From Calculators to Power Plants: PV Systems in Action Photovoltaics systems are quite
different from traditional methods of generating electricity. Their
power production is directly affected by the weather and the time of
day – I. e. they can’t produce electricity without sunshine.
Ironically, photovoltaic cells are also affected by the sun’s heat
becoming less efficient at high temperatures. One important quality of
photovoltaics is their flexibility. Unlike nuclear plants, for
example, PV systems can be made small enough to power a hand-held
calculator, or large enough to power an entire community. When the
demand for electricity increases, a PV system can simply be
enlarged, provided the owner can afford it. Photovoltaic is a young
technology and important questions remain as to how it should best
be used. Among these questions are:
What kind of backup should be provided for nighttime or
cloudy days? should
solar systems be installed at homes or at special generating
stations? and should
utilities or individuals own and operate solar electric systems? Despite these questions,
photovoltaics are already used in hundreds of different ways.
Those applications fall into four broad categories:
stand-alone systems, grid-connected units, central utility stations,
and consumer products. Each
category is discussed below.
As the name implies, stand-alone
photovoltaic systems are virtually self-sufficient.
They provide all the electricity for a particular
application, operating without a utility-line backup.
They are typically small systems, generating less than 10
kilowatts of electricity. Stand-alone systems are most
commonly found in areas far from power lines.
Ranchers, for example, often install solar-powered water
pumps to replenish livestock watering holes in distant grazing
areas. These solar
systems do not require constant refueling like diesel generators and
often cost half as much as power line extensions. More than 1200 Arizona homes or
cabins rely on stand-alone photovoltaics as their main source of
electricity. In particular, PV powered homes are becoming a common sight
on Arizona’s Native American reservations.
More than 140 homes on the Navajo Reservation get their
electricity from the sun, and the Hopis are working toward 350 such
homes. The Hopis do not
allow power lines to enter their villages and have relied for years
on diesel generators or batteries, or have simply lived without
electricity. “Four villages have (utility)
power lines running 1/8 to 1/4 miles from the village”, said Doran
Dalton, sales manager for the Hopi Solar Electric Enterprise.
“We don’t hook up to the power lines because of our
long-standing tradition of self-sufficiency.
We don’t have an objection to electricity, we simply want
to own the source by which we get it. Most Hopis live with very little
electricity, and rely on small PV systems that power only a few
lights and perhaps a television set.
Families that consume more energy install larger PV systems
that provide electricity for all the conveniences of a modern home
– microwave ovens computers, stereo systems, washing machines,
evaporative coolers and lights.
Home PV systems can also power energy-consuming air
conditioners and clothes dryers but these sizeable PV systems can
cost more than $40,000. Despite their vast electricity resources, utility companies also use stand-alone photovoltaic systems instead of extending costly power lines. Many warning sirens at Palo Verde Nuclear Generating Station draw their power from solar panels. Photovoltaics also furnish electricity for mountaintop microwave repeater stations owned by Salt River Project. Stand-alone photovoltaics are
used for numerous other applications in Arizona, including:
nearly 100 monitoring stations owned by the U.S. Geological
Survey, some fire watch towers, a commercial radio station
transmitter near Prescott, numerous emergency roadside telephones,
billboards, lights and fans at the Picacho Peak Rest Area, and many
irrigation or watering system controls.
At a recreation area near Roosevelt Lake, photovoltaics
furnish all the electricity for indoor and outdoor lights and toilet
fans.
Homes or devices connected to
photovoltaic systems as well as utility power lines are called
“grid-connected systems.” The
utility can supply power at night, when electricity is cheapest, or
simply serve as a backup. Federal law mandates that utility
companies must purchase any excess power produced by the PV system,
although at a reduced rate. Homes or devices that employ
both types of power are not common in Arizona, and the biggest
reason is probably money. Photovoltaics
can be extremely cost-effective when compared to the price of
extending power lines. However,
for individuals who live in cities or already have grid electricity,
the choice to install PV is more difficult.
Homeowners must weigh the cost of utility bills – a few
hundred dollars a month at the most – against the price of a
photovoltaic system that can cost thousands of dollars for a typical
house and a typical energy lifestyle. As
the price of PV decreases, and the cost of utility power increases,
PV systems will compare more favorably for grid-connected
applications. In fact,
utility companies like Salt River Project are already preparing for
that possibility. SRP
has been researching a house in Chandler that employs electricity
from both the grid and a small PV system.
“Our experience is that (the PV system) is reliable.
It produces power day after day with very little
maintenance,” said Biff Hoffman, manager of SRP’s Research and
Development division. Individuals who are not
connected to the utility grid sometimes install utility lines to
power only one or two home items.
One Scottsdale couple lived without utility power for seven
years, relying entirely on a PV system and propane generator backup.
This off-grid system was sufficient to power an evaporative
cooler and the other electrical devices in their home.
When in 1990, temperatures reach 122 degrees Fahrenheit, they
decided to install an air conditioner.
To meet the unit’s electrical demand, the couple had to
choose between doubling the size of the solar system (another
$17,000), or paying for a power line at less than $8,000.
The rest, as they say, is history.
