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Renewable Energy Terms & Phrases

Absorptance: The ratio of the radiation absorbed by a surface and the total energy falling on that surface measured as a percentage.

AC or Alternating Current: Alternating Current (AC) is the electrical power available from household outlets. In the United States AC current oscillates between plus 120 volts and -120 volts sixty times a second (60 hertz).

Acid Rain: Also called “acid precipitation” or “acid deposition,” acid rain is precipitation containing harmful amounts of nitric and sulfuric acids formed primarily by nitrogen oxides and sulfur oxides released into the atmosphere when fossil fuels are burned. It can be wet precipitation (rain, snow, or fog) or dry precipitation (absorbed gaseous and particulate matter, aerosol particles, or dust). Acid rain has a pH below 5.6. Normal rain has a pH of about 5.6, which is slightly acidic. (The pH value is a measure of acidity or alkalinity, ranging from 0 to 14. A pH measurement of 7 is regarded as neutral. Measurements below 7 indicate increased acidity, and those above 7 indicate increased alkalinity.)

Active Solar Heating: An active solar heating system uses a pump or a fan to move hot air or liquid heated in a solar collector to a storage tank or area of use.

Adobe: A sun-dried, unburned brick of clay (earth) and straw used in construction; in the United States, primarily in the Southwest.

Ambient temperature: Surrounding temperature,as around a house.

Ampere (amp) : An ampere is a measure of electrical current, the amount of electricity that flows in a line.

Amorphous Silicon: An alloy of silica and hydrogen, with a disordered, noncrystalline internal atomic arrangement, that can be deposited in thin-layers (a few micrometers in thickness) by a number of deposition methods to produce thin-film photovoltaic cells on glass, metal, or plastic substrates.

Angle of incidence: The angle that the sun’s rays make with a line perpendicular to a surface. The angle of incidence determines the percentage of direct sunshine intercepted by a surface. The sun’s rays that are perpendicular to a surface are said to be “normal” to that surface.

Annualized Growth Rates: Calculated as follows: (xn / x1) 1/n , where x is the value under consideration and n is the number of periods. Aquifer: A subsurface rock unit from which water can be produced.

Array: A group of collectors, modules, or panels.

ASHRAE: Abbreviation for the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Auxiliary system: A supplementary heating unit to provide heat when the primary unit cannot, usually occurring during periods of cloudiness or intense cold and the solar system cannot provide enough heat.

Availability Factor: A percentage representing the number of hours a generating unit is available to produce power (regardless of the amount of power) in a given period, compared to the number of hours in the period. Avoided Costs: The incremental costs of energy and/or capacity, except for the purchase from a qualifying facility, a utility would incur itself in the generation of the energy or its purchase from another source.

Azimuth: The angular distance between true south and the point on the horizon directly below the sun.

Ballast : A charging device in fluorescent lights which give a “jump start” to the gas inside the tube to make it start glowing steadily.

Battery: A source of electric power that has been stored as chemical energy in the battery. Batteries produce direct current when connected to an electrical circuit. A battery supplies power at a given voltage and the current depends on the load in the circuit.

Berm: A manmade mound or small hill of earth.

Biomass: Material produced by living organisms(wood, vegetation, manure, etc.).

Biomass fuels: Trees and forest residues, animal manure and waste, grains, crops and aquatic plants are some common sources of biomass fuels.

Black body: A theoretically perfect absorber of incident radiation with also the highest possible emittance.

Black Liquor: A byproduct of the paper production process that can be used as a source of energy.

BTU or British Thermal Unit: A measure of heat energy; the amount needed to raise the temperature of one pound of water by one degree Fahrenheit. One Btu equals 252 Calories.

Busbar Cost: The cost per kilowatthour to produce electricity, including the cost of capital, debt service, operation and maintenance, and fuel. The power plant “bus” or “busbar” is that point beyond the generator but prior to the voltage transformation point in the plant switchyard.

