While reading, pay attention to the basic rules of light measurement. Our historical affinity for burning fuels has been paired with an affinity and strong awareness of temperature, but we lack an awareness for measures of light, found through radiometry. Think of how you “know” what 80 °F versus 65 °F means. Why not measure the irradiance/irradiation along with the temperature? What if we, as a solar culture, were to learn what 1200 W/m2 meant versus 500 W/m2 for an average hourly or minute irradiation upon an exposed surface (vertical wall or sloped rooftop)?
For solar resource assessment, radiometry (the measure of electromagnetic or radiant energy) is more valuable than thermometry (the measure of temperature). Solar energy is comprised of the shortwave band of light found between 250-2500 nanometer wavelengths of light ( , or one billionth of a meter). Within the solar field, we measure the shortwave band in terms of irradiance, as W/m2 (Watts per square meter, a flux of light per receiving area). If we group irradiance over a block of time, say an hour, we call the measure irradiation, in units of Wh/m2 (Watt-hours per area, similar units to electrical energy measure).
To convert energy values as electron volts from nanometer wavelengths: divide 1239.8 nm•eV by the wavelengths that you found in nanometers to find their respective energies in electron volts (eV). Because the error is negligible for us in the field, we can even use 1234.5 nm•eV (just count to five and use the right decimal point).
For more information on how we get to this equation, see the Key Equations [1] page in the Resources box.
Traditional silicon photovoltaics collect energy for electricity production from wavelengths <1100 nm. Other systems like solar hot water panels will collect almost all of the spectrum from the Sun. In fact, almost all of the opaque surfaces about our environment (buildings, roads, water) tend to absorb strongly over the entire shortwave range (leading to an increase in temperature!).
And yet, our wonderfully adaptive human eyes only capture the tiny visible band from 380-780 nm. Thanks to the complex feedback systems from our irises, lenses, and eyelids, eyes can adapt well to extreme conditions of bright or dim lighting but cannot be used to quantitatively gauge irradiance conditions that energetically drive our buildings and photovoltaic systems. Our eyes impart important information, but because they are highly sensitive and adaptive systems, our eyes cannot be used to reliably distinguish and evaluate the solar resource (bare skin may actually be a better receptor for the shortwave band on days that are not windy).
Notice that the "visibility" of a photon is really limited only by the type of detector being used. Let's review the distribution of photons across the electromagnetic spectrum, and then we will discuss types of photon detectors beyond the rods and cones in our eyes. As a reminder, 1 micrometer wavelengths (also called 1 micron wavelengths) are equivalent to 1000 nanometer wavelengths (by unit conversion). It is often easier to use units of nanometers (nm) for the shortwave band, while using units of micrometers (um) for the longwave band. Either will work, but it is helpful to be aware of the relationship.
Now, let's zoom in to the shortwave and longwave bands important to SECSs. Wavelengths are expressed either in nanometers (nm) or micrometers ($\mu m$, also called microns). For wavelengths greater than 380 nm, electromagnetic radiation transitions from the ultraviolet (UV) to the violet visible range for the human eye. As we will observe in the Granqvist figure below, the red visible range ends for wavelengths greater than 780 nm (to the human eye). Notice how the "infrared" crosses over both the shortwave and longwave bands of light.
All objects glow with energy in proportion to temperature. As temperatures increase, the wavelength oscillations become shorter and the density of photons measured is effectively increased. So, the light becomes more intense (more photons), and each photon is packed with more energy.
Most photons are emitted from a thermal surface, where the constituent atoms are vibrating in the material (this means: temperature). Every material has a thermal property, and hence "everything glows." This concept works well for something called "blackbody emission" (zero emittance) or "greybody emission" (non-zero emittance) but not so well to describe your standard laser pointer or microwave oven. So, just to clarify all cases, there are also photons emitted from the Sun and from solid state devices that are "stimulated emission" (lasers), which can allow for photon emission from surfaces that are high in energy, but not high in thermal temperature.
