Sunlight is a composite of waves in the ultraviolet (10–380 nm), Visible (380–750 nm) and infrared (750–1,000,000 nm) spectrums. The light used by solar panels is mostly in the visible light regime, with some incursion into the infrared. Figure 1 [2] shows typical quantum efficiency over the light spectrum for several PV materials.
Much of the heat from the sun is contained within the infrared regime, which is not used for generating electricity. Instead, the panels get hot and then re-emit that heat to the atmosphere via both radiation and convection.
An effective albedo is determined for a given PV panel, or any device, by using two pyranometers. One outward-facing pyranometer measures radiation from the sun, while the other faces towards the PV panel. The ratio of panel-to-sky readings is the effective albedo. In fact, albedo is a misnomer. While the panel is generating electricity, that energy is not actually radiated outward. A non-operative panel may have an albedo of only around .05, this indicating the poor reflectivity of the surface. The effective albedo, including the energy captured to produce electricity, ranges from .15 to .24 [3]. More advanced PV materials may increase efficiency in the future.
This measurement is made when the panel is first started up, that is, before it has a chance to become hot. If the measurement is made in a state of thermal equilibrium, the panel would be hot and the pyranometer would read much higher albedo.
If the effective albedo is, for example, around 0.2, then the rest of the sun’s energy, around 80%, is converted to sensible heat. This would be unimportant if the landscape where the panels are installed has a natural albedo of 0.2. However, in many cases panels may be installed in regions of high natural albedo, or high reflectivity. In this case the panels will release more heat to the atmosphere than the natural landscape would supply. Such a situation could compromise the objectives of the installation if it is being claimed as a credit towards reducing global warming.
Accordingly, it is important to define the albedo of the natural landscapes where PV panels are to be installed. Figure 2 shows a table drawn up from references [2, 4, 5, 6].