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Photovoltaics:
Basic Design Principles and Components
If you are thinking of generating your own
electricity, you should consider a photovoltaic (PV) system—a way to generate
electricity by using energy from the sun. These systems have several advantages:
they are cost-effective alternatives in areas where extending a utility power
line is very expensive; they have no moving parts and require little
maintenance; and they produce electricity without polluting the environment.
This publication will introduce you to the
basic design principles and components of PV systems. It will also help you
discuss these systems knowledgeably with an equipment supplier or system
installer. Because this publication is not intended to cover everything about
designing and installing a PV system, a list of additional PV resources is
provided at the end.
Introduction to PV
Technology
Single PV cells (also known as
"solar cells") are connected electrically to form PV modules,
which are the building blocks of PV systems. The module is the smallest PV
unit that can be used to generate substantial amounts of PV power. Although
individual PV cells produce only small amounts of electricity, PV modules are
manufactured with varying electrical outputs ranging from a few watts to more
than 100 watts of direct current (DC) electricity. The modules can be
connected into PV arrays for powering a wide variety of electrical
equipment.
Two primary types of PV technologies
available commercially are crystalline silicon and thin film. In
crystalline-silicon technologies, individual PV cells are cut from large
single crystals or from ingots of crystalline silicon. In thin-film PV
technologies, the PV material is deposited on glass or thin metal that
mechanically supports the cell or module. Thin-film-based modules are produced
in sheets that are sized for specified electrical outputs.
In addition to PV modules, the
components needed to complete a PV system may include a battery charge
controller, batteries, an inverter or power control unit (for
alternating-current loads), safety disconnects and fuses, a grounding circuit,
and wiring. (See Balance-of-System Equipment
section.)
PV System Applications
Many people are familiar with PV-powered
calculators and watches, the most common small-scale applications of PV.
However, there are numerous large-scale, cost-effective PV applications,
including:
- Water pumping for
small-scale remote irrigation, stock watering, residential uses, remote
villages, and marine sump pumps;
- Lighting for
residential needs, billboards, security, highway signs, streets and
parking lots, pathways, recreational vehicles, remote villages and
schools, and marine navigational buoys;
- Communications by
remote relay stations, emergency radios, orbiting satellites, and cellular
telephones;
- Refrigeration for
medical and recreational uses;
- Corrosion protection
for pipelines and docks, petroleum and water wells, and underground tanks;
- Utility grids that
produce utility- or commercial-scale electricity; and
- Household appliances such
as ventilation fans, swamp coolers, televisions, blenders, stereos, and
other appliances.
The decreasing cost of PV systems and
the increasing number of manufacturers and dealers for PV equipment have
contributed to widespread use of the technology. In PV's early days,
do-it-yourselfers had to search for companies that manufactured PV modules and
often had to adapt or reconfigure components from other non-PV systems. Today,
dealers offer ready-to-use systems and state-of-the-art equipment designed
specifically for PV systems. Many dealers have computer software that helps to
design systems and specify appropriate components. As PV markets expand,
dealers are gaining greater experience with PV applications, making it cheaper
and easier to purchase PV systems.
How Do I Select a PV
Dealer?
Choosing a PV professional will be one
of your most important decisions. If you choose a competent dealer, you won't
need to know all the details of designing, purchasing, and installing your PV
system. Instead, you can rely on the dealer's expertise to design and install
a system that meets your needs. However, just like buying a car or a
television, you must have confidence in the dealer's products and services and
be an informed consumer. With the growth of the PV industry, the number of
regional dealers, mail-order businesses, and local distributors has expanded
rapidly. Many telephone directories contain listings for PV dealers under the
"Solar" heading.
Professional credentials are one
indication of a PV dealer's knowledge and qualifications. Ask dealers what
PV-related courses they have taken, certifications they have earned, and
licenses they have received.
A second consideration is the dealer's
experience in the field. How long has the company been in business? The local
Better Business Bureau can advise you whether any customers have registered
complaints about the dealer. You should also ask the dealer how many systems
like yours he or she has designed and installed. Ask to see installations, and
talk with owners of systems similar to the one you want to purchase.
A third consideration in selecting a
system installer is the variety and quality of products offered for each
component of the system. Because PV systems are often designed for a specific
site, one company's products may not be appropriate for all applications.
