You couldn't ever power a jet aircraft with solar panels, but a 747 might one day be solar powered. It sounds like science fiction, but there is hard science and hard currency being spent to research "artificial photosynthesis" and emulate nature's conversion of photons into powerful chemical bonds that release large amounts of energy.
Energy storage for PV systems is a hot topic that is currently gaining traction as a means of overcoming the proverbial renewable stumbling block: intermittency.
The Joint Centre for Artificial Photosynthesis (JCAP) was founded in 2010 by Steven Chu's Department of Energy with US$122m in funding over five years to research ways to turn sunlight, water and CO2 into hydrogen that could be stored as liquid or in a fuel cell.
The National Renewable Energy Laboratory is conducting similar research into photoelectrochemical (PEC) "light harvesting systems" using multi-junction cell technology developed by the photovoltaic industry. But one of the key differences of JCAP's work is synthesising membranes that scatter light.
JCAP's director, Professor Nate Lewis, has described this "solar to fuels" process as not only an attempt to mimic nature, but to improve on a plant's photosynthetic efficiency of 3% to 6%.
"It’s actually not alchemy – it’s bioinspired. Once you knew that a bird could fly, you didn’t build an airplane out of feathers. We built it with other materials," he said in a 2010 interview.
"We’re going to have to be able to store energy in chemical bonds, just like a plant does, storing it in the bonds through photosynthesis."
When it comes to energy storage, nothing comes close to the energy density of chemical fuels, said Prof Lewis. He estimated that even the best battery has 200 watt hours in a kilogram while a gallon of gasoline contains 13,000 watt hours per kilogram.
"Chemical fuels and chemical bonds are by far the best way to store energy we know of in the universe other than … getting it from the atom," he said.
JCAP, which is a partnership between the California Institute of Technology and Lawrence Berkeley National Laboratory, has yet to make any major announcements two years into its five-year programme as part of the DoE's Energy Innovation Hub. But that doesn't mean it has nothing to show for it, said Josh Spurgeon, one of JCAP's lead researchers.
"Our biggest goal is to have a really effective storage medium for solar energy to solve that intermittency issue because that will always limit how much solar energy can play into the overall global energy mix.
"Transportation is another big issue if you want to eventually move to an entirely carbon neutral or carbon free energy system. We're always going to want to be able to fly jets and you can't really fly a jet on solar panels.
"You need a really energy-dense way to do that. Being able to produce hydrocarbon fuels from sunlight is a really attractive way to do that. You could end up driving cars flying planes on essentially solar energy."
JCAP's goal under the DoE grant programme is to accelerate the rate of discovery of abundant, robust materials that can capture and convert the energy of sunlight into chemical fuels.
"The mission of JCAP is to demonstrate a scalable and cost‐effective solar fuels generator that, without use of rare materials or wires, robustly produces fuel from the sun 10 times more efficiently than typical current crops. To achieve this goal, JCAP will address the critical R&D gaps … to comprise a full artificial photosynthetic system prototype," said JCAP's mission statement.
One of JCAP's major challenges to commercialization is finding cheap and abundant materials for the catalysts, said Spurgeon.
"Most of the catalysts that do really well are noble metals and expensive materials. For hydrogen, platinum is the best catalyst out there but we want to solve big global energy problems so we don't think we can work with something like platinum on that scale. So we're trying to develop new catalysts and find more effective ways to use other catalysts that use earth abundant materials, things that are available in very large quantities in the earth's crust.
“Nickel molybendum catalyst has some promise,” he added.
JCAP's collaborative high throughput for screening potential catalyst materials achieves much more and more quickly than single labs operating alone, said Spurgeon.
It took billions of years for nature to optimize its measly rate of energy conversion. But Spurgeon anticipates great progress in the five-year programme.
"We will demonstrate something that will work really well within our five-year time window. The question is will we make it work well enough and cheap enough to on its own break into the market and that's going to be the big challenge. Hopefully we have a few big discoveries that allow us to accomplish that, but you never know."
Commercial-scale solar fuels are still cost prohibitive and JCAP is not yet in a position to estimate prices. But with new breakthroughs in materials and the macro-trends in energy economics, the price point at which solar fuels can compete with traditional resources may change over time, said Spurgeon.
"You want this to be combined with utility-scale solar to make electricity that is competitive with that produced from fossil fuels. That's very challenging because coal is dirt cheap. The good thing is that as time goes on we get cheaper and fossil fuels get more expensive."