Technical terms from A to Z

• A
• B
• C
• D
• E
• F
• G
• H
• I
• K
• L
• M
• O
• P
• R
• S
• T
• V
• W

A

Absorber

Receives the solar radiation, blackened or coated with a selective coating and equipped with an integrated piping system. On the surface, the solar radiation is converted into heat and transferred to a heat transfer medium (water/antifreeze mixture).

Absorptivity

Specifies the portion of the solar radiation absorbed by the roof that is converted into heat.

AC

Abbreviation for alternating current

Aligning a PV system

To ensure optimum alignment, the module surfaces should face south and be tilted 30°.

Alternating current (AC)

Electricity whose polarity constantly changes. In the German power supply grid, the alternating current has a frequency of 50 Hz (hertz), i.e. it assumes the positive or negative value of a (ideally) sinusoidal half-wave 50 times in one second. Alternating voltage or current is generated by rotating generators or inverters. 

Amorphous silicon

(Abb. a-Si) Atoms in amorphous material form an irregular structure (amorph: Greek for formless) Due to the high level of absorption capability of the a-Si, a solar cell made of this material requires layer thicknesses of only a few µm (micrometers) = thin-film technology

Anti reflection layer

A transparent layer on the surface of solar cells for reducing reflection losses. This special coating allows more light to penetrate into the cells, since the silicon surface would otherwise reflect back 30% of the sunlight.

Aperture area

Glass surface of a collector through which the solar radiation can enter. The aperture area is the reference unit for the collector efficiency according to DIN 4757 or in EN 12975.

Azimuth angle

The azimuth angle specifies how many degrees the surface of the thermal collector or photovoltaic module deviates from the exact south-facing direction.

B

Bypass diode

If a solar cell is partially or completely shaded, less current flows through it.
The series connection in a module causes the shaded solar cell to heat up and can destroy it. Bypass diodes provide protection of shaded cells. Depending on the number of cells, a solar module has between one and three bypass diodes.

C

Charge controller

Used as a connecting element in stand-alone systems, it is connected between photovoltaic generators and accumulators. It controls and monitors charging. Additional functions include, among other things, protection against the deep discharging of the accumulator.

Collector

An elementary component of a collector system that converts the solar radiation into heat as efficiently as possible. The heat is collected and transported by a heat transfer medium and transferred to a storage tank by a heat exchanger. A flat collector consists of a well-insulated housing that contains an absorber. The housing is covered with a sheet of solar glass.

Collector circuit (solar circuit)

Circuit between the collector and a collector system storage tank that consists of collector, piping, heat exchanger, pump and safety equipment.

Collector circuit yield

The thermal energy that is transferred from the collector circuit (solar circuit) to the solar storage tank. In most cases, a specific collector circuit yield is specified. An annual collector circuit yield of more than 400 kWh/(m2 *a) is a good result for a flat collector system; an amount greater than 500 kWh/( m2 *a) is an excellent result for a large-scale system.

D

DC

Abbreviation for direct current

Diffuse radiation

The solar radiation that falls on the Earth’s surface is divided into direct and diffuse radiation. Diffuse radiation is all the radiation that does not coincide with the geometrical straight-line path from the sun to the observation point, but is rather reflected or scattered, for example, through the atmospheric components.

Direct current (DC)

Flow of electricity without a shift in direction, such as the current generated by batteries or photovoltaic modules.

Direct radiation

Solar radiation that reaches the Earth’s surface on a direct path from the sun. The diffuse radiation is added to the direct solar radiation.

E

Efficiency

The efficiency specifies the relationship of two measurements on a system - output power to input power. Efficiency is a current value and depends on the operating conditions of the system at the observed point of time. The efficiency of a solar cell or module is defined as the relationship between the power irradiated and the emitted electrical power. Since the efficiency is surface-dependent, make sure to observe the data that was used for calculating the surface, e.g. the entire module surface or only the active cell surface within the module.
The efficiency of a collector is defined as the ratio of the heat output of the collector (output) compared to the irradiation level in the collector plane (input). The module or collector efficiency is only one of several factors that influence the efficiency of a solar power system.

Electricity

The strength of the charge quantity flowing through a conductor (e.g. in the form of electrons flowing through a copper cable) is referred to as electrical current. The unit of measure for current is ampere (abb. A).

F

Façade installation

A PV system that is mounted on a building façade or permanently integrated into the façade.

Feed-in tariff

Tariff for feeding electricity produced from renewable energy sources into the general power supply grid.

