Today’s guest post reminds me of the more low-tech solution I noted in my 2008 post, Keep it Cool – Refrigeration after Doomsday. “DriedUpGuppy” sent me a lengthy email describing an idea he has for converting propane fired refrigerators to solar. He seemed to be asking for input, but my response was something more like, “Dude, let me post this action. Many readers know more about this stuff than I do.” DriedUpGuppy re-worked the email a bit and … shazam! Guest post.
If you’re a refrigeration expert or otherwise just a cool person, offer your thoughts in the comments section.
After TSHTF, refrigeration will be more than a luxury for many people. It will be a critical capability for diabetics and others on temperature-sensitive medication. The ability to keep food cold will also provide a buffer to stretch the prepper’s stock of stored and homegrown food. This article is presented as a possible solution which is only in the idea stage. Feedback and testing from the user community is encouraged to flesh this out into working plans for others to duplicate.
This focuses on using propane powered refrigerators modified to use solar energy as a replacement for propane or electricity. Propane fridges use a heat source (either a flame or an electrical heating element) to power a heat cycle with a working fluid in a closed system.
The technology is mature and the equipment service life is measured in decades. It is also a technology that can be duplicated by skilled hands as a post-collapse local industry. Used fridges found in RV’s and campers can be had for around $200, and in sizes from the small dorm-type cube to 5 foot tall units with separate freezer compartments. Older units from the late 1970’s to early 1980’s have no electronic or electrical controls. The switching between gas and electricity is done with manual valves and switches.
Gas is controlled through manually operated valves and uses your stash of strike-anywhere matches to light the flame. One repair technician who converts regular fridges to propane as a hobby described the older Dometic and Norcold brands as very reliable workhorses. As long as the pipes remain sound, they will continue working.
Propane powered refrigerators are great off-grid appliances, but they need to be fed propane. Eventually that fuel will be hard to come by. The propane flame which powers the adsorption cycle of the working fluid could be replaced with solar heated hot oil. Specifically, a parabolic trough heating a pipe in the focal plane. Motor oil is circulated through the system, and the hot oil is passed through a copper pipe which is in contact with the previously-flame-heated tube on the back of the fridge. The section of copper pipe contacting the heated fridge tube could be formed to provide a greater area of contact, rather than simply two round tubes with a thin line of contact between them. The efficiency of the heat transfer depends largely on this area of interface. A parabolic trough was selected for its intermediate temperature generation, being between the low temps of a solar hot water “greenhouse box”, and the burn-almost-anything temps of a satellite dish turned into a light concentrator.
A Youtube video of someone troubleshooting a propane fridge used a non-contact thermometer to measure the temp of the propane-heated tube. Just above the flame impinged area the tube was 400 degrees. That would be your target oil temperature. A typical RV fridge uses about 1100 BTU/hour. That will set the flow rate needed at the temperature of 400 degrees when designing the size of the parabolic trough.
For nighttime or cloudy days, excess hot oil from an oversized trough can be stored in a highly insulated tank. The builder would need to calculate how many BTUs to store for X hours at 1100 BTU’s per hour of consumption, thus sizing the tank, as the energy in the hot oil can be calculated. Storing solar heat for after-dark electricity making is impractical, but heat storage would be feasible for the relatively small amount of energy needed by the fridge. Some implementation issues include:
- Transport of high temperature oil from the collector to the fridge
- Design of fail-safe elements given the dangerous oil temperatures in use
- Development of a design matrix with BTU’s required, latitude, desired off-sun run time, and seasonal insolation rates to aid the DIY builder in sizing the components for an effective system
The basic idea can be expanded to include the development of other heat sources, such as exhaust gas or wood stove heat exchange. Experimentation into heat storage methods and capacities would be beneficial. For example, if one builds a fire and uses a heat transfer coil, how much heat energy can be stored in X hours of burning? Is heat energy more effectively withdrawn from coils placed in the exhaust plume, underneath the coals, or both? Could the heating coils be incorporated into a masonry structure like an outdoor bread-baking oven, to recover heat from something used frequently anyway?
Other areas of study include parts sourcing. What pumps can be used for this application? Can a gear pump from a 19xx such-and-such motorcycle be used? What are other good off grid ways to pump the oil?
There are projects out there to build your own solar adsorption cooling from the ground up, but why reinvent the wheel when reliable fridges are sitting in old RV’s and campers across the country? The most complex part of the system has already been built for you.