Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 1
www.nabcep.org
V.5.0 / 10.11
Prepared ...
2 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 3
Thank you to our Resource Guide Sponso...
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1 Introduction..........................
Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 5
What makes our Classic Comp Mount the ...
6 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 7
Introduction
This Photovoltaic (PV) In...
8 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
PV systems are electrical power genera...
Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 9
Types of PV systems are classified bas...
10 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
2. Verify System Design
While the P...
Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 11
Commercial
• Clean Power Estimator:...
12 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
Power and Energy Basics
An understand...
Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 13
A number of tools, measuring devices,...
14 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
• Will the array be subjected to ...
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Sun Position and the Solar Window
The...
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the sun paths are identical, and defi...
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The solar window represents the range...
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array has a zero degree tilt angle, a...
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Varying the array tilt angle results ...
20 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
erly orientations tend to shift the p...
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Contour charts may also be used to pl...
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trees, etc.). Even a small amount of ...
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For example, if the height of an arra...
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2.2.4 Array Mounting Methods
PV arra...
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• Terrain, elevations and grading r...
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since the weight of the array is
bala...
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system to use for the array. Span tab...
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Ballasted mounting systems are signif...
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2.3 Confirm System Sizing
2.3.1 Size...
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shade-free at the same time. It is po...
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The lead-acid cell is the most common...
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ventilation than necessary. A good ru...
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used in cold weather applications. Co...
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Racks and trays are used to support b...
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2.4.1 Determine Loads
The sizing of ...
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When the two phases (buses) in the pa...
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on the importance of the application,...
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  • 1. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 1 www.nabcep.org V.5.0 / 10.11 Prepared by: William Brooks, PE James Dunlop, PE Brooks Engineering Jim Dunlop Solar N A B C E P PV Installation Professional Resource Guide v.6/2013 www.nabcep.org Raising Standards. Promoting Confidence
  • 2. 2 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
  • 3. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 3 Thank you to our Resource Guide Sponsors Acknowledgements: NABCEP wishes to thank the companies and individuals who have made this Resource Guide possible. This document is the result of the efforts of its principal authors: Bill Brooks (Brooks Engineering) and Jim Dunlop (Jim Dunlop Solar). It is also the result of the tireless and myriad contributions of the Study Guide Committee. We are grateful to the following individuals for their contributions: Johan Alfsen (Quick Mount PV) Jason Fisher (SunPower Corp) Brian Goldojarb (Itron Corp) Mike Holt (Mike Holt Enterprises) Tommy Jacoby (Jacoby Solar Consulting) Mark Mrohs (EchoFirst) Mark Skidmore (Solon) Richard Stovall (SolPowerPeople, Inc.) We could not have produced a document of such high qual- ity without the support of our sponsors. We wish to thank the following companies who made financial contributions for the production of this guide: SMA Affordable Solar Quick Mount PV Solar Pro North Carolina Solar Center at NCSU North Carolina State University OnGrid Solar Renewable Energy World Solar Energy International Solectria, LLC Spec Tech Materials and Enclosures Stahlin Enclosures Morningstar Corporation Outback Power Renova Solar Non Endorsement Statement: The North American Board of Certified Energy Practi- tioners (NABCEP) does not assume any legal liability or responsibility for the products and services listed or linked to in NABCEP publications and website. Reference to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not constitute or imply NABCEP’s endorsement or recommendation. NABCEP 56 Clifton Country Road, Suite 202 Clifton Park, NY 12065 800-654-0021 / info@nabcep.org www.nabcep.org Design: Brownstone Graphics / ALBANY, NY Forward/Scope This document was developed to provide an overview of some of the basic requirements for solar photovol- taic (PV) system installations and those who install them. Readers should use this document along with the 2011 National Electrical Code® (NEC® ), the governing building codes and other applicable standards. These codes and standards are referenced often throughout this document, and are the principal rules that govern the installation of PV systems and any other electrical equipment. A thorough understanding of these require- ments is essential for PV system designers and installers. This document is a collaborative effort, and is consid- ered a work in progress. Future editions of this guide will incorporate comments, corrections and new content as appropriate to reflect new types of products, instal- lation methods or code requirements. Public comments are welcomed and can be directed to the following: www.pvstudyguide.org. Units of Measure Both the International System of Units (SI) and the U.S./ Imperial customary units of measure are used through- out this document. While SI units are generally used for solar radiation and electrical parameters, U.S./Impe- rial customary units are used most commonly in the U.S. construction industry for weights or measure. PV professionals are expected to be comfortable with using both systems of measurement and converting between the two given the appropriate unit conversion factors.
  • 4. 4 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 1 Introduction.........................................................................8 2 Verify System Design...............................................10 3 Managing the Project................................................82 4 Installing Electrical Components.................93 5 Installing Mechanical Components.........104 6 Completing System Installation.................108 7 Conducting Maintenance and Troubleshooting Activities...............................113 8 Appendixes.......................................................................123 References..............................................................................123 Case Study Examples.......................................................149 Sample NABCEP Exam Questions..........................156 Table of Contents Welcome to the 2013-14 edition of the NABCEP Certified PV Installation Professional Study and Resource Guide. This edition follows the most recent version of the NABCEP PV Installation Professional Job Task Analysis, which can be found at www.nabcep.org. Over the years we have received many suggestions for improving our Resource Guide. We often receive suggested corrections to perceived inaccuracies in the copy. With the publication of the 2012 resource guide, NABCEP launched an on-line forum (www.pvstudyguide.org/) where comments and suggestions can be post- ed. NABCEP Study Guide Committee members monitor this forum. As a result, the newest edition of the PV Installation Professional Study Guide includes the most relevant and appropriate suggestions we have received. We think that this open comment approach ultimately improved the Study Guide and we hope that you will find the guide to be relevant and useful. We also hope that you will contribute to our online forum if you have suggestions for improving the 2013 version. We always welcome your thoughts and constructive feedback. As ever, we wish to remind all readers of this Study and Resource Guide that it is in no way intended to be the definitive word on PV installation and design. The guide is not intended to be viewed as the sole study resource for the NABCEP PV Installation Professional Certification Examination preparation. The text and the resources in the appendix of this document are an excellent starting point for candidates preparing for the exam, however all candidates should recognize that there are many other sources of good information on the topics covered by the JTA, and they should use them. You may also find a list of references on the “resource” page at nabcep.org. The preferred method of preparation for the NABCEP exam is to review the Job Task Analysis (Exam Blueprint) to see what areas in the body of knowledge are required to pass the exam, and do an honest and thorough self-evaluation to determine what areas you may need to study the most.
  • 5. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 5 What makes our Classic Comp Mount the industry’s most trusted protection against roof leaks? The QBlock Elevated Water Seal Our patented QBlock technology encloses the EPDM rubber seal – the ultimate barrier between the rafter and the rain –  inside a cast-aluminum block and raises it 7/10 of an inch above the flashing where the rainwater flows. This completely protects the rubber seal from the elements for the life of the solar array. 925-478-8269 www.quickmountpv.com MADEIN THEUSA See how our patented QBlock technology prevents future roof leaks at quickmountpv.com/noleaks Don’t risk disastrous roof leaks with inadequate solar mounting products and methods. Insist on Quick Mount PV and install it right – and enjoy peace of mind for the full life of every PV system you install. Rubber seal raised .7" above the flashing and rainwater above the flashing and rainwater
  • 6. 6 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6
  • 7. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 7 Introduction This Photovoltaic (PV) Installation Professional Resource Guide is an informational resource intended for individu- als pursuing the PV Installation Professional Certification credential offered by North American Board of Certified Energy Practitioners (NABCEP). This guide covers some of the basic requirements for the design and installation of PV systems. Additionally, it includes numerous references to books, articles, websites, and other resources. Individuals should use this guide in conjunction with other resources in preparation for the NABCEP exam. In order to qualify for the exam, candidates should first carefully read the NABCEP Certification Handbook, which outlines certain prerequisites for education, training, and system installation experience in a responsible role, to qual- ify for the certification exam. For further information on the certification program, and how to apply, and to download the latest NABCEP Certification Information Handbook, go to: http://www.nabcep.org/certification/how-to-apply-2. This guide is organized and closely associated with the NABCEP PV Installation Professional Job Task Analysis (JTA). The JTA outlines the expected duties of a qualified PV installation professional, and defines the general knowl- edge, skills, and abilities required of those who specify, install and maintain PV systems. The JTA is the basis for the NABCEP PV Installation Professional Certification pro- gram and examination content, and should be referenced often when reviewing this document. The JTA is available for download from the NABCEP website, at: http://www. nabcep.org/certification/pv-installer-certification. The objectives of this guide are to provide general informa- tion, and additional resources concerning the key areas of the JTA. Following are the major content areas addressed in the JTA and in this guide, which serve as the specification for developing the professional examinations. The percent- ages indicate the relative numbers of exam items based on each content area. • Verify System Design (30%) • Managing the Project (17%) JTA Job Description for NABCEP Certified PV Installation Professional Given a potential site for a solar PV system installation, and given basic instructions, major components, schematics, and drawings, the PV installation professional will: specify, adapt, implement, configure, install, inspect, and maintain any type of photovoltaic system, including grid- connected and stand-alone systems with or without battery storage, that meet the performance and reliability needs of customers by incorporating quality craftsmanship and complying with all applicable codes, standards, and safety requirements. • Installing Electrical Components (22%) • Installing Mechanical Components (08%) • Completing System Installation (12%) • Conducting Maintenance and Troubleshooting Activities (11%) This guide is not an all-inclusive or definitive study guide for the exam, and exam questions are not nec- essarily based on the contents in this resource guide. Sample problems and scenarios are presented solely for example purposes, and are not to be considered representative of exam questions. A limited number of actual exam items that have been retired from the item bank are contained at the end of this document.
