These energy sources are all carbon dioxide neutral. Growing vegetation absorbs carbon dioxide from the atmosphere and burning the biomass for energy returns it to the atmosphere. Looked at in this light, the various "forestry carbon dioxide sequestration schemes" cannot affect the growing amount of that gas in the atmosphere in the long term. Sure, the forest will absorb carbon dioxide as it grows and matures. However, the ultimate fate of every tree in that forest is one of the following:
There are no other possibilities. All these outcomes put the carbon dioxide back in the atmosphere. Result: its another zero-sum game as far as atmospheric carbon dioxide is concerned and anybody who tells you different is either fraudulent, a liar or both.
So, the possibilities are horses, biofuel, electricity generation from wood and, possibly, biogenic hydrogen as untapped biological energy sources.
The United Kingdom probably cannot feed its current population without importing food, so horse-drawn transport is impossible here without a return to 19th century population and city sizes. The USA does not have the farmland to feed its required horses and has not had this capability since the late 1960s. Thats even assuming that farms are not needed to feed people. Now knock off all the areas that require major pumped irrigation and other energy inputs for farming or for people to live there: much of the Californian farmland, virtually everybody living in Arizona, Nevada and the Columbia Valley. None of these are killer problems but they can still cause major upheavals, like mass resettlement of the Las Vegas blue-rinses and/or another Dust Bowl.
Biofuel is a general term for the biological production of liquid fuels which will be burnt in conventional engines or heating systems.
Growing plants absorb carbon dioxide, which is returned to the atmosphere when the biofuel is burnt, so biofuel use is carbon neutral. Biofuel production is also carbon dioxide neutral provided that biofuel and the associated waste biomass supplies all the required energy for farm machinery and agrochemical production, crop cultivation, harvesting and converting biomass into fuel.
Overall conversion efficiency is important because it governs the amount of land needed to produce a given amount of fuel. This is almost certainly the limiting factor for biofuel production on land.
An alternative is offered by algae. This is pond scum, rich in lipids or starches, so it provides potential sources of biodiesel or bioethanol respectively. Efficiency should be comparable to systems that convert entire plants to fuel. Additionally, algae can be grown on brackish ponds, in seawater or in enclosed systems installed on deserts. This potentially sidesteps production limits due to available farmland and fresh water, which is either in the sea or recycled within an enclosed system. However, its far from clear that algal systems can avoid either diverting fertiliser from conventional farming or boosting fertiliser production to unsustainable levels. Another drawback is that wild microbes and algae tend to contaminate open ponds and consume the oil producing algae and/or reduce the yield. Solutions are:
Bio Fuel Systems (BFS), a Spanish startup in Alicante, is using cyanobacteria and sunlight to convert carbon dioxide from a cement works into crude oil via a patented system using heat and pressure. It claims this process is carbon negative because, after oil extraction, a lot of the waste is in the form of a carbonate sludge that can be buried or used to make concrete. The carbon negative claim arises because the inputs needed to make a barrel of oil (159 litres) are 2000 kg of flue-gas carbon dioxide and 226 kg of fuel to provide the heat (producing 700 kg of carbon dioxide in the process). Burning the resulting oil produces another 450 kg of carbon dioxide. This leaves 850 kg of carbon-dioxide permanently trapped in the solid residue: hence the carbon negative claim. In theory, if this system was scaled up to produce 90 million barrels of oil a day (the world's crude oil consumption in 2012), thus replacing all oil extraction, it would occupy just 25% of the Libyan desert, or 1% of the global pasture land. However, it produces rather expensive oil: the estimated cost is at least $US5 (£3.10) a litre. This is mostly due to the cost of the polycarbonate reactors and the electricity needed to stir them. These figures are extrapolated from a pilot plant that produces about 2.5 barrels of oil a day. - New Scientist, 8 December 2012, page 36.
Algae Systems, an American startup, would bypass this by using 25 metre plastic bags floating just offshore in the ocean where wave action provides the necessary stirring. A pilot plant should be operational in 2013. It should be carbon negative because its algal processing methods will be similar to thise used by BFS. However, they point out that a major limit to using algae is the expense and limited sources of available nitrogen and phosphorus fertilisers. Calculations show that the entire US supply of these fertilisers would enable less than 10% of the US liquid fuel demand. Available carbon dioxide from American smokestacks set a similar limit. - New Scientist, 8 December 2012, page 36.
