1.3 Solar Radiation
In account of solar energy always should starts with the energy source as the sun. The mass of the sun is approximately 1024 tonnes, a diameter of 1,392,082 km, and energy radiation takes place at the extend of 3.8 × 1020 megawatts [9]. For a billion of years, the stated results remain unchanged in a present scenario, but definitely from time to time, there are minor changes occur in the radiation of sun energy. Based on these factors, we have been considering the constant solar energy.
Solar constant may be as the average of incoming solar energy per unit area, and it has been measured on outer surface atmosphere in a plane perpendicular to the rays with a numerical value of 0.1353 W/cm2. Solar Constant (S) = 1350 W/m2 (approximately) [10].
1.4 Heat Transfer Principles
Heat transfer is one of the energy transfers from high body region to a lower body region due to the potential difference of temperature gradient. There are numerous applications involved under this disciplines, which are as follows:
✓ Internal combustion engines
✓ Refrigerators and air conditioning systems
✓ Heating and cooling of fluids
✓ Radiators, etc.
The heat transfer is classified into three various modes: heat conduction, convention, and radiation. The same has been depicted in Figure 1.1.
1.4.1 Conduction
Heat conduction is a mechanism of heat transmission that takes place from one part of a material to another part of same material or from one region to another region through some physical contact that extends it. Pure conduction heat transfer takes place only in solids.
1.4.2 Convection
Convection heat transfer takes place between a solid body and fluid medium where the temperatures are different. Convection process always involves in case of fluid medium.
1.4.3 Radiation
Radiation is a process of heat transfer that will occur between two different body temperatures without a solid or a fluid medium as transmitting medium. The example of this process is electromagnetic waves where no medium is mandatory for its propagation.
Figure 1.1 Various modes of heat transfer.
All the electromagnetic waves are categorized in terms of wavelength and it travels at a rate of speed as light which is C = 2.998 × 108 m/s.
Properties of emission: The emission radiation depends on the following aspects by a body:
✓ Surface temperature
✓ Nature of surface
✓ Radiation frequency/wavelength
The following are the important parameters to deal the emission properties that are noted as emissive power (Eb), monochromatic emissive power (Ebʎ), emissivity (€), radiation intensity, radiosity, density, and pressure.
Concept of Black Body
A black body is an object which absorbs all the energy radiation from outer atmosphere to its receiving surface. A black body is a perfect emitter. For a black body α = 1, α = 0, and τ = 0.
Black body is an ideal surface that has the following properties:
✓ Black body absorbs all incident radiation irrespective of wavelength and directions.
✓ At a prescribed temperature, it emits larger amount of thermal radiation.
1.5 Basic Laws of Radiation
1.5.1 Stefan-Boltzmann Law
The Stefan-Boltzmann law is as the emissive power of a black body that is directly proportional to fourth power of its absolute temperature.
where Eb - Black body emissive power (W/m2)
σ - Stefan Boltzmann constant—5.67 × 10–8 W/m2 K4
T - Absolute temperature (K)
1.5.2 Planck’s Law
Plank’s Law states that spectral distribution of radiation intensity of perfect black body is expressed by the following equation:
where
(Eb)λ - Single wavelength emissive power of a body
c - Light velocity
h - Planck’s constant
ʎ - Wavelength
k - Boltzmann constant
T - Absolute temperature
1.5.3 Wien’s Displacement Law
The relation between maximum emissive power wavelength and temperature of black body in given by the following expression:
1.6 Solar Energy Conversion
Solar energy can be converted into some other useful form of energy sources using different conversion technologies as in Table 1.4. The methods are photoelectric, photoelectric-photothermal, photothermal, photochemical, photocatalytic, and photo-electrochemical conversion.
Table 1.4 Energy conversion [11].
| Energy | From light | From chemical |
From
electrical
|