Competent dealers will stock components manufactured by several companies. A
variety of product options will help ensure that the most appropriate
components are available for your system. When a dealer recommends a product,
ask what the recommendation is based on, whether there are consumer or
independent testing facility reports you can read, and whether the products
are listed with Underwriters Laboratories (UL).
Fourth, consider the service agreements
and performance guarantees the dealer provides and the warranties given by the
product manufacturers. No system is maintenance-free, nor will all components
function flawlessly forever. When problems emerge with your system, what
services will the dealer provide? What warranties do the manufacturers
provide? What costs should you expect to pay, and which costs will be assumed
by the dealer and/or the manufacturer?
Finally, you should compare prices from
different dealers. Because distribution channels and dealer networks have
expanded dramatically, the opportunity to "shop around" is much
greater today. If possible, approach more than one dealer about a draft design
and cost estimate for your system.
When Are PV Systems
Appropriate?
People select PV systems for a variety
of reasons. Some common reasons for selecting a PV system include:
- Cost—When the cost
is high for extending the utility power line or using another
electricity-generating system in a remote location, a PV system is often
the most cost-effective source of electricity.
- Reliability—PV
modules have no moving parts and require little maintenance compared to
other electricity-generating systems.
- Modularity—PV
systems can be expanded to meet increased power requirements by adding
more modules to an existing system.
- Environment—PV
systems generate electricity without polluting the environment and without
creating noise.
- Ability to combine systems—PV
systems can be combined with other types of electric generators (wind,
hydro, and diesel, for example) to charge batteries and provide power on
demand.
PV systems are not cost-effective for
all applications. The following discussion gives some general guidelines to
consider when deciding whether a PV system is appropriate for your situation.
First, if your site is already connected
to a utility grid, or within one-quarter mile of the grid, a PV system will
probably not be cost-effective. Each utility company spreads the cost of its
power plants and fuel costs among all its customers. Most utilities can
provide electricity to consumers for about 6 cents to 14 cents per
kilowatt-hour. When you install a PV system, you are essentially installing
your own mini-utility system. You pay all the costs of generating the
electricity you consume. Although the sun's energy is free, the PV equipment
is not free. The electricity generated by PV systems at current module and
balance-of-system prices can cost 20 cents to 40 cents per kilowatt-hour,
depending on the installation cost and intensity and duration of the sunlight
at the site.
Second, small PV systems are not
practical for powering space-heating systems, water heaters, air conditioners,
electric stoves, or electric clothes dryers. These loads require a large
amount of energy to operate, which will increase the size and cost of your PV
system. Therefore, select the most energy-efficient loads available. For
example, if your PV system will power lights, look for the most
energy-efficient light bulbs. If your system will pump water for toilets and
showers, look for the most water-conserving fixtures.
Is My Site Adequate for
PV?
A PV system designer can conduct a
detailed site assessment for you. To save the dealer time (and possibly save
yourself some money), you can conduct a preliminary assessment to determine
whether your site has potential for a PV system. Contact the Energy Efficiency
and Renewable Energy Clearinghouse (EREC—see Source
List) for more information on conducting a detailed site feasibility
assessment.
There are three factors to consider when
determining whether your site is appropriate.
First, systems installed in the United
States must have a southern exposure. For maximum daily power output, PV
modules should be exposed to the sun for as much of the day as possible,
especially during the peak sun hours of 10 a.m. to 3 p.m.
Second, the southern exposure must be
free of obstructions such as trees, mountains, and buildings that might shade
the modules. Consider both summer and winter paths of the sun, as well as the
growth of trees and future construction that may cause shading problems.
Finally, the unobstructed southern
exposure must also have appropriate terrain and sufficient space to install
the PV system. A flat, grassy site is appropriate terrain, whereas a steep,
rocky hillside is not.
How Does Weather Affect
PV Module Output?
Unlike utility power plants, which
produce electricity constantly despite the time of day and year or the
weather, the output of PV modules is directly related to these two factors.
Where you live will affect the number of
PV modules you will need for power, because different geographic regions
experience different weather patterns. Seasonal variations affect the amount
of sunlight available to power a PV system.
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How to Size
Your PV System
To size your PV system, you must
first know your energy needs, which you figure by listing all your daily
loads. A load includes anything that uses electricity from your power
source, such as lights, televisions, radios, or batteries. Some loads
need electricity all the time, such as refrigerators, whereas others use
electricity less often, such as power saws. To determine your total
energy consumption, multiply the wattage of the appliance by the number
of hours it is used in a day. Some appliances do not give the wattage,
so you may have to calculate the wattage by multiplying the amperes
times the volts. After adding the totals for each appliance, you can
decide what power output you need for your PV system.
