Unit 2 | MNGT 801 Notes | Solar Energy Technology and Applications Notes | AKTU Notes

2.1 What is Solar Thermal Energy?

Solar thermal energy means using sunlight to produce heat. This heat can be used for:

• Heating water (domestic and industrial)
• Cooking food
• Drying crops
• Generating electricity (through steam turbines)

The device that captures solar energy and converts it to heat is called a Solar Collector.

2.2 Fundamentals of Thermal Collectors

A solar thermal collector absorbs sunlight and transfers the heat to a fluid (usually water or air).

Basic Principle:

• Sunlight falls on a dark-colored absorber surface (black absorbs maximum heat).
• The absorber heats up.
• This heat is transferred to a fluid flowing through or over the absorber.
• The hot fluid is then used for heating purposes.

Key Parameters:

• Absorptivity (α): How well the surface absorbs solar radiation. (Should be high)
• Emissivity (ε): How much heat the surface radiates back. (Should be low)
• Transmissivity (τ): For glazing — how much light passes through glass.
• Reflectivity (ρ): How much light is reflected. (Should be low)

Ideal absorber surface: High α, Low ε — called a selective surface.

2.3 Flat Plate Collectors (FPC)

This is the most widely used solar thermal device in homes and small industries.

Construction:

• Absorber Plate – Dark-colored metal plate (copper or aluminum) that absorbs sunlight.
• Tubes/Channels – Water flows through tubes attached to the absorber plate.
• Glazing (Glass Cover) – One or two glass covers on top. Glass allows sunlight in but traps the heat (greenhouse effect).
• Insulation – At the back and sides to reduce heat loss.
• Casing – A box to hold everything together.

Working:

• Sunlight passes through glass and hits the black absorber plate.
• Plate gets hot and heats the water in the tubes.
• Hot water rises up (natural convection) and is collected in a storage tank.
• Cold water enters from the bottom — this is called thermosyphon flow.

Types of FPC:

• Liquid-type: Water or antifreeze flows through tubes.
• Air-type: Air flows over the absorber plate. Used for space heating and crop drying.

Advantages:

• Simple design, low cost
• Works even on cloudy days (uses diffuse radiation too)
• Low maintenance

Limitations:

• Cannot achieve very high temperatures (max ~80°C)
• Less efficient in cold/cloudy climates

2.4 Thermal Energy Storage Systems

The Sun doesn't shine 24 hours. So we need to store the heat collected during the day for use at night or on cloudy days.

Types of Storage:

1. Sensible Heat Storage:

• Heat is stored by raising the temperature of a material.
• Most common material: Water (cheap, high heat capacity)
• Also used: rocks, sand, soil, concrete
• Example: A hot water tank stores solar-heated water.

2. Latent Heat Storage:

• Heat is stored when a material changes its phase (solid to liquid or liquid to gas).
• Material used = Phase Change Material (PCM)
• Example: Paraffin wax, sodium sulphate decahydrate (Glauber's salt)
• Advantage: Stores much more energy in a smaller volume than sensible storage.

3. Thermochemical Storage:

• Heat is stored through reversible chemical reactions.
• High energy density but complex technology.
• Still in research/development stage.

2.5 Solar Thermal vs Solar PV — Basic Comparison

Parameter	Solar Thermal	Solar PV
Output	Heat	Electricity
Efficiency	50–70%	15–22%
Use	Water/Space Heating	Power Generation
Storage	Hot Water Tank	Battery