Keeping Your Cool, Part 2
Last issue, we learned the basics of refrigeration. To recap, when a substance changes states (solid to liquid or liquid to gas) a heckuvalotta heat gets absorbed. For most of the 'civilized' world, this cycle is sustained by mechanical refrigeration.
Most household refrigerators use a standard configuration which relies on an electric compressor to pressurize and circulate a refrigerant gas, which is then evaporated inside the food compartment, absorbing the heat from the contents. The gas is then caused to condense outside the refrigerator, whereupon it dumps its heat which is radiated to the surrounding air, and the cycle repeats, reusing the same refrigerant gas over and over.
Very early, ammonia was found to be an effective refrigerant gas, although it is extremely toxic. Most modern consumer-style refrigerators use Freon® gas, which, while being less toxic, has a now well-known effect on the Earth's ozone layer. Ammonia is still in use in large industrial refrigeration units.
While compressor refrigeration is a fairly efficient process, with about three time as much energy transferred as consumed, the quantity of energy required on a daily basis is comparatively large when considered in terms of being supplied from storage batteries. Even ultra-efficient refrigerators which are purpose-built to be run from renewable energy power systems use a substantial amount of power, and require a reliable supply of charging current to be available regularly.
In most situations where grid power is not readily available, absorption refrigeration is commonly used, mainly in the form of the gas powered RV refrigerator. These refrigerators use the same basic principles to keep food cold (evaporation/condensation), but are able to do it without the need for a compressor. Instead, these units rely on the properties of thermal motion (heat rises) to move the refrigerant through the closed loop of the system. The heat that drives the refrigerant can come from a variety of sources such as LP gas, electricity, paraffin, diesel, etc.
Because these refrigerators do not have a compressor, they are more sensitive to outside influences such as being out-of-level, and if caused to run too hot can develop 'vapor lock', which prevents them from circulating the refrigerant. You may have heard of the common repair for this vapor lock, which is standing the icebox on it's top for a day or two. It sounds kooky, but it works!
When operating properly, an absorption refrigerator can be used to cool and freeze foods, and will run absolutely silently for many years, as there are no moving parts to wear out (except maybe the door!). My personal experience is that my medium-sized RV refrigerator consumes about ten gallons (38 liters) of LP gas per month. Of course, when grid power is available, this is used instead of the gas to fuel the refrigerator.
Another form of non-compression refrigeration is based on thermoelectric principles. Without going into excruciating details, a device known as a Peltier Junction Semiconductor (kind of like a big transistor) is fed electricity. The effect is that heat is transferred from one side of the junction to the other. Unfortunately, the efficiency of these devices is very poor, with nearly twice as much energy being consumed as is transferred. Small electric picnic coolers are manufactured using these devices, but unless you have quite a supply of inexpensive electricity available, they are nothing more than curiosities.
Recently, some intrepid experimenters have devised a simple machine that uses ammonia gas to produce a goodly quantity of ice using nothing more complicated for power than pure sunlight. Simply described, it uses concentrated solar radiation to pressurize and condense ammonia, which is then stored in a pressure vessel. After the sun sets, the ammonia is then allowed to expand and evaporate in tubing inside an insulated box, where it absorbs the heat from containers of water. This device has been shown to bring a higher standard of living to less developed populations without introducing new and consumptive habits requiring the purchase of fuels and machinery.
Perhaps there is also hope for the rest of the planet...