PV cells, or solar cells, convert sunlight directly into electricity. PV cells are assembled into flat plate systems that can be mounted on rooftops or other sunny areas. They generate electricity with no moving parts, operate quietly with no emissions, and require little maintenance. An individual photovoltaic cell will typically produce between 1 and 2 W. To increase the power output, several cells are interconnected to form a module. Photovoltaic systems are available in the form of small rooftop residential systems (less than 10kWe), medium-sized systems in the range of 10 to 100 kWe, and larger systems greater than 100 kWe connected to utility distribution feeders.
Two semiconductor layers in the solar cell create the electron current. Materials, such as silicon, are suitable for making these semiconducting layers and each has benefits and drawbacks for different applications. In addition to the semiconducting materials, solar cells consist of two metallic grids or electrical contacts. One is placed above the semiconducting material and the other is placed below it. The top grid or contact collects electrons from the semiconductor and transfers them to the external load. The back contact layer is connected to complete the electrical circuit.
Commercially available PV modules convert sunlight into energy with approximately 5 to 15% efficiency. Efforts are under way to improve photovoltaic cell efficiencies as well as reduce capital costs.
Considerable attention is also being given to fully building-integrated PV cells, where the PV cells are an alternative to other construction materials. A principal drawback of PV cells is their reliance on an intermittent power source.