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glossary of energy terms


Click letter to advance to that section of the glossary:

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z


ABSORPTION CHILLER — A refrigeration machine using heat as the power input to generate chilled water.


ACTIVATED CARBON — A form of carbon capable of absorbing odors, anesthetics and vapors.


AEI (Annual Energy Index) — The ratio of the total annual energy consumption of a building or plant in millions of Btu, divided by the total building area in thousands of square feet. The AEI is computed in thousands of Btu per square foot of building per year as a way of characterizing energy usage in the building.


AFUE (Annual Fuel Utilization Efficiency) — The most widely used measure of a furnace’s heating efficiency he measure of seasonal or annual efficiency of a furnace or boiler. It takes into account the cyclic on/off operation and associated energy losses of the heating unit as it responds to changes in the load, which in turn is affected by changes in weather and occupant controls. It measures the amount of heat actually delivered, compared to the amount of fuel that must be supplied to the furnace.


AIR CHANGES — The number of times the air is replaced in a room each hour. Often expressed in cubic feet per minute (CFM) or air changes per hour.


AIR CONDITIONING — The process of treating air to meet the requirements of the conditioned space by controlling simultaneously its temperature, humidity, cleanliness and distribution.


AIR CURTAIN — As used in this context, a jet of air of predetermined thickness and velocity that prevents the penetration of unconditioned air into a conditioned space — for example, at a doorway.


AMBIENT TEMPERATURE — Outside air temperature.


AMMETER — An instrument for measuring the strength of an electric current in amperes.


AMPERE — A rate of measurement by which the flow of electrical current is determined.


APPLIANCE — A device operated by electricity, oil, or gas, especially for use in the home or for performance of domestic chores.


ATTRIBUTED LIFE — With respect to an energy conservation measure, the time period that is equal to the useful life or 15 years, whichever is less; or with respect to a renewable resource energy measure, the time period equal to either the useful life or 25 years, whichever is less.


AUDITOR — Any person who conducts an energy audit and certifies it to be in conformance with an applicable regulation.


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BALANCE TEMPERATURE — The outdoor temperature at which the internal heat gain of a building (from lighting, people and machines) equals the losses through the building envelope.


BALLAST — A device used with gaseous discharge lights such as fluorescent tubes, to limit current flow and to provide voltage at design level.


BARREL — Although seldom put in actual "barrels", crude oil is measured in a unit called the barrel, equal to 42 US gallons; one barrel of crude oil has the same energy as 350 pounds of coal.


BASE LOAD — The energy requirements of a facility that are unaffected by weather changes.


BASE YEAR — A selected year prior to the energy audit, with consumption levels or dollar amounts to which all future usage or costs are compared.


BILLING UNITS — Units of electricity for which a charge is assessed — watts, kilowatts and kilowatt hours.


BLACKOUTS — The failure of an electric power system, often caused by sabotage, storm damage, or equipment failure; blackouts also can occur as the result of overloaded utility equipment.


BLOW DOWN — The discharge of water from a boiler or cooling tower sump that contains a high proportion of total dissolved solids.


BLOWER — A fan used to force air under pressure.


BOILER — A unit that produces hot water or steam for heating.


BOILER CAPACITY — The rate of heat output in Btu/hr at the design pressure and/or temperature, measured at boiler outlet. Rates fuel input at the sites elevation.


BOILER HORSEPOWER — Some older boilers are rated in horsepower rather than in terms of an energy rating. One operating boiler horsepower is defined as 33,500 Btu per hour.


BOUNDARY ENERGY BUDGET — The maximum energy allowance for a building expressed in heating value (Btu's) of the required energy sources as delivered to the project site (electricity, #2 fuel oil, etc.).


BRITISH THERMAL UNIT (Btu) — The quantity of heat needed to raise the temperature of one pound of water one degree Fahrenheit (Roughly, one Btu is approximately equal to the amount of heat released by a burning wooden match).


BROWNOUTS — During periods of acute power shortage, utilities can reduce the voltage on the power lines so that the amount of power delivered to each person is reduced (usually by eight percent).


BUILDING — Any structure that includes provisions for a heating or cooling system, or both, or for a hot water system.


BUILDING ENVELOPE — The elements of a building which enclose conditioned spaces and through which thermal energy is transferred to or from the exterior.


BUILDING SIZE — Gross square feet.


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CALORIE — The amount of heat required to raise the temperature of one gram of water one degree Centigrade.


CAPACITY CHARGE — See demand charge.


CAPACITY FACTOR — The ratio of the average load on the generating plant for the period of time considered to the capacity rating of the plant; sometimes called plant factor. In the US, the average capacity factor is about 52 percent.


