Glossary
O-P
Last Updated Monday, July 24, 2000
compiled by Nick Sundt


O, P

O

Occupancy Sensor. An optical, ultrasonic, or infrared sensor that turns room lights on when they detect a person's presence and off after the space is vacated. (Source: US Department of Energy, 1999a).

Occupied Space. The space within a building or structure that is normally occupied by people, and that may be conditioned (heated, cooled and/or ventilated). (Source: US Department of Energy, 1999a).

Ocean Energy Systems. Energy conversion technologies that harness the energy in tides, waves, and thermal gradients in the oceans. (Source: US Department of Energy, 1999a).

ocean mixing. Processes that involve rates of advection, upwelling, downwelling, and eddy diffusion and that determine how rapidly excess atmospheric carbon dioxide can be taken up by the oceans. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Off-Peak. The period of low energy demand, as opposed to maximum, or peak, demand. (Source: US Department of Energy, 1999a).

Ohms. A measure of the electrical resistance of a material equal to the resistance of a circuit in which the potential difference of 1 volt produces a current of 1 ampere. (Source: US Department of Energy, 1999a).

Oil (fuel). A product of crude oil that is used for space heating, diesel engines, and electrical generation. (Source: US Department of Energy, 1999a).

One-Axis Tracking. A system capable of rotating about one axis. (Source: US Department of Energy, 1999a).

One Sun. The maximum value of natural solar insolation. (Source: US Department of Energy, 1999a).

On-Peak Energy. Energy supplied during periods of relatively high system demands as specified by the supplier. (Source: US Department of Energy, 1999a).

On-Site Generation. Generation of energy at the location where all or most of it will be used. (Source: US Department of Energy, 1999a).

opacity. The degree of obscuration of light; for example, a glass window has almost 0% opacity, whereas a concrete wall has 100% opacity. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Open Access. The ability to send or wheel electric power to a customer over a transmission and distribution system that is not owned by the generator (seller) of the power. (Source: US Department of Energy, 1999a).

Open-Circuit Voltage. The maximum possible voltage across a photovoltaic cell; the voltage across the cell in sunlight when no current is flowing. (Source: US Department of Energy, 1999a).

Open-Loop System. A heating system, such as a solar water heater or geothermal heat pump, in which the working fluid is heated and used directly; in an open-loop solar system, the domestic water is circulated in the collector loop. (Source: US Department of Energy, 1999a).

Operating Cycle. The processes that a work input/output system undergoes and in which the initial and final states are identical. (Source: US Department of Energy, 1999a).

Operation and Maintenance (O&M) Cost. Operating expenses are associated with operating a facility (i.e., supervising and engineering expenses). Maintenance expenses are that portion of expenses consisting of labor, materials, and other direct and indirect expenses incurred for preserving the operating efficiency or physical condition of utility plants that are used for power production, transmission, and distribution of energy. (Source: US Energy Information Administration, 1997a.)

optical thickness (optical depth). In calculating the transfer of radiant energy, the mass of an absorbing or emitting material lying in a vertical column of unit cross-sectional area and extending between two specified levels. Also, the degree to which a cloud prevents light from passing through it; the optical thickness then depends on the physical constitution (crystals, drops, and/or droplets), the form, the concentration, and the vertical extent of the cloud. (Source: Carbon Dioxide Information Analysis Center, 1990a).

organic compound. Molecule that contains atoms of the element carbon, usu-ally combined with itself and with atoms of one or more other element such as hydrogen, oxygen, nitrogen, sulfur, phosphorus, chlorine, or fluorine. See inorganic compound.(Source: US Environmental Protection Agency, 1999a)

organic fertilizer. Organic material such as manure or compost, applied to cropland as a source of plant nutrients.(Source: US Environmental Protection Agency, 1999a)

Orientation. The alignment of a building along a given axis to face a specific geographical direction. The alignment of a solar collector, in number of degrees east or west of true south. (Source: US Department of Energy, 1999a).

Outage. A discontinuance of electric power supply. (Source: US Department of Energy, 1999a).

outer continental shelf (OCS). The submerged lands extending from the out limit of the historic territorial sea (typically three miles) to some undefined outer limit, usually a depth of 600 feet. In the United States, this is the portion of the shelf under federal jurisdiction. See CONTINENTAL SHELF. (Source: California Energy Commission, 1999a.)

