Introduction to Solar Radiation Measurements
Solar radiation is a term used to describe visible and near-visible (ultraviolet and near-infrared) radiation emitted from the sun. The different regions are described by their wavelength range within the broad band range of 0.20 to 4.0 µm (microns). Terrestrial radiation is a term used to describe infrared radiation emitted from the atmosphere. The following is a list of the components of solar and terrestrial radiation and their approximate wavelength ranges:
- Ultraviolet: 0.20 – 0.39 µm
- Visible: 0.39 – 0.78 µm
- Near-Infrared: 0.78 – 4.00 µm
- Infrared: 4.00 – 100.00 µm
Approximately 99% of solar, or shortwave, radiation at the earth’s surface is contained in the region from 0.3 to 3.0 µm while most of terrestrial, or longwave, radiation is contained in the region from 3.5 to 50 µm.
Outside the earth’s atmosphere, solar radiation has an intensity of approximately 1370 watts/meter2. This is the value at mean earth-sun distance at the top of the atmosphere and is referred to as the Solar Constant. On the surface of the earth on a clear day, at noon, the direct beam radiation will be approximately 1000 watts/meter2 for many locations. While the availability of energy is affected by location (including latitude and elevation), season, and time of day, the biggest factors affecting the available energy are cloud cover and other meteorological conditions which vary with location and time.
For the measurement of sun and sky ultraviolet radiation in the wavelength interval 0.295 to 0.385 µm, which is particularly important in environmental, biological, and pollution studies the Total Ultraviolet Radiometer (Model TUVR) was developed. This instrument utilizes a photoelectric cell protected by a quartz window. A specially designed teflon diffuser not only reduces the radiant flux to acceptable levels but also provides close adherence to the Lambert cosine law. An encapsulated narrow bandpass (interference) filter limits the spectral response of the photocell to the wavelength interval 0.295-.0385 µm.
As solar radiation passes through the earth’s atmosphere, some of it is absorbed or scattered by air molecules, water vapor, aerosols, and clouds. The solar radiation that passes through directly to the earth’s surface is called Direct Normal Irradiance (DNI). The radiation that has been scattered out of the direct beam is called Diffuse Irradiance. The direct component of sunlight and the diffuse component of skylight falling together on a horizontal surface make up Global Irradiance. The three components have a geometrical relationship.
Direct radiation is best measured by use of a pyrheliometer, which measures radiation at normal incidence. The Normal Incidence Pyrheliometer (Model sNIP) consists of a wirewound thermopile at the base of a tube with a viewing angle of approximately 5º which limits the radiation that the thermopile receives to direct solar radiation only.
The pyrheliometer is mounted on a Solar Tracker (Models ST-1 and ST-3) or an Automatic Solar Tracker (Model SMT) for continuous readings.
Diffuse radiation can either be derived from the direct radiation and the global radiation or measured by shading a pyranometer from the direct radiation so that the thermopile is only receiving the diffuse radiation. Eppley has developed Shade Disk Adaption Kit (Model SDK) that mounts on the SMT which allows you to measure the diffuse and direct at the same time. We also manufacture the Shadow Band Stand, (Model SBS) for Diffuse measurements in sites where there is no power available to operate an Automatic Tracker.
Global radiation is measured by a pyranometer. The modern pyranometer manufactured by the Eppley Laboratory, using wirewound plated thermopiles, can be one of three models: the Standard Precision Pyranometer (Model SPP), the Global Precision Pyranometer (Model GPP), and the Black & White Pyranometer (Model 8-48). The SPP has a black sensor protected by two precision ground, polished hemispheres and is the preferred instruments for Global measurements. Based on the SPP, the GPP is specifically designed as a lower cost alternative for the PV/CSP industry. The 8-48 has a black and white sensor that is protected by a single polished hemisphere and is the preferred instrument for Diffuse measurements.
LONGWAVE (INFRARED) MEASUREMENTS
The Precision Infrared Radiometer, (Model PIR) was a development of the PSP Pyranometer (forerunner to the SPP Pyranometer) and continues to be the industry standard for precise measurement of incoming or outgoing longwave radiation. The PIR comprises the same wirewound thermopile detector and temperature compensation circuitry as found in the PSP/SPP. This thermopile detector is used to measure the “net radiation” of the PIR and a case thermistor (YSI 44031) is used to determine the outgoing radiation from the case. A dome thermistor is also included if one wishes to measure the dome temperature as compared to the case temperature to make any “corrections” to the final result.
Albedo is the ratio of the incoming shortwave divided by the reflected shortwave. Eppley believes the best way to measure albedo is with two (2) distinct SPPs instead of mounting the two sensors in the same body. This allows for better calibration results and prevents the cold junctions of the two sensors from affecting each other. We have supplied Albedometers as well a Roof Reflectance Kits based on customers wishes.
NET RADIATION MEASUREMENTS
Net radiation is the sum of four individual measurements: Incoming Shortwave, Reflected Shortwave, Incoming Longwave and Outgoing Longwave. Eppley recommends measuring each of the four componants separately using two (2) SPPs and two (2) PIRs.
SUNSHINE DURATION MEASUREMENTS
Sunshine duration is typically defined as the amount of time that the Direct Normal Irradiance (DNI) is greater than 120 Wm-2. This can be determined by using the data collected from the sNIP.