Oil producing crops, such as rape seed or soybeans, are grown using normal arable farming methods. The oil is extracted and converted into biodiesel which can be used to power diesel engines or heating systems.
This is produced by growing sugar-producing crops, such as sugar-cane, sugar-beet or maize, extracting the sugar and fermenting it into ethanol which is used to fuel internal combustion engines.
Around 2005 Iogen demonstrated a process for producing fuel-grade ethanol from arable crop waste biomaterials. The improvements are, firstly, treating the raw materials to increase the surface area of the plant fibres before using enzymatic hydrolysis to convert much of the cellulose to glucose. This is separated and fermented while the solids are burnt to power the process.
Three new developments were announced in the May 2010 New Scientist:
Cool Planet Energy Systems think pyrolytic decomposition of waste plant biomass, i.e. straw, corn harvest waste and forestry waste wood, offers a cheaper and more sustainable solution. In their system, pyrolysis converts waste plant biomass into hydrocarbon fuel and biochar. The hydrocarbons can be used as fuel with little further processing while the biochar can be used as a soil improver or buried permanently. The last choice makes the process carbon negative because up to half the carbon it processes gets buried as biochar: the rest would be returned to the atmosphere. Ideally the system would be deployed as several hundred small units. This would minimise transport costs by collecting waste biomass within a radius of 50km or so and supplying fuel over a similar area. They estimate that, if biochar contains 33% of the carbon input to the process, they can produce fuel for about $US0.40 (£0.25) a litre. - New Scientist, 8 December 2012, page 37.
A study by David Tilman and colleagues at the University of Minnesota in St. Paul (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0604600103) says probably not. They calculate the result of America turning all its maize and soybean production into biofuel. They further assume that some of this biofuel will be used to manufacture the farm machinery, fertiliser and pesticides needed to grow the crops and to run the machinery used to cultivate and harvest it. The remaining biofuel would meet less than 5% of America's current liquid fuel requirement. As of 2010 this approach to ethanol production is pretty much a dead duck thanks to its impact on food production.
Its clear that ethanol production isn't particularly efficient:
The energy inputs, in the form of fertilisers, cultivation and harvesting are considerable. Studies have put these inputs at anything from 75% to over 100% of the energy content of the resulting ethanol.
- Congressional Record page H211, Congressman Roscoe Bartlett: "THE PEAKING OF WORLD OIL", House of Representatives, February 8, 2006, http://www.peakoil.net/Publications/PeakOilSpclOrder%2315TextCharts020806Low.pdf.
Its not clear what proportion of this added energy is derived from burning the biowaste that's left after the sugar has been extracted and how much is obtained from fossil fuel.
In terms of an energy balance, or on a "comprehensive life cycle basis," ethanol contains about twice the amount of energy required to produce it. This includes the energy used to produce the various inputs of production such as fertilizer and pesticides, the fuel costs associated with grain production, and the cost of transporting grain from the farm to the ethanol plant, and from the ethanol plant to the retailer.
- Agriculture Canada
A study in 2006, by Alex Farrell from the University of California at Berkley, calculated that ethanol made from maize releases only 25% more energy than is required to grow the crop once the large amount of fertiliser it requires is taken into account. This is because making fertiliser is very energy-intensive. In addition, runoff from the fields pollutes streams and creates dead zones as the resulting blooms mop up dissolved oxygen. Growing corn to produce ethanol is not a good idea.
