a really well understood effect, but under light soaking, the system loses some of its efficiency in the first six months of operation.
So, the films start out at 15% and then in six months they drop down to 10 or 12%. This particular degradation mechanism is somewhat understood, but how to control it is not.
Dave Cole: Is moisture an issue in temperate zones where it is not always dry? Is water or humidity a factor?
John Turner: Water is not an issue with current sealing technologies for single-crystal silicon cells.
Dave Cole: Is that true for the amorphous silica also?
John Turner: It is the same for amorphous silica. Amorphous silicon uses the same sealing technique.
Dave Cole: What do you mean by “sealing technique”?
John Turner: Typically, the silicon is deposited either on glass or on stainless steel and sealed with a plastic. The plastic is called ethoxybenzoic acid (EBA), but I've forgotten what that means now. It is a cover that seals everything up. It lasts about 20 years. The degradation of EBA is really what limits the lifetime of silicon solar cells.
David Keith, Carnegie Mellon: I think I heard you say that the use of water in electrolysis to make hydrogen would lead to some water shortages. Is that correct?
John Turner: It is going to be an issue if you only have water—
David Keith: I don't get it. I mean about a meter of water a year falls in the typical temperate zones. That means on each hectare you get 104 square cubic meters of water per year.
John Turner: Yes.
David Keith: Yet the amount of water that you need to make hydrogen, assuming you—
John Turner: Pretty small.
David Keith: It is of order 10 cubic meters. So, there is a difference of 103.
John Turner: Right, but it is the culmination that makes the difference. Water is used for food, people and industry. If you add hydrogen production to that, there may be a problem. If you take it by itself, you are absolutely right.
David Keith: It is down by three orders of magnitude.
John Turner: It is going to be an issue.