Heating, Air, & Solar Energy Glossary

| A |

Absorbers

Dark-colored objects that soak up heat in thermal solar power collectors that are used to produce heat energy.

Absorption Coefficient

The factor by which photons are absorbed as they travel a unit distance through a material.

Active Solar Heater

A solar water or space-heating system that moves heated air or water using pumps or fans.

AGM

Absorbed glass mat; a newer type of battery construction that uses saturated absorbent glass mats rather than gelled or liquid electrolyte; somewhat more expensive than flooded (liquid) batteries, but offers very good reliability.

Alternating Current

Electric current in which the direction of flow is reversed at frequent intervals—usually 100 or 200 times per second (50 or 60 cycles per second or 50/60Hz).

AM

Air mass; the ratio of the mass of atmosphere in the actual observer-sun path to the mass that would exist if the observer was at sea level at standard barometric pressure and the sun was directly overhead. AM0 corresponds to the solar spectrum in outer space; the reference spectrum for STC was defined to be AM1.5.

Amorphous Semiconductor

A non-crystalline semiconductor material; easier and cheaper to make than crystalline, but less efficient and more prone to slowly degrade over time; also called thin film.

Amorphous Silicon

A thin-film PV silicon cell having no crystalline structure; manufactured by depositing layers of doped silicon on a substrate.

Ampere (A) or Amp

The unit for the electric current; the flow of electrons. One amp is one coulomb passing in one second; one amp is produced by an electric force of one volt acting across a resistance of one ohm.

Ampere Hour (Ah)

Quantity of electricity or measure of change; a measurement of how many amps of flow can be provided over a one-hour period. Deep cycle batteries used in solar power systems are rated in Ah (amp hours); the term is used to quantify the energy stored in a battery.

Angle of Incidence

Angle between the normal to a surface and the directions of incident radiation; applies to the aperture plane of a solar collector. Most modern solar panels have only minor reductions in power output within plus/minus 15 degrees.

Anode

The positive electrode in an electrochemical cell (battery); the earth ground in a cathodic protections system; the positive terminal of a diode.

Anti-Reflection Coating

A thin coating of a material, which reduces the light reflection and increases light transmission, applied to a photovoltaic cell surface.

Array

Any number of photovoltaic modules connected together to provide a single electrical output. Arrays are often designed to produce significant amounts of electricity.

Array Current

The electrical current produced by a PV array when it is exposed to sunlight.

Autonomous System

A stand-alone PV system that has no backup generating source; may or may not include storage batteries. Most battery systems are designed for a certain minimum “days of autonomy”, which means that the batteries can supply sufficient power without any sunlight to charge the batteries. This varies from 3 to 5 days in the Sun Belt, to 5 to 10 days elsewhere.

AWG

America wire gauge; a standard system for designating the size of electrical wire—the higher the number, the smaller the wire. Most house wiring is #12 or #14. In most other countries, it is specified by the size in millimeters.

Azimuth

Angle between the north direction and the projection of the surface normal into the horizontal plane; measured clockwise from north. As applied to the PV array, 180 degrees azimuth means the array faces due south.

| B |

Base Load Plants

Electricity production facilities used to meet some or all of a given region’s continuous energy demand 24/7, 365 days a year. The cost of energy from such units is usually the lowest available to the grid.

Binary Cycle Geothermal Plant

A type of geothermal power plant that utilizes a closed-loop heat exchange system in which hot geothermal fluid is used to heat a secondary fluid that has a lower boiling point. The secondary fluid is vaporized and used to run a turbine and generate electricity.

Blocking Diode

A diode used to restrict or block reverse current from flowing backward through a module. Alternatively, a diode connected in a series to a PV string protects its modules from a reverse power flow and, thus, against the risk of thermal destruction of solar cells.

Brine

A geothermal fluid with a high saturation of sodium chloride or other salts.

British Thermal Unit (BTU)

The amount of heat energy required to raise the temperature of one pound of water from 60 degrees Fahrenheit to 61 degrees Fahrenheit at one atmosphere pressure.

