Solar and Earth Radia.on
Solar and Earth Radia.on Solar radia.on Any incoming radia.on measured at the earth s surface Earth radia.on The long- wave band of radia.on emi>ed by the earth What are the typical units of radia.on measurements?
Defini.ons Radiant Flux Amount of radia.on coming from a given source per unit.me in Wa>s Radiant Intensity Radiant flux leaving a point on the source, per unit solid angle of space surrounding the point, in W per steradian Solid Angle - angle in three- dimensional space that an object subtends at a point
Defini.ons Radiance Radiant flux emi>ed by a unit area of a source or sca>ered by a unit area of a surface in Wa>s per square meter per steradian Irradiance Radiant flux incident on a receiving surface from all direc.ons, per unit area of surface, in Wa>s per square meter
Defini.ons Absorptance, Reflectance, and Transmi>ance Frac.ons of the incident flux that are absorbed, reflected, or transmi>ed by a medium Global solar radia.on Solar Irradiance received on a horizontal surface, in Wa>s per square meter; Sum of the direct solar beam plus diffuse component of sky light
Defini.ons Direct Solar Radia.on Radia.on emi>ed from the solid angle of the sun s disc, received on a surface perpendicular to the axis of this cone, comprising mainly unsca>ered and unreflected solar radia.on (Wa>s per square meter) Typically taken to be 1367 Wa>s per square meter at top of atmosphere Direct beam is a>enuated by absorp.on and sca>ering in the atmosphere
Defini.ons Diffuse Solar Radia.on Also known as sky radia.on downward sca>ered and reflected radia.on coming from the whole hemisphere (Wa>s per square meter) Visible Radia.on Spectral range of the standard observer (400 nm 730 nm)
Defini.ons Infrared Radia.on Radia.on at wavelengths greater than 730 nm Ultraviolet Radia.on Radia.on in the wavelengths 100 nm 400nm UVA (315 nm 400 nm) UVB (280 nm 315 nm) UVC (100 nm 280 nm)
Defini.ons Photosynthe.cally Ac.ve Radia.on (PAR) band of solar radia.on between 400 and 700 nm that is used by plants in the photosynthesis process (measured in moles of protons) Black Body Body that, at a given temperature, radiates as much or more, at every wavelength, than any other kind of object at the same temperature
Black Body A black body is a theore.cal object that absorbs 100% of the radia.on that hits it Therefore it reflects no radia.on and appears perfectly black In prac.ce no material has been found to absorb all incoming radia.on, but carbon in its graphite form absorbs all but about 3% All objects emit radia.on above absolute zero
Black Body Stars are approximate black body radiators Most of the light directed at a star is absorbed It is capable of absorbing all wavelengths of electromagne.c radia.on, so is also capable of emibng all wavelengths of electromagne.c radia.on Most approximate blackbodies are solids but stars are an excep.on
Methods of Measurement Two primary methods used in measurement of radia.on Thermal detectors Photovoltaic detectors
Thermal Detectors Respond to the heat gain or loss due to absorp.on of incoming or emission of outgoing radia.on
Photovoltaic Detectors Convert absorbed radia.on to a voltage
Radia.on Instruments Categorized according to their use Generic term for all radia.on measuring instruments is the radiometer
Pyrheliometers Thermal Detector uses a blackened plate with a temperature sensor (thermocouple or Pla.num RTD) Measures direct solar radia.on (shortwave) in the band from 0.3 to 3 micrometers Viewing angle is ~ 5 degrees which is wide enough to encompass the sun and some of the sky around it Requires a mechanism to keep it oriented towards the sun
Pyrheliometer
Absolute Pyrheliometers Can measure irradiance without resor.ng to reference sources or radiators Contains elements of a regular pyrheliometer along with an electrical heater posi.oned close to the thermal detector Current is adjusted in the heater to yield the same detector output as was obtained from the sun Also known as an absolute cavity pyrheliometer
Absolute Pyrheliometers Considered a primary standard if it meets the following condi.ons: At least one instrument of a series has been fully characterized Each must be compared with another that has been characterized A detailed descrip.on of the results of the comparisons must be available Calibra.on must be available to the World Radia.on Reference
World Radia.on Reference Established by the WMO in 1980 Measurement standard represen.ng the SI unit of irradiance which is Introduced in order to ensure world- wide homogeneity of solar radia.on measurements
Pyranometers Thermal detector Measures the temperature change induced by the heat gain (loss) due to absorp.on (emission) of radia.on by a black surface Temperature change is measured rela.ve to a white surface or to the shell of the instrument Also known as a thermopile pyranometer
Thermopile Nested array of thermocouples usually 10 100 thermocouples What is the advantage to mul.ple thermocouples?