Example
| Load |
Daily Use
(hrs) |
|
Wattage |
|
Total Energy
Consumption (watt-hrs) |
| Radio |
2 |
x |
25 |
= |
50 |
Lamps
(fluorescent) |
3 |
x |
27 |
= |
81 |
| VCR |
0.5 |
x |
30 |
= |
15 |
| Television |
6 |
x |
60 |
= |
360 |
| Total
Daily Energy Consumption |
506 watt-hrs |
For the items listed above, you
would need a system that produces an average daily energy output of 506
watt-hours. Obviously, different parts of the country receive varying
amounts of sunlight. Because sunlight is the source of power for PV, you
must determine the daily amount of sunlight in your region. Remember
that PV systems are rated by peak watt, which is the amount of power
produced when the module receives 1,000 watts per square meter of
exposure to the sun (insolation).
Let's examine two locations:
Albuquerque, New Mexico, and Pittsburgh, Pennsylvania. Albuquerque is a
fairly sunny area. In Albuquerque, for each peak watt that a PV module
is rated, it will produce a yearly average of 6.2 watt-hrs* of
electricity daily. In Pittsburgh, a cloudier area, the same module will
produce an average of 2.4 watt-hrs* of electricity daily.
If you wanted to use a PV system
in Albuquerque for the appliances listed in the table, you would divide
506 watt-hrs by 6.2, divide that by 0.8 to account for inefficiency of
the batteries and, finally, multiply by 1.2 to cover anything that may
have been overlooked. You find that you would need a PV system rated at
124 peak watts. If you were buying 50-watt modules, you would need three
modules, because you round up to the next highest number.
| 506 |
÷ |
6.2 |
= |
82 |
| 82 |
÷ |
0.8 |
= |
103 |
| 103 |
x |
1.2 |
= |
124 |
| 124 |
÷ |
50 |
= |
3 modules |
For Pittsburgh, you would divide
506 watt-hrs by 2.4, divide by 0.8, and multiply by 1.2, which yields
317 peak watts, or seven modules at 50 watts each.
| 506 |
÷ |
2.4 |
= |
211 |
| 211 |
÷ |
0.8 |
= |
264 |
| 264 |
x |
1.2 |
= |
317 |
| 317 |
÷ |
50 |
= |
7 modules |
Determining your daily energy
consumption can be done through simple calculations like the example
above or with the aid of sophisticated computer programs. If you are
seriously considering purchasing a PV system, there are also other
factors to consider. You may want to refer to other sources (see Source
List) for more precise ways to make your calculations.
*This is based on the winter
average. For more precise calculations, consult month-by-month averages
and use the lowest monthly average.
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Module temperature also affects output.
The conversion efficiency of crystalline-silicon modules falls significantly
at elevated module temperatures.
When designing a PV system, be sure your
PV installer obtains data specific to your area, rather than relying on
general data. The National Oceanic and Atmospheric Administration began
collecting solar data nearly 20 years ago. The National Renewable Energy
Laboratory's Renewable Resource Data Center (see Source
List) can provide solar radiation information, as can EREC. Some state
energy offices also have solar data-collection programs to assist solar
designers. Finally, books are available that contain solar data on most major
cities in the United States, and a few of these are listed in the Reading
List.
Sizing the System to Meet
Your Needs
After you have assessed the
appropriateness of your site, you need to determine how much electricity your
PV system must generate. This depends on how much electricity your loads
require. Again, your dealer can help you with sizing a system that will meet
your needs. The sidebar illustrates the steps involved in sizing a PV system.
You can also contact EREC for more specific information on sizing.
Balance-of-System
Equipment
In addition to the PV modules, you must
purchase balance-of-system (BOS) equipment. This includes battery charge
controllers, batteries, inverters (for loads requiring alternating current),
wires, conduit, a grounding circuit, fuses, safety disconnects, outlets, metal
structures for supporting the modules, and any additional components that are
part of the PV system. Below, we'll discuss PV and BOS configurations first
for loads requiring direct current, then for loads needing alternating
current.
Note that, in many systems, the cost of
BOS equipment can equal or exceed the cost of the PV modules. When examining
the costs of PV modules, remember that these costs do not include the cost of
BOS equipment. Ask your PV dealer about the BOS equipment required by your
system.
