Glossary: P

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P-N Junction

The semiconductor junction in a photovoltaic cell that shunts electrons into a circuit. Electrons are bumped across this junction by photons (light particles).

Parallel Connection

A wiring configuration used to increase current (amperage). Parallel wiring is positive to positive (+ to +) and negative to negative (- to -). Opposite of a series connection.

Parallel Online UPS

Online UPS technology or installations that provide redundant sources of conditioned backup power so that the critical load is protected even in the event of UPS component failure.

Passive Solar

Any use of the sun's energy in a manner that is found in nature without the use of mechanical aid like pumps or fans. For example, heating a thermal mass (a concrete wall or slab, for instance) during the day with direct sunlight, and using the stored heat in that mass to warm a greenhouse or home at night.

Payback

The period of time it takes for an energy generating device or system to pay for itself in fuel savings.

Peak Load

The maximum load or electrical power draw occurring in a given period of time.

Peak Power Point

Operating point of the IV (current-voltage) curve for a photovoltaic cell or module where the product of the current value times the voltage value is a maximum. Also called the “maximum power point.”

Peak Sun Hours

The equivalent number of hours per day when solar irradiance averages 1,000 watts per meter squared.

Pelton Wheel

A common impulse turbine runner—the wheel that receives the water, changing the pressure and flow of the water to circular motion to drive an alternator, generator, or machine. Pelton wheels (named after inventor Lester Pelton) are made with a series of cups or “buckets” cast onto a hub.

Penstock

The pipe in a hydro system that carries the water from the intake to the turbine.

PF

Power Factor: The ratio of total real power, (W) to the total apparent power in volt-amperes (VA);W/VA.

Phantom Load

A device that consumes energy even when its switch is off, such as the digital clock on a VCR.

Photon

The actual (physical) particle unit of light, as the electron is a particle of electric charge. Solar-electric modules use photons to generate electricity. Photons not captured by the cell are either reflected, pass through the panel, or are converted to heat in the solar array.

Photovoltaic Array

A collection of solar modules connected in series, parallel, or series-parallel combination to provide greater voltage, current, or power than can be furnished by a single solar module. Solar-electric arrays can be designed to furnish any desired voltage, current, or power.

Photovoltaic Cell

A device composed of specially prepared semiconductor material or material combinations exhibiting the ability to convert incident solar energy directly into electrical energy.

Photovoltaic Effect

The phenomenon that occurs when photons, the particles in a beam of light, knock electrons loose from the atoms they strike. When this property of light is combined with the properties of semiconductors, electrons flow in one direction across a junction, setting up a voltage. With the addition of circuitry, electrons will flow and electrical energy will be available.

Photovoltaic Module

A PV module consists of series and/or parallel wired cells typically made from layered silicon crystals that convert light energy to DC electricity. The number of modules in a given system varies depending on the combined load being powered.

Pipe Loss (Frictional Head Loss)

The amount of energy or pressure lost due to friction between a flowing liquid and the inside surface of a pipe.

Pole Mount

A PV mount that is installed on the top or side of a pole usually set in concrete. Can be fixed or seasonally tilted.

Polycrystalline Cell

A wafer of silicon with a multi-grained structure. All grains have the same atomic crystal lattice, however, each grain has a unique orientation in space, producing a unique reflection of light, resulting in a "patchy" mottled appearance. AKA multi crystalline cell.

Post And Beam Construction

A traditional building technique in which post and beam framing units are the basic load-bearing members. Post and beams may be of wood, structural steel, or concrete. In this system, there are fewer framing members, leaving more open space for in-fill. Often used in straw bale construction.

Power

The rate of energy use or generation per unit time, measured in watts. 1 watt = 1 joule per second.

Power Factor

The ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. Power factor describes the offset between voltage and amperage peaks in AC. 1 is called “unity” power factor, and is when voltage and amperage peak at the same time—they are then said to be “in phase.” Power factor is calculated by dividing W by VA.

Power Factor (PF)

The ratio of total real power, (W) to the total apparent power in volt-amperes (VA);W/VA.

Power Management Software

Provides monitoring and shutdown for UPS and connected load.

Power System

A rack module, single rack or several parallel connected racks, providing DC power to a single DC bus.

Preset Voltage

The voltage that a rectifier will default to if communications with the Supervisory Module is lost. Generally, this is set to the float voltage by the Supervisory Module.

Pressure

The “push” behind liquid or gas in a tank, reservoir, or pipe. Water pressure is directly related to “head”—the height of the top of the water over the bottom. Every 2.31 feet of vertical head gives 1 psi (pound per square inch) of water pressure.

Primary Cell

A primary cell is an electrochemical cell (battery) that cannot be recharged. The chemical process within the primary cell is only one way—discharge. When a primary cell is discharged it is discarded. Common flashlight batteries are primary cells; they are disposable batteries that should be avoided.

Propylene Glycol

Glycol: An antifreeze, heat transfer fluid that is circulated through closed loop solar hot water collectors.

PSI

The “push” behind liquid or gas in a tank, reservoir, or pipe. Water pressure is directly related to “head”—the height of the top of the water over the bottom. Every 2.31 feet of vertical head gives 1 psi (pound per square inch) of water pressure.

PSTN

Public Switched Telephone Network

Pulse Width Modulation (PWM)

Varying the amount of DC energy sent to a load or other device by changing the length of time a pulse is left on compared to when it is off. The wider the pulse, the greater the energy transfer.

Purpa

The Public Utility Regulatory Policies Act, passed in 1978. Requires utilities to purchase excess generation from small-scale generators. However, without net metering, this can amount to a fraction of retail prices.

PV

Photovoltaic: A device composed of specially prepared semiconductor material or material combinations exhibiting the ability to convert incident solar energy directly into electrical energy.

PV Array

Two or more photovoltaic modules wired in series or parallel.

PV Array Direct

The use of electricity directly from a photovoltaic array, without batteries or other electrical storage. Many solar water pumps work this way, using a tank to store water.

PWM

Pulse Width Modulation: Varying the amount of DC energy sent to a load or other device by changing the length of time a pulse is left on compared to when it is off. The wider the pulse, the greater the energy transfer.