SOLAR ENERGY
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Physics
Today Article on Photovoltaics
How does solar
energy work?
Solar Electric or Photovoltaic Systems convert some of the energy in sunlight
directly into electricity. Photovoltaic (PV) cells are made primarily of silicon, the
second most abundant element in the earth's crust, and the same semiconductor material
used for computers. When the silicon is combined with one or more other materials, it
exhibits unique electrical properties in the presence of sunlight. Electrons are excited
by the light and move through the silicon. This is known as the photovoltaic effect and
results in direct current (DC) electricity. PV modules have no moving parts, are virtually
maintenance-free, and have a working life of 20 - 30 years.
(click
here to view
a system diagram)
PV cells, the smallest unit of a PV system, are wired together
to form modules.
Modules are usually a sealed, or encapsulated, unit of
convenient size for handling.
Modules are wired together to form panels.
Groups of panels form arrays.
A number of arrays form an array field.
The total system includes the arrays and any other
equipment like charge controllers, storage (batteries) and tracking and monitoring
equipment, collectively called balance of system (BOS) components.
History of Photovolatics
The history of PV's dates back to 1839 and major developments evolved as follows:
1839 Edmund Becquerel, a French physicist observed the
photovoltaic effect.
1880's Selenium PV cells were built that converted
light in the visible spectrum into electricity and were 1% to 2% efficient. Light sensors
for cameras are still made from selenium today.
In the early 1950's the Czochralski meter was developed
for producing highly pure crystalline silicon.
In 1954 Bell Telephone Laboratories produced a silicon
PV cell with a 4% efficiency and later achieved 11% efficiency.
In 1958 the US Vanguard space satellite used a small
(less than one watt) array to power its radio. The space program has played an important
role in the development of PV's ever since.
During the 1973-74 oil embargo the US Department of
Energy funded the Federal Photovoltaic Utilization Program, resulting in the installation
and testing of over 3,100 PV systems, many of which are in operation today.
The 1970s through the 1990s have seen a relative
disinterest in solar power with majority ownership of many United States PV manufacturers
transferring to German and Japanese interests.
The Gulf war of 1990 again sparked Americas interest in
non-fossil fuel energy alternatives.
International markets for solar take off in the mid 1990s.
Glossary
of Solar and Photovoltaic Terms
Cell efficiency - The ratio of the electrical energy produced by a photovoltaic
cell (under full sun conditions or 1 kW/m2) to the energy from sunlight falling upon the
cell.
Charge controller - A component that controls the flow of current to and from the battery
subsystem to protect the batteries from overcharge and over discharge. The charge
controller may also monitor system performance and provide system protection.
Diffuse radiation - Sunlight received indirectly as a result of scattering due to clouds,
fog, haze, dust or other substances in the atmosphere.
Direct radiation - Light that has traveled in a straight path from the sun (also referred
to as beam radiation). An object in the path of direct radiation casts a shadow on a clear
day.
Flat-plate array - A photovoltaic array in which the incident solar radiation strikes a
flat surface and no concentration of sunlight is involved.
Fresnel Lens - A concentrating lens, positioned above and concave to a PV material to
concentrate light on the material.
Grid-connected - An energy producing system connected to the utility transmission grid.
(Also called utility interactive.)
Hybrid system - A power system consisting of two or more power generating subsystems
(e.g., the combination of a wind turbine and a photovoltaic system).
Insolation - The amount of sunlight reaching an area, usually expressed in watts per
square meter per day.
Load - Electrical power being consumed at any given moment. The load that an electric
generating system supplies varies greatly with time of day and to some extent season of
year. Also, in an electrical circuit, the load is any device or appliance that is using
power.
Parallel connected - A method of connection in which positive terminals are connected
together and negative terminals are connected together. Current output adds and voltage
remains the same. (See also series connected.)
Photovoltaic cell - The semiconductor device that converts light into dc electricity. The
building block of photovoltaic modules.
Series connected - A method of connection in which the positive terminal of one device is
connected to the negative terminal of another. The voltages add and the current is limited
to the least of any device in the string. (See also parallel connected.)
Solar constant - The rate at which energy is received from the sun just outside the
earth's atmosphere on a surface perpendicular to the sun's rays. Approximately equal to
1.36 kW/m2.
Thick cells - Conventional cells, such as crystalline silicon cells, which are typically
from 4 to 17 mils thick. In contrast, thin-film cells are several microns thick.
Thin-film cells - Photovoltaic cells made from a number of layers of photo-sensitive
materials. These layers are typically applied using a chemical vapor deposition process in
the presence of an electric field.
Voltage regulator - A device that controls the operating voltage of a photovoltaic array.
Resources:
(1) From
THE AMERICAN SOLAR ENERGY SOCIETY
Photovoltaics (PV), a process whereby electricity is created directly by sunlight, holds exceptional promise now for the future.
Energy from the sun will, by photovoltaic transformation, supply an ever-increasing amount of the world's power needs, and this unending form of energy will be produced without degrading the environment.
Partly because of its perceived energy self-sufficiency, Canada has been slow to appreciate the significance of photovoltaics, however, other countries are very much alert to the benefits that electricity through PV can offer.
For example, Japan is targeting to have 30% of its future residential electricity requirements supplied by PV (specific goals of 250 MW by 2000 and 4,600 MW by 2010); the German target is 10% of its energy needs by 2010. Other developed countries, including the USA, are aiming at meeting 15% of their future residential electricity needs through PV - and all this within a time frame of 25-40 years. Developing countries which lie in the earth's sun belt, consider PV in many instances as providing the only means to provide rural electricity. These objectives alone are compelling reasons for Canrom to develop a fully integrated PV manufacturing and marketing facility capable of competing profitably on the world stage.
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