Solar Energy - Photovoltaics UTI-111 Prof. Park Essex County College

Energy from the Sun The sun’s outer surface, called the photosphere, radiates energy iin the form of light and heat. The sun’s inner core is composed of primarily of dense Hydrogen and Helium at a temperature of approximately 27 million  F. The light and heat from the sun is produced when these gases undergo fusion.

Energy from the Sun Of the total energy from the sun that reaches the earth, about 30% is reflected back to space, 47% is absorbed and converted to heat and 23% drive the water cycle. The solar energy that the earth received is called insolation.

Conversion of Solar Energy Conversion of solar energy into electrical energy involves the use of solar cells. Solar cells, also called photovoltaics (PV), convert sunlight directly into electricity. For example, a calculator can be powered by solar cells.

History of PV In 1839, French physicist Edmond Becquerel (3/24/1820 – 5/11/1891) discovered the process of using sunlight to produce an electric current in a solid material. In 1883, New York electrician Charles Edgar Fritts, constructed a selenium solar cell and filed a patent. In 1953, Bell Lab researchers Chaplin-Fuller- Pearson team produced doped silicon solar cells with an efficiency of 6%.

Photoelectric Effect The Photoelectric Effect is the physical process by which a PV cell converts sunlight into electricity. When light strikes a PV cell, it may be reflected, absorbed, or pass through. The light that is absorbed produces electricity. The light energy absorbed by the PV excites the electrons in the atoms of the PV cell. The electrons escapes from their original positions in the atoms of the semiconductor material and becomes part of the electrical flow, or current, in an electrical circuit.

PV Cells Construction To induce the built-in electric field within a PV cell, two layers of different semiconductor materials are placed in contact with one another. The conductivity of semiconductor increases with temperature and in the presence of impurities. The addition of these impurities is called doping. In a PV cell, photons are absorbed in the p-layer. It is important to optimize this layer to the properties of incoming photons to absorb as many as possible, and thus, to free up as many electrons as possible while keeping the electrons from meeting up with holes and recombining with them before they can escape from the PV cell.

PV Cells Construction Electrical contacts are essential to a PV cell because they bridge the connection between semiconductor material and the external electrical connection. The back contact of a cell consists of a layer of aluminum or molybdenum. The front contact is complicated by the need to design a low resistance point that will collect the maximum current. To do this, contacts must be placed across the entire surface of a PV cell. This is normally done with a gird of metal strips or fingers. Since this grid will absorb light, the design must balance shading and electrical losses.

Solar Cells, Modules, and Arrays Solar cells are typically combined into modules that hold about 40 cells; About 10 of these modules are mounted in a PV arrays that can measure up to several meters on a side. These PV arrays can be mounted at a fixed angle facing south, or they can b mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day. About 10 to 20 PV arrays can provide enough power for a household; for a large utility or industrial applications, hundreds of arrays can be interconnected to form a single large system.

Performance of Solar Cells

Regional Ranking of Global PV Cell Production in 2008 (Crystalline and Thin Film, Percentage Ranking by Production in Watts)