The amount of solar radiation reaching the ground impacts the weather, the operation of renewable energy systems, and human activities (Wallace and Hobbs 2006). Devices for measuring irradiance are known as radiometers, which are usually characterized by their sensitivity to specific frequency bands as well as by how they are operated (e.g., passive versus active). More information can be obtained in the “Best Practices Handbook for the Collection and Use of Solar Resource Data for Solar Energy Applications” (Sengupta et al. 2015).
Solar radiation at the ground is usually divided into two main frequency bands. The shortwave band (wavelengths less than 4 microns, including near-infrared, visible, and ultraviolet bands) includes most of the energy associated with solar radiation, whereas the longwave band (wavelengths greater than 4 microns, including the infrared spectrum) includes most radiation emitted from the atmosphere and the Earth itself.
Other bands that are sometimes measured include ultraviolet bands—used for photosynthesis— and the bands used by different types of solar photovoltaic modules. Measurement devices for surface radiation are summarized in Table 1.
Irradiance can be measured in several different orientations including:
- Horizontal, facing up, to measure the incoming shortwave and/or longwave radiation; together, they provide the global horizontal irradiance
- Horizontal, facing down, to measure the shortwave radiation reflected by the surface and/or the longwave radiation emitted by the surface
- Tilted but at a fixed angle, to measure the irradiance available to a solar panel or some other inclined surface
- Sun-tracking and with a narrow field of view, to measure the direct irradiance from the sun, called the direct normal irradiance
- Sun-tracking but with the sun blocked out, which can be combined with global horizontal irradiance measurements to calculate the diffuse component of solar radiation.
The different types of radiometers are found at the Radiometer instrumentation page. All sensors currently on the market have a TRL of 8 or higher. Current areas of research include investigating more affordable sensors and sensor packages that are easier and more cost-effective to deploy and maintain.
Radiometers use photodiodes or thermocouples to measure the irradiance. For example:
- Thermopile-based radiometers use a thermal detector with black coating that has a flat spectral absorption typically ranging from 200 nanometers (nm) to 50,000 nm. The detector is usually protected by hemispherical precision-ground glass or sapphire domes that have a flat spectral response (typically 280 nm to 3,000 nm).
- Photodiode radiometers have a limited and nonuniform spectral response (typically 400 to 1,100 nm), which covers about 75% of the solar spectrum. The detector is usually installed behind a diffuser that scatters or randomizes the direction of the photons. Photodiodes have a higher temperature and angular sensitivity.
Thermopile radiometers are preferred for research applications because of their flat and wide spectral response. Photodiode radiometers are typically less expensive but more sensitive to ambient conditions.
Surface Radiation takes the units of W/m^2 reflecting the rate at which radiated energy arrives at or leaves the ground surface.