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AC
Short for ‘alternating current’, i.e. an electrical current in which the polarity is constantly alternating. Our power network contains AC with a frequency of 50 Hz (Hertz). It assumes a negative and a positive value 50 times each per second. In the solar plant, AC is generated using an inverter.

Ampere
Ampere, or amp, is the unit of measurement of the electrical current, abbreviated ‘A’. The force of an electric current is the number of charges flowing per unit time. Electrical current is measured as the force with which two parallel electrical lines attract each other. It is indicated as current with the symbol ‘I’ See also Ohm’ s Law.

Anode
The positive pole of an electrical circuit. The electrons travel from the negative pole (cathode) to the anode (positive pole). The definition as positive and negative poles was defined when the actual direction in which the electrons travel was not yet known.

Anti-reflective layer
The anti-reflective layer is a layer with a thickness of only a few millionths of a millimetre, consisting of silicon nitride (SiN) and minimizing reflection losses. Light that is reflected from the surface of a solar cell cannot be absorbed, and hence does not contribute to the generation of electricity. In solar cells, the anti-reflective layer therefore increases the light yield and the efficiency.
 
a-Si
a-Si stands for ‘amorphous silicon’. Atoms in amorphous matter such as amorphous silicon are irregular and appear to be randomly arranged; Greek amorphous means ‘shapeless’. By contrast, the microscopic shape of crystalline matter is regular.

BSW - Bundesverband Solarwirtschaft e. V. (Federal Association of Solar Business)
The Federal Association of the Solar Industry (BSW) is based in Berlin and represents the interests of German solar companies in dialogue with politicians and the public. In 2006, the BSW had over 600 members and represented more than 80 % of the domestic production of solar plants. The BSW was created on 28 March 2006 through a merger between the Federal Association of the Solar Industry (BSi) and the Association of Solar Industry Companies (UVS). See also.

Bypass Diode
A diode that is switched parallel to a solar cell, conducting the electrical current of all remaining solar cells past a single shadowed solar cell (when they are connected in series). The diode thus prevents a hot spot.

CdTe
Cadmium telluride, semiconductor material for the production of thin-film solar cells and modules.

CIS
Copper indium diselenide, a material compound of copper, indium and selenium, is used for the production of thin film solar modules.

CO2
Carbon dioxide (CO2) is a colourless, non-flammable gas with an acidic flavour, making up about 0.3 % of the air. It is generated, for instance, when coal, oil and gas – in other words, non-renewable resources – are burned. During the past 200 years of industrialisation, man has burned more and more of these raw materials, which have been stored for millions of years, thus increasing the concentration of CO2 in the atmosphere. The greenhouse effect is ascribed to the increase in the ratio of CO2, since CO2 reflects the long-wave heat radiation of the earth’s surface on its way into outer space and therefore impedes the earth’s heat loss. As a consequence, the average temperature on earth increases.
 
Crystalline silicon
In nature, silicon occurs only as silicon dioxide or in silicate minerals such as sand, i.e. only in a heavily contaminated state. For photovoltaics and the computer industry, it has to be processed into crystalline silicon with a purity of over 99.99 %. The silicon atoms then arrange themselves not randomly in a mixture of substances, but in a regular, crystalline structure.

c-Si
Crystalline silicon.

Current
Term for the flow of electrical charges, for instance in the form of electrons through a copper cable. The unit of measurement is the ampere (abbreviation: A).

DC
Direct current, i.e. electrical current without change of direction, as it is generated in batteries or solar cells.

Degradation
’Degradation’ or ‘ageing’ refers to the decline of a solar cell’ s efficiency over time. Mainly solar cells made of amorphous silicon are affected by the ageing process.

Dena
Deutsche Energie-Agentur. The German Energy Agency is a government agency set up to promote energy saving and regenerative energies. It implements national and international campaigns, informs end users and develops strategies for future energy supply. It was founded by the German federal government in the autumn of 2000 and has its headquarters in Berlin. More at http://www.dena.de/en/

DGS
Deutsche Gesellschaft für Sonnenenergie e.V. The German Association of Solar Energy was founded in 1975 in Munich. The main objective is transformation of the energy industry through broadly based introduction of renewable energies.