Pyranometers Shielded from the atmosphere with a glass dome Glass is transparent to radia.on from 0.25 micrometers to 2.8 micrometers Upper limit can be extended to 4.5 micrometers by using soda lime glass Why is the glass shield necessary?
Pyranometers Errors can arise when the glass dome heats up Second dome may be necessary to minimize these effects Radiometer case temperature can also be measured to account for the temperature differences
Pyranometer
Shadow Ring / Shadow Disk Used with a pyranometer to block direct solar beam so that just the diffuse sky radia.on is measured Shadow ring blocks direct radia.on for the whole day which requires and adjustment each day for la.tude and solar declina.on Shadow disk blocks direct solar beam and requires solar tracker.
Shadow Disk When combining a shadow disk pyranometer with a standard pyranometer, the difference is the direct solar beam Can be compared directly to the output of a pyrheliometer The prac.ce of using a shadow disk and combining it with a pyrheliometer is considered one of the most accurate measurements of solar radia.on. Why?
Shadow Ring Pyranometer
Net Pyranometer Also known as an albedometer Comprises two pyranometers, one facing up, one facing down Upward facing one measures global solar radia.on Downward facing one measures reflected solar radia.on Difference is the albedo
Albedo The extent to which an object diffusely reflects light from the Sun
Pyrgeometers Measures only earth (longwave) radia.on Glass domes can t be used Silicon window (flat, not domed) is used to measure from 3 to 50 micrometers; more stable than polyethylene windows Field of view is limited to 150 degrees May incorporate an electrical heater to prevent dew/frost forma.on Requires temperature correc.on
Pyrgeometer
Pyrradiometers Measures total global solar radia.on, including shortwave and longwave Glass dome is replaced with a hemisphere of silicon Silicon domes are delicate and degrade with exposure to sunlight Requires maintenance and periodic dome replacement, some.mes as oken as monthly
Pyrradiometers Moisture can condense inside domes which can lead to errors Domes can be pressurized with dry air or nitrogen to avoid these errors Two pyrradiometers, one facing up and one facing down, can be combined to measure net radia.on at the earth s surface; known as a net pyrradiometer or net radiometer
Pyrradiometer
Net Radia.on Combining upward and downward facing pyranometers and pyrradiometers will give net radia.on measurements
Measurement Errors Absolute Calibra.on due to use of an imperfect reference sensor Spectral Response sensor not conforming to ideal spectral response Azimuth change in sensor output as the sensor is rotated about the normal axis at a par.cular angle of incident radia.on (symmetry)
Measurement Errors Linearity sensor output is not linearly propor.onal to input Hysteresis delay in response or difference in response to increasing input versus decreasing input at the same input value Temperature sensi.ve to temperature as well as radia.on Response Time rapid change in input and sensor can t respond
Measurement Errors Long- term stability sensor characteris.cs change with.me User setup and applica.on reflec.ons, obstruc.ons, dust and bird droppings, shock, damage, incorrect calibra.on Wind Speed wind hea.ng or cooling of the dome
Exposure Requirements Incidence of sca>ering par.cles (fog, smoke, pollu.on) should be typical of the surrounding area Instrument windows need to be cleaned, oken daily Domes may need to be aspirated to keep them free of dew and dry them aker rain Downward looking sensors should be representa.ve of the area and not contain a tower leg
Exposure Requirements Instrument must be kept free of internal condensa.on Site must be free of obstruc.ons (shadows) for all sun angles for the en.re year Rookops should not be used for upwelling longwave or reflected shortwave radia.on Reflec.ons of light toward the instrument can affect readings Must be kept level
Pyrheliometer Exposure Equitorial mount or automa.c tracker is required for tracking the sun. Must be protected from environmental influences Must be kept aligned to the sun within 0.25 degrees