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Solar Terminology

(listed alphabetically)

activation voltage(s): The voltage(s) at which the controller will take action to protect the batteries.

adjustable set point: A feature allowing the user to adjust the voltage levels at which the controller will become active.

amorphous silicon: A thin-film PV silicon cell having no crystalline structure.

angle of incidence: The angle that a light ray striking a surface makes with a line perpendicular to the surface.

array: Any number of electrically connected photovoltaic (PV) modules providing a single electrical output. Arrays are designed to produce significant amounts of electricity.

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

array operating voltage: The voltage produced by a PV array when exposed to sunlight and connected to a load.

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

battery: A device that converts the chemical energy contained in its active materials directly into electrical energy by means of an electrochemical oxidation-reduction reaction.

blocking diode: A diode used to prevent undesired current flow. In a PV array the diode is used to prevent current flow toward a failed module or from the battery to the PV array during periods of darkness or low current production.

buydown: Cash rebates on renewable energy electric-generating systems that qualify for programs through the CEC or utilities, designed as an incentive for renewable energy by making it more affordable. Typically rebate funds offers a dollar amount per kilowatt, or a certain percentage off the system purchase price.

bypass diode: A diode connected in parallel with a PV module to provide an alternate current path in case of module shading or failure.

California Energy Commission (CEC): The state agency responsible for energy policy, including promoting energy conservation and efficiency measures, and developing renewable and alternative energy resources.

CEC certified: Standards developed by the CEC for determining whether PV systems and equipment (along with other renewable energy equipment) qualify for CEC buydown programs.

charge controller: A device that controls the charging rate, and/or state of charge applied to the battery system from the PV array. Charge controllers are essential for ensuring that batteries obtain maximum state of charge and longest life.

charge factor: A number representing the time in hours during which a battery can be charged at a constant current without damage to the battery. Usually expressed in relation to the total battery capacity, i.e., C/5 indicates a charge factor of five hours. Related to charge rate.

charge rate: The current used to recharge a battery. Normally expressed as a percentage of total battery capacity. For instance, C/5 indicates a charging current equal to one fifth of the battery's capacity.

collector: A surface or device that absorbs solar heat and transfers it to a fluid. The heated fluid then is used to move the heat energy to where it will be useful, such as in water or space heating equipment.

conversion efficiency: The ratio of the electrical energy produced by a photovoltaic cell to the solar energy impinging on the cell.

converter: Any technology that changes the potential energy in a fuel into a different from of energy such as heat or motion. The term also is used to mean an apparatus that changes the quantity or quality of electrical energy.

crystalline silicon: A type of PV cell made from a single crystal or phlycrystalline slice of silicon.

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

design month: The month having the combination of insolation and load that requires the maximum energy from the array.

duty rating: This rating gives the amount of time the inverter can supply its rated power. Some inverters can operate at their rated power for only a short time without overheating. Exceeding this time may cause hardware failure.

fill factor: For an I-V curve, the ratio of the maximum power to the Vmp Imp product of the open circuit voltage and the short-circuit current. Fill Voc Isc 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 array: A PV array that consists of non-concentrating PV modules. Flat-plate arrays and modules use direct and diffuse sunlight, however, if the array is fixed in position, some direct sunlight is lost because of oblique sun-angles in relation to the array.

frequency: Most loads in the United States require 60 Hz. High-quality equipment requires precise frequency regulation - variations can cause poor performance of clocks and electronic timers.

gen-for-gen: When utilities pay for customer solar generated power at a "generator rate for generation costs," that is, the same wholesale (read: lower) price they pay for their big federally-regulated generators in the 1MW to 100+MW range (considerably larger than the 10kW - 1MW "customer-generators" affected by Net Metering).

grid: Term used to describe an electrical utility distribution network.

grid-connected or grid-tied (PV system): A PV system in which the PV array is wired into buildings or residences that are connected to the utility grid. Energy produced by such PV systems can either be used directly, or if there is an excess, flow out through the utility meter.

high voltage disconnect: The voltage at which the charge controller will disconnect the array from the batteries to prevent overcharging.

high voltage disconnect hysteresis: The voltage difference between the high voltage disconnect setpoint and the voltage at which the full PV array current will be reapplied.

input voltage: This is determined by the total power required by the ac loads and the voltage of any dc loads. Generally, the larger the load, the higher the inverter input voltage. This keeps the current at levels where switches and other components are readily available.

insolation: The solar radiation incident on an area over time. Equivalent to energy and usually expressed in kilowatt-hours per square meter. See also Solar Resource.

inverter: In a PV system, an inverter converts dc power from the PV array/battery to ac power compatible with the utility and ac loads.

