Photovoltaics

These roof shingles are coated with PV cells made of amorphous silicon. When installation is complete, the PV shingles look much like ordinary roofing shingles, but they generate electricity.

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, builder, 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.

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.

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 bottom 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.

PV Systems

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.)

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.

For Consumers

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.