CAULKING — A flexible material used to seal up cracks or spaces in a structure.


CCF (One hundred cubic feet) — Used by natural gas companies for billing purposes.


CEILING PLENUM — The space above a suspended ceiling used as a flow path for air movement.


CELSIUS or CENTIGRADE — The temperature scale on which the freezing point of water is 0 degrees and the boiling point is 100 degrees at sea level.


CENTRAL PLANT BOILERS — A unit that uses gas, oil, or electricity or other heat sources to heat water to an elevated temperature, or until it becomes steam, and carries the heat to occupied spaces.


CENTRAL PLANT CHILLERS — A unit that cools a brine solution using an enclosed refrigeration cycle. The brine is either piped to room units (i.e., unit ventilators) for cooling, or used to cool an air stream that is ducted to rooms.


CFM (Cubic feet per minute) — Usually refers to air changes.


CHEMICAL ENERGY — The energy of food and fuels, or more precisely, the energy tied up in chemical molecules.


CHIMNEY EFFECT — The tendency of air or gas in a duct or other vertical passage to rise when heated due to its lower density compared with that of the surrounding air or gas. In buildings, the tendency toward displacement caused by the difference in temperature of internal heated air by unheated outside air due to the difference in density of outside and inside air.


COEFFICIENT OF PERFORMANCE (COP) — COP is traditionally used to quantify efficiency of heat removal, taking into consideration the removal of heat attributable to the air conditioning system itself. COP = Total Heat Removal/Power Requirements.


COLOR RENDITION — Effect of a light source on the color appearance of objects in comparison with their appearance under a reference light source.


COMPRESSOR — The component of a refrigerating system that increases the pressure of a compressible refrigerant fluid, and simultaneously reduces its volume, while moving the fluid through the device.


CONDENSATE — Water obtained by changing the state of water vapor, (i.e., steam or moisture in air) from a gas to a liquid, usually by cooling.


CONDENSER — A heat exchanger that removes heat from a vapor, changing it to its liquid state. (In refrigeration systems, the component that rejects heat.)


CONDUCTION — Method of heat transfer where heat moves through a solid.


CONNECTED LOAD — The uses which an input energy stream is feeding, i.e., fan motor, lights, heat, hot water, etc.


CONTROL — Any device for regulation of a system or component, manual or automatic.


CONSERVATION — The wise and efficient use of natural resources (including energy.)


CONSUMER — A person who uses goods and services to satisfy his personal needs and desires rather than to resell them or to produce other goods or services with them.


CONSUMPTION (ENERGY) — The amount of energy actually used in a given amount of time.


CONVECTION — Method of heat transfer where heat moves by motion of a liquid or gas, usually air.


COOLED, COOLING — The use of refrigeration or evaporative effects to adjust the inside air temperature below the outside air temperature. Fans, blowers, or other means of ventilation are not considered cooling when used by themselves.


COOLING LOAD — Calculated on a monthly, yearly or seasonal basis by multiplying the overall thermal transmittance (U-Value) of a building (in Btu/hr/°F/ sq ft) times total building surface area 24 hours/day times the number of cooling degree days per time period desired.


CUBIC FEET — A common unit of measurement used by natural gas companies to determine the quantity of gas reserves or production and used by utility companies to determine the amount of gas consumed in a customer's home or business.


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DEGREE-DAYS — The difference between the median temperature for any day [(daily high temperature - daily low temperature)/2] and 65°F. If the median temperature is below 65°F, the value is expressed as "Heating Degree Days" (HDD). If the median temperature is above 65°F, the value is expressed as "Cooling Degree Days" (CDD). Degree days are a measure of the severity of the entire season and are directly proportional to fuel consumption.


DEHUMIDIFIER — A machine used to remove or reduce the moisture from the air, principally used in the summer.


DEMAND (ENERGY) — The amount of energy consumed under given constraints of availability, price, and controls on use (e.g., emissions from smokestack).


DEMAND LOAD — The actual amount of electric power on a circuit (measured in kilowatts) during any 15 minute interval for commercial operations. The price of electricity is directly related to the level of this demand. The higher the demand, the higher the cost per electrical unit.


DESIGN TEMPERATURE — The difference between desired indoor temperature and the outdoor temperature to maintain comfort levels for a given locality.


DEW POINT — The temperature at which moisture content of a given sample of air becomes saturated and starts to condense into a "dew".


DIRECT ENERGY CONSERVATION — The process of changing any other form of energy into electricity without machinery that has missing parts. For example, a battery changes chemical energy into electricity by direct energy conservation.


DIRECT EXPANSION COILS — Pipes or tubes containing refrigerants that are part of a refrigeration cycle. These coils are used to cool the flow of air.