Producing ethanol by fermenting cellulose looked initially as if it could be worthwhile. The base crop is wild grasses that are native to the growing area. Unlike maize, the whole plant is harvested and fermented, vastly improving the efficiency of the process. The yields from farm-scale experiments run by Robert Mitchell, who is based at the University of Nebraska, combined with ethanol production models show that this system should produce up to 93% more energy than is used by growing and processing the grass if fertiliser is used. Jason Hill, University of Minnesota, did a similar experiment but without using fertiliser and got energy yields in the range of 60% to 85%, still much better than maize. When he tried grass mixtures in place of monocultures the energy yield improved by 150% to 238%. Added benefits of this approach are the avoidance of a monoculture, minimal harm to wildlife (the annual harvest has a similar effect to the annual grass fires that affect prairies) and minimal pollution. However, two reports in Science (February, 2008 issue) say that this, still hypothetical, process will always have negative consequences regardless of the overall energy production efficiency. If cropland is used it will adversely affect food supplies regardless of its implementation. If uncultivated land is used, the effect of carbon release from this area combined with the loss of future biological carbon sequestration will result in a nett increase in carbon emissions. More details are available in US scientists puncture the ethanol biofuel bubble, an analysis published in The Register. It seems that cellulose fermentation is probably just another bad idea.
Some studies have shown that British arable farming has more energy input in the form of fertilizers, cultivation and harvesting than it gets from the sun, so producing biofuel in the UK may turn out to be an energy sink rather than a resource.
No efficiency estimates have been released for algal systems. In theory they could be better than converting land plants - providing that preparing the biomass for processing needs less energy than harvesting crops. The jury is still out on algae.
George Monbiot pointed out (Feeding cars, not people, 23/11/2004) that using rapeseed oil to run UK's road transport on biodiesel requires 25.9m hectares of farm land. Unfortunately, there is only 5.7m hectares of farmland in the UK. He thinks production will inevitably switch to Indonesian and Malaysian palm oil, causing widespread deforestation and starvation because British drivers will pay more for petrol than local people can afford to pay for food. An update of this survey ( An agricultural crime against humanity, 06/11/2007) pointed out that governments are using biofuel as a way of avoiding hard decisions and ignoring the harm it does.
In fact the situation is far worse than George Monbiot foresaw in his worst nightmare. Indonesian speculators are clear felling rain forest as fast as they can in order to plant oil palms. The carbon this is releasing from the underlying peat swamps is currently adding an extra 10% to atmospheric emissions from fossil fuel - New Scientist, 1 December 2007, page 50.
Other calculations (New Scientist, 15 December 2007, page 6) show that large scale biofuel production is unsustainable because there simply isn't enough agricultural land or water to support it. Here are two estimates from that article:
This view is supported by more recent American statistics for ethanol production. In 2010 ethanol production from corn provided 8% of the fuel used by transport, consuming nearly 40% of the American corn crop in the process (New Scientist, 8 December 2012, page 35). The bottom line is that ethanol from this source can't provide more than 20% of the US demand for transport fuel if we assume that all the land thats suitable for growing corn already does so. Bear in mind that the damage this would do to food production is ignored and that it has no impact whatsoever on fossil fuels consumed by electricity production, used in industry and for heating.
Even though the assumptions, and hence estimates of land and water requirements differ quite widely I think they show that a biofuelled world is simply a nonstarter once the impact of the world's still expanding population is taken into account. Water seems to be the limiting factor rather than agricultural land: de Fraiture's figures show that it requires 2000 litres of water to produce each litre of biofuel.
An analysis (New Scientist, 16 August 2008, page 34) of the feasibility of using biofuel for aviation supports this view. At the time the article was written aviation consumed around 238 million tonnes of jet fuel a year. The analysis looked at the land required to produce this amount of fuel without impacting food production:
Given the vast amount of land needed for any of these approaches its difficult to see how this can avoid an agricultural impact, although algal production could at least take place on desert bordering the sea. However, this analysis has ignored any growth in air traffic and it skips over possible fertiliser requirements. These may be needed to replace the plant nutrients that normal cropping would plough back into the land as straw and stalks.
Not so's you'd notice. A study by Joseph Fargione and colleagues at the Nature Conservancy, Minneapolis, MN looked at the carbon debt incurred by clearing land for biodiesel production. The carbon debt from clearing Brazillian rain forest to make fuel from soya beans would take 300 years to recoup by reduced carbon emissions. Clearing Indonesian rain forest to produce fuel from palm oil would take 400 years to show a net carbon reduction. - reported in New Scientist, 16 Feb 2008, page 19.
