Solar Resource Glossary
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Absolute Cavity Radiometer — An instrument used for very accurate measurements of solar irradiance. Absolute cavity radiometers absorb radiation on a blackened conical receiver and are electrically self-calibrating. Absolute cavity radiometers determine the solar constant and provide the reference from which other radiometers are calibrated.
Absolute Humidity — The mass (in grams) of water in a volume (cubic meter) of air; units are g/m3.
Absorption — When the substance of interest is captured by another substance, reducing the amount available. For example, solar energy is absorbed by some atmospheric molecules, solar collectors, and the ocean.
Aerosol — Excluding weather and clouds, any small particle that tends to stay in the air, such as smoke, dust, salt, and pollen.
Aerosol Optical Depth — (Technically known as the relative aerosol optical depth) is the "extinction per unit path length due to aerosols alone." The typical means of computing this is to account for the extinction of as many of the other constituents as possible (water vapor, ozone, mixed gases, and "equivalent extinction" represented by Rayliegh scattering of atmospheric molecules) and what is left over is the aerosol extinction. The transmission T and "optical depth" t of an atmospheric constituent are related as follows:
T = Exp(-t*m) , or t = - ln(T)/m,
where m is the airmass, or equivalent path length through the atmosphere.
In simplest terms, for Io = extraterrestrial radiation, and I = radiation at surface
I = Io * ( Tw*To*Tg*Tr*Ta)
where Tw = transmittance of water, To of ozone, Tg of gases, Tr Rayleigh effects, Ta aerosols
Strictly speaking, both extinction coefficients and transmission are a function of wavelength, so optical depths are defined at specific wavelengths (spectral optical depth) or total (integrated over all wavelengths). The total optical depth is sometimes called the "opacity."
AES — Atmospheric Environment Service, the Canadian equivalent of USA's National Oceanic and Atmospheric Administration. AES operates the solar measurement network for Canada. They are also the equivalent of USA's National Climatic Data Center in that they respond to requests for weather data and maintain the data archives.
AHF — Automatic Hickey-Frieden absolute cavity radiometer. This is the model designation given by The Eppley Laboratory, Inc. for their commercial version of an electrically self-calibrating absolute cavity radiometer used to define and transfer the World Radiometric Reference (WRR) to pyrheliometers and pyranometers used for solar irradiance measurements. The WRR is maintained at the World Radiation Center, Davos, Switzerland for the World Meteorological Organization.
Airmass — The relative path length of the direct solar beam radiance through the atmosphere. When the sun is directly above a sea-level location, the path length is defined as airmass 1 (AM 1.0). AM 1.0 is not synonymous with solar noon because the sun is usually not directly overhead at solar noon in most seasons and locations. When the angle of the sun from zenith (directly overhead) increases, the airmass increases approximately by the secant of the zenith angle. A better calculation [Kasten, F. and A. T. Young (1989). Revised Optical Air Mass Tables and Approximation Formula. Applied Optics 28 (22), 4735-4738] follows:
m = 1.0 / [ cos(Z) + 0.50572 * (96.07995 - Z)-1.6364]
where Z is the solar zenith angle.
Albedo — The fraction of solar radiation that is reflected. The solar energy community defines albedo as the fraction of solar radiation that is reflected from the ground, ground cover, and bodies of water on the surface of the earth. Astronomers and meteorologists include reflectance by clouds and air. To reduce confusion, some solar researchers use the term ground reflectance.
Algorithm — The set of simple instructions that combine to accomplish a task. Computer codes are algorithms.
Ambient Temperature — Air temperature measured with a thermometer, similar to dry-bulb temperature.
Anemometer — An instrument that measures wind speed.
Angle of Incidence — The angle that a ray (of solar energy, for example) makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.
Angular Response Characterization — Quantifying the effects of radiance incidence angle on pyranometer measurement performance. If a pyranometer is rotated while a beam of light is shined upon it, it will record the maximum energy when it is directly facing the beam, and the energy will fall to zero when it is sideways to (or facing away from) the beam. A graph of the energy reported by the pyranometer as the angle it makes with the beam of light should look like the cosine of the angle, if the instrument were perfect. Pyranometers have imperfections that keep them from producing this curve. The determination of the true behavior of the pyranometer as the angle it makes with the light beam changes is called angular response characterization
ASHRAE — The American Society of Heating, Refrigerating, and Air-Conditioning Engineers.
Atmosphere — The zone of air that surrounds a planet.
Atmospheric Pressure — The pressure (force per area) created by the weight of the atmosphere. At higher elevations, the atmospheric pressure is lower because there is less air.
Atmospheric Turbidity — Haziness in the atmosphere due to aerosols such as dust (particles ranging from 0.1 to 1+ microns in diameter). If turbidity is zero, the sky has no dust. A sun photometer is used to measure atmospheric turbidity.
Attenuation — Loss of a substance as it is deflected, fragmented, or absorbed. For example, solar irradiance attenuates as it passes through the atmosphere to the surface of the earth.
Azimuth Angle — The angle between the horizontal direction (of the sun, for example) and a reference direction (usually north, although some solar scientists measure the solar azimuth angle from due south).
Barometer — An instrument that measures atmospheric pressure.
Barometric Pressure — The pressure (force per area) created by the weight of the atmosphere, measured by a barometer. At higher elevations, the atmospheric pressure is lower because there is less air.
Beam Radiation — Synonym for direct normal irradiance, the amount of solar radiation from the direction of the sun.
Bias Limit — An indication of the average deviation of the predicted, or true values, from the measured values. Typically expressed as twice the mean bias error (MBE):
yi is the ith predicted, or true value
xi is the ith measured value
N is the number of observations.
