Power from the Sun - 2nd Edition

A Practical Guide to Solar Electricity - Revised 2nd Edition

Dan Chiras

• Publisher: New Society Publishers
• Year: 2016
• Print ISBN: 9780865718296
• eBook ISBN: 9781550926248
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

The completely revised and updated guide to solar energy – everything you need to power your home or small business

Written for the individual or business layman, this is the fully revised and updated guide for generating solar electricity. Practical and accessible, it provides a basic understanding of electricity, wiring, and solar energy, and guides the reader through solar electricity site assessment and determining the type of solar system needed for generating solar electricity, providing a solid understanding of grid-tied and off-grid systems, along with important guidelines on installation.

Power from the Sun – 2nd Edition discusses types of photovoltaic and photovoltaic solar energy systems, and includes comprehensive information on recent changes and improvements in PV modules, charge controllers, inverters, batteries, generators, and net metering policies. It offers an excellent overview of the many options available as a guide for generating solar electricity, allowing the reader to make the best choices for their individual situation during the design, installation, and operation of a solar energy system. The definitive layman's guide for homeowners, business owners, installers, architects, and just about anyone interested in generating solar electricity to lower energy bills and achieve greater independence through solar energy.

Dan Chiras is Director of the Center for Renewable Energy and Green building through which he teaches workshops on solar electricity, wind energy, passive solar design, natural building, and green building. He is also president of Sustainable Systems Design, Inc., a company that installs residential solar electricity and wind energy systems and consults on passive solar design, residential renewable energy, and green building throughout North America. Dan is the author of 32 previous books, including The Homeowner's Guide to Renewable Energy and Power from the Wind .

CHAPTER 1

An Introduction to Solar Electricity

Like many other people throughout the world, I have been dreaming of a future powered by renewable energy for many years. Fortunately, that dream is becoming a reality. Today, solar and wind energy have become major contributors to global energy production. In the United States alone, solar, wind (mostly large-scale wind), biomass, and geothermal now produce about 7% of the nation’s electricity. When you add hydroelectricity to the mix, renewable energy now supplies over 13% of America’s electricity. That’s a recent change, occurring within the last few years. Although we have a ways to go, that’s pretty amazing.

What is even more amazing is that some states now generate about 25% of their electricity from renewables. Kansas and Iowa, for example, both do, mostly with wind power. Colorado generates 18% of its electricity from renewables, but its goal is 30% by 2020. California is aiming for 33% by 2020. While impressive, get this: Germany now produces over 60% of its electricity from renewables.

Although there are many reasons why renewables have gone wild in recent years, one of them has been cost. For years, renewable energy advocates have dreamed of a day when we could say that renewables cost the same or less than conventional energy resources. At least for solar electricity, those days are here. Solar electric systems, even without subsidies, often produce electricity at or below the cost of power from utilities. They have reached price parity, and the future is looking quite bright.

This book focuses on an important element of the renewable energy dream: solar electricity. Solar electric systems are also known as photovoltaic systems or PV systems, for short. In this book, I will focus primarily on residential-scale PV systems, that is, systems suitable for homes and small businesses. These systems generally fall in the range of 1,000 watts for very small, energy-efficient cabins or cottages to 5,000 to 15,000 watts for typical suburban homes. All-electric homes could require even larger systems, on the order of 25,000 watts. What does all this mean?

Understanding Rated Power

One of the most frequent questions solar installers are asked is “What size system do I need?” It’s probably the question that’s burning in your mind right now.

The answer is almost always the same: it depends. More specifically, it depends on how much electricity you and your family use. Once an installer knows how much electricity you consume, he or she can size a system.

The size of a solar electric system is given in its rated power, also known as rated capacity. Rated power of a system is the output of the modules (PV panels) times the number of modules. If your system contains twenty 250-watt modules, its rated capacity is 5,000 watts. Because kilo means 1,000, a 5,000-watt system is also called a 5-kilowatt, or 5-kW system. But what does it mean when I say a module’s rated power is 250 watts?

Where Does Your Electricity Come From?