DIRECT FIRED UNIT — The use of heat from a gas, oil or fuel flame to heat air or water.


DOOR SEAL — Found at loading/receiving entrances, these airtight seals fit to a truck to prevent air infiltration when the loading door is open.


DOUBLE-BUNDLE CONDENSER — Condenser (usually in a refrigeration machine) that contains two separate tube bundles allowing the option of either rejecting heat to the cooling tower or to another building system requiring heat input.


DRY BULB TEMPERATURE — Air temperature as indicated by an ordinary thermometer.


DUAL DUCT SYSTEM — A heating system with two ducts, one supplying warm and the other supplying cool air. Mixers blend air from the two ducts and deliver air at the correct temperature to each space.


DUCTWORK — The conduit or piping system through which ventilated warm or cool air is conveyed from the source of supply to the premises or outlet. Ducts are usually made of galvanized metal or fiberglass; they may be lined or covered with insulating materials.


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ECOLOGY — The totality or pattern of the interrelationship of organisms and their environment; also, the science which studies such patterns.


ECONOMIZER CYCLE — A method of operating a ventilation system to reduce refrigeration load. Whenever the outdoor air conditions are more favorable (lower heat content) than return conditions, outdoor air quantity is increased.


ECOSYSTEM — The carefully balanced network of dependence between living and non-living processes in the environment; disruptions anywhere in the system can effect changes elsewhere with unforeseen results.


EER (Energy Efficiency Ratio) — The most commonly used measure of the efficiency for commercial air conditioning systems. The rating is determined by dividing the cooling output of the ground or water source heat pump (in Btu/hour) by the power input (in Watts). Look for an EER of at least 10.5.


ELASTICITY — The response of the market (e.g., a consumer) to a market change (e.g., the price of energy). A negative elasticity implies that demand will decrease in response to a price increase.


ELECTRICAL ENERGY — Associated with electric currents, usually measured in watts (see wattage).


ELECTRIC POWER TRANSMISSION and DISTRIBUTION — The development, design, construction and operation of systems to transport electrical energy from the generation station to the eventual utilization device. Included are extra high voltage AC systems, DC systems, underground systems and cryogenic systems as well as system security and load management.


ENCLOSED FLOOR AREA — The entire floor area of a facility measured in square feet, using the outside dimensions of the building(s).


ENERGY — The capability to do mechanical work or to produce a change in temperature (e.g., to heat or to cool). Equal to the product of power and time (E = P x T).


ENERGY AUDIT — A systematic inspection of a facility designed to identify opportunities for energy savings.


ENERGY EFFICIENCY RATIO (EER) — The amount of useful work (e.g., cooling, heating, or motion) divided by the fuel or energy input. For example, in electricity power generation, EER is the amount of electricity produced per unit of fuel consumed. For an air conditioner, EER is the unit's cooling capacity (in BTU's per hour) divided by the watts required during the same time period.


ENERGY CONSERVATION MEASURE — An energy saving opportunity requiring retrofit, redesign or renovation of an existing facility (i.e. a capital investment is required).


ENERGY RECOVERY — One of the concepts of resource recovery where a part, or all, of the waste materials going into a recovery facility are burned to produce heat which can be used to produce steam for heating or for the generation of electricity. Alternatively, materials which inherently contain an energy investment (e.g., aluminum) can be recovered, saving energy that otherwise would be required to produce more aluminum from new ore.


ENERGY UTILIZATION INDEX — A reference that expresses the total energy (fossil fuel and electricity) used by a facility in a given period (month, year, etc.) in terms of Btu's/gross conditioned square feet.


ENTHALPY — Thermodynamic property of a substance defined as the sum of its internal energy plus the quantity Pv/J, where P =pressure of the substance, v =its volume, and J = the mechanical equivalent of its heat. For the purpose of air conditioning, enthalpy is the total heat content of air, expressed in units of Btu/lb.


ENVIRONMENT — The sum of all the external conditions, including man-made conditions, and influences affecting the life, development, and ultimately the survival of an organism.


ENVIRONMENTAL CONTROL SYSTEM — An integrated and automatically controlled system of refrigeration, HVAC and dehumidification equipment.


ENVIRONMENTAL EDUCATION — Those instructional processes which are concerned with the total surroundings of the individual. This may include people environments as well as natural man-made technological environments.


ENVIRONMENTAL ETHIC — A value system based on a code of moral commitments for the environmental good.


EVAPORATOR — A device in which a liquid becomes a gas while absorbing heat.


EXHAUST AIR — Air rejected from a space to the outside using fans, blowers, or gravity relief ducts.


