Record single junction solar cell

30 08 2012

The 40 edition of the Efficiency Tables  (Green et al., 2012) announces a new record of power conversion efficiency of any solar cell made with a single semiconductor absorber. An efficiency of 28.8% has been measured at the National Renewable Energy Laboratory (NREL) for a 1-cm2 thin film GaAs (gap 1.43 eV)  device, with photovoltage 1.122 V, photocurrent 29.68 mA cm-2 and FF 86.5. The device was fabricated by Alta Devices, a Santa Clara-based ‘start-up’ seeking to develop low cost, 30% efficient solar modules. Alta Devices was founded by  renowned solar cell scientists Harry Atwater of Cal Tech and Eli Yablonovitch of UCB. Yablonovitch, author of fundamental contributions such as the maximum increase of absorption 2 n^2  by randomization of  light reflection (Yablonovitch, 1982), recently issued a preprint (Miller et al., 2011) in which the connection between photovoltaic performance and light emitting properties of a solar cell device was emphasized. The idea refers to develop solar cells that can actually approach the fundamental limits set by (Shockley and Queisser, 1961) based on the balance between light absorption and radiative emission in the solar cell. The connection between radiative properties, for the solar cell working as a a LED, and photovoltaic performance, has become a popular idea (Green et al., 2003,Kirchartz and Rau, 2008) and seems to give important payoffs, as shown by the Alta device.

In the figure I show how the best silicon solar cell and 28.8% record cell approach the maximal theoretical photocurrent and the maximal efficiency under 1 sun illumination. For a general discussion of the realization of efficiencies see (Nayak et al., 2011)

Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. “Solar cell efficiency tables (version 40)”. Progress in Photovoltaics: Research and Applications 2012, 20, 606–614.

Green, M. A.; Zhao, J.; Wang, A.; Trupke, T. “High-efficiency silicon light emitting diodes”. Physica E: Low-dimensional Systems and Nanostructures 2003, 16, 351-358.

Kirchartz, T.; Rau, U. “Detailed balance and reciprocity in solar cells”. physica status solidi (a) 2008, 205, 2737-2751.

Miller, O. D.; Yablonovitch, E.; Kurtz, S. R. “Intense Internal and External Fluorescence as Solar Cells Approach the Shockley-Queisser Efficiency Limit”. arXiv:1106.1603v3 2011.

Nayak, P. K.; Bisquert, J.; Cahen, D. “Assessing Possibilities and Limits for Solar Cells”. Advanced Materials 2011, 23, 2870-2876.

Shockley, W.; Queisser, H. J. “Detailed balance limit of efficiency of p-n junction solar cells”. Journal of Applied Physics 1961, 32, 510.

Yablonovitch, E. “Statistical ray optics”. J. Opt. Soc. Am. 1982, 72, 899-907.

Advertisements




Practical development of Residential Fuel Cells in Europe

15 08 2012

Fuel cells are a technology that allow the generation of electricity and heat at home from natural gas. Fuel cells have been announced as important energy devices for many years, but now it seems that medium size (a few kw) fuel cells for large houses and business are becoming commercialized and increasingly popular. This rewsidential application takes natural gas and runs it through a membrane to produce hydrogen, and then uses the hydrogen to produce electricity. The fuel cell uses the gas by chemical and electrochemcial transformation, not by combustion.

Strong advances on the market of residential fuel cells are occuring in California as described some time back in an article by Kanello. Now Marge Ryan in Fuel Cell Today reports the recent developments in Europe on fuel cells for combined generation of home heat and power (micro-CHP). With the pioneering Ene-Farm programme in Japan going from strength to strength and thousands of new units being sold there each year, it is apparent that the fuel cell suppliers in this market are beginning to enjoy commercial success. What progress Europe is making towards deployment on a scale similar to Japan?

