Photovoltaics: Basic Design Principles and Components

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DOE/GO-10097-377 FS 231 ENERGY March 1997 EFFICIENCY Photovoltaics: Basic Design AND

RCLEARINGHOUSEENEWABLE Principles and Components ENERGY If you are thinking of generating your Introduction to PV Technology own electricity, you should consider a photovoltaic (PV) system—a way to gen- Single PV cells (also known as “solar cells”) are connected electrically to form erate electricity by using energy from the PV modules, which are the building blocks sun. These systems have several advan- of PV systems. The module is the smallest tages: they are cost-effective alternatives in PV unit that can be used to generate sub- areas where extending a utility power line stantial amounts of PV power. Although is very expensive; they have no moving individual PV cells produce only small parts and require little maintenance; and amounts of electricity, PV modules are they produce electricity without polluting manufactured with varying electrical out- the environment. puts ranging from a few watts to more than 100 watts of direct current (DC) elec- This publication will introduce you to the tricity. The modules can be connected into basic design principles and components of PV arrays for powering a wide variety of PV systems. It will also help you discuss electrical equipment. these systems knowledgeably with an equipment supplier or system installer. Two primary types of PV technologies Because this publication is not intended to available commercially are crystalline sili- cover everything about designing and con and thin film. In crystalline-silicon installing a PV system, a list of additional technologies, individual PV cells are cut PV resources is provided at the end. 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 sup- ports the cell or module. Thin-film-based modules are produced in sheets that are sized for speci- fied 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 Jim Yost / PIX01809 Jim Yost circuit, and wiring. (See This stand-alone PV system consists of four modules, each with Balance-of-System Equip- 36 cells. It provides power for lights, radios, televisions, and other ment section.) loads at remote homes in New Mexico.

NT O ME F E T N R E A R This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE national laboratory. P G

E Y D The document was produced by the Information Services Program, under the DOE Office of Energy Efficiency and Renewable Energy. The Energy Efficiency

A N and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc., for NREL / DOE. The statements contained herein are based on C I I T R E information known to EREC and NREL at the time of printing. No recommendation or endorsement of any product or service is implied if mentioned by EREC. D E M S A TAT ES OF Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 20% postconsumer waste

PV System Applications tem. Instead, you can rely on the dealer’s expertise to design and install a system Many people are familiar with PV-pow- that meets your needs. However, just like ered calculators and watches, the most buying a car or a television, you must common small-scale applications of PV. have confidence in the dealer’s products However, there are numerous large-scale, and services and be an informed con- cost-effective PV applications, including: sumer. With the growth of the PV indus- try, the number of regional dealers, • Water pumping for small-scale remote mail-order businesses, and local distribu- irrigation, stock watering, residential tors has expanded rapidly. Many tele- uses, remote villages, and marine sump phone directories contain listings for PV pumps; dealers under the “Solar” heading. • Lighting for residential needs, bill- boards, security, highway signs, streets Professional credentials are one indication and parking lots, pathways, recreational of a PV dealer’s knowledge and qualifica- vehicles, remote villages and schools, tions. Ask dealers what PV-related courses and marine navigational buoys; they have taken, certifications they have • Communications by remote relay sta- earned, and licenses they have received. tions, emergency radios, orbiting satel- lites, and cellular telephones; A second consideration is the dealer’s • Refrigeration for medical and recre- experience in the field. How long has the ational uses; company been in business? The local Bet- • Corrosion protection for pipelines and ter Business Bureau can advise you docks, petroleum and water wells, and whether any customers have registered underground tanks; complaints about the dealer. You should • Utility grids that produce utility- or also ask the dealer how many systems like commercial-scale electricity; and Hundreds of cost- yours he or she has designed and • Household appliances such as ventila- installed. Ask to see installations, and talk tion fans, swamp coolers, televisions, effective applications with owners of systems similar to the one blenders, stereos, and other appliances. you want to purchase. for PV systems have The decreasing cost of PV systems and the A third consideration in selecting a system increasing number of manufacturers and been developed. installer is the variety and quality of prod- dealers for PV equipment have con- ucts offered for each component of the tributed to widespread use of the technol- system. Because PV systems are often ogy. In PV’s early days, do-it-yourselfers designed for a specific site, one company’s had to search for companies that manufac- products may not be appropriate for all tured PV modules and often had to adapt applications. Competent dealers will stock or reconfigure components from other components manufactured by several non-PV systems. Today, dealers offer companies. A variety of product options ready-to-use systems and state-of-the-art will help ensure that the most appropriate equipment designed specifically for PV components are available for your system. systems. Many dealers have computer When a dealer recommends a product, ask software that helps to design systems and what the recommendation is based on, specify appropriate components. As PV whether there are consumer or indepen- markets expand, dealers are gaining dent testing facility reports you can read, greater experience with PV applications, and whether the products are listed with making it cheaper and easier to purchase Underwriters Laboratories (UL). PV systems. Fourth, consider the service agreements How Do I Select a PV Dealer? and performance guarantees the dealer Choosing a PV professional will be one of provides and the warranties given by the your most important decisions. If you product manufacturers. No system is choose a competent dealer, you won’t maintenance-free, nor will all components need to know all the details of designing, function flawlessly forever. When prob- purchasing, and installing your PV sys- lems emerge with your system, what services will the dealer provide? What

