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Calculating the future of solar fuels

Like any process, converting the sun’s energy into liquid fuels requires a sophisticated, interrelated series of choices. But what makes a “solar refinery” so tricky to map out is that, as one decides how to spark a solar-powered reaction that turns carbon dioxide and water into hydrocarbon fuels, so many steps could potentially involve newly developed and experimental technologies. New methods for capturing carbon dioxide from a coal-fired power plant, the ever-expanding array of materials used for catalysis, the diversity of approaches to capturing and converting the energy in sunlight—all of these are variables researchers and industry must weigh as they ask how to design economically viable and energy-efficient solar refineries. And many of them will be using a powerful new tool developed at UW-Madison.

In a new paper in the journal Energy & Environmental Science, a team led by Chemical and Biological Engineering Professors Christos Maravelias and George Huber craft a framework intended to help plot the future of solar fuels. The framework essentially calculates the financial and energy costs, as well as the productivity, of a solar refinery based on myriad variables that a user can plug in. Renewable-energy researchers at UW-Madison have long emphasized the importance of viewing energy production as a holistic process, and they think the paper will help many different people involved in solar fuels keep the bigger picture in mind. The research was funded by the U.S. Department of Energy.

“The idea is that people who are working on parts of the technology can get a sense of what the milestones are that they might need to reach in the overall scope,” says Jeff Herron, a postdoc in Maravelias’ group. Herron is the lead author of the paper, which also includes contributions from PhD student Aniruddha Upadhye in Huber’s group and postdoc Jiyong Kim in Maravelias’ group. “People tend to be narrowly focused on their particular role within a bigger picture.”

It’s important for everyone involved to keep this in mind—from catalysis experts to financial decision-makers—because even a big improvement in one particular step of the process might yield only a tiny improvement in the efficiency of the process as a whole. “People think that if they work on a given section of the refinery, then everything else is done, but it’s not quite that way,” Maravelias says.

The framework is designed to remain relevant as solar-fuel producers and researchers experiment with new technologies—even ones that haven’t been anticipated yet. “The nice thing about it being general is that if a researcher develops a different technology—and there are many different ways to generate solar fuels—the paper doesn’t care exactly what goes into it, and if someone wants a little more detail, our framework would still be applicable,” Maravelias says.

Along with the economic realities of bringing renewable fuels to market, the framework also aims to help producers avoid the perverse situation of wasting too much energy to create a fuel, which will ultimately be used as a source of energy. Capturing CO2 from power plants already requires burning fossil fuels, and other stages of the process, such as separating the final product from leftover CO2, are also expensive in terms of energy use. 

Maravelias also thinks the framework can play a role in the wider debate about which renewable-energy technologies are ultimately the best for society to pursue on a large scale. When, say, comparing solar fuels to biofuels, things can get complicated pretty quickly—one could argue that plants are already harnessing the sun’s energy without the need to build a lot of new infrastructure, but one could also argue that solar-power technology converts that energy more efficiently than plants.

The versatile, process-oriented analyses UW-Madison engineers offer could bring a great deal of empirical clarity to that conversation. In fact, Herron points out that the framework could easily be adapted to help analyze and plan any number of other processes, not just solar refineries.

The other striking thing about the paper is that it does not come off as a boosterish argument for solar fuels. Instead, the researchers want to give their colleagues in the field a realistic sense of the challenges and opportunities ahead.

“If you read the paper, it maybe comes off as a little bit negative about the technology and its prospects, but I don’t think the message is to be negative,” Herron says. “It’s more about setting goals for people to aspire to.” 

Scott Gordon