EXPONENTIAL GROWTH — A quantity exhibits exponential growth (as opposed to arithmetical growth) when it increases by a constant percentage of the whole during a constant time period. A colony of bacteria in which each bacterium divides into two bacteria every ten minutes grows exponentially.


EXTERNALITY — A cost or benefit associated with an action that is not part of the market price (e.g., environmental pollution).


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FAHRENHEIT — The temperature scale on which the freezing point of water is 32 degrees and the boiling point of water is 212 degrees at sea level.


FAN COIL UNIT — A heating element that has been installed in a space and uses a fan to direct room air over a coil heated by steam or hot water. The essential elements are: fan, motor, coil, and enclosure. A fan coil unit does not introduce outside air.


FENESTRATION — An opening in a building, such as a door or window, through which light infiltrates.


FISSION — The splitting of heavy atomic nucleus, usually thorium, uranium, or plutonium, into two approximately equal parts accompanied by the release of a relatively large amount of energy and generally one or more neutrons.


FLATTENED RATE DESIGN — The reduction or elimination of differential charges per unit of consumption based on quantity of usage. A flat rate structure would charge a constant amount per kWh regardless of the total electricity usage.


FLOODLIGHTING — A lighting system designated to illuminate an area using projector-type luminaires usually capable of being pointed in any direction.


FOOT CANDLE — A measurement of illumination; specifically the illumination on a surface one foot from a standard candle. (1 foot candle = 1 lumen per square foot)


FOSSIL FUELS — Fuels which contain the remains of ancient plant and animal life (e.g., coal, oil, oil shale, tar sands, natural gas).


FUEL OILS — Petroleum fractions with a higher boiling range than kerosene, generally classified as distillates or residuals. Distillates (nos. 1, 2, and 4) are the lighter oils used primarily for central heating of homes, small apartment houses, commercial buildings, and for transportation. Residuals (nos. 5 and 60, often called bunker oils, are heavier, high viscosity oils which usually need to be heated before they can be pumped. They are used in industry, large commercial buildings, and for the generation of electricity.


FUEL ADJUSTMENT CLAUSE or SURCHARGE — This charge, which may be "automatic" (not subject to approval by regulators before imposition), is separately stated on the customer bill to recover (or rebate) the present cost of fuel not anticipated in the rate charge.


FUMAROLES — The name applied to geothermal wells issuing hot gases and liquids.


FURNACE — A self-enclosed appliance that heats an air stream using heat from oil, gas, or other fuel fire or electric resistance. The air stream is then delivered either directly or through ducts.


FUSION — The formation of a heavier nucleus from two lighter ones, usually deuterium and tritium, the heavy isotopes of hydrogen, with the attendant release of energy.


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GENERATOR — A device that converts mechanical energy into electrical energy.


GEOTHERMAL ENERGY — Thermal energy contained in rocks and water deep underground in certain areas of the world. This energy is obtained from crustal movements, vulcanization, and the earth's internal radioactivity.


GLARE — The effect produced by harsh, uncomfortably bright light.


GLAZING — Another term for glass used in windows.


GROSS FLOOR AREA — The sum of areas of all of the floors in a building, including basements, mezzanine, and intermediate-floored tiers and penthouses of headroom height, measured from the exterior walls or from the centerline of walls separating buildings.


GROSS SQUARE FEET — The total number of square feet contained in a building envelope using the floors as area to be measured.


GROSS WALL AREA — The vertical projection of the exterior wall area bounding interior space that is conditioned by an energy using system. This includes opaque wall, window and door areas.


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HEAD PRESSURE — The pressure of the refrigerant gas above the compressor outlet.


HEAT-ABSORBlNG GLASS — Generally blue-green, gray or bronze, the color reduces the light transmission of glass. Either single or double pane.


HEAT EXCHANGER — Any device that transfers heat from one fluid (liquid or gas) to another or to the environment.


HEAT GAlN — As applied to HVAC calculations, it is that amount of heat gained by a space by all sources, including people, lights, machines, sunshine, etc. The total heat gain represents the amount of heat that must be removed from a space to maintain indoor comfort conditions.


HEATlNG LOAD — Calculated on a monthly, yearly or seasonal basis by multiplying the overall thermal transmittance (U-Value) of a building (in Btu/hr/°F/ sq ft) times total building surface area times 24 hours/day times the number of heating degree days per time period desired. By dividing this product by the fuel value, one can estimate the quantity of fuel needed over the time interval.


HEAT-LATENT — The quantity of heat required to effect a change in state. This refers to the water vapor content of air.


HEAT LOSS/GAlN — Refers to the quantity of heat (expressed in Btu's/hr) passing through a given section of material and, therefore, not available for useful work.