Building Integrated Photovoltaics (BIPV)

A term for the design and integration of PV into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades—replacing view glass, spandrel glass, or other facade material into semi-transparent skylight systems, roofing systems, shading “eyebrows” over windows, or other building envelope systems.

Bypass Diode

A diode connected across one or more solar cells in a photovoltaic module—such that the diode will conduct if the cell(s) become reverse biased; a diode connected anti-parallel across a part of the solar cells of a PV module. It protects these solar cells from thermal destruction in case of total or partial shading of individual solar cells while other cells are exposed to full light.

| C |

Cadmium Telluride (CdTe)

A polycrystalline thin-film photovoltaic material.

Capacity

The total number of ampere: hours that can be withdrawn from a fully charged battery at a specified discharge rate and temperature.

Cathode

The negative electrode in an electrochemical cell; the negative terminal of a diode.

Cathodic Protection

A method of preventing oxidation (rusting) of exposed metal structures, such as bridges and pipelines, by imposing between the structure and the ground a small electrical voltage that opposes the flow of electrons and that is greater than the voltage present during oxidation.

Cell

The basic unit of a photovoltaic panel or battery.

Cell Barrier

A very thin region of static electric charge along the interface of the positive and negative layers in a photovoltaic cell; the barrier inhibits the movement of electrons from one layer to the other so that higher energy electrons from one side diffuse preferentially through it in one direction, creating a current and thus a voltage across the cell; also called depletion zone, cell junction, or space charge.

Cell Junction

The area of immediate contact between two layers (positive and negative) of a photovoltaic cell. The junction lies at the center of the cell barrier or depletion zone.

Charge Controller

An electronic device which regulates the voltage applied to the battery system from the PV array. It is essential for ensuring that batteries obtain the maximum state of charge and longest life.

Chemical Vapor Deposition

A method of depositing thin semiconductor films. With this method, a substrate is exposed to one or more vaporized compounds, one or more of which contain desirable constituents. A chemical reaction is initiated, at or near the substrate surface, to produce the desired material that will condense on the substrate.

CIS

Copper indium diselenide

Closed-Loop Heat Pump System

A heat pump system that uses a loop of buried plastic pipe as a heat exchanger; loops can be horizontal or vertical.

Combined Collector

A photovoltaic device or module that provides useful heat energy in addition to electricity.

Compressor

The central part of a heat pump system. The compressor increases the pressure and temperature of the refrigerant and simultaneously reduces the volume while causing the refrigerant to move through the system.

Concentrator

A PV module that uses optical elements to increase the amount of sunlight incident on a PV cell. Concentrating arrays must track the sun and use only the direct sunlight because the diffuse portion cannot be focused onto the PV cells. Efficiency is increased, but lifespan is usually decreased due to the high heat.

Concentrator (Module, Array, or Collector)

An arrangement of photovoltaic cells that includes a lens to concentrate sunlight onto small-area cells. Concentrators can increase the power flux of sunlight hundreds of times.

Condenser

A heat exchanger in which hot, pressurized (gaseous) refrigerant is condensed by transferring heat to cooler surrounding air, water, or earth.

Conversion Efficiency (Cell or Module)

The ratio of the electric energy produced by a photovoltaic device (under one sun conditions) to the energy from sunlight incident upon the cell.

Converter

A unit that converts a DC voltage to another DC voltage.

COP (Coefficient of Performance)

The ratio of heating provided by a heat pump (or other refrigeration machine) to the energy consumed by the system under designated operating conditions—the higher the COP, the more efficient the system.

Crystalline Silicon

A type of PV cell made from a single crystal or polycrystalline slice of silicon.

Current

The flow of electric charge in a conductor between two points having a difference in potential (voltage).

Current at Maximum Power (Imp)

The current at which maximum power is available from a module. [UL 1703]

Cut-Off Voltage

The voltage levels (activation) at which the charge controller disconnects the array from the battery or the load from the battery.