Ryan writes that currently, the largest European project in this area is the Callux field trial in Germany, with systems from Baxi Innotech, Hexis and Vaillant installed in selected homes (the Baxi system is PEMFC, the others SOFC). In June this year, the 260 fuel cell systems installed to date had passed a cumulative one million hours of operation. Logging hours in trial deployments is essential to evaluate the performance of products under real-world conditions as a basis for further improvements. These have been substantial: Callux reports that appliance costs have been reduced by 60%, while costs for maintenance and replacement parts have come down by 90%; availability has increased to over 97%; electrical efficiency has increased to over 33% and overall efficiency to over 96%. To add to this, the systems have become smaller, lighter and more durable.





A boost of the market for lithium ion batteries for transportation

7 08 2012

According to a report by IDTechEx, the rapidly growing market for traction batteries will exceed $55 billion in only ten years

Dr Peter Harrop, Chairman, IDTechEx:

Lithium-ion is the winning type of rechargeable battery for the decade – there is almost a consensus. You can count about 150 manufacturers of these and IDTechEx has profiled nearly all of them. We expect 200 manufacturers of lithium-ion batteries soon, mainly because of the burgeoning number of Chinese companies making poor quality me-too-but-cheaper versions – notably the relatively easy-to-make small cylindrical versions such as the archetypal 18650 for laptops and small pouch versions for mobile phones. Then there are those that add basic flat-boxed “prismatic” versions for e-bikes but not much more. One million lithium-ion powered e-bikes were sold in Europe last year and China is starting to use them instead of the troublesome and polluting lead acid ones that the Chinese government seeks to control. Only 15% of lithium-ion battery manufacturers will continue to stay out of versions that propel vehicles.
There is almost a consensus that traction batteries for electric vehicles – hybrid and pure electric – will be the largest value market for lithium-ion batteries over the coming decade as a whole and certainly that electric vehicles will dominate use in later years by a big margin. IDTechEx projects a market for traction batteries in land and water vehicles plus aircraft of just under $60 billion in 2020 of which about 60% will be lithium-ion and 80% of those will be made by just four winning lithium-ion manufacturers.
For example, the number of mobile phones is becoming limited by the number of people in the world and it is not boosted by huge government support. The opposite is true of electric vehicles and they need the equivalent of thousands to tens of thousands of mobile phone batteries per vehicle.
The disagreement comes with just about everything else concerning lithium-ion batteries. What chemistry? IDTechEx has pointed out that most of the existing manufacturers and nearly all of the new manufacturers of lithium-ion batteries use lithium iron phosphate active cathodes because of advantages such as no materials subject to severe price hikes, low cost materials and easier patent position. They have good temperature performance that can reflect in greater safety though no lithium-ion cell is inherently safe and the first defender of safety is the Battery Management System BMS not the cell. Several recent fires and explosions have been related to something other than the cells.
The Japanese Institute for Information Technology notes that the leaders in sales of lithium-ion batteries for vehicles are in Japan (lowest material cost for the advanced materials) and Korea (fastest production lines), not China. We are told that the Japanese and Koreans, notably Panasonic including its Sanyo subsidiary, AESC and LGChem, base their success on ignoring lithium iron phosphate. This is partly because the primary driver of success of the potentially most popular electric vehicles in ten years’ time – pure electric and plug-in hybrid – is maximum all-electric range. There will be a huge take-off in sales when most people think the range of an affordable pure electric car is “adequate”. Nobody knows that figure for widely acceptable range partly because there is almost no statistical correlation between how people respond to questionnaires and what they then do.
Certainly the main frontier of lithium-ion success is achieving affordably greater energy density meaning range. Any improvement creates sales but we do not know the tipping point. The Japanese Institute for Information Technology believes that if we look at the leaders not the mob, there is a trend from lithium manganese oxide active cathode to nickel cobalt manganese to reflect these market needs. This is an over-simplification of course: there are variants and alternatives even among the leaders.
There is near consensus that lithium-ion battery anodes have to get cleverer. That must result in them being more than a minor part of the cost: today they consist of little more than copper foil coated with carbon such as graphite. Disagreement reigns when it comes to routes and timescales, however. EnerDel, Altairnano and Toshiba have taken the lithium titanate route said to improve power density for fast regenerative braking and fast chargers at the roadside or bus depot. However, rather more organisations are now looking at silicon-based anodes and some are in the market though only in small batteries. It is argued that silicon can increase cycle life and even that vital energy density but does vanadium need to be involved? Nanotechnology? Graphene? Struggling for any agreement here, we can only say that most experts believe that the cleverer anode will cost more than its typically 14% of cost reported today but it will never cost as much as the cathode, with 35% of the total spend.




