There is an inevitable compromise to be met between growing crops to make biofuel from and crops for food. The Earth has around 15 million km^2 of arable land available, which if used in its entirety for the purpose, could provide around 1.5 billion tonnes of rape-seed oil which is equivalent to 11 billion barrels of crude oil, still far shy of the 30 billion barrels of conventional oil currently consumed annually, and we still need to feed a rising global population, predicted by the WHO to transcend 9 billion by 2050.
The problem is that crop production is highly limited in its ability to harvest sunlight through photosynthesis, and as an alternative we might consider growing algae as a source of oil. Many strains of algae are more photosynthetically efficient than many land-based plants, and studies have demonstrated that up to 100 times the quantity of oil can be produced per hectare from algae, e.g. chlorella, than from say rape-seed, i.e. closer to 100 tonnes (700 barrels) as compared with around one tonne (7 barrels).
Algae offer the further advantage that they do not require quality arable land and they can be grown on saline water (e.g. in deserts) on on wastewaters. Thus there is the potential to use them to clean wastewaters in the process as an additional benefit. Algae can be cultivated in open-pond systems or in tubular bioreactors, the latter for example may be similar to the Agri-Drip tubing used for irrigation, and so can be laid out across any land, including land-fill sites or any kind of marginal land, such as is often classified as brown field.
It is the yields of algae that are their most attractive feature and it is estimated that 3,200 km^2 of land area could provide enough algal-oil to fuel the United Kingdom, replacing the equivalent of some 40 million tonnes of crude oil. For comparison, the area of Cornwall is 3,500 km^2, or just 1.4% of the U.K. mainland area. In the latter calculation, it is assumed that transport is converted to more efficient diesel engines which derive 50% more in terms of tank to wheels energy than petrol engines do. It has also been proposed that algae production might be used as part of a carbon-capture strategy, since algae grow more efficiently in an atmosphere containing around 15% CO2, and this is readily provided from the effluent of coal and gas fired power stations.
Thus algae may also contribute to meeting our carbon emissions targets. The non-lipid (oil) component of algae, i.e. carbohydrate and protein, may find additional use as high-value products for food production, or the carbohydrate can be digested and fermented as a source of bioethanol and other fuels. Thermal cracking of the algae per se can form a source of biochar, for carbon sequestration and soil-improvement, and syn-gas which may be burned directly as a fuel or converted via Fischer-Tropsch catalysis to synthetic low-emission diesel fuels.