How sustainable would the global transportation sector be if run entirely on biofuels?
There were about 806 million cars and light trucks on the road in 2007 around the world. They burn over a trillion litres of gasoline and diesel yearly, or about 1200 litres/vehicle/year on average. The number of vehicles is increasing rapidly, especially in China and India. The annual production rate is about 70 million vehicles. The fuel consumption is not uniformly distributed among the world’s vehicles. With 5% of the world’s population, the United States is home to about 31% of the vehicles (250 million), and they consume about 44% of the world’s gasoline: 515 gigalitres or about 2060 litres/vehicle/year. Under favourable circumstances, countries with a low population density and large supply of agricultural land could become self-sufficient in biofuels, if only to meet the needs of their transportation sector, perhaps even produce surpluses for export. Let’s look at some examples.
Can Brazil be self-sufficient? Sugar cane, palms, and coconuts apparently are the best fuel crops. They grow in lush tropical countries which are mostly crowded, and few such countries have the flexibility to divert much arable land from food crops. Brazil is an exception. In Amazonia there is still plenty of rain forest that can be torched to clear space for growing fuel. On the other hand, it bears remembering that the Amazon rain forest happens to be the world’s greatest remaining natural CO2 sink. Maybe the Brazilians shouldn’t rush to destroy it. However, if they chose to do so, they could find the 25 million or so hectares they would need to cultivate sugar cane for bioalcohol. Just bear in mind that this is a lot of land – it’s the size of the entire land mass of the United Kingdom.
Let’s consider the US case. The low yield of temperate zone biodiesel crops negatively influence biodiesel’s prospects as a replacement for fossil fuels in America. If one tried to substitute alcohol for gasoline and diesel, then the same volume of driving would require 733 gigalitres/year of alcohol because of the lower energy content of ethanol vs gasoline or diesel. At an alcohol yield of 4000 litres/hectare/year from corn, this would mean 180 million hectares under cultivation for corn fuel not even considering the energy balance overhead. Suppose instead the US planted soy for biodiesel. With a median yield of 740 litres/hectare/year but energy density comparable to gasoline, this would require 700 million hectares not including the energy balance overhead. Given that Americans need to eat and the US has a total of only 190 million hectares of arable land, it is clear that neither corn ethanol nor soy biodiesel are viable substitutes for petroleum in the US transportation sector, unless Americans very drastically reduce their driving.
Will Green Slime Rescue America?
Algae are familiar to most people. They come in many sizes, but most species are microscopic single-celled organisms, and we know them best as the green slime that chokes many bodies of water. They do have a bright side: Researchers consider oxygenic photosynthesizing algae a promising alternative to other biodiesel crops. Algae produce lipids. If algae species are selected and cultivated for their oil yield, they have the potential to make biodiesel much more efficiently than other crops.The US Department of Energy estimates that algae may be capable of a biodiesel yield 30 times greater than soybeans. Algae can be used to produce other biofuels including alcohols and methane, and via hydrocracking also synthetic hydrocarbon fuels. Other DOE estimates reported elsewhere suggest a potential fuel yield as high as 200 times greater than soybeans, or 160 kilolitres/hectare/year, mostly thanks to a short harvesting cycle. If this extremely optimistic scenario is correct, then replacing all the petroleum fuel in the United States would require only about 4 million hectares, which is about 1.3 Belgiums. At a more realistic yield 30 times better than soybeans, the area required for algae cultivation would be 27 million hectares (270,000 sq.km), or roughly 1.1 United Kingdoms. Of course, none of this considers the energy balance overhead. If the energy balance is 2, then we need to double these land masses to obtain a realistic number. Oh yes, and let’s not forget that growing algae requires a lot of water!
As a practical matter, it turns out to be very difficult to grow algae for fuel. If grown in open ponds, the algae colonies are susceptible to being invaded and colonized by wild rival species. The wild algae tend to be hardier and crowd out the cultivated algae. There are so many algal spores in the air, that it is guaranteed they will find the growers’ precious ponds, even in the middle of a desert. The alternative is growing the desirable algae in enclosed systems where they can receive sunlight and be fed sterile CO2. These systems are sometimes called “photobioreactors”. Naturally this is very costly, particularly if you need a factory the size of the United Kingdom.
A Tentative Conclusion
Simply put, when fossil fuels run out, American drivers will drive much, much less than they do today. Even before that happens, they will be encouraged to cut back by the sky-high prices of all fuels. Eventually most vehicles may be hybrids, with pricey liquid fuel reserved for long trips where a cruising range of 100′s of kilometres is indispensable. Commuting will be done by public transit for those wealthy enough to live in an urban area. The suburban poor will have to use electric power in commuter cars to make it in to work.
However, let’s not give up just yet, because we have one more class of fuels to examine: re-formed fuels & biofuels, also known as synfuels.