PHOTOVOLTAIC CONNECTIONS TO A UTILITY GRID AND NET METERING

Although some homeowners have only a photovoltaic system attached to their home, many solar-powered homes and businesses are connected to transmission power lines outside their homes and businesses. The transmission lines are part of a grid system owned by a utility company. Using grid-connected photovoltaic power can have economic as well as environmental advantages for the homeowner. 


Because such homeowners are using much of their electricity from their own photovoltaic system, the amount of electricity they have to purchase from the utility company each month is reduced. In this cooperative arrangement, the homeowners get some of their power from their photovoltaic systems and some from the utility company’s grid.

What Is Net Metering? 

Net metering is a simple way of metering the energy consumed and produced at a home or business that has its own renewable energy generator, such as a solar energy system. 

Net metering enables homeowners to use their own generation of electricity to off set their consumption over a billing period by allowing their electric meters to turn backward when they generate electricity in excess of their demand. Th is program means that customers receive retail prices from their electrical utility company for the excess electricity they generate. A retail price is the price at which a utility company sells the electrical power to a homeowner or other consumer. 

Without net metering, a second meter is usually installed to measure the electricity that flows back to the provider, a utility company that purchases the power at a rate much lower than the retail rate. As of 2010, net metering for homeowners is available in 42 states. 

John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells


(For solar energy solutions and supplies visit our website: www.eastgreenfields.com or email us at inquiry@eastgreenfields.com)   

SOLAR STORAGE

The biggest problem of solar power technologies is how to store the power generated for those times when sunlight is unavailable. Currently, most solar power plants do not have the capability to store excess energy from sunny days to be used on cloudy days. One option is to use a storage battery bank that will collect and store power anytime the system is producing more energy than is needed.  

Photovoltaic to Battery Storage 

A storage battery is an excellent system for supplying electricity when and where it is needed on non-sunny days when solar power is not available. Photovoltaic systems with a backup battery storage unit are used to provide electricity for power tools, lights, home appliances, telephones, and televisions. Photovoltaic/battery systems work well in remote areas where utility power is unavailable or at a distance that is so far away that it would be too costly to install utility transmission lines to a building. 


 Although batteries make photovoltaic systems more useful, they also require some maintenance. Th e batteries used in photovoltaic systems are referred to as deep-cycling batteries, the kinds that are used on many golf carts. Th e batteries are bigger than the typical car battery. Th ese kinds of batteries allow more stored energy for use each day. 
 Batteries designed for photovoltaic projects need to be handled with care. Th e fl uid needs to be checked in unsealed batteries periodically.

John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells

(For solar energy solutions and supplies visit our website: www.eastgreenfields.com or email us at inquiry@eastgreenfields.com)

PHOTOVOLTAIC CELLS COME IN MANY SIZES AND SHAPES

Photovoltaic cells come in many sizes and shapes—from smaller than a postage stamp to several inches across. Each cell is capable of producing one to two watts of power. Although this is not enough to power most appliances, cells can be linked together in modules. Modules store photovoltaic cells in a weatherproof container and may be up to several feet long and a few feet wide. 

Depending on how much energy is needed, the modules can also be linked together to provide higher voltages. Photovoltaic power plants use a few thousand modules to generate electricity for household or business uses. Because photovoltaic systems come in small and large sizes and shapes, they are able to meet almost any electric power need, whether in a small house or in a large community of homes. 

The modules, in turn, can be combined and connected to form photovoltaic arrays of diff erent sizes and power output. These kinds of arrays can be seen on the Nellis Air Force Base and on large solar farms.  

Power of a Solar Array 

Electrical power is measured in watts. A watt is a measurement of total electrical power: Volts x Amps = Watts. For electrical power, one watt is equal to one ampere of current per second. 

The power output of a single solar cell or an array of solar cells can be measured in watts, watt-hours, kilowatt-hours, kilowatt-hours per day, or even megawatts. A megawatt will produce electricity for 400–900 homes per year. Utility power plants usually measure their output in megawatts. Th e megawatt is equal to one million watts. 

