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Addressing Another Energy Miracle: Ethanol From Carbon Dioxide
People love their energy miracles. If a university or company claims they can make fuel from air or from water, along comes the inevitable hype proclaiming that this is just the sort of breakthrough we need to meet our energy needs in a way that doesn’t increase greenhouse gas emissions. These sorts of claims help drive money to continue the research, but a lot of important details tend to be left out when the story is told.
About a year and a half ago a story like this went viral. It was a claimed breakthrough from Audi, in which they announced production of the first batch of synthetic fuel from ingredients found in the atmosphere. The story generated tremendous interest, and I received numerous queries on whether it was too good to be true. I wrote a number of articles at the time (for example) dealing with the actual meaning of this work.
This week another purported energy miracle went viral when researchers claimed they had “accidentally” discovered how to convert carbon dioxide into ethanol. The takeaway from this story is similar to the one from Audi, and in fact the same sort of takeaway from all such stories. So let’s review what this means.
First, here is the story as told by any number of media outlets:Scientists Accidentally Discover Method to Turn Carbon Dioxide Into Ethanol.
Of course with carbon dioxide emissions steadily climbing, and scientists warning that we need to get this under control, anything that can take carbon dioxide from the air and either store it away or convert it back into a fuel is going to generate a lot of interest. Not to downplay the importance of research like this, but this discovery doesn’t mean what most people think it means.
I have received over a dozen inquiries about this story, with some pointing out that the researchers claimed the energy efficiency of the process was 63%. That’s not true. What they claimed was that the yield was 63%. In other words, 63% of the carbon dioxide fed into the process got turned into ethanol.
The big piece that is missing is the amount of energy it took to drive that process, and the bad news there from thermodynamics is that it necessarily requires more energy to be consumed than is contained in the ethanol that is produced. In other words, if you produced a gallon of ethanol by this process, it is 100% certain that the amount of energy consumed is greater than the ~76,000 British thermal units (BTUs) in a gallon of ethanol.
Carbon dioxide is the product of combustion (as is water), and it exists at a very low energy state. Thus, in any process that claims to convert carbon dioxide into a fuel, there will always be an ingredient in the process that is even more important than carbon dioxide. That ingredient is energy, which generally creates carbon dioxide itself when produced. The fact that it takes more energy to produce the fuel than you can get back from burning it comes from one of the fundamental laws of thermodynamics.
Thus, the process is necessarily an energy sink. I pointed this out to a few people, and a couple responded “It’s no different than crude oil. You never get as much gasoline out as the oil you put in. It’s always less than 100%.”
It is true that if you run a gallon of oil through an oil refinery, you will only get something like 90% of that out as finished products. The rest is consumed in processing the oil. But this is an apples and oranges comparison. Only 10% of the oil was consumed to produce the gasoline, diesel, and jet fuel. The rest passed through and ended up in the finished product.
For an apples to apples comparison, a refinery would have toconsume the equivalent of more than 1 gallon of oil to produce a gallon of finished products. That means that you would have to input more than 2 gallons of oil for output of 1 gallon of finished products. More than 1 gallon would be burned. Gone. Up in smoke. That’s the apples to apples comparison.
Or, perhaps a simpler way to think about it is this. If I want to produce 1 gallon of ethanol via this process, I am going to burn the energy equivalent of more than 76,000 BTUs. If I want to produce 1 gallon of gasoline in a refinery (with an energy content of ~115,000 Btu), I am only going to burn ~10,000 BTUs.
This has significant implications for greenhouse gas emissions. Unless purely renewable sources can be tapped to produce ethanol via this process, it would necessarily result in a net increase of carbon dioxide emissions. Remember, to produce a gallon of ethanol, we already had to burn the energy equivalent of more than 1 gallon. That means that by the time the product ethanol is ultimately burned, you are really putting into the atmosphere the emissions of more than 2 gallons of ethanol. And it could be a lot more than 2 gallons of emissions for every gallon produced. Thus, even though the process is consuming carbon dioxide, it will result in higher net carbon dioxide emissions, unless the following condition is met.
One could envision a process in the future that utilizes primarily wind or solar power inputs if more power is being generated than the grid can handle. In a case like that, you could potentially justify a process that is an energy sink, because otherwise the excess solar power would be wasted. This is true for any process that is producing excess energy that would otherwise be wasted. It may be better to turn 10 BTUs of power into 4 BTUs of ethanol than simply see it wasted. In that case you could see a net reduction of carbon dioxide emissions from the process.
The bottom line is this. I can produce gasoline, ethanol — even acetaminophen — from the ingredients found in the air. But I can’t do it cheaply.
Beyond the energy inputs required to make the process go, it ultimately becomes an issue of economics. You have to have extremely cheap power to make it work economically, and to do it in a way that it is net carbon negative you have to use renewable energy. Ultimately this probably ends up like so many of these stories before. It is interesting research, and ultimately could lead to large scale production of carbon negative fuels. But a great deal of nuance has been lost in the hype, and there are still some very big technical challenges ahead.
Robert Rapier has over 20 years of experience in the energy industry as an engineer and an investor. Follow him on Twitter @rrapier.