Propane and Compressor Refrigerators, by Tunnel Rabbit
This is a brief analysis of propane and compressor refrigerators in long term grid-down appplications.
In Northwest Montana life has not changed radically during the Coronavirus lockdown, and there are plenty of used freezers, and fridges available on Craigslist. However, demand for propane refrigerators is on the increase as there is marked rise in interest in self-reliance. If nothing can be found in your area, then be willing to travel to buy a used propane refrigerators before they are gone. These are expensive and hard to find. At the least, these can preserve meat while you jar it up, and provide back up and off grid refrigeration. These will become increasingly expensive and harder to find as the current situation evolves, so this might be a good time to check in your area and use the funds to buy a propane-burning absorption refrigerator instead of standard freezer, 120AC, or 12VDC compressor type refrigerator, that may or may not be still available in your area, when the propane option might still be. JWR Adds: One option is contacting large recreational vehicle (RV) and fifth wheel trailer dealerships. They often have used RV refrigerators available with cosmetic issues (typically just scratched or dented doors) that make them very affordable.
A typical used 10 to 12 cubic foot propane fridge might run $700 or more. A new one is twice that price, or more. And full size propane refrigerators are like hen’s teeth. New refrigerators of the standard 10.5 cubic foot size start at $1,200, and for the largest size, $2,500 new.
The typical 10.5 cu ft. size, set to the lowest temperature for summer operation, will use no more than three 20 pound propane bottles per month during the summer months, or to be precise, consumes 1,200 to 1,600 BTU per hour on the highest (coldest) settings, depending on the model. The lowest setting (higher lower temperature) is often one half to two thirds of the highest consumption or stated BTU rating. The service manual should provide the lower BTU rating. The highest consumption rate or rating, is recorded on a plate at the bottom of the front door, or in the back lower portion near the burner. One pound of propane contains 22,000 BTU, and there are 4.2 pounds of propane per gallon. Divide 22,000 by the BTU rating to determine the number of pounds of propane used in one hour, then multiply by 24 hours to determine the amount used per day.
Propane Storage
For remote retreats that cannot be accessed by a propane delivery truck, you won’t be able to use a typical domestic 200 to 750 gallon propane tank. You will need to use smaller man-portable tanks. If you can handle the weight, a 100 pound bottle is preferable to the 20 pound barbecue tank, as it use avoids frequent changes of tanks and start ups by one fifth, or from every week, to only every 6 weeks or so. 40 pound bottles are harder to find used, and more expensive, yet the convenience of these in this application might make it worth the extra expense.
All figures are estimates adequate for comparative purposes. Ambient temperatures, the quantity of food stored, the refrigerators rating, and selected internal temperature setting are the important contributing factors in fuel consumption. To increase efficiency, or reduce propane consumption up to 30% or more, use insulation attached to the outside in some way. An aluminized coating on the exterior of the insulation helps a great deal, even it’s only aluminum foil, or a mylar blanket cut to size, or foam board with aluminum foil that is usually available at building centers. To increase efficiency further, use a small fan to circulate air to cool the ‘fins’ on the back side. A computer type “muffin fan”, or other small fan can also be used inside to circulate the air, and move warmer air inside passed the evaporator fins that are usually located in the freezer box. Refrigerators and air conditioners require adequate temperature differentials to cause internal system gases to expand and condense. A fan assist with that process.
Propane refrigerators need not be vented if use inside a home, but should be vented if used in a small space such as an RV. A tiny burner provides the heat that boils the gases inside the tubes located on the backside. There are no moving parts to break. However, over time, usually measured in decades, the hydrogen gas can escape rendering the fridge inoperable. Periodic maintenance is also required once, or twice a year to ensure reliable and efficient operation, and the coldest temperatures possible. Usually the chimney needs to be cleaned, and the burner checked for a nice blue flame. Sometimes the tube feeding the burner and the tiny orifice that regulates the gas needs to be clean out with a solvent to remove oil deposited by the propane gas, perhaps the filter changed, or the regulator from the tank replaced or adjusted, and the cooling fins in the back dusted off. Most of the maintenance can be handled by the average person who can follow the instructions from a service manual. JWR Adds: For safety, just leave any repairs that require getting into the ammonia or hydrogen piping up to someone who is qualified!
For longer term sustainability, purchase the smallest propane refrigerator that meets your needs. I am about to trade a larger propane fridge that sold for $700 on the second day advertised, and accepting for two smaller dorm/RV sized ones as trade-ins, for a $200 credit. These are 3-way propane refrigerators that also run on 12VDC and 120AC. This provides redundancy in terms of power source options and doubles the number of fridge available, and each uses approximately one third the propane that a 10.5 cubic foot fridge does, or only about 600 BTU for a 3 cubic foot dorm sized, versus 1,600 BTU for the mid-, or standard-sized variety that are about three times the size. A small RV fridge can usually run for a month on a 20 pound tank during the hottest part of the summer, where as the large 10.5 cu. ft. needs 3 times as much. As ambient air temperatures decreases, fuel consumption will also decreases.