Cycle

The discharge and subsequent charge of a battery.

Cycle Life

Number of discharge: charge cycles that a battery can tolerate under specified conditions before it fails to meet specified criteria as to performance (e.g., capacity decreases to 80 percent of the nominal capacity).

Cycling Losses

The actual efficiency of a heating or cooling system is reduced due to startup and shutdown losses. Oversizing a heating or cooling system increases cycling losses.

Czochralski Process

A method of growing large-size, high-quality semiconductor crystal by slowly lifting a seed crystal from a molten bath of the material under careful cooling conditions.

| D |

DC to DC Converter

Electronic circuit to convert DC voltages (e.g., PV module voltage) into other levels (e.g., load voltage); can be part of a maximum power point tracker (MPPT).

Deep Cycle

Type of battery that can be discharged to a large fraction of capacity many times without damaging the battery.

Deep Discharge

Discharging a battery to 20 percent or less of its full charge; diffuse insolation, i.e., sunlight received indirectly as a result of scattering due to clouds, fog, haze, dust, or other obstructions in the atmosphere; opposite of direct insolation.

Depth of Discharge

The percent of the rated battery capacity that has been withdrawn.

Desuperheater

A device for recovering superheat from the compressor discharge gas of a heat pump or central air conditioner for use in heating or preheating water.

Diffuse Radiation

Radiation received from the sun after reflection and scattering by the atmosphere and ground.

Diode

Electronic component that allows current flow in one direction only.

Direct Beam Radiation

Radiation received by direct solar rays; measured by a pyrheliometer with a solar aperture of 5.7° to transcribe the solar disc.

Direct Current (DC)

Electric current in which electrons flow in one direction only; opposite of alternating current.

Direct Insolation

Sunlight falling directly upon a collector; opposite of diffuse insolation.

Discharge Rate

The rate, usually expressed in amperes or time, at which electrical current is taken from the battery.

Distributed Systems

Systems that are installed at or near the location where the electricity is used, as opposed to central systems that supply electricity to grids. A residential photovoltaic system is a distributed system.

DOD

Depth of discharge from 100 percent state of charge (SOC) in a battery or battery system.

Duty Cycle

The ratio of active time to total time; used to describe the operating regime of appliances or loads in PV systems.

DVM

Digital volt meter

| E |

EER (Energy Efficiency Ratio)

The ratio of cooling provided by a heat pump (or other refrigeration machine) to the energy consumed by the system under designated operating conditions—the higher the EER, the more efficient the system.

Efficiency

The ratio of output power (or energy) to input power (or energy); expressed in percent.

EFG

Edge-defined film growth; a method for making sheets of polycrystalline silicon in which molten silicon is drawn upward by capillary action through a mold.

Electric Circuit

Path followed by electrons from a power source (generator or battery) through an external line (including devices that use the electricity) and returning through another line to the source.

Electric Current

A flow of electrons; electricity; amps.

Electrical Grid

An integrated system of electricity distribution, usually covering a large area (as in, “off-the-grid”).

Electrolyte

A liquid conductor of electricity. In batteries, this is usually H2SO4 (sulfuric acid), but it may be any number of things. Seawater is the most common electrolyte in the world—by suspending zinc and a steel sheet in it, you can get a little electricity.

Electron Volt

An energy unit equal to the energy an electron acquires when it passes through a potential difference of one volt; it is equal to 1.602 x 10:19 V.

Energy

The ability to do work. Stored energy becomes working energy when we use it.

Energy Density

The ratio of energy available from a battery to its volume (Wh/1) or mass (Wh/kg); watts to weight ratio.

Energy Payback Time

The time required for any energy producing system or device to produce as much energy as was required in its manufacture. For solar electric panels, this is about 16-20 months.

Equalization Charge

The process of mixing the electrolyte in batteries by periodically overcharging the batteries for a short time.

