Terrestrial Solar Radiation


After shading a light on the extraterrestrial solar radiation in the chapter 3 it is important to evaluate the global terrestrial solar radiation and its components. The information on terrestrial solar radiation is required in several different forms depending on the kinds of calculations and kind of application that are to be done. Of course, terrestrial solar radiation on the horizontal plane depends on the different weather conditions such as cloud cover, relative humidity, and ambient temperature. Therefore, the impact of the atmosphere on solar radiation should be considered. One of the most important points of terrestrial solar radiation evaluation is its determination during clear sky conditions. Therefore, in this chapter, the equations that determine the air mass basing on available theories are given and the clear sky conditions are introduced with shading a light on the previous work in identifying clear sky conditions. Taking into consideration that, clear sky solar radiation estimation is of great importance for solar tracking, a detailed review of main available models is given in this chapter. As daily, monthly, seasonally, biannually and yearly mean daily solar radiations are required information for designing and installing long term tracking systems, different available methods are commented regarding their applicability for the estimation of solar radiation information in the desired format from the data that are available. An important accent is paid also on the assessment and comparison of monthly mean daily solar radiation estimation models.
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As solar radiation passes through the Earth's atmosphere, it is absorbed (the reason for some atmospheric heating), reflected (the reason astronauts can see the Earth from outer space), scattered (the reason one can read this book in the shade under a tree), and transmitted directly (the reason there are shadows). At the surface of the Earth, the sun has a lower intensity, a different color, and a different shape from that observed above the atmosphere. Therefore, in solar energy applications, the following parameters are commonly used in practice:

  • Direct normal solar irradiation/irradiance is the component that is involved in thermal (concentrating solar power, CSP) and photovoltaic concentration technology (concentrated photovoltaic, CPV).

  • Global horizontal solar irradiation/irradiance is the sum of direct and diffuse radiation received on a horizontal plane. Global horizontal irradiance is a reference radiation for the comparison of climatic zones; it is also essential parameter for calculation of radiation on a tilted plane.

  • Global tilted solar irradiation/irradiance is the total solar radiation received on a surface with defined tilt and azimuth, fixed or sun-tracking. This is the sum of the scattered radiation, direct and reflected. It is a reference for photovoltaic (PV) applications, and can be occasionally affected by shadow.

On the surface of the Earth, we perceive a beam or direct solar irradiance, the component of solar radiation that is neither reflected nor scattered and which comes directly from the disc of the sun and directly reaches the surface; this is the component that produces shadows. The component of solar radiation that is scattered by the atmosphere, and which reaches the ground is called diffuse radiation. This component appears to come from all directions over the entire sky. In this text we will use the term direct to signify solar irradiance coming directly from the sun’s disc, and the term diffuse to indicate solar irradiance coming from all other directions. We use the traditional subscript “b” to represent the direct component of solar irradiance and the subscript “d” to indicate the diffuse component. The sum of direct and diffuse solar irradiance is called the global or total solar irradiance and is identified by the traditional subscript “t”. In the case of inclined surfaces, there is a small part of the radiation reflected by the surface and reaching an inclined plane is called the reflected radiation. These three components together create global radiation.

On a clear day, direct solar irradiance represents about 80 or 90 percent of the total amount of solar energy reaching the surface of the earth. Local blockage of the direct component of solar irradiance produces shadows. On a cloudy or foggy day when “you can’t see the Sun,” the direct component of solar irradiance is essentially zero and there are no shadows. The direct component of solar irradiance is of the greatest interest to designers of high-temperature solar energy systems because it can be concentrated on small areas using mirrors or lenses, whereas the diffuse component cannot.

The diffuse or scattered component of solar irradiance is what permits us to see in the shade. If there was no diffuse component of solar irradiance, the sky would appear black as at night and stars would be visible throughout the day. The first astronauts vividly described this phenomenon to us from the moon where there is no atmosphere to scatter the solar radiation.

The diffuse radiation is the result of downward scattering of solar irradiance by nitrogen, oxygen, and water molecules, water droplets, and dust particles in the atmosphere. The amount of this scattering depends on the amount of water and dust in the atmosphere and the altitude of the observer above sea level.

Since diffuse solar irradiance cannot be concentrated, only flat-plate (non-concentrating) solar collectors and some low-temperature types of concentrators (having wide acceptance angles) can collect diffuse solar irradiance. Few of the collectors used in industrial applications can utilize the diffuse component of solar radiation. In this book we will use the term global to indicate this sum.

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