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Tuesday, February 17, 2009

Storing solar energy by turning over a new leaf

By Jonathan M. Gitlin

Storing solar energy by turning over a new leaf

One of the most exciting talks at AAAS dealt with a breakthrough that might finally give the much-vaunted hydrogen economy a chance. Daniel Nocera, Professor of Energy and Professor of Chemistry at MIT gave a plenary lecture on a novel catalyst developed in his laboratory, one that takes inspiration from the photosynthetic pathways within plant cells to split water into H2 and O2, allowing the H2 to be used as fuel.

Meeting the world's future energy demand is going to need the sun, according to Nocera, and that's only going to happen if we can store that energy for use when it's not shining. This makes sense; plants evolved to make use of high-energy chemical bonds, and the oil, gas, and coal we currently use is just stored sunshine (via plants), albeit concentrated. Hydrogen is the only viable answer compared to batteries, capacitors, or mechanical storage schemes such as pumped water or compressed air once you look at energy density, he explained, and the work from his lab might help make that possible.

It's no understatement to say that the future of fuels is a hot topic. Dealing with climate change means moving away from burning coal, oil, and gas, but currently these represent the bulk of our energy supplies, and it can be hard to see what could take up their slack. Energy projections for 2050 suggest the planet will need between 28-35 terawatts (TW) a year, up from around 13TW in 2000. Nuclear will certainly be a component (more on that from us later) but 8 TW of nuclear power means 8000 new powerplants, or 1 new plant every 1.4 days. Even the most hardcore proponents of the atom don't think that's likely to happen.

Biomass, whether it be switchgrass, miscanthus, or bioengineered algae, has a theoretical maximum limit at around 7TW. Physics dictates the limit of wind power, and damming every river and stream would yield less than single TW. So you can see why everyone is pinning their hopes on that great fusion reactor in the sky.

As John Timmer reported on Friday, Nocera isn't the only one advocating solar energy as the future, but he offers a solution to the problems raised in that article: energy storage.

Nocera's tack is to copy photosynthesis to convert light energy to chemical energy, but to stop at H2 instead of trying to make sugars, lignins, and so on. Compressed H2 at 300 bar contains about 143 MJ/kg, many orders of magnitude more than the most advanced batteries or capacitors, and since you can get H2 out of water, and combusting it just gives you that water back again, it makes a lot of sense in a post-carbon world.

Other groups have spent a lot of time and money working on artificial photosynthesis, but their efforts have been misdirected, he claims. Artificial oxygen evolving complexes (OECs) tend to be highly unstable, so labs concentrate on making them last longer with exotic and costly materials. This not only pushes up the financial costs, it also makes it harder to push electrons into them, lowering efficiency.

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Nature, on the other hand, uses a simple inorganic redox core that self-assembles from water. It's unstable, so plants repair and replace their OECs every 30 minutes or so, and that's what Nocera's artificial OEC does too. Instead of Mn and Ca, it uses Co and Pi, works in sea water, dirty water, or a glass of water, and repairs itself spontaneously!

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When coupled with more widespread solar panels and more efficient fuel cells, Nocera believes that this CoPi catalyst would provide a household's daily fuel needs from 8 liters of water. I hope he's right.

Original here

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