EVA (Ethylene Vinyl Acetate)

An encapsulate used between the glass cover and the solar cells in PV modules. It is durable, transparent, resistant to corrosion, and flame retardant.

| F |

Fill Factor (FF)

For an I-V curve, the ratio of the maximum power to the product of the open-circuit voltage and the short-circuit current. Fill factor is a measure of the squareness of the I-V curve.

Fixed Tilt Array

A PV array set in at a fixed angle with respect to horizontal.

Flat-Plate PV

Refers to a PV array or module that consists of non-concentrating elements. Flat-plate arrays and modules use direct and diffuse sunlight, but if the array is fixed in position, some portion of the direct sunlight is lost because of oblique sun angles in relation to the array.

Float Charge

Float charge is the voltage required to counteract the self-discharge of the battery at a certain temperature.

Float Life

Number of years that a battery can keep its stated capacity when it is kept at float charge (see float charge).

Fossil Fuel

Any of several types of combustible fuels formed from the decomposition of organic matter. Examples are natural gas, propane, fuel oil, and coal.

Frequency

The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz).

Fuel Cell

A device that converts the energy of a fuel directly to electricity and heat without combustion. Because there is no combustion, fuel cells give off few emissions; because there are no moving parts, fuel cells are quiet.

| G |

Gallium Arsenide (GaAs)

A crystalline, high-efficiency semiconductor/photovoltaic material.

Gassing

Gas by-products, primarily hydrogen, produced when charging a battery; also termed out-gassing.

Gel Type Battery

Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

Geothermal Heat Pump

A heat pump that uses the earth as a heat source and heat sink.

Grid

Term used to describe an electrical utility distribution network.

Grid-Connected (PV System)

A PV system in which the PV array acts like a central generating plant, supplying power to the grid.

Ground Loop

An undesirable feedback condition caused by two or more circuits sharing a common electrical line, usually a grounded conductor.

| H |

Heat Exchanger

A device designed to transfer heat between two physically separated fluids or mediums of different temperatures.

Heat Pump

A mechanical device used for heating and cooling, which operates by moving heat from one location to another. Heat pumps can extract heat from air, water, or the earth. They are classified as either air-source or ground-source (geothermal) units.

Heat Sink

The medium—air, water, or earth—that receives heat rejected from a heat pump.

Heat Source

The medium—air, water, or earth—from which heat is extracted by a heat pump.

Hot Spot

An undesirable phenomenon of PV device operation whereby one or more cells within a PV module or array act as a resistive load, resulting in local overheating or melting of the cell(s).

Hot Water Generator

A device for recovering superheat from the compressor discharge gas of a heat pump or central air conditioner for use in heating or preheating potable water.

Hybrid System

A PV system that includes other sources of electricity generation, such as wind or fossil fuel generators.

| I |

I-U Characteristics

The plot of the current versus voltage characteristics of a photovoltaic cell, module, or array. Three important points on the I-V curve are the open-circuit voltage, short-circuit current, and peak power operating point.

Incident Light

Light that shines onto the face of a solar cell or module.

Insolation

Sunlight, direct or diffused; from incident solar radiation; not to be confused with insulation; equal to about 1,000 watts per square meter at high noon.

Interconnect

A conductor within a module or other means of connection that provides an electrical interconnection between the solar cells. [UL 1703]

Inverters

Devices that convert DC electricity into AC electricity (single or multi-phase), either for standalone systems (not connected to the grid) or for utility interactive systems.

Irradiance

The solar power incident on a surface; usually expressed in kilowatts per square meter. Irradiance multiplied by time equals insolation.

Irradiation

The solar radiation incident on an area over time; equivalent to energy and usually expressed in kilowatt hours per square meter.

I-V Curve

A graphical presentation of the current versus the voltage from a photovoltaic device 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.

I-V Data

The relationship between current and voltage of a photovoltaic device in the power-producing quadrant, as a set of ordered pairs of current and voltage readings in a table, or as a curve plotted in a suitable coordinate system (i.e., Cartesian). [ASTM E 1036]

| J |

Joule (J)

Unit of energy equal to 1/3,600 kilowatt hours.

