Photovoltaic terms explained.

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  • The term photovoltaics derives from the Greek word “phos” meaning light and the word “volt” (named by Alessandro Volta). Photovoltaics is a science, which examines light-electricity conversion, respectively, photon energy-electric current conversion. In other words it stands for light-current conversion. Some important photovoltaic terms are explained below. For detailed explanations additional links are also provided.
  • Alternating current (a.c.): Electric current in which the direction of the flow is reversed at frequent intervals, 100 times per second in Europe (50 cycles per second) and 120 times per second in the USA. Opposite of a.c. is direct current (d.c.).
  • Balance of systems (BOS): The parts of the photovoltaic system other than the PV array: switches, controls, meters, power-conditioning equipment, supporting structure for the array and storage components, if any. The cost of land is sometimes included when comparing total system cost with the cost of other energy sources.
  • Battery: For off-grid systems a battery is used to provide energy storage. Nearly all batteries used for PV systems are of the lead-acid type (with a small quantity of antimony to reduce self-discharge). Nickel-cadmium batteries are also suitable and have the advantage that they cannot be overcharged or discharged, but are considerably more expensive. All PV batteries are deep-cycle i.e. designed to be discharged down to 50% or more without damage so that they can supply power over a long period of time (in contrast to a car battery, for example, which is usually only discharged down to 3 to 5 %).
    The lifetime of a battery varies depending on factors such as how it is used, how it is maintained and charged, and temperature, but is typically between 5 and 10 years.
  • Charge controller: Charge controllers are typically used in off-grid photovoltaic power systems. The primary function of a charge controller (or regulator) is to maintain the battery at the highest possible State Of Charge (SOC) and provide the user with the required quantity of electricity, while protecting the battery from deep discharge (by the loads) or extended overcharge (by the PV array). Most charge controllers operate via voltage regulation set points. However, as voltage is not representative of the true SOC, new algorithms are being developed to evaluate the state of the battery, based on Ah or combined VAh monitoring. Additional features such as battery temperature or wire compensation, meters and alarms can enhance the ability of the charge controller to meet the load demand and extend battery lifetime. Other functions such as MPPT, d.c./d.c. conversion, anti-theft protection, load management, pre-payment and data logging can also now be built into the charge controller.
  • Demonstration programme: Project to demonstrate the operation of photovoltaic power systems and their application to potential users/owners.
  • Direct current (d.c.): Electric current in which electrons are flowing in one direction only. Opposite of alternating current (a.c.).
  • Energy payback time: The time required for any energy producing system or device to produce as much useful energy as was consumed in its manufacture and construction. For PV the energy payback times is 2 to 4 years.
  • Final annual yield: Total photovoltaic energy delivered to the load during one year per kilowatt of power installed. Unit: kWh per kW installed.
  • Grid: Network of transmission lines, substations, distribution lines and transformers used by central power systems
  • Grid-connected distributed photovoltaic power system: System installed on consumers’ premises usually on the demand side of the electricity meter. This includes grid-connected domestic photovoltaic power systems and other grid-connected photovoltaic power systems on commercial buildings, motorway sound barriers, etc. These may be used for support of the utility distribution grid.
  • Grid-connected centralized photovoltaic power system: power production system performing the function of a centralized power station (also said centralized photovoltaic power plant).
  • Installed power: Power delivered by a photovoltaic module or a photovoltaic array, under standard test conditions (irradiance of 1 000 W/m², cell junction temperature of 25°C, AM1,5 solar spectrum). Also said STC output power. Unit: W.
  • Inverter: Device that converts direct current (d.c.) into alternating current (a.c.).
  • I-V curve: A graphical presentation of the current versus the voltage from a photovoltaic cell as the load is increased from the short circuit (no load) condition to the open circuit (maximum voltage) condition. The shape of the curve characterizes cell performance.
  • kWh: Symbol of kilowatt-hour, unit of energy (power expressed in kW multiplied by time expressed in hours)
  • Load: The amount of electric power being consumed at any given moment. Also, in an electrical circuit, any device or appliance that is using power. The load for a utility company varies greatly with time of day and to some extent with season.
