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My concern about this process is the use of ultraviolet light. UV photons can drive useful reactions such as splitting water to produce H2. But in the sunlight striking earth, not much of the energy is in the form of UV photons. So if the process is limited by the amount of UV photons, then the energy efficiency cannot be very good. The review of the process doesn't discuss the efficiency in any detail, but it seems likely that most of the energy in the sunlight goes into heating the sample, producing a good environment for catalysts that produce higher hydrocarbons. But that energy probably is not increasing the energy of the fuel, just helping the catalyst reform it. It seems likely that most of the net energy is coming from the UV - producing H2 which can then react with the CO2 and water to form hydrocarbons. This means the process will be highly inefficient unless one gets very lucky and finds a catalyst that can produce high energy chemicals from low energy visible photons. The efficiency matters because this process will have all the capital cost of a solar thermal electricity generating system. If the efficiency is small compared to the solar thermal system (or a photovoltaic system), then it would be much better to use the solar to make electricity. If liquid fuels are needed, the electricity can be used to make H2, and normal catalysis can produce hydrocarbons.
Note that the 54.5 mpg standard is a CAFE measurement, not the sticker value which is an attempt at a "real world" number. The 3rd generation Prius has a CAFE of about 67 mpg, so it is well beyond 54.5. Also, the CO2 and fuel economy standards for 2025 are based on the footprint of a vehicle (area of a rectangle defined by the points where the 4 wheels touch the ground). So larger vehicles don't have to get 54.5 mpg. EPA has commissioned a number of studies which suggest that the 54.5 mpg average is attainable. There is a little bit of excessive optimism in the EPA studies, but we still have almost a decade until 2025. So there is a fair chance that auto manufacturers can meet the goal at a reasonable cost. Especially if fuel gets back to $4/gallon in the U.S. by 2025, cars which meet the 54.5 mpg standard might have a higher purchase price but a lower cost of ownership. There is currently a lot of interest in 48V microhybrids. But these are not too different from the Saturn Vue (36V) and the GM eAssist (120V, 15kW). Both of these were disappointing in the marketplace. Perhaps 48V electric superchargers will be a good way to provide performance and fuel economy in a very small engine.
The focus of the article is on artificial leaves with a solar cell under water. I still don't understand why this is a good idea. Why not do the obvious: separate the solar cells from the electrolysis cell? The business case for solar requires long lasting solar cells (e.g. 30 years). That may be possible with cells in air. It seems unlikely in a water environment. And the cost of windows, water seals, and gas seals spread out over large area solar arrays seems like a big problem. Also, why should the optimum current density be comparable to the optimum solar density? With a "leaf" under water one has to have an electrolysis system that has about the same area as the solar array. It seems more likely that one can optimize the system by using normal solar arrays to produce electricity, use high efficiency power electronics to get the correct voltage and current, and then optimize the electrolysis cell separately.
As a followup to my previous post, I am not questioning the value of research on solar production of H2. But I am questioning the rationale for an artificial leaf that is really just a photovoltaic cell with electrolysis catalysts plated on part of the cell. I do not think that this can possibly compete with separate photovoltaic and electrolysis modules, where each can be optimized. I will be amazed if a combined "leaf" can have the durability, efficiency, optimized current density, serviceability, etc. of a system with separate photovoltaic and electrolysis modules. In the world of electric vehicles, the power electronics can efficiently charge batteries with a wide variety of cell working voltage. So I don't see any real value in trying to optimize the photovoltaic cell voltage to match the best H2 evolution potential. I think these electronics will be a small fraction of the cost of a solar to H2 system. Some work on solar to H2 production involves more simple approaches in which an active catalyst powder in water absorbs sunlight and produces H2 and O2. If one can find a powder or a set of redox systems that are durable in water, absorb a reasonable fraction of the sunlight, and are reasonably efficient at producing H2, then this could be a real winner. A wide variety of research is following this path, but so far none of it has the kind of efficiency and H2 production rate to be economically viable.
