||Left to right: A solar cell, a photovoltaic module, and a photovoltaic array. The module contains 36 solar cells. The array contains 10 modules (360 solar cells).|
Types of Solar Cells
The performance of a solar or photovoltaic (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.
A solar cell consists of semiconductor materials. 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 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 small solar electric system.
Small Solar Electric Modules
In addition to solar cells, a typical photovoltaic (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
Energy performance ratings for PV modules include the following:
- Peak watt (Wp)
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 (NOCT)
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.
Small Solar Electric System Arrays
For small solar electric systems, the most common array design uses flat-plate photovoltaic (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. Some advantages of fixed arrays are:
- 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 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 watt-hours (Wh), or the power in watts supplied during an hour.
To ensure the consistency and quality of photovoltaic 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.
In addition to the solar cells and modules, a small solar electric (or photovoltaic) system consists of other parts called balance-of-system components.
The balance-of-system equipment required depends which kind of system is being used — grid-connected, stand-alone, or hybrid.
A typical small solar electric system usually includes the following balance-of-system components:
- 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.