  • Mounting structure: With the rapid growth of grid-connected distributed systems, a wide range of products have been developed for installing PV modules on buildings. These include mounting structures for PV facades, roof profiles, flat roofs and even ‘PV tiles’ that can be used to replace conventional roof tiles. New products are addressing the need for ease of integration into the building envelope and aesthetic appeal.
  • Off-grid domestic photovoltaic power system: System installed in households and villages that are not connected to the utility grid. Usually, a means to store electricity is used (most commonly lead-acid battery). Also said “stand-alone photovoltaic power system”.
  • Off-grid non-domestic photovoltaic power system: System used for a variety of applications such as water pumping, remote communications, telecommunication relays, safety and protection devices, etc. which are not connected to the utility grid. Usually a means to store electricity is used. Also said “stand-alone photovoltaic power system”.
  • Open-circuit voltage (Voc): The voltage across a photovoltaic cell or module in sunlight when no current is flowing; the maximum possible voltage. Unit: V.
  • Peak power: PV modules are rated by their total power output. The peak power is the amount of power output a PV modules produces at Standard test conditions (STC) of a module operating temperature of 25 degrees Celsius in full sunshine (irradiance) of 1 000 watts per square meter. This is a clear summer day with sun approximately overhead and the cells faced directly towards the sun. Unit: Watt (W), also wirtten Wp by professionals in the field.
  • Performance ratio: Ratio of the final annual (monthly, daily) yield to the reference annual (monthly, daily) yield, where the reference annual (monthly, daily) yield is the theoretically annual (monthly, daily) available energy per kilowatt of installed power.
  • Photovoltaic array: A mechanically integrated assembly of modules and panels together with support structure to form a d.c. power producing unit.
  • Photovoltaic cell: A basic photovoltaic device, which generates electricity when, exposed to a light such as the solar radiation. All photovoltaic cells produce direct current (d.c.).
  • Photovoltaic module: The smallest complete environmentally protected assembly of interconnected photovoltaic cells. For crystalline silicon cells, after testing and sorting to match the current and voltage, the cells are interconnected and encapsulated between a transparent front, usually glass, and a backing material. This ‘module’ is then typically mounted in an aluminium frame. Modules are normally rated between 50 and 200 W, although several manufacturers now offer modules above 200 W.
  • Photovoltaic panel: A group of modules fastened together, pre-assembled and wired, designed to serve as an installable unit in a photovoltaic array.
  • Photovoltaic power system: Set of interconnected elements such as photovoltaic modules, inverters that convert d.c. current of the modules into a.c. current, storage batteries and all installation and control components with a photovoltaic power capacity of 40 W or more. The typology is different according to application: see off-grid and grid-connected photovoltaic power systems.
  • Photovoltaics: The process that produces electricity from sunlight. “Photo” refers to light and “voltaic” to voltage.
  • PV: Abbreviation of photovoltaic (adjective) or photovoltaics (noun).
  • PVPS: Abbreviation of photovoltaic power system(s).
  • Short circuit current (Isc): The current flowing freely from a photovoltaic cell or module through an external circuit that has no load or resistance; the maximum current possible. Unit: Ampere (A).
  • Stand-alone photovoltaic power system: Autonomous system with storage batteries (see off-grid photovoltaic power system).
  • Standard test conditions (STC): The testing conditions to measure photovoltaic cells or modules nominal output power. Irradiance level is 1 000 W/m², with the reference air mass 1,5 solar spectral irradiance distribution and cell or module junction temperature of 25°C.
  • Turnkey price: Price of an installed photovoltaic system excluding VAT/TVA/sales taxes, operation and maintenance costs but including installation costs. For an off-grid photovoltaic system, the prices associated with storage battery maintenance/replacement are excluded. If additional costs are incurred for reasons not directly related to the photovoltaic system, these are excluded. For example if extra costs are incurred fitting PV modules to a factory roof because special precautions are required to avoid disrupting production, these extra costs are not included. Equally the additional transport costs of installing a telecommunication systems in a remote area are not included.
  • watt (W): SI unit of power. Symbol is W. In this website it is understood power under standard test conditions (STC). Also written Wp (peak watt) by PV professionals to mean peak power at STC. Multiples like kW (one thousand watts = 1 000 W) or MW (one million watts = 1 000 000 W) are also used.

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