Cold weather passive assist refrigeration.

If you live in a cold climate, your refrigerator is often running when the temperature outside is colder than it is in your refrigerator. A simple way of using the "coolth" from the outside to cool your refrigerator is by incorporating a heat pipe. A heat pipe is a passive device that transfers heat by evaporation and condensation. Steam heat and steam cooking vegetables are examples of heat pipes in use. Absorbed heat boils water, the steam then condenses and releases its heat while heating your house or cooking your food, the condensed water then drains back to the boiler.

A heat pipe can be used to transfer heat from a refrigerator compartment to the outside. When the temperature is colder outside than in the refrigerator, the liquid in the refrigerator boils, absorbing heat and cooling the refrigerator compartment. The refrigerant then condenses outside and runs back to the refrigerator (see diagram). The pressure in the tube changes with temperature, so the liquid always boils when the temperature is colder outside than in the refrigerator. This device acts as a one-way thermal valve, if the temperature is hotter outside than in the refrigerator heat is not transferred into the refrigerator.

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Refrigerators we manufactured incorporating a heat pipe also contain an active cooling system. When the temperature outside warms and the refrigerator can no longer be passively cooled, a thermostat with a preset temperature will turn on the compressor and cool the refrigerator compartment. Installation required only drilling a hole for the heat pipe running from the refrigerator to the outside.

In the mid 1980's we built about ten of these hybrid refrigerators. The models without a freezer compartment coupled very effectively with a solar system. During the winter when available sunlight is limited, the refrigerator is passively cooled and requires no-energy to run. With our RF16, the freezer compartment consumes about 60 % of the total energy consumed by the unit. During the winter, the heat pipe would decrease energy use by 40%. One of the passively cooled refrigerators is located at the Rocky Mountain Institute in Colorado, where winters are long and passive cooling is effective. Although these refrigerators operate effectively, sales were limited and we no longer manufacture that model.

Reprinted with permission www.sunfrost.com/passive_refrig.html.

LARRY SCHLUSSLER

SUN FROST