Bidirectional Reflectance — A term for the amount of reflected radiation compared to the amount of incident radiation, or albedo, of a surface. The surface is not perfectly specular. That is, the reflected intensity is not at the same angle with respect to the surface normal as the incoming rays, nor are the two intensities necessarily equal. (In crude terms, when light bounces off the Earth, some of it is absorbed and the rest of it bounces "funny," not as it would off a mirror).
Biomass — Plants, crops, and trees; converted by solar fuel technologies into fuels and byproducts.
Bird Clear Sky Model — Named after Dr. Richard Bird, a scientist at NREL, this physical model uses properties of the atmosphere such as albedo, turbidity, and precipitable water to determine the amount of solar radiation striking the earth's surface from a cloudless sky.
Blackbody — The theoretical "perfect" absorber of light at all wavelengths. As blackbodies heat up, they emit a characteristic double-exponential light frequency (energy) curve, which is imperfectly seen in nature.
Bolometer — The most sensitive thermometer known to science. Invented in 1880 by astronomer Samuel P. Langley, the bolometer is used to measure light from the faintest stars and the sun's heat rays. It consists of a fine wire connected to an electric circuit. When radiation falls on the wire, it becomes very slightly warmer. This increases the electrical resistance of the wire. The difference in conductivity is proportional to the incident irradiance.
BORCAL — Broadband Outdoor Radiometer Calibration. A method of calibrating pyrheliometers and pyranometers based on the summation technique at the Solar Radiation Research Laboratory. Up to three days of clear-sky solar irradiance measurements taken at 30-second intervals from sunrise to sunset are used to compute the individual radiometer responsivities.
For pyrheliometer calibrations, the reference direct normal irradiance is measured with an electrically self-calibrating absolute cavity radiometer traceable to the World Radiometric Reference. The individual pyrheliometer responsivity is computed as the mean ratio of the signal from the pyrheliometer (microvolts DC) to the reference irradiance (watts per square meter) for each of the 30-second data samples.
Pyranometer responsivities are computed from the ratio of the signal from each pyranometer (microvolts DC) to the reference global horizontal irradiance determined by the direct normal irradiance (measured with an absolute cavity radiometer) and the simultaneous diffuse horizontal irradiance (measured by a reference pyranometer placed under a solar-tracking shading disk):
Global Horizontal = Direct Normal x cos(Z) + Diffuse Horizontal
Z = Solar Zenith Angle at the time of measurement.
The reference pyranometer is calibrated prior to use in a BORCAL event by means of the Shade Calibration Technique.
Bright Sunshine — When the sun casts an obvious shadow or when a Campbell-Stokes sunshine recorder is recording. The lower limit for bright sunshine (based on a Campbell-Stokes recorder) is between 70 W/m2 (very dry air) and 280 W/m2 (very humid air).
Broadband Solar Irradiance — Theoretically the solar radiation arriving at the earth from all frequencies or wavelengths, in practice limited to the spectral range of radiometers, typically from 300 nm to 3000 nm wavelength. Meteorologists refer to this band as short-wave radiation.
BSRN — The worldwide Baseline Surface Radiation Network or the program that manages it.
Btu — British thermal unit, the amount of energy required to raise one pound of water one degree Fahrenheit at 60°F, equivalent to 1055 joules or 252.1 calories.
Calibration — The process of comparing an instrument's output signal with reality. Instruments that measure solar energy tend to "drift;" that is, their output signals do not mean the same thing from one time period to another. Because of this, they are periodically (annually or semi-annually) re-calibrated against more reliable instruments.
Calorie — (cal) The amount of energy required to raise the temperature of one gram of water one degree Celsius at 50°C, or 4.1855 joules. Note that the large calorie (Cal) is the amount of energy required to raise one kilogram of water one degree Celsius; that is, 1 Cal = 1000 cal. To confuse matters, dietary Calories are almost never capitalized as they should be and are often mistaken for calories.
Calorimeter — An instrument that measures heat.
Campbell-Stokes Sunshine Recorder — A clear glass sphere that focuses the sun's rays onto a special strip chart, producing a charred path when there is bright sunshine. The length of the path determines the bright sunshine duration. The lower limit for bright sunshine (based on a Campbell-Stokes recorder) is between 70 W/m2 (very dry air) and 280 W/m2 (very humid air).
Celsius Scale — The metric temperature scale for which 0°C is the temperature at which water freezes and 100°C is the temperature at which water boils, at standard atmospheric pressure. The conversion from Fahrenheit to Celsius is:
C = ( F - 32 ) / 1.8.
Central Receiver System — A solar power generator that uses a series of tracking mirrors (heliostats) or a paraboloid (three-dimensional parabola, or dish) of mirrors to focus solar energy onto a single central receiver such as a boiler, engine, or photovoltaic array.
Circumsolar Radiation — The amount of solar radiation coming from a circle in the sky centered on the sun's disk and having a radius of between 2.5 and 3.5 degrees, depending on the type of instrument being used to measure beam radiation (direct normal irradiance).
Climate — The typical or expected (average) weather pattern, as opposed to the actual weather at any given instant.
Cloud Amount — The fraction of the sky dome covered by clouds. This fraction is typically expressed either as tenths (1/10, ..., 10/10) or eighths (1/8, ..., 8/8).
Cloud Cover — The fraction of the sky dome covered by clouds. This fraction is typically expressed either as tenths (1/10, ..., 10/10) or eighths (1/8, ..., 8/8). Some researchers refer to this as cloud amount, to clarify the distinction from cloud type, which is the nature of the cloud cover.
Cloud Type — The type of clouds (e.g., altostratus, cumulonimbus) that form each layer of the sky dome.
CMDL — Climate Monitoring and Diagnostics Laboratory, one of several research laboratories forming the Environmental Research Laboratories within the National Oceanic and Atmospheric Administration (under the U.S. Department of Commerce).