2 warranties do the manufacturers provide? compared to other electricity-generating What costs should you expect to pay, and systems. which costs will be assumed by the dealer • Modularity—PV systems can be and/or the manufacturer? expanded to meet increased power requirements by adding more modules Finally, you should compare prices from to an existing system. different dealers. Because distribution • Environment—PV systems generate channels and dealer networks have electricity without polluting the envi- expanded dramatically, the opportunity to ronment and without creating noise. “shop around” is much greater today. If • Ability to combine systems—PV sys- possible, approach more than one dealer tems can be combined with other types about a draft design and cost estimate for of electric generators (wind, hydro, and In the United States, your system. diesel, for example) to charge batteries and provide power on demand. PV systems must When Are PV Systems Appropriate? PV systems are not cost-effective for all have unobstructed applications. The following discussion People select PV systems for a variety of gives some general guidelines to consider southern exposure. reasons. Some common reasons for select- when deciding whether a PV system is ing a PV system include: appropriate for your situation.

• Cost—When the cost is high for extend- First, if your site is already connected to a ing the utility power line or using utility grid, or within one-quarter mile of another electricity-generating system in a the grid, a PV system will probably not be remote location, a PV system is often the cost-effective. Each utility company most cost-effective source of electricity. spreads the cost of its power plants and • Reliability—PV modules have no mov- fuel costs among all its customers. Most ing parts and require little maintenance utilities can provide electricity to con- sumers for about 6 cents to 14 cents per kilowatt-hour. When you install a PV sys- Sun’s Path in the Sky tem, you are essentially installing your own mini-utility system. You pay all the costs of June 21 generating the electricity you consume. Although the sun’s energy is free, the PV equipment is not free. The electricity gener- ated 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 West North duration of the sunlight at the site.

Second, small PV systems are not practical for powering space-heating systems, December 21 water heaters, air conditioners, electric stoves, or electric clothes dryers. These PV modules 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- South East efficient light bulbs. If your system will pump water for toilets and showers, look for the most water-conserving fixtures.

The sun’s noontime height above the horizon changes seasonally. This is important to consider when siting and positioning a PV array.

3 Where you live will Annual Average Daily Peak Sun Hours determine the number 4 4.5 of PV modules your system will need. 4 4

5 4

4.5 6

5 4.7 4.7 3

This diagram illustrates the annual average daily peak sun hours for the United States.

Is My Site Adequate for PV? winter paths of the sun, as well as the growth of trees and future construction A PV system designer can conduct a that may cause shading problems. detailed site assessment for you. To save the dealer time (and possibly save yourself Finally, the unobstructed southern expo- some money), you can conduct a prelimi- sure must also have appropriate terrain nary assessment to determine whether your and sufficient space to install the PV sys- site has potential for a PV system. Contact tem. A flat, grassy site is appropriate ter- the Energy Efficiency and Renewable rain, whereas a steep, rocky hillside is not. Make sure you have Energy Clearinghouse (EREC—see Source List) for more information on conducting a How Does Weather Affect PV selected the most detailed site feasibility assessment. Module Output? energy-efficient loads There are three factors to consider when Unlike utility power plants, which pro- determining whether your site is appro- duce electricity constantly despite the time possible. priate. of day and year or the weather, the output of PV modules is directly related to these First, systems installed in the United States two factors. must have a southern exposure. For maxi- mum daily power output, PV modules Where you live will affect the number of should be exposed to the sun for as much PV modules you will need for power, of the day as possible, especially during because different geographic regions expe- the peak sun hours of 10 a.m. to 3 p.m. rience different weather patterns. Seasonal variations affect the amount of sunlight Second, the southern exposure must be available to power a PV system. The above free of obstructions such as trees, moun- map shows annual average “peak sun tains, and buildings that might shade the hours” for regions in the United States. modules. Consider both summer and