Did you ever wonder where your electricity comes from? According to the US Energy Information Administration (EIA), in 2014, electricity sold by utilities came from the following sources:

As in many cases, national averages can be deceiving. Your utility may rely almost entirely on coal, nuclear, or hydropower. In the Midwest, for instance, it is not uncommon for 75% of one’s electrical energy to come from coal. In the Pacific Northwest, the vast majority of the electricity comes from hydropower. You may be able to find out where your electricity comes from by calling your local utility and talking with their public information specialists.

Rated power is the instantaneous output of a solar module measured in watts under standard test conditions (STC). Watts is a measure of power. Another way of thinking about it is as the rate of the flow of energy. The higher the wattage, the greater the flow.

Watts is also the measurement used to rate technologies that produce electricity, such as a solar module or a power plant. For example, most solar modules installed these days are rated at 250 to 300 watts. Power plants are rated in millions of watts, or megawatts: a typical coal-fired power plant produces 500 to 1,000 megawatts of power.

Most readers are probably more familiar with the terms watt and wattage when they are used to rate power consumption of a device, such as a light bulb or a hair dryer. You may have a 12-watt compact fluorescent light bulb, for instance, or an 1,800-watt hair dryer. Your microwave may be 1,200 watts. In such cases, wattage indicates power consumption. The higher the wattage, the greater the consumption.

As just noted, the rated output of a PV module is determined under standard test conditions—a set of conditions that all manufacturers use to rate their modules. To test a module, workers set it up in a room that is maintained at 77°F (25°C). A light is flashed on the module at an intensity of 1,000 watts per square meter. That is equivalent to full sun on a cloudless day in most parts of the United States. The light is arranged so light rays strike the module at a 90° angle, that is, perpendicularly. (Light rays striking the module perpendicular to its surface result in the greatest absorption of the light.)

Rating modules under standard test conditions is useful for comparing one manufacturer’s module to another’s. A 285-watt module from company A should perform the same as a 285-watt module from company B.

Installers can use this rating to determine the size of a system required to meet your needs. As noted above, most residential solar electric systems fall within the range of 5,000 to 15,000 watts, or 5 to 15 kW. It is important to note, however, that a 5-kW PV system won’t produce 5,000 watts of electricity all the time the Sun’s shining on it. This rating is based on standard test conditions, which are rarely duplicated in the real world. So, this rating system leaves a bit to be desired.

When mounted outdoors, PV modules typically operate at higher temperatures than those subjected to standard test conditions. That’s because infrared radiation in sunlight striking PV modules causes them to heat up. To achieve a cell temperature of 77°F (25°C) under full sun, like that measured in the laboratory, the air temperature must be quite low—about 23°F to 32°F (0°C to −5°C), not a typical temperature for PV modules in most locations most of the year. This is important to know because higher temperatures decrease the output of a solar module. That’s the reason why it is so important to mount a solar system so that the modules stay as cool as possible. It also explains why solar modules on a ground-mounted rack typically perform better than roof-mounted systems. Roofs are hotter.

To better simulate real-world conditions, the solar industry has developed an alternative rating system. They call it PTC, which stands for PV-USA Test Conditions (which stands for PV for Utility-Scale Applications Test Conditions).

PTC were developed at the PV-USA test site in Davis, California, and more closely approximate real-life operating conditions. In this rating system, the ambient temperature is held at 68°F (20°C). The modules face the Sun and output is measured when the Sun’s irradiance reaches 1,000 watts per square meter. The conditions take into account higher module temperatures and the cooling effect of wind. Wind speed in the test is 2.24 miles per hour (or 1 meter per second) 10 meters above the surface of the ground. (That is, wind speed is monitored at 10 meters, or 33 feet.) Although the cell temperature varies with different modules, it is typically about 113°F (45°C) under PTC. Because power output (in watts) decreases with rising temperature, PTC ratings are roughly 10% lower than STC ratings.

When shopping for PV modules, I used to suggest that readers ignore the nameplate rating the manufacturers derived under STC and look up the PTC ratings. Some manufacturers publish these data on their spec sheets; others don’t. As a rule, though, it’s not that important. Solar installers will estimate the output of an array taking into account local conditions. If you must, you can find PTC ratings on the California Energy Commission’s website: consumerenergycenter.org.

An Overview of Solar Systems

As their name implies, solar electric systems convert sunlight energy into electricity. This conversion takes place in the solar modules, more commonly referred to as solar panels. (The term modules is the correct one; that’s what they are called in the National Electric Code, but few people use this term.)