Collector — A device that receives solar energy and converts it to useful energy forms.
Collector Fluid — The working fluid in a collector that uses the heat of solar energy. These collectors focus the sun's rays on central containers or tubes that contain a fluid that is heated and then used to heat air or water or to power motors or turbines.
Concentrating Parabolic Trough — A system that tracks the path of the sun by pivoting on one axis (typically east-west or north-south), using shiny parabolic troughs to heat the collector fluid that passes through a tube at the focus.
Concentrator — A collector that enhances solar energy by focusing it onto a smaller area through mirrored surfaces or lenses.
Cooling Degree-Days — The amount of air conditioning needed, created by adding up all temperature differences of the form (daily temperature in °F – 65°F) for each day in which the temperature exceeds 65°F.
Cosine Response — The effects of radiance incidence angle on pyranometer measurement performance. If a pyranometer is rotated while a beam of light is shined upon it, it will record the maximum energy when it is directly facing the beam, and the energy will fall to zero when it is sideways to (or facing away from) the beam. A graph of the energy reported by the pyranometer as the angle it makes with the beam of light should look like the cosine of the angle, if the instrument were perfect. Pyranometers have imperfections that keep them from producing this curve.
Dewpoint — The temperature at which the water in the atmosphere will condense as drops on a surface.
Diffuse Horizontal Irradiance — Synonym for diffuse sky radiation.
Diffuse Sky Radiation — The radiation component that strikes a point from the sky, excluding circumsolar radiation. In the absence of atmosphere, there should be almost no diffuse sky radiation. High values are produced by an unclear atmosphere or reflections from clouds.
Direct Normal Irradiance — Synonym for beam radiation, the amount of solar radiation from the direction of the sun.
Diurnal — Daily, or the daily cycle. A diurnal plot is usually a representative midnight-to-midnight graph of values measured at a smaller time interval (e.g., hourly or 5-minute values).
DOE — In this context, always refers to the United States Department of Energy, although other departments may have the same acronym.
Dry-bulb Temperature — Air temperature measured with a thermometer, similar to ambient temperature. The term "dry-bulb" distinguishes it from the wet-bulb temperature measured by a psychrometer to determine relative humidity.
Electromagnetic Radiation — The energy produced by an oscillating electrical (and magnetic) field, transmitted by photons. See Electromagnetic Spectrum.
Electromagnetic Spectrum — The entire energy range of electromagnetic radiation specified by frequency, wavelength, or photon energy. The low end of the spectrum is infrared radiation (heat) and passes through the colors of visual light from red through violet, through ultraviolet radiation, x-rays, and gamma rays. Radio and television are transmitted on specific electromagnetic frequencies.
Emissivity — The ratio of the actual amount of electromagnetic radiation emitted by an object to the amount emitted by an ideal blackbody at the same temperature.
Energy — The ability to do work. Some units of energy, such as foot-pounds, measure the ability to lift a weight a certain height. Other units, such as the calorie, indicate the ability to increase temperature, while units of radiation are usually the frequencies or wavelengths of photons.
Environment Canada — Environment Canada is the federal coordinating agency for all environmental issues pertaining to Canada.
EPRI — The Electric Power Research Institute, a research consortium of electric power companies in the United States.
Equation of Time — The annual east-west swing of the location of the sun, which can be detected by noting the position of the sun at the same time (such as noon) each day. This motion is caused by the libration (wobble) of the Earth and can be estimated by [Spencer, J. W. (1971). Fourier Series Representation of the Position of the Sun. Search 2 (5), 172]:
ET = 229.18 x ( 0.000075 + 0.001868 cos D - 0.032077 sin D -
0.014615 cos 2D - 0.040849 sin 2D )
D = nD ( 360° / 365 )
and nD is the number of the day (e.g., Feb. 1 makes nD = 32).
NREL uses solar position algorithms that do not require the equation of time [Michalsky, J. J. (1988). The Astronomical Almanac's Algorithm for Approximate Solar Position (1950–2050). Solar Energy 40 (3), 227-235] .
Equinox — Literally "equal night," a day when the number of hours of daylight equals the number of hours of night. The vernal equinox, usually March 21, signals the onset of spring, while the autumnal equinox, usually Sept. 21, signals the onset of autumn.
Erg — A metric unit of energy (dyne-cm). A joule is 10,000,000 ergs.
ERSATZ — Literally "inferior substitute," the 222 measurement sites in the 1952–1975 SOLMET/ERSATZ solar and meteorological hourly network that did not measure solar radiation. The solar radiation for these sites was modeled from cloud cover data and other information. The SOLMET/ERSATZ network has been replaced by the 1961–1990 National Solar Radiation Database.
ETR — Extraterrestrial radiation, also known as "top-of-atmosphere" irradiance, is the amount of global horizontal radiation that a location on Earth would receive if there was no atmosphere or clouds (i.e., in outer space). This number is used as the reference amount against which actual solar energy measurements are compared.
Evaporation — The process of converting a substance (such as water) from its liquid phase to its gaseous phase.
Extraterrestrial Radiation — Abbreviated ETR, also known as "top of atmosphere" irradiance, this is the amount of global horizontal radiation that a location on Earth would receive if there was no atmosphere or clouds (i.e., in outer space). This number is used as the reference amount against which actual solar energy measurements are compared.
Fahrenheit Scale — The English temperature scale for which 32°C is the temperature at which water freezes and 212°C is the temperature at which water boils, at standard atmospheric pressure. The conversion from Celsius to Fahrenheit is:
F = 1.8 C + 32.
F-CHART — A solar power system analysis and design program. For more information, see the F-CHART home page.
