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Solar Radiation
K Characteristic of Solar Radiation
K EM radiation from thermonuclear reaction (H à He)K Solar radiation from sun 3.85 x 1023 kW (conversion of mass 4 x 109 kg/s into energy) K Solar radiation above atmosphere 1.79 x 1014 kW (at sun-earth distance of 1.495 x 1011 m) K Spectrum of Solar Radiation
K Solar irradiance, flux, power, intensity, energyK Spectral solar irradiance, solar irradiance tableK Blackbody vs. Grey body radiationK Equivalent temperature of sunK Solar constant 1353 W/m2 (also in other units) K Interaction of EM waves with gas atoms/molecules
K Atoms/molecules have energy at certain definite values K Absorption of energy (upward transmission)K Emission of energy (downward transmission) K Associated energy can be EM energy, heat (kinetic energy due to translation/rotation) K Energy of EM radiation quantum (photon) depends on EM K Electronic transition involves large energyK Vibration/Translation involves intermediate energyK Rotation involves small energy K Atmosphere and Energy balance
K Variation (type, density) of constituent gases (and particles) at K Different gases react differently with EM waves.
K In equilibrium, input energy/power equals output energy/power K Input (solar) = Output (solar plus infrared)K Input (short EM radiation) = Output (short and long EM K Basic types of radiation
K Outside atmosphere (extraterrestrial radiation), in definite direction K Inside atmosphere, intensity and direction changed due to absorption, scattering and reflection. Fluctuating with time.
K Total radiationK Direct or beam radiationK Diffused radiation Total Radiation = Direct Radiation + Diffused radiation K Global radiation (total radiation on a horizontal plane) K Airmass
K Showing effects of atmosphere on radiation within atmosphere K The longer the pathlength inside atmosphere, the bigger the effects.
K Depending on elevation, direction of sunlightK Standard airmass for collector testing (AM2) and PV testing (AM1.5) at specified radiation intensities.
K Outdoor testing vs. indoor testing (under solar simulator) K Interaction of radiation and atmosphere
K Highly complexK Dependent on gaseous constituents (major and trace gases), amount of water, dust, aerosols.
K Adequate theory for clear sky condition (gases and dust only) K Scattering of radiation
K Due to gaseous and water molecules, and dustK Geometrical effect, depends on light wavelength λ and sizes of K Measured scattering in terms of transmission coefficient (forward K Scattering depends on λ, hence monochromatic coefficient K Standard condition vs. real situationK Monochromatic transmission coefficients due to gas molecules, K Corrections for pressure (elevation), dust, precipitable water, K Absorption of radiation
K Occurring at certain wavelengths (vs. scattering occurs at every K Absorption occurs when EM energy (photon energy) equals to energy required in transition of molecules (electronic, vibration, rotation) K Absorption depends on λ, hence monochromatic absorption coefficientK Monochromatic absorption coefficients due to ozone, oxygen and water molecule important in solar spectrum.
K Absorption in near infrared (1-10 µm) determined primarily by CO2, K Hence changes in CO2, CH4 change in earth temperature, cooling or K Solar spectrum at earth surface
K Combined effects of scattering and absorptionK To calculate solar spectrum K Multiply extraterrestrial irradiance with monochromatic transmission coefficients (due to scattering and absorption) to get K Integrate above over entire solar wavelength to get solar K Extinction coefficient
K Comparison of extraterrestrial radiation and direct radiation on earth (a measure of scattering and absorption of solar radiation by K Atmospheric turbidity
K Scattering and absorption by pollutants, smoke, aerosols in addition K Turbidity is an indicator of these additional effects.
K Linke turbidity factor: comparison of extinction in real (polluted)
K Angstrom turbidity coefficient: monochromatic extinction
K Schuepp-Valko-Volz turbidity coefficient: monochromatic
extinction coefficient at 0.5 µm (green/yellow) K Sun Position in the Sky
K Orbital plane, Equatorial planeK Celestial sphere, HorizonK North, North Celestial Pole (NCP), ZenithK Earth orbit: solstices (summer, winter), equinoxes (spring, autumn)K Lengths of days and nights vs. earth positions in orbit K Geometry of Sun-Earth System
K Azimuth angleK Elevation angleK Declination K Solar Time
K Solar noon (reference) or true mid-day occurs when sun crosses the K Solar time is measured as time before or after solar noon.