Junction Box

A PV generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

Junction Diode

A semiconductor device with a junction and a built-in potential that passes current better in one direction than the other. All solar cells are junction diodes.

| K |

Kilowatt (kW)

One thousand watts; a unit of power; 1,000 watts.

Kilowatt Hour (kWh)

One thousand watts acting over a period of one hour; a unit of energy; 1 kWh = 3,600 kJ; power multiplied by time equals energy.

| L |

Light Induced Defects

Defects, such as dangling bonds, induced in an amorphous silicon semiconductor upon initial exposure to light.

Light Trapping

The trapping of light inside a semiconductor material by refracting and reflecting the light at critical angles. Trapped light will travel further in the material, greatly increasing the probability of absorption and hence, of producing charge carriers.

Line Commutated Inverter

An inverter that is tied to a power grid or line. The commutation of power (conversion from DC to AC) is controlled by the power line so that, if there is a failure in the power grid, the PV system cannot feed power into the line.

Load

The amount of electric power used by any electrical unit or appliance at any given time.

Load Current

The current required by the electrical device.

Low Voltage Cutoff

The voltage level at which a controller will disconnect the load from the battery.

| M |

Maintenance Free Battery

A valve-regulated lead-acid (VRLA) battery, also called a sealed lead-acid (SLA) or gel cell battery. This type of battery doesn’t require constant maintenance and is often used in off-grid power systems or large portable electrical devices.

Maximum Power Point (MPP)

The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum.  [UL 1703] For a typical silicon cell panel, this is about 17 volts for a 36 cell configuration.

Maximum Power Point Tracker (MPPT)

A power conditioning unit that automatically operates the PV generator at its MPP under all conditions. An MPPT will typically increase power delivered to the system by 10% to 40%, depending on climate conditions and battery state of charge

Microgroove

A small groove scribed into the surface of a cell, which is filled with metal for contacts.

Module

The smallest replaceable unit in a PV array; an integral, encapsulated unit containing a number of PV cells; a number of PV cells connected together, sealed with an encapsulant, and having a standard size and output power; the smallest building block of the power generating part of a PV array; also called a panel.

Monolithic

Fabricated as a single structure.

MOS-FET

Metal-oxide-silicon field-effect transistor; used as semiconductor power switch in charge regulators, inverters, etc.

MPP

Maximum power point; the point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 V.

MPPT

Maximum power point tracker; the means of a power conditioning unit that automatically operates the PV-generator at its MPP under all conditions.

Multicrystalline

A material composed of variously oriented, small individual crystals (sometimes referred to as polycrystalline or semicrystalline); material that is solidified at such a rate that many small crystals (crystallites) form. The atoms within a single crystallite are symmetrically arranged, whereas crystallites are jumbled together. These numerous grain boundaries reduce the device efficiency.

Multijunction Device

A photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum, to achieve greater overall efficiency.

| N |

NOCT (Nominal Operating Cell Temperature)

The solar cell temperature at a reference environment defined as 800 W/m2 irradiance, 20°C ambient air temperature, and 1 m/s wind speed with the cell or module in an electrically open-circuit state.

N-Type Silicon

Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon.

Nominal Voltage

A reference voltage used to describe batteries, modules, or systems.

| O |

Ohm (Ω)

The unit of electrical resistance in which an electromotive force of one volt maintains a current of one ampere.

One-Axis Tracking

A system capable of rotating around one axis.

Open-Circuit Voltage

The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.

Open-Loop Heat Pump System

A heat pump system that uses groundwater from a well. The water is returned to the environment.

Operating Point

The current and voltage that a module or array produces when connected to a load. The operating point is dependent on the load or the batteries connected to the output terminals of the array.

Overcharge

Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

| P |

Panel

A designation for a number of PV modules assembled in a single mechanical frame.

