A reader asks about those new solar panels:
Is it possible to predict where this will work, i.e. in areas with lots of sunlight. Everywhere in the world? It depends?
It depends.
Let’s start with the ultraviolet side of things. UV consists of two bands, UVA (wavelength of 315–400 nm) and UVB (280–315), courtesy Wikipedia. This division is important because UVA is not absorbed by the ozone layer, while UVB is mildly to mostly absorbed by the ozone layer (there’s also a UVC layer, which is completely absorbed and also carries some energy). From the Windows to the Universe solar emission chart,
we can see that UVA, which is closer to the visible spectrum than UVB, also carries more energy than does UVB. Since UVA isn’t absorbed by the ozone layer, the depth of the ozone layer at the location of interest (or the altitude of the site, for that matter) isn’t as important as it might be. It’s not irrelevant, since UVB is carrying some of the energy we’re interested in harvesting, but it’s not paramount. My quick research suggests the ozone layer becomes thinner at altitude (thus pilots’ concerns about cataracts caused by solar rays and sunburn) as well as at the higher latitudes, i.e., near the poles. So at higher altitudes and latitudes you may receive a little more harvestable energy.
What about infrared? Turns out it, too, has problems reaching the Earth’s surface. Windows to the World has another handy chart:
What is absorbing the radiation? According to the Climate Science Investigations site, CO2 is highly absorbent:
Carbon dioxide (CO2) is also an important greenhouse gas. It has a long lifetime in Earth’s atmosphere. Carbon dioxide strongly absorbs energy with a wavelength of 15 μm (micrometers). This makes carbon dioxide a good absorber of wavelengths falling in the infrared radiation region of the spectrum.
This explains why CO2 is classed as a climate change gas – by absorption, the gas warms. And there’s lots of CO2. More or less evenly distributed, so siting your installation based on infrared considerations doesn’t appear to be an effective strategy, insofar as this amateur (me!) can tell.
There are other factors as well. The tilt of the Earth relative to its orbital plane (the source of our seasons, basically) means differing levels of solar radiation reaching a given location on Earth varies with the season. The location itself defines the angle of incidence, as moderated by the season. Yet, interestingly enough, a location directly on the equator may not be as effective as a location at 20 degrees latitude, because wet tropical forests will screen out the rays, while at 20 degrees there are a number of dry deserts.
So, yeah, it depends. Having gone through all this, I have to wonder if these are going to be a gimmick, or part of a larger array where the visible spectrum is also harvested. The diagram from Windows to the World certainly clarifies just how much radiation the Earth’s atmosphere (and magnetic belts) screen out, and what is the best part of the spectrum to harvest – the visible spectrum.