Calorie: The quantity of heat needed to raise the temperature of 12 grams of water 1°C, one kilo-calorie equals approximately 4 Btu’s.

Capacity Factor: The ratio of the electrical energy produced by a generating unit for the period of time considered to the electrical energy that could have been produced at continuous full-power operation during the same period. Capacity, Gross: The full-load continuous rating of a generator, prime mover, or other electric equipment under specified conditions as designated by the manufacturer. It is usually indicated on a nameplate attached to the equipment. Capital Cost: The cost of field development and plant construction and the equipment required for the generation of electricity.

Carbon Dioxide: Carbon Dioxide (CO2) is a colorless, odorless gas formed during combustion of fossil and other carbon based fuels. CO2 is also produced during organic decomposition and respiration. Plants use carbon dioxide during photosynthesis to produce sugars and Oxygen. Carbon dioxide is a greenhouse gas that lets light pass through and traps heat from re-radiating into space.

Cast Silicon: Crystalline silicon obtained by pouring pure molten silicon into a vertical mold and adjusting the temperature gradient along the mold volume during cooling to obtain slow, vertically-advancing crystallization of the silicon. The polycrystalline ingot thus formed is composed of large, relatively parallel, interlocking crystals. The cast ingots are sawed into wafers for further fabrication into photovoltaic cells. Cast-silicon wafers and ribbon-silicon sheets fabricated into cells are usually referred to as polycrystalline photovoltaic cells.

Caulking: Making an airtight seal by filling in cracks around windows and doors.

Circuit: electrical path.

Clerestory: A window that is placed vertically(or near vertically) in a wall above one’s line of vision to provide natural light into a building.

Climate Change (Greenhouse Effect): The increasing mean global surface temperature of the Earth caused by gases in the atmosphere (including carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons). The greenhouse effect allows solar radiation to penetrate the Earth’s atmosphere but absorbs the infrared radiation returning to space.

Coefficient of heat transmission, or U-value: The rate of heat loss in Btu per hour through a square foot of a wall or other building surface when the difference between indoor and outdoor air temperatures is 1°F.

Cogeneration: The production of electrical energy and another form of useful energy (such as heat or steam) through the sequential use of energy.

Collector Efficiency: The ratio of heat energy extracted from a collector to the solar energy striking the cover, expressed in percent.

Collector, solar: a device for capturing solar energy (usually solar thermal), ranging from ordinary windows to complex mechanical devices.

Combined Cycle: An electric generating technology in which electricity is produced from otherwise lost waste heat exiting from one or more gas (combustion) turbines. The exiting heat is routed to a conventional boiler or to a heat recovery steam generator for utilization by a steam turbine in the production of electricity. Such designs increase the efficiency of the electric generating unit. Concentrator: A reflective or refractive device that focuses incident insolation onto an area smaller than the reflective or refractive surface, resulting in increased insolation at the point of focus. Conductance(C): The quantity of heat (Btu’s) which will flow through one square foot of material in one hour, when there is a 1°F temperature difference between both surfaces. Conductance values are given for a specific thickness of material, not per inch of thickness. For homogeneous materials, such as concrete, dividing the conductivity (k) of the material by its thickness (x) gives the conductance (C).

Conduction: Transfer of heat through a quantity of material due to the molecular motion.

Conductivity(k): The quantity of heat (Btu’s) that will flow through one square foot of material, one inch think, in one hour, when there is a temperature difference of 1°F between its surface.

Convection: Motion in a fluid or plastic material due to some parts being buoyant because of their higher temperature. Convection is a means of transferring heat through mass flow rather than through simple thermal conduction.

Dead air space: A confined space of air. A dead air space tends to reduce both conduction and convection of heat. This fact is utilized in virtually all insulating material and systems, such as double glazing, Beadwall, fiberglass batts, rigid foam panels, fur and hair, and loose-fill insulations like pumice, vermiculite, rock wool and goose down.

DC or Direct Current: Direct Current (DC) is the flow of electricity through a circuit that doesn’t cyclically alternate in polarity. Batteries and solar cells are typical sources of direct current.