  • 8. 8 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 PV systems are electrical power generation systems that produce energy. They vary greatly in size and their applica- tions, and can be designed to meet very small loads from a few watts or less up to large utility-scale power plants producing tens of megawatts or more. PV systems can be designed to supply power to any type of electrical load at any service voltage. The major component in all PV systems is an array of PV modules that produces dc electricity when exposed to sunlight. Other major components may include power conditioning equipment, energy storage devices, other power sources and the electrical loads. Power conditioning equipment includes inverters, chargers, charge and load controllers, and maximum power point trackers. Energy storage devices used in PV systems are mainly batteries, but may also include advanced technologies like flywheels or other forms of storing electrical energy or the product, such as storing water delivered by a PV water pumping system. Other energy sources coupled with PV systems may include electrical generators, wind turbines, fuel cells and the electric utility grid. See Fig. 1. Balance-of-system (BOS) components include all mechanical or electrical equipment and hardware used to assemble and integrate the major components in a PV system together. Electrical BOS components are used to conduct, distribute and control the flow of power in the system. Examples of BOS components include: • Conductors and wiring methods • Raceways and conduits • Junction and combiner boxes • Disconnect switches • Fuses and circuit breakers • Terminals and connectors • Grounding equipment • Array mounting and other structural hardware  2011 Jim Dunlop Solar Solar Radiation: 2 - 2 PV System Components 1. PV modules and array 2. Combiner box 3. DC disconnect 4. Inverter (charger & controller) 5. AC disconnect 6. Utility service panel 7. Battery (optional) 1 2 3 4 5 7 6 An Introduction to Photovoltaic Systems Figure 1. - PV system components
  • 9. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 9 Types of PV systems are classified based on the loads they are designed to operate, and their connections with other electrical systems and sources. The specific components needed depend on the type of system and its functional and operational requirements. Stand-alone PV systems operate independently of other electrical systems, and are commonly used for remote power or backup applications, including lighting, water pumping, transportation safety devices, communications, off-grid homes and many others. Stand-alone systems may be designed to power dc and/or ac electrical loads, and with a few exceptions, use batteries for energy storage. A stand-alone system may use a PV array as the only power source, or may additionally use wind turbines, an engine- generator, or another auxiliary source. Stand-alone PV systems are not intended to produce output that operates in parallel with the electric utility system or other sources. See Fig. 2. Interactive PV systems operate in parallel and are intercon- nected and synchronized with the electric utility grid. When connected to local distribution systems, interactive systems supplement utility-supplied energy to a building or facility. The ac power produced by interactive systems either supplies on-site electrical loads or is back-fed to the grid when the PV system output is greater than the site load demand. At night, during cloudy weather or any other periods when the electrical loads are greater than the PV system output, the additional power required is received from the electric utility. Interactive PV systems are required to disconnect from the grid during utility outages or disturbances for safety reasons. Only special battery-based interactive inverters can provide stand-alone power for critical loads independent from the grid during outages. See Fig. 3. 2011 Jim Dunlop Solar System Components and Configurations: 4 - 2 Figure 2. Stand-alone PV systems operate autonomously and are designed to meet specific electrical loads. DC LoadPV Array Battery Charge Controller Inverter/ Charger AC Load AC Source (to Charger Only) Figure 2. Stand-alone PV systems operate autonomously and are designed to meet specific electrical loads.  2011 Jim Dunlop Solar System Components and Configurations: 4 - 3 Figure 3. Utility-interactive PV systems operate in parallel with the electric utility grid and supplement site electrical loads. Load Center PV Array Inverter AC Loads Electric Utility Figure 3. Utility-interactive PV systems operate in parallel with the electric utility grid and supplement site electrical loads. PV systems can be designed to supply power to any type of electrical load at any service voltage.
  • 10. 10 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 2. Verify System Design While the PV installer may not actually design PV systems, they must know how to interpret and review system designs and effectively execute the installation based on the plans. They must also be able to evaluate site issues affecting the design, identify discrepancies in the design or with code compliance, and recommend and implement appropriate corrective actions or alternatives. Experienced PV installers have a thorough understanding of system designs, including their major components, functions and installation requirements. 2.1 Determine Client Needs An accurate assessment of the customer’s needs is the starting point for specifying, de- signing and installing PV systems. Developing and planning PV projects requires an un- derstanding of the customer’s expectations from both financial and energy perspectives. Companies and individuals offering PV installation services must interpret the customer’s desires, and based on the site conditions, clearly explain the options, their trade-offs and costs. They must also explain the functions and operating principles for different types of PV systems, and estimate their performance relative to the customer’s electrical loads. Customer development includes addressing all other issues affecting the proposed instal- lation, such as applicable incentives, legal matters, location of equipment and appearance. Fundamentally, knowledge of the client’s needs and desires become the basis for prepar- ing proposals, quotations, and construction contracts. There are several public domain and commercial software resources available in the PV industry that address different aspects of project development and systems design. The capabilities of these tools range from simple solar resource and energy production es- timates, to site survey and system design tools, to complex financial analysis software. Some tools also provide assistance with rebate programs applications and tax incentives, while other programs and worksheets focus on the technical aspects of system sizing and design. The following lists some of the popular software tools used in the PV industry: Public Domain (NREL/DOE) • PVWATTS: www.nrel.gov/rredc/pvwatts/ • In My Back Yard (IMBY): www.nrel.gov/eis/imby/ • System Advisor Model (SAM): www.nrel.gov/analysis/sam/ NABCEP PV Technical Sales Certification The NABCEP PV Techni- cal Sales Certification is a credential offered for those specifically engaged in marketing and the customer development process for PV installations. Further infor- mation on this certification program is available on the NABCEP website: http://www.nabcep.org/ certification/pv-technical- sales-certification
  • 11. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 11 Commercial • Clean Power Estimator: www.cleanpower.com • PVSYST: www.pvsyst.com • OnGrid: www.ongrid.net • PVSol: www.solardesign.co.uk/ • PV F-Chart: www.fchart.com • Maui Solar Software: www.mauisolarsoftware.com/ • HOMER: www.homerenergy.com/ Manufacturers and Integrators • Inverter string sizing and various system sizing and design tools Assessing Energy Use Knowledge of the customer’s electrical loads and energy use are important considerations for any type of PV installation. The energy produced by PV systems will offset energy derived from another source, and represents a return on the customer’s financial investment. Be prepared to evaluate and discuss the customer’s energy use relative to the PV system options and their expected performance. This can be as simple as reviewing electrical bills for the past year or longer if available. See Fig 5. For new construction or off-grid applications, the energy use can be estimated from equipment ratings and expected load use profiles, but estimates can be highly inaccurate. Actual measurements are always preferred, and there are a number of low-cost electronic watt-hour meters available that can be readily installed to mea- sure specific loads, branch circuits or entire electrical services. Load information is used to size and design PV systems, estimate their performance and to conduct financial evaluations. For stand-alone PV applications, load energy consumption dictates the size and cost of the PV system required, and is a critical design parameter. For these systems, accurate load as- DSIRE Many websites provide information concern- ing local and state regulations for PV installations, including incentive programs, utility interconnection rules, and require- ments for contractor licensing, permitting and inspection. The Database of State Incentives for Renew- able Energy (DSIRE) is an excellent source for this information, and includes up-to-date summary information and numerous links to federal, state and local websites. For addition- al details, see: www. dsireusa.org Figure 4. The Database of State Incentives for Renewable Energy (DSIRE) contains information on rules, regulations and policies for renewable energy and energy efficiency programs in all states. Figure 5. Electric bills are reviewed as part of a site survey to evaluate customer energy use. System Components and Configurations: 4 - 5 Figure 5. Electric bills are reviewed as part of a site survey to evaluate customer energy use. sessments are a must. In many cases, a customer could have a greater benefit in changing equipment or practices to minimize their energy use, rather than installing a larger PV system to offset inefficient loads or habits. Interactive (grid-connected) PV systems may be designed to satisfy a portion of existing site electrical loads, but gen- erally no more than the total energy requirements on a net basis. Systems us- ing energy storage (batteries) for off-grid and utility back-up applications require a detailed load analysis, to adequately size the array, battery and inverter for stand-alone operation. Many PV system
  • 12. 12 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 Power and Energy Basics An understanding of power and energy fundamentals is essential for the PV professional. Electrical power is expressed in units of watts (W): 1 megawatt (MW) = 1,000 kilowatts (kW) = 1,000,000 watts (W) Electrical energy is expressed in units of watt-hours (Wh): 1 kilowatt-hour (kWh) = 1000 Wh Power and energy are related by time. Power is the rate of transferring work or energy, and analogous to an hourly wage ($/hr) or the speed of a vehicle (mi/hr). Energy is the total amount of work performed over time, and analogous to total income earned ($) or distance traveled (mi). Simply stated, energy is equal to the average power multiplied by time: Energy (Wh) = Avg. Power (W) × time (hr) sizing worksheets and software tools incorporate means to input a given electrical load and estimate the PV to load energy contribution in the results. Electrical loads are any type of device, equipment or appliance that consumes electri- cal power. Electrical loads are characterized by their voltage, power consumption and use profile. Many types of electrical loads and appliances are available in high- efficiency models. Alternating-current (ac) loads are powered by inverters, generators or the utility grid. Direct-current (dc) loads operate from a dc source, such as a bat- tery. Some small off-grid PV system applications use only dc loads, and avoid having to use an inverter to power ac loads. 2.2 Review Site Survey Site surveys are used to collect information about the local conditions and issues affect- ing a proposed PV installation. This information is documented through records, notes, photographs, measurements and other observations and is the starting point for a PV project. Ultimately, information from site surveys is used in combination with the cus- tomer desires as the basis for preparing final quotations, system designs, and planning the overall installation. There are many aspects to conducting a thorough site survey. The level of detail depends on the size and scope of the project, the type of PV system to be installed, and where and how it will be installed. Greater considerations are usually associated with commercial projects, due to the larger equipment and increased safety hazards involved. Obtaining the necessary information during a site survey helps plan and execute PV installations in a timely and cost-effective manner. It also begins the process of assembling the system manuals and project documentation.
