Why sustainable power is unsustainable

Colin Barras, New Scientist 6 Feb 09;

Renewable energy needs to become a lot more renewable – a theme that emerged at the Financial Times Energy Conference in London this week.

Although scientists are agreed that we must cut carbon emissions from transport and electricity generation to prevent the globe's climate becoming hotter, and more unpredictable, the most advanced "renewable" technologies are too often based upon non-renewable resources, attendees heard.

Supratik Guha of IBM told the conference that sales of silicon solar cells are booming, with 2008 being the first year that the silicon wafers for solar cells outstripped those used for microelectronic devices.

But although silicon is the most abundant element in the Earth's crust after oxygen, it makes relatively inefficient cells that struggle to compete with electricity generated from fossil fuels. And the most advanced solar-cell technologies rely on much rarer materials than silicon.

Rare metal

The efficiency of solar cells is measured as a percentage of light energy they convert to electricity. Silicon solar cells finally reached 25% in late December. But multi-junction solar cells can achieve efficiencies greater than 40%.

Although touted as the future of solar power, those and most other multiple-junction cells owe their performance to the rare metal indium, which is far from abundant. There are fewer than 10 indium-containing minerals, and none present in significant deposits – in total the metal accounts for a paltry 0.25 parts per million of the Earth's crust.

Most of the rare and expensive element is used to manufacture LCD screens, an industry that has driven indium prices to $1000 per kilogram in recent years. Estimates that did not factor in an explosion in indium-containing solar panels reckon we have only a 10 year supply of it left.

If power from the Sun is to become a major source of electricity, solar panels would have to cover huge areas, making an alternative to indium essential.
Precious platinum

The dream of the hydrogen economy faces similar challenges, said Paul Adcock of UK firm Intelligent Energy.

A cheap way to generate hydrogen has so far proved elusive. New approaches, such as using bacterial enzymes to "split" water, have a long way to go before they are commercially viable.

So far, fuel cells are still the most effective way to turn the gas into electricity. But these mostly rely on expensive platinum to catalyse the reaction.

The trouble is, platinum makes indium appear super-abundant. It is present in the Earth's crust at just 0.003 parts per billion and is priced in $ per gram, not per kilogram. Estimates say that, if the 500 million vehicles in use today were fitted with fuel cells, all the world's platinum would be exhausted within 15 years.

Unfortunately platinum-free fuel cells are still a long way from the test track. A nickel-catalysed fuel cell developed at Wuhan University, China, has a maximum output only around 10% of that a platinum catalyst can offer.

A new approach announced yesterday demonstrates that carbon nanotubes could be more effective, as well as cheaper, than platinum. But again it will be many years before platinum-free fuel cells become a commercial prospect.
Fuel vs food?

Biofuels, like ethanol fermented from maize, are the most infamous examples of the doubtful sustainability of supposedly renewable forms of energy. This time the non-renewable resource at risk is the world's arable land, Ausilio Bauen of Imperial College London said at the meeting.

Again, there are potential solutions, but none that are ready for market. Biofuels from cellulose or even lignin can be derived from inedible plant material and wood rather than food crops. Algae, grown in outdoor tanks, continues to attract attention, and extracting biofuel from marine algae or seaweed could sidestep land use issues altogether.

Renewable energy technologies remain the great hope for the future, and are guaranteed research funds in the short term. But unless a second generation of sustainable energy ideas based on truly sustainable resources is established, the renewable light could be in danger of dimming.