Degree-day(DD): cooling, see degree day for heating, except that the base temperature is established at 75°, and cooling degree days are measured above that base.

Degree-day(heating): An expression of climatic heating requirement expressed by the difference in degree F below the average outdoor temperature for each day and an established indoor temperature of 65°F. (The assumption behind selecting this base is that average construction will provide interior comfort when the exterior temperature is 65°F). The total number of degree-days over the heating season indicates the relative severity of the winter in that area.

Density(p): The mass of a substance which is expressed in pounds per cubic foot. Design Heat Load: The total heat loss from a house under the most severe winter conditions likely to occur.

Design temperature: A temperature close to the lowest expected for a location, used to determine the design heat load.

Daylighting: Allowing the sun to provide direct and non direct light to interior spaces.

Diffuse radiation: Radiation that has traveled an indirect path from the sun because it has been scattered by particles in the atmosphere, such as air molecules, dust and water vapor; indirect sunlight comes from the entire skydome.

Direct methods: Techniques of solar heating in which sunlight enters a house through the windows and is absorbed inside.

Direct Radiation Solar: Radiation that comes straight from the sun, casting shadows on a clear day.

Efficiency: Efficiency is the ratio of desired energy output to the energy input, often expressed in %. For example a typical solar cell has a 15% efficiency which means that 15% of the incident solar radiation on the solar cell is turned into electricity. In solar energy it pertains to the percentage of the solar energy incident on the face of the collector (glazing), that is used for space heating. In solar application this measure pertains to the percentage of the solar energy incident on the face of the collector(glazing), that is used for space heating.

Electricity: Electricity is the flow of electrical energy though a circuit.

Embodied Energy: The total amount of energy needed to manufacture a finished product from raw materials,including the energy used to transport the product.

Emission: The release or discharge of a substance into the environment; generally refers to the release of gases or particulates into the air.

Emissivity: The property of emitting heat radiation; possessed by all materials to a varying extent.“Emittance” is the numerical value of this property, expressed as a decimal fraction, for a particular material. Normal emittance is the value measured at 90° to the sample and hemispherical emittance is the total amount emitted in all directions. We are generally interested in hemispherical, rather than normal emittance. Emittance values range from .05 for brightly polished metals to 0.96 for flat black paint. Most non metals have high values of emittance.

Energy: The capacity to do work.

Energy Conservation: Wise use and careful management of energy resources by reducing wasteful energy use, using energy for a given purpose more efficiently, or reducing energy use altogether.

Energy Conversion: The process of changing one form of energy into another. For example, the chemical energy stored in gasoline can be converted into kinetic energy (energy of motion) by an automobile engine.

Energy Sources : There are many sources of energy. Among these are: 1. Renewables (solar, wind, geothermal,biomass, hydro, tides). 2. Fossil fuels (coal, oil, gas); 3. Nuclear (fission and fusion);

Equinox: Either of two times during a year when the sun crosses the celestial equator and when the length of day and night are approximately equal. These are the autumnal equinox on or about September 22 and the vernal equinox on or about March 22.

Eutectic salts: Salts used for storing heat. At a given temperature, salts melt, absorbing large amounts of heat which will be released as the salts freeze. Xample: Glauber’s salts. The melt-freeze temperatures vary with different salts and some occur at convenient temperatures for thermal storage such as in the range of 80° to 120°F.

Evacuated Tube: In a solar thermal collector, an absorber tube, which is contained in an evacuated glass cylinder, through which collector fluids flows. Flat Plate Pumped: A medium-temperature solar thermal collector that typically consists of a metal frame, glazing, absorbers (usually metal), and insulation and that uses a pump liquid as the heat-transfer medium: predominant use is in water heating applications.

Fluorescent light: The glow discharge of an electrified gas for illumination rather than the glow of an electrically heated conductive filament. More efficient because they produce the same amount of light (lumens) using less electrical energy than incandescent electric light bulbs. Florescent lights produce little heat, and my have reflectors to concentrate or direct the light output.

