Solar and Earth Radiation

Solar radiation – Any incoming radiation measured at the earth’s surface Earth radiation – The long-wave band of radiation emitted by the earth What are the typical units of radiation measurements?

Definitions Radiant Flux – Amount of radiation coming from a given source per unit time in Watts 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

Definitions Radiance – Radiant flux emitted by a unit area of a source or scattered by a unit area of a surface in Watts per square meter per steradian Irradiance – Radiant flux incident on a receiving surface from all directions, per unit area of surface, in Watts per square meter

Definitions Absorptance, Reflectance, and Transmittance – Fractions of the incident flux that are absorbed, reflected, or transmitted by a medium Global solar radiation – Solar Irradiance received on a horizontal surface, in Watts per square meter; Sum of the direct solar beam plus diffuse component of sky light

Definitions Direct Solar Radiation – Radiation emitted from the solid angle of the sun’s disc, received on a surface perpendicular to the axis of this cone, comprising mainly unscattered and unreflected solar radiation (Watts per square meter) – Typically taken to be 1367 Watts per square meter at top of atmosphere – Direct beam is attenuated by absorption and scattering in the atmosphere

Definitions Diffuse Solar Radiation – Also known as sky radiation – downward scattered and reflected radiation coming from the whole hemisphere (Watts per square meter) Visible Radiation – Spectral range of the standard observer (400 nm – 730 nm)

Definitions Infrared Radiation – Radiation at wavelengths greater than 730 nm Ultraviolet Radiation – Radiation in the wavelengths 100 nm – 400nm – UVA (315 nm – 400 nm) – UVB (280 nm – 315 nm) – UVC (100 nm – 280 nm)

Definitions Photosynthetically Active Radiation (PAR) – band of solar radiation 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 theoretical object that absorbs 100% of the radiation that hits it Therefore it reflects no radiation and appears perfectly black In practice no material has been found to absorb all incoming radiation, but carbon in its graphite form absorbs all but about 3% All objects emit radiation 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 electromagnetic radiation, so is also capable of emitting all wavelengths of electromagnetic radiation Most approximate blackbodies are solids but stars are an exception

Methods of Measurement Two primary methods used in measurement of radiation – Thermal detectors – Photovoltaic detectors

Thermal Detectors Respond to the heat gain or loss due to absorption of incoming or emission of outgoing radiation

Photovoltaic Detectors Convert absorbed radiation to a voltage

Radiation Instruments Categorized according to their use Generic term for all radiation measuring instruments is the radiometer

Pyrheliometers Thermal Detector – uses a blackened plate with a temperature sensor (thermocouple or Platinum RTD) Measures direct solar radiation (shortwave) in the bad 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 resorting to reference sources or radiators Contains elements of a regular pyrheliometer along with an electrical heater positioned 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 conditions: – At least one instrument of a series has been fully characterized – Each must be compared with another that has been characterized – A detailed description of the results of the comparisons must be available – Calibration must be available to the World Radiation Reference

World Radiation Reference Established by the WMO in 1980 Measurement standard representing the SI unit of irradiance which is ______ Introduced in order to ensure world-wide homogeneity of solar radiation measurements

Pyranometers Thermal detector – Measures the temperature change induced by the heat gain (loss) due to absorption (emission) of radiation by a black surface Temperature change is measured relative 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 multiple thermocouples?

Pyranometers Shielded from the atmosphere with a glass dome Glass is transparent to radiation 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 radiation is measured Shadow ring blocks direct radiation for the whole day which requires and adjustment each day for latitude and solar declination 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 practice of using a shadow disk and combining it with a pyrheliometer is considered one of the most accurate measurements of solar radiation. 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 radiation Downward facing one measures reflected solar radiation Difference is the albedo

Albedo The extent to which and object diffusely reflects light from the Sun

Pyrgeometers Measures only earth (longwave) radiation 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 formation Requires temperature correction

Pyrgeometer

Pyrradiometers Measures total global solar radiation, 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, sometimes as often 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 radiation at the earth’s surface; known as a net pyrradiometer or net radiometer

Pyrradiometer

Net Radiation Combining upward and downward facing pyranometers and pyrradiometers will give net radiation measurements

Measurement Errors Absolute Calibration – 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 particular angle of incident radiation (symmetry)

Measurement Errors Linearity – sensor output is not linearly proportional to input Hysteresis – delay in response or difference in response to increasing input versus decreasing input at the same input value Temperature – sensitive to temperature as well as radiation Response Time – rapid change in input and sensor can’t respond

Measurement Errors Long-term stability – sensor characteristics change with time User setup and application – reflections, obstructions, dust and bird droppings, shock, damage, incorrect calibration Wind Speed – wind heating or cooling of the dome

Exposure Requirements Incidence of scattering particles (fog, smoke, pollution) should be typical of the surrounding area Instrument windows need to be cleaned, often daily Domes may need to be aspirated to keep them free of dew and dry them after rain Downward looking sensors should be representative of the area and not contain a tower leg

Exposure Requirements Instrument must be kept free of internal condensation Site must be free of obstructions (shadows) for all sun angles for the entire year Rooftops should not be used for upwelling longwave or reflected shortwave radiation Reflections of light toward the instrument can affect readings Must be kept level

Pyrheliometer Exposure Equitorial mount or automatic tracker is required for tracking the sun. Must be protected from environmental influences Must be kept aligned to the sun within 0.25 degrees