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.

  Ground-Level Solar Radiation Characteristics

As solar radiation passes through the earth 's atmosphere, it undergo various processes. The four main process that occur with solar radiations are Absorption, reflection, scattering and transmittance.

  • it is absorbed by water vapours and various particles and gases present in Earth; this is the reason for some atmospheric heating;
  • many radiation get reflected back, the reason astronauts can see the earth from outer space;
  • sunlight also get scattered, the reason one can read book in the shade under a tree;
  • and transmitted directly, the reason there are shadows

    As solar radiation passes through the earth 's atmosphere, it undergo various processes. The four main process that occur with solar radiations are Absorption, reflection, scattering and transmittance.

    • it is absorbed by water vapours and various particles and gases present in Earth; this is the reason for some atmospheric heating;
    • many radiation get reflected back, the reason astronauts can see the earth from outer space;
    • sunlight also get scattered, the reason one can read 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. On the surface of the earth, we perceive two types of solar radiation; direct radiation and diffused radiation

  • beamor direct solar irradiance that comes directly from the disc of the sun
  • diffuseor scattered solar irradiance that appear to come from all directions over the entire sky.

In this tutorial 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

  Measurement of Solar Irradiance

As we are talking about the radiation characteristics so it will be appropriate to discuss briefly about the basic instruments used for measuring solar energy in meteorology.

PyranometersThe instrument measures the sum of the direct and the diffuse solar irradiance and is called the global solar irradiance.

Pyrheliometers - To measure the direct normal component of the solar irradiance only, an instrument called a Normal Incidence Pyrheliometer or NIP also. The tube type structure only allow the direct radiation and does not allow the diffused portion to enter the sensor.

Campbell-Stokes Sunshine Recorder - this instrument consists of a glass sphere that focuses the direct solar radiation and burns a trace on a special pasteboard card. The instrument is installed in all meteorological observatories to record sunshine hours and solar intensity.




In Physics, energy is defined as the ability or potential to perform work or cause changes. We have seen in the previous section that energy from sun undergo changes. The major changes that we the Environmentalist are concern about are:

Reflection –

Part of the solar energy that comes to Earth is reflected back out to space in the same, short wavelengths in which it came to Earth. The percentage of solar energy that is reflected back to space is called the Albedo. Average albedo of Earth is 30%  (Earth’s albedo = 0.3, 1 = perfect reflection). Examples:

  • Ocean surfaces -26%
  • Rain forests - 15%
  • Deserts 40%
  • Cloud usually has a higher albedo than land surface

Emission -

Another part of the energy going back to space from the Earth is the long wavelength radiation emitted by the Earth. This solar radiation when absorbed by the Earth increases the planet's temperature. The process of absorption of outgoing long wavelength radiation by various gases and water vapours present on earth is known as Green House Effect. The greenhouse effect is mainly due to water vapour in the atmosphere. However, various anthropogenic activities have enhanced carbon dioxide, methane and other infrared-absorbing gases which is escalating the process. This green house effect then cause increase in atmospheric temperature which is termed as Global Warming.

 Point to remember:-

The term albedo is used only for short wavelength (means high energy radiation) whereas the green house effect is only caused by long wavelength (means low energy radiation).



Coal is a combustible black or brownish-black sedimentary rock composed mostly of carbon and hydrocarbons. The energy in coal comes from the energy stored by plants that lived hundreds of millions of years ago, when the earth was partly covered with swampy forests.

During the formation of coal, carbonaceous matter was first compressed into a spongy material called "peat," which is about 90% water. As the peat became more deeply buried, the increased pressure and temperature of earth turned it into hard coal.

Types of Coal

Graphite is technically the highest rank/grade of coal. But, it is difficult to ignite graphite and thus it is not used as fuel. It is mostly used in pencils lead and can be used as lubricant when in powdered form.

Anthracite: It is the highest quality of hard coal

Bituminous: This coal has been buried deep in earth and subjected to increased temperatures and pressure. It is the most popular coal in commercial use. This grade of coal has high sulfur content. The category is further divided into sub-bituminous coal.