Fossil fuels: Fuels formed eons ago from decayed plants and animals. Oil, coal and natural gas are such fuels. Such devices are usually of the fluorescent type, which produce little heat, and may have reflectors to concentrate or direct the light output.

Fuel: A material which is consumed in the process of changing its chemically stored energy into heat or light.

Fuel cell: A battery like device which produces electricity more efficiently than normal combustion by having the fuel chemically react with an oxidizer at two separate electrical terminals to produce an electric current.

Fuel efficiency: The amount of useful obtained for the amount of fuel consumed. For automobiles, a fuel efficient car travels more miles per gallon of gasoline than a fuel inefficient car.

Generation (Electricity): The process of producing electric energy from other forms of energy; also, the amount of electric energy produced, expressed in watthours (Wh).

Geopressured: A type of geothermal resource occurring in deep basins in which the fluid is under very high pressure.

Geothermal: Pertaining to heat energy in the earth’s interior.

Geothermal energy: Heat generated by natural processes within the earth. Chief energy resources are hot dry rock, magma (molten rock), hydrothermal (water/steam from geysers and fissures) and geopressure (water saturated with methane under tremendous pressure at great depths).

Geothermal Plant: A plant in which a turbine is driven either from hot water or by natural steam that derives its energy from heat found in rocks or fluids at various depths beneath the surface of the earth. The fluids are extracted by drilling and/or pumping.

Glazing: A covering of transparent or translucent material (glass or plastic) used for admitting light.Glazing retards heat losses from reradiation and convection. Examples: windows, skylights, greenhouse and collector coverings.

Glazing, double: A sandwich of two separated layers of glass or plastic enclosing air to create and insulating barrier.

Global warming: The gradual warming of the earth due to buildup of greenhouse gases such as methane and carbon dioxide in the earth’s atmosphere:

Geyser: A special type of thermal spring that periodically ejects water with great force. Giga: One billion.

Greenhouse effect: Refers to the characteristic tendency of some transparent materials such as glass to transmit radiation shorter than about 2.5 microns and block radiation of longer wavelengths The greenhouse effect happens with visible light passes though a material and gas and the heat generated by the light can not pass through the material or gas. In cars and buildings the radiant energy is trapped by glass: in the earth’s atmosphere the radiant energy is trapped by gases such as carbon dioxide and methane.

Grid: The layout of an electrical distribution system. Groundwater: Water occurring in the subsurface zone where all spaces are filled with water under pressure greater than that of the atmosphere.

Heat: Heat is a form of energy related to molecular movement and hence the temperature of an object.Heat can be transferred by conduction, convection, or radiation.

Heat capacity (volumetric): The number of Btu’s a cubic foot of material can store with a one degree increase in its temperature.

Heat gain: An increase in the amount of contained in a space, resulting from direct solar radiation and the heat given off by people, lights, equipment, machinery and other sources.

Heat loss: a decrease in the amount of heat contained in a space resulting from heat flow through walls, windows, roof and other building envelope components.

High-Temperature Collector: A solar thermal collector designed to operate at a temperature of 180 degrees Fahrenheit or higher.

Horsepower: A unit of power. One horsepower equals 550 foot-pounds per second or 746 watts.

Hydrocarbon: An organic compound containing carbon and hydrogen. Large, heavy hydrocarbons may have up to 90 carbon atoms per molecule while small, light hydrocarbons may have only one to four carbon atoms per molecule. Different types and sizes of hydrocarbons molecules make up crude oil and natural gas.

Hydro: A prefix meaning produced by or derived from water or the movement of water, as in “hydroelectricity”.

Hydroelectric Power: Hydropower or electricity produced by the conversion of kinetic energy from flowing water using a generator.

Incident light: The light falling on a surface, such as sunlight shining on a solar panel.