K Solar time can be expressed as K Hours (e.g. 0900, 1130, 1200, 1520, 1740 hrs)K Hour angle K Hour angle
K Sun crosses 360 degrees of longitude (360 local meridians) in 24 K 4 minutes to move from one local meridian to the next (east K 1 hour to cross 15 meridians (15 degrees of longitude on K Hour angle is a measure of solar time at one local meridian before K Hour angle can be expressed as degrees (of longitude on equator)
or hours and minutes
K ω = 15t degreesK t is time (in hour) from solar noonK ω is positive before solar noon and negative after solar noon.
K Calculation of sun position
K Day in a year n (n between 1 to 365)
K Declination angle δ
K δ is angle between sunlight direction at solar noon and K δ is between -23.45o to +23.45o. Positive if measured due Elevation angle
K γ is angle between sunlight direction and horizon.
K γ is 0o at sunrise and sunset, 90o at zenith.
sin γ = cosϕ cos δ cosω + sinϕ sin δ ϕ is latitude of location (observation point).
K Azimuth angle ψ
ψ is angle between local meridian and projection of sunlight direction onto the horizontal plane at local meridian.
ψ is positive if measured from south to west (0o to 180o).
ψ is negative if measured from south to east ( 0o to -180o).
K True Solar Time
K Solar time depends on sun position at a particular point.
K It is not local clock time or standard time.
K Different from standard time due to
K Local meridian at a particular point differs from meridian K Areas between different meridians may have same standard time (same time zone) if sun positions not much different.
K Thailand (98o - 105o E) has one time zone.
K Standard time based on sun position at Ubon Ratchathani Solar time t
solar vs. Standard time standard
K ∆L is difference between the meridian used in calculating standard time and the local meridian.
K Plus or minus depends on whether which meridian is eastward, K E is called equation of time, correction term due to perturbationK E = 87 K Extraterrestrial Radiation H0
cosφ cosδ sinωs + I sc = solar constant = 4.871 MJ/m2-hr K Radiation Measurement and Estimation
K Radiation Measurement
K Total (global), direct (beam), diffused radiationK Sunshine hours K Pyranometer (global radiation)
K Basic radiation measuring equipment
K Thermopile: converting absorbed light into heat (and
temperature rise/difference detected by thermocouples) K Solar cell: converting absorbed light into electricity
K Bimetallic
K Classes of PyranometersK Modifications to measure spectrum and diffused radiationK Shading ring correction cos δ [ω sin φ sin δ + cosφ cosδ sin ω ] X = fraction of diffused radiation from hemisphere blocked by shading φ = latitude of location
ωs = sunset hour angle in radian
K Pyrheliometer (direct radiation)
K Detection principle similar to pyranometers.
K Must point directly at sun (need tracking mechanism) and
collimator tube (blocking diffused radiation)
K Sunshine hour recorder
K Measuring instances that radiation intense enough to activate K Campbell-Stokes sunshine recorder: focusing sunlight to K Correction factors for various burns on sunshine hour paperK Sunshine hours are different from daylength.
K Estimation of Total Radiation
K Total radiation, if cannot be measured, can be estimated.
K Estimation based on correlation between radiation and sunshine K Correlation coefficient is a measure of relationship between two
variables (e.g. radiation and sunshine hour) K Relationship between variables expressed as regression equations
(e.g. linear, non linear, expontential, transcendental etc.) K Statistical analyses used to determine correlation coefficients and
regressional equations among variables.
K Variables can be used as raw data or dimensionless data.
K Angstrom Type Regression equation
K G and S are measured (raw) values of total radiation and m are chosen values of radiation and K Scatter diagramK Analyses of radiation and sunshine hour K Location (OK within about 50 km.),K Time (monthly, yearly, certain durations in a year) andK Synoptics (atmospheric circulation pattern) K Radiation Map
K Potential of radiation over a country normally expressed as K Map is made from measures of radiation or estimates of radiation from related variables (sunshine hour, cloud cover) K Relationship between Total, Direct and Diffused Radiation
(Liu and Jordan)
K Universal relationship between
K Ratio of daily diffused radiation to daily total radiation, or diffused fraction and
K Ratio of daily total radiation to daily extraterrestrial radiation, or cloudiness index

Source: http://www.cssckmutt.in.th/cssc/cssc_classroom/Solarenergy/LectureNoteSummary.pdf