Photovoltaics, or PV for short, is a technology in which light is converted into electricity using photovoltaic modules that have no moving parts, operate quietly without emissions, and are capable of long-term use with minimal maintenance. Today, PV technologies can convert about 10-20 percent of the sun’s energy directly into electricity. Many areas of the U.S. have the potential to deploy PV technologies to produce a significant portion of their electricity needs with little or no environmental pollution. With reasonable government support incentives for the PV market, it is possible to grow this industry 30 percent per year over the next 30 years, from 340 MW of installed capacity to 9,600 MW.
Solar PV systems are comprised of solar modules that contain solar cells. Solar cells are small, square-shaped panel semiconductors made from silicon and other conductive materials. They are manufactured in thin film layers. When sunlight strikes a solar cell, chemical reactions release electrons, generating electric current. Solar cells are also called photovoltaic cells – or PV cells for short – and can be found on many small appliances, like calculators, toys and even hats. Crystalline silicon, the same material commonly used by the semiconductor industry, is the material used in 94% of all PV modules today.
The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain wave lengths or energies will work efficiently to create electricity, and much of it is reflected or absorbed by the material that makes up the cell. Because of this, a typical commercial solar cell has an efficiency of 15%-about one-sixth of the sunlight striking the cell generates electricity. Low efficiencies mean that larger arrays are needed, and that means higher cost. Improving solar cell efficiencies while holding down the cost per cell is an important goal of the PV industry, NREL researchers, and other U.S. Department of Energy (DOE) laboratories, and they have made significant progress. The first solar cells, built in the 1950s, had efficiencies of less than 4%. Individual PV cells are arranged together in a PV module and the modules are grouped together in an array. Some of the arrays are set on special tracking devices to follow sunlight all day long.
The electrical energy from solar cells can then be used directly. It can be used in a home for lights and appliances. It can be used to power a business. Solar energy can be stored in batteries to light a roadside billboard at night. Or the energy can be stored in a battery for an emergency roadside cellular telephone when no telephone wires are around.
There are two primary PV markets. Off-grid systems are used where the cost of a PV system is cheaper than stringing electrical power lines long distances from the local utility. Grid-connected PV systems usually cannot compete directly with the cost of utility-produced power. Because of state incentives and federal tax credits, many people are considering grid-connected PV systems. If the PV system provides more power than the home or business uses, additional electricity is fed back into the grid for other people to use. This effectively spins an electricity meter backward in what is known as "net metering."
Because they do not produce polluting air emissions or water effluents, solar PV systems are prime candidates for supplying electricity at locations where such environmental impacts are unacceptable, for example, in parks and places where preserving high levels of environmental quality is important.
Types of Systems
Generally speaking, solar electric systems may be categorized into three primary types, stand alone, back-up, and utility connected. Any of these types systems may be designed to meet all or part of the user’s electrical requirements.
Stand Alone PV System
Stand alone type systems usually take the place of grid connected electricity. They generally include solar charging modules, storage batteries and controls/regulator. Ground or roof mounted systems will require a mounting structure, and if 120/240 volt AC power (typical household current) is desired, a DC (direct current) to AC (alternating current) inverter will also be required.
The batteries used for most stand alone type solar electric systems are different than the ones used in an automobile. Storage batteries for solar electricity are called deep cycle batteries. These batteries are designed to be recharged many times and be able to provide a steady amount of power over a long period of time. New high quality batteris designed especially for solar applications with lifetimes of up to 15 years are now available, if properly manged and maintained.
Stand alone solar power systems use a charge controller to prevent over-charging the system’s battery. The charge controller is wired between the solar modules and the battery to monitor and control the current from the modules and shut the current off when the battery is fully charged. This prevents over-charging and damage to the batteries.
Efficient DC appliances help make solar electricity even more economical in many cases and you may be surprised by the variety of DC appliances available. There are TV’s, stereos and fluorescent lights, to name a few. Folks that travel in an RV (recreational vehicle) are usually very familiar with the abundant supply of DC powered appliances.
Some applications need a system that includes a fuel power backup generator, wind turbine or water turbine. Typically, such "hybrid" systems share the load between the solar chargers and the back-up generator. Batteries are still required, plus the DC to AC inverter if regular AC loads will be powered.
The small stand alone DC system
The small stand-alone system is in excellent replacement for kerosene lamps and noisy generators in a remote home, a recreational vehicle or a boat. The size of the PV array and battery bank will depend upon individual requirements. The actual sizing depends on the wattage of the loads and how often they are to be run.
The PV array charges the battery during daylight hours and the battery supplies power to the loads when needed. The charge regulator terminates the charging when the battery reaches full charge. The load center may contain meters to monitor system operation and necessary fuses to protect wiring in the event of a malfunction or short circuit in the building.
Stand alone AC-DC System
This system is the same as the previous system, except for the use of a DC to AC inverter. With the addition of an inverter, commonly available household appliances such as computers, power tools, vacuum cleaners, washing machines and kitchen appliances can used.
High quality DC to AC inverters are available with power outputs ranging from one hundred watts to ten kilowatts and more, and conversion efficiencies greater than 90 percent. To ensure reliable system operation, the inverter should be carefully matched to the loads that will be run. Most larger inverters also have the ability to serve as battery chargers from a backup generator when more power is needed than can be supplied by the solar modules. Often the generator will be wired to an automatic cranking device. This redundancy is important for continuously operating critical loads, such as information systems, refrigerators and other critical equipment.
As the loads (power requirements) on the system are increased, a larger solar array and more battery storage will be required. The more efficient the appliances – the lower the cost for a system will be.