Outgassing. The process by which materials expel or release gasses. (Source: US Department of Energy, 1999a).

Outside Air. Air that is taken from the outdoors. (Source: US Department of Energy, 1999a).

Outside Coil. The heat-transfer (exchanger) component of a heat pump, located outdoors, from which heat is collected in the heating mode, or expelled in the cooling mode. (Source: US Department of Energy, 1999a).

Overhang. A building element that shades windows, walls, and doors from direct solar radiation and protects these elements from precipitation. (Source: US Department of Energy, 1999a).

Overload. To exceed the design capacity of a device. (Source: US Department of Energy, 1999a).

Ovonic. A device that converts heat or sunlight directly to electricity, invented by Standford Ovshinsky, that has a unique glass composition that changes from an electrically non-conducting state to a semiconducting state. (Source: US Department of Energy, 1999a).

oxides of nitrogen. See NOx. (Source: California Energy Commission, 1999a.)

oxidize. To chemically transform a substance by combining it with oxygen.(Source: US Environmental Protection Agency, 1999a)

oxygen cycle. Cyclic movement of oxygen in different chemical forms from the environment, to organisms, and then back to the environment. (Source: US Environmental Protection Agency, 1999a)

Oxygenates. Gasoline fuel additives such as ethanol, ETBE, or MTBE that add extra oxygen to gasoline to reduce carbon monoxide pollution produced by vehicles. (Source: US Department of Energy, 1999a).

ozone. A molecule made up of three atoms of oxygen. In the stratosphere, it occurs naturally and it provides a protective layer shielding the Earth from ultraviolet radiation and subsequent harmful health effects on humans and the environment. In the troposphere, it is a chemical oxidant and major component of photochemical smog. (Source: Carbon Dioxide Information Analysis Center, 1990a).

ozone depleting substance (ODS). a compound that contributes to stratospheric ozone depletion. ODS include CFCs, HCFCs, halons, methyl bromide, carbon tetrachloride, and methyl chloroform. ODS are generally very stable in the troposphere and only degrade under intense ultraviolet light in the stratosphere. When they break down, they release chlorine or bromine atoms, which then deplete ozone. A detailed list of class I and class II substances with their ODPs, GWPs, and CAS numbers are available. (Source: US Environmental Protection Agency, 1999b).

Ozone Depletion. Chemical destruction of the stratospheric ozone layer beyond natural reactions. Stratospheric ozone is constantly being created and destroyed through natural cycles. Various ozone-depleting substances (ODS), however, accelerate the destruction processes, resulting in lower than normal ozone levels. The science page offers much more detail on the science of ozone depletion. (Source: US Environmental Protection Agency, 1999b).

Ozone Depletion Potential (ODP). a number that refers to the amount of ozone depletion caused by a substance. The ODP is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC-11. Thus, the ODP of CFC-11 is defined to be 1.0. Other CFCs and HCFCs have ODPs that range from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetrachloride has an ODP of 1.2, and methyl chloroform's ODP is 0.11. HFCs have zero ODP because they do not contain chlorine. A table of all ozone-depleting substances shows their ODPs, GWPs, and CAS numbers. (Source: US Environmental Protection Agency, 1999b).

ozone layer. the region of the stratosphere containing the bulk of atmospheric ozone. The ozone layer lies approximately 15-40 kilometers (10-25 miles) above the Earth's surface, in the stratosphere. Depletion of this layer by ODS will lead to higher UVB levels, which in turn will cause increased skin cancers and cataracts and potential damage to some marine organisms, plants, and plastics. The science page offers much more detail on the science of ozone depletion. (Source: US Environmental Protection Agency, 1999b).

ozone precursors. Chemical compounds, such as carbon monoxide, methane, non-methane hydrocarbons, and nitrogen oxides, which in the presence of solar radiation react with other chemical compounds to form ozone, mainly in the troposphere. See troposphere(Source: US Environmental Protection Agency, 1999a)


P

Pane (Window). The area of glass that fits in the window frame. (Source: US Department of Energy, 1999a).

Panel (Solar). A term generally applied to individual solar collectors, and typically to solar photovoltaic collectors or modules. (Source: US Department of Energy, 1999a).

