Science Spin May 2009

Harvesting solar power

By Tom Kennedy

Even if we dam every river in the world, cover the hills with wind turbines, and build thousands of nuclear power stations, we do not have the capacity to keep up with the growing demand for energy.

At the SFI Science Summit last November, Dan Nocera, Professor of Energy and Chemistry at MIT, explained that it is simply not possible to get a high enough energy yield from existing technology, and the gap between supply and demand is rapidly growing. "By the year 2050," he said, "we are going to need about 30 terawatts of energy, that's double what we are using now, and by the end of the century we might triple the amount of energy required." As Prof Nocera explained, this is a conservative projection, and "even if we can deliver 100 per cent energy savings, we will still need a lot of energy."

Demand

At present the world demand is about 15 terawatts, and even if the population remained static, millions of people would still be left waiting for the sort of connection that we in the west take for granted.

The drive to replace existing energy sources, said Prof Nocera is not going to solve our problems, and he dismisses many of the claims being made for the alternatives as fanciful. To take biomass as an example, he said we would be doing very well to get a one per cent energy yield. "The whole thing about corn ethanol, is ridiculous," he said, "people push it because there is money to be made."

On wind, Prof Nocera, waving his hand through the air, remarked, that if it's so easy to do that, how can we expect to collect much energy? In his view, developers have made a big mistake in scaling up their calculations. "The experts have got it right for small groups of mills," he said, "but on a larger scale, these calculations will not hold up. Energy has already been extracted from the wind, so there is less for the next wind farm."

Issues

The biggest problem is one of scale, and this even applies to nuclear power. "The problem with nuclear is that you can only put out a gigawatt," he said. On that basis, it would take 8,000 nuclear stations to generate eight terawatts, and that still leaves us short, which leads us on to some startling conclusions. To meet existing world demand, "you could end up building a new nuclear plant every 1.6 days for the next fifty years. Then, of course, you would have to start decommissioning. Nuclear plants last about 50 years, so its not just 1.6 new plants for the next fifty years, but forever."

Not that Prof Nocera wanted to throw cold water on everyone's expectations by showing up the errors in over-optimistic calculations. "Everything should be on the table," he said, for there is no one single fix available to us, and besides, western society is not just going to walk away from a massive dependence on grid-based technology. "Three billion people in the developed world are doomed to the grid," quipped Prof Nocera.

If something works, use it, is Prof Nocera's advice. However, he does not believe that the sums can ever add up unless we can come up with a more efficient way to harvest energy. That energy, he said, is there for the taking.

The world is bombarded with more than enough solar energy to meet almost any demand, but at present the most we can recover is ten per cent, and unless we have efficient storage, a high proportion of this is going to go to waste. "Storage is one of the big problems," said Prof Nocera, and to give some idea of the current limitations, he mentioned that if America has to depend on pumped storage, the 639 sq km Lake Mead reservoir behind the Hoover Dam would have to be emptied and filled five thousand times a day.

We hear a lot about battery improvements, and again, Prof Nocera, while acknowledging their usefulness, said we should not pin too many hopes on them. The improvements we have seen, he said, are in the speed at which they can deliver power, rather than any increase in energy density.

Solutions

So, where does that leave us? Strangely enough, the solutions, both to capture and storage, has been staring us in the face, and as is often the case, Nature got there first. Photosynthesis converts solar energy into chemical bonds, and guess where that energy can be stored so efficiently that enough can be released from less than a litre to enable us break the speed limit.

The highest energy density that we can get, said Prof Nocera, is in chemical bonds. "It is orders of magnitude bigger than other energy storage forms," he said, "Society already has this figured out," he said, "because most of our stored energy is not from batteries, but from chemical fuels. That's a bias I have, and that's a bias Nature has."

When it became important to store energy, it would have been just as easy for Nature to arrange transfer of electrons across membranes, and indeed, while that mechanism exists and is essential for life, it is not used for storage. Nature saves energy in chemical bonds.

As Prof Nocera notes, the process is as elegant as it is efficient. "When you burn a fuel, nothing exotic happens," he said, "you just rearrange bonds," and extract the spare energy.Nature, said Prof Nocera, choose water for a very good reason, and not just because of its abundance. "In one litre you have thirteen megajoules," and he commented on how this knowledge made him look at swimming pools with something akin to awe.

Water comes into the energy equation during the first stage of photosynthesis, and as Prof Nocera observed, most people only think of this process in terms of its final products. As far as he is concerned, the earlier part of the process, where hydrogen is split from oxygen is of bigger interest, because a lot of what follows goes into making sugars, starch and fuelling growth.

Sugar is an energy store, but from our point of view, far from ideal, so Prof Nocera's group at MIT has been concentrating on the catalytically assisted splitting of water into hydrogen and oxygen. Working with Matthew Sanan, a post doc, the group developed catalysts from cobalt, phosphate and platinum with some very useful properties, not least being activity at normal room temperature.

As he remarked, the artificial photosynthetic system is quite easy to set up and is very efficient way to make hydrogen and oxygen. For a small input of energy, their relatively simple laboratory apparatus splits water into oxygen and hydrogen.

Liquid fuel

There, the parallel with photosynthesis ends, for Prof Nocera's group does not want to lock the hydrogen up in sugars or end up producing alcohols. At this stage some attractive options start to open up. For example, the hydrogen could be used directly as a gas fuel, or the gases could be used in fuel cells to produce energy, but Prof Nocera does not think this is the best way to go. "A lot of people argue in favour of using the hydrogen directly," said Prof Nocera, but "I would prefer to take the hydrogen and combine it with carbon dioxide and make a liquid fuel."

As he observes, we are well accustomed to liquid fuels, they are easy to transport and store, they are packed with energy, and in this case, with artificial photosynthesis, the process is totally non-polluting. "I take light and use it to rearrange the bonds in water, and when the hydrogen and oxygen recombined, you get water back again. You are not even using up the water."

Cost

So, if this is so good, how come we are not using it? Up to now, said Prof Nocera, the problem has been cost, but the new lower cost catalysts, that can work well at normal room temperature, are likely to get over that problem.

At present synthetic photosynthesis has an overall 18 per cent efficiency, and Prof Nocera believes that this is all we need to meet our projected energy needs. Hot deserts, he remarked, could become highly productive sources of liquid fuels.

Science, he said, can deliver on solar power within twenty years. Admittedly, there is a lot of work for the chemists, but just how fast they can work depends on just how willing we are to support what they are doing.

 

 

 

Small scale generation of power is likely to be the way to the future, but as Prof Nocera commented, we in the west are condemned to the grid. As we try to patch on extra generation capacity, people in less developed parts of the world may end up with an advantage as local schemes could give more security of supply. With synthetic photosynthesis, said Prof Nocera, "five litres of water could turn a house into a combined power and gas station."