Jawaharlal Nehru Technological University Hyderabad B.Tech III Year NON-CONVENTIONAL POWER GENERATION Previous Question Paper (August - 2022 - R16)

A pyranometer is an instrument used to measure the total solar radiation (global radiation) received from the entire hemisphere on a horizontal surface. It typically consists of a thermopile sensor protected by a glass dome.

The working principle is based on the Seebeck effect. The sensor consists of a series of thermocouples arranged in a thermopile configuration. One set of junctions (hot junctions) is blackened to absorb incident solar radiation, while the other set (cold junctions) is kept in thermal contact with the instrument body, which acts as a heat sink. When solar radiation strikes the blackened surface, it heats up, creating a temperature difference between the hot and cold junctions. This temperature gradient generates a small electromotive force (EMF) proportional to the intensity of the incident solar radiation. This voltage is measured by a sensitive millivoltmeter or data logger to determine the solar irradiance in W/m².

The glass dome serves two purposes: it protects the sensor from environmental elements and acts as a filter, allowing short-wave solar radiation to pass through while blocking long-wave thermal radiation emitted by the instrument itself.

Altitude angle (α): The angle between the sun's rays and the horizontal plane. It is the complement of the zenith angle and varies throughout the day based on the sun's position.

Incident angle (θ): The angle between the sun's rays and the normal (perpendicular) to the surface receiving the radiation. It determines the effective intensity of solar radiation falling on a tilted or horizontal surface.

Zenith angle (θ_z): The angle between the sun's rays and the vertical (the line perpendicular to the horizontal ground). It is calculated as 90 degrees minus the altitude angle.