Do you think that your eyes are great at seeing and measuring light? Surprisingly, your wonderfully adaptive human eyes only capture a tiny fraction (380-780 nm) of the electromagnetic spectrum and cannot reliably distinguish and evaluate the solar resource. In fact, some sources suggest that your bare skin may actually be a better receptor for the shortwave band than your eyes (under no-wind conditions).
Like meteorologists, solar energy specialists have to rely on both ground-based instruments (pyranometers and pyrheliometers) and satellites (such as GOES East and GOES West) to measure sunlight before and after its interaction with the atmosphere. Satellite radiometers measure the intensity of radiation scattered back to space, producing images that reveal clouds, smoke, volcanic ash, and other features that influence surface sunlight.
In this lesson, we will use the concept of components to break the sky dome and the ground into digestible chunks of surfaces with common emission/absorption/ scattering characteristics - direct (Gb), diffuse (Gd ), circumsolar diffuse, ground reflected diffuse (Gg). Components of global irradiance relate to the sources of light within the sky dome. A component is a term for the groups of physical orientations and scattering of light (e.g., diffuse component, beam component). The degree of light scattering on a horizontal surface is assessed via the various clearness indices (e.g., kT, KT, K-barT ). There is a lot to learn!
We shall see that when it is challenging or costly to measure multiple components of light (scattered and unscattered), we have old and somewhat dated tools to attempt broad estimations on the contributions of each component to the total irradiation incident on the aperture of interest. You will see how we often rely on historical observations and empirical correlations by solar scientists and engineers for hourly, daily, and monthly average day data. The main tools used for these older equations are both measured hourly Global Horizontal Irradiation (GHI, or I) gathered from a horizontally mounted pyranometer, and daily extraterrestrial irradiance (Air Mass Zero = AM0, or Top Of Atmosphere = TOA, or just I0), which you learned about in the last chapter. We shall also find that one can infer more than just the components of light from the ratios of measured irradiation to AM0 calculated irradiation--however, there can be significant errors included in the process. We can also describe the fractions of days in a given month where lighting conditions will be clear or overcast/cloudy.
You will also see reference to the Typical Meteorological Year. Keep an eye out for that...it will be a major part of SAM simulation software.
Math warning!
You will observe several equations that are long and complicated. They are empirical relations that will be used to estimate the solar resource on non-horizontal surfaces. As with prior lessons, these equations are at the core of software like SAM, and a student completing this course should be very familiar with their application. Stick with it!