With the energy policy debate churning on, I thought I would look back at what has sometimes been seen as useful metric for comparison of energy options - Energy Return on Energy Invested (EROEI). That’s a measure of how much energy you have to use to build (and where relevant, run) an energy supply system, compared with how much energy you get out of it over its lifetime of operation. For example, as fossil fuel resources get scarcer and harder to extract, the EROEI ratios for their use are falling (e.g. from 44:1 in 1950, to 8:1 in 2020 for oil) this being one more reason why they are on the way out.
In terms of the non-fossil energy options, in some (but not all) studies I have seen, renewables generally do much better than nuclear, their only supply side rival, which should be no surprise, since (bio energy systems apart) renewables do not need any fuel to run. With nuclear, energy has to be used to mine, mill and process uranium ore into fuel, and the amount of energy needed for that will increase as high-grade uranium reserves deplete.
Early studies by Luc Gagnon from Hydro Quebec and an analysis by Canadian Energy Prof. Danny Harvey in 2010 suggested that typical PWR nuclear plants only got about 15 times more energy out than was needed to build and run them, and it was noted that this ratio might fall to 10: 1 or even 5:1 later on, as lower grade ores had to be used. By contrast, wind turbines on good sites could have EROEI ratios of 50: 1, or even up to 80:1. With occasional turbine renewals, hydro projects can do even better: since they can deliver power for centuries, once built, their EROEI ratio is usually very high, up to 200:1 or more. However, solar PV is not so good at present: the manufacture of Solar PV cells is very energy intense, with EROEI ratios only ranging up to 25:1, although that is improving with new types of cell, cell use and fabrication techniques. Focused solar-thermal ‘CSP’ systems can have EROEI ratios of up to 40:1. So far, few wave or tidal stream systems have been built, so EROEI estimates are less solid, but 20:1 seem likely, rising subsequently as the tech matures.
However, the early EROEI estimates/ranges were tentative, sometimes based on limited, and site specific, data. Subsequently, as the technology developed, there have been many papers updating the data e.g. on solar, but also many disagreements. For example, perhaps unsurprisingly, WNA, the nuclear lobbyists, has put the nuclear EROEI at up to 70:1. And methodological disputes continue, for example about where to draw the system boundary. For example, if you include any backup capacity needed to balance variable renewables, then the total system EROEI will be lower, and of course if you exclude the energy needed to make reactor fuel, then the nuclear EROEI is much higher.
A 2013 overview by Carbon Brief, quoted a Scientific American article which put hydro’s EROEI at over 40:1, wind at 20:1 and nuclear at 5:1, but noted that estimates varied and also that EROEIs were not the only important metric for assessing sustainability. Be that as it may, a 2015 assessment at least produced a simple to understand summary graphic, even if not all will agree with it - some see the EROEIs for renewables as being much lower.
Moving more up to date, the debate has continued, although without too much clarity. Wikipedia and Google AI come up with some ambivalent and dated results. Although Google AI relayed the WNA figures for nuclear, it did at least note, in its more detailed coverage, that there were opposing views and other estimates – not just high (as in a 2013 study) but also low (as in a 1988 study). But it didn’t initially include references to the 2010 Harvey data I cited above - until I prompted it.
Google AI also seems to focus on the coverage of the impact of storage on renewables which see it as very high, with EROEI figures falling rapidly as the renewable share expanded, and more storage backup was required. And until I asked, it didn’t mention the more recent view of Diesondorf and Wiedmann, challenging this assessment, with, as I noted in a 2020 post, the impact of storage being seen as depending on the type of storage needed, and also the way the wider system was developed. For example, hydro pumped storage would presumably share hydro’s good EROEIs and the use of renewables along with supply and demand balancing would increase whole system efficiency, so less energy was wasted. Google AI also seems to have missed (until I prompted it) the linked and fascinating broad-frame overview by White and Karmer, which I also mentioned in my 2020 post. That argues that, in a balanced sustainable future world, EROEIs won’t matter too much anymore in overall system terms, since we would then have a non-carbon system. Interesting.
To be fair, Google AI did spontaneously mention a subsequent, equally interesting, 2024 Nature paper, which I hadn’t seen. That examines the net energy performance of nine decarbonisation global energy transition scenarios until 2050 by applying a systemwide energy return on investment EROEI model developed by LUT University Finland. It accepts that achieving higher renewable energy shares requires significant enabling back up/storage technologies, so that system-wide EROEI ‘continually declines as the shares increase’, but staying above 16:1. And then, as renewables reached nearly a 100% share by 2050, the system EROEI moved up to around 20:1 in some cases. That’s still quite a low EROEI, but the paper argues that there were other issues: for example, costs and emissions are not automatically reflected in EROEI. So it seems that, for optimal sustainable system choice, we may need a more comprehensive metric than just simple EROEI.
There may be problems with EROEIs and they are not the whole story, but arguably, if done comprehensively, they may still be useful as general guides and may also give us some feeling for the carbon intensity of developing new green energy systems long term. That's important since initially, it will mostly be fossil fuels that are used to build them, though in time, renewables sources will take over that role, so reducing the carbon debt, even when storage is included. And then, if we are moving to a steady-state zero-growth future, apart from upgrades and replacement of worn parts and components, longer term, there would seem to be little need to use energy to build new systems or energy to extract/process new materials for them! Utopia? Certainly a different world! But for now, EROEI analysis may still matter- even if it is contentious.
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