Photovoltaic Cells

Picture: DOE/EERE

Photovoltaic cells (PVs) (also known as "solar cells") work by transforming light that comes from the sun directly into electricity without an intermediate mechanical device or thermal process. The term photovoltaic is derived by combining the Greek word for light, "phos", with the word "voltaic". The term "volt" is a measure of electricity named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. Photovoltaics literally means light-electricity.

The basic building unit of PV technology is the photovoltaic cell (PV cell). PV cells are made of a semiconductor material, typically silicon, which is treated chemically. When light hits the cell, a field of electricity is created within the layers causing the electricity to flow. This "photovoltaic effect" results in direct current (DC) electricity which is the same type of current produced by batteries.

In order to use this energy in most homes, an inverter is used to change the DC electricity to AC. Once electricity is generated, it can go to power anything in your house or be stored in batteries for later use. The greater the intensity of the light, the greater the flow across the layers and so the more electricity generated. But such a system does not necessary require direct sunlight to work.

Single PV cells are connected electrically to form PV modules, which are the building blocks of PV systems. Depending upon the application, the solar modules are typically wired together to form an array. Individual PV cells – averaging about 4 inches per side – typically converts 15% of the available solar radiation into about 1 or 2 watts of electrical power. Larger modules or arrays of modules are used to generate power for the grid.

Solar Heating Systems

Solar heating can be used to heat the space in homes and buildings or to heat the water. There are two basic types of solar heating systems: passive solar heating and active solar heating.

The appropriate use of windows along with building design is called passive solar heating. The buildings can be designed to make the best use of the sun in winter while keeping the heat out in summer. South-facing, large windows, building materials that absorb and retain heat (such as stones and bricks), and efficient airflow are among the design features of a home that takes advantage of passive solar.

Active solar heating systems use mechanical equipment, such as pumps and fans, to increase the usable heat in a system. The heat is primarily used for heating water in homes, commercial buildings and industrial facilities. Active solar heating can be further divaded into liquid-based and air-based systems according to the kind of energy transfer fluid that is used.

Passive Solar, Active Solar and Photovoltaics

There are three different ways to harness the sun's energy: passive solar, using architectural design and natural materials to absorb the sun's energy; active solar, utilizing the sun's heat by means of solar collectors; and a third way in which solar energy can be harnessed is through the use of photovoltaic systems.

Passive solar is the capturing and storing the suns' energy - light and heat - without the use of any mechanical devices. As the solar radiation strikes windows, walls, floors, and other objects within the room it is converted to heat. A good example of a passive solar energy system is a greenhouse.

Active solar uses devices to collect, store, and circulate heat produced from solar energy. Active solar energy technologies convert sunlight into heat by using a particular energy transfer fluid. This is most often water or air but can also be a variety of other substances.

Photovoltaic systems directly convert sunlight into electricity using a semiconductor material such as silicon. The electrical energy from PVs can be stored in batteries for use when there is no sun (during cloudy days or at night).

Solar Energy: from Sun to Earth

It takes millions of years for the energy in the sun's core to make its way to the solar surface, and then just a little over eight minutes to travel the 93 million miles (149,596,000 km) to earth. The solar energy travels to the earth primarily in the form of electromagnetic radiation similar to radio waves, but in a different frequency range.

The amount of energy from the sun that reaches Earth each day is enormous. But only a small portion of the energy radiated by the sun into space strikes the earth. About 15 percent of the sun's energy is reflected into space before reaching the earth's surface. Another 30 percent of the energy is used in the planet's heat cycle, evaporation of water to run the water - weather cycle, wind, and waves. Only a small fraction of the energy is actually utilized for photosynthesis in plants. The rest could be used to supply our energy needs.

What Does Solar Energy Mean and How It is Generate?

Let's begin with some basic information about solar energy. Solar energy is one of the three main renewable sources of energy along with hidropower and wind energy. The word “Solar” is the Latin word for “sun”. Solar energy means energy that comes from the sun - heat and light.

The sun is our nearest star and like other stars it is a giant gas ball made up mostly of hydrogen and helium. The sun generates energy in its core in a process called nuclear fusion. During nuclear fusion, the sun's extremely high pressure and hot temperature cause hydrogen atoms to come apart and their nuclei to fuse or combine. Hydrogen nuclei (protons) fuse to create helium-4 atom. But the helium atom is less massive than the protons that combined to form it. Some matter is lost during fusion process. The difference in mass was converted to energy which is then emitted into space as radiant waves.