Solar Photovoltaic Basics

A Study Guide for the NABCEP Associate Exam

Sean White

• Editorial: Routledge
• Año: 2018
• ISBN:
  9781351594936
• Edición: 2
• Categoría: Biological Sciences
• Categoría: Ecology

This book explains the science of photovoltaics (PV) in a way that most people can understand, using the curriculum which reflects the core modules of the NABCEP Associate Exam. Whether or not you are taking the NABCEP Associate Exam, learning the material covered in this book is the best investment you can make insuring your place and moving up in the solar industry.

Providing complete coverage of the NABCEP syllabus in easily accessible chapters, this book addresses all of the core objectives required to pass the exam, including the ten main skill sets:

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• PV Markets and Applications

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• Safety Basics

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• Electricity Basics

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• Solar Energy Fundamentals

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• PV Module Fundamentals

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• System Components

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• PV System Sizing Principles

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• PV System Electrical Design

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• PV System Mechanical Design

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• Performance Analysis, Maintenance and Troubleshooting.

You will learn the importance of surveying a site and how to carry out a survey, how to use the tools that determine shading and annual production, and the necessity of safety on site. This guide also includes technical math and equations that are suitable and understandable to those without engineering degrees, but are necessary in understanding the principles of solar PV.

This new edition of Sean White's highly successful study guide has been updated throughout and reflects recent changes in the industry.

Chapter 1
PV markets and applications

Key Contributions to the Development of PV Technology

• 1839: Edmond Becquerel discovered the photovoltaic effect.
• 1905: Einstein described the photoelectric effect and how light (photons) can excite electrons.
• 1922: Einstein received Nobel Prize for describing photoelectric effect.
• 1954: Bell Labs developed the “Bell Solar Battery”.

The “Bell Solar Battery” is what is technically called a solar module today.

Often people incorrectly call a solar module a solar panel.

• 1958: First solar powered satellite sent into space by US Navy. The Vanguard 1 is currently the oldest man-made object in space.
• 1999: World total installed PV capacity 1 GW
  • 1 Gigawatt = 1000 MW (megawatts)
  • 1 MW = 1000 kW (kilowatts)
  • 1 kW = 1000 W (watts)
• 2012: World total installed PV capacity 100 GW, 31 GW of which was installed in 2012
• 2017: World total installed PV capacity over 300 GW
  • 1000GW = 1TW. By the time many of you read this book, there
  • will be 1TW of PV installed in the world.

Figure 1.1 1956 PV advertisement from Bell Labs

Types of PV Systems and the Basics of How They Operate

Grid-Tied, Aka Utility-Interactive PV Systems

Grid-tied PV systems are connected directly to and synchronize with the utility. They are the most popular type of system. The inverter is sized based upon the size of the PV array.

The main components of a utility-interactive PV system are:

• Solar modules
• Inverter.

Self-consumption

We typically do not see batteries installed with interactive inverters, but times are changing and with an increased potential to overload the grid with solar energy in some places on a sunny day, there are reasons why batteries are added to an interactive system so that we will selectively have our inverters send out power, not just when it is sunny. When we have a system with batteries and the purpose is to operate when connected to the grid, we call this process self-consumption. Self-consumption systems are complicated and many solar installers try to avoid installing batteries. The purpose of self-consumption is to work when the grid is operating, as opposed to battery-backup systems which can operate when the grid is down. Some systems can do both self-consumption and battery backup. They say our car batteries will eventually be able to send out power to the grid. Exciting times!

Grid-tied PV systems have to be able to disconnect from the grid whenever the grid is down or not within specifications. This is called anti-islanding and means that the inverter cannot operate alone as an island of power. If a grid-tied system did feed the grid when the grid was down, it could be dangerous to utility workers who are fixing the problem. Some solar customers are surprised to find out that their utility-interactive PV systems will not work during a power outage.

Utility-interactive inverters are interactive inverters that work with the utility. Sometimes these interactive inverters are used in some micro-grid systems or other systems where there is no utility, so they are just called interactive inverters in the National Electric Code (NEC). A microgrid is a small grid of electricity and can be as small as your house or as large as a village.

Stand-Alone, Aka Off-Grid, Systems

Systems that work independent of the utility grid. Usually used for remote homes. Stand-alone systems are designed to fulfill all of the electricity requirements.

There are two basic types of stand-alone systems.

Direct current is the electricity from a solar module or a battery with a positive and a negative connection. Alternating current is what comes from your house and alternates very fast between positive and negative.

AC coupled systems	(ac = Alternating Current)
DC coupled systems	(dc = Direct Current)

DC coupled systems are the most common and simple off-grid systems. Their main components are:

• Solar modules
• Charge controller (prevents battery over- and under-charging)
• Battery
• Inverter.

