A pyranometer is an instrument used to measure global solar radiation (beam + diffuse) on a horizontal surface over a hemispherical field of view. It is widely used in meteorology, climatology, solar energy studies, and agriculture.
2. Principle of Operation
- A pyranometer works on the thermoelectric effect.
- It consists of a thermopile with:
- Hot junctions: blackened and exposed to solar radiation.
- Cold junctions: shaded from sunlight.
- When solar radiation strikes the hot junctions, a temperature difference is created between the hot and cold junctions.
- This temperature difference generates a small voltage (e.m.f) which is proportional to the intensity of solar radiation.
3. Construction Details
- Thermopile Sensor: Circular black surface where hot junctions are exposed; cold junctions are insulated.
- Glass Domes: Two concentric hemispherical domes protect the sensor from:
- Wind
- Rain
- Dust
→ Also reduce convection losses.
- Radiation Shield: Prevents heating of the base by direct solar radiation.
- Output Voltage: Typically in the range of 0 to 10 mV, calibrated to read in W/m².
- Sensitivity: About 9 μV/W/m² with an output impedance of 650 Ω.
4. Working Mechanism
- Sunlight (beam + diffuse radiation) passes through the glass dome.
- It falls on the blackened surface (hot junctions) of the thermopile.
- Cold junctions remain shaded → temperature difference is created.
- The thermopile converts this temperature difference into a measurable voltage.
- This voltage is directly proportional to solar radiation intensity.
5. Types of Pyranometers
- Eppley Pyranometer
- Yellot Solarimeter (PV-based)
- Moll-Gorczyheski Solarimeter
- Rabitzsch Type Bimetallic Actinograph
- Thermoelectric Pyranometer
- Kipp & Zonen Precision Pyranometers (e.g., PSP)
6. Neat Sketch of a Pyranometer
7. Applications
- Solar panel performance monitoring
- Solar resource assessment for power plants
- Weather stations and atmospheric studies
- Building energy modeling