Solar Radiation Part 1

Solar Radiation Part 1

• Sun's Structure:
  • Chromosphere, Photosphere, Convective Zone, Radiative Zone, Core, Sunspots
  • Corona (outer layer)
  • Hydrogen and Helium (primary elements)
• Solar Radiation Facts:
  • Sun produces energy through nuclear fusion (4H → He + 2e + 2y + E)
  • The Earth receives significantly more energy in less than one hour than human consumption in a whole year.
  • The solar constant is 1367 (W m⁻²)
  • 92.1% hydrogen, 7.8% helium.
• Earth-Sun Distance:
  • 1.496 x 10¹¹m = 149.6 million km
  • ±1.7% variation in distance

Solar Energy: Main Advantages

• Primary source of energy on Earth
• Inexhaustible resource
• Abundant supply across the planet

Solar Energy: Disadvantages

• Variable supply throughout the day and year
• Low power density
• Expensive technologies required to harness it
• Non-standard design considerations (location specific)

Sun Structure

• Core: High temperature (16 million K), High density (160 g/cm³)
• Radiative Zone: Energy transport via radiation
• Convective Zone: Energy transport via convection
• Photosphere: “Surface of the Sun”
• Chromosphere: Above the Photosphere
• Corona: Outermost part of the Sun, Very Hot

Elements in The Sun

• Element | % Volume | % Mass
• Hydrogen | 92,100 | 73.46
• Helium | 7,800 | 24.85
• Oxygen | 0.061 | 0.77
• Carbon | 0.030 | 0.29
• Other (N,Ne,Fe,Si,Mg,S) | 0.0015 - 0.0084 | 0.06-0.16

Earth Motions

• Earth rotates on its axis, one rotation taking 24 hours.
• Earth orbits the Sun, taking nearly one year for one revolution.
• Earth's axis tilt (23.5°) causes the seasons.

Earth-Sun Distance

• Average distance: 1.496 x 10¹¹ m
• Earth's orbit is elliptical, causing distance variation.
• Ratio (average distance / distance on any given day) is calculated using the following formula r = 1 + 0.033 cos (360n/365)

Earth-Sun Variables

• The solar constant is 1367 W/m²
• The earth's orbit eccentricity causes a variation in the distance between the sun and the earth.
• The variation on distance is around 3.3%

Nuclear Fusion Reaction

• The sun's core generates energy through nuclear fusion.
• 600 million metric tons of hydrogen are converted to helium each second.
• Significant amount of energy is released.

Radiation Crossing The Atmosphere

• Various substances absorb and scatter solar radiation.
• Ozone (O3) absorbs between 0.15 and 0.4 μm.
• Water vapor (H2O) absorbs between 0.8 and 2 μm.
• Carbon dioxide (CO2) absorbs between 2.5 and 4.5 μm.
• Types of dispersion: Rayleigh Scattering, Mie Scattering, Reflection

Air Mass

• Air mass (AM) is a measure of the amount of atmosphere traversed by solar radiation.
• Calculated using the following equation AM = 1 / cos z / sin (h)

Solar Constant

• Standard value: 1367 W/m²
• Represents solar power per square meter normal to solar rays, outside atmosphere, atmean earth-sun distance.

Global Emissive Power

• Total solar power radiated by the Sun, approximately 3.826 x 10²⁶ W
• Flux received by the Earth is calculated based on average solar irradiance

Uptake of Direct and Diffuse Radiation

• Direct radiation comes in straight lines from the sun.
• Diffuse radiation comes from dispersed light in the sky.
• The percentage of diffuse light varies depending on atmospheric conditions.

Solar Radiation Spectrum

• The solar spectrum is the distribution of solar energy across different wavelengths (nm).
• The spectrum varies between UV and infrared radiation.
• Different atmospheric components absorb light at specific wavelengths.

Measuring Solar Radiation

• Pyrheliometers measure direct normal irradiance (DNI).
• Pyranometers measure global irradiance (GHI).
• Solar trackers follow the sun's movement for accurate measurements.

Pyranometer classification

• Pyrheliometers and Pyranometers are instruments whose objective is to measure the solar radiation.
• Classification criteria, for example, response time, spectral sensitivity and zero offset.
• Criteria of classification: response time, zero offset, stability, resolution, and non-linearity.

Solar Spectrum (extraterrestrial and AM = 1)

• Shows how much solar energy is at each wavelength and how atmospheric components impact that energy.
• extraterrestrial radiation is used as a base to compare the effects of the atmosphere with varying air masses.

Horizontal Plane Irradiation:

• Daily average of horizontal global (GHI) and diffuse horizontal irradiance (DHI)
• Typical values can be obtained from weather data.
• Measurement methods are used to record these quantities.

Sun's Position

• Declination (δ) : Angle between the Earth's equatorial plane and the line connecting the center of the Earth to the center of the sun.
• Hour Angle (ω) : Angle made between the local meridian and the plane of the sun.

Sun Path

• Calculating sun's path on a surface (apparent position of the sun in the sky).
• Includes factors such as the time of day, time of year, and latitude.
• The sun's path is affected by the Earth's rotation and its orbital path around the sun.

Concentrating Collectors

• Different designs used to concentrate solar radiation.
• Configurations vary: tubular, parabolic, Fresnel, heliostat
• Used in applications where high temperature is needed.
• High concentration ratios lead to high temperatures.

Yearly Global Irradiation

• Data represents the amount of solar energy received on a horizontal surface over a period of one year.
• Geographic locations with higher numbers represent areas that receive more solar energy per year.