Fixed-Tilt Array — A set (array) of solar power collectors that do not pivot to follow the track of the sun in the sky. In the Northern Hemisphere, they are usually mounted with a southern tilt that will maximize the amount of energy that they can receive.
Flat-Plate Collector — A solar power collector that absorbs the sun's energy on a flat surface without concentrating or refocusing it.
Flux — The rate at which a substance flows. The watt is a unit of energy flux because it indicates the amount of energy (in joules) that flows every second.
Focusing Collector — A collector that enhances solar energy by focusing it onto a smaller area through mirrored surfaces or lenses.
Gigawatt — 1,000,000,000 (or 109) watts. This unit rose to public prominence in the 1985 motion picture Back to the Future, in which 1.21 gigawatts of power were required for the time-travelling vehicle.
Global Horizontal Radiation — Also called Global Horizontal Irradiance; total solar radiation; the sum of Direct Normal Irradiance (DNI), Diffuse Horizontal Irradiance (DHI), and ground-reflected radiation; however, because ground-reflected radiation is usually insignificant compared to direct and diffuse, for all practical purposes global radiation is said to be the sum of direct and diffuse radiation only:
GHI = DHI + DNI * cos (Z)
Z is the solar zenith angle.
Greenhouse Effect — The warming of the Earth by the atmosphere because of water vapor and gases such as carbon dioxide, which absorb and emit infrared radiation, or heat. Thus, the high-energy photons such as light and ultraviolet radiation are passed through the atmosphere to the Earth, which tends to absorb them and emit lower-energy photons, which are then captured in the atmosphere and partially sent back to Earth. As the presence of infrared absorbers rises in the atmosphere, the more solar energy is retained as heat in the atmosphere and on the surface of the Earth. Because glass also passes light and tends to absorb and reflect heat, this effect is compared to that of a greenhouse.
Ground-Reflected Radiation — The radiation from the sun that is reflected back into the atmosphere after striking the Earth.
Heating Degree-Days — The amount of building heating needed, created by adding up all temperature differences of the form (65°F – daily temperature in °F) for each day in which the temperature falls below 65°F.
Heliostat — A large flat mirror, usually on a tracker so that it can continuously reflect the sun's rays onto a central receiver. A typical central receiver system requires hundreds of heliostats.
Humidity — The amount of water vapor in the air. Because the common measure of water vapor is the ratio between the measured amount and the maximum possible amount (the saturation point at which water condenses as dew), humidity and relative humidity are generally used interchangeably.
Hybrid System — A heating system or energy generator that does not rely on one exclusive energy source. For example, many domestic photovoltaic systems use gasoline generators as backup systems during periods of extreme cloudiness. Utility-scale hybrid solar thermal power plants use sunlight to boil water and an alternative energy source such as natural gas when there is insufficient sunlight.
IEA — The International Energy Agency, an autonomous agency linked to the Organisation for Economic Co-operation and Development.
IEEE — The Institute of Electrical and Electronics Engineers Inc.
Illuminance — Solar radiation in the visible region of the solar spectrum to which the human eye responds.
Incident Angle — The angle that a ray (of solar energy, for example) makes with a line perpendicular to the surface. For example, a surface that directly faces the sun has a solar angle of incidence of zero, but if the surface is parallel to the sun (for example, sunrise striking a horizontal rooftop), the angle of incidence is 90°.
Incident Radiation — Incoming radiation; i.e., radiation that strikes a surface.
Infrared Radiation — Radiation with wavelengths greater than those of the visible light (at about 8000 Angstroms or 800 nanometers) but shorter than those of microwaves (at about 1,000,000 Angstroms or 800,000 nanometers). Infrared radiation is associated with heat energy.
Insolation — Solar radiation on the surface of the Earth. This term has been generally replaced by solar irradiance because of the confusion of the word with insulation.
Inversion — Typically, a temperature inversion, or a zone in the atmosphere in which the temperature increases with altitude, instead of the expected decrease. In general, an inversion is any reversal of the normal trend of the property of an atmospheric substance with respect to altitude.
Interferometer — An instrument for determining the spectral distribution of irradiance. A light interferometer divides a beam of light into two or more beams and brings the beams back together. The recombined beams shine on a screen or another object like a detector surface. The resulting interference fringes can be used to determine the spectral nature of light.
Irradiance — The rate at which radiant energy arrives at a specific area of surface during a specific time interval. This is known as radiant flux density. A typical unit is W/m2.
Joule — A metric energy unit (Newton-meter) equal to approximately 0.2389 calories.
Kelvin — The temperature unit based on the theoretical minimum temperature, used in many physical calculations. The relationship between Kelvin and Celsius is:
K = C + 273.16.
KD — The ratio between diffuse sky radiation and extraterrestrial radiation, used as a normalization of the diffuse sky radiation because KD removes the effect of low sun angle and reduces the scale of values to between 0 and 1.
KT = KN + KD
KN — The ratio between direct normal irradiance and tracking extraterrestrial radiation, used as a normalization of the direct normal irradiance because KN removes the effect of low sun angle and reduces the scale of values to between 0 and 1.
KT = KN + KD
KT — The ratio between global horizontal radiation and extraterrestrial radiation, used as a normalization of the global horizontal radiation because KT removes the effect of low sun angle and reduces the scale of values to between 0 and 1.
KT = KN + KD
kW — Kilowatt, or 1000 watts, a unit of power.
kWh — Kilowatt-hour, a unit of energy equivalent to (1000 = kilo) x (3600 seconds) = 3,600,000 Joules, or 3.6 MJ; the amount of energy a kilowatt source produces in one hour
Langley — Named for Samuel P. Langley (1834–1906), a pioneering solar energy researcher at the Smithsonian Institution, this unit of radiant flux is one calorie per square centimeter (cal/cm2).
