Absorber The surface of a collector (normally black) that absorbs the solar radiation and converts it to heat energy.
Absorptance (α) The ratio of solar energy absorbed by a surface to the solar energy striking it. Energy not absorbed is transmitted or reflected.
Acceleration The time rate of change of velocity.
Active solar system A solar heating or cooling system that requires external mechanical power to move the collected heat.
AFU Annual fuel utilization efficiency is a measure of how efficient a furnace is as a ratio of heat output compared to the total energy consumed.
AGW Anthropogenic global warning.
Altitude The angle of the sun’s position in the sky with respect to the earth’s horizontal.
Ambient temperature Temperature of the surroundings (i.e., for collectors, outdoor temperature).
Angle of incidence The angle measured between an incoming beam of radiation and a line drawn perpendicular to the surface that it strikes.
Anthropogenic Caused or produced by humans.
Aperture, solar The effective radiant energy collection area of a solar collector.
ASHRAE The American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc.
Azimuth The angular distance between true south and the point on the horizon directly below the sun.
Black body emitter An ideal body, which absorbs all radiation falling upon it and emits nothing.
British thermal unit (BTU) The quantity of heat energy needed to raise the temperature of one pound of water to 1 °F.
Calorie The quantity of heat energy needed to raise the temperature of 1 g of water to 1 °C.
CEC Protocol for testing PTC testing standards adopted by the California Energy Commission. Reference PTC.
Centrifugal pump A type of pump, which has blades that rotate and whirl the fluid around so that it acquires sufficient momentum to discharge from the pump body.
Closed loop Any loop in a system that is not exposed to the atmosphere.
Coefficient A number that serves as a measure of some property or characteristic.
Collector Any of a wide variety of solar devices used to collect radiant energy and convert it to heat or electricity.
Collector efficiency The ratio of the heat energy or electricity extracted from a collector to the solar energy striking the cover, expressed in percent.
Collector tilt The angle between the horizontal plane and the solar collector plane.
Concentrating collector A device that concentrates the sun’s rays on an absorber surface, which is significantly smaller than the overall collector area.
Conductance See Thermal conductance.
Conduction The transfer of heat energy through a material by the motion of adjacent atoms and molecules.
Conductivity The ease with which heat will flow through a material as determined by its physical characteristics.
Convection, forced Heat transfer through moving currents of air or liquid induced mechanically by a pump or blower in order to increase mass flow rates and velocities to yield a maximum heat transfer.
Convection, natural Heat transfer through moving currents of air or liquid as a result of thermal gradients and resulting density differences creating the necessary mass flow to promote heat transfer.
Corrosion Deterioration of metal by the chemical action of a fluid or components of a fluid.
CPI Consumer Price Index – An index of the change in the price of consumer goods and services from one base period to another.
Declination The angle between the plane of the earth’s orbit and the equatorial plane.
Demand load Domestic water heating or electricity needs to be supplied by solar or conventional energy.
Density Weight per unit or mass per unit volume.
Design life The period of time for which a solar energy system is expected to perform its intended function without requiring major maintenance or replacement.
DHW Abbreviation for Domestic Hot Water. Also referred to as SHW (Solar Hot Water) and Solar Water Heating (SWH).
Diffuse radiation Solar radiation that has been scattered by clouds and particles in the atmosphere and casts no shadow. Flat plate collector can absorb it but concentrating collectors cannot.
Direct-beam radiation Solar energy received at the earth’s surface uninterrupted by particles in the atmosphere and casting shadows on a clear day.
Ecliptic plane The plane of the earth’s orbit extended to meet the celestial sphere.
Effectiveness See Heat exchanger effectiveness.
Efficiency The ratio of the useful energy supplied by a system (output) to the energy supplied to the system (input).
Elastomer Any of various elastic substances resembling rubber.
Electron An elementary particle consisting of a charge of negative electricity.
Emissivity (ε) A measure of the thermal energy reradiated from a solar collector surface as a fraction of the energy which would be radiated by a totally black body surface at the same temperature.
Energy Defined as the ability to do work. A conserved quantity, which is neither created nor destroyed. It can, however, be converted from one form to another or interconverted with matter according to Einstein’s equation, E = mc2 where m is mass and c is the speed of light.