I-V curve: 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.

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.

low voltage disconnect: The voltage at which the charge controller will disconnect the array from the batteries to prevent overcharging.

low voltage disconnect hysteresis: The voltage difference between the low voltage disconnect setpoint and the voltage at which the load will be reconnected.

low voltage warning: A warning buzzer or light that indicates the low battery voltage setpoint has been reached.

maximum power point or peak power point: That point on an I-V curve that represents the largest area rectangle that can be drawn under the curve. Operating a PV array at that voltage will produce maximum power.

maximum power tracing or peak power tracking: Operating the array at the peak power point of the array's I-V curve where maximum power is obtained.

module: A number of PV cells connected together and sealed with an encapsulant, having a standard size and output power; the smallest building block of the power generating part of a PV array. Also called panel.

modularity: In some systems it is advantageous to use multiple inverters. These can be connected in parallel to service different loads. Manual load switching is sometimes provided to allow one inverter to meet critical loads in case of failure. This added redundancy increases system reliability.

monocrystalline solar cell: A form of solar cell made from a thin slice of a single large crystal of silicon.

multicrystalline: A solar cell composed of variously oriented, small individual crystals. (Sometimes referred to as polycrystalline or semicrystalline). Material that is solidified at such as rate that many small crystals form. The atoms within a single crystallite are symmetrically arranged with a particular orientation, whereas the crystallites themselves are differently oriented. The multitude of grain boundaries in the material (between the crystallites) reduce the cell efficiency. Multicrystalline is also referred to as polycrystalline.

multi-stage controller: Unit that allows different charging currents as the battery nears full SOC.

National Renewable Energy Laboratory (NREL): A U.S. national Laboratory specializing in developing technologies and procedures for using renewable energy sources.

net metering: An arrangement in which any excess energy produced by grid-tied PV systems flows in reverse back through the utility meter. In some cases, utilities give credit for such excess power, enabling it to be "banked" for later use.

normal operating cell temperature (NOCT): The estimated temperature of a PV module when operating under 800 w/m2 irradiance, 20 C ambient temperature and wind speed of one meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

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.

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.

parallel connection: Term used to describe the interconnecting of PV modules or batteries in which like terminals are connected together. Increases the current at the same voltage.

peak load: The maximum load demand on a system.

peak power current: Amperes produced by a module or array operating at the voltage of the I-V curve that will produce maximum power from the module. See I-V curve.

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

photovoltaic cell: A semiconductor that converts light directly into electricity.

photovoltaic (PV) module: An integrated assembly of interconnected photovoltaic cells designed to deliver a selected level of working voltage and current at its output terminals, packaged for protection against environment degradation, and suited for incorporation in photovoltaic power systems.

photovoltaic (PV) panel: Used interchangeably with PV module (especially in one-module systems), PV panels more accurately refer to a physically connected collection of modules used to achieve a required voltage and current.

polycrystalline: See multicrystalline.

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

power conversion efficiency: This value gives the ratio of output power to input power of the inverter. Efficiency of stand-alone inverters will vary significantly with the load. Values found in manufacturers' specifications are the maximum that can be expected.

power factor: The cosine of the angle between the current and voltage waveforms produced by the inverter is the power factor. For resistive loads, the power factor will be 1.0 but for inductive loads, the most common load in residential systems, the power factor will drop, sometimes as low as 0.5. Power factor is determined by the load, not the inverter.

