Monday, May 16, 2016

An energy miracle? But we already have it!

Silicon is a material with properties close to the optimal for a solar cell. It is also one of the most abundant elements in the earth's crust, and, finally, we know how to use it to manufacture cells with efficiency close to the theoretical maximum. Isn't it a miracle?

"EnergySkeptic" recently commented on an article appeared in "Nature" in 2014 on the possibility of cheap photovoltaic cells entering the market of solar energy. The post is short enough that I can reproduce it in full, below. It is interesting because it shows the problems with the idea of the "miracle breakthrough" in energy that Bill Gates advocates.

Here, the discussion is on perovskite solar cells; a technology that promises to be cheaper than that based on silicon. Perovskites are a large class of materials; those being studied as solar cell materials have several advantages, including the fact that they can be manufactured in the form of thin films, don't need to be so extremely pure as silicon, have a band gap close to the theoretical optimum.

That, however, doesn't necessarily make perovskites a "breakthrough" in the field. Even assuming that perovskite cells could reach an efficiency high enough to be marketable, the problem is that, at present, the cost of the cells is only about 30% of the total cost of a solar plant. Even if perovskite cells were to cost half as much in comparison to silicon ones, that would be no improvement unless their efficiency were to match or exceed that of silicon. Otherwise, the whole plant would probably cost more because it would have to occupy more space.

In practice, to have a breakthrough in solar power, we would need a technology which is 1) significantly cheaper than silicon, 2) much more efficient, 3) that uses no rare and non-renewable elements (that rules out, in the long run, cells that use tellurium or gallium). That's a tall order, especially considering that we are bumping into the physical limits of single-junction cells; which cannot have efficiencies higher than a little more than 30%. Silicon, because of some quirks of the way the universe works, happens to be placed almost in an optimal position in terms of band-gap and, at the same time, to be a widely available element in the earth's crust. So, it is, in many respects, an optimal choice for solar cells, and already not so far away from its theoretical limits. I think that we'll stay with silicon for a long, long time. Surely we will improve the technology, but don't expect miracles. That silicon works so well is already a miracle!


Further notes:

1. Here, in Florence, a colleague of mine has built a nice solar plant that uses multi-junction GaAs cells and concentrating mirrors, attaining, I think, around 50% efficiency. I saw it: it is a wonder of technology, full of gears, motors, optics, sensors, computers, and things. But I didn't dare to ask him how much it would cost to buy one for the roof of my house!

2. True breakthroughs may occur "downstream" with respect to energy production; for instance with batteries and the diffusion of a new generation of electric vehicles. There is no thermodynamic limit to the number of times that a battery can be recharged without degrading.

3. "heavy-duty trucks, locomotives, and ships run on diesel fuel" in the article below is, in part, a canard. Here in Europe, locomotives already run on electricity. Trucks can run on electricity, too, ( For ships, the problem is not so much how to push them on, there are ways. It is another one, much more difficult (see e.g. And the only way to solve that problem is to rush into renewable energy as fast as possible.


Van Noorden, R. September 24, 2014. Cheap solar cells tempt businesses. Nature #513 470-471.

[Excerpts. Of interest because rarely do obstacles get mentioned in the news. Most are optimistic hype making it sound like a solution to the energy crisis is just around the corner. And forget that electricity does not solve our main problem — heavy-duty trucks, locomotives, and ships run on diesel fuel ]
Large, commercial silicon modules convert 17–25% of solar radiation into electricity, and much smaller perovskite cells have already reached a widely reproduced rate of 16–18% in the lab — occasionally spiking higher.
The cells, composed of perovskite film sandwiched between conducting layers, are still about the size of postage stamps. To be practical, they must be scaled up, which causes efficiency to drop. Seok says that he has achieved 12% efficiency with 10 small cells wired together.
Doubts remain over whether the materials can survive for years when exposed to conditions outside the lab, such as humidity, temperature fluctuations and ultraviolet light. Researchers have also reported that ions inside some perovskite structures might shift positions in response to cycles of light and dark, potentially degrading performance.
The need for complex engineering might create another setback, says Arthur Nozik, a chemist at the University of Colorado Boulder. After plummeting in past years, the price of crystalline silicon modules — which make up 90% of the solar-cell market — has leveled off but is expected to keep falling slowly. As a result, most of the cost of today’s photovoltaic systems is not in the material itself, but in the protective glass and wiring, racking, cabling and engineering work.
When all these costs are factored in, perov­skites might save money only if they can overtake silicon in efficiency. In the short term, firms are focusing on depositing the films on silicon wafers, with the perovskites tuned to capture wavelengths of light that silicon does not. On 10 September, Oxford PV announced that it was working with companies to make prototypes of these ‘tandem’ cells by 2015, and that this could boost silicon solar cells’ efficiencies by one-fifth, so that they approach 30%. Malinkiewicz’s hope is to find a niche that silicon cannot fill: ultra-cheap, flexible solar cells that might not last for years, but could be rolled out on roof tiles, or used as a portable back-up power source.
There is another potential snag: perovskites contain a small amount of toxic lead, in a form that would be soluble in any water leaching through the cells’ protection. Although Snaith and others have made films with tin instead, the efficiency of these cells is only just above 6%.


Ugo Bardi is a member of the Club of Rome, faculty member of the University of Florence, and the author of "Extracted" (Chelsea Green 2014), "The Seneca Effect" (Springer 2017), and Before the Collapse (Springer 2019)