Flat roof installation

In order to achieve an optimum energy yield, the PV modules on a flat roof must be mounted with a tilt of approximately 30° using a supporting structure.

G

Generator

A generator or solar generator refers to all of the interconnected PV modules as a whole.

Global radiation

Sum of the direct radiation and diffuse radiation that falls on a horizontal surface. The Earth’s atmosphere decreases the irradiance of the extra-terrestrial solar radiation (solar constant) by absorption, reflection and scattering, so that the irradiation level on the Earth’s surface is reduced at our latitudes to 1,000 W/m2 (summer, clear sky, noon). Depending on metrological conditions and astronomical laws (that, among other things, determine the course of the seasons), the solar energy supply fluctuates.
The average annual sum of global radiation on a horizontal receiving surface is, for example, approximately 1,000 kWh/(m2 *a) in the region of Hanover/Germany. That corresponds to the energy content of 100 liters of heating oil or 100 cmb (m3) of gas.

Grid connection

(Grid-parallel operation) Connection of a photovoltaic system to the grid of the electricity supplier by an inverter for completely or partially feeding the photovoltaically generated electricity into the grid. Grid-connected photovoltaic systems do not require energy storage (this function is virtually performed by the public grid).

Ground mounting

Ground mounting refers to a PV system that is installed on an open surface, for example, a field.

H

Heat transfer medium

Fluid in the collector circuit of a solar energy system that transports the heat from the collector to the storage tank. The heat transfer medium is usually a mixture of water and glycol to ensure frost protection.

I

Inverter

Converts the direct current produced by the modules into grid-compliant alternating current. Using a MPP controller, the inverter takes the power from the photovoltaic generator at the maximum power point of the I-U characteristic curve.

K

Kilowatt peak

(Abb. kWp) Measurement for the performance of a PV system (1 kWp = 1,000 Wp).

Kilowatt hour

(Abb. kWh) Unit of the energy (1 kWh = 1,000 Wh)

L

Lightning protection

A PV system generally does not increase the risk of a lightning strike. For safety reasons and to prevent damages, PV systems are, however, constructed according to lightning protection standards.

M

Maximum Power Point

(Abb. MPP) The point of maximum power. The maximum power output can be derived from this operating point on the I-U characteristic curve of a solar cell or module. MMP tracking allows this point to be detected and set during any operating status.

Module

The electrical wiring of several solar cells that are weather-resistant, protected against environmental influences and both electrically insulated and encapsulated. A module forms the basic unit of a photovoltaic system.

Monocrystalline silicon

Name for the silicon that has only one crystal.

Module string

Series connection of several modules

Multicrystalline silicon

Polycrystalline silicon that consists of several crystals.

O

On-roof installation

For on-roof installations, the solar modules are installed parallel to the roof covering using special mounting systems.

Open circuit voltage

(Abb. UO) The electrical voltage of a solar cell or module when both poles are not interconnected, in other words, no current flows between the two poles.

P

Photovoltaics

(Abb. PV) is the technology that uses solar cells to convert solar energy (photons) into electric energy.

Photovoltaic generator

The individual modules are first connected in a series (in a row) to form strings and these strings are then connected parallel to the PV generator to achieve sufficiently high voltages and currents, for example, to feed into the public grid using inverters.

Photovoltaic principle

Describes the formation of an electrical current in a semiconductor when charge carriers are excited (internal photon effect) during the irradiance of light (photons). If the charge carriers are extracted, electrical energy is gained in the form of electrical current.

Polycrystalline silicon

Consists of small-scale connected crystals that range in size from several millimeters to several centimeters. The block casting method is a standard manufacturing process used for polycrystalline silicon.

Primary energy

Energy that can be extracted from a natural source like crude oil, coal, natural gas, hydropower, solar radiation, etc. Primary energy sources can to some extent be used directly by the end user. In most cases, primary energy is, however, first converted into secondary energy.

Primary circuit (collector circuit, solar circuit)

As seen from the heat source (collector), it is the first closed loop. It is the circuit in which the energy is generated.

Power

The energy provided or consumed per unit of time. The measurement unit of power is watt (W) or kilowatt (kW).
1 kW = 1,000 W = 1,000 J/s.

R

Rated output

Maximum possible power output of a solar cell or module. The rated output is defined as the peak output at the maximum power point under standard test conditions (STC) and is indicated in watt peak (abb. Wp).

Reflection losses

Radiation that is reflected by the surface of a collector or module or solar cell and thus can no longer contribute to the generation of heat or electricity.

