Measurement of solar radiation

The precise measurement of solar is important in many fields including remote sensing, meteorology, climate change, solar energy, and various aspects of agriculture. The two common methods that characterize solar radiation are the solar irradiance (or radiation) and solar insolation.

Global average solar insulationGlobal average solar insulation

Definitions


Solar irradiance (symbol: E) is the radiometry term for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. The SI units for irradiance are watts per square meter (W/m2). Solar irradiance is strongly dependent on location and local weather. Irradiance measurements consist of global and/or direct radiation measurements taken periodically throughout the day.

Solar insolation is the total amount of solar energy received at a particular location during a specified time period, often in units of W/(m2·day). While the units of solar insolation and solar irradiance are both a power density (for solar insolation the "hours" in the numerator are a time measurement as is the "day" in the denominator), solar insolation is quite different than the solar irradiance because the solar insolation is the instantaneous solar irradiance averaged over a given time period. Solar insolation is also expressed in units of MJ/m2 per year.

Instrumentation

Actinometer is the general name for any instrument used to measure the intensity of radiant energy, particularly that of the sun. Actinometers are classified according to the quantities that they measure:

  • A pyrheliometer measures measures the intensity of direct solar radiation. It is so designed that it measures only the radiation from the sun's disk (which has an apparent diameter of ½°) and from a narrow annulus of sky of diameter 5° around the sun's disk.
  • A pyranometer measures global radiation (the combined intensity of direct solar radiation and diffuse sky radiation). It measures solar irradiance from the solid angle 2pi onto a plane surface. When mounted horizontally facing upwards it measures global solar irradiance. If it is provided with a shade that prevents beam solar radiation from reaching the receiver, it measures diffuse solar irradiance.
  • A pyrgeometer measures the effective terrestrial radiation. It measures the atmospheric infrared radiation spectrum that extends approximately from 4.5 µm to 100 µm.

A multi-filter rotating shadowband radiometer measures direct normal, diffuse horizontal and total horizontal solar irradiance.A multi-filter rotating shadowband radiometer measures direct normal, diffuse horizontal and total horizontal solar irradiance.

A radiometer is an instrument designed to measure the radiated electromagnetic power. When used in solar energy applications, it is usually desirable for radiometers to respond the same to equal amounts of energy at all wavelengths over the wavelength range of the radiation to be measured. Most radiometers therefore work by using a thermopile to measure the temperature rise of a sensitive element whose receiving surface is painted dull black. Instruments for measuring solar irradiance using a photovoltaic cell as the sensitive element have a non-uniform spectral response.

An alternative method of measuring solar radiation, which is less accurate but also less expensive, is a sunshine recorder. Sunshine recorders measure the number of hours in the day during which the sunshine is above a certain level (typically 200 mW/cm2). Data collected in this way are used to determine the solar insolation by comparing the measured number of sunshine hours to those based on calculations and including several correction factors.

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