The Black Swan: A book

2 08 2012

The Black Swan is a book written in 2007 by Nassim N. Taleb, an american writer of libanese origin. He has a prodigious record on financial trading and here he took to write about the importance of events that seem rare according to the normal explanation or understanding of a state of affairs, but that end up having an enormous sigificance. The major thesis is a warning against taking averages, and it is a very powerful idea, in my opinion. One example he uses is scientific scitation. Some papers get a disproportionately large number of citations, while about 50% of the publsihed paper are never read by anybody. This is totally true and some top journals (so-called boutique journals) make it their business to search only for those papers that will prospectively be largely cited, so they look systematically for the “Black Swan” paper. Such papers can be made out of fashionable subject, high level  abilities for writing titles, very top quality design of figures, and previously highly rewarded names as coauthors. In practice it is not so easy. So there are labs in very important places devoted to explore a potentially fashionable subject for a very short time, to make the Nature paper and then abandon the subject. Other scientists call this to “take the cream out of the coffee”. The rewards for the potentially highly cited paper is so large that it has a considerable influence on the high level science nowadays. Fortunately there still seems to exist a range of scientists that take their time to solve a problem they think important, or interesting, irrespective of the immediate reward, just for the sake of it. Some scientists become crazy about their impact and the awards, others do not care at all. There is every kind of people. In my research filed, there are scientists of both kinds. I especially admire those scientists that pursue an idea relentlessly, with all the power they have been given and all that that they can gather. It is also nice to look at those scientists that seem to have a very good time with their job, that create graciously and effortlessly.

Another instance is the top level science projects in Europe, which is the ERC Advanced Grant. In this call it is necessary to present a “high risk” project, looking again for the Black Swan. It is also a highly demanding art to write such projects, in such a way that you can actually do it, if it is awarded, so that you don’t have to return the 2.5 Meuro if you get no results at all (as it should be expected in most of the projects, if their risk was so high). A balance of this type is called in Spanish “nadar y guardar la ropa”, swim and take care of your clothes.

Scientific credit, finnancial reward, literary success, and the major benefits in other occupations in life, go in a great proportion to a few individuals. Maybe those individuals have really a great merit, but it is clear that others with similar skills, ability and dedication do not make it, so Taleb is arguing for the significance of luck,. The book is not trivial at all, it makes you think how you view statistics, how you give credit to stories (in the press…) and how you infer consequences from the past toward the future. So just by looking at the winners one should not infer the tratis that explain the success, one should look at the whole bunch of people that was trying hard. Noneteheless this year I read the bio of Steve Jobs and one concludes that there is a set of very very special individuals that do very big things, of course if they have the right environment and the right time.

One interesting distinction is between scalable and nonscalable jobs. Some occupations require a dedicated work, and the production depends on the effort and time devoted. But some others do not, and especially in thinking activities, the reward can be small or very large for the same effort. So it is part of the world that some people works in organization of other people’s works. The “idea” people sells an intellectual product in the form of a transaction or a piece of work, while the “labor” person, sells you his work. “If you are an idea person, you do not have to work hard, only think hard. You do the same work whether you produce a hundred units or a thousand… A scalable profession is good if you are successful; they are more competitive, produce monstruous inqualities, and are far more random, with huge disparities between efforts and rewards-a few can take a large share of the pie, leaving others out entirely at no fault of their own.”

“There is more money in designing a shoe than in actually making it: Nike, Dell, and Boeing can get paid for just thinking, organizing, and leveraging their know-how and ideas while subcontracted factories in developing countries do the grunt work and engineers in cultured and mathematical states do the noncreative technical grind.”