 SOLAR STORAGE 

The biggest problem of solar power technologies is how to store the power generated for those times when sunlight is unavailable. Currently, most solar power plants do not have the capability to store excess energy from sunny days to be used on cloudy days. One option is to use a storage battery bank that will collect and store power anytime the system is producing more energy than is needed.  

Photovoltaic to Battery Storage 

A storage battery is an excellent system for supplying electricity when and where it is needed on non-sunny days when solar power is not available. Photovoltaic systems with a backup battery storage unit are used to provide electricity for power tools, lights, home appliances, telephones, and televisions. Photovoltaic/battery systems work well in remote areas where utility power is unavailable or at a distance that is so far away that it would be too costly to install utility transmission lines to a building. 


Although batteries make photovoltaic systems more useful, they also require some maintenance. The batteries used in photovoltaic systems are referred to as deep-cycling batteries, the kinds that are used on many golf carts. Th e batteries are bigger than the typical car battery. These kinds of batteries allow more stored energy for use each day. 

Batteries designed for photovoltaic projects need to be handled with care. Th e fl uid needs to be checked in unsealed batteries periodically. 


John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells

A NEW GENERATION OF SOLAR CELLS

In 2010 many institutions and companies are developing, testing, and manufacturing a new generation of solar cells. Traditional solar cells are made from crystalline silicon, which has been in high demand, at times resulting in a shortage of crystalline silicon supply throughout the world, and which is a high-cost material to purchase. Therefore, the solar photo voltaic industry is researching and developing non-silicon-based technology, including thin-film solar cells.  

Thin-Film Solar Cells 

Solar engineers refer to second-generation solar cells as thin-film solar cells. Materials used in the production of the thin fi lms include semiconductor materials, such as copper indium diselenide (CuInSe 2  or “CIS”), gallium arsenide, and cadmium telluride. 

Engineer Steve Robbins displays a sheet of “thin film” solar cells at the National Renewable Energy Laboratory in Golden, Colorado, in March 2009. Thin film solar panels are relatively low in cost and their flexible design makes them highly adaptable. (John Moore/Getty Images)


 The term  thin film  refers not to the thinness of the film, but to the manufacturing process used to produce the solar cells. Th in-film cells are deposited in very thin, consecutive layers of atoms, molecules, or ions onto wafer-like material. Th e finished solar cell is only a few micrometers or nanometers thick.  

Thick versus Thin: Any Differences? 

Th in-film cells have some advantages over the thick-film silicon cells. For example, less material is needed in the manufacturing of thin-film cells. A thin cell is usually only 1–10 micrometers thick, whereas thick crystalline films typically are 100–300 micrometers thick and have to be cut from ribbons. 

Also, thin-film cells can usually be manufactured in a continuous, automated production process. Automated processes can assure improved product quality, lower operating costs, and better safety conditions. Finally, the thin-fi lm cells can be deposited on flexible materials such as ultra thin glass, stainless steel, or plastic in any shape. Because of their flexibility, thin-film solar cells can be used as rooftop shingles, on the exterior part of a building or facade, or in the glazing for skylights.  

How Do Thin-Film Solar Cells Produce Electricity? 

Under a microscope the thin-fi lm cells have two layers sandwiched together. The top layer is made of a different semiconductor material than the material used for the bottom semiconductor layer. Th e top layer is called an N-type—the negative charge. Chemicals in this layer allow the sunlight through to the absorbing layer, called the P-type—the positive charge. When the sunlight passes through a conducting chemical on the surface, the sunlight makes contact with a series of chemicals (i.e., indium, copper, gallium, and diselenide) that allow electrons to fl ow through a wire to create electricity from the N-type and P-type. 


John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells

WHAT MATERIALS ARE USED TO MAKE PHOTOVOLTAIC CELLS?

