Solar irradiance is the power per unit area received from the Sun. This instantaneous measurement, typically expressed in watts per square meter (\(\text{W/m}^2\)), is distinct from solar insolation, which is the cumulative energy over a period of time. Measuring solar irradiance is important for designing efficient solar power systems, predicting agricultural yields, and developing accurate climate models. Because the energy arriving at the Earth’s surface is not uniform, specialized instruments are required to capture its various components accurately.
Defining the Components of Solar Irradiance
The total solar energy reaching a location is separated into three components based on the path the light travels through the atmosphere.
Global Horizontal Irradiance (GHI) is the total solar radiation received on a flat, horizontal surface. This measurement is widely used for designing photovoltaic (PV) systems, as it represents all the sunlight available to a stationary, level panel.
GHI is composed of two elements: Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI). DNI is the solar radiation that travels in a straight line directly from the Sun to the Earth’s surface. It is measured on a surface held perpendicular (“normal”) to the sun’s rays and is utilized by concentrated solar power technologies.
DHI is the sunlight scattered by atmospheric elements like clouds, aerosols, and air molecules. This scattered radiation arrives at the surface from all directions. The relationship between these components is additive: GHI equals DHI plus the direct beam component on the horizontal plane (DNI multiplied by the cosine of the solar zenith angle).
Instrumentation Used for Measurement
Specialized instruments are required to isolate and quantify these different components of solar irradiance with high precision. The primary tool for measuring Global Horizontal Irradiance (GHI) and Diffuse Horizontal Irradiance (DHI) is the pyranometer. This instrument is designed to measure the total solar radiation flux density from the entire hemisphere above it.
A pyranometer is mounted horizontally and often uses protective domes to shield the sensor while allowing solar radiation to pass through. To measure DHI, a second pyranometer is used in conjunction with a shading mechanism, such as a shadow ball or ring, that physically blocks the direct beam from the Sun. This ensures the shaded pyranometer registers only the scattered, diffuse light.
The instrument dedicated to measuring Direct Normal Irradiance (DNI) is the pyrheliometer. Unlike the pyranometer, the pyrheliometer has a long, narrow tube that limits its view angle to only the direct solar disk. To capture DNI accurately, the pyrheliometer must be mounted on a solar tracking system that continuously points the tube directly at the Sun.
Lower-cost alternatives, such as photovoltaic cell sensors or photometers, can be used for less demanding applications. However, they do not match the spectral accuracy of the high-precision instruments.
Principles of Sensor Operation
The most accurate pyranometers and pyrheliometers rely on a thermopile sensor to convert absorbed radiant energy into a measurable electrical signal. This sensor uses a black coating to absorb incoming solar radiation, causing the surface to heat up. The absorbed heat energy is then converted into electricity through the thermoelectric effect.
The thermopile is constructed from multiple pairs of junctions, called thermocouples, made of two different conducting materials connected in series. One set of junctions, the “hot” junctions, is placed beneath the black-coated receiver surface. The other set, the “cold” junctions, is kept at the instrument’s reference temperature, typically the body of the sensor.
When the black surface is heated by the sun, a temperature difference is created between the hot and cold junctions. This temperature gradient generates a small voltage across the thermopile through the Seebeck effect. The resulting voltage output is directly proportional to the temperature difference and thus the intensity of the solar irradiance.
Alternatively, some lower-cost sensors utilize silicon photovoltaic (PV) cell technology. These sensors generate a current proportional to the light intensity falling on the cell. However, silicon cells are spectrally limited, making them less accurate than thermopiles, which respond uniformly across the full solar spectrum (approximately 0.3 to 3 micrometers).
Practical Setup and Data Collection
Achieving accurate solar irradiance measurements requires careful attention to the physical setup and ongoing maintenance of the instruments. The measurement site must be selected to provide an unobstructed view of the sky and the sun throughout the day and across all seasons. Any nearby structures, trees, or objects that could cast a shadow or reflect light onto the sensor must be avoided.
Pyranometers must be mounted on a stable platform and carefully leveled to ensure the sensor surface is perfectly horizontal. Leveling is crucial because any tilt will cause the instrument to incorrectly measure the horizontal component of the irradiance. For pyrheliometers, the solar tracker must be precisely calibrated to follow the Sun’s path with high accuracy.
Instrument Maintenance
Regular maintenance is necessary to prevent fouling from affecting the readings, as dust, dirt, or condensation on the sensor domes will cause the instrument to underestimate the true irradiance. The glass domes should be cleaned routinely. Many high-end instruments include internal heaters or ventilation systems to prevent dew or frost accumulation.
Data Processing
The voltage signals from the sensors must be recorded using a reliable data logger that records the measurements with a precise timestamp. The raw voltage readings are then converted into irradiance units (\(\text{W/m}^2\)) using the specific calibration factor provided for each instrument.