Module temperature also affects output. How to Size Your PV System The conversion efficiency of crystalline- To size your PV system, you must first know your energy needs, which you silicon modules falls significantly at figure by listing all your daily loads. A load includes anything that uses elec- elevated module temperatures. tricity from your power source, such as lights, televisions, radios, or batteries. Some loads need electricity all the time, such as refrigerators, whereas others When designing a PV system, be sure your use electricity less often, such as power saws. To determine your total energy PV installer obtains data specific to your 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 area, rather than relying on general data. calculate the wattage by multiplying the amperes times the volts. After adding The National Oceanic and Atmospheric the totals for each appliance, you can decide what power output you need for Administration began collecting solar data your PV system. nearly 20 years ago. The National Renew- Example able Energy Laboratory’s Renewable Daily Use Total Energy Resource Data Center (see Source List) can Load (hrs) Wattage Consumption (watt-hrs) provide solar radiation information, as can Radio 2 x 25 = 50 EREC. Some state energy offices also have solar data-collection programs to assist Lamps 3 x 27 = 81 (fluorescent) solar designers. Finally, books are available that contain solar data on most major VCR 0.5 x 30 = 15 cities in the United States, and a few of Television 6 x 60 = 360 these are listed in the Reading List. Total Daily Energy Consumption 506 watt-hrs Sizing the System to Meet 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 coun- Your Needs try receive varying amounts of sunlight. Because sunlight is the source of After you have assessed the appropriate- 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 ness of your site, you need to determine amount of power produced when the module receives 1,000 watts per square how much electricity your PV system meter of exposure to the sun (insolation). must generate. This depends on how Let’s examine two locations: Albuquerque, New Mexico, and Pittsburgh, Penn- much electricity your loads require. Again, sylvania. Albuquerque is a fairly sunny area. In Albuquerque, for each peak your dealer can help you with sizing a sys- watt that a PV module is rated, it will produce a yearly average of 6.2 watt- tem that will meet your needs. The sidebar hrs* of electricity daily. In Pittsburgh, a cloudier area, the same module will illustrates the steps involved in sizing a produce an average of 2.4 watt-hrs* of electricity daily. PV system. You can also contact EREC for If you wanted to use a PV system in Albuquerque for the appliances listed in more specific information on sizing. 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 Balance-of-System Equipment 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. In addition to the PV modules, you must purchase balance-of-system (BOS) equip- 506 ÷ 6.2 = 82 ment. This includes battery charge con- 82 ÷ 0.8 = 103 trollers, batteries, inverters (for loads 103 x 1.2 = 124 requiring alternating current), wires, con- 124 ÷ 50 = 3 modules duit, a grounding circuit, fuses, safety disconnects, outlets, metal structures for 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. supporting the modules, and any additional components that are part of the PV 506 ÷ 2.4 = 211 system. Below, we’ll discuss PV and BOS 211 ÷ 0.8 = 264 configurations first for loads requiring 264 x 1.2 = 317 direct current, then for loads needing 317 ÷ 50 = 7 modules alternating current. Determining your daily energy consumption can be done through simple cal- Note that, in many systems, the cost of culations like the example above or with the aid of sophisticated computer programs. If you are seriously considering purchasing a PV system, there are BOS equipment can equal or exceed the also other factors to consider. You may want to refer to other sources (see cost of the PV modules. When examining Source List) for more precise ways to make your calculations. the costs of PV modules, remember that *This is based on the winter average. For more precise calculations, consult month-by- these costs do not include the cost of BOS month averages and use the lowest monthly average. equipment. Ask your PV dealer about the BOS equipment required by your system.

