Solar Energy

Fundamental Considerations

Electromagnetic Energy

The term electromagnetic energy comprises all types of energy that travels from its source through space in the form of harmonic waves along straight paths at the uniform speed of light (3x108 m/sec). Radiation is the term that pertains to the emission and propagation of electromagnetic energy in the form of waves.

There are many types of electromagnetic energy, but consideration of the subject is necessarily limited to those of solar origin that provide pertinent background information for the proper utilization of solar radiation for disinfection purposes. It should be recognized that solar radiation constitutes only a portion of the entire electromagnetic energy spectrum. In fact, sunlight as sensed by the human eye essentially represents that part which is visible, although it also includes other invisible radiation components. Strictly speaking, the terms light and sunlight refer to radiation wavelengths detectable by the human eye (400 to 700 nm).

Electromagnetic radiation, as well as solar radiation, is commonly classified on the basis of radiation wavelength into several regions, or bands. The wavelength bands of solar radiation, both visible and invisible, are mentioned in Table 1 along with some useful remarks. Similar information is also illustrated diagrammatically in Figure 1 for purposes of visual clarification. Note that the colours shown for the various wavelength bands in the visible region of the solar radiation spectrum are approximate representations of the colours of light within each band.

    Band Wavelength (nm) Atmospheric Effects
    Gamma ray <0.03 Completely absorbed by the upper atmosphere
    X-Ray 0.03 - 3 Completely absorbed by the upper atmosphere
    Ultraviolet, UV
        UV (B) 3 - 300 Completely absorbed by oxygen, nitrogen, and ozone in the upper atmosphere
        UV (A) 300 - 400 Transmitted through the atmosphere, but atmospheric scattering is severe
    Visible 400 - 700 Transmitted through the atmosphere, with moderate scattering of the shorter waves
    Infrared, IR
        Reflected IR 700 - 3000 Mostly reflected radiation
        Thermal IR 3000 - 14000 Absorption at specific wavelengths by carbon dioxide, ozone, and water vapour, with two major atmospheric windows

      Table 1. Spectral bands of incoming solar energy and atmospheric effects

    Figure 1. Solar radiation spectrum showing the different radiation bands and their wavelength ranges.

Propagation of Solar Energy

The sun continuously radiates enormous amounts of solar energy at wavelengths that cover the ultraviolet, visible, and infrared bands. The maximum intensity of the emitted solar energy occurs at a wavelength of about 555 nm, which falls within the band of green light (Figure 1).

Solar radiation moves freely in outer space because of the vacuum, unless its path is obstructed by planets, satellites, meteorites, or other space objects. Whatever portion of it reaches the earth and its surrounding atmosphere may encounter a variety of atmospheric or terrestrial objects. When solar radiation strikes any object whether in the form of solid, liquid or gas, changes in its magnitude, direction, and wavelength are expected to occur depending upon the nature and characteristics of the intervening object. These changes may come as a result of any of the following possible phenomena:

  • Radiation may be transmitted through a transparent object such as air, water, or glass with a change in speed and direction.
  • Radiation may be partially or completely absorbed by an object, the components thus absorbed being dependent on the wavelength of the specific radiation and the characteristics of the object. Blue tinted glass, for instance, would transmit radiation with wavelengths of 425 to 490 nm and, at the same time, absorb radiation in the other wavelength bands.
  • Radiation may be scattered by being deflected in all directions, a common example being the scattering of sunlight as it traverses the atmosphere.
  • Radiation may be reflected by being returned from the surface of an object in an unchanged form except for the deviation whereby the angle of reflection would be equal and opposite to the angle of incidence.