Solar Energy

Nature of solar energy

Ultimate Source

• Solar energy represents the ultimate source of all energy on earth.
• All energy sources we have seen so far is simply energy from sun that was captured, be it in the form of ATP in plant biomass or energy in hydrocarbons in fossils.
• However, the amount of energy plant biomass or fossils have captured is a small proportion of energy from the sun.
• Thus, if we can find ways to access energy from the sun directly, we can increase the energy captured.

Intermittent Nature

• Besides since energy from the sun is accessible only during the day, solar energy, like other renewable sources, suffer from what is called as intermittency problem.
• As a result, an essential component of renewable energy including solar energy is energy storage.
• This is why India is mooting an Energy Storage Policy towards large-scale integration of renewable energy to the grid.

Decentralised Source

• Solar energy represents decentralized energy source as sunlight represents distributed energy resource.
• This said, India’s share of decentralized renewable energy is meagre 6% of the total renewable energy capacity.
• The low share of decentralized solar energy, about 6.5 GW as against the target of 40GW, is the main reason for missing the target of 175GW of RE by 2022.

Pathways of Capturing Energy From The Sun

There are different ways to capture energy from the sun including:

• Solar thermal
• Solar concentrators
• Solar PV cells

Solar Thermal

• The basic principle behind solar thermal is the green house effect. It is way to capture sun’s energy in the form of heat.
• Since heat is low quality energy it has limited applications and thus suitable only for domestic applications like solar water heaters, solar pumps etc.

Principle: Greenhouse effect

• Solar energy is an admixture of various electromagnetic radiation with visible light constituting the maximum proportion. See figure for sun’s emission spectrum.

• Thus, in order to gain maximum from sunlight we need to capture visible light from the sun.
• In order to do this, we use the principle of greenhouse effect demonstrated by glass.
• Glass is transparent to visible light and opaque to infrared light.
• So, glass cover is used in the top layer to allow visible light and block the infrared from the sun.
• This visible light is made to fall on a metal plate coated black which absorbs most of the visible light. (absorber in the figure)
• As a result of absorption of energy, the black metal emits infrared radiation which cannot pass through the glass cover.

• Thus, energy from visible light is trapped between glass cover and metal plate.
• This energy is in the form of heat (convection current) which can be carried by water or air.
• Besides the direction of the set up should be such that it should be able maximize the sun’s rays incident on it.
• In northern hemisphere, it should be inclined at an angle equal to the latitude and south-facing.

Advantage

• The efficiency of solar thermal can reach upto 60%.

Disadvantage

• It is suitable for domestic applications as it cannot be scalable. (water is heated upto 60-70 degree celsius)

Solar concentrators

• In applications where you need temperatures above the boiling point of water you use solar concentrators.
• Solar concentrators use a bunch of mirrors in such a way to focus all the incoming solar energy at one point from where it can be collected and carried.
• Solar towers are an example of solar concentrators.
• At the top of the tower is a sodium salt which can hold heat for longer time thereby increasing the amount of heat that can be captured.

Advantage

• You can heat the salt and keep it for later use when there is no sunlight.
• Efficiency is as high as 50%.

Disadvantage

• Capital cost is high as mirrors are expensive.
• High land requirement: 1.5 hectare per mega watt.

Solar towers in India

• Bhadla solar park in Jodhpur Rajasthan.  (biggest plant in the world)