Infiltration: the uncontrolled movement of outdoor air into the interior of a building through cracks around windows and doors or in walls, roofs and floors. This may work by cold air leaking in during the winter, or the reverse in the summer.

Insolation: The total amount of solar radiation direct, diffuse and reflected-striking a surface exposed to the sky. This incident solar radiation is measured in langleys per minute, or Btu’s per square foot per hour or per day.

Insulation: Materials or systems used to prevent loss or gain of heat, usually employing very small dead air spaces to limit conduction and/or convection.

Internal Collector Storage (ICS): A solar thermal collector in which incident solar radiation is absorbed by the storage medium.

Inverter: A device which changes direct current (DC) into alternating current (AC). Direct current is created by photovoltaic modules or batteries and is converted to AC through the use of an inverter.

Kilowatt: A unit of power equal to 1,000 watts.

Kilowatt-hour: An amount of energy that results from the steady production or consumption of one kilowatt of power for a period of one hour. A kilowatt-hour is a unit of energy equal to 3,413 Btu or 3,600,000 joules.

Light: The portion of the electromagnetic radiation that is visible to the eye. Light consists of many different wavelengths. This result in rainbows when the light is scattered by rain clouds or passes through crystal glass. Light from lasers consists of only one wavelength. Other portions of the electromagnetic spectrum are radio waves, infraredradiation, and ultraviolet radiation.

Liquid Collector: A medium-temperature solar thermal collector, employed predominantly in water heating, which uses pumped liquid as the heat-transfer medium.

Low-Temperature Collectors: Metallic or nonmetallic solar thermal collectors that generally operate at temperatures below 110 degrees Fahrenheit and use pumped liquid or air as the heat transfer medium. They usually contain no glazing and no insulation,and they are often made of plastic or rubber, although some are made of metal.

Lumen: A measurement of visible light.

Marginal Cost: The change in cost associated with a unit change in quantity supplied or produced.

Medium-Temperature Collectors: Solar thermal collectors designed to operate in the temperature range of 140 degrees to 180 degrees Fahrenheit, but that can also operate at a temperature as low as 110 degrees Fahrenheit. The collector typically consists of a metal frame, metal absorption panels with integral flow channels (attached tubing for liquid collectors or integral ducting for air collectors), and glazing and insulation on the sides and back.

Megawatt: A unit of power equal to one million watts or 1,000 kilowatts.

Module: a photovoltaic panel.Night time Insulation: Movable insulation placed on interior of windows to retain heat.

Methane: The most common gas formed in coal mines; a major component of natural gas.

N-layer: An N-layer is created by implanting atoms that have an extra valence electron into a crystalline structure, thus turning the material into a semiconductor. The excess valence electrons are free to move about the material as part of an electric current. This material is used in computer chips, transistors, and solar cells.

Nocturnal cooling: The cooling of a building by the radiation of the excess heat to the night sky.

Nonrenewable Energy Resource: Energy resource that is either replenished very slowly or not replenished at all by natural processes. A nonrenewable resource can ultimately be totally depleted or depleted to the point where it is too expensive to extract and process for human use. Fossil fuels are examples of nonrenewable energy resources.

Nonutility Generation: Electric generation by end-users, independent power producers, or small power producers under the Public Utility Regulatory Policies Act, to supply electric power for industrial, commercial, and military operations, or sales to electric utilities.

Ohm: A unit quantifying electrical resistance.

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. Ozone: Three-atom oxygen compound (O3) found in two layers of the Earth’s atmosphere. One layer of beneficial ozone occurs at 7 to 18 miles above the surface and shields the Earth from ultraviolet light. Several holes in this protective layer have been documented by scientists. Ozone also concentrates at the surface as a result of reactions between byproducts of fossil fuel combustion and sunlight, having harmful health effects.

Parabolic Dish: A high-temperature (above 180 degrees Fahrenheit) solar thermal concentrator, generally bowl-shaped, with two-axis tracking.