Parallel Connection

A way of joining two or more electricity-producing devices (i.e., PV cells or modules) by connecting positive leads together and negative leads together; such a configuration increases the current.

Passive Solar Home

A house that uses a room or another part of the building as a solar collector, as opposed to active solar, such as PV.

Payback

A method of calculating how long it will take to recover the difference in costs of two different heating and cooling systems by using the energy and maintenance cost savings from the more efficient system.

Peak Load

The maximum load demand on a system.

Peak Load Peak Demand

The maximum load, or usage, of electrical power occurring in a given period of time, typically a day.

Peak Sun Hours

The equivalent number of hours per day when solar irradiance averages 1,000 W/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 W/m2.

Peak Watt (Wp)

The amount of power a photovoltaic module will produce at standard test conditions (normally 1,000 W/m2 and 25° cell temperature).

Photon

A particle of light that acts as an individual unit of energy. Its energy depends on wavelength.

Photovoltaic (PV)

Pertaining to the direct conversion of light into electricity.

Photovoltaic (PV) Array

An interconnected system of PV modules that function as a single electricity-producing unit. The modules are assembled as a discrete structure, with common support or mounting. In smaller systems, an array can consist of a single module.

Photovoltaic (PV) Cell

The smallest semiconductor element within a PV module to perform the immediate conversion of light into electrical energy (DC voltage and current).

Photovoltaic (PV) Conversion Efficiency

The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device.

Photovoltaic (PV) Efficiency

The ratio of electric power produced by a cell at any instant to the power of the sunlight striking the cell. This is typically about 9% to 14% for commercially available cells.

Photovoltaic (PV) Generator

The total of all PV strings of a PV power supply system, which are electrically interconnected.

Photovoltaic (PV) Module

The smallest environmentally protected, essentially planar, assembly of solar cells and ancillary parts—such as interconnections, terminals, and protective devices like diodes—intended to generate DC power under unconcentrated sunlight. The structural (load carrying) member of a module can either be the top layer (superstrate) or the back layer (substrate). [UL 1703]

Photovoltaic (PV) Panel

Often used interchangeably with PV module (especially in one-module systems), but more accurately used to refer to a physically connected collection of modules (i.e., a laminate string of modules used to achieve a required voltage and current).

Photovoltaic (PV) Peak Watt

Maximum rated output of a cell, module, or system. Typical rating conditions are 0.645 watts per square inch (1,000 watts per square meter) of sunlight, 68 degrees Fahrenheit (20 degrees Celsius) ambient air temperature and 6.2 x 10:3 mi/s (1 m/s) wind speed.

Photovoltaic System

An installation of PV modules and other components designed to produce power from sunlight and meet the power demand for a designated load.

Photovoltaic–Thermal (PV/T) System

A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form; also called a total energy system.

Polycrystalline

See multicrystalline.

Polycrystalline Silicon

A material used to make PV cells, which consist of many crystals as contrasted with single crystal silicon.

Power Conditioning Equipment

Electrical equipment, or power electronics, used to convert power from a photovoltaic array into a form suitable for subsequent use; a collective term for inverter, converter, battery charge regulator, and blocking diode.

Power Factor

The cosine of the phase angle between the voltage and the current waveforms in an AC circuit; used as a designator for inverter performance; a power factor of one indicates current and voltage are in phase and power is equal to the product of volt-amperes (no reactive power).

Primary Battery

A battery whose initial capacity cannot be restored by charging.

Pulse-Width Modulated (PWM) Wave Inverter

The most expensive, but produce a high quality of output signal at minimum current harmonics. The output voltage is very close to sinusoidal.

PV

Abbreviation for photovoltaic(s).

Pulse Width Modulated (PWM)

PWM inverters are the most expensive, but produce a high quality of output signal at minimum current harmonics. The output voltage is very close to sinusoidal

Pyrheliometer

An instrument used for measuring direct beam solar irradiance; uses an aperture of 5.7° to transcribe the solar disc.

