Solar Radiation And Pyranometer

Clean Energy Technology Made By: Umair N. Mughal Asstt. Professor Mechanical Engineering Deptt. NED University of Engineering and Technology Karachi, Pakistan

Earth Vs Solar Radiation

Atmospheric Windows

Radiation and Objects

Rayleigh Scattering Selective scattering (or Rayleigh scattering) occurs when certain particles are more effective at scattering a particular wavelength of light. Air molecules, like oxygen and nitrogen for example, are small in size and thus more effective at scattering shorter wavelengths of light (blue and violet). The selective scattering by air molecules is responsible for producing our blue skies on a clear sunny day.

Thermal Radiation

Actinometers They are instruments used to measure the heating power of radiation. Actinometer is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time. e.g.

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Solutions of iron(III) oxalate (e.g. potassium ferrioxalate) as a chemical actinometer Bolometers, thermopiles, & photodiodes are physical devices giving a reading that can be correlated to the number of photons detected.

Pyranometers 

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A pyranometer is a type of actinometer used to measure broadband solar irradiance on a planar surface and is a sensor that is designed to measure the solar radiation flux density (in watts per metre square) from a field of view of 180 degrees. The name pyranometer stems from Greek, "pyr" meaning "fire" and "ano" meaning "sky". A typical pyranometer does not require any power to operate The solar radiation spectrum extends approximately from 300 to 2,800 nm. Pyranometers usually cover that spectrum For a flux density or irradiance measurement it is required by definition that the response to “beam” radiation varies with the cosine of the angle of incidence; i.e. full response at when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0 degrees angle of incidence), zero response when the sun is at the horizon (90 degrees angle of incidence, 90 degrees zenith angle), and 0.5 at 60 degrees angle of incidence. It follows from the definition that a pyranometer should have a so-called “directional response” or “cosine response” that is close to the ideal cosine characteristic.

Design of Pyranometers 

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In order to attain the proper directional and spectral characteristics, a pyranometer’s main components are: A thermopile sensor with a black coating. This sensor absorbs all solar radiation, has a flat spectrum covering the 300 to 50,000 nm range, & has a near-perfect cosine response. A glass dome. This dome limits the spectral response from 300 to 2,800 nanometers (cutting off the part above 2,800 nm), while preserving the 180 degrees field of view. Another function of the dome is that it shields the thermopile sensor from convection. The black coating on the thermopile sensor absorbs the solar radiation. This radiation is converted to heat. The heat flows through the sensor to the pyranometer housing. The thermopile sensor generates a voltage output signal that is proportional to the solar radiation

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Main components: thermopile sensor(1), domes(2), glass dome(2,3), radiation screen(4), signal cable(5), gland(6), leveling feet(7), printed circuit board(8),desiccant (9), level(11).

Cosine Response of A “cosine corrected” sensor is designed to Pyranometer: maintain its accuracy when radiation comes from different angles. For pyranometers, the 

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test of cosine response is to measure extreme zenith angles. Cosine response is synonymous to the term Lambertian response. Lambert’s Cosine law states that radiation intensity on a flat surface decreases as the angle of the surface decreases from perpendicular (normal or 0˚ zenith angle). This is expressed as: Eθ = E * cos(θ) All radiation sensors have some azimuth error, which shows up as the difference between the am and pm response. This error is typically smaller than the cosine error. About 90% of the sunlight energy is between 300 to 1100 nm so silicon-cell photodiode pyranometers can be calibrated to estimate all of the shortwave energy from sunlight.