Energy Resources : Basics

  Basic of Energy Resources : Solar Energy

All forms of energy are derived by Sun. Sun is the direct or indirect form of energy in every energy resource that we consume today, either it’s renewable of non-renewable. For example all fossil fuels formed by the carbon and biomass accumulated by plant and animals millions years ago making coal, petroleum and natural gas, winds move due to unequal heating of earth causing wind energy, hydro power is also indirectly connected to sun as the hydrological cycle solely depends on evaporation caused by solar heating. In this way sun control all forms of energy.

Thus, Sun is said to be ultimate source of energy

The sun, our singular source of energy, sits at the centre of the solar system and emits energy as electromagnetic radiation at an extremely large and relatively constant rate, 24 hours per day, 365 days of the year.

  Solar irradiance / Insolation

The rate at which solar energy reaches a unit area at the earth is known as Solar Irradiance or Insolation.  It is measured in Watts per square meter (W/m2). It is very important to know how much solar energy has fallen on a collector over a period of time such as a day, week or year; if you are designing a solar energy collection system.

Therefore, its not just the area that receive the solar radiation but also the time factor that plays very important role in knowing the energy coming from sun. The summation of solar energy reaching a unit area per unit time is called solar radiation or irradiation. The units of measure for solar radiation are Joules per square meter (J/m2) but often watt-hours per square meter (Wh/m2) are used.

Solar radiation here truly refers to the electromagnetic radiation (EMR) that reaches the Earth from the Sun. At an average distance of Earth to be 150 million kilometres from the Sun, the outer atmosphere of Earth receives approximately 1367 W/m² of insolation (World Meteorological Organisation). This varies by around ±2% due to fluctuations in emissions from the Sun itself as well as by ±3.5% due to seasonal variations in distance and solar altitude.

  Irradiance, Irradiation and Illuminance

Irradiance (E) - The total specific radiant power, or radiant flux, per unit area that reaches a terrestrial receiver surface is called irradiance. Irradiance is measured in W/m² and has the symbol E.

Irradiation (H) - When integrating the irradiance over a certain time period it becomes solar irradiation. Irradiation is measured in either   J/m² or Wh/m², and represented by the symbol H.

Illuminance - For daylighting purposes, only the visible part of the sunlight is considered. The analogous quantity to the irradiance for visible light is the illuminance. This uses the unit lm/m² (lumen/m²) or lx (lux). 

  Solar Spectrum

It is the surface temperature of the sun that mainly characterizes the solar spectrum. This spectrum defines the corresponding spectral irradiance for all wavelengths of sunlight. Visible light, with wavelengths between 0.4 µm and 0.75 µm, has a 46% share of the spectrum, infrared light 47%, and ultraviolet light only 7%

The earth’s atmosphere reduces the irradiance that reaches the earth’s surface. Ozone, water vapour and carbon dioxide absorb radiation with certain wavelengths as it passes through the atmosphere.

Solar radiations undergo various changes during its transmission from sun till it reaches the target on the Earth. As we are dealing with energy resource studies so we will focus basically on the ground level solar radiation characteristics. The atmospheric level radition characteristics will be discussed in other related tutorials.