Parabolic Trough: A high-temperature (above 180 degrees Fahrenheit) solar thermal concentrator with the capacity for tracking the sun using one axis of rotation. Particulates: Visible air pollutants consisting of particles appearing in smoke or mist. Passive Solar: A system in which solar energy alone is used for the transfer of thermal energy. Pumps, blowers, or other heat transfer devices that use energy other than solar are not used. Peak Watt: A manufacturer’s unit indicating the amount of power a photovoltaic cell or module will produce at standard test conditions (normally 1,000 watts per square meter and 25 degrees Celsius). Photovoltaic Cell: An electronic device consisting of layers of semiconductor materials fabricated to form a junction (adjacent layers of materials with different electronic characteristics) and electrical contacts and being capable of converting incident light directly into electricity (direct current). Photovoltaic Module: An integrated assembly of interconnected photovoltaic cells designed to deliver a selected level of working voltage and current at its output terminals, packaged for protection against environment degradation, and suited for incorporation in photovoltaic power systems. Pollution: Any substances in water, soil, or air that degrade the natural quality of the environment, offend the senses of sight, taste, and smell, and/or cause a health hazard. The usefulness of a natural resource is usually impaired by the presence of pollutants and contaminants.

Passive Solar Heating: Windows on south wall to allow sun to hit thermal mass and retain heat.

Parabolic collector: A polished dish or u-shaped collector that focuses solar thermal energy on a tube or a point (as in heat-troughs and solar cookers).

Parallel Circuit: An electrical circuit in which each component of the circuit (e.g., a set of light bulbs) is connected across a voltage source (e.g., a battery) so that an electric current flows through each component along a separate path.

PV or Photovoltaic: Pertaining to the production of electricity from light.

P-layer: A P-layer is created by implanting atoms that are short one valence electron into a crystalline structure, thus turning the material into a semiconductor. The implanted atoms in the P-Lay are looking for electrons to complete bonds with neighboring atoms. This absence of an electron is called a hole and “holes” move about the material much like positive electrons. This material is used in computer chips, transistors, and solar cells.

P-N Junction: A P-N Junction is created when a Player is brought into contact with an N-layer, the free electrons in the N-layer drift over to fill the holes in the P-layer. This creates an electric field because of the imbalance of charges on one side as compare to the other. In a solar cell, when light from the sun knocks an electron from orbit around an atom in the P-N junction region, the electric field helps separate the resulting electron and hole, and this is how solar cells produce electricity.

Quadrillion Btu: Equivalent to 10 to the 15th power Btu.

Radiation: The flow of energy across open space via electronic waves, such as visible light. Radiation can be atomic size or smaller particles, or electromagnetic radiation emitted from a source. Light from the sun, radio waves, and gamma rays are all electromagnetic radiation. Emission of alpha particles, electrons, and other particles from radioactive materials is also call radiation.

Radiation, infrared: Electromagnetic radiation, whether from the sun or a warm body, that has wavelengths longer than the red end of the visible spectrum(greater than .75 microns). We experience infrared radiation as heat; 49% of the radiation emitted by the sun is in the infrared band.

Reflected radiation: Sunlight that is reflected from surrounding trees, terrain or buildings onto a surface exposed to the sky.

Reflectance: The ratio or percentage of the amount of light reflected by a surface to the amount incident. The remainder that is not reflected is either absorbed by the material or transmitted through it. Good light reflectors are not necessarily good heat reflectors.

Renewable Energy: Energy from sources that can be renewed and used on a sustainable basis: sunshine, water flow, wind, waves, and vegetation.

Renewable Energy Devices: Solar collectors, wind turbines, hydroelectric turbines, and other devices that produce energy from a sustainable source. Certain renewable resources will always be available no matter how they are used. (e.g., solar energy), while other renewable resources can be depleted when their rate of use exceeds their rate of replacement (e.g., wood).

Resistance (R): R is the reciprocal of conductivity or x/k (x equals thickness of the material in inches).

Retrofitting: Installing solar water heating and/or cooling systems in existing buildings not originally designed for re the purpose.

