A

alternating current (AC)

A type of electrical current, the direction of which is reversed at regular intervals or cycles. In the United States, the standard is 120 reversals or 60 cycles per second. Electricity transmission networks use AC because voltage can be controlled with relative ease.

alternative energy sources

Energy sources different from those in widespread use at the moment (which are referred to as conventional). Alternative energy usually includes solar, wind, wave, tidal, hydroelectric and geothermal energy. Although they each have their own drawbacks, none of these energy sources produces significant air pollution, unlike conventional sources.

ampere (amp)

A unit of electrical current or rate of flow of electrons. One volt across one ohm of resistance causes a current flow of one ampere.

ampere-hour (Ah/AH)

A measure of the flow of current (in amperes) over one hour; used to measure battery capacity.

angle of Incidence

The angle that a ray of sun makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90�.

annual solar savings

The annual solar savings of a solar building is the energy savings attributable to a solar feature relative to the energy requirements of a non-solar building.

array

see photovoltaic (PV) array.

array current

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

availability

The quality or condition of a photovoltaic system being available to provide power to a load. Usually measured in hours per year. One minus availability equals downtime.

azimuth angle

The angle between true south and the point on the horizon directly below the sun.

B

battery

Two or more electrochemical cells enclosed in a container and electrically interconnected in an appropriate series/parallel arrangement to provide the required operating voltage and current levels. Under common usage, the term battery also applies to a single cell if it constitutes the entire electrochemical storage system.

battery cycle life

The number of cycles, to a specified depth of discharge, that a cell or battery can undergo before failing to meet its specified capacity or efficiency performance criteria.

BIPV (building-integrated photovoltaics)

A term for the design and integration of photovoltaic (PV) technology 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 semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.

BTU (British Thermal Unit)

The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

C

cell (battery)

A single unit of an electrochemical device capable of producing direct voltage by converting chemical energy into electrical energy. A battery usually consists of several cells electrically connected together to produce higher voltages. (Sometimes the terms cell and battery are used interchangeably.)

charge

The process of adding electrical energy to a battery.

combined collector

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

concentrator

A photovoltaic module, which includes optical components such as lenses (fresnel lens) to direct and concentrate sunlight onto a solar cell of smaller area. Most concentrator arrays must directly face or track the sun. They can increase the power flux of sunlight hundreds of times.

conductor

The material through which electricity is transmitted, such as an electrical wire, or transmission or distribution line.

converter

A unit that converts a direct current (dc) voltage to another dc voltage.

cycle

The discharge and subsequent charge of a battery.

D

direct current (DC)

A type of electricity transmission and distribution by which electricity flows in one direction through the conductor, usually relatively low voltage and high current. To be used for typical 120 volt or 220 volt household appliances, DC must be converted to alternating current, its opposite.

direct insolation

Sunlight falling directly upon a collector. Opposite of diffuse insolation.

discharge

The withdrawal of electrical energy from a battery.

distributed generation

A popular term for localized or on-site power generation.

downtime

Time when the photovoltaic system cannot provide power for the load. Usually expressed in hours per year or that percentage.

drain down system

Potable water is circulated from the storage tank through the collector loop. Freeze protection is provided by solenoid valves opening and dumping the water at a preset low temperature. Collectors and piping must be pitched so that the system can drain down, and must be assembled carefully to withstand 100 psi city water line pressures. Pressure reducing valves are recommended when city water pressure is greater than the working pressure of the system.

drain back system

The solar heat transfer fluid automatically drains into a tank by gravity. Drain back systems are available in one or two tank configurations. A heat exchanger is necessary, because the city inlet pressure would prevent draining. The heat transfer fluid in the collector loop may be distilled or city water if the loop plumbing is copper. If the plumbing is threaded galvanized pipe, inhibitors may be added to prevent corrosion. Most inhibitors are non-potable and require a double wall heat exchanger. The pump used must be sized to overcome static head.

E

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.

electricity

Energy resulting from the flow of charge particles, such as electrons or ions.

electron

An elementary particle of an atom with a negative electrical charge and a mass of 1/1837 of a proton; electrons surround the positively charged nucleus of an atom and determine the chemical properties of an atom. The movement of electrons in an electrical conductor constitutes an electric current.

energy

The capability of doing work; different forms of energy can be converted to other forms, but the total amount of energy remains the same.

energy audit

A survey that shows how much energy used in a home, which helps find ways to use less energy.

F

fixed tilt array

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

fresnel lens

An optical device that focuses light like a magnifying glass; concentric rings are faced at slightly different angles so that light falling on any ring is focused to the same point.

full sun

The amount of power density in sunlight received at the earth's surface at noon on a clear day (about 1,000 watts/square meter).

G

gigawatt (GW)

A unit of power equal to 1 billion watts; 1 million kilowatts, or 1,000 megawatts.

grid-connected system

A solar electric or photovoltaic (PV) system in which the PV array acts like a central generating plant, supplying power to the grid.