The smaller 600 BTU fridges provide space to preserve several meals. If more refrigeration space is needed, both smaller fridges can be run. We are setting ourselves up for the long haul and should strive to understand and use resources wisely. This is all the cold food storage space that one or two persons actually needs, and it is a luxury item from my perspective. Propane is inexpensive right now ($1.38/gallon around here as of this writing), yet storage costs and fuel consumption can be reduced by one third using the smaller 600 BTU refrigerator, or we can have three times the capacity for the long haul without propane delivery. A 100 pound bottle can run one of these RV, or dorm room-sized fridges for about 6 months, or longer if it is well-insulated, and until the colder seasons arrives again, making a refrigerator unnecessary. A 10.5 cu. ft. refrigerator that is rated at 1,600 BTU would require the equivalent of three 100 pound tanks (24 gallons) during the warm-to-hot six months of the year.
Insulating these fridges reduces the propane consumption by at least by 30%. This means a 10.5 cubic foot fridge that usually uses three 20 pound tanks a month or three 100 pound tanks a season, would then only need two 20 pound tanks a month, or two 100 pound tanks. During the winter when a refrigerator is not all that necessary up here in the frozen end of the American Redoubt, blocks of ice made just outside the door can turn a fridge into an ice box. To store items that need to remain cool, but not frozen, use an ice chest outside, but be certain that the bears are already in hibernation. A super-insulated box full of frozen items, can remain cold into early spring, weather permitting.
One Forced Quick Expedient
One of my freezers failed in mid summer. So I moved about 100 pounds of frozen trout in to a large plastic bin with lid. No ice was added. Two layers of foam board were placed on the floor, and the box on top was then wrapped with several layers of R-11 fiberglass insulation around, and over the top of the box. To complete this field expedient ice box, it was then covered with a large sheet of mylar from HVAC heating duct that reflected radiant heat from the barn’s uninsulated metal roof. It had to be protected from critters in a on old barn that became very hot, approaching 100 degrees during the day. But the fish stayed icy for more than a week while it was canned up.
If one does not have a freezer, frozen or unfrozen meat can be stored temporarily using ice in a super insulated box or container in this way. Placing into the ground would increase storage time as well, and can double as a root cellar. This would work if your freezer failed, or as a method that can be used to extend storage of cold items into warmer spring months by several weeks if improvements were made, and as a method to reduce propane consumption, yet further.
Testing Used Propane Refrigerators
Some propane refrigerators in good working order can also be adjusted cold enough to freeze the entire contents. Ask the person selling a propane fridge to freeze a cup of water before you come out to look and possible buy a used propane fridge. If the freezer portion cannot freeze a cup of water, then do not buy it. If they want big bucks for an old fridge, the least they can do is freeze a cup of water if you do not have a thermometer to test. The refrigerator part of the interior should be at least 40 degrees or colder.
Of course this might be a good time to also consider the newer compressor type refrigerators. This technology is now well proven, and popular for use in recreational vehicles (RVs). These are expensive as well, yet lend themselves to be powered by a PV system. With enough panels and no batteries, these fridges can freeze items during the day when the sun shines, and thaw at night when the sun does not shine. If storage batteries are used, instead of consuming propane, we will be consuming storage batteries every few years.
Relative Costs
Buying a propane fired unit is about the same price of a bank of two Trojan T105 batteries, and additional PV panels to charge them up that might cost $700 for the part of a small system, and last 3 to 4 years, or as long as the batteries last. The cost for this small PV system will run my 3 cubic foot propane fridge that cost less to purchase used or the same new as a Dometic compressor fridge, for 21 summers if one could store that much propane. The math is simple: $700.00/$1.38 = 507 gallons of propane X 4.2 pounds/gallons = 2,129.4 pounds divided by 100 pounds = twenty one 100 pound tanks. And batteries will be harder to come by and more expensive in the future than propane. There is no longer any lead being mined in this country. Most of it is in China.
One 100 pound tank would run the 3 cu ft. fridge for 6 months, from spring into fall. Extra insulation can also be added to the outside to improve efficiency, and reduce consumption another 30%. Propane fridges are proven to last for decades, and their fuel can be stored for decades as well, yet replacing batteries every 3 years to run a compressor fridge may not be possible. It would be nice to have both options. Of course the 3 way fridges can also be run off on inverter, or directly off 12vdc, however they are not designed to run primarily on electricity, and are not as efficient overall on electricity, or become as cold as when run on electricity as the newer compressor types, yet it can be done. They require about 8 to 11 amps 12 VDC, and would need 4 to 6 Trojan T-105 deep cycle batteries, or other deep cycle batteries rated at 200Ah or more, and 2 to 3 times the number PV panels providing 400 to 600 watts. Not good. The better compressor types sip power, using typically less than 4 amps at 12VDC, when the cycle.
If your budget for off-grid refrigerator limits your choice, then propane also happens to be the least expensive, according to the math. And the propane refrigerator has no moving parts to fail. The compressor types have not been tested to the same extent or nearly 100 years as are the propane type, and the more moving parts there are, then the more likely it is to fail. Of course the older the propane refrigerator, the shorter is it’s remaining service life, especially if it is very old when purchased used. They may last as long as 40 to 50 years. Often they are 20 years old, or older when purchased. And the colder the fridge, or the better is runs now, is an indication of how much time it may yet run into the future.
My unprofessional guesstimate is that an old propane fridge that can produce the temperatures that it was originally designed to achieve, will last many more years. The process of losing the hydrogen gas is usually a very slow process, akin to a very slow leaking tire that take years to deflate. If it currently operates as designed, this process as yet to begin.
Lastly, I should mention that I believe that owning two or more small units is better than buying one larger one.