Want to win a Nobel Prize and also go down in history as the greatest inventor of the 21st century? Then find the breakthrough that makes artificial photosynthesis a practical, large-scale reality. Why you ask? Read on.
Last week’s column, “Methane Hydrate Part II: All You Really Need to Know about Global Warming”, was rather grim. Here is the capsule summary. The burning of fossil fuels during the Industrial Age has already increased the concentration of carbon dioxide in the atmosphere from 280 to 390 ppm resulting in a 0.7 oC rise in the global average temperature. The United Nations Intergovernmental Panel on Climate Change has concluded that the largest temperature rise we could endure while still maintaining some semblance of our current lifestyle and population is 2 oC, which corresponds to an atmospheric CO2 concentration of 450 ppm. We will need to restrict ourselves to using 1/3 to 1/4 of the current known reserves of petroleum, natural gas and coal in order to stay below that 450 ppm. As there is still no sign of a meaningful global commitment to reduce fossil fuel use, it now seems inevitable that we will blow right by the U.N. target and proceed toward a climate catastrophe. The addition of the vast reserves of methane hydrate as a new fossil fuel source would exacerbate the problem.
The realization that we are not on track to stay below the U.N. target of 450 ppm is starting to make its way out of technical journals and into the public consciousness. There are two possible approaches to addressing the problem of increasing CO2 content in the atmosphere. The one most commonly mentioned is a rapid global conversion to non-fossil fuel energy sources. While this does address the root of the problem, as I will explain in next week’s column, I do not believe this solution can be implemented rapidly enough to both solve the climate crisis while steering clear of other serious problems. The possible approach is carbon sequestration, the removal of CO2 from the air and its long-term storage.
Below is a partial list of proposed CO2 sequestration methods, along with an explanation of the potential shortcomings of each.
- Subterranean Injection: In this method, CO2 would be captured from the smoke stacks of power plants and injected into depleted oil and gas wells or other underground formations with available empty volume. There are several drawbacks to this approach which are likely to keep it from becoming a viable solution. It’s very expensive. The CO2 may leak back out. It’s only viable for point sources of carbon dioxide like a smoke stack and, therefore, would not apply to cars and other dispersed CO2 sources. Finally, the volume of CO2 gas generated from burning fossil fuels far, far exceeds the amount of available underground storage space.
- Biochar: This is an interesting technology which is garnering increasing attention in progressive media outlets. To make biochar you heat organic matter (which would otherwise decompose and release CO2 back into the atmosphere) to 400-500 oC in an oxygen-free atmosphere, which converts it to a charcoal like material. Biochar is slow to oxidize back to CO2 and can be incorporated into the soil where it improves water retention and nutrient availability. Unfortunately, biochar has several key shortcomings as a global warming response. First and foremost, while it can prevent the release of carbon dioxide from decomposing plant matter, it cannot directly remove CO2 from the air. This limits the rate of sequestration available from biochar to a level which is too low to make much of a dent in the CO2 concentration in the air. Furthermore, the high temperatures needed to produce biochar require significant fuel consumption, thus offsetting its benefit, as well as the use of expensive, specialized processing equipment.
- Carbonate Formation – Carbon dioxide reacts with minerals such as magnesium and calcium to form solid carbonate deposits. While this process does sequester the CO2, the rates of formation are too slow to be of use unless the reaction is carried out at elevated temperatures, making this approach suffer from some of the same limitations as biochar.
Even if one or more of these possible sequestration approaches overcame the hurdles I described, they suffer from an important, additional problem which is common to all. They don’t allow for the carbon to be recycled. To understand why that matters, you need to come with me on a short journey of the history of energy on the earth.
All fossil fuel or biomass energy comes from photosynthesis, in which plants absorb energy from the sun and store it in the chemical bonds of the hydrocarbon molecules that they make from CO2 that they capture from the air. We recover and use that stored solar energy by burning those hydrocarbons, which releases heat and emits carbon dioxide.
