The Sun has been radiating enormous energy for billions of
years without showing any signs of getting cold. Where does
this energy come from? It is now known that the most important
source of energy of the Sun is the nuclear burning of protons
to create helium.
(The Sun - This photo was downloaded
from NASA Discovery Missions Genesis website http://discovery.nasa.gov/genesis.html)
The first step in energy production in stars is the fusion
of four hydrogen nuclei to make one helium nucleus. There
are two chains of reactions by which the conversion of hydrogen
to helium is effected: the proton-proton cycle and the carbon-nitrogen-oxygen
cycle (sometimes referred to simply as the carbon cycle).
They were both first studied and proposed as sources of stellar
energy by H. Bethe (1906-) and independently by C. von Weiszacker
(1912-). The proton-proton cycle operates in less massive
and luminous stars like the Sun, while the carbon-nitrogen-oxygen
cycle (which speeds up dramatically at higher temperatures)
dominates in more massive and luminous stars.
In the proton-proton cycle, two hydrogen nuclei (protons)
are fused and one of these protons is converted to a neutron
by beta decay to make a deuterium nucleus (which consists
of one proton and one neutron). An electron and a very light
particle called neutrino are also emitted during the process.
Then a third proton is added to deuterium to form the light
isotope of helium, helium-3,  .
When two nuclei collide, they form a nucleus of ordinary helium,
helium-4, (two
protons and two neutrons), and release two protons (refer
to figure 1).
(Figure 1)
During fusion, a tiny amount of mass is lost. One helium
atom weighs just a little bit less than two hydrogen atoms.
That little bit of mass is transformed into an enormous amount
of energy, mainly infrared and visible light, which radiates
in all directions through space. But only a very small fraction
of solar radiation (one part in two billion) reaches the Earth.
Even so, the Sun is the source of almost all the energy on
Earth, including our food and our fuel.
Solar energy, like all radiant energy, travels through space
in electromagnetic waves. What happens to the tiny fraction
of the Sun's energy that reaches the Earth?
- About 30% is reflected back into space by clouds,
deserts, and snow.
- Some, in the ultraviolet part of the electromagnetic
spectrum, is absorbed by the Earth's ozone layer.
- Some drives the water cycle: Solar energy is absorbed
by water which evaporates and later falls back to earth
as rain and snow.
- Some drives the winds: Solar energy heats up the Earth's
surface and the blanket of air surrounding it. Since the
air is heated unevenly, currents of hot air expand upward
while cool air moves down to take its place. The Sun is
the source of all our weather.
- Some is captured and stored by green plants and green
one-celled organisms in the process called photosynthesis.
The Sun is the source of food for almost all life on Earth.
In the 1830s, the British astronomer John Herschel used a
solar collector box to cook food during an expedition to Africa
. Now, people are trying to use the Sun's energy for lots
of things. Solar technologies use the Sun's energy and light
to provide heat, light, hot water, electricity, and even cooling,
for homes, businesses, and industry.
(Figure 2 Solar Thermal Collector
- This photo was downloaded from APS Project Sol website
http://projectsol.aps.com/
Photo Courtesy: APS)
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Solar thermal technology
uses mirrors to concentrate the sun's heat. The heat
is used to boil water which drives a steam turbine.
Solar radiation can also be used to heat a gas which
drives a piston. The kinetic energy of the moving turbine
or piston is then converted into electrical energy.
Solar thermal energy is a large scale application, used
by a power plant in place of nuclear or fossil fuels.
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(Figure 3 Photovoltaic Cell Panels
- This photo was downloaded
from APS Project Sol website http://projectsol.aps.com/
Photo Courtesy: APS ) |
Photovoltaic technology uses
devices called solar cells which can change light energy
directly into electrical energy. This technology is
used in small applications like calculators and wrist-watches;
in medium-sized arrays that power highway lighting,
recreational vehicles, space stations and other electrical
systems not connected to power lines, and in large arrays
owned and operated by power companies. No mechanical
means are employed in solar heating. Incorporating solar
designs can reduce heating bills as much as 50 percent.
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Photovoltaic solar cells, which directly convert sunlight
into electricity, are made of semiconducting materials. Photovoltaic
stands for photo (light) and voltaic (electricity), whereby
sunlight photons free electrons from common silicon. The photovoltaic
cell was discovered in 1954 by Bell Telephone researchers,
examining the sensitivity of a properly prepared silicon wafer
to sunlight. Initially it has been used primarily for applications
in space research . The Hubble telescope and the international
space station utilize solar panels for their energy requirements.
The photovoltaic (PV) cells are made of special materials
called semiconductors such as silicon, which is currently
the most commonly used. Basically, when light strikes the
cell, a certain portion of it is absorbed within the semiconductor
material. This means that the energy of the absorbed light
is transferred to the semiconductor . The energy knocks electrons
loose, allowing them to flow freely. PV cells also have one
or more electric fields that act to force the electrons freed
by light absorption to flow in a certain direction. This flow
of electrons is an electric current. By placing metal contacts
on the top and bottom of the PV cell, we can draw that current
off to use it externally. For example, the current can power
a calculator. This current, together with the cell's voltage
(which is a result of its built-in electric field or fields),
defines the power (or wattage) that the solar cell can produce.
The performance of a photovoltaic array is dependent upon
sunlight. Climate conditions (e.g., clouds, fog) have a significant
effect on the amount of solar energy received by a PV array
and, in turn, on its performance. Most "current technology"
photovoltaic modules are about 10 percent efficient in converting
sunlight to electricity with further research being conducted
to raise this efficiency to 15 percent.
Photovoltaic conversion is useful for several reasons. Conversion
from sunlight to electricity is direct, so that bulky mechanical
generator systems are unnecessary. The modular characteristic
of photovoltaic energy allows arrays to be installed quickly
and in any size required or allowed.
Also, the environmental impact of a photovoltaic system is
minimal, requiring no water for system cooling and generating
no by-products. Hence solar energy is the cleanest energy
source on Earth.
The major drawbacks (problems, or issues to overcome) of
the solar energy are: (1) the intermittent and variable manner
in which it arrives at the earth's surface and, (2) the large
area required to collect it at a useful rate.
The success of solar power will depend on the answer to the
following question: 'What do you do when the sun goes down?
The simplest answer is to build an auxiliary system that
will store energy when the sun is out. However, the problem
is that such storage systems are unavailable today. Simple
systems, like water pipes surrounded by vacuum, do exist.
They are based on the concept that provided the pipes are
insulated, the water will store thermal energy.
(Figure 4) |
There were 14 solar thermal
electric units operating in the US at the end of 2001,
with more on the way. Most of these are in California,
though Nevada, Arizona, Texas, and Virginia have them,
too. The 30-megawatt Solar Electric Generating Systems
(SEGS) are operated by The Kramer Junction Company,
and consist of five devices. Each of these SEGS comprises
150 to 354 megawatts of installed parabolic converters,
trough which solar thermal energy is converted into
electric energy at a capacity located in California's
Mojave Desert. The combined California facilities produce
more than 90% of the world's commercially available
solar thermal electric power.
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