Title: Solar energy
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Permanent Link: http://ufdc.ufl.edu/CA01300662/00001
 Material Information
Title: Solar energy
Physical Description: Book
Language: English
Creator: Virgin Islands Energy Office
Publisher: Virgin Islands Energy Office
Publication Date: 2007
 Subjects
Subject: Caribbean   ( lcsh )
Spatial Coverage: North America -- United States Virgin Islands
Caribbean
 Record Information
Bibliographic ID: CA01300662
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.

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DEPARTMENT OF PLANNING AND NATURAL RESOURCES
VIRGIN ISLANDS ENERGY OFFICE
45 ESTATE MARS HILL
FREDERIKSTED, VIRGIN ISLANDS 00840
TELEPHONE 340 773-1082 STX FAX 340 772-0063
340 774-3320 STT FAX 340 714-9531
www.vienergy.org


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SOLAR ENERGY

INTRODUCTION:
Solar energy is energy that comes from the sun.
Every day the sun radiates an enormous amount
of energy. The sun radiates more energy in one
second than people have used since the Hydgen Hydrogen
beginning of time. All this energy comes from -,
+ Energy
within the sun itself. Like other stars, the sun is \' 1 F Ener
a big gas ball made up mostly of hydrogen and Hydgen Hydrogen
helium. The sun generates energy in its core in
a process called nuclear fusion.
During nuclear fusion, the sun's extremely high NUCLEAR FUSION
pressure and hot temperature cause hydrogen
atoms to come apart and their nuclei to fuse or combine. Some matter is lost
during nuclear fusion. The lost matter is emitted into space as radiant energy.
It takes millions of years for the energy in the sun's core to make its way to the
solar surface, and then approximately eight minutes to travel the 93 million miles
to earth. The solar energy travels to the earth at a speed of 186,000 miles per
second, the speed of light.
Only a small portion of the energy radiated by the sun into space strikes the earth,
one part in two billion. Yet this amount of energy is enormous. Every day enough
energy strikes the United States to supply the nation's energy needs for one and a
half years! About 15 percent of the sun's energy that hits the earth is reflected
back into space. Another 30 percent is used to evaporate water, which, lifted into
the atmosphere, produces rainfall. Plants, the land, and the oceans also absorb
solar energy. The rest could be used to supply our needs.

APPLICATIONS


Photovoltaics:


Photovoltaic are solar cells that
produce electricity directly from
sunlight. The solar cells are made
of thin layers of material, usually
silicon. The layers, after treatment
with special compounds, have
either too many or too few
electrons. When light strikes a




















Solar Thermal:


sandwich of the different layers, electrons start flowing and an
electric current results.
Photovoltaic are used throughout the nation and elsewhere to
operate appliances, provide lighting, and to power navigation
and communication aids. Photovoltaic panels provide power
for equipment in space ships and satellites. PV cells supply
power needed to operate many kinds of consumer products such
as calculators and watches. Photovoltaic systems provide
electricity to remote villages, residences, medical centers, and
other isolated sites where the cost of photovoltaic equipment is
less than the expense of extending utility power lines or using
diesel-generated electricity.

Solar Thermal

energy obtained
by exposing a
collecting device
to the rays of the
sun. A solar
thermal system
makes use of the
warmth absorbed
by the collector to -
heat water or another working fluid, or to make steam. Hot
water is used in homes or commercial buildings and for
industrial processes. Steam is used for process heat or for
operating a turbine generator to produce electricity or industrial
power.
There are several basic kinds of solar thermal power systems
including "flat plate" solar water heaters; concentrating
collectors, such as central tower receivers; and parabolic trough
and dish collectors.
Flat plate solar water heaters Water flows through tubes
that are attached to a black metal
absorber plate. The plate is
enclosed in an insulated box with a
transparent window to let in
sunlight. The heated water is
transferred to a tank where it is
available for home, commercial or
institutional use.
Central tower receivers In order to produce steam and
electricity with solar thermal energy, central receivers have a



















Solar Stills:


field of tracking mirrors called heliostats to focus sunlight onto
a single receiver mounted on a tower. Water or other heat
transfer fluid in the tower is heated and used directly or
converted into steam for electricity.
Parabolic dishes or troughs curved panels which follow the
direction of the sun's rays and focus the sunlight onto receivers.
A liquid inside the pipes at the receivers' focal point absorbs
the thermal energy. The thermal energy received can be
converted to electricity at each unit or transported to a central
point for conversion to electricity.