Latitude — The angular distance from the equator to the pole. The equator is 0°, the North Pole is 90° north, and the South Pole is 90° south.
Light — Usually the visual portion of the electromagnetic spectrum between infrared (about 8000 Angstroms or 800 nanometers) and ultraviolet (about 4,000 Angstroms or 400 nanometers); however, the term is sometimes used as a synonym for all electromagnetic radiation.
Line-Focusing Concentrator — A system that tracks the path of the sun by pivoting on one axis (typically east-west or north-south), using shiny parabolic troughs to heat the collector fluid that passes through a tube at the focus. It derives its name from the fact that solar radiation is focused on a line instead of on a point.
Local Apparent Time — The time of day based strictly on the longitude of the locality and not on "blocky" time zones. For example, when it is 12:00 Pacific Standard Time (USA) (assumed to be 120° west longitude), it is 11:51 Local Apparent Time in Seattle, Washington (USA), at 122° 18' west longitude.
Local Standard Time — The time of day based on the longitude of the zone meridian associated with a locality. In the USA, the zone meridians are known by the following names:
|75° West||Eastern Standard Time|
|90° West||Central Standard Time|
|105° West||Mountain Standard Time|
|120° West||Western Standard Time|
|135° West||Alaska Standard Time|
|150° West||Hawaii Standard Time|
Longitude — The east-west angular distance of a locality from the Prime Meridian. The Prime Meridian is the location of the Greenwich Observatory in England and all points north and south of it.
Longwave Radiation — Infrared radiation, radiation with wavelengths greater than those of the visible light (at about 8000 Angstroms or 800 nanometers) but shorter than those of microwaves (at about 1,000,000 Angstroms or 800,000 nanometers). Longwave radiation is associated with heat energy.
Lux — The International Standard unit of measure for luminous flux density at a surface. One Lux equals one lumen per square meter.
Macroclimate — The general climate of a large region such as the Rocky Mountains or the Northern Great Plains.
Megawatt— A unit of power equal to 1,000,000 (106) watts.
Measurement Uncertainty — The bounds that should be placed on a measured value because of uncertainties in the measurement. If there are several factors pertaining to the measurement, such as voltage bias and temperature bias and precision of measurement scale, the total measurement uncertainty can be difficult to calculate and may be larger than the largest individual uncertainty of any one factor depending on the sensitivity of the measurement to the significant factors. There is no such thing as a perfect measurement, although some measurements are so precise that errors are negligible. Solar irradiance measurements are notoriously unreliable with the best methods (1% to 3% uncertainty, which means that an "excellent" method can produce results that may differ as much as 50 W/m2) and can become worthless (10% to 30% uncertainty) with careless methods.
Mesoclimate — The climate that is peculiar to a small natural feature such as a hill or a small lake. This climate tends to be different from the general climate of the region in predictable ways. Statements such as "it always rains more in Hunter's Glen in the spring" or "it snows more at the airport than downtown" are statements about mesoclimates.
Meteorology — The study of the atmosphere.
METSTAT — The METeorological/STATistical solar irradiance model developed to produce the 1961–1990 National Solar Radiation Data Base. METSTAT uses meteorological inputs such as cloud cover and precipitable water to produce a nominal value of solar irradiance and randomly varies this value via statistical tables derived from measured solar irradiance data. METSTAT produces hourly solar irradiance data sets that are intended to behave like observed data, although the randomizer tends to guarantee that there will not be an hour-by-hour match of measured data to METSTAT-derived data.
Microclimate — The local climate near the ground that is peculiar to a small area (usually, the radius is less than a kilometer and can be as small as a centimeter). A microclimate region is defined by changes in behavior of the atmosphere's surface boundary layer and not by obvious physical features.
Mie Scattering — The scattering of solar radiation by (mathematically spherical) particles in the atmosphere that have an approximate size of the wavelength of light, analyzed by Gustav Mie. While Rayleigh scattering explains the blue sky, Mie scattering explains why wet, coastal skies are whiter than dry, mountainous skies.
Minutes of Sunshine — A specific instance of bright sunshine duration, the number of minutes per hour during which the sun casts an obvious shadow or when a Campbell-Stokes sunshine recorder is recording, usually above 210 W/m2.
Model — A way to represent a system for the purposes of reproducing, simplifying, analyzing, or understanding it. A standard representation is the computer model, but models can be made of any substance such as clay, paper, abstract mathematics, or concepts.
Nadir — Straight down (toward the center of the Earth).
NCDC — The United States National Climatic Data Center.
NIP — A Normal Incident Pyrheliometer, used to determine the amount of solar irradiance emitted from the direction of the sun. These thermopile-based radiometers have a uniform spectral response from 280 nm to 2800 nm and a 5.7° field of view.
NOAA — The United States National Oceanic and Atmospheric Administration.
Non-Renewable Energy Source -— There is no formal definition for this term. Typical usage defines it as any energy source that is used faster than it is replenished. Standard examples are petroleum products, natural gas, coal, and fissionable materials; the replenishment rates of these energy sources are immensely slow, making it likely that they will become depleted at some time.
Normal Radiation — Radiation striking a surface that is facing the sun. Mathematically, the word normal is the vector (direction) that is perpendicular to a surface, and the direction of a normal radiation source is perpendicular to a radiation source. Global (total) normal solar irradiance is all radiation that strikes a flat surface that faces the sun, while direct normal solar irradiance excludes all radiation that does not come from the direction of the sun in the sky.
NREL — The National Renewable Energy Laboratory, a U. S. national laboratory specializing in developing technologies and procedures for using renewable energy sources.
NSRDB — The 1961–1990 >National Solar Radiation Database, which supplies hourly solar and meteorological data from 239 locations in the United States and its territories.
