2.1 Solar geometry and radiation principles

Solar geometry and radiation principles are crucial for understanding how to harness the sun's energy effectively. These concepts help us calculate the sun's position and predict the amount of solar radiation available at different times and locations.

Understanding solar angles, irradiance components, and atmospheric effects is key to designing efficient solar power systems. This knowledge allows engineers to optimize the placement and orientation of solar collectors, maximizing energy capture throughout the year.

Solar Angles and Positions

Understanding Solar Position Angles

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• Zenith angle measures the vertical angle between the sun and the point directly overhead
  • Ranges from 0° (sun directly overhead) to 90° (sun at horizon)
  • Affects the intensity of solar radiation reaching a surface
• Azimuth angle indicates the horizontal direction of the sun relative to a reference point
  • Usually measured clockwise from true north
  • Varies throughout the day as the sun moves across the sky
• Declination angle represents the angular position of the sun at solar noon
  • Varies seasonally due to Earth's axial tilt
  • Ranges from -23.45° to +23.45° throughout the year
• Hour angle describes the sun's east-west movement
  • Measured in degrees or time
  • 15° of rotation per hour (360° in 24 hours)

Solar Time and Position Calculations

• Solar time differs from standard clock time
  • Based on the apparent motion of the sun across the sky
  • Accounts for variations in Earth's orbit and rotation
• Equation of time adjusts for discrepancies between solar time and mean solar time
  • Accounts for Earth's elliptical orbit and axial tilt
  • Varies throughout the year, ranging from -14 to +16 minutes
• Solar position calculations combine multiple angles
  • Used to determine sun's location at any given time and place
  • Essential for optimizing solar energy collection systems

Solar Irradiance Components

Direct and Diffuse Solar Radiation

• Direct normal irradiance (DNI) describes solar radiation received directly from the sun
  • Measured on a surface perpendicular to the sun's rays
  • Highest component of solar radiation on clear days
  • Critical for concentrating solar power systems
• Diffuse irradiance results from scattered sunlight in the atmosphere
  • Comes from all directions of the sky dome
  • Increases on cloudy or hazy days
  • Significant for flat-plate solar collectors
• Global horizontal irradiance (GHI) combines direct and diffuse radiation on a horizontal surface
  • Represents total solar radiation received
  • Calculated as GHI = DNI × cos(zenith angle) + diffuse irradiance
  • Used to assess overall solar resource at a location

Solar Constant and Extraterrestrial Radiation

• Solar constant defines the average solar radiation intensity outside Earth's atmosphere
  • Approximately 1361 W/m² at mean Earth-Sun distance
  • Varies slightly due to solar activity cycles
• Extraterrestrial radiation changes with Earth's orbit
  • Peaks around January when Earth is closest to the sun (perihelion)
  • Lowest around July when Earth is farthest from the sun (aphelion)
  • Variation of about ±3.3% throughout the year

Atmospheric Effects on Solar Radiation

Atmospheric Attenuation Processes

• Air mass quantifies the path length of solar radiation through the atmosphere
  • Air Mass 1 (AM1) occurs when the sun is directly overhead
  • Increases as the zenith angle increases
  • Calculated as AM = 1 / cos(zenith angle) for zenith angles < 70°
• Atmospheric attenuation reduces solar radiation intensity
  • Caused by absorption, reflection, and scattering
  • Varies with atmospheric conditions (humidity, pollution, clouds)
  • Results in lower irradiance values at Earth's surface compared to extraterrestrial levels

Spectral Distribution and Solar Spectrum

• Solar spectrum describes the distribution of solar radiation across different wavelengths
  • Peaks in the visible light range
  • Includes ultraviolet, visible, and infrared radiation
• Atmospheric effects alter the spectral distribution
  • Ozone layer absorbs most ultraviolet radiation
  • Water vapor absorbs some infrared radiation
  • Results in characteristic absorption bands in the terrestrial solar spectrum
• Spectral response affects solar technology performance
  • Photovoltaic cells have specific spectral sensitivity ranges
  • Concentrating solar thermal systems utilize a broader spectrum