  • 13. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 13 A number of tools, measuring devices, special equipment and safety gear may be re- quired for conducting site surveys. See Fig. 6. Some of the basic equipment includes: • Appropriate PPE including hardhats, safety glasses, safety shoes, gloves and fall protection equipment • Basic hand tools, ladders, flashlights, mirrors and magnifying glasses • Tape measures, compasses, levels, protractors and solar shading calculators • Voltmeters, ammeters, watt and watt-hour meters, and power quality analyzers • Graph paper, calculator, audio recorders, cameras and electronic notebooks A PV installer must evaluate whether a proposed site will be suitable for the installation and proper operation of the system. In general, a site assessment involves determining: • A suitable location for the array • Whether the array can operate without being shaded during critical times • The mounting method for the array • Where the balance-of-system (BOS) components will be located • How the PV system will be interfaced with existing electrical systems 2.2.1 Array Location PV arrays can be mounted on the ground, rooftops or any other suitable support struc- ture. The primary considerations for optimal PV array locations include the following: • Is there enough surface area available to install the given size PV array? • Can the array be oriented to maximize the solar energy received? • Is the area minimally shaded, especially during the middle of the day? • Is the structure strong enough to support the array and installers? • How will the array be mounted and secured? • How far will the array be from other system equipment? • How will the array be installed and maintained?  2011 Jim Dunlop Solar System Components and Configurations: 4 - 6 Figure 6. A variety of tools and equipment may be required for a site survey. Figure 6. A variety of tools and equipment may be required for a site survey.
  • 14. 14 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 • Will the array be subjected to damage or accessible to unqualified persons? • Are there local fire codes or wind load concerns that limit rooftop areas for PV installations? • Are there additional safety, installation or maintenance concerns? The answers to these and other questions will help determine the best possible locations for installing PV arrays. There are many trade-offs, and designers and installers need to evaluate potential locations based on the site conditions and other available information, and determine if a PV installation is feasible. Array Area Individual PV module characteristics and their layout dictate the overall surface area required for a PV array with a specified peak power output rating. The surface area required for a given array depends on many factors, including the individual module dimensions, their spacing in the array, and the power conversion efficiency of the mod- ules used. Fire safety codes, wind loads and accessibility to the array for installation and maintenance must also be considered when evaluating suitable array locations and lay- outs, and may limit possible locations to install PV arrays. PV arrays installed in multiple rows of tilted racks or on trackers require additional spacing between each array mount- ing structure to prevent row to row shading. Power densities for PV arrays can vary between 6 and 15 watts per square foot (W/sf) and higher, depending on module efficiency and array layout. For example, the power density of a 175 watt crystalline silicon PV module with a surface area of 14.4 sf is calcu- lated by: 175 W ÷ 14.4 sf = 12.2 W/sf For a 4 kW PV array, the total module surface area required would be: 4000 W ÷ 12.2 W/sf = 328 sf This is approximately the area of 10 sheets of plywood. Additional area is usually required for the overall PV array installation and other equipment. All things considered, it usually takes about 80 to 100 sf of surface area for a 1 kWdc rated PV array using standard crystalline silicon PV modules. For example, assum- ing an array power density of 10 W/sf, a 1 MW PV array would require 100,000 sf of array area, slightly larger than two acres and the approximate size of the rooftops on big box retail establishments. See Fig. 7. Figure 7. For a power density of 10 watts per square foot, a 500 kW PV array can be installed in a 50,000 square foot area.  2011 Jim Dunlop Solar System Components and Configurations: 4 - 7 Figure 7. For a power density of 10 watts per square foot, a 500 kW PV array can be installed in a 50,000 square foot area. 270 ft 370 ft Total roof area: 100,000 sq. ft.
  • 15. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 15 Sun Position and the Solar Window The location of the sun relative to any point on earth is defined by two ever-changing angles. The solar azimuth angle defines the direction of the sun’s horizontal projection relative to a point on earth, usually symbolized by the Greek letter Psi (c). For example, with compass headings, north is 0° or 360°, east is 90°, south is 180° and west is 270°. However, some solar equipment and computer programs use due south as the zero de- gree reference because it simplifies the complex equations used to calculate sun position. In these cases, solar azimuth angles west of south are typically represented by negative angles (due west is -90°), and east of south is represented as a positive angle (due east is +90°). The solar altitude angle defines the sun’s elevation above the horizon, and commonly symbolized by the Greek letter alpha (a). At sunrise and sunset, when the sun is on the horizon, the sun’s altitude is 0°. If the sun is directly overhead, then its altitude is 90° (at the zenith). The sun will be directly overhead at noontime some point during the year only between the Tropic of Cancer and Tropic of Capricorn. This range of tropical latitudes (23.45° north and south of the equator, respectively) is defined by the limits of solar declination and sun position, which also define the beginnings of the seasons. See Fig. 8. A sun path or sun position diagram is a graphical representation of the sun’s altitude and azimuth angles over a given day of the year, for the specified latitude. These charts can be used to determine the sun’s position in the sky, for any latitude, at any time of the day or year. Sun path diagrams are the basis for evaluating the effects of shading on PV arrays and other types of solar collectors. Typically, these charts include the sun paths for the solstices and at the equinoxes, and sometimes the average monthly sun paths or for different seasons. At the equinoxes, Figure 8. Sun position is defined by the azimuth and altitude angles. 2011 Jim Dunlop Solar Solar Radiation: 2 - 8 Figure 8. Sun position is defined by the azimuth and altitude angles. North West South East Zenith Horizontal Plane Altitude Angle Azimuth Angle Zenith Angle Solar Noon Solar noon is the local time when the sun is at its highest point in the sky and crossing the local meridian (line of longi- tude). However, solar noon is not usually the same as 12 p.m. local time due to offsets from Daylight Savings Time, and the site longitude relative to the time zone standard meridi- an, and eccentricities in the earth-sun orbit. A simple method to determine solar noon is to find the local sunrise and sunset times and calculate the midpoint between the two.
  • 16. 16 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 the sun paths are identical, and define the average sun path for the year. The equinox- es define the first days of spring and fall, and everywhere on earth, the sun rises due east and sets due west, and the sun is above the horizon for exactly 12 hours. On the equinoxes, the sun is directly overhead (solar altitude is 90°), at solar noon everywhere along the equator. A sun path chart shows all possible sun positions over a day and the year. See Fig. 9. This chart indicates that on the first day of winter (December 21), the sun rises at about 7 a.m. solar time and sets at about 5 p.m. On December 21, the sun’s highest altitude is about 37° at noontime. On March 21 and September 21, the first days of spring and fall, the sun rises at 6 a.m. at an azimuth of 90° and the highest sun altitude is 60° at solar noon. On June 21, the first day of summer, the sun rises at about 5 a.m., reaches a maximum altitude of about 83° and sets at about 7 p.m. At 9 a.m. on June 21, the azimuth is approximately 95° (slightly north of east) and the altitude is approximately 49° (about half way between the horizon and zenith). The winter and summer solstices define the minimum and maximum solar altitude angles and the range of sun paths over a year. For any location on earth, the maximum solar altitude at solar noon is a function of the solar declination and the local latitude. Since we know solar altitude at solar noon on the equator is 90° at the equinoxes, the solar altitude angle will be lower at higher latitudes by an amount equal to that lati- tude plus the solar declination. For example, at 40° N latitude on the winter solstice, the solar altitude angle at solar noon would be 90° - 40° + (-23.45°) = 26.55°. Converse- ly, on the summer solstice at the same latitude, the maximum solar altitude would be approximately 47° higher or about 73.5°, since the solar declination varies between ±23.45°. At the winter solstice, the sun is directly overhead along the Tropic of Capri- corn (23.45° S) at solar noon, and at the summer solstice, the sun is directly overhead along the Tropic of Cancer (23.45° N). See Figs. 10 a-c.  2011 Jim Dunlop Solar Solar Radiation: 2 - 9 Figure 9. A sun path chart shows the annual range of sun position for a given latitude. Sun Position for 30o N Latitude 8 AM 8 AM 8 AM 10 AM 10 AM 10 AM Noon Noon Noon 11 AM 11 AM 11 AM 1 PM 1 PM 1 PM 2 PM 2 PM 2 PM 4 PM 4 PM 4 PM 0 15 30 45 60 75 90 (180)(150)(120)(90)(60)(30)0306090120150180 << East (positive) << Azimuth Angle >> West (negative) >> AltitudeAngle(positiveabovehorizon) Winter Solstice Summer Solstice Vernal and Autumnal Equinox Figure 9. A sun path chart shows the annual range of sun position for a given latitude.
