When the electrons leave their position, holes are formed.  When many electrons, each carrying a negative charge, travel toward the front surface of the cell, the resulting imbalance of charge between the cell's front and back surfaces creates a voltage potential like the negative and positive terminals of a battery.  When the two surfaces are connected through an external load, electricity flows.

The photovoltaic cell is the basic building block of a PV system.  Individual cells can vary in size from about 1 cm (1/2 inch) to about 10 cm (4 inches) across.  However, one cell only produces 1 or 2 watts, which isn't enough power for most applications.  To increase power output, cells are electrically connected into a packaged weather-tight module.  Modules can be further connected to form an array.  The term array refers to the entire generating plant, whether it is made up of one or several thousand modules.  As many modules as needed can be connected to form the array size (power output) needed. 

The performance of a photovoltaic array is dependent upon sunlight.  Climate conditions (e.g., clouds, fog) have a significant effect on the amount of solar energy received by a PV array and, in turn, its performance.  Most current technology photovoltaic modules are about 10 percent efficient in converting sunlight with further research being conducted to raise this efficiency to 20 percent.

The PV cell was discovered in 1954 by Bell Telephone researchers examining the sensitivity of a properly prepared silicon wafer to sunlight.  Beginning in the late 1950s, pvs were used to power U.S. space satellites.  The success of PVs in space generated commercial applications for pv technology.  The simplest photovoltaic systems power many of the small calculators and wrist watches used everyday.  More complicated systems provide electricity to pump water, power communications equipment, and even provide electricity to our homes.

Photovoltaic conversion is useful for several reasons.  Conversion from sunlight to electricity is direct, so that bulky mechanical generator systems are unnecessary.   The modular characteristic of photovoltaic energy allows arrays to be installed quickly and in any size required or allowed.

The environmental impact of a photovoltaic system is minimal, requiring no water for system cooling and generating no by-products.  Photovoltaic cells, like batteries, generate direct current (DC) which is generally used for small loads (electronic equipment).  When DC from photovoltaic cells is used for commercial applications or sold to electric utilities using the electric grid, it must be converted to alternating current (AC) using inverters, solid state devices that convert DC power to AC.  Historically, pvs have been used at remote sites to provide electricity.  However, a market for distributed generation from PVs may be developing with the unbundling of transmission and distribution costs due to electric deregulation.  The siting of numerous small-scale generators in electric distribution feeders could improve the economics and reliability of the distribution system.

These systems, also known as solar thermal power plants, use the sun's heat to create steam, which then turns a turbine and produces electricity. (Fossil fuel burning power plants also produce electricity by first creating steam in order to turn a turbine.)

The major applications of solar thermal energy at present are heating swimming pools, heating water for domestic use, and space heating of buildings.  For these purposes, the general practice is to use flat-plate solar-energy collectors with a fixed orientation (position).
Where space heating is the main consideration, the highest efficiency with a fixed flat-plate collector is obtained if it faces approximately south and slopes at an angle to the horizon equal to the latitude plus about 15 degrees.

Solar collectors fall into two general categories:  nonconcentrating and concentrating.

In the nonconcentrating type, the collector area (i.e. the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).

In concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. Where temperatures below about 200o F are sufficient, such as for space heating, flat-plate collectors of the nonconcentrating type are generally used.

There are many flat-plate collector designs but generally all consist of:
(1) a flat-plate absorber, which intercepts and absorbs the solar energy,
(2) a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber,
(3) a heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and
(4) a heat insulating backing.

Solar space heating systems can be classified as passive or active. In passive heating systems, the air is circulated past a solar heat surface(s) and through the building by convection (i.e. less dense warm air tends to rise while more dense cooler air moves downward) without the use of mechanical equipment. In active heating systems, fans and pumps are used to circulate the air or the heat absorbing fluid.

Source: www.enecho.meti.go.jp

A solar power tower or central receiver generates electricity from sunlight by focusing concentrated solar energy on a tower-mounted heat exchanger (receiver).
This system uses hundreds to thousands of flat sun-tracking mirrors called heliostats to reflect and concentrate the sun's energy onto a central receiver tower.
The energy can be concentrated as much as 1,500 times that of the energy coming in from the sun.
Energy losses from thermal-energy transport are minimized as solar energy is being directly transferred by reflection from the heliostats to a single receiver, rather than being moved through a transfer medium to one central location, as with parabolic troughs.
Power towers must be large to be economical.
This is a promising technology for large-scale grid-connected power plants.
Though power towers are in the early stages of development compared with parabolic trough technology, a number of test facilities have been constructed around the world.