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The complete physical assembly of PV modules. In a hierarchical sense, arrays are composed of sub-arrays. Each sub-array is composed of PV modules in close physical proximity and with the same orientation.

Back-up system
A source of electricity that is available when the primary system is not producing sufficient power to meet demand. A battery bank is one example; a fossil fuel generator is another.

A large single crystal of semiconductor, cylindrical in shape, often 6 to 12 inches in diameter and over one foot long. Wafers are sawn from the boule and further process into PV cells.

Bypass diode 
A diode that is placed across a portion (or all) of the cells in a PV module. Since the current of a cell is proportional to its illumination, a shaded cell in a series-connected module or string will "choke" the current through the other cells. To prevent this from happening, a bypass diode is placed across a fraction of cells in a module, say eah 1/3 of the cells in a module. In this way if a portion of the module is shaded, the bypass diode can "bypass" the current around those cells, preventing the "current choking" from happening. Unfortunately, the voltage drops to a fraction of a volt, greatly reducing the power available from the bypassed cells. Nonetheless, the total current through the module is not compromised and the output power of a partially shaded module might drop to 2/3 or 1/3 of its potential output, which is better than something close to zero!

Cell (PV) 
A thin circular or rectangular wafer of semiconductor that is doped with impurity atoms to produce a p-n junction and metalized for electrical contacts. A finished cell, fabricated out of silicon, produces approximately 0.6V and 2.5A (for a 10 cm diameter wafer).

CFL - Compact Fluorescent Lamps
These are small, highly-efficient fluorescent lamps that are designed to be replacements for incandescent lamps. For the same light output (measured in lumens) as an incandescent, a CFL consumes about 25% of the power (measured in watts). Although they are more expensive than incandescent lamps, they pay for themselves in reduced electricity bills in as short as 1-2 years. In addition they last much longer than incandescent lamps, typically 5-7 years.

Controller - PV System
In an off-grid PV system, the charge controller regulates the charging current in the battery bank, reducing the charging current as the battery capacity reaches 100%. Battery voltage is an indication of the percent capacity, so it is this voltage that the controller monitors to maintain a fully charged battery. Since the battery voltage is also a function of temperature, charge controllers should also employ a temperature sensor in contact with the battery system.

Controller - Solar Hot Water System
In a hot water system, the controller measures the temperature difference between the hot water storage tank and the solar collectors. There are two temperature differentials that control the water flow. The first the differential required to initiate circulation, this often is set at 10-20 degrees F. The second is the temperature differential required to cease circulation, typically 2-4 degrees F. These two differentials insure that the water in the collectors is circulated only when it is warmer than the water in the storage tank.

Negatively charged carriers of electricity. A photon of sunlight can create an electron-hole pair via the photoelectric effect.

Gallium Arsenide, a compound semiconductor. Compared to silicon, GaAs is a relatively exotic and expensive material with superior photovoltaic efficiencies. GaAs PV modules are used in space applications and in similar situations where the greatest efficiency is required.

Ground Fault Detection and Interrupt. A requirement for PV systems mounted on roofs, for fire prevention (not safety) reasons. A Ground Fault Protection (GFP) device detects currents in the grounded wire in excess of a certain limit, and interrupts the current by opening the circuit between the PV array and load.

PV systems that are connected to the electrical utility grid, which permits the use of utility electricity when the PV system is producing less electricity than needed, and storage of electricity on the grid when excess electricity is being produced.

positively charged carriers of electricity. A hole can be thought of as the "absence of an electron" and practically exist only in semiconductors.

IEEE 929-2000
A standard that sets the utility interface for an inverter, which in most cases will receive its DC power from a PV array. However, since it is aninterface standard, IEEE 929 applies equally well to wind, fuel cell and batteries. If an inverter meets the requirements of the May 1999 version of UL 1741, it will meet the requirements of IEEE 929-2000.

Infrared, Near
The portion of the optical spectrum defined to be 0.76 microns to 3.0 microns.

The instantaneous energy available from sunlight at the surface of the earth per unit area per unit time, measured in Watts per meter squared.

Irradiation (insolation)
The total energy received from sunlight over a day (sometimes averaged over a year). It is typically measured in kilowatt hours per meter squared.

