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Solar Electric

 

Renewable Energy > Solar Electric

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Photovoltaics
 
The U.S. Department of Energy works to provide clean, reliable, affordable solar electricity for the nation through its research programs in photovoltaic (PV) energy systems. The following pages explain the "how's" and "why's" of PV. Whether you are a student, consumer, engineer, or researcher, there is something here for you.

Photovoltaic technology makes use of the abundant energy in the sun, and it has little impact on our environment. Photovoltaics can be used in a wide range of products, from small consumer items to large commercial solar electric systems.

Our goal is to ensure that photovoltaic energy systems make an important contribution to the energy needs of our nation and the world. In these pages, you will learn about DOE's R&D in photovoltaic energy systems, and much more. You will also find out—

Photovoltaic Basics

Have you ever wondered how electricity is produced by a photovoltaic — what we often call a PV or solar electric — system? We'll help you understand by covering the basics of PV technology, which includes the underlying physics, how various PV devices are designed and become fully functional systems, and what's happening today in PV research and development.

The Solar Energy Technologies Program of the U.S. Department of Energy (DOE) and its partners are adding to our fundamental knowledge and expertise in this area while improving the technologies that put the abundant energy of sunlight to work for us.

To help you delve further into this fascinating topic, we've compiled additional information sources at the top of many of these pages that will direct you to other pages within our own Web site, as well as to other helpful Web sites. While perusing this material, you may wonder what a specific term means. If so, visit our solar glossary for a comprehensive listing of renewable energy and electrical terms.

PV Physics

In this section, you'll learn how sunlight can be converted into electricity. We'll explain the basics by using crystalline silicon as a common PV material to illustrate some fundamental principles. You'll understand what's going on at the atomic level when sunlight shines on a solar cell. We'll also review some basic aspects of light itself.

PV Devices

Solar materials need to have certain important qualities. You'll first learn what these characteristics are. Then we'll describe the major families of PV materials currently being developed, including various types of silicon, thin films, and new concepts. Finally, we'll show you how we design these materials to be used with other materials to become useful solar cells.
 

Here, you'll learn how solar cells are combined to become a larger photovoltaic system. You'll discover that PV systems come in two basic designs — flat-plate and concentrator systems. Other components, known as balance-of-system equipment, make the entire system fully functional to supply electricity to important energy applications.

Energy Payback Times for Photovoltaic Technologies

Energy payback time (EPBT) is the length of deployment required for a photovoltaic system to generate an amount of energy equal to the total energy that went into its production. Roof-mounted photovoltaic systems have impressively low energy payback times, as documented by recent (year 2004) engineering studies. The value of EPBT is dependent on three factors: (i) the conversion efficiency of the photovoltaic system; (ii) the amount of illumination (insolation) that the system receives (about 1700 kWh/m2/yr average for southern Europe and about 1800 kWh/m2/yr average for the United States); and (iii) the manufacturing technology that was used to make the photovoltaic (solar) cells.

With respect to the third factor, i.e., manufacturing technology, there are three generic approaches for manufacturing commercial solar cells. The most common approach is to process discrete cells on wafers sawed from silicon ingots. Ingots can be either single-crystal or multicrystalline. However, in either case, the growing and sawing of ingots is a highly energy intensive process. A more recent approach which saves energy is to process discrete cells on silicon wafers cut from multicrystalline ribbons. The third approach involves the deposition of thin layers of non-crystalline-silicon materials on inexpensive substrates. It is the least energy intensive of the three generic manufacturing approaches for commercial photovoltaics. This last group of technologies includes amorphous silicon cells deposited on stainless-steel ribbon, cadmium telluride (CdTe) cells deposited on glass, and copper indium gallium diselenide (CIGS) alloy cells deposited on either glass or stainless steel substrates.

Recent research has established battery-free, grid-tied EPBT system values for several (year 2004-early 2005) photovoltaic module technologies (see Table 1). In Table 1, the values in the last column are the reciprocals of the respective values in the third column. It is seen that, even for the most energy intensive of these four common photovoltaic technologies, the energy required for producing the system does not exceed 10% of the total energy generated by the system during its anticipated operational lifetime. Future research will extend the table to include amorphous silicon and CIGS alloys.

Table 1. System Energy Payback Times for Several Different Photovoltaic Module Technologies.

(1700 kWh/m2/yr insolation and 75% performance ratio for the system compared to the module.)

Cell Technology

Energy Payback Time (EPBT)1(yr)

Energy Used to Produce System Compared to Total Generated
Energy 2(%)

Total Energy Generated by System pided by Amount of Energy Used to Produce System2

Single-crystal silicon

2.7

10.0

10

Non-ribbon multicrystalline silicon

2.2

8.1

12

Ribbon multicrystalline silicon

1.7

6.3

16

Cadmium telluride

1.0

3.7

27

1. V. Fthenakis and E. Alsema, "Photovoltaics energy payback times, greenhouse gas emissions and external costs: 2004-early 2005 status," Progress in Photovoltaics, vol. 14, no. 3, pp. 275-280, 2006.

2. Assumes 30-year period of performance and 80% maximum rated power at end of lifetime.

PV Research and Development

What's next? Find out here, as you discover what's going on in the world of photovoltaic research and development — or "PV R&D," for short. Our discussion of R&D activities follows under three broad categories: Fundamental Research, Advanced Materials and Devices, and Technology Development.

