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05.08
Solar
Energy – An Alternative Energy Source
By George Zobrist
According to the U.S. Energy
Information Administration, an independent
statistical agency within the U.S. Department of
Energy, in 2005, 86 percent of worldwide energy
production was derived from fossil
fuels. With a global push to find viable
alternatives to coveted — and finite — fossil
fuel reserves, renewable and sustainable
resources, such as solar energy, are enjoying a
resurgence of interest from the engineering and
political communities.
From time immemorial, solar energy has been used in
one form or another for generating heat and
other forms of energy.
All renewable energy sources — including wind,
rain, tides and geothermal heat — are dependent upon the energy
from the sun. By some accounts, Earth's oil
deposits were originally formed with help from
the sun’s energy. Early Greeks, Native Americans,
the Chinese and countless others warmed their buildings
by orienting them towards the available
sunlight. Today, solar technology provides
heating, lighting, conversion to electricity,
and some contend that it can be used, through a
chemical reaction, to obtain oil.
According to Oak Ridge National
Laboratory scientists, the Earth receives
approximately 174 petawatts of solar energy at
any given instance. The oceans, which comprise
most of the Earth’s surface, absorb a large
percentage of this energy. And, while huge amounts
of solar energy reach the land mass, it is
difficult to make use of because it is spread
over such a vast area. Atmospheric conditions
also reduce the amount of solar energy impinging upon the earth,
with roughly half of it being re-radiated into
space.
Numerous technologies
hold promise for harnessing and utilizing the sun’s energy. They
include: agricultural and architectural
technologies, solar
lighting, solar thermal, HVAC, solar cookers, electricity
generation, solar to petroleum, transportation,
and satellites to mention a few. As might be
expected, most of these applications require
reliable sunlight, large open spaces, and
expensive technologies for successful
conversion.
Following are brief descriptions of some of these
technologies:
Solar power satellites (SPS)
are proposed to be built in a high earth orbit
and will use microwave power transmission to
beam the solar power to earth. The main
advantage that solar power satellites have is an
unobstructed view of the sun (about 99 percent). The
obvious impediment is the construction and
launch costs. The only way to compete with
conventional sources is to have low launch costs
and a space-based manufacturing system that
constructs the SPS in space. Peter Glaser was
granted a patent in 1973 for a method of
transmitting power from space to a ground based
receiving antenna. This system is composed of: a
solar collector, a microwave antenna on the
satellite, and a system of much larger antennas
on the earth’s surface for collection. There is
presently no line item in either the NASA or DOE
budget for SPS. A minimal effort is expended
through discretionary budget monies.
Photovoltaic (PV) solar
cells can convert sunlight directly into
electrical energy. The early work on this
technology was performed by Charles Fritts in
1883. Bell Telephone Lab researchers, while
researching silicon wafers, discovered the PV
cell in 1954. Early cells had about 1 percent
efficiency; they are now about 10 percent
efficient. PVs are being used successfully in
numerous satellites. In 1999, the worldwide
capacity of PVs had reached 1,000 MWs. It was
projected to reach 9,000 MWs in 2007. Also, PV’s
are DC generation systems, so the power must be
converted to AC for typical residential and
industrial usage (an added cost).
Concentrating solar power
systems can be used to generate electricity
by focusing the sun’s rays to obtain steam and
thus turn electrical generators. The
concentration of the suns rays to perform other
useful tasks dates back to at least ancient China,
and legend claims that Archimedes concentrated
sunlight on an invading fleet to repel them.
Some of the main methods for concentrating solar
energy are the solar trough, power tower and
parabolic dish. The Solar Electric Generating
System plants in California and Plataforma Solar
de Almerica’s SSPS-DCS plant in Spain are
representative of this technology. Abengoa Solar Inc.
plans to build a solar electric generating
plant. The plant will use thousands of mirrors
over a three square mile area, southwest of
Phoenix. The system would focus the sun’s rays
on fluid-filled pipes which in turn would use
the heat generated to run generators/turbines
for electricity. The system would generate 280
Megawatts of power at an average of 14
cents/KWH. Abengoa Solar is a joint venture
between Arizona Public Service and a Spanish
developer.
Solar water heating systems
utilize a collector and a storage tank. Many
large commercial buildings utilize this
technology, for not only hot water systems but
to also supplement their heating systems.
Solar vehicles have been
under development since at least the1980s. There
is a bi-annual World Challenge Race in which
numerous Universities from around the world
compete. In 1987 the winner’s average speed was
42 mph over 1,877 miles in Australia. The
electricity used to power these vehicles is
generated by PV panels. Solar boats and aircraft
deriving power from PVs have also been developed.
Sunshine to petroleum
technology is under research at Oak Ridge
National Laboratories and seeks to use heat from
sunlight to facilitate a chemical reaction to
break down carbon dioxide gas into oxygen and
carbon monoxide. The carbon monoxide can be used
to artificially synthesize gasoline.
Of course, other than
available/reliable sunlight, and
efficiencies/cost, a reliable method of storage
must also be available. Two main storage systems
are thermal mass and electrical. These are used
for short- and long-term storage. Thermal storage
systems are usually composed of organic
materials, such as paraffin. Inorganic systems
are composed of salts, metals and various
alloys. United Technologies and the US Renewable
Group are in the process of commercializing a
molten salt system to generate steam which can
drive an electrical generator. The system
consists of a solar concentrator used to heat
molten salt, where the steam generated is cycled
through a turbine which is then used to drive an
electrical generator. It is stated that one
plant, using this method, can supply the
electricity needs of about 50,000 households.
Electrical storage can be
utilized via a PV system connected to a
rechargeable battery bank. Excess PV energy can
also be connected into the existing power grid.
Net metering laws, which allow the generator of
excess energy to feed it back into the power
grid, are being enacted world-wide.
IEEE-USA has historically
supported solar and other
renewable electricity technologies as essential
components of a diverse set of options for
electric power generation in the coming decades.
Benefits of solar power as
an alternative energy source include:
Solar energy does pose some indirect impacts on the
environment, primarily during the manufacturing process.
Additionally, large solar farms in the desert
may be harmful to the ecosystem. The downside to
solar power includes:
-
low
efficiency
-
large numbers of installations
required to
achieve large amounts of power
-
the need for large
unobstructed space, and mostly desert
installations
-
overcast skies can inhibit the energy
output
-
unavailable at night
-
storage
mechanisms
-
seasonal variations create
interruptions
The cost to produce one Kilo Watt Hour (KWH) is
estimated 10-20 cents, while most
electric utilities charge 9 cents/KWH, or less (large industrial users are charged somewhat
less). One of the main impediments for any
alternative energy source is cycling in the
users' cost of petroleum. In recent years there
have been numerous false starts in the
development of alternative energy sources.
Perhaps the current bleak global energy outlook
will catalyze the action needed to overcome the
hurdles that have prevented solar energy from
becoming a viable alternative energy source.
For some of the articles from which
this information was obtained, follow:
-
J. Lynn Lunsford, “Solar
Venture - Molten Salt and Solar Project for
Arizona,” Wall Street Journal, 2
January 2008.
Dr. George W. Zobrist is
professor emeritus at the University of
Missouri-Rolla, Department of Computer Science,
IEEE-USA's Member Activities editor, and former
editor of IEEE Potentials. Comments may
be submitted to
todaysengineer@ieee.org. Opinions expressed
are the author's.
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