Equinox The point of intersection of the ecliptic and celestial equator when the declination is zero.
External manifold A distribution pipe that runs outside the collector housing and connects to the inside header of each absorber plate.
Fill-drain assembly Comprised of two boiler drains and a check valve; installed for the filling and draining of transfer fluids in a closed loop freeze resistant system.
Flash point The temperature at which fluid vapors will flashover if an ignition source is present.
Flat plate collector A solar collector that converts sunlight to heat or electricity on a plane surface without the aid of reflecting surfaces to concentrate the sun’s rays.
Fluid Any substance, gas, or liquid, used to capture heat in the collector and transport the energy from the point of collection to storage or direct use.
Flux Magnetic field lines.
Fossil fuels Combustible substances of organic origin established in past geologic ages consisting of hydrocarbons formed from the decay of vegetation under heat and pressure (coal, oil, natural gas).
Fusion The union of atomic nuclei to form heavier nuclei resulting in the release of enormous quantities of energy when certain light elements unite.
Galvanic corrosion Material degradation caused by an electrochemical reaction between two or more different metals in a system, which are not properly isolated from one another.
Galvanic series Metals ranked from electrically positive to electrically negative to provide a relative measure of “corrodability” of each metal when used in a multimetal system.
Gauge pressure That pressure measured above atmospheric pressure.
Generic Relating to or characteristic of an entire class.
GHG Greenhouse gas
Glazing A transparent/translucent sheet of glass/fiberglass that reduces heat loss from a solar collector and traps the thermal energy.
Greenhouse effect A retention of solar radiation in a solar panel whereby a cover glazing traps a layer of still air next to the absorber plate and reduces the convection heat loss. In terms of environmental considerations, it is a phenomenon whereby the earth’s atmosphere traps the solar radiation by the presence of gases in the atmosphere such as carbon dioxide, water vapor, and methane, allowing incoming sunlight to pass through but absorbing heat radiated back from the earth’s surface.
Grid An interconnected system for the distribution of electricity over a wide area through a network of high-tension cables and power stations.
Head For pumping considerations, the vertical rise to the highest point of a piping system.
Header The pipe that runs across the top or bottom of an absorber plate, gathering or distributing the heat transfer fluid to or from the risers that run across the absorber surface. (Also called manifolds if connected internal to each collector housing.)
Heat The sum total of all molecular energy of a body; a vector quantity.
Heat exchanger A device that transfers heat from one substance to another without mixing the two.
Heat exchanger effectiveness The ratio of the actual rate of heat transfer to the theoretical maximum rate of heat transfer.
Heat transfer medium Air or liquid that is heated and used to transmit energy from its point of collection to its point of storage and/or end use.
Hydrostatic Relating to liquids at rest or to the pressure they exert or transmit.
Inflation A persistent increase in the level of consumer prices or persistent decline in the purchasing power of money.
Infrared radiation Electromagnetic radiation from the sun that has wavelengths slightly longer than visible light, not visible to the naked eye.
Insolation The total amount of solar energy received at the earth’s surface at any location and time (BTU/ft2-h).
Insulation A material with a high thermal resistance (R) to heat flow.
Intercept The distance from the origin to a point where a graph crosses a coordinate axis.
Internal manifold A distribution pipe that connects the headers of the collectors internally and in turn becomes the header itself.
IPCC United Nations, Intergovernmental Panel on Climate Change.
Isogonic chart A chart depicting magnetic compass deviations from true south.
Index of refraction The measure of the bending of a ray of light when passing from one medium to another (ratio of the speed of light in a vacuum divided by the speed of light in a medium).
Inverter An electrical power converter that changes direct current to alternating current.
Kilowatt-hour (KWh) The amount of energy equivalent to1 kilowatt of power being used for 1 h (3413 BTU).
Kinetic energy Energy of motion.
Langley A unit of measure of insolation named for the American astronomer Samuel P. Langley (3.687 BTU/ft2).
Latitude Referring to a point on the earth as determined by an angle formed by a line intersecting the center of the earth to a particular point on the its surface and the plane cutting the earth at the equator.
Liquid-type collector A collector that uses a liquid as the heat transfer fluid.
