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| Glossary
of Solar Energy Terms
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Sacrificial
anode: A piece of metal electrically connected to a buried or
submerged structure that is to be protected
from corrosion. The metal of the sacrificial anode is selected to corrode
preferentially to the metal of the protected structure and so reduce its corrosion
rate.
Sealed battery: A battery with a captive electrolyte
and a re-sealing vent cap to which electrolyte cannot
be added. Also called a valve-regulated battery.
Sealed lead-acid battery: A form of lead-acid
battery where the electrolyte is immobilized, either by being contained in an
absorbent fibre separator or gel between the batteries plates.
Secondary battery: A battery that can be
recharged; a rechargeable battery.
Self discharge: Self discharge represents
energy lost to internal chemical reactions within the cell.
Self discharge rate: The rate at which a battery
will lose its charge when at open circuit (with no load connected).
Semiconductor: A material that has an
electrical conductivity in between that of a metal and an insulator.
Transistors and other electronic devices are made from semiconducting materials,
and are often called semiconductors. Typical semiconductors for PV cells include
silicon, gallium arsenide, copper indium diselenide, and cadmium telluride.
Series connected: A method of connection in
which the positive terminal of one device is connected to the negative terminal
of another. The voltages add and the current is limited to the least of any device
in the string.
Series regulator: A type of battery charge
controller or regulator in which the charging current is controlled by a switch, transistor,
or field-effect transistor connected in series with the PV module or array.
Series resistance: Resistance to current flow
within a cell due to factors such as the bulk resistance of the cell materials
and contact resistances.
Shallow-cycle battery: A battery with small
plates that cannot withstand many deep discharges (i.e. to a low state of
charge).
Shelf life:
The time for which a device can be stored and still retain its specified
performance.
Short circuit current (Isc) : The current
generated by an illuminated solar PV cell, module, or array when its output terminals
are shorted; the maximum current possible.
Shunt controller: A controller or regulator that
re-directs,
or shunts, the charging current away from the battery. Generally used for smaller systems.
Silicon (Si): A chemical element with atomic
number 14, a dark gray semi-metal. Occurs in a wide range of silicate minerals
and makes up approximately 28% of the earth's crust (by weight). Silicon
has a face-centered cubic lattice structure like diamond. The most common semiconductor
material used in making PV cells either traditionally in its crystalline form
or more recently as an amorphous thin film.
Sine wave: A waveform that has is defined by
an equation in which one variable is proportional to the sine of the other, as
generated by an oscillator in simple harmonic motion. The sine wave is the most ideal form of electricity for running
more sensitive appliances, such as radios, TVs, computers and the like.
Sine wave inverter: An inverter that produces
grid-quality, sine wave AC electricity.
Single-crystal silicon: Silicon material with a single
crystal structure. A common material for the construction of solar PV cells.
Solar constant:
The power density of solar radiation on a plane perpendicular to the direction
of the sun at the mean earth-sun distance outside the earth's atmosphere; its
value is 1.37 kW per square meter.
Solar energy: Energy from the sun.
Solar noon: The mid-point between sunrise
and sunset; the time when the sun reaches its highest point in its daily arc
across the sky.
Solar power: Electricity generated by conversion
of sunlight, either directly through the use of photovoltaic panels, or indirectly
through solar-thermal processes.
Solar
module: A device used to convert light from the sun directly into DC
electricity by using the photovoltaic effect. Usually made of multiple solar cells
bonded between glass and a backing material. A typical Solar Module would be 100
Watts of power output (but module powers can range from 1 Watt to 300 Watts) and
have dimensions of 2 feet by 4 feet.
Solar resource: The amount of solar insolation
received at a site, normally measured in units of kWh/m2/day which equates to the number
of peak sun hours.
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 turbogenerator.
Solar thermal: A form of power generation
using concentrated sunlight to heat water or other fluid that may then used to
drive a motor or turbine.
Solar-grade silicon: Intermediate-grade silicon
used in the manufacture of solar cells. Less expensive than electronic-grade silicon.
Specific
gravity:
The ratio of the weight of a solution to the weight of an equal volume of water
at a specified temperature; used with reference to the sulfuric acid electrolyte
solution in a lead acid battery as an indicator of battery state of charge.
More recently called relative density.
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.
Square wave: A train of rectangular voltage
pulses that alternate between two fixed values for equal lengths of time.
