Photovoltaic Cells, Modules and Arrays

Photovoltaic cells, aka solar cells, photoelectric cells, or just PV cells, are a type of solar technology that takes the energy found in light and directly converts it to electrical energy. When sunlight strikes a PV cell electrons are dislodged creating an electrical current.

Photovoltaic cells power many of the small calculators and wrist watches in use every day. More complex systems provide electricity to pump water, power communications equipment light homes, and run appliances. Beyond the utility power line, PV is often the lowest-cost means to provide electricity, and almost always simplest and cleanest to operate.

The cost of PV has fallen by 90 percent since the early 1970s. Photovoltaics are producing electricity for critical loads from the polar ice caps to the tropics to satellites in outer space. There is a strong market today in developing countries to provide rural electrification with solar panels, which replace kerosene lamps, batteries, and wood fires at a far lower cost than the central station power plants.

Anatomy of a Solar Cell

photovotaic cell diagram

The diagram above illustrates the operation of a basic photovoltaic cell, also called a solar cell. Solar cells are made of the same kinds of semiconductor materials used in microelectronics, such as silicon (melted sand) or cadmium telluride. For solar cells, a thin semiconductor wafer is specially treated to form an electric field, positive on one side and negative on the other. When light energy strikes the solar cell, electrons are knocked loose from the atoms in the semiconductor material. If electrical conductors are attached to the positive and negative sides, forming an electrical circuit, the electrons can be captured in the form of an electric current. This electricity can then be used to power a load, such as a light or a tool. Each PV cell converts about 5 to 15 percent of the sunlight that hits it into electrical current.

Photovoltaic Modules and Arrays

Photovoltaic cells are modular. That is, one can be used to make a very small amount of electricity, or many can be used together to make a large amount of electricity. A 3.9-inch (10-centimeter) diameter PV cell can make about one watt of power if the sun is directly overhead and the conditions are clear.

photovoltaic cell fits into a module which fits into an arrayBecause each photovoltaic cell produces only about one-half volt of electricity, cells are often mounted together in groups called modules. Each module holds about forty photovoltaic cells. By being put into modules, the current from a number of cells can be combined. PV cells can be strung together in a series of modules or strung together in a parallel placement to increase the electrical output.

When multiple PV cell modules are put together, they can form an arrangement called an array or array field. In general, the larger the area of a module or array, the more electricity that will be produced. Photovoltaic modules and arrays produce direct current (dc) electricity. They can be connected in both series and parallel electrical arrangements to produce any required voltage and current combination.

Types of photovoltaic cells

A monocrystalline PV cell is blue or gray-black in color. At the rounded corner of each cell is a white backing. This backing shows through and makes a pattern that is easy to see. Some people do not use monocrystalline PV cells on their home or businesses because of their appearance. A module of PV cells is usually covered with tempered glass and surrounded by an aluminum frame .

A polycrystalline PV cell looks a little different than a monocrystalline PV cell. Polycrystalline PV cells are shaped like rectangles and colored sparkling blue. There is no white background showing. Thus, these PV cells look more uniform in appearance. Like monocrystalline cells, they are often covered in tempered glass and placed in an aluminum frame.

thin film cellsAnother type is the amorphous or thin-film cell. This type of PV cell is less durable and not as efficient for energy conversion. Thin-film cells are the future of PV cell technology because they use less semiconductor material, do not need as much energy to manufacture, and are easier to mass produce than other PV cells. Light weight and flexible, this technology can be weaved into the fabric of a building in applications where surface area is not an issue. Applications include building canopies, facades and rain screens.

Additional components

Sometimes, photovoltaic systems have other components to make them useful for providing electricity. Two such components are an inverter and a storage device. The inverter helps change the DC power (direct current) produced by the cells to the AC (alternating current) used by most equipment, homes, and businesses that run on electricity in the United States.

The storage unit stores the energy created by the photovoltaic cells for use when there is little or no sun. One storage unit that works well with photovoltaic cells is a battery, which stores the energy created electrochemically. The energy created by PV cells can also be stored as potential energy. Pumped water and compressed air are two types of potential energy. All of these storage types are used where the PV cells are located.

Uses for Photovoltaic Cells

The first use for the first practical photovoltaic cell was a source of electricity for satellites orbiting Earth. PV cells were chosen because they were considered safer than nuclear power, another option being considered. On Earth, photovoltaic cells are used to make electricity in places not connected to the power grid or where it is too costly to use electricity produced by the grid. This often happens in remote areas.

People who live in isolated houses or who want to be independent of the power grid use photovoltaic cells to provide electricity for their homes because of their adaptability. PV cells can power most household appliances, such as televisions, refrigerators, and computers, and they can also power electric fences and feeders for livestock. Photovoltaic systems can be used on farms to power pumps that provide water for livestock on grazing areas that are far away from the main farm.

Other independent, often isolated, objects use photovoltaic cells in similar ways. Navigation beacons can be powered by PV cells, as can remote monitoring equipment stations for pipeline systems, water quality systems, and meteorological information. Many traffic signals, street signs, billboards, bus stop lights, highway signs, security lighting, and roadside emergency telephones also use this technology.

Benefits and disadvantages of photovoltaic cells

The use of photovoltaic cells has many positive aspects. They make no noise, require little to no maintenance, and are reliable. No special training is needed to operate a PV cell system. In addition, PV cells can be made a variety of sizes from very small to very large, providing flexibility in use. Moreover, many PV cells can be used anywhere because they can use both direct sunlight and diffuse sunlight. Finally, PV cell systems are long-lasting, maintaining their effectiveness for twenty to thirty years. Thus, they produce much more energy through their operation over their lifetime than is used to manufacture them.

Like many solar energy technologies, however, one major drawback to photovoltaic cells is that no power is produced when there is no sunshine. If the weather is poor and the sun is blocked, as when it rains or snows, these cells do not produce power. Photovoltaic cells also do not produce power at night. Because of this situation, some sort of backup system or alternate power supply is needed.

Photovoltaic Cells, Modules and Arrays copyright 2011

photograph of photovoltaic array