HEAT PUMP — A device which transfers heat from a cold reservoir to a hotter one by the expenditure of mechanical energy with the primary purpose of heating the hot reservoir rather than refrigerating the cold reservoir. A heat pump is a reversed refrigeration apparatus. Heat pumps are far more efficient for residential heating than electrical resistive heating.


HEAT RECOVERY — The capture and use of heat generated by lighting, cooling and refrigeration systems for purposes requiring heat. Also, the removal of such heat from a conditioned space to reduce cooling or refrigeration requirements.


HEAT-REFLECTIVE GLASS — Glass specifically treated, usually on the inside surface with a reflective metallic coating that reduces solar radiation heat gain, either single or double pane.


HEAT TRANSFER — The movement of heat from a hot substance to a cooler substance; accomplished by radiation, conduction or convection.


HID (HIGH lNTENSITY DISCHARGE) — Light bulbs which use bright glowing gases. Mercury vapor and high pressure sodium are examples of HID lighting.


HORSEPOWER — British unit of power, 1 HP = 746 watts, 42.41 Btu/minute, or 2545 Btu/hr.


HSPF (Heating Seasonal Performance Factor) — The most commonly used measure of the heating efficiency of heat pumps. Technically speaking, it is the heat pump’s estimated seasonal heating output in Btus divided by the amount of energy that it consumes in Watt-hours. New heat pumps manufactured after 2005 in the US are required to have an HSPF of at least 7.7. The most efficient heat pumps have an HSPF of 10.


HUMIDIFIER — A machine used to inject moisture into the air, usually in the winter, thereby increasing the relative humidity of air.


HUMIDISTAT — A switch for activating any of several components of an air

conditioning system to control the relative humidity of a conditioned space, which responds automatically to changes in relative humidity.


HUMIDITY — The amount of water vapor in the air within a given space. Higher humidity allows lower temperatures to provide acceptable comfort levels.


HUMIDITY (RELATIVE) — A measure of the amount of moisture in the air, expressed in terms of percent of saturation at a given temperature.


HVAC — A system that provides heating, ventilating, and/or air conditioning within or associated with a building.


HYDROELECTRIC — Electricity production by water-powered turbine generators.


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INDIRECT LIGHTING — Lighting by luminaires distributing 90 to 100 percent of the emitted light upward so it will reflect off walls or ceilings.


INFILTRATION — The flow of air into a building. Air leakage into or out of a building through cracks and around windows and doors


INSULATING or DOUBLE-PANE GLASS — Two or more panes of glass separated by an airtight space.


INSULATION — Any material that provides a high resistance to the flow of heat or electricity. Used to retard heat flow.


INTERIOR AREA — Measured in square feet using the outside dimensions of a building. A plat plan, architectural plans, or specifications may have this information readily available.


INTERNAL COMBUSTION ENERGY — Energy is supplied by a burning fuel, which is directly transformed into mechanical energy by controlled combustion.


INVERTED RATE DESIGN — A rate design for a customer class that inverts the prevailing declining block rate design such that the unit charge for electricity increases instead of decreases as usage increases.


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KILOWATT (kW) — A unit of power, equivalent to 1,000 watts.


KILOWATT HOUR (kWh) — A unit of electrical energy representing the use of 1,000 watts (1 kW) of power for one hour. For example, a 100-watt bulb burning for 10 hours will consume one kilowatt-hour of energy.


KINETIC ENERGY — Energy of motion; for instance, mechanical energy.


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LAW OF THE CONSERVATION OF ENERGY AND MASS — The old law (formulated in the 1840's) stated that energy can be changed in form but cannot be created or destroyed. However, with the advent of Einstein's Special Theory of Relativity, it became evident that energy could be changed in form or created from mass and vice versa. The sum of energy and mass in the universe is constant.


LIFE-CYCLE COST — The lifetime cost of a new piece of equipment; the sum of purchase price plus operating and maintenance over the life of the product.


LOAD FACTOR — The fraction of available power being used at a given time.


LUMEN — A measure of the amount of light produced by an electric light bulb. Two bulbs the same rated wattage may differ in the number of lumens they produce. For example, 100 watt bulbs are available which produce from 1000 to 1750 lumens.


LUMINAIRE — Light fixture designed to produce a desired effect.


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M — Thousands (For example, Mcf on gas billings equals thousand cubic feet.)


MM — Million (For example, MMBtu equals million Btu's.)


MAGNET HYDRODYNAMICS (MHD) — The process that uses a magnetic field to produce electricity directly from the hot smoke and gases given off by burning fuels like coal and oil.


MAINTENANCE CHANGES — A modification or energy-efficiency opportunity that involves no or low costs as distinguished from a modification that is more capital intensive.