I continue to be puzzled by the logic of these attempts at artificial leaves. In this case, they have made a photovoltaic system (GaAs/GaInP2) and then incorporated catalysts for producing H2 and O2. How can this be more cost effective than using a quality photovoltaic cell to produce electricity and then run the electricity into a quality electrolyzer to produce H2? (The electrolyzer can either use precious metal catalysts or one of the new "earth abundant" material catalysts). The economics of photovoltaic electricity require very long life systems (people often suggest a 30 year useful life). How can one hope to make a long life system that incorporates the photovoltaic cell into a system with windows, water, and catalysts? The authors made a cell that lasts for 40 hours. And how can this combined system be less costly than separate photovoltaic modules and electrolyzers? With the artificial leaf, each module can be ruined by poisoning from impurities in the water, or corrosion of the semiconductor material, or a leak that lets the H2 escape. With a separate system, the relatively expensive photovoltaic part is independent. Electrolyzer modules can be replaced at relatively little cost, leaving the photovoltaic modules to continue to function.
The EPA sticker fuel economy is derived from a weighted average of 5 driving cycles: city, highway, city with cold start, US06 (more aggressive driving), and SC03 (hot ambient with air conditioner set for maximum cooling rate). In the recent past, auto companies had the option of just using the city and highway driving cycles and using a linear correlation to get "real world" driving numbers for the EPA sticker. EPA created the linear correlation to produce reasonably accurate estimates for 2008 vintage vehicles. I'm not sure if auto makers now have to use the 5 tests, or if they can still use the linear correlation with the city and highway tests. Detailed test result data is available at and for many of the cars there are only city and highway test results. Given this background, there are several possible reasons why the EPA sticker numbers might not be accurate for turbos and for the Ford Fusion hybrid. Perhaps the weighted average of the 5 tests does not work very well for these vehicles. Or perhaps the linear correlation for the city and highway tests does not work very well for these vehicles. Of course, if drivers are much more aggressive in terms of acceleration and maximum speed, then their fuel economy will be worse than the EPA sticker would predict. Similarly, the Consumer Reports tests may or may not do a good job of predicting real world driving for these vehicles.
Most of the ACKinetics discussion is for AC induction motors, but except for the Tesla, most vehicles with electric motors use permanent magnet motors. I believe the permanent magnet motors are chosen because they are more efficient over a wide range of speeds and loads. Here is a reference to some Oak Ridge measurements of motor efficiency for the Camry motor: Over a wide range of (steady state) speeds and loads, the motor is more than 90% efficient. Perhaps Neil Singer will comment on whether the ACKinetics drives would likely improve the efficiency of the Prius or Camry Hybrids. Note that high efficiency is needed for power split hybrids like the Prius, where a second motor/generator is built into the planetary gear to create an electrically variable transmission. Sometimes part of the power from the engine is used to drive the generator which then supplies power to the motor. Both the generator and motor have to be efficient to minimize the losses. High efficiency is also necessary to get the most out of regenerative braking over a driving cycle. Toyota has worked hard to optimize the efficiency of their motors and drive electronics. It would be interesting to know whether the ACKinetics system can improve on what Toyota does.
There are a number of problems with Gaylor and Viscusi's analysis. I have talked with several automotive leaders who firmly believe that consumers do not rationally value fuel economy. These leaders believe that (at least in the past), consumers would not pay more for better fuel economy if the true payback time was more than about 1 year. That is clearly irrational. The authors also focus on the consumer, and ignore any barriers for the industry to take risks to develop more fuel efficient vehicles. There is plenty of evidence that many automotive companies are focused on the near future, and will not do the hard work to improve fuel economy without a strong push from regulations. Every time EPA provides a new regulation to limit pollution, industry leaders claim that they can't possibly afford to meet the regulations. Then they meet the regulations for less cost than even EPA predicted. (Examples include the first introduction of the catalytic converter and the reduction of SO2 contributions to acid rain). Gaylor and Viscusi claim that EPA and NHTSA only consider cost and fuel economy, but that is not true. I have read the proposed regulations and supporting material carefully. EPA and NHTSA consider cost effective ways to improve the fuel economy of many different kinds of vehicles, while keeping the utility of each vehicle constant. So consumers will have the same range of choices of vehicles they have now. The EPA and NHTSA analyses show that one can make cost effective improvements that will greatly reduce fuel consumption while having many other benefits including reduced greenhouse gas emissions. One can question whether EPA and NHTSA have correctly assessed the long term cost and efficacy of improvements in technology. (The latest EPA/NHTSA estimates are optimistic compared to a 2011 study by the National Research Council). But any cost benefit study should clearly attempt to consider all the costs and benefits, as EPA and NHTSA have tried to do. Limiting the benefits to just the reduction in pollution and intentionally ignoring other benefits does not make economic sense.