Why PV is Important

The University of Wyoming and several rural electric companies have set up demonstration projects to study the durability, maintenance requirements, and useful life of solar-powered water-pumping systems. Solar electric systems are an ideal choice in Wyoming's vast rural areas, especially those at a considerable distance from conventional power lines.

Photovoltaics (PV) is an important energy technology for many reasons. As a solar energy technology, it has numerous environmental benefits. As a domestic source of electricity, it contributes to the nation's energy security. As a relatively young, high-tech industry, it helps to create jobs and strengthen the economy. As it costs increasingly less to produce and use, it becomes more affordable and available. And there are many more reasons, as we shall see.

Few power-generation technologies have as little impact on the environment as photovoltaics. As it quietly generates electricity from light, PV produces no air pollution or hazardous waste. It doesn't require liquid or gaseous fuels to be transported or combusted. And because its energy source - sunlight - is free and abundant, PV systems can guarantee access to electric power.

PV frees us from the cost and uncertainties surrounding energy supplies from politically volatile regions. And in addition to reducing our trade deficit, a robust domestic PV industry creates new jobs and strengthens the U.S. economy.

The Benefits of PV

Let's take a look at the many ways PV is bettering our world — today. We will explore why PV is important —

To the Economy
To Energy Assurance
To the Environment
To You

Because—
It's highly reliable and needs little maintenance.
It costs little to build and operate.
It has virtually no environmental impact.
It's produced domestically, strengthening our economy and reducing our trade deficit.
It's modular and thus flexible in terms of size and applications.
It meets the demand and capacity challenges facing energy service providers.
It helps energy service providers manage uncertainty and mitigate risk.
It serves both form and function in a building.

PV in Use: Getting the Job Done with Solar Electricity

Photo of Sojourner exploring Mars.

Photo of the portable, highly cold-tolerant runway lighting systems for Antarctic Support Associates.

DOE and one of its partners, the West Bengal Renewable Energy Development Agency, are working to improve socioeconomic conditions in the Sunderbans region of West Bengal, India. These rooftop PV modules on a village health center in West Bengal provide power for refrigerators containing medicines and vaccines, for lights, and for other important needs.

After decades of use on Earth and in space, solar electricity made its debut on another planet in 1997 when "Sojourner" began exploring Mars. High-efficiency photovoltaic (PV) cells located on top of the Sojourner vehicle generated 16 watts of power at noon on Mars, which was enough to carry out a day's mission.

The Space and Naval Warfare Command contracted with Northern Power Systems to design and install these portable, highly cold-tolerant runway lighting systems for Antarctic Support Associates, to ensure safe landings for cargo planes on the south polar ice.

Modern solar electric power-generation systems such as photovoltaics (or PV) are some of the most elegant and environmentally benign energy systems ever invented. But do people actually use them? The answer is yes — and not just in space. PV systems can also be found in the most isolated spots on Earth as well as in the heart of some of our largest cities. And every place in between.

Today's PV systems are used to generate electricity to pump water, light up the night, activate switches, charge batteries, supply power to the utility grid, and much more. PV has so many uses today that it probably already touches your life in some way. You might have noticed the small PV systems attached to emergency telephones along the highways. But PV provides power in many ways we can't see — for all kinds of satellites in space, including those that keep modern communication systems "up and running."

To make PV systems even more efficient, affordable, and available, the U.S. Department of Energy (DOE) and its partners in universities and industry continue to conduct advanced research and development in this exciting, important energy technology. Here, we describe some of PV's current major uses, or applications, grouped in these categories:

We've also included illustrative examples in some case studies that illustrate many of today's everyday uses for PV energy systems.

For Consumers

A photo of PV shingles being installed on a roof.

Each PV shingle being installed on this roof will produce 17 watts under full sun, for a total system size of 1.2 kW. The shingles mount directly on to the roof structure and take the place of asphalt shingles. The whole PV system is connected to the utility grid through an inverter and produces electricity on customer's side of the meter.

Solar-electricity, or photovoltaics (PV) converts sunlight directly into electricity. You may be more familiar with PV cells as solar cells that power watches and calculators. But PV can do much more. It can provide electricity for residential and commercial buildings, including power for lights and air conditioning. PV can also be a convenient source of power for pumping water, electrifying fences, or aerating ponds in remote applications.

As an energy-conscious consumer, you want to do all you can to use energy efficiently and add more clean, renewable energy to your life. This section contains information and analysis tools to help you evaluate your options and make an informed decision. If you've already decided to install solar electric panels (also called photovoltaic or PV panels) on your roof, you've given it careful thought and considered all the benefits of using a reliable, abundant, and environmentally smart source of energy — the sun. If you haven't decided yet, we hope the information in these pages will help you make a decision about purchasing PV panels for your home or business.

How does it work?

Here you can view entertaining and educational presentations about how solar energy is produced.

How can I learn more?

To learn more about the fundamentals of photovoltaics, please visit our PV Basics section for a more in depth look. And, if you are interested in educational materials for the classroom, please visit our educational section for students and educators.

How do I get and use it?

If you're interested in putting renewable energy to work in your home or office and want to know more about solar and other renewable technologies, see So You Want to Put PV on Your Roof!

Why is solar electricity important?

In addition, if you would like to read about the environmental, socio-economic, and security impacts of PV and other solar energy technologies on the world we live in, please see our "Why PV is Important" section.

What kind of impact am I making on the environment?

We compiled several useful "calculators" and decision-making tools so you can evaluate how much energy you consume and its environmental impacts. Some good energy-saving tips are also included with these decision-making tools.


U.S. Department of Energy - Energy Efficiency and Renewable Energy