Pyronometer

An instrument for measuring total hemispherical solar irradiance on a flat surface, or global irradiance. Thermopile sensors have been generally identified as pyranometers; however, silicon sensors are also referred to as pyranometers.

| Q |

Qualification Test (PV)

A procedure applied to a selected set of PV modules, involving the application of defined electrical, mechanical, or thermal stress in a prescribed manner and amount. Test results are subject to a list of defined requirements.

| R |

Rated Module Current

The current output of a PV module measured at standard test conditions of 1,000 W/m2 and 25°C cell temperature.

Reactive Power

The sine of the phase angle between the current and voltage waveforms in an AC system.

Rectifier

A device that converts AC to DC, such as a battery charger or converter. See inverter.

Remote Site

A site not serviced by an electrical utility grid.

Remote Systems

Systems off the utility grid; resistive voltage drop; the voltage developed across a cell by the current flow through the resistance of the cell.

Resistance

The property of a conductor which opposes the flow of an electric current and results in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.

Reverse Bias

Condition where the current producing capability of a PV cell is significantly less than that of other cells in its series string. This can occur when a cell is shaded, cracked, or otherwise degraded, or when it is electrically poorly matched with other cells in its string.

| S |

Schottky Barrier

A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

Secondary Battery

A battery that can be recharged.

Self Discharge

The rate at which a battery, without a load, will lose its charge.

Self Distance

The rate at which a battery, without a load, will lose its charge

Semiconductor

Any material that has a limited capacity for conducting an electric current; generally falls between a metal and an insulator in conductivity. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

Semicrystalline

See multicrystalline.

Series Connection

A way of joining photovoltaic cells or batteries by connecting positive leads to negative leads; such a configuration increases the voltage.

Series Regulator

Type of battery charge regulator where the charging current is controlled by a switch connected in series with the PV module or array.

Series Regulator

Type of battery charge regulator where the charging current is controlled by a switch, transistor, or FET connected in series with the PV module or array.

Series Resistance

Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

Shelf Life of Batteries

The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

Short-Circuit Current (Isc)

The current flowing freely from a photovoltaic cell through an external circuit that has no load or resistance; the maximum current possible.

Shunt Regulator

Type of a battery charge regulator where the charging current is controlled by a switch or transistor connected in parallel with the PV panel. Overcharging of the battery is prevented by shorting the PV output. Shunt regulators are common in PV systems as they are relatively cheap to build and simple to design. Series regulators usually have better control and charge characteristics.

Silicon (Si)

A chemical element, atomic number 14, semi-metallic in nature, dark gray, an excellent semiconductor material; a common constituent of sand and quartz (as the oxide); crystallizes in face-centered cubic lattice like a diamond; the most common semiconductor material used in making photovoltaic devices.

Sine Wave Inverter

An inverter that produces utility-quality, sine wave power forms.

Single Crystal Material

A material that is composed of a single crystal or a few large crystals.

Single Crystal Silicon

Material with a single crystalline formation. Many PV cells are made from single crystal silicon.

Solar Cell

See “photovoltaic cell”.

Solar Constant

The strength of sunlight; 1,353 watts per square meter in space and about 1,000 watts per square meter at sea level at the equator at solar noon.

Solar Energy

Energy from the sun. The heat that builds up in your car when it is parked in the sun is an example of solar energy.

Solar Grade Silicon

Intermediate-grade silicon used in the manufacture of solar cells; less expensive than electronic-grade silicon.

Solar Noon

That moment of the day that divides the daylight hours for that day exactly in half. To determine solar noon, calculate the length of the day from the time of sunset and sunrise and divide by two. Solar noon may be quite a bit different from “clock” noon.

Solar Spectrum

The total distribution of electromagnetic radiation emanating from the sun.

Solar Thermal Electric

Method of producing electricity from solar energy by using focused sunlight to heat a working fluid, which in turn drives a turbo generator.

Split Spectrum Cell

A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See “multi-junction device”.

