Hydrogen's role in an uncertain energy future
Introduction
Two questions are of crucial importance in discussing the future of energy in the 21st century: how much energy will we consume annually, and what sources of energy will we use? The answers to these questions are by no means certain. In 2006, total global primary energy consumption was about 493 EJ [1], [2]. (EJ = exajoule = 1018 J.) Following the International Energy Agency (IEA) convention, energy generated from hydroelectricity and other renewable primary electricity sources is converted to primary energy on a one-to-one basis [2]. Again following IEA practice, primary energy in this paper includes non-commercial fuel wood. Table 1 shows total global primary energy from 1970 to 2006, illustrating both the steady growth in energy use over this period, and the recent rising share of fossil fuels. For electricity production, their share has been rising for several decades [1], [3].
How much energy will we need in the future? A more equitable future world would require reductions in the present large differences in per capita primary energy consumption. Among the high-income countries, in 2004 Italy had the lowest per capita energy use at 132.7 GJ [2]. (GJ = gigajoule = 109 J.) The UN median estimate for 2050 global population is 9191 million [4]; if all used energy at this rate, global primary energy use would be 1220 EJ. This value is similar to the maximum value of 1173 EJ in 2050 in the various scenarios in Riahi et al. [5]. Various other researchers present futures with roughly 1000 EJ or more primary energy for 2050 [6], [7], [8], [9], [10], [11], with some envisaging even higher values later in the century. While these researchers do not necessarily view their figures as projections, they clearly regard 1000 EJ by 2050 as at least possible.
Because of the various serious constraints facing high energy use, low energy futures must also be considered. The energy conservation needed for these can occur in two ways; from increasing technical energy efficiency of power generation and energy-using equipment, or from less use of energy-consuming equipment. One recent study [12] estimated that for their ‘2 °C’ scenario, global primary energy consumption in 2050 could be held to 422 EJ, lower than today's value. Amory Lovins is a strong advocate of the potential for technical energy efficiency, arguing that energy efficiency in a variety of applications can be increased by a factor of 10–100, and that an overall four-fold reduction in energy use is possible [13]. An annual primary energy use of 300 EJ could thus be considered as illustrative of a low energy future.
We focus here on primary energy because our chief concern is with climate change impacts and fossil fuel depletion. But what relation does primary energy have to the energy available for consumption? In 1973, the ratio of global primary energy to total final consumption (secondary energy) was 1.34, but had risen to 1.47 by 2004 [2]; primary energy rose faster than final energy use. If all energy was derived from coal fired power stations this ratio would approach 3.0; but if derived solely from hydroelecticity, the ratio falls to 1.0. Greater use of renewable energy does not necessarily guarantee a better secondary energy return, as it is likely that increased renewable energy would see greater need for energy storage and conversion. Each additional process will act to reduce the energy available for consumption. Increased fossil fuel use would ultimately need greater use of coal and non-conventional oil sources, again raising the primary/secondary ratio.
Within the range of possible energy futures, what role will hydrogen play? In this paper we limit primary energy consumption in 2050 to two cases: a high energy energy future (He = 1000 EJ) and a low energy future (Le = 300 EJ). Other values for future energy are of course possible, but they will probably fall within these limits. We then explore hydrogen's role by considering the energy supply mix for each energy future. To do this we define two energy supply mixes, namely a high (Hf) and a low (Lf) fossil fuel share, where ‘high’ means 80% fossil fuels (roughly their present share in global energy supply—Table 1) and ‘low’, 20% fossil fuels. We impose one final limit on future energy; for simplicity we assume the primary/secondary energy ratio remains constant, while acknowledging that it may alter with primary energy level and mix.
We argue that neither high energy case is probable, because of resource limitations, and rising energy, environmental and money costs per unit of delivered energy as annual energy demand rises far beyond present levels. The low energy/low fossil case is most likely, followed by the low energy/high fossil case, although both require large cuts in energy use, and most probably, lifestyle changes in high energy use countries. Hydrogen production would be best favoured in the low fossil fuel options, with production both greater, and implemented earlier, in the high energy case. It is thus least likely in the low energy/high fossil fuel case.
Section snippets
Challenges to future sustainable energy provision
As shown in Table 1, the present global energy system is dominated by fossil fuels, and the official forecasts discussed above see little change in this pattern before 2030. An important question is for how long these business-as-usual projections can continue without running into constraints in the form of limited reserves of fossil fuels, or severe environmental problems from their combustion, including not only global climate change from CO2 and methane emissions, but also air pollution
High fossil fuel supply mix
The reports discussed above all assume high energy consumption in the future, with 80% or more coming from fossil fuels in their reference scenarios in 2030, and for the WETO report, still over 70% in 2050 [2], [3], [20]. Since fossil fuel use in recent years has grown strongly, forming a rising share of primary energy, a high energy future based largely on fossil fuels (He–Hf case) deserves serious consideration. This option also benefits from the massive past investment in the energy supply
Low fossil fuel energy supply mix
The He–Lf future could be based on renewable energy (RE) sources and/or nuclear energy. Several researchers think that RE sources could readily supply the 800 EJ required [38], [39]. Fossil fuels would supply 200 EJ, only half today's use, but still unsustainable under ASPO/EWG assumptions (Fig. 1). If CO2 emissions were held to 15% of year 2000 values, 10.7 Gt of CO2 would need to be captured and sequestered.
We doubt that non-carbon sources could supply 800 EJ by 2050, or even later. RE sources
Implications for the future of hydrogen
Given the various possibilities for future energy, what do the official reports mentioned in Section 1 have to say about hydrogen production? Only the WETO study provides detailed projections. For their hydrogen (H2) case—the most optimistic for H2—the report projects that production globally will rise from about 4.6 EJ in 2030 to 43.8 EJ in 2050, compared with total primary energy production in 2050 of 850 EJ [20]. The reference case has H2 production in 2030/2050 of only 1.3/14.7 EJ. The hydrogen
Conclusions
Energy use in this century and beyond faces deep uncertainties. There are widely conflicting opinions on the size of ultimately recoverable fossil fuel reserves, and the extent to which unconventional resources can be tapped. If, as expected in most forecasts, fossil fuel use continues to grow, the sequestration of vast amounts of CO2 would be needed if we are to limit global warming. Large emitters such as power plants could probably only capture around a third of the amount needed, requiring
Acknowledgements
Patrick Moriarty would like to acknowledge the financial support of the Australasian Centre for the Governance and Management of Urban Transport (GAMUT) in the preparation of this paper.
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