  • 17. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 17 The solar window represents the range of sun paths for a specific latitude between the winter and summer solstices. Wherever possible, PV arrays should be ori- ented toward the solar window for maximum solar energy collection. As latitudes increase to the north from the equator, the solar window is inclined at a closer angle to the southern horizon. The sun paths and days are longer during summer and shorter dur- ing winter. For any location, the maximum altitude of the sun paths at solar noon varies 47° between the winter and summer solstices. Figures 10a -10c. The solar window is defined by the limits of sun paths between the winter and summer solstices. Figure 10c. Figure 10b. Solar Declination Solar declination (d) is the ever changing angle between the earth’s equatorial plane and the sun’s rays. This is the primary geo- metric factor affecting the sun position and the solar energy received at any point on earth. Solar declination varies continuously from –23.45° to +23.45° over the year in a sinusoidal fashion, due the earth’s constant tilt and elliptical orbit around the sun. The limits of solar declination define the tropi- cal and arctic latitudes, and the range of sun position in the sky relative to any point on earth. The winter and summer solstices are defined by the minimum and maximum limits of solar declination, respectively. Solar declination is 0° at the equinoxes, when the earth’s equatorial plane is aligned directly toward the sun’s rays.  2011 Jim Dunlop Solar Solar Radiation: 2 - 11 Figure 10b. Winter Solstice Equinoxes Summer Solstice N W S E Zenith 47 Tropic of Cancer  2011 Jim Dunlop Solar Solar Radiation: 2 - 12 Figure 10c. Winter Solstice Equinoxes Summer Solstice N W S E Zenith 47 47° N Figure 10a.  2011 Jim Dunlop Solar Solar Radiation: 2 - 10 Figure 10. The solar window is defined by the limits of sun paths between the winter and summer solstices. Winter Solstice Equinoxes Summer Solstice N W S E Zenith 47 Equator 2.2.2 Array Orientation PV arrays should be oriented toward the solar window to receive the maximum amount of solar radiation available at a site, at any time. The closer an array surface faces the sun throughout every day and over a year without being shaded, the more energy that system will produce, and the more cost-effective the PV system becomes with respect to alternative power options. Similar to sun position, the orientation of PV arrays is defined by two angles. The array azimuth angle is the direction an array surface faces based on a compass heading or relative to due south. North is 0° or 360°, east is 90°, south is 180° and west is 270°. Unless site shading or local weather patterns dictate otherwise, the optimal azimuth angle for facing tilted PV arrays is due south (180° compass heading) in the Northern Hemisphere, and due north in the Southern Hemisphere. The array tilt angle is the angle between the array surface and the horizontal plane. Generally, the higher the site latitude, the higher the optimal tilt angle will be to maximize solar energy gain. A horizontal
  • 18. 18 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 array has a zero degree tilt angle, and a vertical array has a 90° tilt angle. The array azimuth angle has no significance for horizontal arrays, because they are always oriented horizontally no matter how they are rotated. See Fig. 13. nlop Solar Solar Radiation: 2 - 13 Figure 11. Magnetic compass readings must be corrected for magnetic declination. West East Geographic North South - 180 Magnetic North 270 90 0 180 Magnetic Declination (Positive, Eastern)  2011 Jim Dunlop Solar Solar Radiation: 2 - 14 Figure 12. The western U.S. has positive (easterly) declination, and will cause a compass needle to point east of geographic north. USGS East Declination (positive) West Declination (negative) Figure 11. Magnetic compass readings must be corrected for magnetic declination. Figure 12. The western U.S. has positive (easterly) declination, and will cause a compass needle to point east of geographic north. For unshaded locations, the maximum annual solar energy is received on a surface that faces due south, with a tilt angle slightly less than the local latitude. This is due to longer days and sun paths and generally sunnier skies during summer months, especially at temperate lati- tudes. Fall and winter performance can be enhanced by tilting arrays at angles greater than the local latitude, while spring and summer per- formance is enhanced by tilting arrays at angles lower than the local latitude. Adjustable-tilt or sun-tracking arrays can be used to increase the amount of solar energy received on a daily, seasonal or annual basis, but have higher costs and complexity than fixed-tilt arrays.  2011 Jim Dunlop Solar Solar Radiation: 2 - 1 Figure 13. The orientation of PV arrays is defined by the surface azimuth and tilt angles. West North East South Zenith South-facing array Southwest-facing array Tilt Angle Azimuth Angle Surface Normal Surface Direction Figure 13. The orientation of PV arrays is defined by the surface azimuth and tilt angles. Magnetic Declination Magnetic declination is the angle between mag- netic north and the true geographic North Pole, and varies with location and over time. Magnet- ic declination adjustments are made when using a magnetic compass and with some solar shad- ing devices to accurately determine due south. Magnetic compasses and devices incorporating them usually have a revolving bezel to adjust for magnetic declination. See Fig. 11. Magnetic declination is considered positive when magnetic north is east of true north and negative when magnetic north is west of true north. The western U.S. has positive (easterly) declination, and the eastern U.S. has negative (westerly) declination. Magnetic declination is near zero on a line running through Pensacola, FL, Springfield, IL and Duluth, MN, called an agonic line. The greatest magnetic declination occurs in the northeastern and northwestern most parts of the U.S. and North America. For example, a compass needle points 15° east of geographic north in Central California. Con- versely, a compass needle points about 13° west of geographic north in New Jersey. In most of the central and southern U.S., magnetic declina- tion is small and can usually be neglected, espe- cially considering the small effects of changing array azimuth angle by a few degrees. See Fig. 12.
  • 19. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 19 Varying the array tilt angle results in significant seasonal differences in the amount of solar energy received, but has a smaller impact on the total annual solar energy received. See Fig 14. For stand-alone PV systems installed at higher than tropical latitudes, the optimal tilt angle can significantly reduce the size and cost of the system required to meet a given load. For systems that have winter-dominant loads, arrays should be tilted at an angle of latitude +15°. If the array is being designed to meet a summer-dominant load, the array should to be tilted at an angle of latitude –15° to maximize solar energy collection during summer months.  2011 Jim Dunlop Solar Solar Radiation: 2 - 16 Figure 14. Array tilt angle affects seasonal performance. West North East South Winter Solstice Equinoxes Summer Solstice Zenith Latitude+15 tilt maximizes fall and winter performance Close to Latitude tilt maximizes annual performance Latitude-15 tilt maximizes spring and summer performance Figure 14. Array tilt angle affects seasonal performance. The effects of non- optimal array orienta- tion are of particular interest to PV installers and customers, be- cause many potential array locations, such as rooftops do not have optimal solar orienta- tions. When trade-offs are being made be- tween orientation and aesthetics, having this information available can help the prospec- tive owner and in- staller make decisions about the best possible array locations and their orientation. Multiplication factors can be used to adjust PV system annual energy production for various tilt angles relative to the orientation that achieves the maximum annual energy production, and are region specific. See Table 1. These tables help provide a better un- derstanding of the impacts of array orientation on the amount of solar energy received, and the total energy produced by a PV system. In fact, the amount of annual solar energy received varies little with small changes in the array azimuth and tilt angles. For south-facing arrays, array tilt angles close to 30º (a 7:12 pitch roof) produce nearly the maximum amount of energy on an annual basis for much of the continental U.S. How- ever, arrays oriented within 45º of due south (SE and SW) produce very close to the same energy (within 7%) as a south-facing array. Since shading losses are often much higher, these orientation losses tend to be smaller than one might expect. Even horizontally mounted (flat) arrays will produce more energy than systems using tilted arrays facing to the east or west. For some utility-interactive PV system installations, it may be desirable to face an array toward the southwest or even due west, provided that the array tilt is below 45º. West-
  • 20. 20 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 erly orientations tend to shift the peak array power output to the afternoon during utility peak hours, but do not necessarily maximize the energy production or financial benefit to the system owner if they are not the utility. Some net metering programs offer time- of-use rate structures to encourage the production of energy during utility peak hours. A careful analysis using an hourly computer simulation program is necessary to determine the cost benefit of these orientations. A minimum of six hours of unshaded operation is still important for best system performance. Note: The tables and charts showing the effects array orientation on the solar energy received and the energy produced by PV arrays were derived with data generated from PVWatts running simulations for various locations with different array tilt and azimuth angles. Table 1. Array orientation factors can be used to adjust the maximum available solar radiation for non-optimal orientations.  2011 Jim Dunlop Solar Solar Radiation: 2 - 18 Figure 16. PVWatts is an online tool used to estimate the performance of interactive PV systems. NREL PVWatts™ PVWatts™ is an online software model produced by the National Renewable Energy Laboratory to estimate the performance of grid-connected PV systems. See Fig 16. The user defines the site location, the maximum power for the PV array, the array mount- ing and orientation, and selects the appropriate derating factors. The software models the PV system output at each hour over a typical year, using archived solar resource and weather data. This tool can be used to evaluate the solar energy collected and energy produced by grid-tied PV systems for any location and for any array azimuth and tilt angles. To run PVWatts™ online, see: http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/. Figure 16. PVWatts is an online tool used to estimate the performance of interactive PV systems.
  • 21. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 21 Contour charts may also be used to plot similar data comparing the effects of array orientation on the amount of solar energy received. See Fig. 15. These charts clearly show that for lower latitudes and array tilt angles closer to horizontal, array azimuth angles as much as 90º from due south have a minimal effect on the solar energy received. The reduction in solar energy received for off-azimuth orientations increases with increasing tilt angles and at higher latitudes. Generally, for most of the central and southern U.S., fixed-tilt arrays with azimuth angles ±45 degrees from due south and tilt angles ±15 of the local latitude will receive at least 90% of the annual solar energy as for optimally tilted south-facing surfaces. 2.2.3 Perform a Shading Analysis A shading analysis evaluates and quantifies the impacts of shading on PV arrays. Shad- ing may be caused by any obstructions in the vicinity of PV arrays that interfere with the solar window, especially obstructions to the east, south and west of an array. This includes trees, towers, power lines, buildings and other structures, as well as obstruc- tions close to and immediately around the array, such as antennas, chimneys, plumbing  2011 Jim Dunlop Solar Solar Radiation: 2 - 17 270 240 210 180 150 120 90 0 15 30 45 60 Azimuth (deg) Tilt(deg) Available Irradiation (% of maximum) 95-100 90-95 85-90 80-85 75-80 70-75 270 240 210 180 150 120 90 0 15 30 45 60 Azimuth (deg) Tilt(deg) Available Irradiation (% of maximum) 95-100 90-95 85-90 80-85 75-80 70-75 Miami, FL Boston, MA Figure 15. The effects of varying array tilt and azimuth angles are location dependent. Figure 17. Shading of PV arrays can be caused by any obstructions interfering with the solar window.  2011 Jim Dunlop Solar Solar Radiation: 2 - 1 Figure 17. Shading of PV arrays can be caused by any obstructions interfering with the solar window. LADWP vents, dormer windows and even from other parts of the array itself. See Fig 17. Shading of PV arrays can also be caused by accumulated soiling on the array surface, which can be particularly severe in more arid regions like the western U.S., requiring regular cleaning to en- sure maximum system output. PV arrays should be unshaded at least 6 hours during the middle of the day to produce the maximum energy possible. Ideally, there should be no shading on arrays between the hours of 9 a.m. and 3 p.m. solar time over the year, since the majority of solar radiation and peak system output occur during this period. However, this is not always achievable and tradeoffs are made concern- ing the specific array location, or mitigating the shad- ing obstructions if possible (e.g., trimming or removing
  • 22. 22 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 trees, etc.). Even a small amount of shading on PV arrays during peak generation times can dramatically reduce the output of the system. Sun path charts are the basis for conducting shading evaluations. By measuring the worst-case altitude and azimuth angles of a shading object from an array location, a scale image of the obstruction can be plotted on a sun position chart for the given latitude. This shows the portion of the solar window that is obstructed by shading. Knowing the amount of receivable solar energy during different periods of a day, the shading analysis can be used to estimate the reduction in solar radiation received during the shaded times of the day and year, and ultimately estimate the reduced energy production for a PV system. These are the fundamental principles used for a shading analysis. Most system design and performance estimating tools also incorporate shading factors to derate the system output accordingly.  2011 Jim Dunlop Solar Solar Radiation: 2 - 20 Figure 18. Various devices are used to determine the extent of shading for potential PV array locations. Solar Pathfinder Solmetric SunEye Wiley ASSET Figure 18. Various devices are used to determine the extent of shading for potential PV array locations. To simplify shading evaluations, several devices and software tools have been com- mercially developed. See Fig. 18. These devices are all based on sun path charts and viewing the solar window at pro- posed array locations. The devices project or record obstructions in the solar window, and estimate the net solar energy received after shading. PV installers should be familiar with these tools, their principles of operation and how to obtain accurate results. More elaborate architectural software tools, such as Google Sketch-up and CAD programs can allow designers to simulate complex shading problems and provide detailed designs and renderings of proposed PV installations. Sources for shading evaluation tools and software include: • Solar Pathfinder™: www.solarpathfinder.com • Solmetric SunEye™: www.solmetric.com • Wiley ASSET™: www.we-llc.com • Google SketchUp™: sketchup.google.com For larger PV systems with multiple parallel rows one in front of another in the array, one row of modules can shade the one in back during winter months if the rows are too closely spaced. A six-inch shadow from an adjacent row of modules is capable of shutting down an entire string or row of modules depending on the direction of the shadows and the electrical configuration of the array. A simple rule for minimum spacing between rows is to allow a space equal to three times the height of the top of the row or obstruction in front of an array. This rule applies to the spacing for any obstructions in front of an array.