Square Wave Inverter

The inverter consists of a DC source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence at a certain frequency. The square wave inverter is the simplest and the least expensive to purchase, but it produces the lowest quality of power.

Stand-Alone System

A photovoltaic system that operates independent of the utility grid; an autonomous or hybrid energy system not connected to a grid; may or may not have storage, but most stand-alone systems require batteries or some other form of storage.

Stand-Off Mounting

Technique for mounting a PV array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

Standard Test Conditions

Conditions under which a module is typically tested in a laboratory: irradiance intensity of 1,000 W/m2, AM1.5 solar reference spectrum, a cell (module) temperature of 25°C, plus or minus 2°C.

State of Charge (SOC)

The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

String

A number of modules or panels interconnected electrically in series to produce the operating voltage required by the load.

Substrate

The physical material upon which a photovoltaic cell is made.

Subsystem

Any one of several components in a PV system (i.e., array, controller, batteries, inverter, load).

Sulfation

A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

Sun Path Diagram

Graphical representation of the sun’s height and azimuth.

Superstrate

The covering on the sun side of a PV module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

Supplemental Heating

A heating system used during extremely cold weather when additional heat is needed to moderate indoor temperatures. May be in the form of electric resistance or fossil fuel.

System Operating Voltage

The array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

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Thermal Electric

Electric energy derived from heat energy, usually by heating a working fluid, which drives a turbo generator. See “solar thermal electric”.

Thermophotovoltaic (TPV) Device

A device that converts secondary thermal radiation, re-emitted by an absorber or heat source, into electricity; the device is designed for maximum efficiency at the wavelength of the secondary radiation.

Thick Crystalline Materials

Semiconductor material, typically measuring from 200 to 400 microns thick, that is cut from ingots or ribbons.

Tilt Angle

The angle of inclination of a solar collector measured from the horizontal.

Thin-Film PV Module

A PV module constructed with sequential layers of thin-film semiconductor materials—such as copper indium diselenide, cadmium telluride, gallium arsenide, or amorphous silicon—a few microns or less in thickness, used to make photovoltaic cells; commonly called amorphous.

Total Internal Reflection

The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must eventually be absorbed by the semiconductor.

Tracking Array

PV array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy. Typically, a single axis tracker will give you 15% to 25% more power per day, and dual axis tracking will add about 5% to that.  This depends somewhat on the altitude and season.

Transformer

Converts the generator’s low-voltage electricity to higher voltage levels for transmission to the load center, such as a city or factory; steps AC voltage up or down, depending on the application.

Trickle Charge

A charge at a low rate, balancing through self-discharge losses, to maintain a cell or battery in a fully charged condition.

Two Axis Tracking

A system capable of rotating independently around two axes (e.g., vertical and horizontal) and following the sun for maximum efficiency of the solar array.

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Uninterruptible Power Supply (UPS)

The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

Utility Interactive Inverter

An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the PV system’s output is fully synchronized with the utility power.

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VAC

Volts AC

VDC

Volts of direct current

VMP

Voltage at maximum power

VOC

Open-circuit voltage

Varistor

A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

Vented Cell

A battery designed with a vent mechanism to expel gases generated during charging.

Volt (V)

A unit of measure of the force, or “push”, given the electrons in an electric circuit. One volt produces one ampere of current when acting through the resistance of one ohm.

Voltage at Maximum Power

The voltage at which maximum power is available from a module.

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Wafer

A thin sheet of semiconductor material made by mechanically sawing it from a single-crystal or multi-crystal ingot or casting.

Watt (W)

The unit of electric power. One ampere of current flowing at a potential of one volt produces one watt of power.

Watt Hour (Wh)

A unit of energy equal to one watt of power connected for one hour.

Waveform

The shape of the curve graphically representing the change in the AC signal voltage and current amplitude, with respect to time.

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Zenith Angle

The angle between directly overhead and the line intersecting the sun; (90°: zenith) is the elevation angle of the sun above the horizon.