Optical Depth — (Technically known as the relative aerosol optical depth) usually considered to be synonymous with the airmass, is the approximate number of aerosols in a path through the atmosphere relative to the standard number of aerosols in a vertical path through a clean, dry atmosphere at sea level.
Orientation — The direction that a solar energy collector faces. The two components of orientation are the tilt angle (the angle the collector makes from the horizontal) and the aspect angle (the angle the collector makes from north).
Ozone Layer — The layer in the atmosphere with the most ozone, usually at an altitude of 25 km. Ozone is created from oxygen by ultraviolet radiation bombardment. Because ozone tends to absorb and block ultraviolet radiation, a substantial ozone layer reduces the risk of skin cancer.
Parabolic Collector Trough — A system that tracks the path of the sun by pivoting on one axis (typically east-west or north-south), using shiny parabolic troughs to heat the collector fluid that passes through a tube at the focus.
Passive Solar — Technology for using sunlight to light and heat buildings directly, with no circulating fluid or energy conversion system.
Peak Watt — The maximum amount of power produced or demanded in a time interval, measured in watts.
Percent Possible Sunshine — The ratio of measured bright sunshine to the total possible bright sunshine in a given time period such as an hour or a day, expressed as a percent.
Photoelectric — Pertaining to the conversion of light (radiant energy) to electricity.
Photometer — An instrument that measures illuminance.
Photon — The fundamental particle or quantum of electromagnetic radiation (radiant energy).
Photovoltaic — Technology for converting sunlight directly into electricity, usually with photovoltaic cells.
Photovoltaic Cell — A single semiconducting element of small size (for example, 1 cm2) that absorbs light or other bands of the electromagnetic spectrum and emits electricity.
Photovoltaic Module — A unit composed of several photovoltaic cells that is the principal unit of photovoltaic array. A photovoltaic module's size is on the order of 1 m2, although its size is governed by convenience and application.
Photovoltaic Array — A photovoltaic module or set of modules used for converting solar radiation to energy.
PMOD/WRC — The Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center, at Davos, Switzerland. PMOD/WRC determines and maintains worldwide standards for measurement of solar radiation, including the World Radiometric Reference (WRR), for the World Meteorological Organization.
Point-Focusing Concentrator — A solar power generator that uses a series of tracking mirrors (heliostats), Fresnel lenses, or a paraboloid (three-dimensional parabola, or dish) of mirrors to focus solar energy onto a single central receiver such as a boiler, engine, or photovoltaic array.
Polarimeter — An instrument for determining the amount of polarization of light. Ordinary light from the sun or a lamp is composed of disorderly waves that vibrate in all directions perpendicular to the light beam. But polarized light consists of orderly waves that vibrate in only one direction.
Power — The amount of work or energy expended in a given amount of time. For example, the watt is a unit of power, which is defined as a joule per second.
Precipitable Water — The amount of water in a vertical column of atmosphere. The unit of measure is typically the depth to which the water would fill the vertical column if it were condensed to a liquid. For example, 6 centimeters of precipitable water (in the absence of clouds) indicates a very moist atmosphere. Precipitable water is often used as a synonym for water vapor.
PSP — Precision spectral pyranometer, a pyranometer with an outer clear dome that can be replaced by colored domes that transmit specific bandwidths of the solar spectrum. The PSP is a specific instrument of Eppley Laboratory Inc.
Pyranometer — An instrument with a hemispherical field of view, used for measuring total or global solar radiation, specifically global horizontal radiation; a pyranometer with a shadow band or shading disk blocking the direct beam measures the diffuse sky radiation, as is illustrated in the picture below.
Pyrheliometer — Instrument with a narrow (circumsolar) field of view that measures direct normal irradiance. Pyrheliometers are mounted on sun-following trackers so that the instrument is always aimed at the sun.
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Radiation — Synonymous with electromagnetic radiation, or the energy produced by an oscillating electrical (and magnetic) field, transmitted by photons. See Electromagnetic Spectrum.
Radiometer — An instrument that measures radiance, or the radiation emitted by an object.
Radiosonde — An instrument package that moves through the atmosphere, usually attached to a balloon, and transmits data over a radio frequency.
Raman Scattering — Sir Chandrasekhara Venkata Raman (1888–1970) was the Indian physicist who discovered that when a beam of light passes through a liquid or a gas, it is scattered, and the frequency of some of the scattered light is changed. The amount of change is a function of the scattering particles and the wavelengths of light.
Random Error — The difference between the actual and the desired quantity that varies randomly; that is, if a probability distribution of differences is produced, it is the Gaussian error function.
Rayleigh Scattering — The scattering of solar radiation by (mathematically spherical) particles in the atmosphere that are much smaller than the wavelength of light, analyzed by Lord Rayleigh. Rayleigh scattering explains the blue sky.
RCC — Radiometer Calibration & Characterization (RCC) software is used to automate the BORCAL process. The RCC controls all data acquisition from the reference radiometers and those under calibration, displays several color-coded fields representing the present sky condition and instrument performance, builds an instrument calibration database, and generates the final calibration report.
Renewable Energy — There is no formal definition for this term. Typical usage defines it as any energy source that is replenished at least as fast as it is used. Standard examples are solar, wind, hydroelectric, and biomass products.
Receiver — A device that receives solar energy and converts it to useful energy forms.
Reflectance — The fraction or percent of a particular frequency or wavelength of electromagnetic radiation that is reflected from the surface of a substance without being absorbed or transmitted.
Refraction — The bending of electromagnetic radiation by its passage through a medium of a high refractive index. Light is refracted by passing through a lens, water, or the atmosphere.