MARKET PRICE — The price for a commodity or service that is actually associated with its purchase, irrespective of other components of its "total" price such as subsidies and externalities.


MEGAWATT (MW) — A unit of power equal to 1,000 kilowatts or one million watts.


MERCURY VAPOR — An illumination type which uses electrically-excited gas as the source of light.


MULTI-ZONE SYSTEM — A system which heats one air stream and cools another. Air from the two streams is blended to the correct temperature and then delivered by duct to each space. The blending is done at the central heating plant.


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NEMA (National Electrical Manufacturers Association) — An American association that establishes standards of manufacture for electrical equipment.


NIGHT THERMOSTAT — A method of night control by which room temperatures are reduced at night to 55°F to 60°F. The thermostats are adjusted either manually or automatically.


NONUNIFORM LIGHTING — Task lighting only where needed within a space, in contrast to lighting levels provided generally throughout the space.


NUCLEAR ENERGY — The energy liberated by a nuclear reaction (fission or fusion) or by radioactive decay; sometimes called "atomic energy."


NUCLEAR POWER PLANT — Any device, machine, or assembly that converts nuclear energy into some form of useful power, such as mechanical or electrical power. In a nuclear-electric-power plant, heat produced by a reactor is generally used to make steam to drive a turbine that, in turn, drives an electric generator.


NUCLEAR REACTOR — A device in which a fission chain reaction can be initiated, maintained, and controlled. Sometimes called an atomic "furnace", it is the basic machine of nuclear energy.


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OCCUPIED HOURS — The time when a commercial, industrial or institutional building is normally occupied by people functioning in their jobs.


OIL SHALE — A sedimentary rock which contains an oil-yielding organic material called "kerogen". When heated, the oil shale may yields as much as 60 gallons of oil per ton of rock.


OPAQUE AREA — All exposed areas of a building that enclose conditioned space, except openings for windows, doors, skylights or building service areas.


OPERATIONAL & MAINTENANCE OPTION (O & M) — A simple low- or no-cost opportunity to reduce energy consumption by modifying scheduling and/or increasing efficiency of energy systems.


OSHA — The Occupational Safety and Health Administration.


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PAYBACK — The time required for the cost of energy saved to equal the purchase and installation costs.


PEAK DEMAND — The instantaneous use of electrical energy by a building or facility. The maximum yearly demand is the highest demand for a single month in a year. In addition to electrical charges per kilowatt hour of consumption, commercial electric bills may include a monthly charge per kilowatt or demand. Peak demand is based on fifteen- or thirty-minute averages. Some utility companies employ a ratchet principle in which billed demand is either the peak demand for the month or a fraction of the peak demand registered for the previous twelve months, whichever is greater. If this is the case, one fifteen- to thirty-minute period with all electrical systems switched on results in increased charges for the entire year. This penalty demand charge may be in effect all year, or just during peak use seasons.


PEAK LOAD — Measure of power delivered at the time demand is maximum (e.g., daily peak, seasonal peak, or yearly peak). Peak periods of power usage vary across the country mainly due to differing climatic conditions.


PETROCHEMICALS — Chemicals removed from crude oil at the refinery and used to make a wide range of products, such as plastics, synthetic fibers, detergents, and drugs.


PHOTOVOLTAIC (PV) CELL — Directly converts light energy into electrical energy. It is light from the sun—not heat—that is used. Each cell develops about half a volt of DC electrical potential. The maximum amperage of the cell is proportional to its surface area.


PHOTOVOLTAIC (PV) MODULE — A series string, typically of about 36 cells, put together to charge a 12 volt system. This is packaged in a weather-tight frame in a size that can typically be carried by one person.


PHOTOVOLTAIC (PV) ARRAY — Describes the whole group of PV modules in a system. These can be on a single rack or multiple racks. The term "subarray" is sometimes used to mean one rack of modules in a multiple-rack array.


PLANT FACTOR — Same as capacity factor.


PLENUM — A large duct used as an air distributor from a furnace.


PORTABLES — Any transportable, relocatable, portable, or mobile buildings.


POWER — The rate at which energy is produced and consumed. Power is usually measured in kilowatts (electrical power) and horsepower (mechanical power).


POWER FACTOR — The ratio of actual power being used in a circuit (expressed in watts or kilowatts) to the power which is apparently being drawn from the line (expressed in volt-amperes or kilovolt amperes). A low power factor means poor electrical efficiency and is always costly to the consumer.


POWER FACTOR CHARGE — A charge added to an electric bill to compensate the utility for the loss of useful power caused by a customer's equipment.


PRESENT VALUE — The present worth of a dollar saved or spent at a determined point of time in the future. The concept reflects the time value of money.