Panel Radiator. A mainly flat surface for transmitting radiant energy. (Source: US Department of Energy, 1999a).

Parabolic Aluminized Reflector Lamp. A type of lamp having a lens of heavy durable glass that focuses the light. They have longer lifetimes with less lumen depreciation than standard incandescent lamps. (Source: US Department of Energy, 1999a).

Parabolic Dish. A solar energy conversion device that has a bowl shaped dish covered with a highly reflective surface that tracks the sun and concentrates sunlight on a fixed absorber, thereby achieving high temperatures, for process heating or to operate a heat (Stirling) engine to produce power or electricity. (Source: US Department of Energy, 1999a).

Parabolic Trough. A solar energy conversion device that uses a trough covered with a highly reflective surface to focus sunlight onto a linear absorber containing a working fluid that can be used for medium temperature space or process heat or to operate a steam turbine for power or electricity generation. (Source: US Department of Energy, 1999a).

Parallel. A configuration of an electrical circuit in which the voltage is the same across the terminals. The positive reference direction for each resistor current is down through the resistor with the same voltage across each resistor. (Source: US Department of Energy, 1999a).

Particulates. The fine liquid or solid particles contained in combustion gases. The quantity and size of particulates emitted by cars, power and industrial plants, wood stoves, etc are regulated by the U.S. Environmental Protection Agency. (Source: US Department of Energy, 1999a).

parts per billion (ppb). Number of parts of a chemical found in one billion parts of a particular gas, liquid, or solid mixture. See concentration. (Source: US Environmental Protection Agency, 1999a)

parts per million (ppm). Number of parts of a chemical found in one million parts of a particular gas, liquid, or solid. See concentration. (Source: US Environmental Protection Agency, 1999a)

Parallel Connection. A way of joining photovoltaic cells or modules by connecting positive leads together and negative leads together; such a configuration increases the current, but not the voltage. (Source: US Department of Energy, 1999a).

particulate matter. Very small pieces of solid or liquid matter, such as particles of soot, dust, aerosols, fumes, or mists. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Passive/Natural Cooling. To allow or augment the natural movement of cooler air from exterior, shaded areas of a building through or around a building. (Source: US Department of Energy, 1999a).

Passive Solar (Building) Design. A building design that uses structural elements of a building to heat and cool a building, without the use of mechanical equipment, which requires careful consideration of the local climate and solar energy resource, building orientation, and landscape features, to name a few. The principal elements include proper building orientation, proper window sizing and placement and design of window overhangs to reduce summer heat gain and ensure winter heat gain, and proper sizing of thermal energy storage mass (for example a Trombe wall or masonry tiles). The heat is distributed primarily by natural convection and radiation, though fans can also be used to circulate room air or ensure proper ventilation. (Source: US Department of Energy, 1999a).

Passive Solar Heater. A solar water or space-heating system in which solar energy is collected, and/or moved by natural convection without using pumps or fans. Passive systems are typically integral collector/storage (ICS; or batch collectors) or thermosyphon systems. The major advantage of these systems is that they do not use controls, pumps, sensors, or other mechanical parts, so little or no maintenance is required over the lifetime of the system. (Source: US Department of Energy, 1999a).

Passive Solar Home. A house built using passive solar design techniques. (Source: US Department of Energy, 1999a).

past climate analogs. The reconstructing of past climates at a given locality from modern climatic conditions in a different elevation or latitudinal zone to infer past climatic conditions. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Payback. The amount of time required for positive cash flows to equal the total investment costs. (Source: US Department of Energy, 1999a).

PCBs (polychloronated biphenyls). A group of organic compounds used in the manufacture of plastics and formerly used as a coolant in electric transformers. In the environment, PCBs are highly toxic to aquatic life. They persist in the environment for long periods of time and are biologically accumulative. (Source: California Energy Commission, 1999a.)

Peak Clipping/Shaving. The process of implementing measures to reduce peak power demands on a system. (Source: US Department of Energy, 1999a).

Peak Demand/Load. The maximum energy demand or load in a specified time period. (Source: US Department of Energy, 1999a).

