Thursday, November 6, 2008

Solar Power: a response

This source ( indicates that inefficiency of photovoltaics at a low angle is a minimal concern compared to subtended angle (which I knew about) and longevity (which I hadn't considered). Of course you know the problem with a shallow angle: a six-inch photovoltaic will only really be catching three inches of sunlight if it's at a 60 degree angle to the sun. I just picked up the impression from the article that photovoltaic inefficiency dropped even faster than that, but he thinks not and he sounds like he knows what he's talking about. I guess the chief concern would be durability, though: the fact that solar has to last 20-30 years to be economical is well-taken.
As with so many "breakthroughs" in photovoltaics, the new antireflective coating developed at RPI seems to be overhyped:
"Lin's discovery could antiquate these automated solar arrays, as his antireflective coating absorbs sunlight evenly and equally from all angles. This means that a stationary solar panel treated with the coating would absorb 96.21 percent of sunlight no matter the position of the sun in the sky. So along with significantly better absorption of sunlight, Lin's discovery could also enable a new generation of stationary, more cost-efficient solar arrays."
While an improved AR coating might boost overall photovoltaic efficiency by a few percent, the primary reason for heliostatic tracking is not to maximize absorption, but to maximize the subtended solid angle of the panel or array. That is, when the sun is at an angle away from normal incidence (away from perpendicular to the surface) the light incident on the panel is proportional to the cosine of this angle, so it is maximum when the angle is zero and falls to zero when the angle approaches +/- 90 degrees. Absorbing 96.21 percent of the sunlight is not terribly helpful if the panel, because of it's angle, is only receiving 30 percent of the light it would if optimally oriented.
Since I worked in the field 25 years ago, there have been hundreds of "breakthroughs" announced aimed at improving the efficiency of mass market photovoltaics. With very few exceptions, the added cost of these "breakthroughs" makes them not viable, or they are found to degrade over the 20-30 year service life required for payback of investment. The high volume solar panels being sold today are only a couple percent more efficient than what was being sold back then.
In fairness, some of the more exotic technologies are used in space power applications, where efficiency is king and low cost is not a high priority.
Doug Ely
"The presentation or 'gift' of the Holy Ghost simply confers upon a man the right to receive at any time, when he is worthy of it and desires it, the power and light of truth of the Holy Ghost, although he may often be left to his own spirit and judgment." --Joseph F. Smith (manual, p. 69)
"Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me."

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