PROCESS ENERGY — Energy that is directly related to a manufacturing process. It can be estimated by various mathematical or graphical techniques.


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R-VALUE (RESISTANCE) — A measurement that indicates the difficulty that a unit of heat has in flowing through a given material (expressed in units of sq ft x hr x °F/Btu). The important thing to keep in mind is that R-value relates to specific material in a wall, not to the complete wall. The reciprocal of the sum of R-values for a composite barrier is the overall transmittance or U-value.


RADIANT ENERGY — Any energy that flows outward or radiates in all directions from a source, with characteristics of both particle motion and wave motion. Light is a visible form of radiant energy, but radio waves, X-rays, infrared, and ultraviolet rays are also examples.


RADIATION — The transfer of heat from one body to another by electromagnetic waves without heating the air between the bodies.


RATE STRUCTURE — The system of prices which historically have resulted in lower prices per unit consumption for larger users. Some people argue that rates should be flat (i.e., not decrease with amount used) or even inverted (i.e., increase with amount used).


REACTIVE POWER — The product of the reactive current and the operating voltage. The greater the reactive current, in proportion to the useful current, the greater the reactive power and the lower the power factor.


REACTOR — See nuclear reactor.


RECOVERED ENERGY — Utilized energy that would otherwise be lost.


RECYCLING — The process by which waste materials are transformed into new products in such a manner that the original products may lose their identity.


REFLECTANCE — The fraction of incident radiation that is reflected from a surface. For an opaque surface, the sum of absorptance and reflectance must be equal to unity. For a transparent surface, the sum of reflectance, absorptance and transmittance must equal unity.


REFRIGERANT — The fluid used for heat transfer in a refrigerating system. It absorbs heat at a low temperature and pressure and rejects heat at a higher temperature and pressure and usually changes state between liquid and gaseous phases.


REHEAT — The application of sensible heat to supply air that has been previously cooled below the temperature of the conditioned space by either mechanical refrigeration or the introduction of outdoor air to provide cooling.


REFRIGERATION, TON OF — Equivalent to the removal of heat at a rate of 200 Btu's/minute, 12,000 Btu's/hr or 288,000 Btu's/day.


RETROFIT — Alteration of an existing system (e.g., a house) so that it uses less energy to achieve the same function (e.g., space heating.)


RETURN GRILLES — Openings in the return duct work of an air-conditioning system through which used air is brought back to the primary unit to be filtered and heated or cooled. Removal of this stale air is necessary for the efficient distribution of newly conditioned air.


RETURN or SUCTION LINE — A pipe containing gaseous refrigerant from an evaporator to the inlet or suction of a compressor.


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SEASONAL PEAK — The maximum demand placed on the utility's capacity resulting from seasonal factors. Some utilities have summer peaks, some winter peaks, some both.


SECOND LAW OF THERMODYNAMICS — First formulated by the German physicist Rudolf Clausius, who stated, "It is important for a self-acting machine, unaided by any external agency, to convey heat from one body to another at a higher temperature". The essence of the law is this: Heat will not flow of its own accord from a cold place to a hot one.


SEER (Seasonal Energy Efficiency Ratio) — The most commonly used measure of the efficiency of consumer central air conditioning systems and heat pumps. Technically speaking, it is the air conditioning system or heat pump’s estimated seasonal cooling output in Btus divided by the amount of energy that it consumes by the system in Watt-hours. As of January 2006, a central air conditioner or heat pump must have a SEER of at least 13 to be sold in the United States. Higher efficiency models have a SEER of up to 21.


SENSIBLE HEAT — The heat which, when added or subtracted, causes a temperature change.


SETBACK — Reducing the level of heating from a system to the lowest practical point especially during periods when the activities or occupation patterns allow it


SIMPLE PAYBACK — The length of time required for an investment to pay for itself; determined by dividing the initial investment by first year energy savings.


SINGLE DUCT SYSTEM — A heating system which has the heating (and cooling) elements in a single air flow path. All spaces receive heated air from this main duct.


SINGLE ZONE SYSTEM — A single heating and/or cooling source and serves a single temperature control zone.


SOLAR ENERGY — Energy derived from the sun's radiation. This takes the form of direct (thermal radiation); indirect (e.g., wind, hydro, wood); and fossil (e.g., coal, oil, gas.)


SPACE CONDITIONING — The act of heating or cooling the air in the home to a comfortable level.


STEAM TRAP — A device for allowing the passage of condensate, or air or condensate, yet preventing the passage of steam.


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TECHNOLOGY — The machines and the materials made and used by people.


TEMPERATURE SETTING — A temperature reading, manually or automatically set in a thermostat, for temperature control.


