|
HOW IT
WORKS
PV technology converts
sunlight directly into electricity. It works any time the sun is shining,
but more electricity will be produced when the light is more intense (a
sunny day) and is striking the PV modules directly (when the rays of
sunlight are perpendicular to the PV modules). Unlike solar systems for
heating water, PV technology does not use the sun's heat to make
electricity. Instead, PV produces electricity directly from the electrons
freed by the interaction of sunlight with semiconductor materials in the PV
cells. In order to produce electricity, however, PV cells need to be
oriented to the solar window or they will not receive enough
incidental sunlight to excite the cells to make electricity. The
solar window is, depending on where you are in the world (lets say the
northern hemisphere), usually facing south, at approximately a 30° angle
from the horizontal. The graphic at the right shows the approximate
geometry which describes the solar window.
SOLAR CELL CONSTRUCTION
Solar cells,
composed of semi-conductor materials such as silicon, are the basic building
block of PV technologies. An individual PV cell typically produces between 1
and 2 Watts, hardly enough to meet household needs. To increase the power
output, cells are commonly connected to form larger PV modules. Modules,
which are also sometimes called panels, range in power output from about 10
Watts to 300 Watts for residential and business power applications. A
typical PV module consists of a protective weatherproof enclosure for the
semi-conductor materials and the electric wiring needed to connect the
module with the rest of the system. By connecting modules together, system
designers can create PV arrays that have power outputs of 15,000 Watts
(15kW) or more. In Vermont, 15kW is the current size limit for PV systems to
be eligible for net metering.
BENEFITS OF
SOLAR
But you don't need to
understand the detailed physics of how PV works to understand its appeal:
investing in PV allows you to produce your own electricity with no noise, no
air pollution, and no moving parts while using a clean, renewable resource.
A PV system will never run out of fuel, and it won't increase our oil
imports from overseas. In fact, it may not even contribute to the trade
deficit, because many PV system components are manufactured in the United
States. Due to these unique characteristics, PV technology has been called
"the ultimate energy source for the 21st century."
Global warming and the frequency and severity
of the atmospheric disturbances it causes can be directly tied to the
quantity of carbon dioxide in the atmosphere at any one time. There
are many types of atmospheric disturbances, but the ones most devastating to
man are the typhoons, severe hurricanes, tornadoes, cyclones, and tropical
storms of the Pacific Ocean. Note the far right of the graph...that
represents current condition of the atmosphere. Can you recall now
many hurricanes in our own country have wreaked havoc on our coastlines and
how many tornadoes are now appearing in places where tornadoes have not
historically appeared? If we don't individually take responsibility at
some point, the earth will take care of itself and sweep away with violent
storms, the polluters of its atmosphere.
The earth will balance itself out...all we
have to do is stop adding to the problem and the atmospheric CO2 will
decline to normal planetary levels and the atmosphere will gradually cool to
normal levels. Then normal rain patterns will return and the dry
barren parts of the earth will be green and lush again and the severe storms
will abate.
PV POWER PRODUCTION
PV systems produce power
intermittently because they work only when the sun is shining. This is not a
problem for PV systems connected to the utility grid, because additional
electricity you need is automatically delivered to you by your utility.
Systems that are independent of the grid use battery banks to provide power
when the sun is not out.
The graphic to the right
illustrates typical arrangements for (a) a direct current off-grid, and (b)
an alternating current grid-connected, residential PV system.
GRID-TIED SYSTEMS
A PV system that is
independent of the grid typically consists of a battery bank and charge
controller. This type of system, a) in the following graphic, can be used to
provide direct-current (DC) power, or with an inverter, can be used to
supply power for alternating current (AC) loads. A PV system tied to the
utility grid, b) in the following graphic, typically consists of one or more
PV modules connected to an
inverter (or power conditioner) that changes the system's DC output to AC,
which is compatible with the utility grid. You may include batteries in the
system to provide reliable back-up power in case your utility experiences
power outage.
THE COST OF SOLAR
Historically, PV-generated electricity has usually
been more expensive than conventional utility-supplied electricity. Improved
manufacturing has reduced the cost to less than one percent of what it was
in the 1970s, but the cost (amortized over the life of the system)
is now about 12 cents per kilowatt-hour. This is about two and a
half times the retail price that Vermont residents now pay for electricity
from their utilities. Financial incentives can help make PV more affordable,
and with today's continually rising utility costs it can match and exceed
today's price for electricity from your utility.
Unlike electricity
purchased month by month from a utility, PV power comes with a high initial
investment and no monthly charge thereafter. This means that buying a PV
system is like paying years of electric bills up front. You'll probably
appreciate the reduction in your monthly electric bills, but the initial
expense may be significant. By financing your PV system, you can spread the
cost over many years, and incentives can also lighten the strain on your
wallet
|