Peak-hour demand (water heater terminology). The maximum water usage, in gallons/hour, during the time of day when your family is likely to use the greatest amount of hot water. (Source: Lawrence Berkeley Laboratory, 1999)

Peak Load Plant. A plant usually housing old, low-efficiency steam units; gas turbines; diesels; or pumped-storage hydroelectric equipment normally used during the peak-load periods. (Source: US Energy Information Administration, 1999b)

Peak Power. Power generated by a utility unit that operates at a very low capacity factor; generally used to meet short-lived and variable high demand periods. (Source: US Department of Energy, 1999a).

Peak Shifting. The process of moving existing loads to off-peak periods. (Source: US Department of Energy, 1999a).

Peak Watt. A unit used to rate the performance of a solar photovoltaic (PV) cells, modules, or arrays; the maximum nominal output of a PV device, in Watts (Wp) under standardized test conditions, usually 1000 Watts per square meter of sunlight with other conditions, such as temperature specified. (Source: US Department of Energy, 1999a).

Peaking Capacity. Power generation equipment or system capacity to meet peak power demands. (Source: US Department of Energy, 1999a).

Peaking Hydropower. A hydropower plant that is operated at maximum allowable capacity for part of the day and is either shut down for the remainder of the time or operated at minimal capacity level. (Source: US Department of Energy, 1999a).

Pellets. Solid fuels made from primarily wood sawdust that is compacted under high pressure to form small (about the size of rabbit feed) pellets for use in a pellet stove. (Source: US Department of Energy, 1999a).

Pellet Stove. A space heating device that burns pellets; are more efficient, clean burning, and easier to operate relative to conventional cord wood burning appliances. (Source: US Department of Energy, 1999a).

percolation. The movement of water downward and radially through the subsurface soil layers, usually continuing downward to the groundwater. (Source: Carbon Dioxide Information Analysis Center, 1990a).

perfluorocarbons (PFCs). A group of human-made chemicals composed of carbon and fluorine only. These chemicals (predominantly CF 4 and C 2 F 6 ) were introduced as alternatives, along with hydrofluorocarbons, to the ozone-depleting substances. In addition, PFCs are emitted as by-products of industrial processes and are also used in manufacturing. PFCs do not harm the stratospheric ozone layer, but they are powerful greenhouse gases: CF 4 has a global warming poten-tial (GWP) of 6,500 and C 2 F 6 has a GWP of 9,200.(Source: US Environmental Protection Agency, 1999a)

Perfluorocarbon Tracer Gas Technique (PFT). An air infiltration measurement technique developed by the Brookhaven National Laboratory to measure changes over time (one week to five months) when determining a building's air infiltration rate. This test cannot locate exact points of infiltration, but it does reveal long-term infiltration problems. (Source: US Department of Energy, 1999a).

Performance Ratings. Solar collector thermal performance ratings based on collector efficiencies, usually expressed in Btu per hour for solar collectors under standard test or operating conditions for solar radiation intensity, inlet working fluid temperatures, and ambient temperatures. (Source: US Department of Energy, 1999a).

Perimeter Heating. A term applied to warm-air heating systems that deliver heated air to rooms by means of registers or baseboards located along exterior walls. (Source: US Department of Energy, 1999a).

Perlite Insulation. Perlite is a white gravelly, yet extremely light material. It is the same material found in garden centers and used as part of potting mixtures. Perlite is no longer used as an insulating material, except for the occasional do-it-yourselfer, although it is not uncommon to find it in existing houses. (Source: Lawrence Berkeley Laboratory, 1999)

Permeance. A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury. (Source: US Department of Energy, 1999a).

permafrost. Perennially frozen ground that occurs wherever the temperature remains below 0 degrees C for several years. (Source: Carbon Dioxide Information Analysis Center, 1990a).

petrochemicals. Chemicals obtained by refining (i.e., distilling) crude oil. They are used as raw materials in the manufacture of most industrial chemicals, fertilizers, pesticides, plastics, synthetic fibers, paints, medicines, and many other products. See crude oil.(Source: US Environmental Protection Agency, 1999a)

petroleum. A generic term applied to oil and oil products in all forms, such as crude oil, lease condensate, unfinished oils, petro-leum products, natural gas plant liquids, and non-hydro-carbon compounds blended into finished petroleum products. See crude oil.(Source: US Environmental Protection Agency, 1999a)

Phase. Alternating current is carried by conductors and a ground to residential, commercial, or industrial consumers. The waveform of the phase power appears as a single continuous sine wave at the system frequency whose amplitude is the rated voltage of the power. (Source: US Department of Energy, 1999a).