This is our first principle of seven for the behavior of light. So, let's begin with the simple yet important statement: Light is directional. Light, as a photon, is born (emitted) from a surface in many directions and then impinges upon other surfaces, where it is either absorbed, reflected, or transmitted (and refracted). We use that directionality to describe the resource. In the text chapter, we have defined a visual shorthand for light that is emitted, transmitted, reflected (scattered), and/or absorbed by various surfaces.
Notice that the "Ir-" instructs us that the light is incident upon a surface. For example, we measure "irradiance" on a pyranometer, because light is absorbed by the device. Also, notice that my choice of "-iance" or "-iation" determines if the light is a measure of rate (as in power: energy per time) or energy (rate integrated over a span of time).
All objects glow (even the gases in the sky); it is just the intensity and peak wavelength that shifts with respect to their thermal temperature. This brings us to the second of the seven principles of light behavior. Light (e.g., derived from the sun) has a broad spectrum of energies, which are discussed as wavelengths. We tend to group similar wavelengths in the spectrum according to the properties of their emitting surface or receiving surface.
This is a "group" of high-energy wavelengths of light that are emitted from the Sun (See Light is Directional), comprising the majority of the total energy collected by the Earth's surface or our designed SECS, given the decrease in the light power density with distance to Earth (93 million miles, 150 million km). This bundle of wavelengths of light emitted from the sun includes the ultraviolet (UV), the visible, and the near infrared (IR) sub-bands.
The Sun is effectively the only surface from our surrounding environment that regularly and naturally emits shortwave radiance. We can deliver shortwave band energy to a new surface using a reflector, though, as is the purpose of light concentration onto a central receiver.
Long wave band consists of low energy light. The wavelengths of light that are emitted from our skin, from pavement, the sky, ice cubes, and even hot ovens are coming from surfaces that are quite cool relative to the surface of the Sun, right? We are comparing 250-500K surfaces to a 6000K surface. Planck's Law (discussed in the next page) suggests that the distribution of wavelengths will be very different from cool bodies than from extraterrestrial bodies with internal fusion reactors.
The group of wavelengths from cool bodies on Earth is termed the longwave band. This is >2500nm at Earth's surface, but also can be >3000nm if we were to group the wavelengths from space. The longwave band photons are of lower energy, but there is a very wide band of wavelengths included in the group. The longwave band contributes to the greenhouse effect, and keeps our atmosphere comfortably warm. With the addition of CO2 and water vapor to the atmosphere, we make the sky into a better reflector for keeping longwave energy instead of allowing it to escape into space.
Now, we're going to take a moment to explore the following multiple figure graphic from Smith and Granqvist. There is a lot of information in here, so take your time and revisit it as the class progresses.
Light intensity decreases in proportion to the square of the distance between emitter and receiver. This is called the Inverse Square Law, which applies to solar energy as well as to any other light source. In case of the Sun, the emitted energy flux is 6.33x107 W/m2 at the surface of the Sun drops down to 9126 W/m2 at the surface of Mercury (d=58 million km), and down to 1361 W/m2 at the exterior of Earth's atmosphere (d=150 million km).
As shown in the next figure, the reason for the inverse square law is geometric in nature. As light is emitted from a point, like the Sun at a very large distance, the same quantity of photons is spread out over an increasingly larger area with distance. Area is a spatial term in units of distance squared. This same principle works for ordinary surfaces, as photographers well know.Mathematically, this process of light “dilution” can be represented by the following equation (which is another form of Equation 3.2 in the SECS textbook)
\[G_2^{} = {G_1}\left( {\frac{{d_1^2}}{{d_2^2}}} \right)\]where G1 is the light intensity (or flux) at distance d1, and G2 is the light intensity at distance d2.
If we take G1 as the radiative energy flux at the surface of the Sun, we can then estimate the radiative energy flux at the Earth’s orbit (G2) using this equation simply based on the planetary distances. Try to make this calculation with d1 being the Sun’s radius, and d2 being the distance from the Sun to the Earth and see what you get. You will have a chance to share your result in the Lesson 3 Activity further in this lesson.