Low-carbon fuels from sunlight and waste carbon dioxide: It is possible, is it practical?

A Thought Leader Series Piece


By Ellen B. Stechel

Note: Ellen B. Stechel is the Deputy Director of ASU's LightWorks and Managing Director of LightSpeed Solutions, communicating global efforts of leading scientists and researchers working towards sustainable transportation energy based on liquid hydrocarbon fuels from the sun.

A network of issues buried beneath the strategic and economic importance of petroleum and the increasing concentration of atmospheric carbon dioxide is complex; however, until addressed, no measure of global sustainability will be obtainable.

If we accept that, any solution to such issues yield lower net carbon emissions by 50-80 percent, then despite obvious advantages, alternative fossil fuel pathways cannot be the ultimate solution for transportation.

The economics of carbon

A stable policy environment to level the playing field and allow time for low-carbon options to develop, deploy, and decrease costs through experience, learning, scale, and innovation is necessary, but insufficient.

Higher carbon fuels from Canadian tar sands; coal or gas-to-liquids projects; and natural gas switching (with modest carbon reductions) rapidly entering the transportation sector may block market penetration of low-carbon innovations, discouraging investment in emerging technologies. Long-lived assets could "lock-in" a high-carbon transportation infrastructure and all but eliminate viable options for transitioning to a low-carbon future.

Innovation policy that enables a balanced portfolio of promising options would stimulate development of viable possibilities by focusing on solving the problem as opposed to choosing a limited set of specified approaches, thereby excluding opportunities for novel solutions, including hybrids, integrated systems, and new concepts.

Is liquid hydrocarbon fuel still a good option?

New low-carbon domestic energy sources and transportation innovation, such as increased fuel economy, biofuels, electrification, and possibly hydrogen, would reduce total demand for petroleum and carbon emissions, but not enough.

Could liquid hydrocarbon-based fuel remain a viable and sustainable option in large quantities? Often overlooked, liquid hydrocarbon fuels are unrivaled in the rate of delivery to on-board, usable energy storage. They are also unsurpassed in having high energy densities accommodating both space and weight requirements. Consequently, there are no credible alternatives for air, heavy-duty, or commercial ocean applications save some penetration of compressed or liquefied natural gas.

Furthermore, it is neither useful nor accurate to think of petroleum as a primary energy resource. It is more appropriate and instructive to recognize that conventional fossil fuels are in fact, "stored (ancient) sunlight" in the form of energy dense, sequestered carbon and hydrogen that nature took millions of years to produce and modern civilization is taking only centuries to consume. Carbon dioxide and water are simply the energy-depleted, oxidized form of the carbon and hydrogen making up the hydrocarbon. Thus, we might consider reframing the problem as a techno-economic challenge to reverse combustion fast enough to match consumption.

Recycling carbon dioxide

This reframing suggests searching for large-scale options that convert, store, and upgrade sunlight to a higher energy value and transportable form as nature did, but faster. An underexplored emerging strategy is to develop solar technologies that recycle—rather than bury—waste carbon dioxide into new supplies of liquid hydrocarbon fuels.

For example, synthetic solar thermochemical fuel processes can convert solar energy, excess carbon dioxide, and low quality water into gasoline, diesel, and aviation fuel—fuels that are compatible with the existing energy infrastructure. This process recycles carbon dioxide back into fuel at rates considerably faster and more efficiently than the biosphere naturally captures and fixes carbon dioxide from the atmosphere.

To achieve societal objectives, such options will need to do so efficiently, affordably, and sustainably. Many challenges are avoided by utilizing existing infrastructure whenever possible and using waste carbon dioxide as a carbon source feedstock initially from concentrated sources, but ultimately directly or indirectly captured from the excess in the atmosphere.

Opportunities and challenges

Large-scale industrial conversion of solar energy that transforms carbon dioxide and water into infrastructure compatible hydrocarbon fuels is an attractive option to facilitate a smooth and continuous transition, affecting the existing vehicle fleet and co-evolving with the future fleet. However, such an option while certainly possible, still has significant resource, economic, and technical challenges before becoming practical, especially if it is going to achieve scale and be sustainable.

A general examination identifies a number of challenges, such as achieving high solar energy-to-fuel system-level efficiency, low material intensity in the solar collectors, high material accessibility, and good material durability; limited and no additional arable land use; and low water consumption. Opportunities to meet each of these challenges are already encouraging.

Using the sunlight to re-energize carbon dioxide both directly and in hybrids (with biomass or fossil feedstocks) can produce net lower and ultimately net neutral carbon-based fuels with most of the carbon in the initial feedstock making it into the fuel product. Researchers in several countries, including the U.S., working on solar-based recycling of carbon dioxide have prototypes and some making it to large-scale demonstrations.

Such innovations could unite solar energy interests with fossil fuel and biofuel interests, and could preserve an option for a low-carbon future and a smooth transition that maximizes the use of installed infrastructure and new investments in natural gas.

A promising energy future

These opportunities offer significant promise for a platform of technologies that store sunlight and sequester carbon above ground as an energy-dense fuel with affordable economics, closing the-carbon cycle, and scalable to global demand.

Despite challenges, there are promising advances already happening and opportunities to leverage developments in related industry segments. By working across stovepipes, we can drive sustainable economic growth, create many high-quality jobs, and produce viable and scalable solar alternatives to petroleum.

About the author: Ellen B. Stechel is trained in mathematics, chemistry, and physics. Early in her career, she was a technical staff member at Sandia National Laboratories before moving to the Scientific Research Lab and later Product Development at Ford Motor Company. While at Ford, her responsibilities included emissions and fuel chemistries, climate change and sustainability, and deployment of new technologies for low emission vehicles. Later in her career, she returned to Sandia National Labs to build and manage research efforts in applied energy, making fuels from the sun and concentrating solar technologies. She is now a professor of practice at ASU’s Department of Chemistry and Biochemistry.