A reader at Scientific American raises some healthy skepticism about whether or not the exponential trend in solar watts / dollar can continue.
This is a fine thing to be skeptical about. As I mentioned in the original post, we shouldn’t expect exponential trends to continue for ever. Most run up against external limitations at some point and level out or reverse.
It’s also worth noting that the solar gains are far slower than gains in computing. Computing gains have been roughly 60% in circuit density per year, and more or less the same in the annual gain of computing per dollar. Solar gains are much more modest, at roughly 7.5% gain per year. While computing performance per dollar seems to double roughly every 18 months, solar power per dollar doubles every 9 years. They’re both exponential trends (at least so far), but they have different slopes.
That said, the comparison between the gains in solar watts / dollar and the Moore’s Law increase of transistors per area (or the later morphing of this to computations / dollar) is fairly apt.
In both cases, they’re driven by three factors:
- Nearly insatiable consumer demand for more of the resource (computing and energy, respectively)
- Industry expectations. Any company working on a new microprocessor has to expect that their competition is going to be improving at a rate around that dictated by Moore’s Law (roughly, a doubling of transistors on the same size chip every 18 month). That gives companies working on new microprocessors a goal post to aim for. If they don’t hit that post, they can expect to be behind their competition. Similar factors apply in memory, in storage, and in bandwidth, each of which have their industry-noted exponential trends.The same dynamics work in solar photovoltaic power. Solar PV manufacturers have observed the same trend discussed here. As I noted in the original article, the trend is now 31 years old. Whether or not it will continue for 31 more years, any PV manufacturer has to expect that it will continue for at least the next few. That gives PV manufacturers their own goal posts to shoot for. And the PV market is crowded. Wikipedia lists more than 50 notable solar PV manufacturers.
- Progress Made by Reducing Materials Per Output. The final factor is the one that makes the gains physically possible. In both computing and in solar, the gains being made in performance per dollar are being made by reducing the amount of material required to achieve each unit of output. By etching thinner lines, the semiconductor industry crams more transistors onto the same amount of silicon. They’re using less silicon per transistor. The solar PV industry, similarly, is using less silicon per watt. Solar manufacturers are doing this by reducing the thickness of solar cells, increasing the efficiency of solar cells, and increasingly by looking at techniques that use materials other than silicon.For a look at how industry thinks about this, here is a graph of silicon per watt that Sun Power presented at the SEMICON West Conference in 2007.And here’s a chart of decreasing silicon wafer thicknesses out to 2012 from an article by researchers at Applied Materials Switzerland, specifically focused on reducing silicon grams / watt.
The similarity of the three factors tells me that the analogy is an apt one. That does not guarantee that it will continue forever. We will eventually hit the limit of what can be physically done to reduce materials needed for solar cells. But that looks likely to happen significantly after solar PV becomes less expensive than coal electricity.