Can Solar Fuels Avert an Imminent Petroleum Fuel Crisis?

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Keywords: solar fuels, energy, solar energy, peak oil, hydrogen, Fischer Tropsch, Transition Town, Iocalisation, globalisation

The problem of energy

The world population of 7 billion humans uses energy at a mean rate of 16 TW (16 terawatts). This amounts to an annual 504 EJ (504 exajoules = 5.04 x [10.sup.20] J), and is provided from fossil fuels (oil, gas and coal), plus nuclear and hydro (hydroelectric) power along with all other forms of renewable energy (1). The breakdown of these various contributions is given in Table I. Around one third of the total energy used by humans on Earth is provided by crude oil, and close to one quarter each by natural gas and coal, although the amount of coal being burned is rising, particularly in China. Nuclear and hydro-power each contribute around 6% of the total energy mix, while the combination of renewable energy from all sources, wind, wave, geothermal, wood, solar etc. amounts to just a little above 1%. In 2004, humanity used 471 EJ of energy (2), and while the relative proportion of each contributing energy source has remained modestly constant, it is clear that at a rise to 504 EJ in 2010, the demand placed upon these energy sources is rising relentlessly. This follows not merely a growing human population per se, but an increasingly affluent industrialised consumer society. We need to consider two aspects: firstly, that the C[O.sub.2] produced by burning fossil fuels is believed to contribute to global-warming and that this may lead to unwelcome or even catastrophic changes to the global climate (3).

Secondly, and more immediately, the fossil fuels and uranium too (for nuclear power) are in finite supply, and there is compelling evidence that each source will meet its own production peak within the next two decades. Most vulnerable appears to be crude oil (petroleum), world supplies of which are predicted to peak ("peak oil") probably during the next 5 years (4). Irrespective of the exact timing of peak oil, there are salient predictions (see below) that a gap will emerge in the supply of oil against demand for it, from the end of this year (2012), rising to a shortfall of 10 million barrels a day by 2015 (5). This situation has been termed "gap oil" (6), and can only be exacerbated by peak oil, when supply must draw-down against rising demand, thus enlarging the gap from both sides. Thus, in order to curb carbon-emissions and to extend limited resources, alternative and ideally renewable sources of energy are needed.

Liquid fuels and transportation

A simple comparison of the energy content delivered from different energy sources, as is made in Table 1, is somewhat misleading, since it seems to imply that if the production of one of them begins to fail, it can be readily substituted by another. The issue of transportation is a singular example where this is not the case, since practically all the vehicles used in the world--cars, lorries, buses, trains, ships and planes--have been engineered to run on liquid fuels that are refined from crude oil. Therefore the likely consequences of "peak oil", with dwindling supplies and escalating costs of liquid fuels, are very serious. Ward's, the US based publisher, estimated that as of 2010 there were 1.015 billion motor vehicles in use in the world (7). This figure represents the number of cars, light, medium and heavy duty trucks, and buses, but does not include off-road vehicles or heavy construction equipment. Between 1950 and 1970, the world vehicle population doubled roughly every 10 years, passing the 250 million mark in 1970, and exceeding 500 million in 1986. It has been estimated that the world's road transportation fleet will reach 2 billion by 2020, of which at least 50% will be cars. China's and India's automobile fleets are expected to grow at an annual rate of around 7 or 8%, while in the United States, it will be under 1% a year, and around 1 to 2% in Western Europe, but this depends tacitly on finding an expanding liquid fuel supply, and it is this which is at issue. Indeed, the International Energy Agency (lEA) has issued a report (8) to the effect that a shortfall in oil production of 64 million barrels a day (mbd) can be expected by 2030, which represents a loss of 62% of the world supply of conventional crude oil currently 84 mbd, assuming a demand by 2030 of 96 mbd, a figure significantly downgraded from prior estimates by the lEA of 120-130 mbd. At a mean decline rate of 2.9 mbd/year (-3.4%/year) this value accords closely with the prediction in a recent U.S. Army report (5) that there will be a deficiency of 10 mbd by 2015, following a loss of any spare capacity for crude oil against demand for it by the end of this year (2012).

While it is possible to run cars and other road vehicles on electricity, provided either from batteries or hydrogen/fuel cells, actually converting their number substantially to these alternative energy carriers (neither electrons nor hydrogen being primary fuels, i.e. they must be created from primary sources) would be such a considerable undertaking that the scheme is not feasible. Vehicles can be adapted to run on gas but a peak in natural gas production is expected within 20 years, following oil, and converting them all would take many decades, so this is no solution either. It is, therefore, a new source of liquid fuels that must be sought, since they would be far more compatible with a transportation fleet and distribution infrastructure designed for liquid petroleum fuels. These, ideally, should be "carbon-neutral" in order to reduce carbon emissions, a condition which certainly does not apply (4) to coal-to-liquids processing (with probably twice the carbon emissions overall that are incurred in the production and burning of diesel or petrol derived from petroleum), nor gas-to-liquids either, unless the C[O.sub.2] is captured and stored in some way that prevents it escaping into the atmosphere. Biofuels are attended by a number of vexed issues (6): the competition for arable land between growing crops for fuel or crops for food, the increased amount of freshwater required to grow fuel crops above that already needed for agriculture, the clearing of rainforest to produce high-energy fuel-crops, e.g. palm-oil, and the fundamental EROEI which for bioethanol may only marginally exceed the overall energy costs of its production, or in some cases not quite break-even. Clearly, some other strategy is necessary.

[FIGURE 1 OMITTED]

Figure I summarises the quantity and fate of solar radiation striking the top of the earth's atmosphere (2). We see that 52 PW ([10.sup.15] W) is reflected back into space (i.e. 30% of the total). Thus, in outer space, there is more solar energy available to be collected, which has prompted potential schemes to launch photovoltaic arrays into space on satellites (2), with which to capture the sun's energy and then beam it back to earth in the form of microwaves for terrestrial applications. At the top of the atmosphere, with the sun directly overhead, the radiation flux provides around 1.4 kW [m.sup.-2] of energy, the "solar constant" (2). Since the total amount of energy (1) (oil, gas, …