Square wave inverter: The simplest and the
least expensive type of inverter, but which produces the lowest quality of power.
The inverter uses switches that can carry a large current and withstand a high
voltage that are turned on and off in the correct sequence and at a certain frequency.
Stand-alone (PV system): A solar PV system that
operates without connection to a grid a supply of electricity.
Standard test conditions (STC): Conditions
under which a module is typically tested in a laboratory: (1) Irradiance intensity
of 1000 W/square meter (0.645 watts per square inch), AM1.5 solar reference spectrum,
and (3) a cell (module) temperature of 25 degrees C, plus or minus 2 degrees C
(77 degrees F, plus or minus 3.6 degrees F). [IEC 1215]
Standby current: The current used by the inverter when no load is active,
corresponding to lost power.
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.
State of charge (SOC): The capacity
of a battery at a particular time expressed at a percentage of its rated capacity.
Static head: The height of the
water level above the point of free discharge of the water, normally measured when the
pump is off.
Storage: Storing energy in a battery or
battery stack. In water pumping, storage can be achieved by pumping water to a storage
tank.
Storage density: The capacity of a battery,
in amp-hours compared to its weight. Measured in Watt-hours per kilogram.
Stratification:
Occurs in a liquid electrolyte solution when its concentration varies from top to bottom.
Can be solved by periodic controlled charging at voltages that produce
gassing to mix the electrolyte solution.
String: A number of cells, modules or panels interconnected
electrically in series to produce the required operating voltage.
Substrate: The physical material upon which
a photovoltaic cell is made. Sub-system: Any one of several components in a PV
system (i.e., array, controller, batteries, inverter, load).
Suction head: The height of pump above the
surface of the water source when the pump is located
above the water level.
Sulfation: The formation of lead-sulfate crystals
on the plates of a lead-acid battery; large crystals of lead sulfate grow on the
plate, instead of the usual tiny crystals, making the battery extremely difficult
to recharge. If the crystals get large enough, shorting of the cell may occur.
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.
Surge capacity: The ability of an inverter
or generator to deliver instantaneous high currents when starting motors, for
example.
Surge:
An excessive amount of power drawn by an appliance when it is first switched
on. An unexpected flow of excessive current, usually caused by excessive voltage,
that can damage appliances and other electrical equipment.
Switch: a common device which breaks an electrical
circuit thereby halting the flow electricity through the circuit.
Switch-mode: A
form of converting one form of electricity to another by rapidly switching it
on and off and feeding it through a transformer to effect a voltage change.
System availability: The proportion of time
(usually expressed in hours per year) that a solar PV system will be able to meet fully
the load demand.
System operating voltage: The output
voltage of a solar PV array under load, dependent on the electrical load and
size of the battery stack
connected to the output terminals. |
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| T |
Temperature
compensation: Adjustment via the use of electronic circuitry to change
the charge controller activation points depending on battery temperature. This
is desirable if the battery temperature is expected to vary by more than 5 deg
C from the ambient temperature. The temperature coefficient for lead acid batteries
is typically -3 to -5 millivolts/deg C per cell.
Temperature factors: Are used to decrease
battery capacity at cold temperatures, to decrease PV module voltage
at high temperatures and to increase the resistance of wire at high temperatures.
Thermal electric: Electric energy derived
from heat energy, usually by heating a working fluid, which drives a turbogenerator.
Thermophotovoltaic (TPV) device: A device
in which solar energy is concentrated on to a radiator which reaches a high temperature
and emits the energy in a different part of the spectrum, better matched to the
bandgap of the matched solar cell. This approach should enable high cell
efficiencies to be obtained.
Thick cells : Conventional solar cells in
most types of PV modules, such as crystalline silicon cells, which are typically
from 200-400 micrometers thick. In contrast, thin-film cells are several microns
thick.
Thick-crystalline materials: Semiconductor
material, typically measuring from 200-400 micromterers thick, that is cut from
boules, ingots or ribbons.
Thin film PV module: A solar PV module constructed
with sequential layers of thin film semiconductor materials usually only micrometers
thick. Currently, thin film technologies account for around 12% of all solar modules
sold around the world. This share is expected to increase, since thin film technologies
represent a potential route to lower costs.
Thin film: A layer of semiconductor material,
such as copper indium diselenide, cadmium telluride, gallium arsenide, or amorphous
silicon, a few microns or less in thickness, used to make photovoltaic cells.
