A Brief Introduction to Solar Radiation and Pyranometers

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Stars generate huge amounts of energy through the process of nuclear fusion. Our own sun, an unremarkable medium-sized star, produces a total energy E of about 3.9x10²⁶W. This energy is radiated into space uniformly in all directions. Fundamental physical laws tell us that the intensity of this radiation decreases as the inverse square of the distance from the sun. The solar constant (S_o) is defined as the average energy per unit area of solar radiation falling on the surface of an imaginary sphere of radius R around the sun:

S_o = E/(4π R²) = 1370 W/m²

where R is the average Earth/sun distance, about 150,000,000,000 m. The solar "constant" actually fluctuates by a few Watts per square meter, and the energy the Earth receives varies regularly with the seasonal change in the Earth/sun distance -- from a maximum of about 1417 W/m² in early January to a minimum of about 1324 W/m² six months later. (Yes, the sun is closest to Earth in January!)

Not all of the energy that reaches the top of the Earth/atmosphere system reaches Earth's surface. Some of it is reflected back to space and some of it is absorbed in the atmosphere. Around noon on a typical summer day at temperate latitudes, about 1000 W/m² actually reaches Earth's surface. This is a lot of energy -- enough to power 10 100-W light bulbs. Currently, it is not possible to convert even a majority of this energy into a more directly usable form. Still, there is great potential for solar energy to contribute to a global economy that must eventually wean itself from its current overwhelming dependence on fossil fuels. There are already many useful applications of solar energy, especially when it is converted directly into electrical or heat energy.

It is not hard to measure energy from the sun at Earth's surface, a quantity called insolation. Pyranometers (from the Greek word for fire) are designed specifically for this purpose. In high-quality pyranometers, several devices called thermocouples are electrically connected to form thermopiles. When sunlight strikes a thermopile, it creates a temperature difference across the junctions of the thermopile. This produces a small voltage proportional to the amount of incident energy. The advantage of thermopile-based pyranometers is that they respond across a very broad range of wavelengths, as required to measure insolation. A high-quality thermopile-based pyranometer is shown in Figure 1. (This instrument is roughly 20 cm in diameter.)

Figure 1. Eppley PSP pyranometer.

Unfortunately, accurate thermopile-based pyranometers are very expensive. Fortunately, there is a much less expensive alternative that also works reasonably well -- silicon-based solar cells. Solar cells for generating electrical energy directly from sunlight are widely used. As a result, they are very inexpensive. With careful design, it is possible to build solar cell-based pyranometers. These might more accurately be called "surrogate pyranometers" because they do not respond uniformly to the entire range of radiation emitted by the sun.

Figure 2 shows a "homemade" solar cell-based pyranometer. There are two small solar cells under the round Teflon® diffuser on the left of the light-colored case. (The diffuser is about 1.9 cm in diameter.) A bubble level for leveling the instrument is mounted on the right of the case. The output from the solar cells drives the analong milliammeter in the dark blue case. A small commercial solarcell-based pyranometer is also shown. This can be used to calibrate the "homemade" pyranometer.

Figure 2. A "homemade" solarcell-based pyranometer with an analog current meter, shown with a commercial solar cell-based pyranometer used for calibration.

The basic output from a pyranometer is solar energy, measured in units of Watts per square meter. However, it is interesting to note that pyranometer output can also be used to track the motion of clouds. This is important to atmospheric and Earth scientists who need to understand the effects of clouds.

Figure 3 shows insolation on 1 September, 2004, at Drexel University in Philadelphia, PA, USA, sampled every 30 seconds. Early in the morning, skies are clear, but clouds arrive in late morning. The variability in insolation as a function of time, with interspersed "clear" and very low insolation values, suggests the passage of individual cumulus clouds over the sun. Note the insolation values right at and just after 750 minutes. Here, it appears that the insolation is actually larger than that seen under the clear skies just before 750 minutes. This phenomenon, which is not unusual, is due to reflection of sunlight from the sides of clouds -- further evidence that these are well-defined cumulus clouds.

Figure 3. Insolation data showing arrival of clouds.

You can find more information about the sun and instruments for measuring solar radiation here. You can find instructions for building your own solar-cell based pyranometer here.