Diode
Electronic semiconductor component which lets the electrical current pass in only one direction. If a power line is compared to a water pipe, the diode would be a (one-way) valve in the water pipe. Diodes are made of semiconductor crystals and have a characteristic resistance. A solar cell therefore is basically a large, illuminated diode.

Doping
By the precise addition of small amounts of so-called doping atoms, the electrical conducting ability of a semiconductor can be significantly enhanced. Doped semiconductors are used for the production of pn junctions in solar cells and electronic components.

Input Charges
Amount which according to the REA is paid for the input of regeneratively produced electricity into the public electricity grid. The rate is different for each type of renewable energy, such as wind, solar energy, waterpower and geothermal energy, and is reduced by 5% every year for solar energy. In 2006, it was between 40.6 cents (large outdoor plants) and 51.8 cents (roof systems) per kWh for solar plants.

Efficiency
The efficiency η (pronounced ‘eta’) of a solar cell or a module is defined as the ratio between the power output and the power input (the absorbed light). In other words, it indicates how well the solar cell converts light into electricity. In addition, the efficiency of the module indicates how much energy is lost by connecting the individual cells in the module.

Electrical field
An electrical field is generated when current flows through a conductor. The field exerts a force on charge carriers; due to their different charges, electrons and holes in semiconductor material move in opposite directions when an electrical field is applied and they can therefore be separated by a field. In most solar cells the pn junction generates an electrical field.

Electrons
Electric particles with a negative charge, whose movement in a conductor, such as a wire, is called ‘current flow’. Electric particles that are at rest do not constitute electrical current.

Fossil resources
The fossil resources oil, gas and coal were formed millions of years ago, when the remains of plants and animals were deposited on the ocean floor and over the years were covered by sediments such as clay and sand. Underneath the sediments, pressure and temperature increased, converting the organic material into hydrogen and carbon compounds. The carbon was withdrawn from the atmosphere. When these resources are burned today, the carbon is released as  CO2, causing the greenhouse effect. There is only a limited supply of fossil resources: according to the IEA, fossil oil, for example, will last approximately another 40 years, fossil gas about 50 years, uranium about 60 years and coal about 100 to 120 years.

GaAs
Gallium arsenide, a semiconductor material for the production of thin-film solar cells.

Hot Spot
Hot spot of a solar module, which is generated on a module when a single solar cell    in a series connection is shadowed (see shading). Such a cell behaves like an (Ohm) resistance and can heat up to the point of destruction when the current in the other cells of the module flows through it. If this is the case, the solar cell burns out like an overcharged resistor.

Grain boundary
When molten metal solidifies, fields are formed that can be recognised under a microscope and look like grains. The collision of two grains is called ‘grain boundary’. The size of the grains can be controlled through the speed of the cooling process. In multicrystalline silicon as the source material of solar cells, the boundaries between the individual grains represent obstacles in the transport of the charge. Therefore the efficiencies of monocrystalline solar cells are typically higher than those of multicrystalline cells.

I
The symbol for electrical power measured in ampere . The relationship between current, voltage and resistance is stated in Ohm's law:  Current = voltage divided by resistance (I = V/R). In other words, the larger the voltage and the lower the resistance between the anode and the cathode, the higher the current.

Inverter
Photovoltaic modules generate DC. An inverter transforms DC into AC. This enables the electrical energy produced with solar energy to be used by end consumers at 230 volts of alternating voltage or the electricity can be supplied to the public power network. Central inverters are used in large photovoltaic systems, and string inverters in small photovoltaic systems.

Open circuit voltage
Term for terminal voltage, abbreviated Voc. The voltage of a solar cell whose two poles are not connected with each other and between which there is therefore no current. In other words, Voc refers to the voltage which a cell builds up (see also Ohm’ s Law).