R-factor: A unit of thermal resistance used for comparing insulating values of different materials; the reciprocal of the conductivity; the higher the Rfactor of a material, the greater its insulating properties.

Seasonal efficiency: The ratio of solar energy collected and used to that which strikes the collector, over an entire heating season.

Selective surface: An absorber coating that absorbs most of the sunlight hitting it but emits very little thermal radiation.

Semiconductor: A substance that is normally an insulator but is changed into a conductor by implanting (or doping) with atoms with a different number of valence electrons. Substances like silicon and gallium arsenide can be made into semiconductors.

Series: When batteries or solar cells are linked together so that the positive terminal of one is connected to the negative terminal of the next. This is also used when there is one electrical path in an electric circuit.

Shading coefficient: The ration of the solar heat gain through a specific glazing system to the total solar heat gain through a single layer of clear, double strength glass.

Shading mask: A section of a circle that is characteristic of a particular shading device,superimposed on a circular sun path diagram to determine the time of day and the months of the year when a window will be shadowed by the device.

Silicon: A semiconductor material made from silica, purified for photovoltaic applications. Single Crystal Silicon (Czochralski): An extremely pure form of crystalline silicon produced by the Czochralski method of dipping a single crystal seed into a pool of molten silicon under high vacuum conditions and slowly withdrawing a solidifying single crystal boule rod of silicon. The boule is sawed into thin wafers and fabricated into single-crystal photovoltaic cells. Smog: Air pollution associated with oxidants.

Skydome (sky vault): The visible hemisphere of sky, above the horizon, in all directions.

Skytherm ™ system: A form of movable insulation and a roof pond system developed by Harold Hay. The system involves motor-driven sliding insulation panels.

Skylight: A clear or translucent panel set into a roof to admit sunlight into a building.

Solar altitude: The angle of the sun above the horizon measured in a vertical plane.

Solar cell: Device often made of semiconductor materials that generates electricity when exposed to the sun or other radiant source. Sometimes referred to as a photovoltaic cell. Solar cells are similar to batteries except they use solar energy instead of chemical energy to produce the power.

Solar collector: A device or structure used to collect solar energy. This term is most often used for a solar water heating panel that is enclosed in a box with a glass or plastic cover. The term is sometime applied to systems from parabolic troughs to solar greenhouses.

Solar constant: The amount of radiation or heat energy that reaches the outside of the earth’s atmosphere.

Solar cooling: The use of devices that utilize heat from solar collectors to drive cooling systems.

Solar energy: Radiant energy from the sun, which can be converted into other forms of energy, such as heat or electricity.

Solar electricity: Electricity produced using solar energy technologies.

Solar greenhouse: A conventional greenhouse in which mass is added for heat storage, double glazing is used, and the north side is attached to a house or berm.

Solar Heater: A water or space heating system that uses the sun’s energy to produce heat.

Solar heating : Processes, active or passive, that derive heat directly from the sun.

Solar house or Solar tempered house: A dwelling that obtains a large part of its heat from the sun.

Solar radiation: Electromagnetic radiation emitted by the sun.

Solar process heat: The use of sunlight to produce heat to drive industrial processes.

Solar thermal energy systems: Systems using concentrating collectors to focus the sun’s radiant energy onto or into receivers to produce heat.

Solar Window: Openings that are designed or placed primarily to admit solar energy into a space.

Specific heat (Cp): The number of Btu’s required to raise the temperature of one pound of a substance 1°F in temperature.

Stand-Alone system : A photovoltaic (PV) installation not connected to a utility power line. A ‘direct system’ uses the PV-produced electricity as it is produced, e.g. a solar-powered water-pumping station. A ‘battery storage system’ stores the excess PV-produced electricity for use a later time, e.g. at night or on cloudy days.

Sun path diagram: A circular projection of the sky vault, similar to a map, that can be used to determine solar positions and to calculate shading.