Roof-integrated installation

In roof-integrated installations, the modules are integrated into the roof covering. To do so, the existing roof covering is removed or in case of new buildings, the planned module surface is cut out.

Roof slope

Roof slope is the angle of a roof with the horizontal surface. Roof slopes between 20° and 50° are suitable to ensure the harmonic integration of a PV system. Corresponding mounting systems can bring the modules into a suitable position, even with an unfavorable roof slope.

S

Secondary energy

Results from the conversion of primary energy, e.g. coke or briquettes from coal and gasoline, diesel fuel or heating oil from crude oil.

Secondary circuit

As seen from the heat source (collector), it is the second closed loop. It is the circuit that transports the energy to the consumer device.

Selective coating

Special coating on the absorber of a collector that absorbs nearly all of the (short-wave) solar radiation and simultaneously reduces the radiation losses that increase with the rising temperature by radiating (emission) the (long-wave) thermal emission.

Shading

The shade on the roof cast by trees, buildings or antennas is the enemy of every PV system, since the solar cells are connected in a series and each solar cell that is in the shade interrupts the smooth energy flow and thus adversely affects the performance of the system.

Short-circuit current

(Abb. ISC) The current provided by a solar cell or module when both clamps are connected without any additional resistance (short-circuit).

Silicon

Chemical element that makes four bonds with neighboring atoms and in doing so, can form hard and brittle crystals with a stabile diamond structure. After oxygen, silicon is the second most common element in the Earth’s crust, however only occurring as silicon dioxide SiO2 (quartz, sand). Silicon is the semiconductor that plays the most important role in the electronic and photovoltaic industry to date. The raw material silicon dioxide can be processed into monocrystalline, polycrystalline or amorphous silicon.

Solar energy

In the narrower sense, the energy that travels from the Sun to the Earth in the form of photons.

Solar thermal energy

Using solar energy for the direct generation of heat. The corresponding energy converter is known as a collector.

Solar yield

Specifies the utilizable solar heat, in other words, the remaining heat that can be used from the storage tank after deducting all thermal losses of the collector system.

Standard test conditions (STC)

Standardized conditions for determining the rated output (measurement of the I-U curve characteristics) of PV modules: irradiation level 1,000 W/m2 with perpendicular light incidence; radiation spectrum corresponds to AM 1.5; cell temperature of 25°C.

String

Name for several solar cells connected in a series within one module.

Stand-alone system

Autonomous photovoltaic system that is used for supplying energy to remote (grid-remote) consumer devices, for example, in the mountains or in rural regions.

T

Temperature coefficient

Specifies how much the open circuit voltage or power output and thus the efficiency of a solar cell or a module decreases for each degree Celsius as the cell temperature increases. Since the above sizes of crystalline solar cells have comparably high negative temperature coefficients, modules, particularly those made of crystalline silicon cells, should be well-ventilated.

Temperature difference controller

Controls the heat transport from the collector to the storage tank by switching the collector pump on and off. The control signal is the temperature difference at the collector outlet and in the lower part of the storage tank.

Thin-layer technology

Name for the manufacturing technology of thin-film solar cells that are deposited directly onto a low-cost substrate (glass, metal foil, plastic foil). The advantages of thin-film technology lie within the material and energy savings during the industrial manufacturing process, the simple doping ability and the ability to produce large-scale solar cells with integrated series interconnection.

Tilt angle

Angle between a tilted receiving surface and the horizontal surface. Depending on the latitude of the installation location, there are different ideal tilt angles for a solar energy system.

Tracking

Using a tracking system, the module surface of a PV system is rotated and follows the position of the sun during the course of the day. The system energy yield can thus be increased by approximately 30%.

V

Volt

(Abb. V) Unit of measure for electrical current.

Voltage

Potential difference between two points, e.g. both poles of a battery. The voltage (U) is the cause of the electrical current (I); both measurements are linked together by the resistance (R) of a conductor according to the ohmic law (U = R * I). The unit of measure of the electrical current is volt (abb. V).

W

Wafer

Name for a thin slice of semiconductor material (e.g. silicon). Wafers are used as the basic material for manufacturing computer chips and crystalline solar cells. The crystalline slices are general sawed from semiconductor blocks and are between 0.2 and 0.3 millimeters thick.

Watt peak

(Abb. Wp) Measurement for the performance (rated output) of solar cells and modules. For the purpose of comparison, module prices are usually listed in Euro/Wp.

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