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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.
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.
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 Divided 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.
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Content Last Updated: 10/04/2006