Source of Solar Energy

Solar energy is energy emitted by a star. Figure 4-1 shows the anatomy of a star. Energy emitted by a star is generated by nuclear fusion. The fusion process occurs in the core, or center, of the star. Energy released by the fusion process propagates away from the core by radiating from one atom to another in the radiation zone of the star. As the energy moves away from the core and passes through the radiation zone, it reaches the part of the star where energy continues its journey towards the surface of the star as heat associated with thermal gradients. This part of the star is called the convection zone. The surface of the star, called the photosphere, emits light in the visible part of the electromagnetic spectrum. The star is engulfed in a stellar atmosphere called the chromosphere. The chromosphere is a layer of hot gases surrounding the photosphere.

Chromosphere

Convection Zone

Photosphere (Surface)

Corona (outer atmosphere)

Figure 4-1. Anatomy of a Star

Chromosphere

Convection Zone

Photosphere (Surface)

Corona (outer atmosphere)

Figure 4-1. Anatomy of a Star

The amount of sunlight that reaches the Earth depends on the motion of the Earth around the Sun. Figure 4-2 illustrates the orbit of the Earth around the Sun. Most planetary orbits lie in the ecliptic plane. The ecliptic plane is the plane of the orbit that intersects the Sun. It is shown in Figure 4-2. The line of intersection between the orbital plane and the ecliptic plane is the line of nodes.

The luminosity of a star is the total energy radiated per second by the star. The amount of radiation from the Sun that reaches the Earth's atmosphere is called the solar constant. The solar constant varies with time because the Earth follows an elliptical orbit around the Sun and the axis of rotation of the Earth is inclined relative to the plane of the Earth's orbit. The distance between points on the surface of the Earth and the Sun varies throughout the year.

The amount of solar radiation that reaches the surface of the Earth depends on the factors illustrated in Figure 4-3. The flux of solar radiation incident on a surface placed at the edge of the Earth's atmosphere depends on the time of day and year, and the geographical location of the surface.

Some incident solar radiation is reflected by the Earth's atmosphere. The fraction of solar radiation that is reflected back into space by the Earth-atmosphere system is called the albedo. Approximately thirty-five percent of the light from the Sun does not reach the surface of the Earth. This is due to clouds (20%), atmospheric particles (10%), and reflection by the Earth's surface (5%). The solar flux that enters the atmosphere is reduced by the albedo. Once in the atmosphere, solar radiation can be absorbed in the atmosphere or scattered away from the Earth's surface by atmospheric particulates such as air, water vapor, dust particles, and aerosols. Some of the light that is scattered by the atmosphere eventually reaches the surface of the Earth as diffused light. Solar radiation that reaches the Earth's surface from the disk of the Sun is called direct solar radiation if it has experienced negligible change in its original direction of propagation.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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