Solar Energy Overview
Solar energy is usually divided into two categories,
although they can be employed together in solar installations. Solar
thermal energy is generated from heat and employs heat directly to heat
water, the ambient temperature in buildings, or steam to power
electricity generators. Solar photovoltaic electricity is generated from
light, employing photovoltaic modules or cells, which convert sunlight
into electricity using cells with semi conductors. Solar thermal
technologies use the sun's heat. They include non-grid solar thermal
technologies; water heating systems, solar cookers, solar drying
applications and solar thermal building designs. These technologies help
to conserve energy in heating and cooling applications. Solar thermal
devices use direct heat from the sun, concentrating it to produce heat
at useful temperatures. As with many other advances in the energy
sector, modern solar thermal industry began with the oil embargo of
1973-1974 and was strengthened with the second embargo in 1979. In the
early 1980's, a 354 MW solar power plant was built in the Mojave Desert,
in California. The heat contained in solar rays, concentrated by
reflecting troughs and raised to 400oC, produces steam that runs a
conventional power generator. When the sun is not shining, the plant
switches to natural gas. The latest generation of this type of plant
incorporates new engineering solutions and new scientific principles
such as non-imaging optics, which makes it possible to build much more
efficient concentrators at lower costs. Solar thermal technology has
many applications both for grid-connected power generation, in isolated
locations where grid connected electricity is not viable, and in
domestic and commercial situations.
Solar photovoltaics or solar PV are solid-state semiconductor devices
that convert light directly into electricity. Solar PV's are mostly made
of silicon with traces of other elements and are closely related to
transistors, LEDs and other electronic devices. The electricity is
direct current but can be converted to alternating current or stored for
later use.
Bell Telephone researchers discovered the PV cell in 1954 when examining
the sensitivity of a properly prepared silicon wafer to sunlight. From
the late 1950s, PVs were used to power US space satellites, which
generated commercial applications for PV technology. The simplest PV
systems power many of the small calculators and watches in everyday use.
More complex systems provide electricity in off-grid applications and
generate electricity for the grids. Advanced technology is required to
manufacture PV cells and modules, but the cells themselves are simple to
use. PV modules are usually low-voltage DC devices with no moving or
wearing parts, although arrays of PV modules can be wired for higher
voltages. Once installed, a PV array does not require much maintenance
except for an occasional cleaning, and even that is not imperative. Most
PV systems contain storage batteries, which require some water and
maintenance similar to that required by the battery in a car.
A solar cell consists of layers of semiconductor materials with
different electronic properties. In a typical solar crystalline silicon
cell, most of the material is silicon. The silicon is doped with a small
quantity of boron to give it a positive or p-type character. A thin
layer on the front of the cell is doped with phosphorous to give it a
negative or n-type character. The interface between these two layers
contains an electric field and is called a junction. Light consists of
particles called photons and when the light hits the solar cell, some of
the photons are absorbed in the region of the junction, freeing
electrons in the silicon crystal. If the photons have enough energy, the
electrons are able to overcome the electric field at the junction and
are free to move through the silicon and into an external circuit. As
they flow through the external circuit they give up their energy as
useful work (turning motors, lighting lamps, etc.) and return to the
solar cell. The photovoltaic process is entirely solid-state and
self-contained; there are no moving parts and no materials are consumed
or emitted.
About the author
A graduate of Cambridge University, Euan Blauvelt was trained in market research in London, later moving to Southeast Asia for twelve years where he was responsible for many research studies for a wide range of industries and governments. On his return to London he was a co-founder of ABS Energy Research seventeen years ago, which specializes in energy and environmental services market research .