Relative Humidity — The amount of water vapor in the air expressed as the ratio between the measured amount and the maximum possible amount (the saturation point at which water condenses as dew).
Remote Sensing — The determination of a quantity by detecting it from a distance. A common application of remote sensing is the use of satellite-borne instruments to determine the location and amount of resources on the surface of the Earth.
Re-Radiation — The re-emission of electromagnetic radiation that had been previously absorbed by a substance.
Rotating Shadow Band Radiometer — An instrument that determines total solar radiation and diffuse sky radiation by periodically shading the total sky sensor from the sun with a rotating shadow band. Once every minute, the curved black shadowband rotates 180° to obscure the sun for a few seconds and then returns to its resting position.
Saturated Air — Air that has the maximum amount of water vapor; any increase in water vapor will cause condensation.
Scattered Radiation — Radiation that has been reflected from particles, disrupting the original direction of the beam.
Semiconductor — A material that has much lower resistance to the flow of electrical current in one direction than in another. Diodes, transistors, and many photovoltaic cells contain semiconductive materials.
SERI — The Solar Energy Research Institute, which became the National Renewable Energy Laboratory in 1991.
Shading Disk — A disk on a tracking arm that blocks the direct normal irradiance so as to allow a pyranometer to measure only the diffuse sky radiation.
Shadow Band — A metal strip that blocks the direct normal radiation so as to allow a pyranometer to measure only the diffuse sky radiation.
Shortwave Radiation — The principal portion of the solar spectrum that spans from approximately 300 nanometers (nm) to 4000 nm in the electromagnetic spectrum. Longwave radiation is infrared radiation (>4000 nm).
Silicon Sensor — A photovoltaic cell that is being used to measure solar irradiance. Because its spectral response is not as exact as that of thermopile instruments, it has a higher uncertainty.
Sky Dome — Refers to the appearance of the entire sky, from horizon to zenith in all directions.
Sky Radiation — Synonymous with diffuse sky radiation, the radiation component that strikes a point from the sky, excluding circumsolar radiation. In the absence of atmosphere, there should be almost no sky radiation. High values are produced by an unclear atmosphere or reflections from clouds.
Solar Absorber — A sheet of material—usually copper, aluminum, or steel—that forms the surface of a solar collector. It collects and retains solar radiation, which is passed to a heat transfer medium.
Solar Cell — A photovoltaic cell that is used to convert solar energy into electricity.
Solar Collector — A device that receives solar energy and converts it to useful energy forms.
Solar Concentrator — A solar collector that enhances solar energy by focusing it onto a smaller area through mirrored surfaces or lenses.
Solar Constant — Although not strictly constant, this number is the amount of solar power flux that passes through the mean Earth orbit. The currently accepted value is 1366 W/m2. Note that Earth-based instruments record lower values of solar power flux because of atmospheric attenuation.
Solar Conversion Technologies — Collective name for all methods for converting the sun's energy into usable energy.
Solar Detoxification — Technology for using concentrated sunlight to break down and destroy hazardous waste.
Solar Electric — Technology for converting sunlight directly into electricity.
Solar Energy Technology — Method for harnessing, storing, and using the sun's energy.
Solar Fuel Technology — Methods for converting biomass into fuels and byproducts.
Solar Heat — Technology to harness the sun's energy for heating buildings, air, and water for industrial and household uses.
Solar Irradiance — The amount of solar energy that arrives at a specific area of a surface during a specific time interval (radiant flux density). A typical unit is W/m2.
Solar Noon — The time at which the position of the sun is at its highest elevation in the sky. At this time, the sun is either due south (typically in the Northern Hemisphere) or due north (typically in the Southern Hemisphere). This time can be quite different from noon according to local standard time.
Solar Radiation — The electromagnetic radiation emitted by the sun.
Solar Spectrum — The electromagnetic spectral distribution emitted by the sun or received by a collector or instrument on Earth.
Solar Thermal Electric — Technology for using the sun's energy to produce steam to run turbines that generate electricity.
SOLMET — The 26 measurement sites in the 1952–1975 SOLMET/ERSATZ solar and meteorological hourly network that measured global horizontal solar radiation. The SOLMET/ERSATZ network has been replaced by the 1961–1990 National Solar Radiation Database.
Spatial — Pertaining to space, or pertaining to distance such as spatial variation (variation over distance).
Specific Humidity — The mass of water vapor per unit mass of air, including the water vapor (usually expressed as grams of water vapor per kilogram of air).
Spectral Distribution — The solar spectral distribution.
Spectral Irradiance — The amount of radiant energy flux expressed in terms of the solar spectrum. NREL's Solar Spectral Radiation Data Base contains thousands of irradiance spectra.
Spectroradiometer — An instrument designed to measure spectral irradiance. Spectroradiometers gathered the data in NREL's Solar Spectral Radiation Data Base.
SRRL — The Solar Radiation Research Laboratory at the National Renewable Energy Laboratory.
Stratosphere — The relatively isothermal (constant temperature) layer of the atmosphere above the troposphere and below the mesosphere.
Sunshine — Used interchangeably with the more precise term bright sunshine, when the sun casts an obvious shadow or when a Campbell-Stokes sunshine recorder is recording, usually above 210 W/m2.
Sunshine Duration — The length of time for which the sun casts an obvious shadow or when a Campbell-Stokes sunshine recorder is recording. The lower limit for bright sunshine (based on a Campbell-Stokes recorder) is between 70 W/m2 (very dry air) and 280 W/m2 (very humid air).
Sun Position — The location of the sun in the sky, expressed in terms of azimuth angle and zenith angle.
Suns — A unit that multiplies the amount of energy the Earth can receive from the sun, typically used by the solar concentrator community; e.g., a concentrator might focus the energy of 40 suns onto a central receiver. Note that this unit is not precisely defined and is usually less than the solar constant.
