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Harvesting energy from algae

Algae hold great promise as a possible source of biodiesel because they grow rapidly, are rich in vegetable oil and can be cultivated in ponds of seawater, reducing the need for fertile land and fresh water. Many companies are seeking ways to produce algal oil on a commercial scale, but they face significant hurdles.

February 15, 2008
by WENDEL BROERE

Guests and reporters watched David Parker, New Zealand’s minister for energy and climate change issues, pour a green-brown liquid from a plastic jerry can into the fuel tank of a red Land Rover. It was no ordinary fuel, but a blend containing biodiesel made with the oil from microscopic algae grown in sewage ponds. For an awkward moment the four-wheel drive refused to start, but then the engine caught and Parker took it for a 10-minute spin around the parliament building in Wellington.

The publicity stunt put the spotlight on algae as a potential source of biodiesel that doesn’t compete with food crops — in contrast to today’s conventional biofuels. Indeed, waterborne algae hold much promise. The tiny marine plant needs only water (salt or fresh), the energy of the sun, nutrients and carbon dioxide (CO2) to produce vegetable oil through photosynthesis. Some strains yield at least 15 times more vegetable oil per hectare than crops commonly used for biofuels, such as rapeseed, palm or soya. Some algae species grow so fast that they double their size three or four times in one day. That means they can be harvested frequently, an advantage over crops which are only harvestable a few times a year. Moreover, installations to grow algae can be located in areas unsuitable for agriculture, even deserts.

However, significant hurdles must be overcome before algae-based biofuel can be produced cost-effectively in the large volumes that would be needed to make a difference in the world’s overall supply of transport fuel. To work on a large commercial scale, the process calls for huge amounts of water, which could limit where it is produced. There is no tried and tested method to harvest algae efficiently in large volumes. And research must still be done to identify the best strains for producing oil.

There are no obstacles to engineering design for microalgae production, says Peter Williams, Emeritus Professor of biogeochemistry at the School of Ocean Sciences at the University of Wales in Bangor. However, he singles out three uncertainties around the viability of commercial production: “The capital and operational cost of the plant itself, whether we need to do development work on the productivity of algae, and the cost of harvesting algae.”

Biofuel demand set to rise

Demand for biofuel is set to increase as the European Union, the USA, and other countries ratchet up requirements for the use of biofuels in transport. That and historically high oil prices have spurred numerous start-ups, university research teams and large companies to find ways to produce biodiesel from algae on a commercial scale. Honeywell company UOP recently unveiled a project to produce military jet fuel from vegetable and algal oils. Chevron announced a collaboration agreement with the U.S. Department of Energy’s National Renewable Energy Laboratory to identify and develop algae strains for transport fuels. And in December 2007, Shell and HR Biopetroleum formed a joint venture company, Cellana, to build a pilot plant in Hawaii to produce small volumes of vegetable oil for conversion into biofuel.

Algae have been grown successfully on a small scale for years in the pharmaceutical industry and for health foods, but never cost-effectively on a large scale. There are two common methods to grow algae. The first uses a series of storage tanks linked by transparent tubes that rest on support structures. Algae and water are pumped through the pipes to ensure maximum exposure to sunlight. CO2 piped into the installation feeds the algae. There is little risk of contamination of the algae as they are grown in a closed environment resembling laboratory conditions. Productivity per hectare is also high so the equipment takes up less land than open systems. However, the equipment is expensive — several kilometres of tubes are necessary to produce commercial amounts of oil — as are maintenance costs to keep it clean and working. Some estimates say biodiesel from these closed systems could only compete if crude oil cost $800 per barrel.

The alternative method pumps water around a continuous loop of a man-made, open-air channel to expose the algae to sunlight. The raceways at existing open pond algae farms hold about as much water as a municipal swimming pool. Such open ponds are cheaper than closed systems, but they have their drawbacks too: light only reaches the algae near the surface, water easily evaporates and the temperature is harder to control. The risk of contamination is also greater than in closed systems. Predators such as small shrimp-like organisms that eat algae can enter open ponds, carried by wind or spread by waterfowl. “Once they are there, it’s like letting a dog loose in a meat shop. They propagate and take over,” Williams says.

