EME 810
Solar Resource Assessment and Economics

4.3 Measurement devices: Technology of irradiance transducers


Reading Assignment

OK, so the eyes don't have it. So, how do we go about measuring the solar resource? The Chapter represents a beginning overview to the subject of measurement. The assigned white paper from the National Renewable Energy Laboratory covers our topic in much greater depth.

Why Measure?

Measurement is an important aspect of all scientific endeavors. It is especially important in the proper and efficient design of solar energy collection systems. Proper solar assessment involves metrological and climate data, and correct measurement of global (beam and diffuse) radiation is essential to any solar design effort. Without adequate and precise measurement of the solar resources, system designers and engineers would essentially be "flying blind." In this section, we will discuss the equipment used to perform the required measurements.

Video: Measuring sunlight - Edited version for World Meteorological Day 2019 (4:36)

Click here for a transcript of the Measuring sunlight video.

The Sun. This constant companion delivers the energy that powers life on Earth. But for over four and a half billion years of Earth history the amount of energy the earth has received from the Sun has varied, with major consequences for the climate and for living things. Even the relatively stable climate humanity has enjoyed since the end of the last Ice Age is regularly affected by small changes in the amount of solar radiation reaching the Earth's surface.

These small variations are caused by long-term cycles affecting the Earth's orbit around the sun, changes in cloud cover, and other fluctuations here on Earth. They can have major impacts on our lives. The 1991 volcanic eruption of Mount Pinatubo, for example, spewed huge clouds of sulfur aerosols into the air. This reduced the sun's irradiance by up to 5 percent for around ten months, which cooled the Earth by some point 5 degrees Celsius for several years. This is why measuring the sunlight hitting the earth is so important. It is critical to our understanding of the weather and climate system.

Scientists need radiation measurements in order to study climate variability and change and to forecast the weather. Radiation measurements are also essential for decision-makers in the solar energy industry. To calculate how much electricity a proposed solar energy installation will produce, they need to know how much sunlight will be available on sunny days, and cloudy days, or on short winter days versus long summer days. Measuring sunlight, however, is not as easy as it may sound. We need long-term measurements that are comparable from place to place, from time to time and from instrument to instrument. This requires a special effort to finally calibrate thousands of ground-based instruments all around the world.

The P.M.O.D. Institute in Davos, Switzerland has been studying how to measure sunlight for over 100 years. Since 1959 it has organized a meeting every five years bringing together scientists from around the world to simultaneously calibrate their instruments. In 1971 the World Meteorological Organization invited the P.M.O.D. to serve as the world radiation Center. The center maintains the primary standard for measuring the Sun's irradiance, the so-called world Radiometric Reference. This ensures that these highly sensitive instruments, known as pyrheliometer, are accurate and their data are comparable. Accuracy is particularly important for the solar energy industry, which needs to know the absolute amount of sunlight that is available, while comparability is critical for climate science, which tracks trends and changes over time. Wolfgang Finserele is responsible for solar radiometry at the WMO World Radiation Center. He maintains the “World Standard Group” comprising the six measuring instruments against which instruments around the world are compared.

Our role is to make sure that everybody uses the same accurate scale for measuring solar irradiance. To make those measurements comparable to each other and two measurements that I can take in the past and we'll be taking in the future. Without international collaboration on carrying out this rigorous behind-the-scenes work, we would have a much weaker understanding of the climate system. Without the IPC, countries can't compare their radiation data. We would not be able to look at effective climate records throughout the globe, so this is a very vital important part of the role of the WMO. The World Radiation Center will organize the next global intercomparison meeting in 2020 and will continue its work on assuring the quality of solar radiation measurements far into the future.

Pyranometers and Pyrheliometers

The Pyranometer: Global Irradiation Measurements

Pyranometers act as solar energy transducers, in that they collect irradiance signals and transform them into electrical information signals. That information is passed on to a data logger and computer, and then we either present the data in short bursts (1 second) or integrate and average the data over longer periods of 1 minute to 1 hour.

Research grade pyranometers use a film of opaque material to collect thermal energy. The thermal energy diffuses into a thermal transducer called a thermopile (a stack of thermal devices) that produces a small current proportional to the temperature. We should note that metals (in general) are very good reflectors, making them also very poor absorbers. So, how do we get a material that functions on thermal gradients to make use of the radiation from the sun?

The key is in the absorber material: Parson's black is a paint with very low reflectance across shortwave and longwave bands of light (~300-50,000 nm; making it an effective blackbody). However, if covered by glass (a selective surface), the "window" of light acceptance from the Sun is about 300-2800 nm. This system assembly forms a shortwave (band) global (component) pyranometer. Now imagine, if we develop a thermopile with a thin coating of a black absorber, but replace the glass with a material that is transparent in the longwave band (many organopolymers/plastics), we will have created a longwave (band) global (component) pyranometer.

On the other hand, inexpensive pyranometers can use photodiodes. Photodiodes are photovoltaics (just small). They are semiconductor films that directly convert shortwave band radiation into electrical signals (no thermal conversion step necessary). While the cutoff for a silicon photodiode is <1100nm, the integrated power response is fairly comparable to that of a Parson's black-coated thermopile detector. However, they do not perform as well (relative to thermopile detectors) near sunrise and sunset due to a cosine response error.