AC coupled systems are complicated and less common. They use two types of inverters: battery inverters to create voltage so that interactive inverters can work when there is no utility. Their main components are:

• Solar modules
• Grid-tied inverters
• Off-grid inverter/charger
• Batteries.

Figure 1.2 DC coupled PV system

The National Electric Code (NEC) aka “The Code”

The NEC includes the rules for installing safe PV systems. The NEC is published every three years, such as the years 2014, 2017, 2020, 2023 and 2026. Different places adopt different versions of the Code and often most of the solar installed in the USA is installed in places that adopt a version of the Code three years after it has been published. This book is not a book designed to teach you all about the NEC (we have other books for that). The NEC changed the definition of a PV system to not include batteries in the 2017 NEC. People always call solar modules solar panels and people will always call batteries part of the PV system; however we should mention this to give you an example of how the NEC evolves. If you are studying for the NABCEP Associate exam, have no fear, NABCEP is saving this complicated material for when you take the NABCEP PV Installation Professional Exam in the future.

Hybrid PV Systems

Hybrid systems include another source of power besides PV, batteries or the utility.

Typical other sources of power include:

• Generator (internal combustion engine)
• Wind turbine
• Micro-hydro (small hydroelectric turbine).

Grid-Tied-Battery-Backup

Multimodal systems can operate as grid-tied systems and off-grid systems. They are typically the most complex systems to design.

The inverters will typically produce as much power as possible when operating in utility-interactive mode. When the utility is interrupted, the inverters disconnect from the grid and switch to stand-alone mode and make as much power as the loads require. (Loads are devices that consume electricity.)

Figure 1.3 SMA Sunny Island AC coupled PV system

Multimodal systems have to disconnect from the grid completely when the grid is down, but still have to feed power to the building. These systems usually power a subpanel of specific loads and not the entire building.

Multimodal inverters

Multimodal inverters are inverters that can operate in multiple modes, such as interactive mode (connected to the utility) and stand-alone mode. These inverters are used for backup power for when the grid is down. Multimodal inverters are sometimes referred to as bimodal inverters.

Direct, Aka Direct-Coupled PV System

This is the simplest type of PV system. The only components are PV and a load (usually an electric motor).

A good example of a direct-coupled system is a solar attic fan. A solar attic fan consists of a PV module and a fan. When the sun is out, the fan works, when it is brighter, the fan works better, which is convenient, since we need a fan more when the sun is out.

Another common direct-coupled system is water pumping. In sunny times, more water is needed and water can be stored with elevation and used at night.

Direct water pumping system variation

Often times, water pumping systems use a linear current booster (LCB) which boosts current and sacrifices voltage at times of lowlight, such as mornings.

Informational note: You will learn later in this book that voltage x current = power and that power x time = energy.

There are no direct-coupled lighting systems, since when the sun is out the direct sunlight is the most efficient light. Perhaps modern-day cave dwellers could benefit from direct-coupled lighting systems.

Self-Regulating System

A self-regulating system is a stand-alone system without a charge controller. In most cases, not having a charge controller would damage a battery by under- and over-charging the battery.

According to the National Electric Code, a self-regulating system has to be designed so that it will not charge over 3% battery capacity in an hour. This way the battery will not be over-charged. Over-charging a battery can not only damage the battery, but also can split water molecules into hydrogen and oxygen, which is an explosive combination (rocket fuel).

There are no safety issues with under-charging batteries, but as anyone with a car knows, letting a battery die is not good for the life of the battery, so self-regulating systems are designed with loads that are small relative to the size of the battery and PV, so that they can survive dark winter days.

A good example of a self-regulating system is a coastguard buoy, which has a big battery bank relative to a modest blinking light.

Advantages and Disadvantages of PV

PV works when the sun is up and that is when people use electricity the most. We say that it generates at “peak” times, which makes it most valuable. Other conventional forms of energy production can be easily stored; however, when we are burning things to make electricity, we are doing something that causes pollution.

Wind power is good when mixed with solar, because wind can work at night, however most people would rather live in sunny places than windy places. Also, wind is intermittent, and sunshine is much more predictable. Even on cloudy days, solar systems make power. Solar works 365 days per year.

One of the problems that we have with solar is storage of energy so that we can use the solar energy at night. When solar was adopted early and solar energy was less than 1% of the energy produced on the grid, storage was not such a big deal; however, as the industry grows exponentially, storage becomes more important in places where solar is adopted to a greater degree. Energy storage is becoming more important in places where solar PV is more saturated, s...