Direct-Current System
Equipment
Charge Controller. The
charge controller regulates the flow of electricity from the PV modules to the
battery and the load. The controller keeps the battery fully charged without
overcharging it. When the load is drawing power, the controller allows charge
to flow from the modules into the battery, the load, or both. When the
controller senses that the battery is fully charged, it stops the flow of
charge from the modules. Many controllers will also sense when loads have
taken too much electricity from batteries and will stop the flow until
sufficient charge is restored to the batteries. This last feature can greatly
extend the battery's lifetime.
Controllers generally cost between $20
and $400, depending on the ampere capacity at which your PV system will
operate and the monitoring features you want. When selecting a controller,
make sure it has the features you need; cost should be a secondary
consideration.
Battery. The battery
stores electricity for use at night or for meeting loads during the day when
the modules are not generating sufficient power to meet load requirements. To
provide electricity over long periods, PV systems require deep-cycle
batteries. These batteries, usually lead-acid, are designed to gradually
discharge and recharge 80% of their capacity hundreds of times. Automotive
batteries are shallow-cycle batteries and should not be used in PV systems
because they are designed to discharge only about 20% of their capacity. If
drawn much below 20% capacity more than a few dozen times, the battery will be
damaged and will no longer be able to take a charge.
Deep-cycle batteries cost from about $65
up to $3,000. The cost depends on the type, capacity (ampere-hours), the
climatic conditions in which it will operate, how frequently it will receive
maintenance, and the types of chemicals it uses to store and release
electricity. A PV system may have to be sized to store a sufficient amount of
power in the batteries to meet power demand during several days of cloudy
weather. This is known as "days of autonomy." Consult with your PV
dealer before selecting batteries for your system.
Most types of batteries contain toxic
materials that may pose serious health and safety problems. The National
Electric Code (NEC), battery companies, and PV system designers recommend that
lead-acid and wet cell batteries, which give off explosive hydrogen gas when
recharging, be located in a well-ventilated space isolated from the other
electrical components of the system and away from living spaces. Allow enough
space for easy access during maintenance, repair, and replacement. Most
important, maintain the battery according to the manufacturer's instructions,
and recycle the batteries properly when they wear out.
Alternating-Current
System Equipment
Inverter. AC systems
also require an inverter, which changes the DC electricity produced by PV
modules and stored in batteries into AC electricity. Different types of
inverters produce a different "quality" of electricity. For example,
lights, televisions, and power tools can operate on lower-quality electricity,
but computers, laser printers, and other sophisticated electronic equipment
require the highest-quality electricity. So, you must match the power quality
required by your loads with the power quality produced by the inverter.
Inverters for most stand-alone
applications (i.e., those systems not connected to the utility grid) cost less
than $1 per rated output watt. The cost is affected by several factors,
including the quality of the electricity it needs to produce; whether the
incoming DC voltage is 12, 24, 36, or 48 volts; the number of AC watts your
loads require when they are operating normally; the amount of extra surge
power your AC loads need for short periods; and whether the inverter has any
additional features such as meters and indicator lights.
Tell your PV dealer if you plan to add
additional AC loads in the future. If you are considering building another
room onto your house or adding electrical loads, consider purchasing an
inverter with a larger input and output rating than you currently need. This
may be less costly than replacing it with a larger one later.
The National Electric
Code
The National Electric Code (NEC) was
established in 1897 to ensure safety in all systems that generate, store,
transport, and consume electricity. You or the dealer who installs your PV
system should be careful to follow NEC's equipment requirements so that the PV
system can be approved by local electric code officials. Be aware that many
states require all electrical equipment to be installed by a licensed
electrician.
However, many local code officials are
not familiar with PV systems. Though you follow the provisions of NEC, you may
have difficulty convincing a code official that you have installed a
code-approved system. Contact and educate (if necessary) local code officials
before you purchase and install the system. Throughout the installation
process, invite them to observe what you or your dealer have done before you
build any enclosure around wiring, connections, or other components. This will
help ensure that your system receives approval and will also help future PV
installers to get code approval.
Local insurance providers and lenders
may also need to be educated about the safety, reliability, and
cost-effectiveness of PV systems. Obtaining insurance will be easier in states
where PV systems are more common.
What Else Do I Need to
Consider?
No PV system is maintenance-free.
Schedule regular inspections of your system to ensure that the wiring and
contacts are free from corrosion, the modules are clear of debris, and the
mounting equipment has tight fasteners.
