22 November 2012

Running on sunshine

Drawing on billions of years of evolution, artificial photosynthesis represents an elegant solution to our energy problems – but we’ve a way to go before the potential benefits outweigh the hidden costs.

Plants, algae and many species of bacteria convert sunlight into chemical energy by a process called photosynthesis. Credit: Wikimedia

More than three decades ago, Japanese chemist Akira Fujishima developed a process now known as artificial photosynthesis to mimic the way plants harness the Sun’s energy.

“The principle is to covert sunlight into electric energy,” says Pei Zhai, a postdoctoral fellow at America’s Lawrence Berkeley National Laboratory, “then covert electric energy to chemical energy.”

Plants do this by making glucose, a high-energy sugar that can be combined with other nutrients to make more complex chemicals.

Artificial photosynthesis, however, hasn’t progressed that far. “Basically,” Zhai said while presenting her results last week in Long Beach, California, at a meeting of the Society of Toxicology and Environmental Chemistry, “this technology harnesses solar energy and water to produce hydrogen and oxygen.”

The oxygen goes into the air. The hydrogen is collected for fuel.

That’s great as far as it goes, because hydrogen is a powerful fuel, but there’s a hitch: hydrogen isn’t easy to store and ship. Instead of simply being put in a tanker truck, it has to be compressed or liquefied – processes that requires additional equipment and, more importantly, significant amounts of electric power. In other words, hydrogen fuel made in this manner might require more energy to make and process than the fuel itself contains.

To find out if this is the case, Zhai conducted a “net energy assessment” of everything that goes into making artificial-photosynthesis-derived fuel. This includes not only the energy required to run the compressors and liquefiers, but the “embodied energy” of the materials used to construct the entire hydrogen-generating array.

What she found was that the process could work, but that designers need to think about how they will do it. Liquefaction, for example, could be a deal-breaker, sapping more energy from the fuel used for electrical power generation than is contained in the resulting hydrogen fuel. (Compression, on the other hand, takes considerably less energy.)

Also, if electrical power to do any of this is pulled off the existing power grid, it not only reduces the overall energy yield, but may come from coal- or oil-burning power plants, both of which produce the very pollutants ‘clean’ hydrogen fuel is designed to avoid.

But that needn’t be the case, she said. Instead, compressors (and, if necessary, liquefiers) could be powered by on-site photovoltaic cells – an approach rendered even more attractive by the fact that they would produce the most power at exactly the times when it is needed: when intense midday sun is also causing the artificial-photosynthesis array to run at peak.

Such a setup would produce no carbon dioxide emissions from its day-to-day operations, while yielding about 90 kilograms of hydrogen a day from an array about the size of a soccer field, she said.

To put that in perspective, hydrogen contains about 130 megajoules of energy per kilogram, compared to 47 for a kilogram of petrol or gasoline (about 1.3 litres). Ninety kilograms a day is enough, therefore, to replace about 330 litres of petrol – or to drive a fuel-efficient car all the way across Australia. It’s about equivalent to the production from a small group of low-yield oil wells.

Not that it’s currently economical. “I bet it would cost a fortune,” says Eric Johnson, of Atlantic Consulting, Gattikon, Switzerland. “I’m not saying people shouldn’t look into this stuff. It’s [just] hard to imagine it’s anywhere near cost-competitive.”

Thomas McKone, an environmental health researcher at Lawrence Berkley National Laboratories in California, USA, agrees. But he believes it’s important to continue working on the dream of turning sunlight directly into fuels – even if the current end product, hydrogen, isn’t ideal. “The long-term benefits are tremendous,” he says. Meanwhile, he notes that the technology is still in its infancy. “It’s like looking at computers in the 1950s,” he says. The ultimate dream, he adds, is to somehow convert that energy into mixed alkanes – the chemical constituents of petrol.

“You don’t have to compress gasoline,” adds Mikhail Chester, a civil engineer from Arizona State University.

Zhai’s study is just one such examination of the total ‘lifecycle’ costs of alternative-energy technologies.

Johnson, for example, has examined the effect of using biomass – specifically trees – as power plant fuel. People seeking carbon-neutral alternative fuels see this type of power generation as a plus because it replaces fossil fuels. But cutting down forests to burn as an alternative to coal or oil still releases stored carbon into the atmosphere. If the forests are slow to regrow, he says, the most effective way to offset carbon-dioxide build-up in the air might be to preserve them as protected woodlands.

Chester, on the other hand, has questioned the traditional wisdom regarding electric cars, trying to find out if – when all factors are taken into account – they are indeed ‘greener’ than conventional-fuel vehicles.

Electric cars may produce zero tailpipe emissions, but they increase emissions from battery production. And the farther a car can go between recharges, the bigger this problem becomes. “More batteries does not necessarily mean everything is greener,” Chester said. “More batteries means more range, but higher cost and weight.”

There’s also the problem of where the electricity to recharge the battery comes from.

In hybrids, it comes from energy that would otherwise be wasted, in braking. In fully electric cars, it comes from whatever power source is used by the local electrical utility, which could range from carbon-free hydroelectric power to coal.

Conventional-fuel vehicles have high environmental costs. “But the battery-electric vehicle has the potential to be even higher as we go away from clean electricity,” he said. On balance, he concluded, today’s best solution may be small-battery cars such as hybrids or plug-in hybrids.

All in all, Chester said, it’s a matter of the “hidden cost of energy”.

And when everything is taken into account, that can be a lot more complex than it initially looks.

Richard A. Lovett is an award-winning writer and science fiction author and regular COSMOS contributor.

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