Phase Change. The process of changing from one physical state (solid, liquid, or gas) to another, with a necessary or coincidental input or release of energy. (Source: US Department of Energy, 1999a).

Phase-Change Material. A material that can be used to store thermal energy as latent heat. Various types of materials have been and are being investigated such as inorganic salts, eutectic compounds, and paraffins, for a variety of applications, including solar energy storage (solar energy heats and melts the material during the day and at night it releases the stored heat and reverts to a solid state). (Source: US Department of Energy, 1999a).

phenology. The study of periodic biological phenomena with relation to climate, particularly seasonal changes. These phenomena can be used to interpret local seasons and the climatic zones. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Photobiological Hydrogen Production. A hydrogen production process that process uses algae. Under certain conditions, the pigments in certain types of algae absorb solar energy. An enzyme in the cell acts as a catalyst to split water molecules. Some of the bacteria produces hydrogen after they grow on a substrate. (Source: US Department of Energy, 1999a).

photochemical smog. Air pollution caused by chemical reactions among various substances and pollutants in the atmosphere. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Photocurrent. An electric current induced by radiant energy. (Source: US Department of Energy, 1999a).

photoelectric. Of or relating to the electrical effects of light, including the emission of electrons, the generation of a voltage, or a change in resistance. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Photoelectric Cell. A device for measuring light intensity that works by converting light falling on, or reach it, to electricity, and then measuring the current; used in photometers. (Source: US Department of Energy, 1999a).

Photoelectrochemical Cell. A type of photovoltaic device in which the electricity induced in the cell is used immediately within the cell to produce a chemical, such as hydrogen, which can then be withdrawn for use. (Source: US Department of Energy, 1999a).

Photoelectrolysis Hydrogen Production. The production of hydrogen using a photoelectrochemical cell. (Source: US Department of Energy, 1999a).

Photogalvanic Processes. The production of electrical current from light. (Source: US Department of Energy, 1999a).

Photon. A particle of light that acts as an individual unit of energy. (Source: US Department of Energy, 1999a).

photosynthesis. The manufacture by plants of carbohydrates and oxygen from carbon dioxide and water in the presence of chlorophyll with sunlight as the energy source. Oxygen and water vapor are released in the process. Photosynthesis is dependent on favorable temperature and moisture conditions as well as on the atmospheric carbon dioxide concentration. Increased levels of carbon dioxide can increase net photosynthesis in many plants. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Photovoltaic (Conversion) Efficiency. The ratio of the electric power produced by a photovoltaic device to the power of the sunlight incident on the device. (Source: US Department of Energy, 1999a).

Photovoltaic (PV; Solar) Array. A group of solar photovoltaic modules connected together. (Source: US Department of Energy, 1999a).

Photovoltaic (Solar) Cell. A single photovoltaic device. (Source: US Department of Energy, 1999a).

Photovoltaic Device. A solid-state electrical device that converts light directly into direct current electricity of voltage-current characteristics that are a function of the characteristics of the light source and the materials in and design of the device. Solar photovoltaic devices are made of various semi-conductor materials including silicon, cadmium sulfide, cadmium telluride, and gallium arsenide, and in single crystalline, multi-crystalline, or amorphous forms. (Source: US Department of Energy, 1999a).

Photovoltaic (Solar) Module or Panel. A solar photovoltaic product that generally consists of groups of PV cells electrically connected together to produce a specified power output under standard test conditions, mounted on a substrate, sealed with an encapsulant, and covered with a protective glazing. Maybe further mounted on an aluminum frame. A junction box, on the back or underside of the module is used to allow for connecting the module circuit conductors to external conductors. (Source: US Department of Energy, 1999a).

Photovoltaic Peak Watt. see Peak Watt. (Source: US Department of Energy, 1999a).

Photovoltaic (Solar) System. A complete PV power system composed of the module (or array), and balance-of-system (BOS) components including the array supports, electrical conductors/wiring, fuses, safety disconnects, and grounds, charge controllers, inverters, battery storage, etc. (Source: US Department of Energy, 1999a).