Losses (Wiring)
All current carrying conductors have a non-zero resistance (unless they happen to be superconducting!). As a result, current in a conductor produces a voltage drop -- along the conductor -- which results in less power being delivered to the load. This voltage drop, called the ohmic or IR drop, is the product of the current and the resistance of the wire carrying the current. NEC and other electricians' guides have tables for the resistance per foot of common copper and aluminum wire gauges. For example, #10 AWG copper wire (specifically 7-strand USE #10 AWG at 75 deg C) has a resistivity of 1.29 ohms per thousand feet.. A 100 foot length of USE 10 carrying 5A from a 24 volt source will suffer a 0.65V drop, yielding 23.35V at the load end of the wire. This represents a 2.7% voltage (and power) reduction at the load. 3% is the maximum voltage drop representing good design practice, with many installers designing to 1% or less.

Maximum Power Point. This refers to functionality of charge controllers and inverters whereby they present a dynamically variable load to a PV module or array that maximizes the power sourced by the PV module or array. The MPP point is near the knee of the I-V characteristic. Since the I-V characteristic changes with the intensity of the sunlight and ambient temperature, MPP tracking insures that the maximum power is available under varying operating conditions.

Module (PV) 
A collection of PV cells, wired together and encapsulated in a glass/plastic sandwich. Modules usually come in an aluminum frame and provide pigtailed or connectorized leads.

MPP current
A measure of PV current production under MPP operation.

MPP power
A measure of PV power production under MPP operation.

MPP voltage
A measure of PV voltage under MPP operation.

The National Electric Code, currently at revision 2002. The code book and a handbook can be ordered through any bookstore.

Net metering
A set of rules that utility companies need to comply with for their customers who want to install their own electricity generation equipment and tie it into the grid.

Noon-equivalent hours
For a fixed PV array, oriented South and tilted approximately at the local latitude, the Sun describes an arc through the sky which results in a varying intensity over the day: less intensity in the early morning and late afternoon and maximum intensity at Noon. If, for a given location, the intensity is integrated over a complete day and the result normalized to the peak Sun intensity, the result is "Noon equivalent hours". For example, in Los Angeles we receive about 5.5 "Noon-equivalent hours". This means that the integrated intensity is the same as if the Sun shone at the "noon intensity" for 5.5 hours.

This refers to a electric generation system (e.g. wind, PV, hydro) that is not tied to the local electric utility. As a result, some sort of back-up system is usually employed, such as batteries or a fossil fuel generator.

Open circuit voltage
This is the voltage that a PV module will exhibit when exposed to sunlight, with zero current being drawn. This number is not sensitive to the light intensity, but is a function of the PV module physical temperature.

The direction that the surface of the PV module is pointing. It is usually expressed in a compass bearing (e.g. South-east) and a co-elevation or degrees from the Zenith (e.g. 30 degrees). The optimum orientation is South and a co-elevation approximately equal to the local latitude.

The amount of time it takes an initial investment to be recouped by revenue or cost savings over time. A PV system can pay for itself in 5-10 years depending on electricity rates and system costs. Solar hot water systems can pay for themselves in as little as 2-3 years.

Having the property of turning light directly into electricity.

Pitch (Roof)
The angle of a roof, measured from 0 degrees being "flat". Most roofs in southern California are less than 30 degrees, and are a good match to our local latitude (33 degrees north). Roof pitches are also designated as a ratio of "rise to run". For example, a 3:12 roof pitch means that the roof rises 3" for every 12" of horizontal distance, and corresponds to 14 degrees.

p-n junction 
A p-n junction is a region in a semiconductor where the doping profile changes rapidly from "p-type" to "n-type". This results in a region void of free carriers and a built-in electric field.

A semiconductor that is composed of many small crystalline regions, randomly oriented throughout the volume of material. PV modules made out of a polycrystalline semiconductor have a lower efficiency but are cheaper to manufacture. Currently, poly-crystalline PV modules are cost-effective where space is not a premium, compared with single crystal PV modules.

Power is the measure of energy change, or energy flow, per unit time. In electrical systems, power is usually measured as the product of the potential difference (measured in volts) times the current flow (measured in amperes) and is expressed in watts or kilowatts (thousands of watts). In thermal systems, and in specific solar hot water systems, power is measured in BTU/hr (British Thermal Units per hour). A BTU is the amount of thermal energy required to raise one pound of water 1 degree Fahrenheit.

Abbreviation for photovoltaic.

Polyvinyl acetate. A highly transparent, water-insoluble resin belonging to the family of organic polymers, prepared by treating its monomer, vinyl acetate, with peroxide catalysts. PVA is used in the lamination process of PV modules.

A cash payment made to the purchaser of solar energy system. Currently in California, PV systems are eligible for rebates that reduce the cost of systems by almost 50%.

