Glossary of Alternative Energy Terms
Absorbed Glass Mat (AGM) Battery: A battery with fiberglass mat which
suspends the electrolyte between the lead plates. The mat allows the battery to
be used in many positions and assists the recombination of hydrogen and
oxygen into water after it is released from the electrolyte during charging.
Alternating Current (AC): Electric current from the power utility, a generator or
inverter which changes direction of flow at frequent intervals. In the Western
Hemisphere this is 120 times per second (60 cycles or hz). Most of the rest
of the world operates on 50 cycles.
Amorphous Silicon: A non-crystalline semiconductor material that is used in
"thin-film" photovoltaic modules such as those made by Uni-Solar and some of
the earlier Solarex modules. The doped silicon is sprayed onto a sub-strate
such as glass or stainless steel. This type of module uses less photovoltaic grade
silicon than a single or multi-crystal module and also allows for a flexible panel
with a metal or synthetic backing. They also will produce power in lower light
conditions than conventional modules.
Ampere (A) or Amp: The unit for electric current as in the flow of electrons. It
describes the quantity of electrons passing a point in one second. It will often be
shown on a photovoltaic module label as "I" for intensity.
Ampere-Hour (AH): The quantity of electrical energy equal to the flow of one
ampere of current for one hour. Deep cycle storage battery capacity is often rated
in amp hours.
Array: A number of photovoltaic modules wired together with a single output.
Autonomous System: A photovoltaic system that is stand-alone and has no
backup power source. These usually incorporate a battery bank for power
storage.
Balance of System: The items in a photovoltaic system, with the exception of
the pv modules themselves, that make up a complete system.
Battery: Two of more cells connected together to store electricity.
Battery Capacity: The number of amp-hours that a battery can store.
Battery Cycle Life: The number of charge and dis-charge cycles a battery can
endure before it is no longer useful. This is based on a specific depth of discharge (drawing the battery down to a given amp-hour capacity remaining.)
The more the battery is drawn down before recharging, the lower the number of
charge/dis-charge cycles that will be left.
Blocking Diode: In the "Olden Days" most photovoltaic charge controllers were
either on/off or diversion types. At night power could drain from the batteries back
into the photovoltaic modules. The modules would actually glow (when
viewed with light amplification equipment) like a huge led light. In those instances
a blocking diode (or one-way electrical gate), usually a Schottky diode, was used
to prevent this. Today’s charge controllers either have a built-in diode or
automatically disconnect the modules from the battery bank at night.
By-Pass Diode: PV modules can be connected in a series string (positive to
negative) to increase voltage. If one of the modules is shaded, the output from
the other modules in that series string can feed into that shaded module causing
damage or a fire. When the modules have a by-pass diode in the junction box
that diode will pass the power through without allowing it to go into the cells of the
shaded module. Once the shading is removed, the diode has a higher resistance
than the cells in the module and the module starts producing power again. UL
Listed modules will show a maximum series string voltage which
the by-pass diode in the module can handle.
Cell: The basic unit of a pv module or battery. In a pv module the cell produces
around 0.45 to 0.5 volt, the more cells the higher the voltage. A lead acid battery
cell produces about 2.0 volts (a 12 volt battery will have 6 cells.)
Charge Controller: This is placed between the alternative energy power source
and the battery bank to prevent over-charging.
Charge Rate: The rate in amp hours of charging current supplied to a battery.
Compact Fluorescent Lamp (CFL): Fluorescent lights which have an Edison
(screw-in) base and use either short looped or spiral fluorescent tubes.
Crystalline Silicon: A pv cell made of either a single crystalline or poly
crystalline structure which have a controlled direction of the crystals. An
Amorphous structure has random patterns and directions.
Current: The flow of electricity (electrons) between two points having a
difference in potential (voltage.)
Current at Maximum Power (Imp): The current rating of a pv module when the
maximum power (watts) is being produced.
Days of Autonomy: The number of days a stand-alone system can power loads
without incoming power from pv module input.
DC to DC Converter: A device which takes a direct current voltage and either
steps it up or steps it down.
Deep Cycle Battery: A battery which is designed (usually with thick plates) to
withstand being drawn down to a low state of charge and still being able to be
recharged.
Diode: A one-way electrical gate which allows current to travel in one direction
only.
Direct Current: Electric current which travels in one direction only, such as that
produced by a pv module or storage battery.
Discharge Rate: Usually shown in amp-hours (amps over time) which shows
how much current is being taken from a battery.
Disconnect: Also a safety disconnect. It is a circuit opening device which can be
placed between an alternative energy power source and the charge controller,
between the charge controller and batteries, as well as between the batteries and
the loads.
Efficiency: This is the ratio of power input verses power output. It is usually
shown as a percentage.
Electrical Circuit: The pathway from the power source to the load.
Electrical Grid: The system of power distribution from power generating plants
to the customers. It’s usually operated by a public utility company.
Electrolyte: The liquid in a storage battery. In a lead acid battery it is composed
of water and sulfuric acid.
