May 19 2023

Making Fuel from Sunshine

When it comes to big problems it’s generally a good idea to remember some basic principles. One is that there is no free lunch. This is a cliche because it’s true. Another way to put this is – there are no solutions, only trade offs. Sometimes there is a genuine advance that does improve the calculus, and there are certainly more or less efficient ways to do things. But when making decisions that affect the technological infrastructure for a world-spanning civilization of billions of people, everything has consequences.

As I have been writing about frequently, perhaps the biggest such decisions we face involve where we get the energy to power our civilization. On the one hand we have the technology of what’s possible. On the other we have economics, which will tend to favor the cheapest option regardless of other concerns. But then we also have – other concerns. That is generally where governments and regulations come in, ways for the public to exert their common interests other than making individual purchasing decisions. Free market forces are powerful at generating information and homeostatic systems, but are generally blind to long term or strategic planning. In my opinion, we need to have an optimal blend of both.

But in the background, science and technology is slowly, incrementally, advancing. We no longer have the luxury of just waiting for technology to solve our problems, but we do want to keep pushing the ball forward and make sure we include scientific progress in our strategic planning, and efficiently incorporate new technology when it’s available. That is partly why I like to peek a little ahead at potential technologies that might be coming down the pike.

In that vein, here is an incremental advance that is at the proof-of-concept stage – using solar energy to make liquid fuels directly from CO2 and H2O. This kind of technology is sometimes referred to as “artificial leaf” technology, since it makes high energy compounds powered by sunshine. Such systems involve a mechanism to capture photons and a catalyst that promotes a specific chemical reaction. In this case:

Here we assembled artificial leaf devices by integrating an oxide-derived Cu94Pd6 electrocatalyst with perovskite–BiVO4 tandem light absorbers that couple CO2 reduction with water oxidation.

The catalyst is made from copper and palladium. The light absorber is perovskite, which is actively being developed as perhaps the next type of solar panel to replace silicon. The process makes multicarbon alcohols ethanol and n-propanol. This is an advance over current technology, which can make either hydrogen or syngas, which then needs another step to make liquid fuel. Going directly to liquid fuel in one step is a huge efficiency gain.

Ethanol is already a fuel additive, making up about 10% of fuel in American cars. The problem with ethanol is that it is currently sources mostly from corn, taking up a significant amount of agricultural land. Also, the energy advantage is pretty marginal – it does have a positive energy balance, by about a thirdN-propanol is actually better than ethanol. It has a high octane and energy density, and is the most desirable alcohol for use in gasoline engines, but is currently too expensive for general use. Blends of up to 50% n-propanol are available. Flex fuel vehicles are designed to use alcohol blends, and can burn up to 83% alcohol mixed with gasoline. There are also flex fuel cars with small batteries that can run on 100% ethanol. Pure ethanol has problems in the winter, but this can be fixed with hybrids.

The advantage of alcohol fuels is that they can immediately be mixed into our transportation infrastructure, with existing cars burning 10-20% ethanol or 30-50% n-propanol. Flex fuel cars and hybrids can burn 80-100% alcohol fuel. Gasoline, in other words, can be phased out without too much infrastructure disruption. Biofuels are not suitable for this application because of the land-intensive and energy intensive nature of their manufacture. But we could replace oil fields with solar fields making ethanol and n-propanol.

The technology currently is not ready for prime time. Again, we are at the proof of concept stage. One challenge is that the Faradaic efficiency is only 7.5%. That is the percentage of energy that could theoretically be converted to fuel that is actually converted. This efficiency needs to go up. And second, the technology needs to be scaled up from the lab. Unless we can make millions of gallons per day, this will not make a dent in our carbon footprint (the US alone burns 369 million gallons of gasoline per day).

Finally, for any new technology we need to consider the trade-offs and the alternatives. Is using a field of solar panels to make liquid fuel better than just making electricity for battery electric vehicles? I don’t know the answer, but that would be an illuminating calculation. I suspect that will depend on the efficiency of the whole process, and at 7.5% it probably cannot compete with photovoltaics. But, using sunlight to make liquid fuel can be a useful part of the whole system. It can be a way to store energy when excess electricity is available. Zero carbon liquid fuel cars may also have a niche for situations in which BEVs are not optimal, such as in rural areas or for long-haul applications.

There is also something to be said for spreading out our technology, rather than relying on any single solution. First, this can help phase our fossil fuel faster. And second, this may reduce the demand for raw material for car batteries.

What about using technology like this to make hydrogen instead of liquid fuel? We already have working hydrogen fuel cell cars. All of my previously stated concerns about hydrogen still apply. Hydrogen would require a new infrastructure, and compressing gas has inherent problems. Liquid fuels are better, and can be fed into our existing infrastructure much more easily.

As always, the devil is in the details. If this kind of technology can become mature so that it has reasonable efficiency and can scale up economically for widespread industrial use, I can envision a role for it. At least for the next 20-30 years, while battery technology is still improving, it may be a way to more quickly get to net zero while releasing the least amount of CO2. It may also have a longer term niche for certain applications – trucks, big equipment, aircraft, remote areas. There are certain advantages to energy dense liquid fuel that would probably give such technology long utility.

And of course, this technology may go nowhere if it cannot be scaled up efficiently. Or it may take so long that it gets eclipsed by other technology.


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