Algal biofuel production should be better in this respect, simply because there is no land clearance involved. There's only the carbon cost of building a tank system off the coast or an enclosed system in a desert and of the associated fuel production facility.
If you grow biofuel you'll have to find the land somewhere. You have just three choices: do without the previous produce, displace previous production to new land or grow the biofuel on new land. Doing without is a non-starter due to its impact on food availability and price. Both the other options require new land to be brought into production by clearing forests, draining swamps, etc. This will certainly damage wildlife and may have additional unforseen ecological effects. It will certainly boost carbon emissions: see above.
Plantations of fast-growing trees, such as willows, are coppiced and burnt in power plants near or in the plantations. Solid plant waste from ethanol or biodiesel plantations is also suitable fuel for electricity generation. The ash should probably be returned to the plantations as fertiliser.
Judging by the rate that North American forests are vanishing there is no spare capacity there for energy generation, renewable or not. From what I heard and saw back in 1991 on the Olympic Peninsular near Seattle in Washington State, the forestry companies are making no attempt to replant what they cut. Rape and pillage seems to be the name of that game. Besides, even if there was replanting, the great northern forests in the USA and Canada are very slow growing. I have been quoted 150 years as the expected regeneration time.
United Kingdom forests are more or less static and trees are pretty slow growing here too, so electricity generation from wood is unlikely to have much impact on the demand for energy.
New Zealand has extensive exotic forests for the paper pulp export trade. The Mediterranean pine, pinus radiata, grows very fast on the volcanic plateau near Rotorua. In the early 1970s, when forest planting had been completed and the trees had matured enough for harvesting, the Kaingaroa Forest provided a sustainable harvest of 8000 tonnes of wood a day. If the export trade for the resulting newsprint is reduced by the oil crisis, the output of this forest could fairly easily be diverted to sustainable electricity generation.
Some interesting work is being done in Holland on direct electricity production by growing plants. This relies on the fact that a significant proportion of the sugars and proteins synthesised by green plants leaks out into the soil from their roots where they are metabolised by bacteria, which in turn release electrons. The electricity is harvested by wiring the soil the plants are growing in so it forms a microbial fuel cell. Given suitable development, the researchers expect that an output of 3.2 W/m2 is possible under laboratory conditions while an outdoor system, based on marsh grasses growing in waterlogged soil, might realistically yield 1.6 W/m2. It has a big advantage over solar power because it will continue to provide electricity after dark.
Comparisons with other renewables are interesting. For the same annual electricity output, this type of PMFS requires five times the land area that wind farms or PV solar generators would occupy. However, to get the same electricity output from growing biofuel and using it to generate electricity would need 35 times the land area required by a PMFS system.
Research, reported in New Scientist (25 Feb 2006, p37), has demonstrated direct hydrogen production by the green alga Chlamydomonas reinhardtii. The idea is to grow the algae in transparent, water-filled polythene tubes. Supply the algae with nutrients and sunlight and they produce hydrogen and oxygen. Then you remove the gas mixture from the array of tubes, separate and dry the hydrogen and ship it off to be used as fuel.
Water use should be minimal because it would be recycled. The impact on arable land should also be tiny because because this type of hydrogen farm can be installed in empty desert.
Its been calculated that enough hydrogen could be produced to replace all the petrol used in the USA by 25,000 sq.km of hydrogen farms given a solar conversion efficiency of 10%. To put this in perspective, this is a tenth of the area that American farmers currently use to grow soya beans. That's 0.3% of the land area the USA or 8.4% of Arizona. If it works, it could provide an income source to developing countries by allowing the Middle East and North Africa to supply Eurasia with hydrogen.
The snag is that the current conversion efficiency is under 0.1%, so some serious genetic engineering is needed before this becomes a viable energy source.
It has been estimated that the human ecological impact already exceeds the sustainable capacity of the globe. We are approaching and even exceeding the sustainable fresh water supply in many parts of the world. Both these factors are likely to severely limit biological energy sources.
With the possible exception of biogenic hydrogen, scratch the biological options without a severe dose of human depopulation.