The world is now used to the idea of generating energy from sunlight — the installed capacity of photovoltaic solar panels passed 200 gigawatts in 2015 — but PV is only part of the story when it comes to harnessing the sun’s power. View on FT.com (paywall)
“Many people mix up energy and electricity. Only about one-sixth of energy comes from electricity, the rest is in the form of fuels,” says Professor Leif Hammarström, chair of the Solar Energy Platform Sweden. “Renewable electricity needs to be complemented with renewable fuels.”
“We’re accustomed now to the idea that we can create electricity from sunlight but there’s a problem with that — electricity still can’t be stored,” says Professor Harry Atwater, director of the Joint Center for Artificial Photosynthesis, a collaboration between five research institutes in California. “Artificial photosynthesis enables the direct conversion of sunlight into chemical fuels, which means storable solar fuel.”
Natural photosynthesis is the process by which plants take CO2, one of the key greenhouse gases causing climate change, from the atmosphere and create the precursors of starch and sugar, with the addition of water, Prof Atwater explains. Humankind already takes advantage of this process to produce low-carbon fuels in the form of biofuels, but, says Prof Atwater: “Natural photosynthesis is not optimised for efficiency, it’s optimised for the sustainability of plants. We have a biofuel infrastructure but its efficiency is capped by the efficiency of natural photosynthesis, which only converts about 1 per cent of the sunlight into fuel.”
He adds: “If we turned all of the available arable land in the US over to biofuels production, we still wouldn’t produce enough fuel to run the American vehicle fleet. We can’t go carbon-free using traditional biofuels.”
In the pursuit of developing artificial conversion of sunshine into chemical power, researchers have been able to build and operate integrated solar fuel generators that split water to produce combustible hydrogen with an overall efficiency of 10 per cent. By adding CO2 to the mix, it is also possible to make hydrocarbons that can be made into fuels, chemicals or plastics, although this demands chemically more complex techniques.
As the field develops, decisions will need to be made on whether to focus on producing hydrogen or hydrocarbons such as methanol.
“The scientific maturity of splitting water [to produce hydrogen] is greater and some people think we should focus on hydrogen but that would require building a hydrogen infrastructure that does not yet exist,” says Prof Atwater. “One of the attractive things about making methanol is that there is already a 95 per cent efficient process to turn methanol into gasoline, so if we have a carbon-free source of methanol we can create low-carbon gasoline pretty easily.”
The other benefit of producing hydrocarbons is that CO2 captured from power stations and industrial facilities could be used as a feedstock, creating a new market for gases and thus making carbon capture and storage more viable.
“If we wanted to preserve the current fuel infrastructure, we would probably want to make hydrocarbon fuels,” Prof Atwater adds. “JCAP has just changed its focus from hydrogen production to directly making hydrocarbons.”
Both Profs Atwater and Hammarström are quick to point out that the technology remains in its infancy, even though research has been going on since the 1970s. “At the moment, we don’t have an artificial photosynthesis industry,” Prof Hammarström says. “Current devices are expensive and don’t last long enough but we have shown that it works — it is no longer just an idea.”
“It’s been really exciting to see the tremendous growth in the field in the last five years, in countries ranging from the US to Korea, Japan and China,” he adds.
The field is starting to attract some of the world’s biggest companies — Shell and Total are members of the Solar Fuels Institute based at Northwestern University in Chicago. Siemens researchers are working with universities in Switzerland and Germany on developing catalysts that will help to produce hydrocarbons.
The first commercial products made using artificial photosynthesis are likely to be speciality chemicals rather than fuel as these are more expensive and so the technology is more likely to be competitive. “I think we are in the same place we were in 1985 with PV. Solar panels were more than $100 a watt. Now they are $0.50 a watt. There is a real opportunity here,” says Prof Atwater.