The Absorption-Cycle Heat Pump

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Photo by MOTHER EARTH NEWS Staff
After ten years of development, the regenerative absorption-cycle heat pump shoes promise of revoluntinizing the way we heat our homes. Left: Kim Dao, project director; right: Joseph Rasson.

It’s probably fair to say that most of us consider heat pumps to be pretty mysterious devices. Somehow or other they manage to pull heat from outside air that should be too cold to keep us warm; or, at the flip of a switch, they take heat out of indoor air and keep us cool. To add to the mystery, heat pumps have a performance rating, called a coefficient of performance (COP), that is greater than one — suggesting that there is a free lunch, after all.

Of course, it isn’t true. Heat pumps don’t create energy; they take advantage of a fact that Lord Kelvin (as in “Kelvinator”) figured out about a century ago: Anything that’s at a temperature higher than absolute zero (0 Kelvin, -273° Celsius, or -460° Fahrenheit) has heat, and if you can put something cooler next to it that heat will be transferred. Additionally, the fact that liquids absorb heat when they vaporize and give up heat when they condense allows more heat to be transferred. Therefore, heat pumps deliver more energy than they use, simply because they take advantage of the heat that’s present in air (or water, earth, or whatever medium in which the evaporator is placed).

Today most heat pumps — whether they’re used to warm buildings, to cool buildings, or to refrigerate food — employ a compressor to increase the pressure and temperature of the working fluid, thereby effecting heat transfer, vaporization, and condensation. But another system, one known as the absorption cycle, has also been used where electricity is unavailable to run a compressor motor. These devices are similar to the compressor type but use heat directly to raise temperature and pressure. Absorption refrigerators were widespread before electricity came to remote areas of the U.S. and are still used in recreational vehicles and at remote sites.

For obvious reasons, the absorption cycle lends itself well to solar-thermal energy. Converting the heat output of a solar panel into rotating motion (to spin a compressor) would be a complicated and expensive procedure; an absorption-cycle heat pump, however, has no moving parts and requires only heat at fairly low temperatures to operate. Technicians have been working on solar-driven absorption-cycle heat pumps for over a decade, but the hang-up has been that the best COP’s have only been about 0.7. This requires a single-dwelling collector area of 600 to 700 square feet, a prohibitive amount.

One of the people who was caught long ago by the allure of the absorption cycle is Kim Dao, a mechanical engineer at Lawrence Berkeley Laboratories. In 1975, Dr. Dao began attempting to improve the performance of the home-scale, conventional absorption cycle heat pump. After a good many years of frustration, he and his coworkers — Rollin Armer, Joseph Rasson, and Michael Wahlig — recognized that attempting to make incremental improvements in the conventional heat pumps simply wasn’t going to take them where they wanted to go: to COP’s of better than one.

  • Published on Nov 1, 1984
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