A small solar electric or photovoltaic (PV) system can be a reliable and pollution-free producer of electricity for your home or office. Small PV systems also provide a cost-effective power supply in locations where it is expensive or impossible to send electricity through conventional power lines.
Because PV technologies use both direct and scattered sunlight to create electricity, the solar resource across the United States is ample for home solar electric systems. However, the amount of power generated by a solar system at a particular site depends on how much of the sun's energy reaches it. Thus, PV systems, like all solar technologies, function most efficiently in the southwestern United States, which receives the greatest amount of solar energy.
Because of their modularity, PV systems can be designed to meet any electrical requirement, no matter how large or how small. You can connect them to an electric distribution system (grid-connected), or they can stand alone (off-grid). You can also use PV technology to provide outdoor lighting.
Solar cells—the basic building blocks of a PV system -- consist of semiconductor materials. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms. This phenomenon is called the "photoelectric effect." These free electrons then travel into a circuit built into the solar cell to form electrical current. Only sunlight of certain wavelengths will work efficiently to create electricity. PV systems can still produce electricity on cloudy days, but not as much as on a sunny day.
The performance of a solar (or PV) cell is measured in terms of its efficiency at converting sunlight into electricity. There are a variety of solar cell materials available, which vary in conversion efficiency.
Silicon remains the most popular material for solar cells, including these types:
The absorption coefficient of a material indicates how far light with a specific wavelength (or energy) can penetrate the material before being absorbed. A small absorption coefficient means that light is not readily absorbed by the material. Again, the absorption coefficient of a solar cell depends on two factors: the material making up the cell, and the wavelength or energy of the light being absorbed.
The bandgap of a semiconductor material is an amount of energy. Specifically, the bandgap is the minimum energy needed to move an electron from its bound state within an atom to a free state. This free state is where the electron can be involved in conduction. The lower energy level of a semiconductor is called the "valence band." The higher energy level where an electron is free to roam is called the "conduction band." The bandgap (often symbolized by Eg) is the energy difference between the conduction band and valence band.
Solar cell material has an abrupt edge in its absorption coefficient; because light with energy below the material's bandgap cannot free an electron, it isn't absorbed.
Thin film solar cells use layers of semiconductor materials only a few micrometers thick. Thin film technology has made it possible for solar cells to now double as these materials:
Thin-film rooftop or solar shingles, made with various non-crystalline materials, are just now starting to enter the residential market. The following are benefits of these solar shingles:
Current issues with commercially available solar shingles include their lower efficiencies and greater expense compared with the standard home solar electric system.
The basic PV or solar cell typically produces only a small amount of power. To produce more power, solar cells (about 40) can be interconnected to form panels or modules. PV modules range in output from 10 to 300 Watts. If more power is needed, several modules can be installed on a building or at ground-level in a rack to form a PV array.
In addition to solar cells, a typical PV module or solar panel consists of these components:
Energy performance ratings for PV modules include the following:
For home solar electric systems, the most common array design uses flat-plate PV modules or panels. These panels can either be fixed in place or allowed to track the movement of the sun.
The simplest PV array consists of flat-plate PV modules in a fixed position. These are some advantages of fixed arrays:
These features make them suitable for many locations, including most residential roofs. Because the panels are fixed in place, their orientation to the sun is usually at an angle that is less than optimal. Therefore, less energy per unit area of array is collected compared with that from a tracking array. This drawback, however, must be balanced against the higher cost of the tracking system.
Solar arrays are designed to provide specified amounts of electricity under certain conditions. The following factors are usually considered when determining array energy performance:
The amount of electricity required may be defined by any one or a combination of the following performance criteria:
This last parameter is often given as a power efficiency, equal to "power output from array" ÷ "power input from sun" × 100%. Power is typically given in units of Watts (W), and energy is typically in units of Wh, or the power in Watts supplied during an hour.
To ensure the consistency and quality of PV systems and increase consumer confidence in system performance, various groups -- such as the Institute of Electrical and Electronics Engineers (IEEE), the International Electrotechnical Commission (IEC), and the American Society for Testing and Materials (ASTM) -- are working on standards and performance criteria for PV systems.
A typical home solar electric system consists of these components:
The balance-of-system equipment required depends on whether the system is a stand-alone system, connected to the electric grid, or a hybrid system. Balance-of-system equipment can include:
U.S. Department of Energy