Production challenges

The huge amounts of water needed for large-scale production in both systems poses another challenge. The water holding algae must be refreshed each day and taken out of light to kill any predators or weeds. That involves pumping it out of the production system and back in again. “It’s like any agricultural system, but at very high intensity,” says Ian Archibald, a scientist with Shell Global Solutions International B.V. The smallest practical size for an algal biodiesel plant is 1,000 hectares, which pumps about 1 million cubic metres (28 million cubic feet) of salt water a day. This is about twice the amount of fresh water used for agricultural irrigation. Researchers expect to reduce the amount of salt water needed to produce oil from algae.

Harvesting algae and extracting oil present other technical and cost hurdles. The tiny plants have a diameter one-tenth the size of a human hair — too small to collect by straining the water. Other options, such as adding chemicals to make the algae stick together or spinning the liquid to separate the water from algae, are used at commercial plants that grow algae in small quantities for pharmaceutical products and foods. Both methods are expensive when applied in large-scale commercial production. Researchers are seeking the best of a variety of approaches.

There are also competing methods to extract the lipids and treatments to make biodiesel. Common methods to extract oil from algae involve pressing it out of dried algae or adding chemicals to remove the oil. However, vegetable oil must be treated before it can be used as a fuel. A diesel engine can run on vegetable oil but gummy deposits will form in the motor and the fuel will go solid at low temperatures. The standard treatment to make oil from land crops more palatable for engines can also be applied to algae. A chemical reaction breaks their carbon chains and adding methanol yields oil that can be blended with regular diesel. The treated algal oil can be used in diesel blends of up to 10%. An alternative method, hydrotreatment, uses hydrogen instead of methanol and catalysts to speed up the chemical reaction. This approach yields a fuel that is identical to fossil fuel, but it is complex and more expensive as it uses costly-to-produce hydrogen and catalysts.

Seeking a winning strain

There are more than 100,000 known strains of microalgae in the world. The search for the best one for biofuel production is still under way. A Shell research programme involving several universities is looking for the strain with a winning combination of high oil content and a rapid growth rate. Algae reproduce by dividing their cells. Usually algae with high oil contents grow relatively slowly. For example, algae containing 80% oil will only divide once every 10 days, whereas algae containing 30% oil may divide three times daily. Moreover, strains capable of producing large amounts of lipids tend to do so when they are starved of nutrients. But starvation also slows growth. Some researchers hope to genetically modify algae to produce more oil. Others have taken a different approach and produced yellow algae that allow light to penetrate further into ponds, promoting more uniform growth.

Recent history serves as a cautionary tale to rapid commercialisation of algae-based biofuel on a large scale. The 1970s’ oil crises prompted several government-funded studies into algal fuel in France, Germany, Japan and the USA, and other countries in a bid to increase energy security. But the technology was expensive and development costs were too high. The U.S. Department of energy’s Aquatic Species Programme, for example, built two 1,000-square-metre open pond systems and found that algae were only economically viable as a biofuel at oil prices of more than $60 a barrel. The Clinton administration ended the programme 11 years ago after spending about $25 million as low oil prices of the day made it unattractive economically. In Japan the Research for Innovative Technology of the Earth programme extensively studied uses for microalgae. The programme concentrated on closed systems to grow algae but it was stopped after an investment of more than $100 million as the technology was seen as unfeasible. New approaches combine the best of both of these worlds.

Today’s all-time high oil prices and advances in biochemical science and technology have breathed new life into researching the green slime. Whether algae-based fuels will thrive depends on whether the technological and commercial hurdles can be overcome. If they can, algae – one of the world’s oldest organisms – may prove a boon to the transport needs of the future.