Editor’s Note: EarthTechling, always looking to bring you innovative cleantech reading, is proud to repost this article courtesy of Do The Math. Author credit goes to Tom Murphy, a physics professor at UC San Diego.
A common question I get when discussing solar photovoltaic (PV) power is: “What is the typical efficiency for panels now?” When I answer that mass-market polycrystalline panels are typically about 15–16%, I often see the questioner’s nose wrinkle, followed by dismissive mumbling that 15% is still too low, and maybe they’ll wait for higher numbers before personally pursuing solar. By the end of this post, you will understand why this response is annoying to me. At 15%, we’re in great shape: it’s plenty good for our needs. Let’s do the math and fight the snobbery.
First, let’s look at the efficiencies of other familiar uses of energy to put PV into perspective. I will act as if I’m directly addressing the PV efficiency snob, because it’s fun—and I would never be this rude in person. This may not apply to you, the reader, so please take the truculent tone in stride.
So 15% is far too low for you? Perhaps you reason that laboratory prototypes and expensive spacecraft applications can get 40%-plus results, so let’s not take the plunge prematurely, given the abysmal 15%.
Perhaps you drive a car. Maybe you’ll stop when you realize that it converts thermal energy from burning gasoline into locomotive power at an efficiency around 15–25% (and this on a finite resource). We should wait for better.
Electric cars deliver battery-stored energy to the wheels at something like 85% efficiency. Now we’re talking. But the charging process imposes another 85% efficiency, and the real kicker is that the fossil fuel (or nuclear) plant supplying the electrical power is only 35% efficient for a net fossil-to-wheels efficiency around 25% (same ballpark as the gasoline car).
Hydrogen fuel cells offer no efficiency advantage in practice, achieving 20–40% for the round-trip hydrogen conversions, not including the efficiency of creating and delivering the electrical power to drive the formation of hydrogen.
If you’re low on energy, you might consider eating. But on second thought, our metabolic efficiency of converting chemical energy into mechanical output is similar to that of a car, so why bother? Turn up your nose.
Perhaps you are a fan of biofuels. This is perhaps the best apples-to-apples comparison to PV, being solar-driven. An Iowa corn field captures solar energy at a paltry efficiency of 1.5%! Okay, but we know by now that corn ethanol has a number of problems. Algae can be far more efficient, right? But even here, photosynthesis tops out at something like 5–6% efficiency under ideal conditions.
PV is Actually Rather Remarkable
Considering this last point, I think it’s rather impressive that we beat biology by a factor of 3 in just a few decades of effort (biology had much longer to work on the problem). Moreover, 15% is perfectly adequate for our needs, as we’ll see at the end.
Qualitative assessments aside, it is rewarding to understand the origin of PV efficiency, and to appreciate that we’re not terribly far from the theoretical limit. The point is that we shouldn’t hold out for some arbitrary efficiency before we embrace solar PV: we don’t really need the extra efficiency, and in any case, physics has something to say about how high we might expect to go.
A photovoltaic cell is most typically a slice of crystalline silicon 200—300 μm thick. (μm = micron = micro-meter = one-millionth of a meter). The construction can either bemonocrystalline—slowly grown from a large single-crystal boule, or polycrystalline, cast in an ingot and with a patchwork of crystal domains in varying orientations (translation: pretty to look at). Monocrystalline varieties have a slight advantage in efficiency: like 18% vs. 15%. The cell is doped into what we call a p-n junction, which is basically a diode. What is important here is that the junction is very near the front surface of the cell, and it is here that energy is effectively harvested.
It works like this: a photon of light comes in from the sky, penetrating some depth into the silicon. If it has enough energy (imagine a sign out front: “you must be this tall to go on this ride”), it can pop an electron out of the lattice, leaving a “hole” behind.