Energy: The ability to do work - it is usually measured in watts.
Energy Density: The ratio of energy available in a battery (storage device) to the
mass or weight of the battery.
Equalization Charge: Intentionally over-charging a battery for short periods of
time at a higher than normal voltage to help mix up the acid and water in the
electrolyte and equalize, or balance, the cells in a battery.
Float Charge: This is a light maintenance charge to offset the battery losses
caused by self-discharge in storage.
Float Life: This is the number of years a battery will retain its stated amp hour
capacity when kept at a float charge.
Gassing Current: It is portion of the charging current which is consumed by the
electrolytic breakdown of the water into oxygen and hydrogen during charging.
Gel Battery or Gel Cell: A sealed battery where the electrolyte is captured in a
silica gel.
Grid-Tie: An alternative energy system that output alternating current into the
building's wiring. Any power not used will feed into the electrical grid.
Hybrid System: A power generating system that has more than one power
source such as: photovoltaic, wind, hydro or an auxiliary generator.
Insolation: Sunlight, either direct or reflected which is usually measured in watts
per square meter. It is used to determine the available sunshine in a given
location for sizing a photovoltaic system.
Interconnect: Wired connections between solar cells as well as between
photovoltaic modules.
Inverter: A device which converts direct current from a battery (or photovoltaic
modules in a grid-tie system) into 120 or 240 volt a.c.
Junction Box: An electrical enclosure where wiring circuits are connected. On
the back of a pv module this is commonly referred to as the "J" box.
Kilowatt: One thousand watts.
Kilowatt Hour: On thousand watt hours (i.e. 1000 watts either produced or
consumed in an hour.)
Life Cycle Cost: The total system cost from the time of installation to the time it
heads to the bone yard.
Load: Anything that draws power from a circuit (i.e. lights, fan, radio, etc.)
Maximum Power Point Tracker (MPPT): A charge controller that operates a pv
array at its maximum power point voltage/amperage.
Megawatt: One million watts.
Module: A single photovoltaic module (solar panel).
Multi-crystalline: Photovoltaic cells that have semi-random orientation of the
structure. They look like broken sheets of colored glass.
NEC: National Electrical Code. Sets standards for electrical installations - article
690 of the code refers to pv systems.
Nominal Operating Cell Temperature (NOCT): A reference temperature
(usually 25 degrees C) for basing the output voltage and amperage of a pv cell or
module. Some modules show both a NOCT on the sticker for both 25 degrees C
and 45 degrees C. The higher temperature will have lower outputs.
Nominal Voltage: The reference voltage for describing battery, pv modules and
systems (i.e. 12, 24 or 48 volt.) A 12 volt system usually operates from 12 to 14.5
volts.
Ohm: A unit of measurement dealing with the resistance to flow of electrons
through a circuit.
One Axis Tracking: A pv system where the modules track the sun from East to
West and return to East in the morning.
Open Circuit Voltage: The highest possible voltage a pv cell or module will
produce when in full sun and without being connected to a load.
Orientation: Aiming the pv module(s) to a compass heading, usually to true
South in the Northern Hemisphere.
Parallel Connection: Connecting two or more electrical producing devices, such
as pv modules, or storage devices, such as batteries, so the capacity (amperage
or amp/hours) increases but the voltage remains the same. This is
done by connecting positive to positive and negative to negative.
Peak Load / Peak Demand: The most power consumed at a given moment in
the day.
Peak Sun Hours: The number of hours in the day when solar irradiance (or
insolation) averages 1000 watts per square meter - also known as full sun.
Photovoltaic (PV): Referring to the direct conversion of light into electricity.
Photovoltaic Array: Connecting of one or more pv module(s) so they act as one
electricity producing unit.
Photovoltaic Cell: The smallest semi-conductor part in a pv module, the building
block of the module.
Photovoltaic Module: A series of photovoltaic cells mounted in a self-contained
package, also known as
a solar panel.
Photovoltaic System: A complete assembly from the pv module(s) to the power
consuming loads.
Polycrystalline: Photovoltaic cells that have semi-random orientation of the
structure. They look like broken sheets
of colored glass.
Pulse-Width-Modulated Charge Controller: This is placed between the pv
module(s) and the batteries. It operates by allowing different width and different
heights of voltage spikes to flow into the battery from the pv power source.
PV: Photovoltaic.
Remote Site: A location where the only power available is that which is produced
on-site.
Resistance: The property of a conductor or electrical device to oppose the flow
of electrons.
Series Connection: Connecting two or more electrical producing devices, such
as pv modules, or storage devices, such as batteries, so the the voltage
increases but the capacity (amperage or amp/hours) remains the same. This is
done by connecting positive to negative and so on.
Short Circuit Current (Isc): The current (amps) flowing from a pv cell or module
through a circuit that has no load or resistance - the highest possible current.
Shunt Regulator: A photovoltaic charge controller that shorts out the pv module
when the battery is charged.
Silicon: The primary material in most photovoltaic modules. Commonly found in
sand and quartz.