Photovoltaic-Thermal (PV/T) Systems. A solar energy system that produces electricity with a PV module, and collects thermal energy from the module for heating. There are no commercially available systems available (as of 11/97). (Source: US Department of Energy, 1999a).

phytoplankton. That portion of the plankton community comprised of tiny plants (e.g., algae and diatoms). (Source: Carbon Dioxide Information Analysis Center, 1990a).

planetary albedo. The fraction (approximately 30%) of incident solar radiation that is reflected by the earth-atmosphere system and returned to space, mostly by backscatter from clouds in the atmosphere. (Source: Carbon Dioxide Information Analysis Center, 1990a).

planetary boundary layer. The transition region between the turbulent surface layer and the normally nonturbulent free atmosphere. This region is about 1 km in thickness and is characterized by a well-developed mixing generated by frictional drag as the air masses move over the Earth's surface. This layer contains approximately 10% of the mass of the atmosphere. Also called the "atmospheric boundary layer" or "frictional layer". (Source: Carbon Dioxide Information Analysis Center, 1990a).

plankton. Passively floating or weakly motile aquatic plants (phytoplankton) and animals (zooplankton). (Source: Carbon Dioxide Information Analysis Center, 1990a).

Pleistocene. The earlier of the two epochs of the Quaternary period, starting 2 to 3 million years before the present and ending about 10,000 years ago. It was a time of glacial activity. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Plenum. The space between a hanging ceiling and the floor above or roof; usually contains HVAC ducts, electrical wiring, fire suppression system piping, etc. (Source: US Department of Energy, 1999a).

point source. A single identifiable source that discharges pollutants into the environment. Examples are smokestack, sewer, ditch, or pipe. See non-point source. (Source: US Environmental Protection Agency, 1999a)

Polluter Pays Principle (PPP). The PPP suggests that the polluter should bear the cost of preventing and controlling pollution. Its intent is to force polluters to internalize all the environmental costs of their activities so that these are fully reflected in the costs of the goods and services they provide. (Source: Global Environment Facility, 2000a).

Polycrystalline. A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. (Source: US Department of Energy, 1999a).

Polyethylene. A registered trademark for plastic sheeting material that can be used as a vapor retarder. This plastic is used to make grocery bags. It is a long chain of carbon atoms with 2 hydrogen atoms attached to each carbon atom. (Source: US Department of Energy, 1999a).

Polystyrene. (see Foam Insulation) (Source: US Department of Energy, 1999a).

Porous Media. A solid that contains pores; normally, it refers to interconnected pores that can transmit the flow of fluids. (The term refers to the aquifer geology when discussing sites for CAES.) (Source: US Department of Energy, 1999a).

Portfolio Standard. The requirement that an electric power utility generate or purchase a specified percentage of the power it supplies/sells from renewable energy resources, and thereby guarantee a market for electricity generated from renewable energy resources. (Source: US Department of Energy, 1999a).

positive feedback. An interaction that amplifies the response of the system in which it is incorporated. See feedback mechanisms. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Potable Water. Water that is suitable for drinking, as defined by local health officials. (Source: US Department of Energy, 1999a).

Potential Energy. Energy available due to position. (Source: US Department of Energy, 1999a).

Pound of Steam. One pound of water in vapor phase; is NOT steam pressure, which is expressed as pounds per square inch (psi). (Source: US Department of Energy, 1999a).

Pound Per Square Inch Absolute (psia). A unit of pressure [hydraulic (liquid) or pneumatic (gas)] that does not include atmospheric pressure. (Source: US Department of Energy, 1999a).

Power. Energy that is capable or available for doing work; the time rate at which work is performed, measured in horsepower, Watts, or Btu per hour. Electric power is the product of electric current and electromotive force. (Source: US Department of Energy, 1999a).

Power Coefficient. The ratio of power produced by a wind energy conversion device to the power in a reference area of the free windstream. (Source: US Department of Energy, 1999a).

Power Conditioning. The process of modifying the characteristics of electrical power (for e.g., inverting dc to ac). (Source: US Department of Energy, 1999a).

Power Density. The amount of power per unit area of a free windstream. (Source: US Department of Energy, 1999a).

