Users Guide to Off-Grid Energy Solutions

Solar Cells and the Photo-electric Effect Fact Sheet

Solar electricity is the direct conversion of sunlight to electricity. Light striking solar cells is converted into electric energy. This occurs according to a principle called the photo electric effect. Solar electric devices are also called photovoltaic or PV devices.

Solar cell: The basic unit of solar electric production is the solar cell. Light striking solar cells creates a current powered by incoming light energy.

Mono - and polycrystalline type cells do no get used up or damaged while generating electric power. Their life is shortened only by breakage or exposure to the such factors as rain, heat, dust, cold, etc. If a cell is properly protected it should last for more than 20 years.

Several varieties of silicon-type solar cells and solar cell modules are available. These include modules with cells made from monocrystalline , polycrystalline and amorphous silicon. Monocrystalline refers to cells cut from single crystals of silicon. A crystal is the regular geometric state taken up by elements in certain conditions; silicon crystals are specially grown for the computer industry. Polycrystalline refers to cells made from many crystals. Amorphous type cells - made from silicon that is not in crystalline form - are less costly than poly- or monocrystalline cells. When choosing modules, it is important to consider the advantages and disadvantages of the various types.

Solar Modules

Solar cells are connected in series to increase the voltage. Arrangements of many solar cells wired in series, sealed between glass and plastic, and supported inside a metal frame are called solar cell modules. Groups of modules mounted together are called arrays.

All solar cell modules are rated according to their maximum output, or peak power. The peak power, abbreviated Wp, is defined as the amount of power a solar cell module can be expected to deliver at noon on a sunny day when it is facing directly towards the sun at standard temperature conditions (STC). Modules almost always produce less power than their rated peak power in field conditions.

Terms and definitions

hybrid system: a hybrid system refers usually to the combination of two energy generating applications (e.g., diesel generator with PV system) to provide electricity at all times, or in all critical times. Generally, a hybrid system will be designed to ensure reliability (e.g., hospitals, industrial processes, computer operations), where one energy source (e.g., PV from the sun, wind) is intermittent, ie, not always available.

insolation: incident solar radiation. A measure of the solar energy incident on a given area over a specific period of time. Usually expressed in kilowatt-hours per square metre per day or indicated in peak sun hours

inverter: An electronic device that converts low voltage DC to high voltage AC power. In solar-electric systems, an inverter may take the 12, 24, or 48 volts DC and convert it to 115 or 230 volts AC, conventional household power.

irradiance: the solar radiation incident or a surface per unit time. Expressed in watts or kilowatts per square metre

open circuit voltage (Voc): the maximum possible voltage across a solar module or array. Open circuit voltage occurs in sunlight when no current is flowing

peak power (Wp): the amount of power a solar cell module can be expected to deliver at noon on a sunny day (i.e. at Standard Test Conditions) when it is facing directly towards the sun

photovoltaic (PV): The phenomenon of converting light to electric power. Photo = light, Volt = electricity. Abbreviation: PV.

PV Array-Direct: The use of electric power directly from a photovoltaic array, without storage batteries to store or stabilize it. Most solar water pumps work this way, utilizing a tank to store water.

PV Array: A group of PV (photovoltaic) modules (also called panels) arranged to produce the voltage and power desired.

PV Cell: The individual photovoltaic device. The most common PV modules are made with 33 to 36 silicon cells each producing 1/2 volt.

PV Lighting System: a system that includes at leat a PV module, a battery, an inverter and a light. Can include a large number of lights, preferably high efficiency fluourescent, LED, halogen lamps.

PV light suppliers: there are a number of international photovoltaic suppliers who range from specialist suppliers, to renewable energy equipment suppliers, to suppliers of leisure and camping equipment.

PV Module: An assembly of PV cells framed into a weatherproof unit. Commonly called a "PV panel". See PV array.

PV: The common abbreviation for photovoltaic.

silicon: a semi-conductor material commonly used to make photovoltaic cells

solar cell module: groups of encapsulated solar cells framed in glass or plastic units, usually the smallest unit of solar electric equipment available to the consumer

solar cell: a specially-made semiconductor material (i.e. silicon) which converts light energy into electric energy

solar electricity: electricity that is generated by the sun's rays. The most common form of solar electricity in use today is from photovoltaics. However, solar electricity is being produced from solar thermal arrays (typically mirrors focusing the sun's heat on turbines) in several parts of the world.

solar energy: energy that is received from the sun.

solar radiation: see insolation.

solar module: a collection of PV cells wired to generate electricity from the sun, and read to be directly connected to a system, or arranged into an array.

solar tracker: A mounting rack for a PV array that automatically tilts to follow the daily path of the sun through the sky. A "tracking array" will produce more energy through the course of the day, than a "fixed array" (non-tracking) particularly during the long days of summer.

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