Longitude Referring to a point on the earth as determined by an angle formed by the intersection of a line from the center of the earth to a particular point on the earth’s surface and the plane cutting vertically through the center of the earth.
Manifold See Header.
Mercator projection A graphical depiction of altitude and azimuth onto a flat map for each variation of latitude. Used for plotting obstacles, which might block energy collection in the “solar window”.
Miscible Capable of being mixed.
NABCEP North American Board of Certified Energy Practitioners.
Net metering A renewable energy incentive allowing the energy produced from photovoltaic arrays to be fed into the utility power grid so that retail credit is received for a portion of the electricity generated.
Nonselective surface An absorber coating that absorbs most of the incident sunlight but which emits a high level of thermal radiation in return. Typically, it is a flat-black paint.
Normalized curve Conformed to a standardized reference base.
NREL National Renewable Energy Laboratory previously known as the Solar Energy Research Institute.
Opaque Impervious to forms of radiant energy other than visible light.
Open loop Any loop in a system that is vented to the atmosphere.
Ordinate The vertical coordinate of a point in a two-dimensional plane, parallel to the y-axis.
Orientation Number of degrees to the east or west of south that a solar collection surface faces.
Overall coefficient of transmittance (U-value) The reciprocal value of the sum of thermal resistances (BTU/hr-ft2-°F). This value is the combined thermal conduction value of all the materials in a cross section including air spaces and air films. The lower the U-value, the higher the insulating value.
Passive solar system A system that uses gravity, heat flows, evaporation, or other naturally occurring phenomena without the use of external mechanical devices to transfer the collected energy (i.e., south facing windows).
Payback The time at which the initial cost and annual operating and maintenance expenses of a solar energy system equal the total savings generated by the system when compared with conventional energy sources. Both systems costs are computed at compounded interest rates of inflation for the same amount of energy generated.
Peak sun hours The equivalent number of hours per day when solar insolation averages 1 KW/m2.
Percentage of possible sunlight The percentage of daytime hours during which there is enough direct solar radiation to cast a shadow.
pH Measure of solution acidity.
Photochemical The effect of radiant energy in producing chemical changes.
Photon A quantum of electromagnetic radiation that has zero rest mass and an energy of Plank’s constant times the frequency of the radiation. Photons are generated in collisions between nuclei or electrons and in any other process in which an electrically charged particle changes its momentum. Photons can also be absorbed by any charged particle.
Photovoltaic Concerning the generation of an electromotive force when radiant energy falls on the boundary between dissimilar substances.
Photovoltaic cell An electrical device that converts light energy directly into electricity.
Pitch The ratio of vertical rise to horizontal span where rise is the distance from the attic floor to roof peak and span is the width of the house.
Plank’s constant A universal constant of nature relating the energy of a quantum of radiation to the frequency of the oscillator from which it was emitted. Its numerical value is 6.6260755 × 10−27 erg-seconds = 6.6260755 × 10−34 joules per hertz.
Potable water Water free from impurities present in amounts sufficient to cause disease or harmful physiological effects and conforming in its bacteriological and chemical quality to the requirements of the Public Health Service Drinking Water Standards or the regulations of the public health authority having jurisdiction.
Pound The basic unit of force in the English system of measure, defined as the force that gives a standard pound (0.4535924277 kg) an acceleration equal to the standard acceleration of earth’s gravity, which is 32.174 ft/s2.
Pressure The ratio of force per unit area.
Pressure drop The loss in static pressure through a component such as a heat exchanger, length of pipe, or duct, which may include fittings such as elbows or the combined losses throughout the entire length of fluid flow travel. Normally stated in terms of pounds per square inch (psi).
Proton An elementary particle consisting of a charge of positive electricity.
PTC Photovoltaic industry protocol called the “Photovoltaics for Utility Scale Applications” Test Conditions known as PVUSA Test Conditions or PTC used for testing comparisons of PV modules.
Radiant energy The flow of energy across open space via electromagnetic waves (i.e., visible light).
Reflectivity The ratio of the radiant energy reflected from a surface, to the radiant energy incident upon that surface.
Refraction The bending of light or sound waves when passing from one medium to another of different optical density.