Note that the study assumed the car is driven only 150k km. That is far less than the life of a vehicle, and this would definitely affect the "life cycle" carbon footprint. Probably this low number was chosen because it might represent the life of a battery, especially for first generation EV. If one has to replace the battery in an EV, then the manufacturing carbon footprint will go up. As with biofuel life cycle analysis, the results can vary widely depending on the assumptions. Argonne created the software GREET to provide a more complete and transparent analysis of biofuels. We probably need similar analysis of the carbon footprint for car manufacturing and service in order to get closer to a meaningful answer.
@SJC - please provide a reference for your comment above. All reports of Nocera's "artificial leaf" claim the device uses sunlight and has no wires. See and
The work of Daniel Nocera at MIT is related, and he has published a lot of discussion about the rationale and challenges for solar produced H2. See for example an account of his recent work on an "artificial leaf": The basic idea is to couple a semiconductor (to absorb photons and produce electrons and holes) with O2 and H2 evolving catalysts (to use the electrons and holes to produce O2 and H2). Developing an economical system will be a challenge, but a variety of research groups are making some progress.
A recent study at MIT of methanol as a fuel (Bromberg and Cheng )concludes that methanol is relatively safe in terms of groundwater pollution. A search using Google Scholar seems to confirm this. The main concern is that methanol and ethanol can change the transport and degradation of other pollutants such as benzene. While methanol is somewhat worse than ethanol, there is not a very big difference (Gomez and Alvarez 2010 Methanol contamination by itself would be dangerous, but in most scenarios the dispersal would be fast enough to limit the danger.
Reel$$ states that "there is little science to justify the hand wringing over GHG. The world knows it. And its nations are voting to dismiss it." This is very inaccurate. Compare the statements of a recent U.S. National Academy of Sciences report: "The scientific understanding of climate change is now sufficiently clear to begin taking steps to prepare for climate change and to slow it. Human actions over the next few decades will have a major influence on the magnitude and rate of future warming. Large, disruptive changes are much more likely if greenhouse gases are allowed to continue building up in the atmosphere at their present rate. However, reducing greenhouse gas emissions will require strong national and international commitments, technological innovation, and human willpower." See Similar statements are made by the National Academy of Science of Russia, China, India, Japan, Canada, France, Great Britain, ... (See
The comparison of gasoline hybrids and diesels needs to be done with more care than is shown in some of the comments for this post. The comments by usbseawolf2000 are meaningful because they compare the mpg for the hybrid and the diesel on the same EU driving schedule and accurately point out that the hybrid has much lower NOx emissions. Diesel fuel does have 11-15% more carbon per liter than gasoline, so if the diesel and the hybrid have the same mpg, then the diesel will emit more CO2. (This ignores CO2 produced in refining; most sources say this is about the same for gasoline and diesel). With the right refining processes, one can choose how much gasoline vs diesel to produce from a barrel of oil, so there is no clear winner in that regard. Within the context of U.S. emission laws, the comparison between the 2009 VW Jetta TDI and the 3rd generation Prius is probably meaningful. The Prius has better fuel economy (50 mpg vs 34 mpg)and less emission of pollutants. The Prius has technologies like electric water pump and electric steering assist that the Jetta probably does not have. If one took all the technologies of the Prius and added them to the Jetta, the difference in fuel economy might not be so great. The cost of the 2 vehicles is not too different, given the uncertainty in resale prices. At a 2009 diesel conference in the U.S., most engineers were pessimistic about the prospects for diesel passenger cars in the U.S. The main concern was anticipated new pollution rules in California (which also apply to about half the U.S.) These would tighten up the requirements for hydrocarbon and NOx emissions. The engineers felt these would be extremely hard to meet with diesel powertrains. So I say - cheers to all the good engineers that are trying to improve hybrids and diesels. It will be interesting to see what is the best technology for each market.
Methanol has serious disadvantages as a common fuel since it is soluble in water and toxic. California was pushing this as a low emission diesel fuel in the past, but when people started to think about the problems with contaminating groundwater, they gave up on it.
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Mar 3, 2010