  • 23. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 23 For example, if the height of an array is three (3) feet, the minimum separation distance should be nine (9) feet since the height of the adjacent row if it is three feet above the front of the next row. See Fig. 19. In the southern half of the United States, a closer spac- ing may be possible, depending on the prescribed limits to avoid shading. However, even at the lowest latitudes the spacing should not be less than two times the height of the top of the adjacent module. Multiple rows of PV arrays can also be more closely spaced using lower tilt angles, and even with the orientation penalty of a lesser tilt angle, it is usually a better option than to suffer shading losses. The minimum required separation distances between PV array rows and other obstruc- tions depends on latitude, the height of the obstruction, and the time of day and year that shading is desired to be avoided. To avoid shading at the winter solstice between  2011 Jim Dunlop Solar Solar Radiation: 2 - 21 Figure 19. Multiple rows of rack-mounted PV arrays must be separated far enough apart to prevent shading. D Sun PV Array H β Figure 19. Multiple rows of rack-mounted PV arrays must be separated far enough apart to prevent shading.  2011 Jim Dunlop Solar Solar Radiation: 2 - 22 Figure 20. The minimum required separation distances between PV array rows and other obstructions depends on latitude, the height of the obstruction, and the time of day and year. D Separation Factor vs. Latitude for South-Facing Array Rows To Avoid Shading on Winter Solstice at Specified Solar Time 0 2 4 6 8 10 12 10 15 20 25 30 35 40 45 50 55 60 Latitude (deg N) SeparationFactor,Distance/Height(D/H) 8 am - 4 pm 9 am - 3 pm 10 am - 2 pm 11 am - 1 pm Figure 20. The minimum required separation distances between PV array rows or other obstructions depends on latitude, the height of the obstruction, and the time of day and year to avoid shading. 9 a.m. and 3 p.m. solar time, the separation distance between PV arrays and ob- structions should be at least 2 times the height of the ob- struction at latitudes around 30°, 2-1/2 times the height at latitudes around 35°, 3 times the height at 40° latitude and 4 times the height at 45° latitude. See Fig. 20.
  • 24. 24 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 2.2.4 Array Mounting Methods PV arrays can be mounted on the ground, rooftops and other structures that provide adequate protection, support and solar access. The site conditions usually dictate the best mounting system location and approach to use. Rooftops are very popular locations for installing PV arrays. Because they are elevated, roof mounts offer some physical protection and limited access to the array for safety, and usually provide better sun exposure. Rooftop PV installations also do not occupy space on the ground that might be needed for other purposes. Rooftop and other building- mounted PV arrays must be structurally secured and any attachments and penetrations must be properly weathersealed. Available rooftop areas for mounting PV arrays may be limited by any number of factors, including required spaces about the array for instal- lation and service, pathways and ventilation access for fire codes, wind load setbacks, and spaces for other equipment. Sloped roofs also present a significant fall hazard, and require appropriate fall protection systems and/or personal fall arrest systems (PFAS) for installers and maintenance workers. The layout of a PV array can have a significant effect on its natural cooling and operating temperatures. A landscape (horizontal) layout may have a slight benefit over a portrait (vertical) layout when considering the passive cooling of the modules. Landscape is when the dimension parallel to the eaves is longer than the dimension perpendicular to the eaves. In a landscape layout, air spends less time under the module before escap- ing and provides more uniform cooling. Standoff mounts operate coolest when they are mounted at least 3 inches above a roof. Key items to evaluate during a site survey for roof-mounted PV arrays include: • Building type and roof design • Roof dimensions, slope and orientation • Roof surface type, condition and structural support • Fall protection methods required • Access for installation and maintenance   Ground-mounted PV arrays are commonly used for larger systems, or where rooftop in- stallations are not possible or practical. Ground-mounts can use a variety of racks, poles and other foundations to support the arrays. Ground-mounted arrays are generally more susceptible to damage than roof-mounted arrays, although their location and orientation is less constrained than for rooftop installations. If an array is mounted at ground level, NEC 690.31(A) requires that the wiring be protected from ready access. Several options may be possible to meet this requirement, including protecting the wiring with non- conductive screening like PVC, limiting access with security fencing, or by elevating the array. Elevating arrays also provides physical protection, and usually helps avoid shad- ing concerns that may exist at lower heights. Site surveys for ground-mounted PV arrays should consider: • Zoning and land use restrictions
  • 25. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 25 • Terrain, elevations and grading requirements • Soil type and array ground-cover • Water table, flood zones and drainage • Array foundation requirements • Security requirements and fencing • Access for vehicles, equipment and maintenance The following are common types of PV array mounting systems: Integral mounting systems are where modules are integrated into the roofing or building exterior. These systems are sometimes referred to as building-integrated PV or BIPV. Standoff mounting, referred to by some as flush mounting, uses standoffs attached to the roof to support rails on which PV modules are attached. This is the most common method for residential installations. See Fig. 21. Figure 21. Standoff mounts are the most common way PV arrays are attached to sloped rooftops.  2011 Jim Dunlop Solar Solar Radiation: 2 - 23 Figure 21. Standoff mounts are the most common way PV arrays are attached to sloped rooftops. Gary Lee Sharp Solar op Solar Solar Radiation: 2 - 23 Figure 21. Standoff mounts are the most common way PV arrays are attached to sloped rooftops. y Lee Sharp Solar  2011 Jim Dunlop Solar Solar Radiation: 2 - 23 Figure 21. Standoff mounts are the most common way PV arrays are attached to sloped rooftops. Gary Lee Sharp Solar Ballasted mounting systems are often used in large-scale flat roof commercial projects. These mounting systems require engineering for roof structural loading and ballast re- quirements. Often roof tethers augment the ballast for seismic concerns or excessive wind requirements. See Fig. 22.  2011 Jim Dunlop Solar Solar Radiation: 2 - 24 Figure 22. Self-ballasted PV arrays are a type of rack mount that relies on the weight of a the PV modules, support structure and additional ballast material to secure the array. Ascension Technology University of Wyoming  2011 Jim Dunlop Solar Solar Radiation: 2 - 24 Figure 22. Self-ballasted PV arrays are a type of rack mount that relies on the weight of a the PV modules, support structure and additional ballast material to secure the array. Ascension Technology University of Wyoming  2011 Jim Dunlop Solar Solar Radiation: 2 - 24 Figure 22. Self-ballasted PV arrays are a type of rack mount that relies on the weight of a the PV modules, support structure and additional ballast material to secure the array. Ascension Technology University of Wyoming Figure 22. Self-ballasted PV arrays are a type of rack mount that relies on the weight of a the PV modules, support structure and additional ballast material to secure the array. Rack mounting is typically used for non-tracking systems at ground level and on flat rooftops. This method is typical on large commercial or utility-scale arrays. Pole mounting, is typically used with manufactured racks mounted on top or attached to the side of a steel pole. Pole-top arrays are common for off-grid residential PV systems,
  • 26. 26 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 since the weight of the array is balanced over the pole, allow- ing easy seasonal adjustment. Side-of-pole mounts are most common in small one- or two- module applications where the entire system, such as remote telemetry application, is mount- ed on a single pole. See Fig. 23. Tracking mounting systems are systems that follow the sun on a daily or seasonal basis. Track- ing may increase summer gain by 30% or more, but winter gain may be 15% or less. Tracking may be two-axis for maximum performance or single-axis for simplicity and reliability. See Fig. 24.  2011 Jim Dunlop Solar Solar Radiation: 2 - 25 Figure 23. Pole-mounted arrays use either fixed, adjustable, or sun-tracking arrays installed on a rigid metal pipe.  2011 Jim Dunlop Solar Solar Radiation: 2 - 26 Figure 24. Sun-tracking arrays are typically mounted on poles and increase the amount of solar energy received. NREL, Warren Gretz Figure 24. Sun-tracking arrays are typically mounted on poles and increase the amount of solar energy received. Roof Structure and Condition An important consideration for roof-mounted PV arrays is to assess the condition of the roofing system and determine whether the roof and its underlying structure can support the additional load. Structural loads on buildings are due to the weight of building materials, equipment and workers, as well as contributions from outside forces like hydrostatic loads on founda- tions, wind loads and seismic loads. The requirements for determining structural loads on buildings and other structures are given in the standard ASCE 7 – Minimum Design Loads for Buildings and other Structures, which has been adopted into the building codes. A structural engineer should be consulted if the roof structure is in question, or if specific load calculations are required for local code compliance. Common stand-off roof-mounted PV arrays, including the support structures generally weigh between 3 and 5 pounds per square foot (psf), which should be fine for most roofs designed to recent standards. Generally, houses built since the early 1970’s have been through more rigorous inspection and tend to have more standard roof structures than those built prior to that period. If the attic is accessible, a quick inspection of the type of roof construction is worthwhile, and will help determine the appropriate attachment