Solar cells are made from many diff erent semiconductors. A semiconductor is a material that has the properties of an insulator as well as those of an excellent conductor. Some of the semiconductors used for making solar cells include silicon, gallium arsenide, copper indium diselenide, and cadmium telluride. All of these materials are suited to the development of solar cells to conduct electricity. 

CRYSTALLINE SILICON SOLAR CELLS 

Silicon (Si) is a semiconductor and the most common and important element in computer chip and solar technologies. It is a semimetallic, chemical element that crystallizes in a cubic shape similar to the pattern of a diamond. Silicon crystals are found in sand and quartz and are used extensively in the manufacturing of solar cells.

How Do Solar Cells Work? 

Most solar photovoltaic cells are made from two layers of crystalline silicon that have been chemically treated using a process called doping. Th e doping process gives one silicon layer a negative charge (N) and the other a positive charge (P). A solar cell is essentially a PN junction sandwiched between two layers of semiconductor materials. 

The process of producing electricity from a solar cell begins with sunlight. When the particles of light strike a solar cell, they cause electrons to be ejected from the silicon atoms. The electrons move freely from the negative layer to the positive layer through metal terminals producing electricity. A four-inch cell can produce about one watt of direct current. (Illustrator: Jeff Dixon)

Now how does the solar cell work? Th e process of producing electricity from a solar cell begins with sunlight, which contains energy in the form of photons or particles of light. When photons strike a solar cell, they cause electrons to be ejected from the silicon atoms located near the junction. An electron is a subatomic particle with a negative charge. Th e stream of ejected electrons can move freely from the negative layer to the positive layer through the metal terminals. 

How much electricity is generated from a solar cell? The typical fourinch solar cell can produce about one watt of direct current (DC) electricity when exposed to sunlight. To generate more electricity, you need to wire together many solar cells in a panel called a solar array that is encased in a watertight container for weather protection. If you need to produce a greater amount of electricity, then the panels, in turn, can be wired together. Th ese kinds of solar panels are placed on the roofs of homes and businesses to generate electricity. 

***

John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells

WHAT ARE SOME USES FOR PHOTO VOLTAIC CELLS?

According to research, about 40 percent of all photo voltaic cells sold are used for producing electrical power for homes and for pumping water in remote areas. About 35 percent of them are used in transmitting and communication operations. Many lighthouses, off shore petroleum drilling operations, highway signs, and radio and telephone transmitters are also powered by solar panels. 

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John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells

A BRIEF HISTORY OF SOLAR ENERGY

The early Greeks and Romans used passive solar energy to warm up the rooms in their homes. The Romans covered south-facing openings in the inside walls with glass or other transparent materials to keep in the heat, particularly during the winter months. 

In the 1830s the British astronomer John Herschel used a solar box, similar to today’s solar oven, on a trip to Africa to cook his food during his stay. Later in the 19th century, European scientists developed large cone shaped collectors that were used to boil ammonia for use in refrigeration plants and for other products. 

In the United States, John Ericsson did much research to harness solar power. He designed the parabolic trough collector, which operates much like the parabolic trough collectors we have today. (Parabolic trough collectors are discussed further in chapter 2.) 

William Adams used mirrors and the energy of the sun to help power a steam engine during the 1870s. His design, called the power tower concept, is still in use in the 21st century. (Solar power towers are discussed in chapter 2.) 

In the 1950s Gerald Pearson, Calvin Fuller, and Daryl Chaplin of Bell Laboratories (now AT&T Labs) discovered the use of silicon as a semiconductor. 

In the 21st century, Silicon, an element, is utilized as a major ingredient in the production of solar cells and solar panels. In 1953 the same three scientists developed the first silicon solar cell. Th e cell generated small measure of electric current. In the 1950s and 1960s, satellites and spacecrafts used solar panels for electricity. In the early 21st century the photovoltaic market is growing by 30 percent per year, as the costs of manufacturing the cells decrease.

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John F. Mongillo
A Student Guide to Energy 
Copyright 2011
Greenwood Publishing Group
Volume 2 Solar Energy and Hydrogen Fuel Cells