H

header

The pipe that runs across the edge of an array of solar collectors, gathering or distributing the heat transfer fluid from, or to the risers in the individual collectors. This assures that equal flow rates and pressure are maintained.

hybrid system

A solar electric or photovoltaic system that includes other sources of electricity generation, such as wind or diesel generators.

I

incident light

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

infrared radiation

Electromagnetic radiation whose wavelengths lie in the range from 0.75 micrometer to 1000 micrometers; invisible long wavelength radiation (heat) capable of producing a thermal or photovoltaic effect, though less effective than visible light.

insolation

The solar power density incident on a surface of stated area and orientation, usually expressed as watts per square meter or BTU per square foot per hour. Sunlight, direct or diffuse; from 'incident solar radiation.' Not to be confused with 'insulation.' Equal to about 1000 watts per square meter at high noon in Dodge City.

inverter

A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid.

J

joule

A metric unit of energy or work; 1 joule per second equals 1 watt or 0.737 foot-pounds; 1 BTU equals 1,055 joules.

junction box

A photovoltaic (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.

K

kilowatt (kW)

A standard unit of electrical power equal to 1000 watts, or to the energy consumption at a rate of 1000 joules per second.

kilowatt-hour (kWh)

1 thousand watts over a period of 1 hour. The kWh is a unit of energy. 1 kWh=3600 kJ.

L

life-cycle cost

The estimated cost of owning and operating a photovoltaic system for the period of its useful life.

load

The demand on an energy-producing system; the energy consumption or requirement of a piece or group of equipment. Usually expressed in terms of amperes or watts in reference to electricity.

M

maintenance-free battery

A sealed battery to which water can�t be added to maintain electrolyte level.

megawatt (MW)

1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

megawatt-hour

1,000 kilowatt-hours or 1 million watt-hours.

N

National Electrical Code (NEC)

Contains guidelines for all types of electrical installations. The 1984 and later editions of the NEC contain Article 690, "Solar Photovoltaic Systems" which should be followed when installing PV systems.

O

ohm

A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere.

one-tank closed-loop system

A conventional DHW tank, usually electrically heated, is converted to a solar DHW storage tank by installing an external heat exchanger coil. The lower electrical element is removed, leaving the uppermost of the usual two elements to provide auxiliary water heating and to achieve good stratification (layering of hotter water over progressively colder water).

open system

Some part of the system is open to the atmosphere, or system contains fresh or changeable water.

orientation

Placement with respect to the cardinal directions, N, S, E, W; azimuth measures orientation from north.

P

photocurrent

An electric current induced by radiant energy.

photon

A particle of light that acts as an individual unit of energy.

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) 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 string of modules used to achieve a required voltage and current).

photovoltaic (PV) system

A complete set of components for converting sunlight into electricity by the photovoltaic process, including the array and balance of system components.

PV

Abbreviation for photovoltaic.

Q

quad

One quadrillion BTU (1,000,000,000,000,000 BTU).

R

rectifier

A device that converts alternating current to direct current.

regulator

Prevents overcharging of batteries by controlling charge cycle; usually adjustable to conform to specific battery needs.

resistance (R)

The property of a conductor, which opposes the flow of an electric current resulting 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.

S

solar constant

The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 Watts per square meter or 492 BTU per square foot.

solar cooling

The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers, heat-pumps, and air conditioners.

solar energy

Energy derived ultimately from the sun. The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours. It can be divided into direct and indirect categories. Most energy sources on Earth are forms of indirect solar energy, although we usually don�t think of them in that way. Coal, oil and natural gas derive from ancient biological material which took its energy from the sun (via plant photosynthesis) millions of years ago. All the energy in wood and foodstuffs also comes from the sun. Movement of the wind (which causes waves at sea), and the evaporation of water to form rainfall which accumulates in rivers and lakes, are also powered by the sun. Therefore, hydroelectric power and wind and wave power are forms of indirect solar energy. Direct solar energy is what we usually mean when we speak of solar power � it is the use of sunlight for heating or generating electricity.

solar noon

The time of the day, at a specific location, when the sun reaches its highest apparent point in the sky; equal to true or due, geographic south.

solar rights

A legal issue concerning the right of access to sunlight.

solar resource

The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

stand-off mounting

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

string

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

sun path diagram

A circular projection of the sky vault, similar to a map, that can be used to determine solar positions and to calculate shading.

system availability

The percentage of time (expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.

T

tilt angle

The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

tracking array

A photovoltaic (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.

transformer

An electromagnetic device that changes the voltage of alternating current electricity.

U

uninterruptible power supply (UPS)

The designation of a power supply providing continuous uninterruptible service. The UPS contains 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 assure that the photovoltaic system's output is fully synchronized with the utility power.

V

volt (V)

A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

voltage

The amount of electromotive force, measured in volts, that exists between two points.

voltage protection

Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

voltage regulation

This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

W

watt

The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

window

A wide-band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

Z

zenith angle

The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).