Power Factor (PF). The ratio of actual power being used in a circuit, expressed in watts or kilowatts, to the power that is apparently being drawn from a power source, expressed in volt-amperes or kilovolt-amperes. (Source: US Department of Energy, 1999a).

power pool. An entity established to coordinate short-term operations to maintain system stability and achieve least-cost dispatch. The dispatch provides backup supplies, short-term excess sales, reactive power support, and spinning reserve. Historically, some of these services were provided on an unpriced basis as part of the members' utility franchise obligations. Coordinating short-term operations includes the aggregation and firming of power from various generators, arranging exchanges between generators, and establishing (or enforcing) the rules of conduct for wholesale transactions. The pool may own, manage and/or operate the transmission lines ("wires") or be an independent entity that manages the transactions between entities. Often,the power pool is not meant to provide transmission access and pricing, or settlement mechanisms if differences between contracted volumes among buyers and sellers exist. (Source: California Energy Commission, 1999a.)

Power Transmission Line. An electrical conductor/cable that carries electricity from a generator to other locations for distribution. (Source: US Department of Energy, 1999a).

ppm (parts per million). The unit commonly used to represent the degree of pollutant concentration where the concentrations are small. (Source: California Energy Commission, 1999a.)

Preheater (Solar). A solar heating system that preheats water or air that is then heated more by another heating appliance. (Source: US Department of Energy, 1999a).

precipitation. Any or all forms of liquid or solid water particles that fall from the atmosphere and reach the Earth's surface. It includes drizzle, rain, snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. The ratio of precipitation to evaporation is the most important factor in the distribution of vegetation zones. Precipitation is also defined as a measure of the quantity, expressed in centimeters or milliliters of liquid water depth, of the water substance that has fallen at a given location in a specified amount of time. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Precautionary principle. The principle holds that where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation. (Source: Global Environment Facility, 2000a).

prescribed burning. Deliberate setting and careful control of surface fires in forests to help prevent more destructive fires and to kill off unwanted plants that compete with commercial species for plant nutrients; may also be used on grasslands. (Source: US Environmental Protection Agency, 1999a)

Present Value. The amount of money required to secure a specified cash flow at a future date at a specified return. (Source: US Department of Energy, 1999a).

Pressure Drop. The loss in static pressure of a fluid (liquid or gas) in a system due to friction from obstructions in pipes, from valves, fittings, regulators, burners, etc, or by a breech or rupture of the system.

Pressurization Testing. A technique used by energy auditors, using a blower door, to locate areas of air infiltration by exaggerating the defects in the building shell. This test only measures air infiltration at the time of the test. It does not take into account changes in atmospheric pressure, weather, wind velocity, or any activities the occupants conduct that may affect air infiltration rates over a period of time. (Source: US Department of Energy, 1999a).

Primary Air. The air that is supplied to the combustion chamber of a furnace. (Source: US Department of Energy, 1999a).

primary productivity. See gross primary production and net primary production. (Source: Carbon Dioxide Information Analysis Center, 1990a).

primary succession. The natural development of vegetation and soil on a site that had not previously borne vegetation (e.g., a sand dune or lava flow), which vegetation will be replaced by other, successive plant communities. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Prime Mover. Any machine capable of producing power to do work. (Source: US Department of Energy, 1999a).

Process Heat. Thermal energy that is used in agricultural and industrial operations. (Source: US Department of Energy, 1999a).

Products of Combustion. The elements and compounds that result from the combustion of a fuel. (Source: US Department of Energy, 1999a).

Producer Gas. Low or medium Btu content gas, composed mainly of carbon monoxide, nitrogen(2), and hydrogen(2) made by the gasification of wood or coal. (Source: US Department of Energy, 1999a).

programmable controller.. A device that controls the operation of electrical equipment (such as air conditioning units and lights) according to a preset time schedule. (Source: California Energy Commission, 1999a.)

Programmable Thermostat. A thermostat with the ability to record different temperature settings for different times for your heating and/or cooling equipment. Programmable thermostats can be electronic, or mechanical.  (Source: Lawrence Berkeley Laboratory, 1999)

Projected Area. The net south-facing glazing area projected on a vertical plane. Also, the solid area covered at any instant by a wind turbine's blades from the perspective of the direction of the windstream (as opposed to the swept area). (Source: US Department of Energy, 1999a).

