What is Global Solar Irradiance?
The sun’s emitted energy is 3.72 X 1020 MW. The mean irradiance reaching the outside of the Earth’s atmosphere, normally to the sun’s beams, is the Solar Constant. The accepted value, obtained by NASA from extra-terrestrial measurements in 2008, is 1360.8 ± 0.5 W/m2. Actually it is not constant, due to the Earth’s elliptical orbit and the sun spot activity cycle.
The radiation at the Earth’s surface is strongest when the sun is directly overhead at a ‘solar zenith angle’ (θ)of 0° and the thickness of the atmosphere is at its minimum – Relative Air Mass of 1.0 for that location. As the sun moves down, the Direct Normal Irradiance (DNI) beam strikes the Earth’s surface obliquely, and spreads out, reducing the amount of energy per unit area as a cosine function. At the horizon (θ = 90°) the Air Mass is approximately 11 times larger than at the shortest path.
When passing through the atmosphere, the solar radiation is scattered, reflected, and absorbed by air molecules, aerosol particles, water droplets and ice crystals in clouds. This produces diffuse solar radiation. The intensity and the spectrum of the solar radiation received at the Earth’s surface changes markedly depending upon the location, time, date and atmospheric conditions.
Global Horizontal Irradiance (GHI), from the hemisphere above a horizontal plane surface, is a combination of DNI, corrected for the angle of incidence of the beam (θ), and Diffuse Horizontal Irradiance (DHI). GHI = DNI*cosθ + DHI. All these parameters are measured in units of W/m2.
What is a Pyranometer?
Pyranometers are defined by ISO 9060:1990 as the instruments for the measurement of hemispherical (global) solar radiation for solar energy. Specifically, in the wavelength range from at least 300 nm (10-9 m) to 3000 nm this is often referred to as ‘short-wave’ solar radiation.
ISO 9060:1990 has three categories; rising in performance from Second Class, through First Class, to Secondary Standard. The best pyranometers considerably exceed the Secondary Standard requirements. The Primary Standard is, in effect, the World Radiometric Reference (WRR) maintained by the World Radiation Centre in Davos, Switzerland. All ISO Pyranometer Calibrations must be traceable back to the WRR.
Photodiode detectors (or PV cells) cannot meet the ISO 9060 requirements for equal response to solar radiation of a spectral range. Compliant pyranometers use the thermoelectric detection principle. Incoming radiation is almost completely absorbed by a horizontal blackened surface and the resulting temperature increase is measured via thermocouples, connected to make a thermopile.
It is necessary to protect the black detector coating against external influences such as precipitation, dirt and wind. Field Pyranometers have a hemispherical dome made from optical quality glass, which can improve the directional response of the detector. Double domes further reduce the effects of dynamically changing environmental conditions and a white sun shield minimises housing heating effects.
Thermopile pyranometers do not require a power supply. The detector generates a small voltage in proportion to the temperature difference between the black absorbing surface and the instrument housing. This is of the order of 10 µV (microvolts) per W/m2, so on a sunny day the output will be around 10 mV (millivolts). Each pyranometer has a unique sensitivity, defined during the calibration process, which is used to convert the output signal in microvolts into global irradiance in W/m2. To maintain performance, recalibration is usually recommended every two years, and a high quality water-proof connector for the signal cable greatly simplifies the process.
The measurement’s uncertainty is affected by factors that are functions of the Pyranometer design and construction. The best pyranometers with glass domes can achieve an uncertainty in the daily GHI total of less than 2%. The best model uses quartz domes and can measure within 1%.
The Smart Pyranometer
Smart pyranometers require a small amount of power, but provide advantages over traditional passive instruments. They have integrated digital signal processing and industry standard RS-485 Modbus data communication. A range of status and configuration information can be accessed and the instruments are individually addressable, enabling daisy-chaining in networks and saving on cabling costs.
By: Clive Lee at Kipp & Zonen
Steve Sherman says
Clive,
I’m a retired IC designer, and I have a son who’s working on solar cell
chemistry in college. I’m trying to understand how I can make use of GHI and GNI data from the NSRDB to calculate the annual energy production of east-facing panels, tilted at 32 degrees from the horizontal, mounted on my house in Lexington, MA, at 42 degrees latitude.
Many articles exist that define GHI and GNI, but none seem to describe the process of using the NSRDB data to calculate annual energy production in kWh.
Your help would be appreciated.
Thanks,
Steve