ISE
ISE, the Fraunhofer Institute of Solar Energy Systems in Freiburg, is one of the leading institutes of solar technology in Germany. In addition to photovoltaics, ISE deals with solar construction, chemical energy conversion, energy storage and rational energy use. The ISE researcher and institute director, Prof. Joachim Luther, received the German Environment Award in 2005 for his outstanding merits in the field of solar energy. See also www.ise.fhg.de

Kilowatt (kW)
The output of photovoltaic plants – in other words, the electrical current produced by a solar plant – is measured in kilowatts (1 kW = 1,000 W).

Kilowatt peak (kWp)
Unit of measurement for the nominal capacity of a photovoltaic cell or a photovoltaic module. This value is measured under standardised test conditions. While these conditions do not reflect the true conditions in an installed system, they make a direct comparison of the output of different modules possible. Measurements are made at a module temperature of 25 degrees and a radiation of 1,000 W/m².

Kilowatt hour (kWh)
The energy produced by a solar plant is indicated by the amount of electricity in watts produced per hour. The kilowatt hour is the product of constant power generation times the duration of power generation.

Megawatt (MW)
Unit of measurement for power. 1 megawatt = 1,000 kilowatts or 1,000,000 watts.

Megawatt Peak (MWp)
1 megawatt peak equals 1,000 kilowatts peak.

Monocrystal
A monocrystal or single crystal is characterized by a completely regular arrangement of the atoms over the entire block of material.

Monocrystalline silicon
Term for silicon available as single crystals.

Network input
The REA obliges power grid operators to pass the electricity generated by renewable energy sources from their networks on to the consumers and to pay the charges established by the REA (see input charges) for the electricity. This charge is then allocated to all electricity customers nationwide. The input charge varies for the different renewable energy sources such as sun, wind and water, and decreases each year by fixed percentage points.

Network-connected plants
Energy production plants are usually connected to the general power network. The opposite of this are insular plants, which are only used for supplying buildings, for example, that are not connected to the network. These solar plants then store the energy in batteries. The advantage of network operation is that the energy will be consumed even if the producer does not currently need the energy.

Network connection
Connection of a photovoltaic plant to the power network using an inverter; the goal is to supply the electricity produced in the PV plant completely or partially to the grid.

Ohmic Resistance
The electrical resistance of a material (symbol: R). Georg Simon Ohm (1789–1854) was a German physicist who examined the conductivity of metals systematically for the first time.

Ohm’s Law
Ohm’s Law is the basic principle of electrical engineering. It describes the connection between current, voltage and resistance: current = voltage divided by resistance (I = U/R). In other words: the greater the voltage and the lower the resistance between anode and cathode, the higher the current. See also Ohmic resistance   .

Photovoltaics
Photovoltaics deal with the conversion of radiation energy, in particular solar energy, into electrical energy. It has been used for energy supply (initially via satellites) since 1958. The name is made up of the components photo – Greek for ‘light’ – and volta – after Alessandro Volta, the pioneer in electricity.

Photovoltaic cell
Photovoltaic or solar cells convert light energy (usually sunlight) into DC by exploiting the photovoltaic effect. When photons hit semiconductor material, this releases electrons. These generate electric current.

Photovoltaic module
A photovoltaic or solar module consists of several solar cells that are connected with one another. They are placed between two glass or plastic discs to protect them against the weather conditions. Several photovoltaic cells are connected with each other in a solar module in order to achieve more usable voltages. Typically, photovoltaic modules are attached inside a frame on the roof or to a supporting frame.

Photovoltaic system
System (power plant) for the generation of electrical current from solar energy. The DC generated in the photovoltaic modules can be used to operate engines or charge batteries. If it is to be fed into the public electricity network or used for the operation of electrical machines, an inverter is required to convert DC into AC.

pn junction
When a p-doped and an n-doped semiconductor touch, a pn transition is generated. Through diffusion of the majority charge carriers, a space charge zone is generated in the transition zone, resulting in an electrical field. This can separate electron hole pairs.

