## Ahhh EROEI

The Ratio of the Energy Returned divided by the Energy Invested in producing electricity. The Green bars are global estimates and the purple bars apply to the US. There is considerable uncertainty in all the numbers.

How should we decide on the mix of technologies to use to generate electricity? There are pros and cons for all the choices.

• Coal is cheap but emits carbon dioxide.
• Gas is a bit more expensive but emits 50%  less carbon dioxide.
• Nuclear requires eye-watering up-front investment but is low carbon.
• Wind energy is intermittent but sustainable

So it is interesting to make quantitative comparisons between the differing technologies. We have many choices in comparing parameters. Initial costs; running costs;  immunity to world fuel prices; sustainability – the list goes on.

One interesting choice is EROEI: the Energy Return on Energy Invested. It is the answer to the sum:

EROEI = Useful energy produced ÷ Energy invested

So for example, if I use one unit of energy to dig coal from the ground, ship it around the world,  and then burn it to power a steam turbine and make electricity, how many units of electrical energy do I generate?

This is a simple question to ask, but a difficult one to answer. For example, one would obviously consider the energy used in shipping the coal. But what about the energy used in building the ship? Or some fraction of it? Using standardised rules one can produce estimates of EROEI and the results – in a chart at the top of the article are interesting.

• First there is massive discrepancy between world-wide coal (18) and US coal (80). This is presumably because of the ease of extraction of US coal, and the short distance from mines to coal-powered  electricity-generating plant. The large numbers in each case help explain the popularity of coal in generating electricity both world-wide and in the US. The energy return of course takes no account for energy which might be needed to cope with the consequences of the massive carbon dioxide emissions, or the appalling environmental legacy of coal mines.
• Second is the number for wind (20 or 18) – which is more-or less the same as coal. At Protons for Breakfast many people ask whether in energy terms wind power is ‘worth it’. The answer from these studies is a definite ‘Yes’. However I suspect that the time to reap this return on investment may be longer which affects the financial return on investment.
• On reflection I was not surprised that hydroelectricity represents the best EROEI, but of course this does not cover the environmental costs of such schemes.
• The low value for gas (7 or 10) surprised me. I suppose this reflects the costs of discovery, transport, storage and delivery.
• And finally the numbers for solar energy more or less match the numbers for nuclear energy. These are not specific to the UK and so the same numbers are unlikely to hold here. However I was surprised at the low number for nuclear power and the relatively high value of Solar Photovoltaic generation.

EROEI is not a magic number – but it is a fundamental number. If this number is below unity, then in energy terms the activity makes no sense. And if the number is close to unity, then the activity is barely worthwhile unless there is some other benefit. Scientific American suggest that activities where the EROEI is below 5 represent a borderline below which electricity -generating technologies are no longer worthwhile. It is interesting that several current technologies – including nuclear power –  come close to that suggested border.

References

Mason Inman: Scientific American 2013: This contains lots of links to his sources – but many of these are behind pay walls 😦

Wikipedia EROEI  This contains lots of links to sources – but many of these are behind pay walls as well 😦

### One Response to “Ahhh EROEI”

1. Solar power gets real | Protons for Breakfast Blog Says:

[…] – this makes complete sense. I can’t speak for the finances, but in terms of EROEI, this looks a sensible energy […]