Cosine Response Error

Remember the cosine projection effect that we discussed in Lesson 2? It matters here for solar measurement. In the morning and evening, at low solar altitude angles ( α s), some of the radiation incident on the detector is reflected, which produces a reading less than it should be. Some correction can be made for this using a black cylinder casing and a small white plastic disk cover (with a low reflectance at low angles to minimize the cosine error).

Review of a pyranometer in operation:

For standard research, technicians mount pyranometers in a horizontal orientation. Pyranometers produce a voltage in response to incident solar radiation. Provided that a pyranometer uses a thermopile (thermoelectric detector), the device acts as an "integrator" of all components and bands of light. In the case of a glass enclosure, even a thermopile detector will operate only in the shortwave band. Pyranometers based on photodiodes are used only for shortwave global radiation measurements. The following two images are explained in detail at the University of Oregon's Solar Radiation Monitoring Laboratory (maintained by Dr. Frank Vignola). The left image is a LI-COR pyranometer, which uses a silicon photodiode to measure irradiance (a little PV cell). The right image, which looks like a flying saucer from the 1950s, is an Eppley Precision Spectral Pyranometer (PSP). The Eppley is a First Class Radiometer, and uses a thermopile to measure irradiance. The white ring is to reflect stray light away, such that the system does not heat up and so that the influence of the ground reflectance (the albedo) is minimal.

LI-COR pyranometer (L), Eppley PSP (R). Described in caption and text.
Figure 4.2: LI-COR pyranometer (left), Eppley Precision Spectral Pyranometer (PSP) (right)
Credit: UO SRML

Standard pyranometers are designed to be mounted horizontally in shadow-free areas, with the normal vector relative to the surface of the collector (which is horizontal) pointing vertically. Measurements of downwelling shortwave band irradiance from a horizontal pyranometer collect Global Horizontal Irradiance, or GHI. However, through a simple modification, a pyranometer may also be used to measure diffuse irradiance. By using an occulting disk or band, beam radiation can be blocked from the sensor surface of the pyranometer, leaving only diffuse radiation to be measured.

The Pyrheliometer: Beam Component Measurements

If we wished to measure only the direct component of downwelling irradiation, we would use a pyrheliometer. The device is a combination of a long tube with a thermopile at the base of the tube and a two-axis tracking system to always point the aperture of the device directly normal to the surface of the Sun. A measure of irradiance from a pyrheliometer is therefore called Direct Normal Irradiance (DNI) (Gb,n) data. An Eppley Normal Incidence Pyrheliometer is displayed below on the left, while an Eppley Solar Tracker is displayed on the right.

Eppley Normal Pyrheliometer (L), Eppley Solar Tracker system (R). Described in caption and text.
Figure 4.3: Left: Eppley Normal Pyrheliometer (tracking system not displayed). Right: Eppley Solar Tracker system (2-axis tracking ability). The Eppley Normal Incidence Pyrheliometer is to be mounted on the Solar Tracker.
Credit: UO SRML

Curious side note: The World Meteorological Organization (WMO) has a definition for "sunshine." Sunshine means irradiance conditions of >120 W/m2 from the direct component of solar radiation. Really, sunshine has a definition!

Satellite-Based Methods

Until now, we have assumed that measurements of GHI or DNI will come from surface-based measurement methods. By reading Ch. 4 of the CSP Best Practices, we also see that satellites can be used to retrieve GHI (not typically DNI). Geostationary Satellites are used to collect GHI data.

Geostationary Satellites:

  • GOES-West ( λ=115°) located to observe the eastern Pacific and the western half of the United States. The actual satellite is GOES-15 (in place as of late 2011, also, soon to be replaced in 2015).
  • GOES-East ( λ=75°) located in a good spot to keenly observe Atlantic weather systems and weather over the eastern half of the United States. The actual satellite is GOES-13 (in place as of 2010, soon to be replaced in 2015).
  • Meteosat-9 ( λ=0°)
  • Meteosat-7 ( λ=+57.5°)
  • MTSAT ( λ=+140°) Australia/Asia

In the United States, the National Oceanic and Atmospheric Administration's geostationary satellites go by the name of "GOES," which is an acronym for "Geostationary Operational Environmental Satellite." Two operational geostationary satellites, GOES-13 and GOES-11, currently orbit over the equator at 75 and 135 degrees longitude West, respectively. As an aside, GOES-12 is currently drifting east toward ϕ=60°, where it will provide images of South America.

To access images from GOES or geostationary weather satellites operated by other countries visit:

Geostationary satellites are far from perfect. Consider that images of clouds at high latitudes will become highly distorted due to the cosine projection effect, or from viewing the Earth at increasingly oblique angles. For latitudes poleward of approximately 70 degrees, geostationary satellites become essentially useless. But, this is also where the solar resource becomes quite limited. Polar-orbiting satellites can therefore collect at high latitudes where geostationary satellites are not efficient. Each polar orbiter has its cycle effectively fixed in space, completing 14 orbits per day while the Earth rotates.