Lignite: It is a low grade brown coal, which is soft with high moisture content.

Peat: this type of coal is only 10% carbon and 90% moisture therefore cannot be used as fuel. Decaying plants in swamps produce peat, which has low carbon content and high moisture contents with the result low heating capacity. Peat is a highly effective absorbent for fuel and oil spills on land and water. It is also used as a conditioner for soil to make it more able to retain and slowly release water.

The table given below gives a brief idea about the carbon content and calorific value of coal

Just to revise; Heating Value/Calorific Value is the amount of heat produced when unit mass of fuel is burned (1kg of solid/ 1 lt of liquid/ 1 cubic meter of gas).

The commercial usage of type of coal in thermal power plants is subjected to its heating value, ash content and sulfur content etc parameters.

 Point to remember:-

India is bestowed with high coal reserves mainly concentrated in BiharJharkhandOrissaMadhya Pradesh, Chhattisgarh and West Bengal. The Indian coal produces excess of fly ash as well as bottom ash with lower calorific value.

Before going further to other fossil fuels, it is important to know briefly the mining process of coal.

Types of Coal Mining-

  • Surface Mining/Open pit mining – this type of mining is done when coal is prsent in upper layer of soil generally at 200 ft depth. The activity spoils the fertile top layer of soil and produce lot of dust and erosion in surrounding areas.


  • Underground Mining/Shaft Mining - this type of mining is done when coal is prsent in deep layer of soil generally at 1000 ft depth. The mining is done by making underground passages/tunnels. The risks to workers is much more in this type of mining as they are subjected to underground fires (due to methane trapped inside) and land subsidence.


General law that applies to mining activity is -

Hilt’s Law : is a geological term that states that, in a small area, the deeper the coal, the higher its rank (grade). The law holds true if the thermal gradient is entirely vertical, but metamorphism may cause lateral changes of rank, irrespective of depth.

 Point to remember:-

As Coal mining is a major industry in India so Government have stringent rules for approval process for its mining. Environment Impact Assessment (EIA) for getting Environmental Clearance (EC) has different set of thresholds for mining. The screenshot of Government Notification for EIA states that if the mine lease area for coal mining is more than 150 hactares then the proponent needs to get EC from Central Government (MoEFCC) and if the area is from 5 hactares to 150 hactares then the EC will be given by respective State Government (SEIA).


Coal as a fuel

Approximately 60% of electricity production in India is through thermal power plants which uses coal as a fuel.

When coal is used for electricity generation, it is usually pulverized (powwdered) and then combusted (burned) in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity.


The process in thermal power plant thus converts heat energy into mechanical energy (fuel burned produce steam which spin turbine) and to electrical energy (the shaft of the turbine spins a magnet inside coils of conductor which induces current)

Thermal efficiencies of old coal fired plants are 25%. It means if we burn 100 kg coal we actually get energy equivalent to 25kg coal rest all the energy go waste. The efficiency depends on fuel, engine and many other factors. Research is continously been done to increase the efficiency of thermal power plant but most of the research till date are not applied on commercial scale.

Approaches to increase efficiency of Coal Fired Thermal Power Plants

An alternative approach of using coal for electricity generation with improved efficiency is the Integrated Gasification Combined Cycle (IGCC) power plant. Instead of pulverizing the coal and burning it directly as fuel in the steam-generating boiler, the coal can be first gasified to create syngas, which is burned in a gas turbine to produce electricity . This technique can increase the thermal efficiencies of plants to a range of 39-42%.

Coal Liquefaction (CTL technology) – In this technique coal is liquified by applying high temprature or pressure to burn it as clean fuel with higher burning efficiceny. But the process increases the energy consumption of plant thus not economically efficient and also have high carbon footprints.

Point to remember:-

Here, we can also go through the Indian Government clearance (EIA) requirment for thermal power plants.

Clearance from Center; if the coal fired thermal power plant will generate more than or equal to 500MW of electricity and if it generate energy in range 50 – 500 MW then the clearnce shall be taken by State Govt.