea though. That should help improve it further!
Jeff Banaszak
Commented on August 31st, 2006
Have you ever used the 12 volt igloo refrigerators they are used for cooling and heating food. Not sure what sizes are available. NO freon it uses a peltier ceramic heat pump.
Brenda
Commented on December 5th, 2006
What if you insulated the front and sides of the inside of a Chest Freezer..I have just purchased one and want to use it as a freezer. I was going to insulate the whole inside but I am wondering if that is a good idea
Martin Vance
Commented on January 5th, 2007
What is your response to #3 regarding use of 12 V igloo coolers?
Sincerely
Martin
Patrick
Commented on June 30th, 2007
peltier ceramic heat pump -
they use TONs of power aka electricity.
It okay if you are travelling down the road feeling fine,
but if you have to generate the volts…. lots of Amps
are needed.
There are certain times when “peltier junction” devices are perfect.
Patrick
Commented on June 30th, 2007
Brenda: Why are you insulating the inside of the freezer?
It is ALREADY insulated
ray
Commented on July 18th, 2007
we were paying 1.99/gal for propane and now the station stopped selling it….so i’m tired of paying up to 3/gal so i’m trying the cheap freezer-fridge trick….and curently modifying my trackers to accept 4 additional panels…..i tried running a small rv chest type refer plus a small 300 watt A/C and noticed my batteries are not keeping up…
Barry Wright
Commented on October 12th, 2007
I reckon it is worth trying to switch the freezer on and off with a thermostatic switch rather than a timer.. More precise with temperature, and I suspect it may be running less time/day than with the current setup.. Also a Night-time override could be easily inserted to save battery strain in the pm, using the thermal mass in the water. Easy to set up to see what highest and lowest temps reached also. Most bits from radio shack style places.
Jack Higginson
Commented on November 17th, 2007
I stumbled across this page. Your fridge system is exactly what is done on many small sailng yachts, as the compressor can only be run whilst the engine is running, they use what are essentially freezers as fridges by running the compressors opportunistically when the engine is switched on (such as when manoevering into port etc.) Failing that, it is necessary to run the engine simply to run the fridge (and also charge batteries used to power the radio, lights etc.) so that is minimised. Very long distance sailors use solar panels, but they don’t have the luxury of fresh food and use packaged instead, and therefore don’t have fridges at all.
Owen Davis
Commented on April 4th, 2008
Have just assembled a Carel micro-controller IR33S series, available in 240 or 24 V that has NTC type probe. The controller is powered and goes between the supply and the freezer. Probe goes into the top opening freezer. Installed this on a Fisher & Paykel Freezer 240V, 0.5 A, 220 Cuft. Am currently running trials but initial results indicate: Initial temperature drop from 25C to 3C takes 19 minutes. Without programming the micro-controller to compensate for over shoot at 3C, overshoots target of 3C down to 0.9C then returns to 5C when controller cuts in and reduces temp to 3C in 4 minutes. At 25C ambient and no internal thermal mass takes 24 minutes to return to 5C from 0.9C and controller to cycle again. This would result in about 10 minutes compressor operation per hour – no internal thermal mass or 20 watt hour – will complete trials with specific thermal mass load and advise results. At the moment this seems to be a good solution to have an efficient, large capacity fridge.
Ralph
Commented on September 9th, 2008
How about trying the tupperware thing with the timer. Drill a hole to run the wires through and then seal it up with some silicone sealant. Then, as a backup, put a hole thru your freezer near the bottom and just keep the timer tupperware in the bottom with the water tupperware. Should stay nice and dry with that frozen air.
Oh, and have you tried using milk jugs instead of the tupperware? Cheaper than tupperware and if you have neighbors with kids and no goats/cows, there’s your source.
Uncle B
Commented on January 29th, 2009
Found these on the web for my “Zero running cost, Zero upkeep” off-grid dream retirement home. The ymay be of interest to you!
Einstein Fridge – http://www.gomestic.com/Consumer-Information/Eco-Fridge-That-Uses-Zero-Electricity.285375
I have a dream home in my plans that is “Zero running cost – Zero upkeep” and survival oriented for the depression we are entering. This fridge and a solar model at: http://www.geekologie.com/2008/08/zero_carbon_footprint_the_sola.php
David
Commented on July 11th, 2009
A thought about thermal mass. If you dissolve salt in your water it will lower the freezing point. Then when the compressor is off the thermal mass will do its work (absorb heat) at a lower temperature. Could be helpful with a freezer.
Timers will stick closed because the switch gets corroded, then produces enough heat to weld the contacts together.
RUSS
Commented on July 20th, 2009
fokes you might want to try this method if you have the room, http://fourmileisland.com/IceBox.htm no moving parts, no energy[fossle fuel of any kind] used, TOTALLY PASSIVE…. .I BUILT ONE AND IT WORKED LIKE A DREAM ! GOOD LUCK
Julio Costa
Commented on August 28th, 2011
The problem with the contacts failing is most likely the arcing due to the inductive load (motor) switching on and off. If you place a “zero voltage switch” in line, then this switching will only occur at a zero voltage point of the mains cycle, so there will be negligible arcing, and your contacts will last much longer. I will post a simple diagram on my website
eric
Commented on September 29th, 2011
by the way you should use a 40 amp timer from Intermatic … yeah they are meant for turning on and off hot water heaters and the such. But it’s better then buying 13.00 timers every year. Not sure how you measure your amps from your fridge. But, normally when you hear the “click” of the compressor kicking on you may notice the lights dim? Well, for about a half a second is when the compressor has to really work, and really sucks down the power for a very short time of about 13-15 amps. So, I think the cheap switches you are using are under rated and you need to step up to at least a 20amp rated switch… Intermatic has been making heavy duty timer/switch for 50 years and it will be the last timer you will ever need to buy.
Rosemary
Commented on October 8th, 2011
now near the end of 2011 how is your system doing? do you still like it? what changes would you make?
I may give this a try in 2012. thanks for the article!