Solar Water Heaters – Principle and Applications

Introduction; Solar water heaters (SWHs) are devices that harness solar energy to heat water. These systems are environment-friendly, renewable, and offer a sustainable alternative to conventional water heating methods, which often rely on fossil fuels.

Advantages of Solar Energy:

• Inexhaustible and renewable
• Pollution-free
• No production of greenhouse gases or toxic by-products
• Reduces dependency on non-renewable energy sources
• Available in abundance in a country like India

Solar Energy Potential in India

• Total land area: 3.28 × 10¹¹ m²
• Average sunshine days/year: ~300 days
• Average solar insolation: 500 W/m²
• Sunshine hours/day: 5 hours

Calculated Potential:

• Total energy/day: 3.28 × 10¹¹ × 500 × 5 × 3600 = 2.952 × 10¹µ J = 2.952 × 10⁹ MJ/day
• Annual energy: 2.952 × 10⁹ MJ/day × 300 = 8.856 × 10¹¹ MJ/year
• If 1% land is used with 10% efficiency: 8.856 × 10¹¹ × 0.01 × 0.1 = 8.856 × 10⁸ MJ/year usable energy

Principle of Solar Water Heater

“Solar radiation is absorbed by a black surface and transferred to water, raising its temperature.”

• Works on the Thermosyphon principle (natural circulation due to density difference in hot and cold water)
• Water circulates without any mechanical pump

Heat Transfer:

• Radiation: Solar rays absorbed by black surface
• Conduction: Heat transferred to water through metal
• Convection: Heated water rises, cool water sinks

Main Components

1. Solar Collector: Absorbs solar radiation and transfers heat
2. Storage Tank: Insulated container to store hot water
3. Piping System: Connects collector to tank and usage points
4. Mounting Stand: Keeps collector at optimal angle (usually 11° facing south)

Types of Solar Collectors

a) Based on Collecting Characteristics:

• Flat-Plate Collectors (FPC): Flat absorbing surface without focusing
• Concentrating Collectors: Use reflectors/lenses to focus sunlight

b) Based on Mounting:

• Fixed Tilt: Angle adjusted seasonally
• Tracking: Moves with sun (expensive)

c) Based on Fluid:

• Liquid-based: Water, glycol solution
• Air-based: Air as heat transfer medium
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Construction of Flat-Plate Collector

• Absorber Plate: Corrugated GI sheet, painted black
• Tubes: 12 mm GI or copper in serpentine form
• Insulation: Wooden box with thermal insulation (glass wool/mineral wool)
• Cover: Glass/plastic sheet (transparent)
• Storage Drum: Insulated tank above the collector
• Stand: Angle iron frame for tilting at 11°

Specifications:

• Absorber plate thickness: 1–2 mm
• Tube diameter: 1–1.5 cm
• Glass cover thickness: 4 mm (tempered)

System Working

• Cold water from overhead tank enters collector
• Absorber plate heats up under sun
• Water in tubes heats and rises to storage tank
• Natural circulation continues as long as sunlight is available
• Hot water available at 55–63°C
• Overnight heat loss: 4–8°C

Capacity and Installation

• Basic unit: 100 litres/day
• Area required: 25 sq. ft for collector + 10 sq. ft for accessories
• Usage: Suitable for 4–6 family members
• Lifespan: 15+ years

Applications

• Domestic: Bathing. Kitchen washing
• Commercial: Hotels. Hospitals. Hostels
• Industrial: Boiler feed water. Food processing (e.g., dairy, jaggery units)
• Agricultural: Cleaning equipment. Pre-heating in solar dryers

Advantages

Limitations