Propane. A hydrocarbon gas, C3H8, occurring in crude oil, natural gas, and refinery cracking gas. It is used as a fuel, a solvent, and a refrigerant. Propane liquefies under pressure and is the major component of liquefied petroleum gas (LPG). (Source: US Department of Energy, 1999a).

protocol. A protocol is linked to an existing convention, but it is a separate and additional agreement that must be signed and ratified by the Parties to the convention. Protocols typically strengthen a convention by adding new, more detailed commitments. (Source: UN Climate Change Secretariat, 1999a)

proxy climate indicators. Dateable evidence of a biological or geological phenomenon whose condition, at least in part, is attributable to climatic conditions at the time of its formation. Proxy data are any material that provides an indirect measure of climate and include documentary evidence of crop yields, harvest dates, glacier movements, tree rings, varves, glaciers and snow lines, insect remains, pollen remains, marine microfauna, isotope measurements: 18O in ice sheets; 18O, 2H, and 13C in tree rings; CaCO3 in sediments; and speleothems. There are three main problems in using proxy data: (1) dating, (2) lag and response time, and (3) meteorological interpretation. Tree rings, pollen deposits from varved lakes, and ice cores are the most promising proxy data sources for reconstructing the climate of the last five millennia because the dating are precise on an annual basis while other proxy data sources may only be precise to +/- 100 years. (Source: Carbon Dioxide Information Analysis Center, 1990a).

pycnocline. In the ocean, a region where the water density increases rapidly with depth. (Source: Carbon Dioxide Information Analysis Center, 1990a).

Psi. Pounds of pressure per square inch. (Source: US Department of Energy, 1999a).

Psia. Pounds/force per square inch absolute. (Source: US Department of Energy, 1999a).

Psig. Pounds/force per square inch gauge. (Source: US Department of Energy, 1999a).

Psychrometer. An instrument for measuring relative humidity by means of wet and dry-bulb temperatures. (Source: US Department of Energy, 1999a).

Psychrometrics. The analysis of atmospheric conditions, particularly moisture in the air. (Source: US Department of Energy, 1999a).

Public Utility Holding Company Act (PUHCA) of 1935. A law to protect consumers and investors. It placed geographic restrictions on mergers and limitations on diversification into non-utility lines of business and takeovers of electric and gas utilities, and also established regulated monopoly markets or service territories for utilities. (Source: US Department of Energy, 1999a).

Public Utilities Regulatory Policy Act (PURPA) of 1978. A law that requires electric utilities to purchase electricity produced from qualifying power producers that use renewable energy resources or are cogenerators. Utilities are required to purchase power at a rate equal to the avoided cost of generating the power themselves. (See Avoided Costs and Qualifying Facility) (Source: US Department of Energy, 1999a).

Public Utility or Services Commissions (PUC or PSC). These are state government agencies responsible for the regulation of public utilities within a state or region. A state legislature oversees the PUC by reviewing changes to utility laws, rules and regulations and approving the PUC's budget. The commission usually has five Commissioners appointed by the Governor or legislature. PUCs typically regulate: electric, natural gas, water, sewer, telephone services, trucks, buses, and taxicabs within the commission's operating region. The PUC tries to balance the interests of consumers, environmentalists, utilities, and stockholders. The PUC makes sure a region's citizens are supplied with adequate, safe utility service at reasonable rates. (Source: US Department of Energy, 1999a).

Purchased Power Adjustment. A clause in a rate schedule that provides for adjustments to the bill when energy from another electric system is acquired and it varies from a specified unit base amount. (Source: US Energy Information Administration, 1999b)

Pyranometer. A device used to measure total incident solar radiation (direct beam, diffuse, and reflected radiation) per unit time per unit area. (Source: US Department of Energy, 1999a).

Pyrheliometer. A device that measures the intensity of direct beam solar radiation. (Source: US Department of Energy, 1999a).

Pyrolysis. The transformation on a compound or material into one or more substances by heat alone (without oxidation). Often called destructive distillation. Pyrolysis of biomass is the thermal degradation of the material in the absence of reacting gases, and occurs prior to or simultaneously with gasification reactions in a gasifier. Pyrolysis products consist of gases, liquids, and char generally. The liquid fraction of pyrolisized biomass consists of an insoluble viscous tar, and pyroligneous acids (acetic acid, methanol, acetone, esters, aldehydes, and furfural). The distribution of pyrolysis products varies depending on the feedstock composition, heating rate, temperature, and pressure. (Source: US Department of Energy, 1999a).