You should also monitor the power output
of your PV modules, the state-of-charge and electrolyte level of your
batteries, and the actual amount of power that your loads use. Writing this
information in a notebook is a good way to track your system's performance and
help you determine whether your system is operating as designed. Monitoring
will also help you understand the relationships between your system's power
production, storage capability, and load requirements.
PV Can Power Your Future
PV systems can be cost-effective options
for providing electricity to your home or remote site. However, they are not
appropriate for all situations. Deciding whether a PV system is right for you
depends on many factors. Therefore, conduct careful research and consult with
PV equipment dealers and others who have installed these systems. If you then
decide that a PV system is right for you, the power of the sun will take on a
new meaning in your life.
Source List
The following are just a few of the many
organizations that can help you with locating PV equipment dealers in your area
and designing and installing PV systems.
The Energy Efficiency and
Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) 363-3732
Fax (703) 893-0400
E-mail: doe.erec@nciinc.com
EREC provides free general and technical
information to the public on the many topics and technologies pertaining to
energy efficiency and renewable energy, including PV systems, solar energy, and
solar radiation data.
Equipment, Dealers, and
Installers
Renewable Energy & Efficiency
Training Institute (RETI)
1800 M Street, NW
Suite 300
Washington, DC 20036
(202) 496-1417
Fax: (202) 496-1494
RETI offers customized PV design,
installation, and maintenance programs to meet the needs of a wide range of
customer groups.
Solar Energy Industries
Association (SEIA)
122 C Street, NW
4th Floor
Washington, DC 20001
(202) 383-2600
Fax: (202) 383-2670
SEIA is the national trade organization of
PV and solar thermal manufacturers and component suppliers.
Training Programs
Florida Solar Energy Center (FSEC)
Photovoltaic System Design Assistance and Training Center
1679 Clearlake Road
Cocoa, FL 32922-5703
(407) 638-1000
Fax: (407) 638-1010
FSEC offers workshops on a variety of
topics related to PV system design and use.
Siemens Solar Industries (formerly
Arco Solar)
Photovoltaic Technology and System Design Training Course
4650 Adohr Lane
Camarillo, CA 93012
(805) 388-6561
Fax: (805) 388-6395
Siemens offers a one-week training program
on PV technology and system design.
Solar Energy International (SEI)
P.O. Box 715
Carbondale, CO 81623
(970) 963-8855
Fax: (970) 963-8866
SEI offers training programs on PV system
design and installation, as well as on wind energy, mini-hydro systems, and
solar home design. SEI also sells books on a variety of renewable energy topics.
On-Line Renewable Energy
Information
Energy Efficiency and Renewable
Energy Network (EREN)
http://www.eren.doe.gov
EREN is the Department of Energy's premier
resource for information about renewable energy and energy efficiency
technologies, including solar radiation and photovoltaic data.
National Renewable Energy
Laboratory (NREL)
http://www.nrel.gov
NREL, one of the Department of Energy's
national laboratories, leads the nation toward a sustainable energy future by
developing renewable energy technologies. Its Web site includes information on
many renewable energy topics. See NREL's Renewable Resource Data Center, at
http://www.rredc.nrel.gov, for solar radiation information.
Reading List
Periodicals, Books, Pamphlets,
and Reports
Consumer Guide to Solar Energy, S. Sklar, Bonus Books, Chicago, 1991.
Home Power Magazine: The Hands-On
Journal of Home-Made Power, Home Power, Inc., P.O. Box 520, Ashland, OR
97520; (916) 475-3179; www.homepower.com.
Photovoltaic Fundamentals, National
Renewable Energy Laboratory, Document No. DE-91015001, available from National
Technical Information Service, U.S. Department of Commerce, 5285 Port Royal
Road, Springfield, VA 22161, 1991.
The Solar Electric House, S.
Strong, Sustainability Press, Still River, MA, 1993.
Solar Electricity: A Practical Guide to
Designing and Installing Small Photovoltaic Systems, S. Roberts, Prentice
Hall, NJ, 1991.
Stand-Alone Photovoltaic Systems:
Handbook of Recommended Design Practices, Sandia National Laboratory,
Document No. SAND87-7023, available from National Technical Information Service,
U.S. Department of Commerce, 5285 Port Royal Road, Springfield, VA 22161, 1991
(revised).
U.S. Department of Energy
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Basic Design Principles and Components
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