Solar energy is an inexhaustible source of energy that can be converted directly or indirectly into heat and electricity. The sun is expected to radiate at a constant rate for billions of years. It has the greatest potential of all renewable energy sources, but its diffuse nature and intermittency present challenges. Solar radiation is measured as the amount of energy received per unit time and area from the top of the atmosphere, known as the solar constant, which NASA estimates is approximately 1353 watts per square meter.

2. Solar Radiation and Measurements.
• Solar Energy can be converted either directly or indirectly
into other forms of energy such as heat and electricity.
• The sun is expected to radiate at an essentially constant rate
for a few billion years.
• So inexhautsible source of energy
• Solar energy reaching at the top of the atmosphere
8% UV radiation
46%Visible Light
46% Infrared Radiation

3. Solar Energy
• Major source of power
• Greatest potential of all the renewable sources
• Its potential – 178 billion MW
• About 20000 times the world’s demand
• Solar power where sun hits atmosphere - 1017W
• Solar power on earth’s surface - 1016W
Main Limitations
• Solar light is diffuse not concentrated
• Large space is required.
• Intermittent and variable manner in which it arrives at the
earth’s surface
• Presence of clouds, wind, haze etc.

4. Solar Constant - Isc
• The rate at which solar energy arrives at the top of the
atmosphere
• Solar Constant is the amount of energy received
• In unit time
• On a unit area
• Perpendicular to the sun’s direction
• At the mean distance of the earth from the Sun
• Since Sun’s activities vary, rate of arrival of solar radiation also
varies
• actual value – avg of these values (may vary about 3% on either
direction)

5. Contd……
NASA standard value of solar constant in three different units
• 1.353 kilowatts per square meter or 1353 watt per sq m
• 116.5 langleys (calories per sq. cm) per hour, or 1165 kcal
per sq m per hr (1 langley = 1cal/cm2)
• 429.2 Btu per sq ft per hour. (1 British thermal Unit Btu =
1055 J)

6. Solar Radiation at the earth’s surface

7. Beam Radiation
• Solar radiation that has not been absorbed or scattered and
reaches the ground directly from the Sun is direct or beam
radiation
•It produces a shadow when interrupted by an opaque object
Diffuse Radiation
•Solar radiation received from the Sun after its direction has
been changed by reflection and scattering from the atmosphere.
•Since scattered in all directions diffuse radiation comes from all
parts of the sky
Total or Global Radiation
Sum of direct and diffuse radiation