Tilt angle: The angle of inclination of a
solar collector or solar module measured from the horizontal.
Total AC load demand: The sum of the AC
loads; its value is important to select the correct Inverter.
Total internal reflection: The trapping of
light within the PV cell by internal reflection of incident light at angles greater
than the critical angles for the interfaces, so that the light cannot escape the
cell and is therefore eventually absorbed by the semiconductor.
Tracker: any device used to direct a PV array
towards the sun.
Tracking array: A PV array that is moved to
follow the path of the sun in order to maintain the maximum incident solar radiation
on its surface. The two most common methods are firstly single-axis tracking in
which the array tracks the sun from east to west, and secondly, two-axis tracking
in which the array points directly at the sun all the time. Two-axis tracking
arrays capture the maximum possible daily energy. Typically, a single axis tracker
will give 15% to 25% more power per day, and dual axis tracking will add a further
5%.
Transformer: A transformer is a device that
changes voltage from one level to another. A device used to transform voltage
levels to facilitate the transfer of power from the generating plant to the customer.
Transistor: A semi-conductor device used to
switch or otherwise control the flow of electricity.
Trickle charge: A small charging current
designed to keep a battery fully charged.
Two-axis tracking: A system capable of rotating
independently about two axes (e.g., vertical and horizontal) and following the
sun for maximum efficiency of the solar array. |
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| U |
Uninterruptible
power supply (UPS): A power supply capable of providing continuous
uninterruptible service; normally containing batteries to provide energy
storage.
Utility-interactive inverter:
An inverter that can operate only when connected to the utility grid supply and
an output voltage frequency fully synchronized with the utility power. |
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| V |
VAC:
Volts alternating current
Varistor: A non-ohmic or voltage-dependent variable resistor.
Normally used as over-voltage limiters to protect sensitive equipment from power spikes or lightning strikes
by shunting the energy to ground.
VDC: Volts direct current
Vented cell: A battery with a vent to expel gases
liberated during charging.
Vmp: The voltage at which a PV device is operating
at maximum power.
Voc: Open-circuit voltage Volt (V): The unit
of electromotive force that will force a current of one ampere through a resistance
of one ohm. Voltage at maximum power
Volt: The unit if electric potential and potential difference. The
amount of work done per unit charge in moving a charge from one place to
another. The potential difference across a resistance of 1 Ohm when a
current of 1 Amp is flowing.
Voltage drop: The voltage lost along a length
of wire or conductor due to the resistance of that conductor. This also applies
to resistors. The voltage drop is calculated by using Ohm's Law.
Voltage protection: A sensing circuit on an
Inverter that will disconnect the unit from the battery if input voltage limits
are exceeded.
Voltage regulator: A device that controls
the operating voltage of a photovoltaic array.
Voltage: Unit of measurement for the
electrical `pressure' of electricity. Measured in Volts (V).
Voltmeter: An electrical or electronic device used to measure voltage. |
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| W |
Wafer:
A thin sheet of crystalline semiconductor material either made by mechanically
sawing it from a single-crystal boule or multicrystalline ingot or block, or made
directly by casting. The wafer is "raw material" for the solar cell.
Watt (W) : The unit of electrical power commonly
used to define the electricity consumption of an appliance. The power developed
when a current of one ampere flows through a potential difference of one volt;
1/746 of a horsepower. 1 Watt = 1 Joule/s.
Watt hour (Wh): A unit of energy equal to
one Watt of power being used for one hour.
Waveform: The shape of a wave or pattern
representing a vibration. The shape characterizing an AC current or voltage output.
Watt peak (User friendly definition): Is
the Direct Current Watts output of a Solar Module as measured under an Industry
standardized Light Test before the Solar Module leaves the Manufacturers facility.
Watt peak: (technical definition): The Watt Power output
of a Solar module is the number of Watts Output when it is illuminated under standard
conditions of 1000 Watts/meter2 intensity, 25°C ambient temperature and a spectrum
that relates to sunlight that has passed through the atmosphere (AM or Air Mass
1.5).
Wet shelf life: The period over which a charged battery, filled
with electrolyte, can remain unused before its performance falls below a specified. |
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| Zenith
angle: The angle between directly overhead and a line through the sun. The elevation angle of the sun above the
horizon is 90° minus the zenith angle. |
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