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.

rated power: Rated power of the inverter. However, some units can not produce rated power continuously. See duty rating. Choose an inverter that will provide at least 125 percent of simultaneous peak load requirements to allow for some growth in load demand.

renewable energy: Energy resources that are naturally replenishing but flow-limited. They are virtually inexhaustible in duration but limited in the amount of energy that is available per unit of time. Renewable energy resources include: biomass, hydro, geothermal, solar, wind, ocean thermal, wave action, and tidal action.

renewable energy credits (RECs): Renewable energy certificates (RECs), also known as green certificates, green tags, or tradable renewable certificates, represent the environmental attributes of the power produced from renewable energy projects and are sold separate from commodity electricity. Customers or businesses can buy green certificates whether or not they have access to green power through their local utility or a competitive electricity marketer. And they can purchase green certificates without having to switch electricity suppliers. Source: US Dept of Energy

reverse current protection: Any method of preventing unwanted current flow from the battery to the PV array (usually at night). See blocking diode.

series connection: Connecting the positive of one module to the negative of the next module. This connection of PV modules or batteries increases the voltage while the current remains the same.

series controller: A controller that interrupts the charging current by open-circuiting the PV array. The control element is in series with the PV array and battery.

short circuit current (Ise): The current produced by an illuminated PV cell, module, or array when its output terminals are shorted.

shunt controller: A controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited PV array. Most shunt controllers are for smaller systems producing 30 amperes or less.

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.

single crystalline: Silicon material with a single crystal structure. A common material for the construction of solar PV cells.

single-stage controller: A unit that redirects all charging current as the battery nears full SOC.

Solar Pathfinder: A device used in PV site assessment for charting the Sun's path through the sky for all months of the year, calibrated by the hours of the day. Also provides other critical, detailed site data.

solar thermal: The process of concentrating sunlight on a relatively small area to create the high temperatures needs to vaporize water or other fluids to drive a turbine for generation of electric power.

stand-alone (PV system): An autonomous or hybrid photovoltaic system not connected to a grid (as opposed to grid connected). May or may not have storage, but most stand-alone systems require batteries or some other form of storage.

standby current: This is the amount of current (power) used by the inverter when no load is active (power loss). This is an important parameter if the inverter will be left on for long periods of time to supply small loads. The inverter efficiency is lowest when load demand is low.

surge capacity: The ability of an inverter or generator to deliver high currents momentarily required when starting motors. Mosst inverters can exceed their rated power for limited periods of time (seconds). Surge requirements of specific loads should be determined or measured. Some transformers and ac motors require Mosting currents several times their operating level for several seconds.

tare loss: Loss caused by the controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.

temperature compensation: A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than +-5 C from ambient temperature. The temperature coefficient for lead acid batteries is typically -3 to -5 millivolts/C per cell.

temperature factors: It is common for three elements in PV system sizing to have distinct temperature corrections: 1) A factor used to decrease battery capacity at cold temperatures; 2) A factor used to decrease PV module voltage at high temperatures; 3) A factor used to decrease the current carrying capability of wire at high temperature.

thin film PV module: A PV module constructed with sequential layers of thin film semiconductor materials. See amorphous silicon .

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

time-of-use meter: An electric meter that measures and records the times during which a customer consumes or generates various amounts of electricity. This type of meter is used for customers who pay time-of-use rates.

time-of-use rates: Electricity prices that vary depending on the time periods in which the energy is consumed. In a time-of-use rate structure, higher prices are charged during utility peak-load times, currently about $.32 - $.38 per kilowatt hour. Off-peak rates are $.08 - $.12 per kilowatt hour. While such rates are intended to provide an incentive for consumers to curb power use during peak times, the higher rates also apply to customers generating power.

total ac load demand: The sum of the ac loads. This value is important when selecting an inverter.

voltage protection: The inverter can be damaged if dc input voltage levels are exceeded. Remember, battery voltage can far exceed nominal if the battery is overcharged. A 12-volt battery may reach 16 volts or more and this could damage some inverters. Many inverters have sensing circuits that will disconnect the unit from the battery if specified voltage limits are exceeded.

voltage regulation: This indicates the variability in the output voltage. Better units will produce a nearly constant root-mean-square (RMS) output voltage for a wide range of loads.

waveform: The shape of a wave or pattern representing a vibration. The shape characterizing an AC current or voltage output.

wind turbine: A device installed on top of a tall tower that collects wind energy and converts it into electricity. Turbine electrical output is converted for utility compatibility and this output is fed into the residence's wiring at the breaker panel.