Temporal — Pertaining to time, such as temporal variation (variation over time).
Terrestrial Radiation — Electromagnetic radiation that is emitted by the Earth, as opposed to solar radiation emitted by the sun.
Thermopile — A set of thermocouple junctions connected in series to boost the voltage to a meaningful amount (usually measured in millivolts). A thermocouple is a metallic strip or wire that produces an electromagnetic potential (voltage) when the two ends (junctions) are at different temperatures. The "cold" junctions of thermopile radiometers are painted white to reflect radiation, and the "hot" junctions are painted black to absorb radiation.
TMY — Typical Meteorological Year, a "typical" year of hourly solar and meteorological values that is designed to produce the expected climate of a location throughout a year.
Total Solar Radiation — Solar radiation that is the sum of direct, diffuse, and ground-reflected radiation; however, because ground-reflected radiation is usually insignificant compared to direct and diffuse, for all practical purposes global radiation is said to be the sum of direct and diffuse radiation only.
Tracking Collector — Any collector that changes its orientation throughout the day to follow the path of the sun in the sky. Two-axis trackers continually face the sun, while one-axis trackers rotate on one axis so that collectors receive the maximum amount of circumsolar radiation that strikes the axis.
Transient Response — The short-term response of an instrument caused by a change of status of the instrument's environment. For example, the switching of a power supply on and off will send very short-term power spikes that can be detected by a volt meter with sufficiently rapid response time.
Transmittance — The fraction or percent of a particular frequency or wavelength of electromagnetic radiation that passes through a substance without being absorbed or reflected.
Transpiration — The transfer of water from the leaves of plants to water vapor in the atmosphere.
TRNSYS — TRNSYS is computer software that is used for designing buildings and systems that use solar energy.
Troposphere — The lowest region of the atmosphere between the surface of the earth and the stratosphere. In the troposphere the temperature usually decreases with increasing altitude.
Trough — A colloquial and descriptive name of the parabolic cylinder (surface of constant parabolic cross-section) used for collecting solar radiation along the focal length. Trough systems follow the path of the sun by pivoting on one axis (typically east-west or north-south), using shiny parabolic troughs to heat the collector fluid that passes through a tube at the focus.
Turbidity — A measure of the opacity of the atmosphere. A perfectly clear sky has a turbidity of 0, and a perfectly opaque sky has a turbidity of 1. Turbidity is affected by air molecules and aerosols.
Ultraviolet Radiation — The range of radiation just beyond the violet in the visible spectrum (at about 4,000 Angstroms or 400 nanometers) to the x-ray region (at about 40 Angstroms or 4 nm). Solar researchers tend to define the energetic region of the electromagnetic spectrum from 1–300 nm as the ultraviolet region. UV-A is in the 320–400 nm range, and UV-B is in the 280–320 nm range.
Uncertainty — The expression of the amount of doubt that remains after a result is obtained. Although uncertainty may be subjective and without foundation ("We are 80% certain that nuclear fusion will be a power source in the 21st century"), many uncertainties are determined by statistical procedures ("Sampling polls indicate that 63% agree, with a possible 3% uncertainty") or measurement uncertainty (e.g., 3.204 ± 0.005 °C).
Visible Radiation — The range of radiation wavelengths that is visible to the human eye; goes from red to violet and then enters the ultraviolet part of the spectrum.
Volt — The metric unit of electric potential.
Water Vapor — Gaseous water (individual water molecules) in the atmosphere.
Watt — A unit of power defined as a joule per second.
Watt-hour — A unit of energy equal to 3600 joules.
Wavelength — The distance between adjacent peaks or troughs of a wave. Wavelengths of light are typically expressed in terms of Angstroms or nanometers (10-9 meters).
WBAN — The Weather Bureau/Army/Navy identifier of weather stations. This five-digit code is unique to each weather measurement station in the WBAN system.
Weather — The state of the atmosphere at a given time. This includes temperature, humidity, cloud cover, precipitation type, and presence of aerosols.
Wet-Bulb Temperature — The temperature to which air will cool when water is evaporated into unsaturated air; measured by a wet-bulb thermometer, which has a wet cloth sleeve that covers its bulb. Wet-bulb temperature and dry-bulb temperature are used to compute relative humidity.
Wind — Horizontal motion of air near the surface of the Earth.
Wind Rose — Polar graphs that indicate the speed and relative duration of wind according to its direction. Wind roses are useful for determining the most prevalent direction of winds of desired strength. Wind roses are reported at NREL's Solar Radiation Research Laboratory.
WMO — The World Meteorological Organization.
WRR — The World Radiometric Reference, which provides the basis for all measurements by radiometers in the world. Every five years, many of the best absolute cavity radiometers undergo an intercomparison at PMOD/WRC (Davos, Switzerland). The most stable, accurate, and precise instruments provide the World Radiometric Reference for the coming years. Any credible radiometer measurement must be traceable to the WRR.
WSG — The World Standard Group (WSG) of absolute cavity radiometers is maintained by the World Meteorological Organization's World Radiation Center (Davos, Switzerland). The WSG is a group of seven well-characterized absolute cavity radiometers used to define the World Radiometric Reference (WRR). International intercomparisons of national standard pyrheliometers with the WSG are held every five years at the WRC to transfer the WRR to national centers. Having participated in such comparisons since 1980, NREL has three absolute cavity radiometers directly traceable to the WRR. The WRR has an uncertainty of less than +/- 0.3%. This means that the best possible measurements of direct normal solar irradiance have at least this uncertainty.
WYEC — The Weather Year for Energy Calculations, a "typical year" used by engineers and architects of ASHRAE.
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Zenith Angle — The angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).