5 lead-acid, are designed to gradually dis- Direct-Current (DC) System charge and recharge 80% of their capacity Charge controller PV hundreds of times. Automotive batteries modules 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 Grounding circuit 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 Electric load (DC) the types of chemicals it uses to store and Grounding circuit release electricity. A PV system may have Battery to be sized to store a sufficient amount of power in the batteries to meet power This figure illustrates the configuration of the PV modules and BOS demand during several days of cloudy equipment in a basic direct-current system with battery storage. (Circuit weather. This is known as “days of auton- breakers and safety fuses are not shown.) omy.” Consult with your PV dealer before selecting batteries for your system. Direct-Current System Equipment Charge Controller. The charge controller Most types of batteries contain toxic mate- regulates the flow of electricity from the rials that may pose serious health and PV modules to the battery and the load. safety problems. The National Electric The controller keeps the battery fully Code (NEC), battery companies, and PV charged without overcharging it. When system designers recommend that lead- the load is drawing power, the controller acid and wet cell batteries, which give off allows charge to flow from the modules explosive hydrogen gas when recharging, into the battery, the load, or both. When be located in a well-ventilated space iso- the controller senses that the battery is lated from the other electrical components fully charged, it stops the flow of charge of the system and away from living In addition to the from the modules. Many controllers will spaces. Allow enough space for easy also sense when loads have taken too access during maintenance, repair, and PV modules, you will much electricity from batteries and will replacement. Most important, maintain stop the flow until sufficient charge is the battery according to the manufac- need to purchase restored to the batteries. This last feature turer’s instructions, and recycle the batter- can greatly extend the battery’s lifetime. ies properly when they wear out. balance-of-system Controllers generally cost between $20 Alternating-Current System (BOS) equipment. and $400, depending on the ampere capac- Equipment ity at which your PV system will operate Inverter. AC systems also require an and the monitoring features you want. inverter, which changes the DC electricity When selecting a controller, make sure it produced by PV modules and stored in has the features you need; cost should be a batteries into AC electricity. Different types secondary consideration. of inverters produce a different “quality” of electricity. For example, lights, televisions, Battery. The battery stores electricity for and power tools can operate on lower- use at night or for meeting loads during quality electricity, but computers, laser the day when the modules are not gener- printers, and other sophisticated electronic ating sufficient power to meet load equipment require the highest-quality requirements. To provide electricity over electricity. So, you must match the power long periods, PV systems require deep- quality required by your loads with the cycle batteries. These batteries, usually power quality produced by the inverter.

6 The National Inverters for most stand-alone applica- However, many local code officials are not tions (i.e., those systems not connected to familiar with PV systems. Though you fol- Electric Code the utility grid) cost less than $1 per rated low the provisions of NEC, you may have output watt. The cost is affected by several difficulty convincing a code official that contains provisions factors, including the quality of the elec- you have installed a code-approved sys- tricity it needs to produce; whether the tem. Contact and educate (if necessary) and requirements incoming DC voltage is 12, 24, 36, or 48 local code officials before you purchase volts; the number of AC watts your loads and install the system. Throughout the require when they are operating normally; installation process, invite them to observe for PV systems. the amount of extra surge power your AC what you or your dealer have done before loads need for short periods; and whether you build any enclosure around wiring, the inverter has any additional features such as meters and indicator lights. connections, or other components. This will help ensure that your system receives Tell your PV dealer if you plan to add approval and will also help future PV additional AC loads in the future. If you installers to get code approval. are considering building another room onto your house or adding electrical loads, Local insurance providers and lenders consider purchasing an inverter with a may also need to be educated about the larger input and output rating than you safety, reliability, and cost-effectiveness of currently need. This may be less costly PV systems. Obtaining insurance will be than replacing it with a larger one later. easier in states where PV systems are more common.

