The sun is the source of nearly all (99.97%) renewable energy on Earth. Not only solar
energy directly – wind, hydroelectric, biomass, and all other renewable energies except
geothermal get their energy from the sun. The total solar energy absorbed by the Earth
in one hour is more energy than the world uses in one year. The amount of solar energy
reaching the surface of the planet in one year is about twice as much as will ever be
obtained from all of the Earth's non-renewable resources of coal, oil, natural gas, and
mined uranium combined.
About 30% of the solar radiation received by the Earth is reflected back to space by the
upper atmosphere. The rest is absorbed by clouds, oceans and land masses, heating
them. Warm air containing evaporated water rises, driving atmospheric circulation
(wind). This air cools when it reaches a high altitude, and the water vapor condenses
into clouds, which rain onto the earth's surface, completing the water cycle. The
conversion of solar energy into chemical energy by photosynthesis produces food,
wood, and the biomass from which fossil fuels are derived.
Since ancient times, solar energy has been used by humans in many ways. There are
two basic types of solar technologies: passive and active. Passive solar techniques are
architectural. Using materials that can absorb heat is a passive solar technique. Spaces
are designed to circulate air naturally. Buildings are positioned to the sun in a way that
minimizes energy requirements for heating or cooling. Clotheslines, which dry clothes
through evaporation by wind and sunlight without consuming electricity or gas, are
another example of passive solar technology. Active solar techniques convert sunlight
into electricity or other useful power. Active solar technologies generate energy, while
passive solar technologies reduce the need for energy.
Solar thermal technologies can be used for water heating, space heating or cooling, and
process heat generation. Solar hot water systems use sunlight to heat water. Solar
cookers use sunlight for cooking, drying and pasteurization. Solar concentrating
technologies, such as parabolic dishes and other reflectors, can provide process heat
for commercial and industrial applications, such as providing power for factories.
Sunlight can be converted into electricity using photovoltaics (PV), concentrating solar
power, and various experimental technologies. PV have mainly been used to power
small and medium-sized applications, from a calculator powered by a single solar cell to
homes powered by a photovoltaic array, but there is a trend toward larger PV power
stations in the US and Europe.
A solar cell, or photovoltaic cell, converts light into direct current using the photoelectric
effect. They were first made in the 1880s. The earliest significant use of solar cells was
as a power source for satellites, where they are still used today. The high cost of solar
cells limited their use on Earth until the early 1970s, when prices fell to levels that made
PV generation more competitive. The 1973 oil crisis also caused a rapid rise in the
production of PV during the 1970s and early 1980s. The price of PV fell during this time,
from $100/watt in 1971 to $7/watt in 1985. Cheaper oil from the early 1980s, however,
slowed growth of the PV industry to about 15% per year from 1984 through 1996.
Some experts believe photovoltaics could cover all the world energy demand.
Photovoltaics are 85 times as efficient as growing corn for ethanol. A one-hectare plot of
adapted from WIKIPEDIA for abcteach
land can produce enough ethanol in one year to drive a car 30,000 miles. Covering the
same land with photo cells can produce enough electricity to drive it 2,500,000 miles.
One experimental idea is the solar updraft tower (also known as a solar chimney or
solar tower), a large greenhouse that funnels into a central tower. As sunlight shines on
the greenhouse, the air inside is heated, and expands. The expanding air flows up the
central tower, past a turbine that converts the air flow into electricity.
Space solar power systems, another experimental idea, would use a large solar array in
geosynchronous orbit to collect sunlight and beam this energy in the form of microwave
radiation to Earth for distribution. In theory, these systems could deliver power about
96% of the time, an advantage over other solar energy sources.
Developing a solar-powered car has been an engineering goal since the 1980s. The
World Solar Challenge is a solar-powered car race, where teams compete to cross
central Australia. In 1987, when it started, the winner's average speed was 42 mph. By
2007, the winner's average speed had improved to 56.46 mph. International interest in
the development of other solar-powered vehicles is also growing. Solar boats have
sailed across the Pacific and Atlantic oceans, and in 1974, the unmanned Sunrise II
plane made the first solar flight. On April 29, 1979, the Solar Riser made the first flight in
a solar-powered, fully controlled, manned flying machine.
Storage is an important issue in the development of solar energy. Solar energy is
available only when the sun shines, not at night or in inclement weather, so storage or
back-up power systems must be used. One method of storing solar power is to use
thermal mass systems. Thermal mass systems store solar energy in the form of heat at
domestically useful temperatures for daily or seasonal durations. Thermal storage
systems generally use common materials with high specific heat capacities such as
water, earth and stone. Such systems can lower peak demand, shift time-of-use to off-
peak hours and reduce overall heating and cooling requirements.
Solar energy can also be stored at high temperatures using molten salts. Salts are an
effective storage medium because they are low-cost, have a high specific heat capacity,
and can deliver heat at temperatures compatible with conventional power systems.
Some PV systems often use rechargeable batteries to store excess electricity. A
photogalvanic device is a type of battery in which the cell solution forms energy-rich
chemical intermediates when illuminated, and can store and produce electricity.
Pumped-storage hydroelectricity stores energy in the form of water pumped when
energy is available from a lower elevation reservoir to a higher elevation one. The
energy is recovered when demand is high by releasing the water to run through a
hydroelectric power generator.
It would seem that solar power could supply all of our energy needs, but there are some
drawbacks. As with wind power, solar power is an intermittent resource. Time will tell if
ways can be found to solve these problems and enable us to use these abundant
energy sources on a large scale.
adapted from WIKIPEDIA for abcteach