The same couple discovered an interesting problem that sometimes occurs with photovoltaic systems. Electricity provided by their PV system did not follow perfect sine-wave form, as does utility power. That change in the quality of electricity ruined their computer printer two times before the couple discovered the reason—a faulty inverter. Because of the occasional
problem in power quality, some utility companies require “power
conditioning systems” between the home PV system and the grid.
These systems serve two functions: they ensure that
electricity entering the grid is of the same quality as power
produced by the utility company, and, they can serve as an off
switch so that line workers are not hurt by electricity traveling
from the photovoltaic system. SRP
determined that power-conditioning systems were necessary from its
research on the Chandler house. Some photovoltaic systems are
connected to the grid for reasons not related to power requirements.
Bus stops in downtown Phoenix, for example, have a utility
line back up to satisfy the city’s insurance requirements.
One day, enormous fields of photovoltaic arrays may stand among the Saguaro and Ocotillo of Arizona’s deserts. Connected to the utility grid, their combined power may equal production at large coal plants such as the Navajo Generating Station near Page. Today, however, the world’s
largest central photovoltaic power plant generates a maximum of only
six megawatts of electricity – 125 times less than each of three
units at the Navajo station. This
plant, called the Carrisa Plant project, was built in 1984 by ARCO
Solar Corporation (now called “Siemens Solar”) and is connected
to the grid owned by Pacific Gas & Electric Company. The Carissa Plain plant covers dozens of acres of land with photovoltaic arrays mounted on two-axis trackers. Mirrors, placed next to the arrays, help reflect light to increase the potential power output. Unfortunately, the intense reflected light has partially destroyed the protective module coatings and has actually decreased production of electricity. Further development of better coatings should solve this problem. For those reasons, the Carissa
Plain plant is slated to be dismantled. Since, 1984, its only
revenue has come from electricity sales to Pacific Gas and Electric
Company. The
electricity is purchased at PG&E’s “avoided cost” of
producing electricity – a price even lower than wholesale.
In the meantime, worldwide demand for photovoltaic panels has
dramatically increased and their value has risen.
Consequently, it is more economical for the owners of Carissa
Plain to sell the individual modules than to sell the electricity
they produce. Near Sacramento, California is
another photovoltaic central power plant that has operated since
1984. The plant was built in two stages, called SMUDPV-1 and SMUD
PV-2 and together they generate up to two megawatts of electricity
at maximum production – enough to power 400 to 500 homes.
The total plant employs more than 58,000 photovoltaic modules
and occupies more than 20 acres of land. So far, three photovoltaic central power plants have been built in Arizona. One completed in 1982 at Phoenix’s Sky Harbor Airport, was once the world’s largest grid-connected photovoltaic power plant. It was designed to produce 225 kilowatts of power using concentrator solar modules. The plant was dismantled in 1987, when the lease was not renewed. APS has donated many of the
panels from the plant to Arizona high schools and others are still
being researched at the Solar Test and Research Center (STAR Center)
in Tempe. The Solar Test and Research
Center was established in 1998 to test the effectiveness of
different photovoltaic equipment in the Arizona climate.
It features five photovoltaic arrays, each producing 2
kilowatts of power used in the APS grid.
Many different types of photovoltaic cells are represented in
the arrays. Data obtained from the STAR
Center provides valuable information about photovoltaic systems and
how they operate. For
example APS has found that single-axis sun tracking systems improve
electrical output by about 20 percent and double-axis trackers
improve output another 20 percent.
They have also found that output
decreases about 10 percent in midsummer, when the weather is
hottest. Arizona’s other photovoltaic central power plant is not utility-operated. It belongs to a 24-home subdivision in Glendale, Arizona called “Solar One.” The first-of-its-kind subdivision was constructed by John F Long Homes, with a photovoltaic field along its south side. The 2600-panel system provides 192 kilowatts of electricity at peak output and provides much of the electricity used by homeowners during daylight hours. The utility company provides nighttime power and purchases any excess produced by the PV system. Until rates changed in 1991, some Solar One homeowners actually received refund checks from the utility company. Photovoltaic central power plants have also received attention abroad. They have been constructed in Denmark, Greece, Spain, Germany, Saudi Arabia and Japan.
From
toys to security systems, ever-growing arrays of consumer products
operate on electricity supplied by photovoltaic cells.
These products are available through catalogs, at many
Arizona photovoltaic companies,
and even in some department stores. An estimated 200 million people
already own PV-powered calculators and wristwatches. Other
solar-powered devices include: portable camping lights,
Frisbee-sized pool cleaners, small fans that roll up in car windows,
and hats with tiny fans for extra cooling. Car manufacturers such as Mazda
and Audi now offer a “solar sunroof” option in some new car
models. These sunroofs,
incorporating see-through photovoltaic cells, power ventilation
systems that help cool a parked car as much as 20 degrees.
PV cells have also been used to power entire electric cars. Landscape lights and security systems are practical, photovoltaic-powered products that are growing in popularity. Depending upon the size and complexity of these systems, owners can avoid hundreds of dollars in expenses for digging power line trenches or hooking-up to a power line.
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