Solar Refrigeration: A Hot Idea for Cooling

By Duane Schrag

SOLAR REFRIGERATION: The sun can be used to power refrigerators rather than the electrical compressors common today.

Fishermen in the village of Maruata, which is located on the Mexican Pacific coast 18 degrees north of the equator, have no electricity. But for the past 16 years they have been able to store their fish on ice: Seven ice makers, powered by nothing but the scorching sun, churn out a half ton of ice every day.

There's a global scramble to drive down emissions of carbon dioxide: the electricity to power just refrigerators in the U.S. contributes 102 million tons annually. Solar refrigeration can also be inexpensive and it would give the electric grid much-needed relief. Electricity demand peaks on hot summer days—150 gigawatts more in summer than winter in the U.S. (A gigawatt equals on billion watts.) That's almost 1.5 times the generating capacity of all the coal-fired power plants west of the Mississippi River. Further, solar is plentiful. The solar energy hitting 54 square feet (five square meters) of land each year is the equivalent of all the electricity used by one American household, according to data from the National Renewable Energy Laboratory and Energy Information Administration, both part of the U.S. Department of Energy.

Making cold out of hot is easier than one might think. A group of students last year at San Jose State University built a solar-powered ice maker with $100 worth of plumbing and a four-by-eight-foot (1.2-by-2.4-meter) sheet of reflecting steel. No moving parts, no electricity but give it a couple hours of sunshine and it can make a large bag of ice.

The key is the energy exchanged when liquids turn to vapor and vice versa—the process that cools you when you sweat. By far the most common approach, the one used by the refrigerator in your house, uses an electric motor to compress a refrigerant—say, Freon—turning it into liquid. When the pressure created by the compressor is released, the liquid evaporates, absorbing heat and lowering the temperature.

Absorptive chillers like solar refrigerators use a heat source rather than a compressor to change the refrigerant from vapor to liquid. The two most common combinations are water mixed with either lithium bromide or ammonia. In each case, the refrigerating gas is absorbed until heat is applied, which raises the temperature and pressure. At higher pressure, the refrigerant condenses into liquid. Turning off the heat lowers the pressure, causing that liquid to evaporate back into a gas, thereby creating the cooling effect.

As with most technologies, the efficiency of such absorptive refrigeration depends on the degree of engineering (and expense) brought to bear. Single-effect devices have a coefficient of performance of 0.6 to 0.7—that is, they create 60 to 70 Btus (British thermal units) of cooling for every 100 Btus of input heat. That low level of efficiency can be achieved with something as crude as some pipe, a bucket of water, some calcium chloride (as absorbant), ammonia (as refrigerant), and a sheet of shiny metal (the solar collector).

If what you want to do is heat or cool, using solar energy this way is probably more efficient—and certainly cheaper—than converting it first into electricity. "That approach ought to be comparable to photovoltaics, or a little better," said Tom Mancini, program manager for solar power at the Sandia National Laboratories in Albuquerque, N.M.

It would take a fair-size collector—86 square feet (eight square meters), assuming 40 percent panel efficiency—just to deliver the cooling of a small (6,000 Btu per hour or half-ton) window air conditioner. And central air-conditioning units are often 30,000 Btu or more; few homeowners could spare the space for that.

But concerns over collector area depend on location. In the developing world, solar powered ice makers allow locals to store the village's food or medicine without any electricity. For example, in May charitable organization, Heifer International, set up three solar ice makers in remote areas of Kenya. Each will be able to keep 26.5 gallons (100 liters) of milk chilled. More than 500 members of two dairy cooperatives are expected to benefit directly.

Most of the interest in such solar refrigeration in Western countries comes from the commercial, not residential, sectors. Cost is one reason—absorption chiller systems typically cost $7,000 to $10,000 per ton of cooling; one-ton window air conditioners from big box retailers start around $250—but companies can save on electric bill as well as enjoy a more benign environmental image.

Building occupancy patterns is another; most Americans are not at home during the day. "We don't have as much daytime occupancy in residential buildings as in commercial," says Pat Hale, sales manager for Yazaki Energy Systems, in Plano, Tex. Other problems include the expense of retrofitting homes to add plumbing to the attic. And the high temperatures associated with concentrating solar collectors raise liability concerns.

But some entrepreneurs think a residential market nevertheless is emerging. Walter Ross is CEO of Austin Solar AC, a start-up that is testing 36,000 and 60,000 Btu solar-fired chillers. The units provide cooling in summer and heating during winter by just using the sun's heat directly. "We're getting a lot of interest from people who have been using propane for heating," he said. "The biggest issue we run into with these is siting: Most neighborhood associations won't allow these things on your roof."

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