The Power of Solar Energy
More
Information About Photovoltaics
Systems that Convert
Sunlight to Electricity
Can Meet Many Different Needs
What are the important terms?
What are PV energy systems?
What are some opportunities for using PV?
What is required?
What does a PV system cost?
Systems that Convert Sunlight to Electricity
Can Meet Many Different Needs
(Source: Federal Energy Management
Program)
Photovoltaics is a technology
that converts radiant light energy (photo) to electricity (voltaics). Photovoltaic
(PV) cells are the basic building blocks of this energy technology.
PV cells (also called solar cells)
are made of semiconductor materials, most typically silicon. The amount of electricity
a PV cell produces depends on its size, its conversion efficiency (see box below),
and the intensity of the light source. Sunlight is the most common source of the
energy used by PV cells to produce an electric current.
It takes just a few PV cells
to produce enough electricity to power a small watch or solar calculator. For
more power, cells are connected together to form larger units called modules.
Modules, in turn, are connected
to form arrays, and arrays can be interconnected to generate electricity for a
large load, such as a group of buildings.
Single-crystal silicon is the
most common semiconductor material used in making PV cells. Polycrystalline silicon,
in the form of a thin film or coating on an inexpensive base of glass or plastic,
is also used, and PV modules can also be made of thin films of amorphous (noncrystalline)
silicon. Thin films are usually less expensive to manufacture because they require
less silicon and the process is less labor-intensive. PV devices are also being
developed using combinations of other materials, such as cadmium, copper, indium,
gallium, selenium, and tellurium.
PV modules are typically installed
on or near a building or other structure. They can also be specially designed
as an integral part of a building's roof, wall, skylight, or other element. This
is called building-integrated PV or BIPV.
What
are the important terms?
Balance of system (BOS)容very
element (and its associated costs) of a PV system except the modules themselves;
this includes the design; land and site preparation; installation; support structures;
and power conditioning, operation and maintenance, and storage equipment.
Break-even cost葉he
cost of a PV system at which the value of the electricity it produces equals the
cost of electricity from an alternative source plus the delivery of this electricity
to the site; a break-even distance is the distance a power line needs to be extended
to match the installation cost of a PV system.
Peak watt葉he
"rated" output of a cell, module, or system; the amount of power a PV
device produces when operating at 25ｰC during tests; the peak rating is usually
determined during indoor tests rather than outdoors.
PV conversion efficiency葉he
percentage of available sunlight converted to electricity by a PV module or cell;
technically, the ratio of electric power produced by a cell to the power of the
sunlight striking the cell.
What
are PV energy systems?
A PV energy system usually includes
a module or array and the structural hardware needed to install it. The simplest
PV systems generate DC electricity, usually for a small load, when the sun is
shining or they are exposed to artificial light. More complex systems include
a power inverter that converts the direct current (DC) generated by PV to alternating
current (AC), and batteries that store energy for use at night or when the sun
isn't shining.
Today, PV is used primarily for
cathodic (corrosion) protection, traffic warning lights, water pumping for irrigation
and livestock, telecommunications, security and lighting systems, resource monitoring,
and electric load management. Many of these uses are remote (or off-grid) power
generation applications, not connected to utility power lines. PV systems are
already used in many off-grid applications in the Federal government, such as
for emergency call boxes near interstate highways.
When the electricity required
for an application exceeds the amount a PV system can supply, a conventional electric
generator can be added to create a hybrid PV/generator system. Wind systems can
also be added.
PV systems actually have many
benefits:
- Portability洋any kinds of PV
systems can be moved about easily.
- Reliability葉hey operate for
long periods with little maintenance.
- Low operating costs葉he fuel
is free and there are no (or few) moving parts.
- Low environmental impact葉hey
are quiet and nonpolluting (no greenhouse gas emissions.
- Stand-alone capability葉hey
operate in remote areas far from power lines.
- Modularity用ower output can
be increased by adding more modules.
- Safety葉hey are not flammable
and meet National Electric Code requirements.
- Versatility葉hey operate well
in almost any climate.
- Short lead time用repackaged
PV systems are available, and utility easements aren't needed.
- Ease of installation溶o heavy
construction equipment is required.
What
are some opportunities for using PV?
Photovoltaics is a good choice
for remote applications in which the daily electric load falls somewhere between
a few watt-hours and about 100 kilowatt-hours. Because it is nonpolluting, PV
should definitely be considered for remote areas that rely on fossil-fueled generators
for electric power.
On a first-cost basis, the installed
cost of a PV system can be less than the cost of utility
service. But PV may also be a
good choice in areas where the reliability of a power plant is questionable, or
where an agency is being charged high rates during peak hours.
The National Park Service has
installed more than 450 PV systems, chiefly to provide power for resource-monitoring
equipment. These are some additional, widely demonstrated, off-grid applications
for PV:
- Lights for walkways, streets,
highways, and common areas
- Residential uses (fans, lights,
refrigerators) in remote areas
- Elctricity for campgrounds,
marinas, and offshore drilling platforms
- Equipment for weather stations
and fire observation towers
- Communications equipment and
facilities (e.g., emergency roadside phones, microwave repeater stations)
- Cathodic (corrosion) protection
for metal pipes and similar objects
- Highway and warning signs, security
systems, transmission tower beacons
- Livestock watering pumps, irrigation
systems, and disinfection equipment
- Emergency power during times
of crisis.
What
is required?
A potential user should evaluate
different PV systems on the basis of cost, system performance, system reliability,
and maintenance needs. These are some additional requirements:
- Modules must face south and
be unshaded; they can be mounted on the application (e.g., a highway sign), on
a roof, or on the ground.
- Batteries are often needed to
meet peak loads or for nighttime use; they require periodic maintenance.
- Power inverters will be needed
if the load requires AC electricity.
What
does a PV system cost?
To determine the economic feasibility
of using a PV system, consider these three main factors:
- The size and nature of the load
- The availability of the solar
resource
- The cost of alternative sources
of power.
On a 20-year, life-cycle-cost
basis, a remote PV system typically costs from 25｢?50｢ per kilowatt-hour. In off-grid
applications, PV can be more cost-effective than many alternatives.
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