Thermal capacity: The quantity of heat needed to warm a collector up to its operating temperature.

Thermolcirculation: The convective circulation of fluid which occurs when warm fluid rises and is displaced by denser cooler fluid in the same system.

Thermosiphon System: A solar collector system for water heating in which circulation of the collection fluid through the storage loop is provided solely by the temperature and density difference between the hot and cold fluids.

Thermal Energy: The total internal kinetic and potential energy of an object due to the random motion of its atoms and molecules. An object that feels hot has more thermal energy inside it than it does after it has cooled down. Although technically incorrect, the word “heat” is often used to mean thermal energy.

Thermal mass: The amount of potential heat storage capacity available in a given assembly or system. Drum walls, concrete floors and adobe walls are examples of thermal mass.

Thermal radiation: Electromagnetic radiation emitted by a warm body.

Thermosiphoning: An event where heated water in a solar collector becomes lighter and rises to the top and cooler water, that is heavier, sinks to the bottom. Some systems use thermosiphoning to transfer hot water from the collector to the storage container.

Time lag: The period of time between the absorption of solar radiation by a material and its release into space. Time lag is an important consideration in sizing a thermal storage wall or Trombe wall.

Tilt angle: The angle that a collector surface forms with the horizontal.

Tracker, flat-plate : A device upon which one or more photovoltaic panels are mounted that continually rotate the panels at the sun throughout the day.

Translucent: the quality of transmitting light but causing sufficient diffusion to eliminate perception of distinct images.

Transmittance: The ratio of the radiant energy transmitted through a substance to the total radiant energy incident on its surface. In solar technology, it is always affected by the thickness and composition of the glass cover plates on a collector, and to major extent by the angle of incidence between the sun’s rays and a line normal to the surface.

Transmission System (Electric): An interconnected group of electric transmission lines and associated equipment for moving or transferring electric energy in bulk between points of supply and points at which it is transformed for delivery over the distribution system lines to consumers, or is delivered to other electric systems.

Turbine: A machine for generating rotary mechanical power from the energy of a stream of fluid (such as water, steam, or hot gas). Turbines convert the kinetic energy of fluids to mechanical energy through the principles of impulse and reaction, or a mixture of the two.

U Value (coefficient of heat transfer): The number of Btu’s that flow through one square foot of roof, wall or floor in one hour, when there is a 1°F difference in temperature between the inside and outside air, under steady state conditions. The U value is the reciprocal of the resistance or R-factor.

Utility-Interactive System : A photovoltaic (PV) installation connected to the utility power grid. This is also referred to as a grid-tied system.

Vapor barrier: A component of construction which is impervious to the flow of moisture and air and is used to prevent condensation in walls and other locations of insulation.

Vapor-Dominated Geothermal System: A conceptual model of a hydrothermal system where steam pervades the rock and is the pressure-controlling fluid phase

Volt: A unit of electromotive force. It is electrical pressure analogous to pressure in a garden hose or a bicycle tire. It is also called potential or potential difference.

Watt (Electric): The electrical unit of power. The rate of energy transfer equivalent to 1 ampere of electric current flowing under a pressure of 1 volt at unity power factor. Watt (Thermal): A unit of power in the metric system, expressed in terms of energy per second, equal to the work done at a rate of 1 joule per second. Watthour (Wh): The electrical energy unit of measure equal to 1 watt of power supplied to, or taken from, an electric circuit steadily for 1 hour. Wheeling: The use of the transmission facilities of one system to transmit power and energy by agreement of, and for, another system with a corresponding wheeling charge, e.g., the transmission of electricity for compensation over a system that is received from one system and delivered to another system).

Wind machine: an electricity driven rotor that moves air to protect orchards from frost.

Windmill: a wind-driven rotor that converts wind energy into mechanical energy (as in water pumping, or grinding grain).

Wind turbine or Generator: Device powered by the wind that produces mechanical or electrical power. A generator specifically designed to convert the kinetic energy in wind into electrical energy (electricity).