Solar stills are systems designed to filter or purify water. The
number of systems designed to filter water have increased
dramatically in recent years. As water supplies have increased
in salinity, have been contaminated, or have experienced
periods of contamination, people have lost trust in their
drinking water supply. Water filtration systems can be as
simple as a filter for taste and odor to complex systems to
remove impurities and toxins. Solar water distillation is one of
the simplest and most effective methods of purifying water.
Solar water distillation replicates the way nature purifies water.
The sun's energy heats water to the point of evaporation. As the
water evaporates, purified water vapor rises, condensing on the
glass surface for collection.

This process removes impurities such as salts and heavy metals,
as well as destroying microbiological organisms. The end result
is water cleaner than the purest rainwater.

Solar energy is allowed into the collector to heat the water. The
water evaporates only to condense on the underside of the glass.
When water evaporates, only the water vapor rises, leaving
contaminants behind. The gentle slope of the glass directs the
condensate to a collection trough, which in turn delivers the
water to the collection bottle.



3 -R






Solar Crop Dryers:


Using the sun to dry crops and grain is one of the oldest
and most widely used applications of solar energy. The
simplest, and least expensive technique is to allow crops
to dry naturally in the field, or to spread grain and fruit
out in the sun after harvesting. The disadvantage of these
methods is that the crops and grain are subject to damage
by birds, rodents, wind, and rain, and contamination by
windblown dust and dirt. More sophisticated solar dryers
protect grain and fruit, reduce losses, dry faster and more
uniformly, and produce a better quality product than open
air methods.

The basic components of a solar dryer are an enclosure or
shed, screened drying trays or racks, and a solar
collector. In hot, arid climates the collector may not even
be necessary. The southern side of the enclosure itself
can be glazed to allow sunlight to dry the material. The
collector can be as simple as a glazed box with a dark
colored interior to absorb the solar energy that heats air.
The air heated in the solar collector moves, either by
natural convection or forced by a fan, up through the
material being dried. The size of the collector and rate of
airflow depends on the amount of material being dried,
the moisture content of the material, the humidity in the
air, and the average amount of solar radiation available
during the drying season.

There are a relatively small number of large solar crop
dryers in the United States. This is because the cost of the
solar collector can be high, and drying rates are not as
controllable as they are with natural gas or propane
powered dryers. Using the collector at other times of the
year, such as for heating farm buildings, may make a
solar dryer more cost-effective. It is possible to make
small, very low cost dryers out of simple materials. These
systems can be useful for drying vegetables and fruit for
home use.




















ADVANTAGES AND DISADVANTAGES OF SOLAR ENERGY

Advantages: Solar energy makes use of a renewable natural
resource that is readily available.


Solar power used by itself creates no carbon dioxide or other
toxic emissions.

Use of solar thermal power to heat water or generate electricity
will help reduce the Territory's complete dependence on fossil
fuels.

Solar water heaters are an established technology, readily
available on the commercial market, and simple enough to
build, install and maintain by yourself.
The production of electricity by the photovoltaic process is
quiet and produces no toxic fumes.

PV cells generate direct-current electricity that can be stored in
batteries and used in a wide range of voltages depending on the
configuration of the battery bank.

Although most electric appliances operate on alternating
current, an increasing number of appliances using direct current
are now available. Where these are not practical, PV-generated
direct current can be changed into alternating current by use of
devices called inverters.






Disadvantages: Solar thermal systems are not cost-effective in areas that have
long periods of cloudy weather or short daylight hours.

The arrays of collecting devices for large systems cover
extensive land areas.

Solar thermal systems only work with sunshine and do not
operate at night or in inclement weather. Storage of hot water
for domestic or commercial use is simple, using insulated tanks,
but storage of fluids at the higher temperatures needed for
electrical generation, or storage of electricity itself, needs
further technical development.

Photovoltaic-produced electricity is presently more expensive
than power supplied by utilities.

Batteries need periodic maintenance and replacement.

High voltage direct-current electricity can pose safety hazards
to inadequately trained home operators or utility personnel.

ADDITIONAL RESOURCES
Photovoltaic Program: www.eren.doe.gov/pv/
www.nrel.gov/ncpv/
Solar Thermal Program: www.eren.doe.2ov/ste/
www.eren.doe.2ov/csp
www.sandia.2ov/Renewable Ener2y/solarthermal/nsttf.html
American Solar Energy Society: www.ases.org/solar




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