FIGURE 1.1. These solar modules being installed by several students at The Evergreen Institute consist of numerous square solar cells. Each solar cell has a voltage of around 0.6 volts. The cells are wired in series to produce higher voltage, which helps move the electrons from cell to cell. Modules are also wired in series to increase voltage. Credit: Dan Chiras.

A solar module consists of numerous solar cells. Most solar modules contain 60 solar cells, although manufacturers are also producing larger modules that contain 72 cells each. Solar cells are made from one of the most abundant chemical substances on Earth, silicon dioxide. (Silicon dioxide makes up 26% of the Earth’s crust.) Silicon dioxide is found in quartz and from a type of sand that contains quartz particles. Silicon is extracted from silicon dioxide.

Solar cells are wired in series inside a module (see sidebar). The modules are encased in glass (in the front) and usually plastic (in the back). The plastic and glass layers protect the solar cells from the elements, especially moisture. Most modules these days have aluminum frames. Because the anodized (silver) aluminum frames are striking, many manufacturers produce modules with black aluminum frames. The aluminum rails and mounting hardware used to attach them to the roof are also black, so the array blends in well, and homeowners associations and neighbors are happy. Some manufacturers are now producing frameless modules to make them even more aesthetically appealing. One manufacturer’s modules passed an unprecedented Class 4 hail testing (that is, they withstood 2-inch hail striking it at 75 mph), making them the most impact-resistant module in the industry. Frameless modules have been designed to make installation quick, easy, and less expensive.

Understanding Electricity: Series Wiring

Although few people know what series wiring is, we use it all the time. Every time you put batteries in a flashlight, for instance, you place the positive end of one battery against the negative end of the next one. Series wiring increases the voltage. Voltage is a force that moves electrons (it is considered an electromotive force). The higher the voltage, the more force. When placing two 1.5-volt batteries in series in a flashlight, you increase the voltage to 3. Place four 1.5-volt batteries in series and the voltage is 6.

In solar modules, each solar cell has a positive and a negative lead (wire). To wire in series, the positive lead of one module is soldered to the negative of the next, and so on and so on. Most solar cells in use today are square and measure 5 × 5 inches (125 × 125 mm) or 6 × 6 inches (156 × 156 mm) and have a voltage of about 0.6 volts. When manufactures wire (actually, solder) 60 solar cells together in series in a module, the voltage increases to 36. Wiring several solar modules together in series, in turn, increases the voltage of an array. Ten 36-volt modules wired in series results in an array voltage of around 360.

A group of modules wired in series is known as a series string. Most residential solar arrays consist of series strings of 10 to 12 modules. String sizes in commercial PV array are larger.

Two or more modules are typically mounted on a rack and wired together. Together, the rack and solar modules are referred to as an array. Ground-mounted arrays are typically anchored to the earth by a steel-reinforced concrete foundation. Together, the rack, modules, and foundation are referred to as an array.

Solar modules produce direct current (DC) electricity. That’s one of two types of electricity in use today. It consists of a flow of tiny subatomic particles called electrons. They flow through conductors, usually copper wires. In direct current electricity, electrons flow in one direction, as illustrated in Figure 1.2. In this case, electrons flow out of the battery through the light bulb, where they give off their energy, creating light and heat. The de-energized electrons then flow back into the battery.

FIGURE 1.2. Solar electric systems produce direct current (DC) electricity just like batteries. As shown here, in DC circuits the electrons flow in one direction. The energy the electrons carry is used to power loads like light bulbs, heaters, and electronics. Credit: Forr...

Table of contents

• Cover
• Title Page
• Copyright Page
• 1. An Introduction to Solar Electricity
• 2. Understanding the Sun and Solar Energy
• 3. Understanding Solar Electricity
• 4. Solar Site Assessment
• 5. Solar Electric Systems: What Are Your Options?
• 6. Understanding Inverters
• 7. Batteries, Charge Controllers, and Gen-sets
• 8. Mounting a PV Array for Maximum Output
• 9. Final Considerations: Permits, Covenants, Utility Interconnection, and Buying a System
• Appendix A: Summary of State Net Metering Policies
• Resource Guide
• Index
• About the Author
• A Note About the Publisher