I've fallen behind in my postings lately, in part because of final exams, grading, and other end-of-semester activities. Now, however, I have some time to write about a talk I attended at Texas Tech back on November 18 as part of the Maddox Solar Energy Series. The speaker was Sarah Kurtz of the National Renewable Energy Laboratory and her presentation was entitled "The science behind high-efficiency solar cells and why we might care."
Thanks to Richard A. Muller's book Physics for Future Presidents (which I reviewed here), I had a little bit of background knowledge of what Dr. Kurtz would likely be talking about.
Besides reinforcing what I learned from Dr. Muller's book, Dr. Kurtz's talk also exposed me to several pieces of solar-energy terminology, which I could use to search the web and discover resources to increase my knowledge of the area. Indeed, I was able to find a number of layperson-friendly summaries of solar energy, which I summarize in the following paragraphs.
As described in this transcript from a PBS NOVA episode on solar energy:
A solar cell is like a sandwich; the top part is for protection, the bottom is its base and the middle layer, made of silicon, is where the action is.
When particles of sunlight, called photons, strike individual atoms of the silicon, they easily break the weak bond between silicon's nucleus and its outer orbit of electrons. Once freed, the electrons travel to the top of the silicon layer, where they move in a current along metal conducting strips. Then it's across the panels to wires that feed the electricity to the house.
Dr. Kurtz participated in this NOVA episode, as seen in this part of the transcript:
NARRATOR: ...rooftop solar arrays are not very efficient. Even now the very best panels and coverings convert only 15 percent to 20 percent of the sun's rays into electricity, about half the conversion efficiency of a coal plant.
SARAH KURTZ (National Renewable Energy Laboratory): We'd like to be able to improve upon that, so we've been researching how to use more sophisticated materials to put together a high efficiency cell.
NARRATOR: Back at the National Renewable Energy Lab, Sarah Kurtz leads a team that is developing something called a multijunction solar cell. It looks like a miniature version of a normal solar cell, but even at this tiny size, it's much more powerful because it contains several micro-thin layers of light-absorbing materials.
The light from the sun is revealed as a band of colors, each color a different wavelength of energy. Traditional silicon solar cells absorb only the red spectrum of the sun's rays. The rest of the energy bands are blocked.
SARAH KURTZ: You could do better if you could use two different types of materials, or even three different types of materials, to attune them to the color of the light that's coming down.
NARRATOR: The extra layers allow the cell to absorb additional wavelengths of light, greatly increasing its efficiency...
Indeed, as Muller discusses on page 82 of his book:
The best efficiency achieved so far, 41%, has been with a "triple junction" cell that has separate layers for different colors of light. But they are expensive, about $65 per square inch. If the cost can be brought down, and if we don't run out of the specialized materials needed to make them, then solar could become an important source of energy in the future.
This U.S. Department of Energy document explains some of the basic physics in converting light to electricity:
When certain semiconducting materials, such as certain kinds of silicon, are exposed to sunlight, they release small amounts of electricity. This process is known as the photoelectric effect. The photoelectric effect refers to the emission, or ejection, of electrons from the surface of a metal in response to light. It is the basic physical process in which a solar electric or photovoltaic (PV) cell converts sunlight to electricity.
Sunlight is made up of photons, or particles of solar energy. Photons contain various amounts of energy, corresponding to the different wavelengths of the solar spectrum. When photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the PV cell (which is actually a semiconductor).
Finally, this document from the Lawrence Berkeley Laboratory reviews some of the leading ideas (as of 2004) on the types of materials that possibly could increase efficiency beyond 50 percent.
I've checked to see if Dr. Kurtz's PowerPoint slides from her Texas Tech talk were available online, but haven't been able to find them. I did, however, find another presentation of hers online, which seems to cover some of the same topics she addressed at Texas Tech.