  • 27. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 27 system to use for the array. Span tables are available in various references, which can help quantify the load-bearing capabilities of roof trusses or beams. For further information see: www.solarabcs.org/permitting Wind loads are a primary concern for PV arrays, especially in hurricane-prone regions. The design wind loads for PV arrays in some Atlantic and Gulf coastal re- gions can exceed 50 PSF and greater on certain portions of a roof or structure. While common stand-off PV arrays do not generally contribute to any additional wind loads on a structure, the array attachment points to the structure or foundation must be of sufficient strength to withstand the design loads. For example, a 15 square-foot PV module could impose an uplift load of 750 pounds under a design load of 50 psf. A panel of four of these modules can impose a load of 3,000 pounds on the entire mounting structure. If the panel is secured by six roof attachments, and if the forces are distributed equally, there would be a 500-pound force on each attachment, and it must be designed and installed to resist this maxi- mum uplift force. Several manufacturers of roof mounting systems provide engi- neering analysis for their mounting systems and attachment hardware. Without this documentation, local inspectors may require that a custom mounting system have a structural analysis from a professional engineer for approval. This engineering documentation easily justifies the additional costs of purchasing mounting hard- ware from a qualified mounting system manufacturer.   The age and condition of the roof covering must also be evaluated. If the roof cover- ing is due for replacement within the next 5 to 10 years, it typically makes sense to roof the building before installing the PV system, as the array would need to be removed and replaced before and after the roofing work.Different types of roof coverings have different lifetime expectations and degradation mechanisms, and wherever roofing issues are a concern for PV installation, it is highly advisable to engage a licensed roofing contractor in the project. Before recommending or deciding on any PV array mounting system, verify with the mounting system supplier that the hardware is appropriate for the given ap- plication.Also, it is generally not advisable to try to fabricate or copy a mounting system design for smaller projects. This usually costs much more than purchasing a pre-engineered system, and may not meet the structural or environmental require- ments of the application. PV array mounting structures also must be electrically connected to the equipment grounding system, and special bonding jumpers and connectors are available to maintain electrical continuity across separate structural components. Oftentimes, local jurisdictions require engineering documentation to certify the structural integrity of the mounting system and attachments. Commercial Roof Mounting Options PV arrays are mounted on large commercial buildings with flat composition roofs using a variety of racking systems. These mounting structures may be secured by fasteners and physical attachments to the building structure, or by using ballasted racking, or a combination of both to hold the array in place.
  • 28. 28 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 Ballasted mounting systems are significantly heavier than mounting systems de- signed for direct structural attachments, depending on the weight of ballast used, and usually require special load calculations. The main advantages of ballasted mounts include easier installation, and by eliminating direct structural attachments and penetrations into the structure, the possibility of roof leaks is greatly dimin- ished. Ballasted mounting systems are engineered for specific wind loads and roof structures, and have very specific requirements on how to install the array. Even when wind loading is not a concern, additional restraints may be required on ballasted arrays for seismic loads.   2.2.5 BOS Locations Any site survey also includes identifying proposed locations for all BOS compo- nents, including inverters, disconnects, overcurrent devices, charge controllers, batteries, junction boxes, raceways, conductors and any other electrical apparatus or mechanical equipment associated with the system. The PV installer must ensure that all equipment locations are suitable for the intended equipment. Considerations for BOS locations include providing for accessibility to the equip- ment for installation and maintenance. Some BOS components may need to be installed in weather-resistant or rain-tight enclosures if they are not installed in- doors. Other components, including many utility-interactive inverters, may already be rated for wet and outdoor locations. Minimum clearances and working spaces are required for electrical equipment that may be serviced in an energized state. Dedi- cated clear spaces are also required above and in front of all electrical equipment. These and many other installation requirements are outlined in Article 110 of the NEC: Requirements for Electrical Installations. Avoid installing electrical equipment in locations exposed to high temperatures and direct sunlight wherever possible, and provide adequate ventilation and cooling for heat-generating equipment such as inverters, generators and chargers. Consid- erations should also be taken to protect equipment from insects, rodents, and other debris. All electrical equipment must be properly protected from the environment unless the equipment has applicable ratings. This includes protection from dust, rain and moisture, chemicals and other environmental factors. All electrical equipment contains instructions on the proper installation of the equipment, and for the environmental conditions for which it is rated. Some equipment has special considerations, covered under different sections of the electrical and building codes, and in manufacturer’s instructions. For example, bat- tery locations should be protected from extreme cold, which reduces their available capacity. Battery containers and installation must follow the requirements in NEC 480. Major components are generally located as close together as possible, and to the electrical loads or services that they supply, in order to minimize the length of conductors, voltage drop and the costs for the installation.
  • 29. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 29 2.3 Confirm System Sizing 2.3.1 Size the Module Mounting Area If a roof is selected for the array location, then it is necessary to determine whether the roof is large enough for the proposed number of PV modules. For roof areas with non- rectangular shapes, determining the amount of useable roof area can be a challenge. When laying out a plan for mounting modules on a roof, access to the modules must be provided for maintenance. For easiest access, a walkway should be provided between rows of modules. However, this consumes valuable roof area, so a balance needs to be made between the area for the array and access. New requirements in the 2012 Inter- national Fire Code [IFC 605.11] require clear space at the edges and peaks of roofs for firefighter access. This poses a challenge to roof-mounted PV systems. Often, only 50% to 80% of the roof area that has a suitable orientation can be used for mounting modules when room for maintenance, wiring paths, firefighter access and aesthetic considerations are taken into account. To determine the size of the PV array (ultimately the power rating of the system) that can be installed, the usable roof area must be first established. The dimensions and orien- tation of individual modules may allow various layouts for the array that ultimately need to fit within the usable areas of the roof. The location of structural attachments, the desired electrical configuration, and wire routing are also important considerations when determining the best layout. Computer-aided drawing tools can be helpful in determin- ing possible acceptable array layouts given module and roof dimensions. Smaller array surface areas are required to generate the same amount of power with higher efficiency modules. By definition, a 10% efficient PV module has a power density of 100 W/m2 (approximately 10 W/sf) peak power output when exposed to 1000 W/m2 solar irradiance. Crystalline silicon PV modules may have efficiencies 12% to 15% and higher for special higher-price models. Higher efficiency modules means less support structure, wiring methods and other installation hardware are required for an array. Most thin-film PV module technologies have efficiencies below 10%, and require correspond- ingly larger array areas to produce an equal amount of power. For example, consider a roof with overall dimensions of 14’ by 25’ (350 sf) with a usable area of 250 sf (71% of total). This roof area would be sufficient for a 2.5 kW crystalline silicon array (250 sf x 10 W/sf= 2500 W) or an 8% efficient thin film array of about 2 kW. 2.3.2 Arrange Modules in Mounting Area Siting the PV array in the available mounting area can have a large impact on the per- formance of a PV array. In addition to shading and orientation, the array layout must be consistent with the electrical string layout. A string is a series-connection of PV modules in an array. Each set of modules in a series string must be oriented in the same direction if the string is to produce its full output potential. For example, if a string has 12 modules in series, all 12 modules must be in the same or parallel planes of a roof and ideally be
  • 30. 30 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 shade-free at the same time. It is possible to split a string between two roof faces, pro- vided the modules face the same direction. The outputs of multiple strings having similar voltage but using different current output modules, or facing different directions may be connected in parallel. This characteristic of string inverters poses a design challenge on many residential proj- ects. For instance, a roof may be large enough to hold 24 modules on the south and west faces together. However, the south face may be large enough to mount 16 modules and the west face only large enough to mount 8 modules. If the inverter requires 12 modules in series, the west face is not usable and the south face will only permit 12 modules to be installed. This means that only half the potential array area can be utilized by that string inverter system. This example suggests that it might be reasonable to find an inverter with lower input voltage that only requires 8 modules in series, or consider using module level micro-inverters to avoid string sizing requirements altogether.   2.4 Review Design Energy Storage Systems A battery converts chemical energy to electrical energy when it is discharged, and con- verts electrical energy to chemical energy when it is charged. Because the power pro- duced by PV arrays does not always coincide with electrical loads, batteries are common- ly used in most stand-alone PV systems to store energy produced by the PV array, for use by system loads as required. Batteries also establish the dc operating voltage for the PV array, charge controllers and dc utilization equipment, including inverters and dc loads, as applicable. Batteries are sometimes used in interactive systems, but only with special types of battery-based inverters intended for interactive operation, also called multi-mode invert- ers. These inverters operate as diversionary charge controllers and dump excess PV array energy to the grid when it is energized [NEC 690.72]. When there is a loss of grid voltage,  2011 Jim Dunlop Solar Solar Radiation: 2 - 27 Figure 25. Utility-interactive systems with battery storage are similar to uninterruptible power supplies, and have many similar components. Inverter/ Charger Critical Load Sub Panel Backup AC Loads Main Panel Primary AC Loads Electric Utility Bypass circuit BatteryPV Array AC Out AC In DC In/out Charge Control Figure 25. Utility-interactive systems with battery storage are similar to uninterruptible power supplies, and have many similar components. these inverters transfer loads from the grid to operate in stand-alone mode. Interactive systems with battery backup cost significantly more to install than simple inter- active systems without batteries, due to the additional equipment required (special inverters, batteries and charge controllers). The design and installation of these systems is also more complex, and usually involves conducting a load analysis and reconfiguring branch circuits in dedicated subpanels. See Fig. 25.
