A pyrheliometer is an instrument used to measure the direct solar radiation at a given location. Since they need to be pointed directly at the sun, pyrheliometers are typically mounted on a tracking device that follows the sun’s movements. After sunlight enters the pyrheliometer, it is converted to an electrical voltage by a thermopile. This voltage can then be calibrated to give units of watts per square meter, the standard units of solar irradiance. Pyrheliometers are used for scientific research and for placing solar panels.
Solar irradiance is a measure of the flux of solar radiation, or the solar energy per unit time, per unit area. It depends on the location of measurement—solar irradiance near the surface of the sun will be much larger than at the distance of Earth. In fact, there are variations in solar irradiance across the surface of Earth; these depend on the amount of atmosphere sunlight must penetrate, and, to a lesser extent, differences in distance from the sun. The average solar irradiance at Earth’s distance from the sun is about 1,366 watts per square meter.
Pyrheliometers positioned on the Earth’s surface measure the solar irradiance after it penetrates the atmosphere. In the atmosphere, some sunlight is scattered; this kind of diffuse sunlight can be measured by a different instrument, called a pyranometer. The sunlight that is not scattered enters the pyrheliometer and hits a thermopile.
A thermopile is a device that uses the Seebeck effect to create an electrical voltage based on temperature differences. The Seebeck effect refers to the phenomenon where two different metals at different temperatures will produce an electrical current when connected. In a pyrheliometer, one end of the thermopile faces the sun while the other end remains shaded. This arrangement will leave the two metals at different temperatures and, therefore, will lead to a flow of current in the thermopile. When the solar irradiance is higher, a higher electrical voltage will be measured.
Pyrheliometers are commonly used to study changes in solar radiation output, such as the 11-year solar cycle. Another application of the pyrheliometer is solar panel placement and design. Different parts of the Earth receive different amounts of solar irradiance, and this data can be collected by pyrheliometers and used to create maps of average radiation levels. Polar latitudes generally have less incoming solar radiation, while other areas have lower levels because of frequently cloudy skies. These maps can help inform where it makes sense to place solar panels.