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Home Power - Home Power/Home-Made Power for Off-Grid Living.
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Posted by rpautrey2 on October 24, 2008, 3:14 pm
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Features
October 20, 2008
Solar Refrigeration: A Hot Idea for Cooling
How to build a solar refrigerator: The brighter the sun, the better it
works
By Duane Schrag
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=97150 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=97the 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=97say, Freon=97turning 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=97that 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=97and certainly cheaper=97than 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=9786 square feet (eight square
meters), assuming 40 percent panel efficiency=97just 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
=97absorption 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=97but 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."
http://www.sciam.com/article.cfm?id=3Dsolar-refrigeration
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