  • 31. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 31 The lead-acid cell is the most common type of storage battery used in PV systems. Oc- casionally nickel-cadmium or other battery technologies are used. Newer battery types like lithium-ion are also becoming possible as the costs of these battery systems continue to decrease and performance improves. A motive power or traction battery is a type of lead-acid battery designed for use in deep discharge applications, such as electric vehicles. Motive power batteries are robust and are commonly used in stand-alone PV systems. A starting, lighting and ignition (SLI) bat- tery has a larger number of thinner plates to provide a greater surface and can deliver higher discharge currents, but are damaged by frequent and deep discharges, and are sel- dom used in PV systems. Deep discharge-type batteries differ from automobile starting batteries in several respects, mainly their designs use heavier, thicker plates and stronger inter-cell connections to better withstand the mechanical stresses on the battery under frequent deep discharges. Flooded batteries have a liquid electrolyte solution. Open-vent flooded types have removable vent caps and permit electrolyte maintenance and water additions. Valve-reg- ulated lead-acid (VRLA) batteries have an immobilized electrolyte in gel form or absorbed in fiberglass separator mats between the plates. VRLA batteries are spill proof and do not require electrolyte maintenance, however they are more expensive and less tolerant of overcharging and higher operating temperatures than flooded types. Charge controllers must use appropriate charge regulation settings for the type of battery used. See Fig 26. Vented lead-acid batteries release hydrogen and oxygen gases, under normal charging conditions. This is due to electrolysis of the electrolyte solution during final charging stages, and results in water loss. Consequently, adequate ventilation must be provided for both vented and sealed battery systems [NEC 480.9 and 480.10]. While it is difficult to determine adequate ventilation requirements, it is generally advisable to provide greater 2011 Jim Dunlop Solar Batteries: 6 - 28 Figure 26. Both flooded and sealed lead-acid batteries are commonly used in PV systems. BATTERY TYPE ADVANTAGES DISADVANTAGES FLOODED LEAD-ACID Lead-Antimony low cost, wide availability, good deep cycle and high temperature performance, can replenish electrolyte high water loss and maintenance Lead-Calcium Open-Vent low cost, wide availability, low water loss, can replenish electrolyte poor deep cycle performance, intolerant to high temperatures and overcharge Lead-Calcium Sealed-Vent low cost, wide availability, low water loss poor deep cycle performance, intolerant to high temperatures and overcharge, can not replenish electrolyte Lead-Antimony/Calcium Hybrid medium cost, low water loss limited availability, potential for stratification VALVE-REGULATED LEAD-ACID Gelled medium cost, little or no maintenance, less susceptible to freezing, install in any orientation fair deep cycle performance, intolerant to overcharge and high temperatures, limited availability Absorbed Glass Mat medium cost, little or no maintenance, less susceptible to freezing, install in any orientation fair deep cycle performance, intolerant to overcharge and high temperatures, limited availability NICKEL-CADMIUM Sealed Sintered-Plate wide availability, excellent low and high temperature performance, maintenance free only available in low capacities, high cost, suffer from ‘memory’ effect Flooded Pocket-Plate excellent deep cycle and low and high temperature performance, tolerance to overcharge limited availability, high cost, water additions required Figure 26. Both flooded and sealed lead-acid batteries are commonly used in PV systems.
  • 32. 32 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 ventilation than necessary. A good rule is to provide similar ventilation to a battery room as is required for a combustion water heater. VRLA batteries do not release gasses under normal charging, and have lower ventilation requirements than flooded open vent types. Capacity is a measure of battery energy storage, commonly rated in ampere-hours (Ah) or kilowatt-hours (kWh). For example, a nominal 6-volt battery rated at 220 Ah stores 1.32 kWh of energy. Battery design features that affect battery capacity include the quan- tity of active material, the number, design and physical size of the plates, and electrolyte specific gravity. Usable capacity is always less than the rated battery capacity. Opera- tional factors that affect the usable battery capacity include discharge rate, cut-off voltage, temperature and age of the battery. See Fig. 27. The rate of charge or discharge is expressed as a ratio of the nominal battery capacity (C) to the charge or discharge time period in hours. For example, a nominal 100 ampere-hour battery discharged at 5 amps for 20 hours is considered a C/20, or 20-hour discharge rate. The higher the discharge rate and lower the temperature, the less capacity that can be withdrawn from a battery to a specified cutoff voltage. See Fig. 28. State-of-charge is the percentage of available battery capacity compared to a fully charged state. Depth-of-discharge is the percentage of capacity that has been removed from a bat- tery compared to a fully charged state. The state-of-charge and depth-of-discharge for a battery add to 100 percent. The allowable depth-of-discharge is the maximum limit of battery discharge in operation. The allowable depth-of-discharge is usually limited to no more than 75 to 80% for deep cycle batteries, and must also be limited to protect lead-acid bat- teries from freezing in extremely cold conditions.   Specific gravity is the ratio of the density of a solution to the density of water. Sulfuric- acid electrolyte concentration is measured by its specific gravity, and related to battery state of charge. A fully charged lead-acid cell has a typical specific gravity between 1.26 and 1.28 at room temperature. The specific gravity may be increased for lead-acid batteries Batteries: 6 - 29 Voltage(V) Capacity (Ah) Cut off voltage High discharge rate Low discharge rate Figure 27. Battery capacity is a measure of the stored energy that a battery can deliver under specified conditions.  2011 Jim Dunlop Solar 30 40 50 60 70 80 90 100 110 120 -30 -20 -10 0 10 20 30 40 50 C/500 C/120 C/50 C/5 C/0.5 Battery Operating Temperature ( o C ) Percentof25o CCapacity Figure 28. The higher the discharge rate and the lower the temperature, the less capacity that can be withdrawn from a battery to a specified cutoff voltage.
  • 33. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 33 used in cold weather applications. Conversely, the specific gravity may be decreased for applications in warm climates to prolong battery life. In very cold climates, batteries must be protected from freezing by installing in a suit- able enclosure, or by limiting the depth of discharge. Because the density of electrolyte decreases with increasing temperature, specific gravity readings must be adjusted for temperature. Inconsistent specific gravity readings between cells in a flooded lead-acid battery indicate the need for an equalizing charge. Many factors and trade-offs are considered in battery selection and systems design, and are often dictated by the application or site requirements. Among the factors to consider in the specification and design of battery systems include: • Electrical properties: voltage, capacity, charge/discharge rates • Performance: cycle life vs. DOD, system autonomy • Physical properties: Size and weight, termination types • Maintenance requirements: Flooded or VRLA • Installation: Location, structural requirements, environmental conditions • Safety and auxiliary systems: Racks, trays, fire protection, electrical BOS • Costs, warranty and availability Most PV systems using batteries require a charge controller to protect the batteries from overcharge by the array. Only certain exceptions apply for special self-regulated systems, which are designed using very low charge rates, special lower voltage PV modules, larger batteries and well-defined, automated loads. If the maximum charge rates from the PV array multiplied by one hour is equal to 3% of the battery nominal amp-hour capacity or greater, a charge controller is required [NEC 690.72]. If a battery is overcharged, it can create a hazardous condition and its life is generally reduced, especially for sealed, valve- regulated lead-acid (VLRA) batteries. Many charge controllers also include overdischarge protection for batteries, by disconnecting loads at a predetermined low-voltage, low state-of-charge condition. Battery installations in dwellings must operate less than 50 volts nominal, unless live parts are not accessible during routine battery maintenance. This requirement generally limits the voltage of lead-acid batteries to no more than 48 volts, nominal. This equates to either 24 series-connected nominal 2-volt lead-acid cells, or 40 series-connected nominal 1.2-volt alkali type nickel cadmium cells. All battery installations in dwellings must have live parts guarded. Live parts must also be guarded for any battery installations 50 volts or greater by elevation, barriers or location in rooms accessible to only qualified persons. Sufficient working spaces and clearances must be provided for any battery installations [NEC 110.26]. If the nominal voltage of a battery bank exceeds 48 V, then the batteries shall not be installed in conductive cases, unless they are VRLA batteries designed for installation with metal cases [NEC 690.71(D)]. Note that 48 V nominal battery banks typically operate above 50 V and exceed the 50 V limit for ungrounded PV systems [NEC 690.41]. Battery systems either must have a system grounded conductor or meet the requirements for ungrounded systems [NEC 690.35].