Reradiation Radiation resulting from the emission of previously absorbed radiation.
Risers The flow channels or pipes that distribute the heat transfer liquid from the headers across the surface of an absorber plate.
R-value See Thermal resistance.
Selective surface A surface that absorbs radiation of one wavelength (i.e., visible light) but emits little radiation of another wavelength (i.e., infrared), thereby reducing heat loss.
Sky vault The entire projection of the sun’s path at any particular latitude. (See Sun path diagram)
Slope The ratio of vertical rise to horizontal run where rise is the distance from the attic floor to roof peak and the run is the horizontal distance from the roof peak to the end of the roof section.
Solar constant The average amount of solar radiation reaching the earth’s outer atmosphere (436.5 BTU/ft2-hr; ±3.5%)
Solar noon The instant of time the sun’s position is true south (azimuth is 0°) and the altitude is a maximum for the day.
Solar radiation (solar energy) Electromagnetic radiation emitted by the sun. The visible part of this spectrum ranges from long red to short violet wavelengths.
Solar window An outline of an area in the sky for a particular latitude through which a maximum amount of direct solar radiation reaches the collectors during any particular time of year and day.
Solstice The time at which the sun reaches its greatest declination, north or south.
SPCAF Single Payment Compound Amount Factor.
Specific heat The quantity of heat, in BTUs, needed to raise the temperature of one pound of material to 1 °F (BTU/lb-°F).
Spectral distribution An energy curve or graph that shows the variation of radiant energy in relation to wavelengths.
SRCC Solar Rating and Certification Corporation—An independent third party certification organization that administers national rating programs for solar energy equipment providing a means to compare thermal performance of solar DHW collectors.
Stagnation A no-flow condition.
Standby heat loss Heat lost through storage tank and piping walls under no flow conditions.
STC Standard Test Conditions—A manufacturer’s testing protocol whereas PTC or CEC protocols are preferred.
Sun path diagram (solar window) A circular projection of the sky vault, similar to a map, which can be used to determine solar position and to calculate shading.
Temperature An indicator of the intensity or degree of heat stored in a body; a scalar quantity.
Temperature gradient A change in temperature in a specific direction.
Thermal conductance (C) A property of a material equal to the quantity of heat per unit time that will pass through a unit area of the material when a unit average temperature is established between the surfaces (BTU/hr-ft2-°F).
Thermal conductivity (K) A measure of the ability of a material to permit the flow of heat (BTU-in/hr-ft2-°F).
Thermal inertia The tendency of a large mass to remain at the same temperature or to fluctuate only very slowly when acted upon by external sources.
Thermal resistance (R-value) A measure of the ability of a material to resist the flow of heat; the higher the R-value, the greater the insulating value of the material (hr-ft2-°F/BTU).
Thermistor A sensing device that changes its electrical resistance with changes in temperature. Used with differential controllers and control monitors to supply collector and storage tank temperature information.
Thermoelectric Involving relations between temperature and the electrical condition in a metal or in contacting metals.
Thermionic Dealing with electrically charged particles emitted by an incandescent substance.
Thermosiphon The natural convection of heat through a fluid that occurs when a warm fluid rises and cool fluid sinks under the influence of gravity.
Tilt angle See Collector tilt.
Toxic fluids Gases or liquids that are poisonous, irritating, and/or suffocating, as classified in the Hazardous Substances Act, Code of Federal Regulations, Title 16, Part 1500.
Translucent Admitting and diffusing light so that objects beyond cannot be distinguished clearly.
Transmissivity (τ) The ratio of solar energy passed through a surface to the radiation striking it. Energy not transmitted is either absorbed and/or reflected.
Transparent Having the ability to transmit light without appreciable scattering so that objects beyond are entirely visible.
Tropic of Cancer The latitude denoting the most northerly position of the sun in which the declination angle is +23.5°.
Tropic of Capricorn The latitude denoting the most southerly position of the sun in which the declination angle is −23.5°.
Ultraviolet radiation Electromagnetic radiation with wavelengths shorter than visible light.
U-value See Overall coefficient of transmittance.
Viscosity The readiness with which a fluid flows when acted upon by an external force (g/cm-s).
Wavelength The distance between the start and finish of an energy pulse.