THERM — A unit of heat energy, equal to 100,000 Btu's . There are 10.3 therms in 1,000 cubic feet of natural gas.


THERMAL BARRIER — A strip of nonconducting material, such as wood, vinyl, or foam rubber, separating the inside and outside surfaces to slow conduction of heat.


THERMAL POLLUTION — The process by which the temperature of a body of water is increased above its normal ecological reading by inflow(s) of water or other effluents from additional source.


THERMONUCLEAR ENERGY — Designation of, employment of, the heat energy released in nuclear fission.


THERMOSTAT — A unit that compares the surrounding air temperature with a set temperature, and turns the heat and/or cooling source on or off to maintain the set temperature.


TON — A weight of 2,240 lbs. (long ton) or 2,000 lbs. (short ton). The latter is applied to coal measurement in the U.S. In addition, it is a unit of measurement used by some international oil transport carriers.


TOTAL COST — The sum of market price plus non-market costs (e.g., environmental costs not included in the market price).


TRANSMITTANCE — The fraction of incident radiation that is transmitted through a surface.


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UNACCOUNTED ENERGY — It is not always possible to account for all of the energy that has been consumed when conducting an energy audit. The attempt is always made to keep unaccounted energy at a low value through successive refinements in the estimate or measurements made. (Also called parasitic loss.)


UNOCCUPIED HOURS — The time when a commercial, industrial, or institutional building is normally empty of people, except for a few attendants or maintenance personnel.


URANIUM (SYMBOL-U) — A radioactive element with the atomic number 92, and, as found in natural ores, an average atomic weight of approximately 238. The two principle natural isotopes are uranium-235 (0.7% of natural uranium), which is fissionable, and uranium-238 (99.3% of natural uranium), which is fertile (capable of being converted into fissionable material.) Natural uranium is the basic raw material of nuclear energy.


USEFUL LIFE — That period of time for which a modification used under specific conditions is able to fulfill its intended function and which does not exceed the period of remaining use of the building being modified.


U-VALUE (THERMAL TRANSMITTANCE) — The overall coefficient of heat transmission (air to air) expressed in Btu/sq ft/hr/°f. The "U"-value applies to combinations of different materials used in series along the heat path flow, including air spaces and surface air films on both sides. The lower the U-value, the less heat is transferred. Numerically equivalent to the reciprocal of the sum of the R-values of materials in combination.


UNIT HEATER — A heating element that is installed in a space to be heated, and uses a fan to direct room air over a heat exchanger that is heated by gas, oil, or other fuel fire, or by electrical resistance. The essential elements of a unit heater are: a fan, motor, heat exchanger, and an enclosure.


UNIT VENTILATOR — An assembly, usually mounted against a wall, that heats and/or cools a space and provides ventilation by drawing in outside air. The essential components of a unit ventilator, all enclosed in a housing, are: fan, motor, heating and/or cooling element, dampers, filters, an outlet grill. Outside air may be taken in or drawn directly through a grill that appears on the outside of the wall opposite the unit, or through cavities in the building walls.


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VARIABLE AIR VOLUME — A system which heats or cools a single air stream to a constant temperature. The temperature of each building space is maintained by controlling the amount of air delivered to it.


VAPOR BARRIER — A predominant insulation material, usually installed at the time of construction. It should go on the heated side of walls and floors to protect the insulation against condensation in winter. The three main types of vapor barrier use are 1) a polyethylene sheet; 2) asphalt-coated paper; and 3) aluminum foil.


VENTILATION — The forced introduction of air into a space by a controlled mechanical system or unit.


VOLT — Unit of electromotive force of a quantity of electricity, measured in volts.


VOLTAGE — The amount of electromotive force of a quantity of electricity, measured in volts.


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WASTE, RADIOACTIVE — Equipment and materials (from nuclear operations) which are radioactive and for which there is no further use.


WATT — The unit of measure for electric power. Wattage is calculated by multiplying voltage times current amps flowing through the circuit (one horsepower equals 746 watts.)


WEATHER SENSITIVE ENERGY — That part of the building's energy consumption, above the base load or constant portion that is required for the heating or cooling season. It can be estimated by various graphical or mathematical techniques.


WEATHER-STRIPPING — Metal, plastic or felt strips designed to seal spaces between windows and door frames to prevent infiltration.


WET BULB TEMPERATURE — The temperature at which liquid or solid water, through evaporation under constant pressure and temperature, brings the air to the saturation point. This is commonly used to calculate percent relative humidity.


WORK PLANE — Plane at which work is usually performed and at which illumination is specified and measured. Unless otherwise indicated, the work plane is assumed to be a horizontal plane 30 inches off the floor.


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ZONE — Section of a heating and/or cooling system separately controllable.


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