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Solar Fuels: How Close, How Real?

There’s a class of fuels that don’t use an intervening biomass to make a fuel — so, though they use biology or waste carbon, they’ve bristled at being called “biofuels." Instead, the technologies that depend on unique pathways to converting CO2 and water to fuels and chemicals prefer “solar fuels."

By any name, they’re fascinating. What’s the latest?

The most well-known of the solar fuels is, without a doubt, those from Joule Unlimited, which pioneered the term “solar fuels” in the first place and is the closest to reaching scale. Using modified cyanobacteria, they form jet fuel, diesel and ethanol under the brand name “Sunflow” — and have been operating a demonstration facility in Hobbs, New Mexico for going on two years now.

But other technologies have come along — not following the same biotechnology path, but utilizing some of the same underlying concepts of waste CO2 utilization, bypassing biomass, and making target fuels and chemicals directly from the same inputs that plants use to make biomass in the first place.

About Liquid Light

One of the most interesting of these is Liquid Light, which emerged from stealth mode in the past year, and focuses on electrocatalytic conversion of CO2 to useful fuels and chemicals. The company’s first process is for the production of ethylene glycol (MEG), with a $27 billion annual market.

In measuring progress, the company notes that “Results consistent with cost-advantaged production have been validated at lab scale for key parts of our process. Ultimately, the company has opportunities in propylene, isopropanol, methyl-methacrylate and acetic acid.”

So, here’s a company that doesn’t use biology, doesn’t use sunlight, and doesn’t use an intervening biomass. Well might you ask, what has this anything to do with a solar fuel?

Back to Solar Fuels

So, let’s focus for a moment back on solar fuels. Is there anything in this class of technologies that might prove out the potential of a new way of storing solar energy, as liquid fuel?

You see, that’s been perhaps the biggest challenge of solar technology to date. Production gets better and cheaper all the time, but how do you store it — and, in real-time, how do you deal with the intermittent nature of key renewable energy technologies?

After all, electro-chemical batteries, that store solar energy in electron form — well, they’re getting better but not nearly fast enough to match the interest in solar, or match the rate of improvement in solar PV technology. Grid-scale systems still generally try to use solar energy in real-time, for peak power periods — which electric cars suffer from short ranges.

A very interesting advance along that from appears this week in the Journal of CO2 Utilization. A research team from Princeton, using Liquid Light catalysts and specially-designed reactor cells , demonstrated that it was possible to produce formates.

What makes it especially interesting is that they used a standard solar panel to power the set of electrocatalytic reaction cells. The project confirmed that renewable power, in this case solar, worked as a power source, and that the intermittent, sometimes-unpredictable nature of renewable sources did not negatively impact process efficiency.

There have been attempts to make formates directly from sunlight before. None have reached the basic processing efficiency benchmark, which was to match or exceed the conversion efficiency of natural photosynthesis.

In this case, the results were as much as 9X better than the best previously reported results by industry or research labs, for converting solar energy to formates, and roughly 2X better than photosynthesis.

The researchers report (BEWARE: sciencespeak alert!)

The storage of solar energy as formic acid generated electrochemically from carbon dioxide has been identified as a viable solar fuel pathway. We report that this transformation can be accomplished by separating light absorption and CO2 reduction through the use of a commercial solar panel illuminated with natural AM1.5 sunlight to power a custom closed-loop electrochemical flow cell stack. Faradaic yields for formate of up to 67 percent have been demonstrated in this system, yielding a solar energy to fuel thermionic conversion efficiency above 1.8 percent.

What Are Formates, Again?

Formate itself is salt or an ester of formic acid — which is best known as the key chemical in ant venom and accounts for the odor from a crushed ant.

For one, there are formic acid fuel cells, where the formic acid can be fed directly to proton exchange membrane fuel cells and provide electricity to power mobile phones and laptops. Though formic acid fuel cells are not yet a practical technology, they’ve been touted as an alternative to hydrogen fuel cells because they don’t require exotic temperatures and pressures like hydrogen does.

For those of you who hate the charge times of Li-ion batteries — well, you are talking about seconds by simply re-fueling. Some of the latest work on catalysts for formic acid fuel cells is here, from work performed at Umea University in Sweden, here . Work from 2012 is also reported here.

Another Technology Path Using Some of the Same Ideas

James Liao’s lab at UCLA has been working on some of the same ideas. Back in 2012, they reported that “We’ve been able to separate the light reaction from the dark reaction and instead of using biological photosynthesis, we are using solar panels to convert the sunlight to electrical energy, then to a chemical intermediate, and using that to power carbon dioxide fixation to produce the fuel. This method could be more efficient than the biological system.”

“Instead of using hydrogen, we use formic acid as the intermediary,” Liao told Gizmag, based on work he published in Science. “We use electricity to generate formic acid and then use the formic acid to power the CO2 fixation in bacteria in the dark to produce isobutanol and higher alcohols.”

Brookhaven and AIST have been pursuing some related ideas. In their case, they think that hydrogen and CO2 can be converted to formic acid, stored more safely and efficiently than cryogenically cooled hydrogen, then convert back to hydrogen when its time to release into the fuel cell. More on that here.

Back to Liquid Light and Chemical Production

For now, the Princeton research confirmed that Liquid Light’s underlying technology could produce compelling results across more than one target chemical. And, the portion of the experiment that measured cell efficiency also supported the high expected efficiency for electrocatalytic conversion of CO2 to other chemicals (such as ethylene glycol) using Liquid Light’s technology.

The Bottom Line

There’s increasing evidence that a scaleable process to store solar energy as a liquid chemical at standard temperature and pressure is on the way. And, that useful targets like MEG (ethylene glycol) are in the mix, there.

That’s a key substantiation that there’s a near-term market available to Liquid Light that will support its path to commercial success. Which will be mighty pleasing to its investors, which include VantagePoint Capital Partners, BP Ventures, Chrysalix Energy Venture Capital, and Osage University Partners.

Longer term, couple of forks in the road ahead. Could be that target fuel molecules will appear in Liquid Light’s future — or, via partners.

More interestingly, there are opportunities here to — in the long term — connect two of the most hyper-efficient technologies around (solar PV and fuel cells). Is formic acid going to be the crucial intermediate that links high-efficiency energy capture to high-efficiency electric engines?

It’s the kind of challenge that ought to deserve an X Prize. We'll see if one emerges.

This article was originally published on Biofuels Digest and was republished with permission.

Lead image: Solar panel via Shutterstock


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Volume 18, Issue 3


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