Solar Energy
The term solar power is used to describe a number of methods of
harnessing energy from the light of the Sun. It has been used in many
traditional technologies for centuries and has come into widespread
use where other power supplies are absent, such as in remote
locations and in space. Its use is spreading as the environmental
costs and limited supply of other power sources such as fossil fuels
are realized.
Energy from the Sun
Theoretical annual mean insolation, at the top of Earth's atmosphere
and at the surface on a horizontal square meter .
Global solar energy resources. The colors in the map show the actual
local solar energy, averaged through the years of 1991-1993. The
scale is in watts per square meter.
The land area required to supply the current global primary energy
demand by solar energy using available technology is represented by
the dark disks.The rate at which solar radiation reaches a unit of area
in space in the region of the Earth's orbit is 1,366 W/m , as measured
upon a surface normal (at a right angle) to the Sun. This number is
referred to as the solar constant.[1] The atmosphere reflects 6% and
absorbs 16% of incoming radiation resulting in a peak power at sea
level of 1,020 W/m . [2] [3] Average cloud cover reduces incoming
radiation by 20% through reflection and 16% through absorption.[4]
The image on the right shows the average solar power available on the
surface in W/m calculated from satellite cloud data averaged over
three years from 1991 to 1993 (24 hours a day). For example, in
North America the average power of the solar radiation lies
somewhere between 125 and 375 W/m , between 3 and 9 kWh/m
/day.
It should be noted that this is the maximum available power, and not
the power delivered by solar power technology. For example,
photovoltaic panels currently have an efficiency of ca. 15% and,
hence, a solar panel delivers 19 to 56 W/m or 0.45-1.35 kWh/m /day
(annual day and night average). The dark disks in the image on the
right are an example for the land areas that, if covered with solar
panels, would produce slightly more energy in the form of electricity
than the total primary energy supply in 2003. [6] That is, solar cells
with an assumed 8% efficiency installed in these areas would deliver
a bit more energy in the form of electricity than what is currently
available from oil, gas, hydropower, nuclear power, etc. combined.
It should also be noted that a recent concern is that of Global
dimming, an effect of pollution that is allowing less and less sunlight
to reach the Earth's surface. It is intricately linked with pollution
particles and Global warming, and is mostly of concern for issues of
Global climate change, but is also of concern to proponents of Solar
Power due to the existing and potential future decreases in available
Solar Energy. The order of magnitude is about 10% less solar energy
available at sea level, mostly due to more intense cloud reflections
back into outer space. That is, the clouds are whiter and brighter
because the pollution dust serves as a vapor-liquid phase change
initiation site and generates clouds where otherwise there would be a
moisture filled but otherwise clear sky.
After passing through the Earth's atmosphere, most of the sun's
energy is in the form of visible and Infrared radiations. Plants use
solar energy to create chemical energy through photosynthesis.
Humans regularly use this energy burning wood or fossil fuels, or
when simply eating the plants.
Solar design in architecture
Solar design can be used to achieve comfortable temperature and
light levels with little or no additional energy. This can be through
passive solar, where maximising the entrance of sunlight in cold
conditions and reducing it in hot weather; and active solar, using
additional devices such as pumps and fans to direct warm and cool
air or fluid.
Solar heating systems
Solar hot water systems use sunlight to heat water. These systems
may be used to heat domestic hot water or for space heating but are
most commonly used to heat pools. These systems are basically
composed of solar thermal collectors and a storage tank. The three
basic classifications of solar water heaters are:
Active systems which use pumps to circulate water or a heat transfer
fluid.
Passive systems which circulate water or a heat transfer fluid by
natural circulation. These are also called thermosiphon systems.
Batch systems using a tank directly heated by sunlight.
A Trombe wall is a thermal mass that is heated by sunlight during the
day and radiates stored heat during the evening.
Solar cookingr
Pictured: Solar CookerA solar box cooker traps the Sun's power in an
insulated box; such boxes have been successfully used for cooking,
pasteurization and fruit canning. Solar cooking is helping many
developing countries, both reducing the demands for local firewood
and maintaining a cleaner environment for the cooks. The first known
western solar oven is attributed to Horace de Saussure.
Photovoltaics
The solar panels (photovoltaic arrays) on this small yacht at sea can
charge the 12 V batteries at up to 9 Amps in full, direct sunlightSolar
cells, also referred to as photovoltaic cells, are devices or banks of
devices that use the photovoltaic effect of semiconductors to generate
electricity directly from sunlight. Until recently, their use has been
limited due to high manufacturing costs. One cost effective use has
been in very low-power devices such as calculators with LCDs.
Another use has been in remote applications such as roadside
emergency telephones, remote sensing, cathodic protection of pipe
lines, and limited "off grid" home power applications. A third use has
been in powering orbiting satellites and other spacecraft.
Total peak power of installed PV is around 5,300 MW as of the end of
2005. This is only one part of solar-generated electric power. For solar
reflector plants see below.
Declining manufacturing costs (dropping at 3 to 5% a year in recent
years) are expanding the range of cost-effective uses. The average
lowest retail cost of a large photovoltaic array declined from $7.50 to
$4 per watt between 1990 and 2005. With many jurisdictions now
giving tax and rebate incentives, solar electric power can now pay for
itself in five to ten years in many places. "Grid-connected" systems -
that is, systems with no battery that connect to the utility grid through
a special inverter - now make up the largest part of the market. In
2004 the worldwide production of solar cells increased by 60%. 2005
is expected to see large growth again, but shortages of refined silicon
have been hampering production worldwide since late 2004.
Solar fibers
A photovoltaic device not using silicon is currently in development.
The device consists of a "solar tape," containing titanium dioxide
(TiO2) in the form of a tape or fiber that could be combined with
clothing or building materials. The material has inferior efficiency to
conventional photovoltaics (5% for an initial commercial version to
"near 12%" in the lab as of 2004, versus 15-30% for conventional
cells). Its advantages are its low manufacturing cost, low weight,
flexibility, function in artificial light, and resulting versatility.
Written by: Tejus Mathew
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