Poly- or multicrystalline silicon
Poly- or multicrystalline silicon is made up of small, linked (individual) crystals ranging from several millimeters up to several centimeters in size.

Primary energy consumption
Primary energy consumption is the entire energy consumed by a national economy. Especially in electrical power generation, regenerative energies such as solar energy, water and wind power are increasingly replacing the traditional energies of lignite, coal and nuclear power. Today their share is already over 10% in Germany and this is set to increase to one quarter by 2020.

Renewable energy
’Renewable energy’, also called ‘regenerative energy’, is a term for energy derived from resources that are replaced and effectively inexhaustible. Regenerative energy is essentially used as solar energy, biomass, the earth’s heat, water power and wind energy. An important criterion is that renewable energy is provided without generating CO2.

Renewable Energy Act (REA)
Since April 2000, the REA has determined the charges for electricity generated from solar, wind, water, geothermal and biomass energy. The German parliamentary majority, made up of Social Democrats, the Green Party and the Christian Democrats, set out to promote regenerative energies with this act, triggering a boom that has surpassed many expectations. The newest version of the REA is from January 01, 2009.

Low light behavior
Performance of the photovoltaic cell    during low radiation, for example when the sky is cloudy.

Semiconductor
Semiconductors acquire electrical conductivity under the influence of light rays. The possibility of influencing conductivity through doping makes semiconductors important for the production of solar cells. A total of 80 % of all solar cells produced worldwide are manufactured from the semiconductor material silicon, the second most frequent element of the earth’s crust.

Shading
When clouds, trees or buildings cast shadows on a solar plant, this leads to reduced yields and, in rare cases, can cause damage through hot spots.

Silicon
Chemical element capable of bonding with four neighbouring atoms. Silicon is the semiconductor which has been the most important for the semiconductor industry and photovoltaics so far. The raw material silicon oxide (sand) can be processed into monocrystalline, multicrystalline or amorphous silicon. See also monocrystalline silicon, multicrystalline silicon.

Silicon Wafer
Silicon disc for the production of computer chips and photovoltaic cells.

Single crystal
The single crystal, or mono-crystal, is characterised by its completely regular arrangement of atoms over the entire material block. There are no non-uniform fields or grains within the crystal. Single silicon crystals are produced by a number of methods including the Czochralski method, among other methods.

Solar cell
See photovoltaic cell.

STC
Acronym for ‘standard test conditions’. The defined test conditions stipulate that a light source radiates vertically with an intensity of 1,000 W/m², at a temperature of 25°C and with an air mass of 1.5.

String
Several serially connected solar cells of a module.

Temperature coefficient
Indicates how much the efficiency of a solar cell is reduced if the temperature is raised by 1°C. Since crystalline solar cells have a relatively high temperature coefficient, modules consisting of crystalline solar cells should always be sufficiently back-ventilated.

Thin film solar modules
Term for solar modules which are not produced with the aid of silicon-based cell technology: today mainly the materials a-Si:H, CdTe, CIS and GaAs are used for manufacturing these modules. They have the advantage of being able to be produced using less semiconductor material and with simpler technical procedures. The efficiency of thin-layer modules is considerably below that of modules with silicon cells, but their production is considerably cheaper.

U
Symbol for voltage, measured in volts (V). Voltage indicates the difference in the current between two poles. In a water circuit, voltage would correspond to the pressure in the pipe. See also Ohm’ s Law.

Volt
Unit of measurement for electrical voltage, abbreviated ‘V’. Volt was named after the Italian physicist Alessandro Volta. One volt equals the voltage between two points in a conductor where one watt is produced with one ampere   . See also Ohm’ s Law.

Watt
Unit of the output of photovoltaic plants, used to measure its performance.

Watt Peak (Wp)
Measuring unit for the standard performance (power rating, or wattage rating) of a photovoltaic cell or a photovoltaic module under standard test conditions (STC). Module prices are typically indicated in €/Wp. 1,000 watts peak = 1 kilowatt peak.