Until about the year 1850, our primary fuel source was biomass, primarily wood, for heating, cooking, and industrial processing like making beer or soap. When human society was dependent on biomass, the total amount of energy which could be produced was the sum of the energy currently being stored by new plant growth plus a drawdown of the energy stored in the trees in the forest.
As the human population grew the forests began to dwindle, inspiring our predecessors to look for new sources of energy. They discovered and began burning ancient biomass in the form of coal and petroleum. The exploitation of these fossil fuels allowed for humanity to unleash hundreds of millions of years of stored solar energy in a little over a century. This energy bonanza has driven the explosion in human population from around 1 billion in 1850 to over 7 billion today, and created the climate crisis we now face.
With that background in mind, let’s consider why is it necessary to recycle carbon rather than simply burying it. Consider if subterranean injection is much more successful than I predict, and enough CO2 could be sequestered to bring its concentration in the atmosphere back to an acceptable range of 300-350 ppm. This approach would allow us to burn our fossil fuel reserves without killing ourselves via the Greenhouse Effect. (Though we may well find another way.) As we proceeded down this path, we would eventually end up with no fossil fuels left and a staggering amount of buried CO2. In this scenario, the amount of biomass-based energy available to human society would plummet back to the same level as in 1850. This gap in energy availability could conceivably be filled through a combination of solar, wind and nuclear power, but another important gap could not be filled.
Long time readers may recall that one of the first Common Science columns I published was called “Everything Comes from Oil, Everything,” in which I detailed how and why a staggeringly large percentage of the products of modern society are produced from petroleum. What makes petroleum so useful is that the sun and plants have already done most of the work for us by providing a veritable cornucopia of useful hydrocarbon molecules which we can, via the chemical industry, manipulate into useful items such as plastics, clothing, medicine, carpets, glue, paint, bowling balls and computers. We have know-how to make these same items from plant matter, but the world cannot grow enough biomass each year to feed, provide energy to, and make products for seven billion humans. So if we don’t want 2150 to look substantially like 1850, we’ll need to find a way to make hydrocarbons in a way that doesn’t compete with food production.
The Holy Grail for both solving our climate crisis and providing a replacement for petroleum as a raw material source would be an artificial photosynthesis process. To be successful, the process would need to operate at room temperature and pressure and be far more efficient than plants. Plants store about 2-4% of the energy from the sunlight that shines on them in the chemical bonds of the hydrocarbons they form. For artificial photosynthesis to be effective in combating climate change, it would need have a sunlight-to-hydrocarbon efficiency of at least 20% and capture more that the current 10 gigatons of carbon which are being added to the atmosphere each year. Some of the resulting hydrocarbons could be recycled as fuel. The remainder could be converted into long-lived items like plastic building materials, effectively sequestering the carbon in useful infrastructure.
There are already a number of research efforts chasing this Holy Grail. To its great credit, in 2010 the Department of Energy, under the leadership of President Obama and with collaboration from Cal Tech, opened the Joint Center for Artificial Photosynthesis. Although this sounds promising, I’m still not entirely convinced it will work. It will be extraordinarily difficult to invent and scale-up a technology with the qualities I’ve outlined above. Materials used would need to have very high surface-to-volume ratios to provide a sufficient number of sites for the CO2 to be captured, and have unique and impressive chemical and electronic properties to drive a photosynthesis-like reaction using only sunlight. I think the answer, if it exists, will be found in nanotechnology. (For more details on nanotechnology check out my recent column here.) But in the meantime, let’s wish our friends at Cal Tech good luck.
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- Hamadri was a teaching assistant and mentor of mine while I was an undergrad at the University of Virgina. Hamadri had come to the U.S. for his undergrad and had the choice of attending either Berkerly or Cal Tech. We used to ask him how he made the choice between these two prestigious institutions. He would give us a sly smile and answer “well Berkeley is Berkeley but . . . Cal Tech is Cal Tech”! Hamadri, if somehow this gets to you, I hope you are well and still proud of your alma mater.