Sine Wave Inverter: A dc to ac inverter which produces alternating current
(usually 220 volt ac. in the Western Hemisphere)
that is of the same wave form as that delivered by the utility company.
It’s also known as a pure or true sine-wave inverter.
Single Crystal Material (PV cell): A pv cell where the crystal orientation is in the
same direction. With current technology these are the most efficient, as well as
the most expensive to manufacture.
Solar Cell: An earlier term for a photovoltaic cell which is the building block of a
pv module.
Solar Constant: The strength of sunlight which is about 1000 watts per square
meter at sea level at the equator.
Solar Energy: Energy from the sun.
Solar Grade Silicon: A grade of silicon which is used to manufacture
photovoltaic cells.
Square Wave Inverter: These are the earliest dc. to ac. inverters. Instead of
producing a curved waveform like a sine wave inverter, these produce a very low
grade and inefficient form which is flatten (square) at the top and bottom.
Staebler-Wronski Effect: The nature of amorphous (thin-film) photovoltaic
devices is that they can lose up to 10% of their output in the first year of being
exposed to light. This is why manufacturers under-rate the output on the pv
module label (it shows the output after the first year) and why the controller and
protection devices in a pv system have to be up-sized to handle the
output of the pv modules for the first year.
Stand-Alone System: A system that relies on power produced by itself with no
incoming power from the utility grid.
Stand-Off Mounting: This is when pv modules are roof mounted a few inches
from the roof surface and tilted at the optimum angle for the site latitude. This
also allows the back of the modules to cool which helps maintain output voltage.
Standard Test Conditions: When pv module output is measured for the labeling
they are tested in a controlled laboratory.
The irradiance (light source) supplies the standard 1000 watts per square meter
and the temperature is 25 degrees C.
State of Charge: The available "useable" capacity remaining in a battery. This is
usually measured in percent. In theory, a 100 amp hour battery that has had 50
amp hours removed from it would have a 50% state of charge.
Sulphation: A lead-acid battery has lead plates and an electrolyte composed of
water and sulfuric acid. As the battery discharges the sulfuric acid migrates into
the lead plates. If it is allowed to remain in the plates sulfate crystals form on
the surface and insulate the lead from the electrolyte. This can permanently
reduce the storage capacity of the battery.
Substrate: This is the top covering of the pv cells in a pv module. It is usually
low-iron tempered glass but can also be a flexible synthetic material.
Surge or Surge Rating: When an electric motor is first started it requires a
greater amount of amperage than the name plate rating. This is because the
motor has to overcome the inertia of the moving parts at rest and get them
moving.
Surge Capacity: This is the ability of an inverter or generator to output a higher
power than the name plate rating for a few seconds to start an electric motor.
Temperature Compensation: This is the ability of a charge controller to adjust
the voltage going into a battery to allow for higher or lower than normal battery
temperature.
Thin-Film: A non-crystalline semiconductor material that is used in "thin-film"
photovoltaic modules such as those made by Uni-Solar and some of the earlier
Solarex modules. The doped silicon is sprayed onto a sub-strate such as glass or
stainless steel. This type of module uses less photovoltaic grade silicon than a
single or multi-crystal module and also allows for a flexible panel with a metal or
synthetic backing. They also will produce power in lower light conditions than
conventional modules.
Tilt-Angle: This is the angle of a pv module in relation to level which will orient it
as close as possible to being perpendicular to the sun for maximum power
production.
Tracking PV Array: A photovoltaic array that follows the path of the sun to
maximize the solar radiation incident to the pv surface. The two most common
orientations are a one axis where the array tracks the sun east to west and a two
axis where the array points directly at the sun at all times. Tracking arrays use
both the direct and diffused sunlight.
The most common drive mechanisms are electric, using pv modules at each end
of the array to keep it centered on the sun and a freon driven which uses the heat
differential between each end of the array.
Transformer: An electromagnetic device which steps up or steps down an
alternating current.
Trickle Charge: A low charge rate which compensates for the self-discharge.
Utility Active Inverter: A d.c. to a.c. inverter which accepts direct current from a
pv array or wind generator and feeds it directly into a buildings' electrical service
entrance (breaker panel) and the utility grid.
VAC: Volts alternating current.
VDC: Volts direct current.
VOC: Volts open circuit - the most a pv module can produce under full sun and
with no load attached to the output.
Volt (V): A unit of electrical force or push. One volt produces one amp of current
when acting against one ohm of resistance.
Voltage at Maximum Power (Vmp): The voltage at which maximum power
(wattage) is available from a photovoltaic module. This is the "sweet spot" where
the curve of amperage and voltage meet to produce the most power.
Watt (W)(James Watt): The unit of electrical power or amount of work done. It is
one ampere of current flowing at a potential of one volt and produces one watt of
power.
Watt - Hour (WH): A quantity of electrical energy when one watt is used for one
hour.
Waveform: The shape of and alternating current curve graphically representing
the change in an AC signal voltage and current amplitude with respect to time.