In the early 19th century, Claude Pouillet first tried to measure solar radiation. Since then, in order to meet the needs of renewable energy utilization and climate research, scholars have never stopped studying the complex interactions between solar radiation and the earth’s atmosphere and surface. In fact, Pouillet’s original work measured the broadband radiation produced by the sun, which is now called total solar irradiance (TSI), and this research topic is still very active. At present, in the average sun-earth distance, the accepted TSI value is 1366±7W/m². Determining this in the introduction will help the following discussion. The earth’s orbit around the sun is elliptical, which makes the solar radiation at the top of the atmosphere the highest at perigee (around January 3), about 1415W/m²; the lowest at apogee (around July 4), about 1321W /m². The radiation outside the atmosphere is relatively predictable. Considering the influence of the atmosphere on radiation transmission, the prediction of surface radiation is more challenging.
As shown in Figure 1, there are three basic components of surface solar radiation related to solar resource forecasting and assessment:
Direct Normal Irradiation (DNI): Sunlight directly shines from the surface of the solar disk to the surface orthogonal to the light path, which can be measured with a direct radiometer at a field of view angle of 5°~5.7°.
Diffuse Horizontal Irradiation (DHI): This type of radiation refers to radiation from the sky dome in addition to DNI. Solar radiation is scattered by atmospheric clouds, aerosols and other components at the surface level when it reaches the surface. It can be measured at a 180° field of view using a direct radiometer with a light-shielding ring.
Global Horizontal Irradiation (GHI): The total solar radiation that the hemispherical radiation reaches down to the horizontal surface, measured with an unshielded pyranometer.
The World Meteorological Organization (WMO) provides detailed solar radiation composition measurement guidelines, including measurement practices, instrument specifications and operating procedures.
The three solar radiation components are interrelated. The sum of the direct and scattered radiation measured on any plane is equal to the total radiation. On the horizontal plane, the solar zenith angle (SZA) can be used to convert the DNI at a certain moment into direct horizontal plane radiation:
GHI=DNI×cos(SZA)+DHI
The time change graph in Figure 2 illustrates the changes in solar radiation components over time under clear and cloudy conditions. Based on these three solar radiation components, the solar radiation that may be received by a collector in any direction can be estimated, that is, “POA radiation” (Perez & Stewart, 1986). When estimating the radiation received by the flat panel collector, the assumed sky and ground conditions increase the uncertainty of the estimation. Therefore, the use of a total pyrometer to determine the solar radiation of the flat panel POA can greatly reduce the uncertainty of the data. Because the observation angles of CPV and CSP collectors are narrow, the POA solar radiation they receive can be measured through DNI. However, DNI data is relatively small, so GHI data is more commonly used to calculate DNI.
Solar energy resource forecasting and evaluation must consider the uncertainty of solar radiation measurement and model estimation in the process of project design and application. In particular, these data form the basis for the development and verification of solar forecasts. The uncertainty in the simulated solar radiation data depends on the methodology and basic input data, but the input data should be larger than the measured data used to develop and verify the solar radiation model.