Alternating-Current (AC) System What Else Do I Need to Consider? Charge controller PV No PV system is maintenance-free. Sched- modules ule 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 Grounding circuit has tight fasteners. Inverter 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 deter- Grounding circuit mine whether your system is operating as Electric load (AC) Battery designed. Monitoring will also help you understand the relationships between your system’s power production, storage Most household appliances operate on alternating current (AC). This illustrates a capability, and load requirements. basic configuration of the PV modules and BOS equipment in an AC system. (Circuit breakers and safety fuses are not shown.) PV Can Power Your Future The National Electric Code PV systems can be cost-effective options for providing electricity to your home or The National Electric Code (NEC) was remote site. However, they are not appro- established in 1897 to ensure safety in all priate for all situations. Deciding whether systems that generate, store, transport, and consume electricity. You or the dealer who a PV system is right for you depends on installs your PV system should be careful many factors. Therefore, conduct careful to follow NEC’s equipment requirements research and consult with PV equipment so that the PV system can be approved by dealers and others who have installed these local electric code officials. Be aware that systems. If you then decide that a PV sys- many states require all electrical equipment tem is right for you, the power of the sun to be installed by a licensed electrician. will take on a new meaning in your life.

7 Source List The following are just a few of the many organizations Solar Energy International (SEI) that can help you with locating PV equipment dealers in P.O. Box 715 your area and designing and installing PV systems. Carbondale, CO 81623 (970) 963-8855 The Energy Efficiency and Renewable Energy Fax: (970) 963-8866 Clearinghouse (EREC) P.O. Box 3048 SEI offers training programs on PV system design and installa- Merrifield, VA 22116 tion, as well as on wind energy, mini-hydro systems, and solar home design. SEI also sells books on a variety of renewable (800) 363-3732 energy topics. Fax (703) 893-0400 E-mail: [email protected] On-Line Renewable Energy Information EREC provides free general and technical information to the Energy Efficiency and Renewable Energy Network public on the many topics and technologies pertaining to (EREN) energy efficiency and renewable energy, including PV systems, http://www.eren.doe.gov solar energy, and solar radiation data. EREN is the Department of Energy’s premier resource for Equipment, Dealers, and Installers information about renewable energy and energy efficiency Renewable Energy & Efficiency Training technologies, including solar radiation and photovoltaic data. Institute (RETI) National Renewable Energy Laboratory (NREL) 1800 M Street, NW http://www.nrel.gov Suite 300 NREL, one of the Department of Energy’s national laborato- Washington, DC 20036 ries, leads the nation toward a sustainable energy future by (202) 496-1417 developing renewable energy technologies. Its Web site Fax: (202) 496-1494 includes information on many renewable energy topics. See RETI offers customized PV design, installation, and mainte- NREL’s Renewable Resource Data Center, at nance programs to meet the needs of a wide range of customer http://www.rredc.nrel.gov, for solar radiation information. groups. Solar Energy Industries Association (SEIA) Reading List 122 C Street, NW 4th Floor Periodicals, Books, Pamphlets, and Reports Washington, DC 20001 Consumer Guide to Solar Energy, S. Sklar, Bonus Books, (202) 383-2600 Chicago, 1991. Fax: (202) 383-2670 Home Power Magazine: The Hands-On Journal of Home- SEIA is the national trade organization of PV and solar thermal Made Power, Home Power, Inc., P.O. Box 520, Ashland, manufacturers and component suppliers. OR 97520; (916) 475-3179; www. homepower.com. Training Programs Photovoltaic Fundamentals, National Renewable Energy Florida Solar Energy Center (FSEC) Laboratory, Document No. DE-91015001, available from Photovoltaic System Design Assistance and National Technical Information Service, U.S. Department Training Center of Commerce, 5285 Port Royal Road, Springfield, VA 1679 Clearlake Road 22161, 1991. Cocoa, FL 32922-5703 (407) 638-1000 The Solar Electric House, S. Strong, Sustainability Press, Fax: (407) 638-1010 Still River, MA, 1993. FSEC offers workshops on a variety of topics related to Solar Electricity: A Practical Guide to Designing and PV system design and use. Installing Small Photovoltaic Systems, S. Roberts, Prentice Siemens Solar Industries (formerly Arco Solar) Hall, NJ, 1991. Photovoltaic Technology and System Design Stand-Alone Photovoltaic Systems: Handbook of Recom- Training Course mended Design Practices, Sandia National Laboratory, 4650 Adohr Lane Document No. SAND87-7023, available from National Camarillo, CA 93012 Technical Information Service, U.S. Department of Com- (805) 388-6561 merce, 5285 Port Royal Road, Springfield, VA 22161, Fax: (805) 388-6395 1991 (revised). Siemens offers a one-week training program on PV technology and system design.