The Red Book
Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors. Contains a 30-year average of monthly solar radiation from 1961 to 1990, compiled by the Weather Bureau, Army and Navy, for about 250 cities in the United States.

Return on investment, usually expressed as a percentage. For example, if a PV system cost $10,000 and saved $700 in electricity bills over the next year, the RoI would be 7%.

Service Agreement
This is the contract between you and your utility. It includes a tariff (the cost of energy) and quality of service (voltage and frequency limits). It can also include reliability of service (outage conditions).

Short circuit current
The current supplied by a PV module when its output leads are shorted together (zero voltage).

Single-axis tracker
A PV module mounting system that tracks the sun's path through the sky. The system has a fixed tilt angle (elevation) an tracks only the sun's azimuth.

Solar modules accumulate dust, dirt, leaves, bird dropping and other detritus over time. These materials block some of the sunlight from striking the PV cells (or thermal absorber in the case of a solar thermal system). For roof mounted systems, this blocking of the sun can amount to 5-10% over a years' time, and it is important to wash solar panels about once a year to keep soiling to a minimum.

Solar electric
A system that converts sunlight into electricity.

Solar thermal
A system that converts sunlight into thermal energy - hot water, warm air or warm flooring.

Either of the two moments in the year when the Sun's apparent path is farthest north or south from the Earth's Equator. Winter solstice, which occurs on December 21st or 22nd, represents a point in time with minimum solar energy availed, while summer solstice represents the maximum.

Spectrum (solar)
A measure of the sun's energy over different wavelengths. The sun emits maximum energy at about 0.45 microns, whereas silicon PV cells are most efficient at 1.1 microns.

String (PV) 
Electrical concept. A series-connection of identical PV modules. The current in a PV string is equal though each module, while the overall voltage of a string is the sum of the individual voltages of each module.

Sub-array (PV) 
Mechanical concept. A portion of the total PV array that is physically co-located and oriented in the same direction. Each module in a sub-array should have the same sun intensity and should have the same electrical characteristics. For this reason, all of the modules in a string should also be composed of modules in the same sub-array -- to the greatest extent possible.

Surge Arrestor
A normally high impedance device that it connected in shunt between the hot and grounded sides of an electrical system. Under a transient high voltage event, the device "breaks down", drawing a large amount of current and dissipating the energy. "Break down" is an irreversible process, and once it occurs the surge arrestor must be replaced.

A brand of polyvinyl fluoride, manufactured by the Dupont Company.

Time of Use
A electrical rate structure that prices energy depending on the time of day that it is consumed. Peak rates are typically in the afternoon (say $0.20 per kWh); shoulder rates in late morning and early evening ($0.13 per kWh); lowest rates in the evening and all weekend ($0.08 per kWh). This rate structure encourages customers to shift their electricity usage to the off-peak time periods, when generally the demand for electricity is the greatest.

Tracking systems
These systems track the sun's movement through the sky, maximizing the output of the PV module. There are one-axis and two-axis trackers. One axis trackers are set at a fixed elevation, usually close to the local latitude, and then track the sun's azimuth as it moves through the sky. Two-axis trackers vary both the elevation and the azimuth of the Sun achieving 100% accurate tracking. One axis trackers can increase the annual energy output of a PV system by 33% at mid latitudes, but the added cost offsets somewhat the benefit from increased output. Two axis trackers can further increase annual energy output by another 4-5% or so, but again add to the cost. In general, trackers yield better performance numbers at higher latitudes.

Underwriter's Laboratories testing required by IEEE 929 to insure that charge controllers and inverters used in PV systems are safe, anti-islanding and operate within strict voltage, frequency and harmonic content limits.

The portion of the optical spectrum short ward of the visible, typically 0.4 microns and shorter.

The portion of the optical spectrum defined to be 0.40 microns (violet) to 0.76 microns (red) .

A measure of PV power production. PTC stands for PVUSA Test Conditions, and consists of an ambient temperature of 20 deg C (68 deg F), irradiance of 1,000 W per square meter squared and wind speed of 1.0 m/s. The actual PV module temperature under these conditions is approximately 40 deg C (104 deg F). PTC is a more realistic estimate of PV power production than STC. PTC ratings are about 10% less than STC ratings.

A measure of PV power production. STC stands for Standard Test Conditions, and consists of a cell temperature of 25 deg C, 1,000 W per square meter, 1.5 air masses, and a ASTM standard spectrum. Manufacturers measure the STC power of a PV module using high-intensity flash lamps and a high-speed I-V curve tracer. This procedure, among other things, prevents the temperature of the PV module from deviating from 25 deg C.

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