  • 34. 34 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 Racks and trays are used to support battery systems and provide electrolyte contain- ment. Racks can be made from metal, fiberglass or other structural nonconductive materials. Metal racks must be painted or otherwise treated to resist degradation from electrolyte and provide insulation between conducting members and the bat- tery cells [NEC 480.9]. Due to the potential for ground faults, metal or other conduc- tive battery racks, trays and cases are not allowed for open-vent flooded lead-acid batteries more than 48 volts nominal. In addition, conductive racks are not permitted to be located within 150 mm (6 in.) of the tops of the nonconductive battery cases [NEC 690.71(D)]. These requirements do not apply to sealed batteries that are manu- factured with conductive cases. Any conductive battery racks, cases or trays must also have proper equipment grounding [NEC 250.110]. If batteries are connected in series to produce more than 48 V (nominal), then the bat- teries must be connected in a manner that allows the series strings of batteries to be separated into strings of 48 V or less for maintenance purposes [NEC 690.71(D-G)]. The means of disconnect may be non-load-break, bolted, or plug-in disconnects. For strings greater than 48 V, there must also be a means of disconnecting the grounded circuit conductors of all battery strings under maintenance without disconnecting the grounded conductors of operating strings. Whenever the available fault current of a battery exceeds the interrupt ratings of nor- mal overcurrent devices, disconnect means or other equipment in a circuit, special current-limiting overcurrent devices must be installed [NEC 690.9, 690.71]. While many dc-rated circuit breakers do not have sufficient interrupt ratings, current limit- ing fuses are available with interrupt rating 20,000 A and higher. Whenever these fuses may be energized from both sides, a disconnect means is required to isolate the fuse from all sources for servicing [NEC 690.16]. A disconnecting means must also be provided for all ungrounded battery circuit conductors, and must be readily acces- sible and located within sight of the battery system [NEC 690.17]. To prevent battery installations from being classified as hazardous locations, venti- lation of explosive battery gasses is required. However, the NEC does not provide specific ventilation requirements. Vented battery cells must incorporate a flame arres- tor to help prevent cell explosions from external ignition sources, and cells for sealed batteries must have pressure relief vents [NEC 480.9, 480.10]. Special safety precautions, equipment and personal protective equipment (PPE) are required when installing and maintaining battery systems. Hazards associated with batteries include caustic electrolyte, high short-circuit currents, and explosive poten- tial due to hydrogen and oxygen gasses produced during battery charging. Insulated tools should be used when working on batteries to prevent short-circuiting. High- voltage battery systems may present arc flash hazards, and special PPE, disconnect- ing means and equipment labeling may apply [See NFPA 70E]. Batteries are also very heavy and should only be lifted or supported by methods approved by the manu- facturer. Battery installations over 400 lbs may also have to meet certain engineering requirements in seismic regions for the design of non-structural electrical compo- nents [See ASCE 7-10].
  • 35. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 35 2.4.1 Determine Loads The sizing of batteries or any other energy storage system is based on the magnitude and duration of the applied electrical loads. The average power consumption of the elec- trical loads defines the maximum discharge rates as well as the total energy withdrawn from the battery on an average daily basis. The size of the battery (total capacity) is selected based on these system parameters and the desired maximum and average daily depth-of-discharge. The maximum battery depth-of-discharge in actual system opera- tion is determined by the low-voltage load disconnect, the discharge rate, temperature and other factors.. Identify all existing and planned electrical loads that will be connected to the system, including their ac or dc operating voltage, their power or current consumption, and their expected average daily use. List all loads and multiply the power use by the aver- age daily time of operation to determine daily energy consumption and peak power demand. See Fig. 29. In practice, the inverter should be large enough to power the total connected load, but is only required to be as large as the single largest load [NEC 690.10(A)]. 2.4.2 Identify Circuits for Required Loads Load circuits supplied by stand-alone PV systems must be clearly identified and limited to the design loads. Additional loads beyond what the system has been designed to supply will ultimately result in decreasing battery state-of-charge and reduced battery lifetime. Ensure that only critical loads are connected and that the most efficient loads and practices are used wherever possible. In all cases, do not exceed the load estimates for which the system was designed unless additional generation resources are used. Multiwire Branch Circuits Many stand-alone PV systems use inverters with 120 Vac output, with the hot leg con- nected to both sides (phases) of a common 120/240 V split-phase load center. Normally with 240 V service, the current on one phase is 180 degrees opposed to the current on the other phase, and results in neutral conductor currents equal to the difference be- tween the two phase currents.  2011 Jim Dunlop Solar Batteries: 6 - 31 Electrical Load Power (W) Avg. Daily Time of Use (hr) Avg. Daily Energy (watt- hours) Lighting 200 6 1200 Refrigerator 300 9.6 (40% duty cycle) 2880 Microwave 1200 0.5 600 Pumps 1000 1 1000 TV and entertainment equipment 400 4 1600 Fans 300 6 1800 Washer 400 0.86 (3 hours 2 times per week) 344 Miscellaneous plug loads 200 12 2400 Total all loads 4000 W (4 kW) 11,824 Wh (11.8 kWh) Figure 29. A load assessment evaluates the magnitude and duration of electrical loads. OSHA requirements for battery installations include the following: • Unsealed batteries must be installed in ventilated enclo- sures to prevent fumes, gases, or electrolyte spray entering other areas, and to prevent the accumulation of an explosive mixture. • Battery racks, trays and floors must be of sufficient strength and resistant to electrolyte. • Face shields, aprons, and rubber gloves must be provided for workers handling acids or batteries, and facilities for quick drenching of the eyes and body must be provided within 25 feet of battery handling areas. • Facilities must be provided for flushing and neutralizing spilled electrolyte and for fire protection. • Battery charging installations are to be located in designated areas and protected from dam- age by trucks. • Vent caps must be in place during battery charging and maintained in a functioning condition.
  • 36. 36 • NABCEP PV Installation Professional Resource Guide Copyright © 2013 NABCEP v. 6 When the two phases (buses) in the panel are connected together to distribute the 120 V source, the currents on both sides of the panel are now in phase with each other and are additive. If multiwire branch circuits that share a neutral conductor for two branch circuits are connected to this modified distribution panel, the neutral conductor can potentially become overloaded and create a fire hazard. For these installations, a special warning sign is required on the panel to prohibit the connection of multiwire branch circuits [NEC 690.10(C)].   2.4.3 Batteries and Battery Conductors The goal of battery wiring is to create a circuit that charges and discharges all batteries equally. If batteries are connected in series, this is automatic, but if batteries are connected in parallel, the currents may be unequal due to subtle differences in cable resistance and connections. All batteries used in a battery bank must be the same type, same manufac- turer, the same age, and must be maintained at equal temperatures. Batteries should have the same charge and discharge properties under these circumstances. Series batteries connections build voltage while capacity stays the same as for one battery. See Fig. 30. Parallel battery connections build capacity while voltage stays the same. See Fig. 31. Parallel connections are made from opposite corners of the battery bank to help equalize the voltage drop and current flow through each string. In general, no more than four batteries or series strings of batteries should be connected in parallel. It is better to use larger batteries with higher ampere-hour ratings than to connect batteries in parallel. Large conductors, such as 2/0 AWG, 4/0 AWG or larger, are typically used to minimize voltage drop in battery connections. Figure 30. Series battery connections increase voltage.  2011 Jim Dunlop Solar Batteries: 6 - 32 Figure 30. Series battery connections increase voltage. Battery 2 12 volts 100 amp-hours 24 volts 100 amp-hours Total: + - + - Battery 1 12 volts 100 amp-hours + -  2011 Jim Dunlop Solar Solar Radiation: 2 - 33 Figure 31. Parallel battery connections increase capacity. Battery 2 12 volts 100 amp-hours 12 volts 200 amp-hours Total: + - Battery 1 12 volts 100 amp-hours + - + - Figure 31. Parallel battery connections increase capacity. Listed flexible cables rated for hard service usage are permitted to be used for battery conductors, and can help reduce excessive terminal stress that can occur with standard stranded conductors [NEC 690.74, Art. 400]. Welding cable (listed or not listed), automo- tive battery cables, diesel locomotive cables (marked DLO only) and the like may not meet NEC requirements for battery connections. Properly rated cable will have a conduit rating such as THW or RHW to meet building wiring requirements. Size Batteries for Loads Battery sizing in most PV systems is based on the average daily electrical load and a de- sired number of days of battery storage. The number of days of storage is selected based
  • 37. Copyright © 2013 NABCEP v. 6 NABCEP PV Installation Professional Resource Guide • 37 on the importance of the application, and the desired average daily depth- of-discharge for the battery. Autonomy is defined as the number of days that a fully charged battery can meet system loads without any recharging. Autonomy is calculated by the nominal battery capacity, the average daily load and the maximum allowable depth-of-discharge. Larger autonomy means a larger battery with higher costs, and shallower aver- age daily depth-of-discharge, lower charge and discharge rates, and usu- ally longer battery life. Figure 32. A charge controller is required in most PV systems that use battery storage to regulate battery state-of-charge.  2011 Jim Dunlop Solar Solar Radiation: 2 - 34 Charge controller protects battery from overcharge by PV array Charge Controller BatteryPV Array  2011 Jim Dunlop Solar Solar Radiation: 2 - 35 Figure 33. Charge controllers used in PV systems vary widely in their size, functions and features. Morningstar TriStar controller Morningstar ProStar controller Morningstar lighting controller Outback MPPT controller Xantrex C-series controller Figure 33. Charge controllers used in PV systems vary widely in their size, functions and features. For example, consider a system load that is 100 Ah per day. A 400 Ah battery is selected, with a desired allowable depth-of-discharge of 75% (300 Ah usable). This battery design would deliver 3 days of autonomy in this system (3 days × 100 Ah/day = 300 Ah). Critical applications, such as vaccine refrigeration systems, telecommunications or defense and public safety applications may be designed for greater than 3 days of autonomy to help improve system reliability. PV hybrid systems using generators or other backup sources may require less autonomy to achieve the same level of system availability. Charge Controller Operation A battery charge controller limits the voltage and or current delivered to a battery from a charging source to regulate state-of-charge [NEC 690.2]. See Fig 32. A charge controller is required in most PV systems that use battery storage, to prevent damage to the batteries or hazardous conditions resulting from overcharging [NEC 690.72(A)]. Many charge con- trollers also provide overdischarge protection for the battery by disconnecting dc loads at low state-of-charge. Additional functions performed by charge controllers include controlling loads or backup energy source and providing monitoring and indicators of battery voltage and other system parameters. Special controllers are also available that regulate battery charge by diverting excess power to auxiliary loads. See Fig 33. Many charge controllers protect the battery from overdischarge by disconnecting dc loads at low battery voltage and state-of-charge, at the allowable maximum depth of discharge limit. See Fig. 34. Some smaller charge controllers incorporate overcharge and overdis-  2011 Jim Dunlop Solar Solar Radiation: 2 - 35 Figure 33. Charge controllers used in PV systems vary widely in their size, functions and features. Morningstar TriStar controller Morningstar ProStar controller Morningstar lighting controller Outback MPPT controller Xantrex C-series controller

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