by Dave Elliott
There is inevitably some energy ‘embedded’ in energy generation systems, and it is useful to compare the energy needed to build and run plants relative to the useful energy out, but estimating ‘Energy Returns of Energy Invested’ (EROEIs) can be tricky. The ratios can range up to 200:1 or more, and down to single figures- very worryingly since then it is hardly worth running the plant.
It’s fairly easy to estimate the energy used in the fuel they consume (if any) and the energy output over the plants lifetime, based on its specifications, but the operational reality may be different and the final results will be plant specific and so hard to average out. It’s even harder to estimate the primary energy used in construction and in the materials used in its construction, not least since that involves assumptions about what type of primary energy source was used and the energy conversion efficiency. It’s similar to the problems facing life cycle analysis. See the useful discussion at www.theoildrum.com/node/3810.
As a result, estimates can vary dramatically. For example it’s usually claimed that the EROEI ratio for hydro is very high 100-200:1, or higher, given the fact that, as well as having no fuel requirement, once built, hydro dams last for centuries. Gagnon’s study in Energy Policy 36 (2008) 3317-3322 quotes up to 267:1 for run of the river plants! But some studies have come up with much lower ratios for hydro, down to 11:1. See www.theoildrum.com/node/3910.
Similarly it’s usually claimed that EROEI ratios for nuclear are low, 14-16:1 (Gagnon) or even lower, depending on the ore grade of uranium used, right down to 5:1 or less, for low grade ores, which need a lot of energy to mine separate and process (see Harvey’s 2010 ‘Carbon Free Energy’ Earthscan book). But some studies have quoted nearly 100:1! See http://nuclearinfo.net/Nuclearpower/TheBenefitsOfNuclearPower.
These divergencies seem odd, given that these technologies have been in use for some time, but perhaps the outlier results are due to looking at non-typical examples, or make unconventional assumptions. The 100:1 nuclear study evidently forgot the energy needed to enrich the uranium fuel rods. More understandably, a study of a geothermal plant in Iceland found that the EROEI could range from 9.5:1 to 33:1 depending on the operating procedures used and crucially on whether the waste heat was used: www.sciencedirect.com/science/article/pii/S0360544213000121.
The ranges quoted by Harvey for wind (20-80:1) solar PV (10-25:1) and CSP (8-40:1) are also wide, which is not surprising given that they are newer technologies, operating in a range of contexts, and for even newer renewables the estimates are even more uncertain, although its been suggested that the Pelamis wave energy device could have an EROI of nearly 15:1. With most renewables, the EROIs will of course be site specific, depending on the solar, wind etc regime, so you would expect a range. A 2006 meta study of wind, using data up to the year 2000, cited projects with EROEIs up to 71.4:1 (that one was in Denmark) although many were around 20-30:1: www.theoildrum.com/node/1863. But EOEIs are likely to improve with time, as these new technologies develop, depending on type. For example, even in 2006 NREL suggested that thin film PV might be at up to 30:1 and it should be better than that now. See www.nrel.gov/pv/thin_film/docs/lce2006.pdf.
You might expect the figures for fossil fuel to be more solid. They are, and they are also very low, having fallen dramatically as the easy to get at resources were exhausted. Once, the direct extraction energy debt may have been low compared to the energy extracted (50-100:1), but it’s now much lower and of course you have to add in processing, transport and final conversion energy debts. Some technologies have improved, but the overall EROEI trend for fossil technologies, like nuclear technologies, seems to be downward. One recent report warned that the overall global EROEI is now down from about 40 in 1990 to 17 in 2010, may decline to 11 by 2020. www.tullettprebon.com/strategyinsights/index.aspx . Indeed it could be even worse. In addition to quoting (future) nuclear EROEI’s down to as low as 2:1 as uranium reserves dwindle, Harvey quotes final current EROEI’s for coal plants at 5-6.7: 1 and gas plants at 2.2:1, while Gagnon puts CCGT at 2.5-5:1 and coal plants at 2.5-5.3:1, either way making most renewables clearly better, with the trend being to improvement.
Inevitably the calculations for biomass are more complex, since there are so many different possible types of feedstock, all with different calorific values and processing chains, and also may different forms of energy conversion, with different efficiencies. For some types of so-called ‘first generation’ biofuels, the EROEI ratio can be less than 1. Gagnon cites the EROEI ratio for power generation using biomass from plantations as 3-5:1, depending on the transportation distance. Newer types of more efficient bioenergy conversion and second generation biofuels may do better, and so should using already existing biomass: Gagnon cites the EROEI for biomass wastes as 27:1.
EROEIs can also be calculated for energy saving technologies, although clearly they are even more varied in type and deployment context. The UK Association for Environment Conscious Building has suggested that EROEI’s for retrofit insulation might be up to 100. http://www.aecb.net/publications/less-is-more-energy-after-oil/.
There have been plenty of dire warnings about potential industrial and economic collapse due to falling EROEIs, some of them very pessimistic. See for example ‘Energy and the Wealth of Nations’, Charles A. S. Hall and Kent A. Klitgaard, Springer, 2011, reviewed at http://www.pelicanweb.org/solisustv08n03page1.html#eroibookreview. But the review above suggests that there is hope, if we switch to, and upgrade, renewables.
Longer term however some say even renewables may face problems. For example, in his 2007 World Energy and Population (WEAP) blog report, Paul Chefurcha said renewables growth will stall and collapse around 2090 or so, since there wont be enough fossil/ nuclear energy, or the high level of technology and manufacturing capacity needed, to sustain expansion. http://www.paulchefurka.ca/WEAP/WEAP.html.
This particular doom scenario seems unlikely, or, rather, unnecessary. If we invest our remaining fossil (and fissile) fuel in building up renewables capacity, we will create a base for further renewables growth, up to maybe a sustainable steady state, which will then only need relatively small energy inputs for upgrades and replacement. However it could also provide a surplus, to allow for more growth, if that’s what we want/need, up to the final planetary limit of renewable energy availability, which is some way off, even given land use constraints. There is an awful lot of desert for CSP/CPV and sea for offshore wind, wave and tidal. The only (!) uncertainty is- will we invest fossil energy in this, or just burn it off, for no long-term benefit. Chefulka is not confident. He thinks we are doomed by our willful short-termism and greed for more. Certainly, as the WEAP study makes clear, and as the Optimum Population Trust also always says, what would help, is less economic growth expectations, or at least less people!