1.1 Global and Indian Energy Scenario
The world runs on energy. Today, most of this energy comes from fossil fuels like coal, oil, and natural gas. But these fuels are limited — they will run out one day. Also, burning them causes pollution and climate change.
Global Scenario:
- World energy demand is increasing every year due to population growth and industrialization.
- Fossil fuels meet about 80% of global energy needs.
- Countries are now shifting towards renewable energy (solar, wind, hydro) to reduce dependence on fossil fuels.
Indian Scenario:
- India is the 3rd largest energy consumer in the world.
- India has huge potential for solar energy because it receives sunlight for 250–300 days per year.
- The Indian government launched the National Solar Mission targeting 100 GW of solar power.
- States like Rajasthan, Gujarat, and Madhya Pradesh have high solar radiation levels.
1.2 Need for Solar Energy
- Fossil fuels are getting expensive and will eventually run out.
- Solar energy is free, clean, and available everywhere.
- It does not produce greenhouse gases or air pollution.
- It can be used in remote areas where electricity grids don't reach.
- It reduces India's dependence on imported oil and gas.
1.3 Propagation of Solar Radiation from Sun to Earth
The Sun is basically a giant ball of hot gases (mostly hydrogen and helium). It produces energy through a process called nuclear fusion — hydrogen atoms combine to form helium, releasing a massive amount of energy.
This energy travels from the Sun to Earth in the form of electromagnetic radiation (light and heat waves). This is called solar radiation.
- Distance from Sun to Earth = approximately 150 million km
- Light takes about 8 minutes 20 seconds to travel from Sun to Earth.
- Solar Constant = 1367 W/m²
1.4 Solar Radiation Spectrum
Solar radiation is not just visible light — it includes a range of wavelengths:
| Type | Wavelength | % of Total Energy |
|---|---|---|
| Ultraviolet (UV) | < 0.4 µm | ~9% |
| Visible Light | 0.4 – 0.7 µm | ~45% |
| Infrared (IR) | > 0.7 µm | ~46% |
Most solar energy reaching Earth is in the visible and near-infrared range.
1.5 Extra-terrestrial and Terrestrial Radiation
Extra-terrestrial Radiation:
- This is the solar radiation available outside Earth's atmosphere.
- It is relatively constant (about 1367 W/m²).
- There is no air, dust, or clouds to block it.
Terrestrial Radiation:
- This is the solar radiation that reaches Earth's surface after passing through the atmosphere.
- On a clear day, about 800–1000 W/m² reaches the ground.
- Absorption by gases like ozone, water vapor, CO₂
- Scattering by air molecules
- Reflection by clouds
1.6 Components of Solar Radiation on Earth's Surface
When sunlight hits Earth's surface, it has two components:
1. Beam (Direct) Radiation: Comes directly from the Sun in a straight line. This is used in concentrating collectors.
2. Diffuse Radiation: Scattered by clouds and atmosphere, comes from all directions of the sky.
3. Reflected Radiation (Albedo): Reflected from the ground and surrounding surfaces.
Global Radiation = Beam + Diffuse + Reflected
1.7 Solar Energy Measuring Instruments
To use solar energy properly, we need to measure it accurately. Here are the main instruments:
1. Pyrheliometer:
- Measures direct (beam) radiation from the Sun.
- Points directly at the Sun.
- Used at weather stations.
2. Pyranometer:
- Measures total (global) radiation — both direct and diffuse.
- Has a glass dome to collect radiation from the entire sky hemisphere.
- Most commonly used instrument.
3. Sunshine Recorder (Campbell-Stokes Recorder):
- Records the number of sunshine hours in a day.
- Uses a glass sphere that focuses sunlight on a card, burning a trace.
4. Albedometer:
- Measures reflected radiation from the ground.
- Two pyranometers — one facing up, one facing down.
1.8 Solar Radiation Estimation
We cannot always measure solar radiation directly. So we estimate it using formulas.
Angstrom Formula (most common):
H/H₀ = a + b(n/N)
Where:
H = monthly average daily radiation on horizontal surface
H₀ = extra-terrestrial radiation
n = actual sunshine hours
N = maximum possible sunshine hours
a, b = constants depending on location
This formula tells us that more sunshine hours = more solar radiation received.
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