Green machine: A new push for pond scum power

Green machine is our weekly column on the latest advances in environmental technologies

Pond scum. The term surfaces in every news report about the decades-long effort to get energy from algae. But more respect for the tiny creatures may soon be in order: industry and governments have started pouring billions into alga power.

Just the basic facts make you wonder why algae aren't powering our civilisation already. The single-celled phytoplankton produce half our planet's oxygen and are the fastest-growing green organisms known. In shallow seawater ponds on land, they can use sunlight and sewage to turn concentrated carbon dioxide – flue gas from coal burning, say – into usable hydrocarbons, half of which can almost be poured straight into a diesel engine.

"Ten million hectares of algae could supply all US transportation fuel," says Greg Mitchell of the Scripps Institution of Oceanography in La Jolla, California. That's less than 3 per cent of the area farmed in the US – and algae can live in seawater in the desert.

Fuel shocks

Too good to be true? Until recently, it seemed so. After the 1970s fuel shocks, the US launched the Aquatic Species Program (ASP) to investigate algae. It eventually built 1000 square metres of ponds at Roswell, New Mexico, and found many promising algal strains. But it concluded in 1996 that diesel from algae would cost twice as much as diesel from oil – at least at oil prices then.

So the ASP ended, and focus shifted to bioethanol from maize. Enthusiasm for that has dimmed, however, as it conflicts with our need for farmland for food.

One option has been to look at making bioethanol from inedible land plants. But interest has also swung back to algae. Last year oil giant ExxonMobil committed $600 million to develop algal fuels with genome pioneer Craig Venter. Since then projects have mushroomed: John Benemann, formerly a lead investigator on the ASP, estimates that $2 billion could be flowing into algal energy projects between 2009 and 2011.

ExxonMobil believes it will be able to sell algal diesel through existing refineries and petrol stations. Last month the first jet engine powered entirely by algae oil flew at an air show in Berlin, Germany.

Energy efficient

Algae can produce 10 times as much energy per hectare as land-grown oil crops, says Mark Hildebrand of Scripps, because "they float. They don't have to spend half their [energy] building stalks to support themselves."

Instead, they make more cells until they run out of nutrients such as nitrogen. Then they stop dividing, but keep photosynthesising, storing the results as an oil droplet that can eventually take up more than half the cell.

One simple chemical reaction makes this oil into a diesel fuel that releases less particulate, sulphur-oxide and nitrogen-oxide pollutants than petroleum-based diesel. The remaining algal biomass can also be burned as fuel, though may be worth more as livestock feed, especially for fish. That means less damage to the environment, and less demand for overtaxed fisheries, soil and water to catch or grow feed.

Problems, problems

Algae do present problems, however. One is that they don't grow and make oil at the same time – and that impedes industrial production. Algae can be grown until they use up all the nitrogen in their water. Then they stop multiplying – but before they switch to making oil, they cannibalise their own proteins for any spare nitrogen. Then they start making oil, but with damaged photosynthetic machinery that is less productive.

One family, the diatoms, solves that problem, says Hildebrand: they make intricate cell walls from silica, and start making oil when there is no more silica to grow. So they can produce oil without damaging themselves.

Scaling up algae production has been plagued by teething trouble and short-term financing. One high-profile start-up, GreenFuels Technologies, built the most ambitious pilot plant yet – it used CO₂ from a power plant in Arizona – but went broke last year, a victim of unruly algal growth and expensive growth chambers.

The cheapest way to grow algae is in large open ponds, but these risk being invaded by algal "weeds" and algae-eating zooplankton. Mixed algal communities, perhaps with different species to match the seasons, might out-compete interlopers, says Hildebrand.

But even if growth problems are resolved, harvesting the oil is expensive. Benemann thinks there is promise in algae that spontaneously settle to the bottom of their tanks, or one kind that excretes oil in a capsule which resists oil-eating bacteria. Industrialising this process will require much R&D.

A little means a lot

And ultimately, production will require suitable climate, land, water, nutrients and CO₂, all at one site. Even using waste land, seawater, sewage and smokestack CO₂, Benemann thinks this will limit the potential for algal biomass to the equivalent of 1 per cent of the CO₂ now being released, or less. "But that's still a gargantuan amount. Let's hope we can do that much."

We won't without more long-term funding, says Mitchell. But he is sanguine about the pay-off. "The US has been subsidising corn ethanol at $5 billion a year. Half that amount for 15 years in algae could solve the water, fuel, energy, feed and farmland crises humanity faces."