Glossary of Solar Radiation Resource Terms

Other relevant glossaries


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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. The picture below illustrates instruments being calibrated at SRRL. The radiometers on the calibration table are calibrated to a reference instrument traceable to the World Radiometric Reference (WRR).

   


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 Farenheit to Celsius is

    C = ( F - 32 ) / 1.8


Central Receiver System - a solar power generator which uses a series of tracking mirrors ( heliostats) or a paraboloid (3-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). The Circumsolar Data Manual has a fuller discussion.


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. Departmentof Commerce).


Collector - a device that receives solar energy and converts it to useful energy forms. The following figure from Shining On illustrates several types of collectors.


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. Below is an illustration from the Solar Radiation Data Manual for Flat Plate and Concentrating Collectors .


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 (see graph below for an example).

Click here for graph


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