Back-up AC System
A back-up or stand-alone AC solar electric system will usually have a PV array of ten or more modules, battery bank and one or more inverters. Two or more stackable inverters are an excellent choice for this type of system since they can work together to supply power to large loads and if one fails, the others can continue to operate at reduced output until repairs are made. The utility will back-up the solar power and run the loads when available and needed. If utility power fails the power from the solar PV system can run the backed up loads. A fossil fuel generator may be included to further back-up the system.
In most businesses and homes, an AC only system simplifies wiring by allowing the use of low cost, readily available switches, outlets, and fixtures. Savings on wire cost are significant, because the large gauge wire required for efficient transmission of low voltage DC power over long runs is avoided.
Utility Intertied System
These are the simplest systems and require no batteries. These systems are not designed for back-up power but are designed to power immediate electricity loads at the source and contribute power back into grid or utility supply if the electricity is not being consumed. These systems automatically ‘shut off’ if utility power goes off line. This protects line workers from power being back fed into the grid during an outage. Once utility power returns the utility intertied inverter resumes normal operation.
By lowering a building’s electric bills these systems will pay for themselves over a number of years and reduce the air pollution produced by utility companies that burn coal or natural gas. These systems also help the utility company reduce ‘peak load’ during the day. Contributing clean, green power from your own roof helps create jobs and is the alternative to buying fossil fuel derived electricity.
Utility intertied systems are generally designed to reduce power demands from the utility by ‘net metering’ power or in some cases to sell power back to the utility. These utility connected renewable energy systems can certainly help offset a building’s utility bills while helping the utility reduce ‘peak hour demands’. A typical system might include solar modules, a mounting structure, and AC inverter/control for the power to be fed back through the building’s 120/208/240 volt AC power distribution system.
The Department of Energy’s Office of Energy Efficiency and Renewable Energy has developed a helpful Consumer’s Guide titled "Get Your Power from the Sun" that is an excellent primer on residential PV systems.
Other PV Technology Applications
Building Integrated PV (BIPV)
The acronym BIPV (Building Integrated Photovoltaics) refers to PV systems that are integrated within a building’s design and architecture so that the solar components also serve as structural or design elements. These can include roofs, walls, awnings and facades. When PV panels are integrated into a building during construction, the incremental costs of the system are reduced while the building owner is provided with tangible, cost-saving advantages such as significantly reduced demand for peak electricity, reduced transmission losses and the ability of back-up power. BIPV gives buildings the opportunity to become more self-sufficient by allowing them to generate their own electricity rather than merely consume energy. PV integrated into a building can, as a second function, also provide shade, insulation and help to control the interior climate.
BIPV doesn’t use any extra space and because the material savings from replacing ordinary construction material with BIPV substantially reduces the cost of the installed PV system and thus the cost of PV electricity. The building in the photo above is 4 Times Square, a 48-story skyscraper at the corner of Broadway and 42nd St., was the first major office building to be constructed in New York City in the 1990s. The building’s most advanced feature is the photovoltaic skin, a system that uses thin-film PV panels to replace traditional glass cladding material. The PV curtain wall extends from the 35th to the 48th floors on the south and east walls of the tower, making it a highly visible part of the midtown New York skyline. Although the surface area for PV is relatively small, the system still provides enough energy to power the equivalent of five to seven homes.
Thin films of special photovoltaic material can produce solar cells with relatively high conversion efficiencies, while using much less material than crystalline silicon cells. The key materials or technologies are amorphous silicon (a-Si), copper indium diselenide (CIS) and its alloys, and cadmium telluride (CdTe). Thin film technologies have been underdevelopment for the past 15 to 20 years and utilize very small amounts of specialized materials to create solar panels. These thin film panels have the potential to produce power significantly cheaper than today’s standard silicon technology. Thin film photovoltaics are typically manufactured in roll or shingle format with amorphous silicon in much smaller amounts than used in a traditional photovoltaic solar panel. The advantages of the product are its light weight, durable construction and a thin profile which is easier to integrate into building architecture and design. A frequent application is for a thin film rolled product to be adhered within the channels of a standing seam roof or for thin film shingles to be integrated into a coordinating asphalt shingled roof.
Thin-film PV panels account for about 12% of all solar modules sold throughout the world. Shares in thin-film PV panels are expected to increase, as thin-film PV has the potential to cost far less than crystalline silicon wafer panels. Advanced thin-film design also promises a high energy return across a wide range of climates.
The term “thin film” describes the method used to place the film, rather than the thin-nature of the film; thin-film cells are deposited in thin, consecutive layers of atoms, molecules or ions. These thin-film cells have several advantages over “thick-film” cells. For starters, thin-film cells use far less material—about one to 10 micrometers thick, as opposed to the 100- to 300- micrometer width of thick film cells. Thin-film cells also lend themselves to relatively simple production, and can also be placed on flexible substrate materials, opening a whole new dimension for use.
Despite these advantages, thin-film cell panels are generally less efficient than their crystalline silicon counterparts, with efficiency rates of 7-10% as compared to silicon’s average of 15% efficiency. Further research and experimentation may yield more efficient thin-film technology.
One promising technology combining “the best of both worlds” is crystalline silicon thin films, deposited on glass. This technology makes use of the advantages of crystalline silicon as a solar cell material, while simultaneously saving costs with the utilization of a thin-film approach. Multi-layer thin-film technology also has greater efficiency rates than that of crystalline silicon panels.
Not surprisingly, thin-film PV cell and module shipments experienced the greatest percentage gain between 2005 and 2006, nearly doubling. Still, conventional crystalline silicon cells and modules shipments continued to dominate all PV technologies with 233,518 peak kilowatts shipped in 2006.
For a more in-depth look at how PV cells work, check out the "HowStuffWorks.com" web page for Solar Cells.