Self-Sufficient Solar Chest Fridge

One of the biggest challenges of the self-sufficient , off-grid life is refrigeration.  Standard refrigerators are out of the question since even the most efficient models consume far too much energy.  Our small solar power system just would not support one.  An alternative many opt for is the propane fridge but, of course, propane is a non-renewable resource and, where we are, no propane truck in its right mind will venture so we would have to haul the tanks ourselves. So, back to solar.

While trying to find a more efficient fridge that doesn't cost half our self-sufficient challenge budget, I came across plans for a solar powered chest fridge.  The Aussie who wrote it explained that freezers were more efficient than fridges due to the simple fact that, since cold air is heavier than warm air, it stays in a chest style freezer when you open the door whereas it falls out of the stand up fridge requiring more power to cool more warm air. That, and the fact that fridges also have fans to eliminate moisture, makes them too inefficient to run on small solar power.  The Aussie solution is to make a fridge out of a chest freezer.  The cold air stays in when you open the door and no fan. He claims a power usage of about  0.1 kWh a day.

The trick to making a chest fridge is to replace the internal thermostat, which is designed to operate at very low, ie. freezing temperatures, with an external thermostat with a higher temperature range.  The Aussie plans call for a typical digital themostat, the kind you see on the living room wall that controls the furnace.  The low end of the temperature range of these thermostats is about 40 degrees which is just about right for a fridge.  The plan calls for the removal of the sensor unit within the digital thermostat, soldering it to a small guage wire and running it inside the fridge.  This is about where the theory ends and reality sets in.

Recognizing the obvious logic of the Aussie plan, in theory at least,  we went out and purchased a small used freezer and a digital thermostat.  When I opened the thermostat, however, what I found was a sensor unit about the size of a gnat's head attached to the circuit board by two of its hairs and, just to make sure the little thing didn't go astray it was securely encased in a teensy metal garage making its removal well nigh impossible. 

Looking for a solution to this problem, I returned to the Aussie's site and noticed that he had added some information to the effect that his digital thermostat began arching and fried itself after six weeks.  This was somewhat disconcerting to me having just disemboweled my fifty dollar digital thermostat.  He did, however, have a fix in the form of a complete description of how to build your own chest fridge thermostat from scratch.  The scratch took the form of a forty item material list of sundry resistors, capacitors and potentiometers and a schematic that looks suspiciously like directions into downtown Manhattan...the long way.   Or, he'll sell you one for $150.

Now, I take nothing away from the Aussie.  His thermostat is worth at least $150. to be able to read that schematic and solder all those teensy diodes onto a circuit board and actually make it work.  With my budget already shot over the digital thermostat, however, I was looking for a simpler, cheaper solution.  I found it at my local used appliance store where I bought my chest freezer.  I let the young woman there know what I was attempting to do to the freezer she sold me so she called Bob out of the back of the store.  He thought about the issue for a few minutes then disappeared into the back and came out with the temperature control from a used fridge.  He suggested wiring that in place of the internal freezer thermostat and run the attached sensor tube through the lid and into the fridge.  The simplicity and the $25. cost captured my interest.  The thing is just a knob you turn to set the temperature. It draws no power and its temperature range is that of a fridge.

So, that's what I did.  The wiring is as simple as pulling two wires off the original thermostat and connecting them to the controller, plug it in and turn it on.  I duct taped it to the back of the fridge with the sensor tube running under the lid and down into the fridge temporarily to test it.  It started up and within two or three minutes the thermometer read 40 degrees.  I have had it running now for a week.  The controller is set at '1' and it runs about three times a day to keep the temperature in the fridge between 34 and 40 degrees F.  I don't have any way to measure how much power the fridge actually draws when operating but my small solar power capacity handles it without undue stress.

We are now self-sufficient in solar refrigeration and all for $125. (not counting the digital thermostat which I have chaulked up to experience and completely blotted it out of my mind so, if you meet me, don't bring it up).