One of the most promising renewable energy technologies is photovoltaics. Photovoltaics (PV) is a truly elegant means of producing electricity on site, directly from the sun, without concern for energy supply or environmental harm. These solid-state devices simply make electricity out of sunlight, silently with no maintenance, no pollution, and no depletion of materials. There is a growing consensus that distributed photovoltaic systems that provide electricity at the point of use will be the first to reach widespread commercialization. Chief among these distributed applications are PV power systems for individual buildings.
The performance of a solar cell is measured in terms of its efficiency at turning sunlight into electricity. Only sunlight of certain energies will work efficiently to create electricity, and much of it is reflected or absorbed by the material that make 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.
Solar panels are rated in terms of peak power (kWp). This is the potential power output in bright sunlight (1000W/m²) and an air temperature of 25 ºC (the output of panels reduces as they increase in temperature). 1 kWp of well-sited photovoltaic array in the UK should be able to produce 700-800 kWh of electricity per year. The amount of incident solar radiation will depend on the latitude of the site, the direction that the panels face and the panels tilt angle. An online calculator is available for obtaining estimates of the potential generated energy for sites in Europe.
There are two basic commercial PV module technologies available on the market today:
Thick crystal products include solar cells made from crystalline silicon either as single or poly-crystalline wafers and deliver about 10-12 watts per ft² of PV array (under full sun).
Thin-film products typically incorporate very thin layers of photovoltaicly active material placed on a glass superstrate or a metal substrate using vacuum-deposition manufacturing techniques similar to those employed in the coating of architectural glass. Presently, commercial thin-film materials deliver about 4-5 watts per ft² of PV array area (under full sun). Thin-film technologies hold out the promise of lower costs due to much lower requirements for active materials and energy in their production when compared to thick-crystal products
There is relatively little mechanical work associated with the installation of photovoltaic panels, and there is a mature market for the supply of flexible mounting systems. However, there is controlled work that requires properly trained electricians, and installation of roof-mounted panels would normally require a trained 'roofing' installer. Photovoltaic modules generate electricity whenever they are exposed to daylight and individual modules cannot be switched off so, unlike most other electrical installations, installing a photovoltaic system involves working on a live system. Typical installations of photovoltaic modules will weigh 10–13 kg/m².
The main types of installation are:
Roof or wall mounting of framed photovoltaics. The photovoltaics are mounted in framing that should generally protrude less than 200mm to satisfy normal planning requirements.
Roof or wall-integrated photovoltaics, where the panel is a weathered section of the surface.
Roof slates and tiles. These will be more expensive as individual components but may reduce expenditure by displacing standard tiles/slates.
Surface-mounted (thin-film) photovoltaics. Semi-flexible thin-film photovoltaics are attached to a building component such as a glass panel or flat roof.
Framed installations (freestanding or attached to building). Plastic or metal frames can sit on roofs or on the ground to provide optimum tilt and orientation.
More recent developments have led to a variety of forms of integrated solar collector, including:
Solar shingles mounted flat on boarded roofs.
Solar slates mounted on battens that can replace standard roof components.
Solar glass laminates, where the photovoltaics take the form of semi-transparent glazing.
