Renewable Energy Vocabulary: N-P

A-C • D-F • G-I • J-M • N-P • R-S • T-Z

NANOHYDRO
Any hydro plant that produces less than 100 watts.

NATIONAL ELECTRICAL CODE (NEC)
A document that describes the legal standards for residential and commercial wiring practices with safety as the prime objective. Many U.S. jurisdictions base their wiring inspections on the NEC.

NET METERING
State by state legislation that requires utilities to purchase renewably produced electricity at the same price that they sell it, until a building’s monthly or annual energy use is offset.

NOMINAL VOLTAGE
A reference voltage used to describe batteries, modules, or systems (for example, a 12 volt or 24 volt battery, module, or system).

NOSECONE
The pointed piece farthest toward the wind on a wind generator, designed primarily for cosmetic purposes, but also protects the blade attachment points and generator from the weather.

OFF-PEAK
The period of low energy demand, as opposed to maximum or peak demand.

ON-PEAK
Energy supplied during periods of relatively high system demands as specified by the utility.

OHM (Ω)
The unit that quantifies a material’s resistance to electron flow.

OHM’S LAW
Basic formula defining the relationship between voltage, amperage, and resistance. Commonly stated as E = I x R, or Voltage = Amperage x Resistance.

OPEN CIRCUIT
When an electrical circuit is interrupted by breaking the path at one or more points, stopping the electrons from flowing. A light switch opens an electrical circuit when it turns off the light.

OPEN CIRCUIT VOLTAGE (VOC)
The maximum possible voltage across a PV array, module, or cell. The voltage across the terminals of a photovoltaic cell, module, or array with no load applied when the cell is exposed to standard insolation conditions, measured with a voltmeter.

OPEN LOOP SYSTEM
A fresh water or “direct” solar hot water system, generally for use in freeze-free climates.

ORIENTATION
Placement according to the directions N, S, E, or W.

OUTGASSING
The emission of gasses by a material. See also GASSING.

OVERCURRENT
Current that exceeds the rated current of the equipment or the ampacity of a conductor, resulting from overload, short circuit, or ground fault.

OVERCURRENT DEVICE
A safety fuse or breaker designed to open a circuit when an overcurrent occurs.

PARALLEL CONNECTION
An electrical circuit with more than one possible path for electron flow. When wiring PV modules, this wiring configuration increases amperage (current), while voltage remains the same. Parallel wiring is positive to positive (+ to +) and negative to negative (- to -). Opposite of a series connection.

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.

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.

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).

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 multicrystalline 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.

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.

PSI
See PRESSURE.

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
See PHOTOVOLTAIC

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.