Small Solar Electric Systems
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.
Types of Solar Cells
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 an 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:
- Monocrystalline or single crystal silicon
- Multicrystalline silicon
- Polycrystalline silicon
- Amorphous silicon.
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 the 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:
- Rooftop or solar shingles
- Roof tiles
- Building facades
- Glazing for skylights or atria.
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:
- Attractive integration into homes
- Dual-purpose -- serves as both roofing material and pollution-free electricity producer
Current issues with commercially available solar shingles include their lower efficiencies and greater expense compared with the standard home solar electric system.
Small Solar Electric Modules
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:
- A transparent top surface, usually glass
- An encapsulant -- usually thin sheets of ethyl vinyl acetate that hold together the top surface, solar cells, and rear surface
- A rear layer -- a thin polymer sheet, typically Tedlar, that prevents the ingress of water and gases
- A frame around the outer edge, typically aluminum.
Energy performance ratings for PV modules include the following:
- Peak Watt -- Measures the maximum power of a module under laboratory conditions of relatively high light level, favorable air mass, and low cell temperature. These conditions are not typical in the real world.
- Normal operating cell temperature -- Measures a module's nominal operating cell temperature after the module first equilibrates with a specified ambient temperature. It results in a lower Watt value than the peak-Watt rating, but it is probably more realistic.
- AMPM Standard -- Measures the performance of a solar module under more realistic operating conditions. It considers the whole day rather than "peak" sunshine hours, based on the description of a standard solar global-average day (or a practical global average) in terms of light levels, ambient temperature, and air mass.
Home Solar Electric System Arrays
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:
- No moving parts
- No need for extra equipment
- A lightweight structure.
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:
- Characterization of solar cell electrical performance
- Determination of degradation factors related to array design and assembly
- Conversion of environmental considerations into solar cell operating temperatures
- Calculation of array power output capability.
The amount of electricity required may be defined by any one or a combination of the following performance criteria:
- Power output -- power (Watts) available at the power regulator, specified either as peak power or average power produced during one day.
- Energy output -- the amount of energy (Watt-hour or Wh) produced during a certain period of time. The parameters are output per unit of array area (Wh/m²), output per unit of array mass (Wh/kg), and output per unit of array cost (Wh/$).
- Conversion efficiency -- defined as "energy output from array" ÷ "energy input from the sun" × 100%.
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.
HOME SOLAR ELECTRIC COMPONENTS
A typical home solar electric system consists of these components:
- Solar cells
- Modules or panels (which consist of solar cells)
- Arrays (which consist of modules)
- Balance-of-system parts.
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:
- Mounting racks and hardware for the panels
- Wiring for electrical connections
- Power conditioning equipment, such as an inverter
- Batteries for electricity storage (optional)
- Stand-by gasoline electric generator.
U.S. Department of Energy