The sun

Solar energy is harvested using solar radiation, so it becomes necessary to understand solar radiation and how it interacts with the energy conversion devices to produce widely usable energy.  Hence, we start with understanding some facts about Sun. Sun, the largest member of the solar system with about 15 million kelvins temperature at its core and 6000K temperature at the surface, is a sphere of intensely hot gaseous matter. 3.8 x 1026 W of energy is radiated from it and roughly about 1.7 x 1018 W of energy reaches the earth. This energy is generated from the fusion reactions taking place at its core. It is approximately 1.5 x 108 km away from earth and at any given time, half of the earth is lit by the sun’s radiation.

Radiation Theories

Now getting back to radiation, there were many theories given by researchers but the two most popular ones which explain radiation were given by Max Planck and James Maxwell. To simply understand these theories, Maxwell describes radiation as electromagnetic waves and Planck describes radiation as photon or quantum of energy. You may be wondered what relevance do they have in PV, but understanding both of them makes us comprehend the photovoltaic devices and their operating principles better. To elucidate this further, when we will discuss solar cells, we will treat light as waves when reflection losses are studied and light as photon when semiconductor properties are studied. Both these theories are used to understand the emission and propagation of light.

It is very important to note here that the results obtained from the electromagnetic theory of Maxwell are used to predict the radiation properties of materials. Modern physics has four differential equations of Maxwell’s theory but we need not list them here. On the other hand, Planck’s theory is used to predict the magnitude of radiation energy emitted by a body at a given temperature.

Blackbody Radiation:

So how does energy reach here on earth from the sun? For this, we have to understand blackbody radiation. What is a blackbody? It is an ideal absorber or emitter of radiation. Anything that is incident on it will be absorbed and it will emit radiation according to its temperature. Anything in nature that is above absolute zero temperature emits radiation. A blackbody is named so because it will absorb all the radiation incident on it and thus appear black. If we consider Sun as an ideal blackbody, the fusion reactions at its core generate heat which is brought to the surface by convection (it is a heat transfer activity where heat is considered to be transferred within fluids from the hotter side to the cooler side). Now the surface being at 6000K, generates radiations which transfer through the vacuum and earth’s atmosphere to reach earth. How do we understand what sort of radiation is generated from blackbody? The spectrum of radiation can be known from Planck’s equation:

\[F( \lambda ) \ =\ \frac{2\pi hc^{2}}{\lambda ^{5}\left( exp\left(\frac{hc}{\lambda kT}\right) -1\right)}\]

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