See also The EROEI of energy balancing.

I’ve been spending a lot of time thinking about EROEI (energy return on energy investment) lately since Tim Helweg-Larsen at PIRC asked me for help with a report. The report, “The Offshore Valuation” was produced by the Offshore Valuation Group chaired by PIRC and it looks at the value to the UK of the offshore renewable energy resource.

Tim, ever present with the big ideas, asked me to have a look at the EROEI of a whole range of energy technologies. What I did was begin a trawl of the literature hunting high and low for figures on EROEI, EROI, life cycle energy, net energy, life cycle analysis, energy payback time, etc and the result was a table on page 61 of the report listing the average EROEIs I found.

The abstract for the study we delivered is below.

“Energy Return on Energy Investment (EROEI) is calculated as the ratio between energy inputs and energy outputs for an energy generating technology. A process that uses more energy than it produces is by definition unsustainable in the long term. Technologies which require fuel generally have a lower EROEI than those which can extract “free” energy from the environment (wind, waves, tides, sunlight). As part of this study we have a carried out a review of published EROEIs and have found a high EROEI for marine technologies, lower EROEIs for solar and nuclear power, and still lower for coal and gas.

“In addition this study has found a suggestion of increasing EROEI for wind turbines and nuclear power, and a falling EROEI for coal, gas and solar power. Of these, the most significant is the reduction in EROEI of coal. The study examines the reasons for these relationships with the greatest reductions expected to come from carbon capture and storage and the greatest increases expected from increased size of wind turbines and lower energy inputs to nuclear fuel enrichment.

“Finally the study looks at the relative lifecycle efficiencies of grid connection and a number of other balancing methods. Grid connection and HVDC interconnectors are found to have a similar lifecycle efficiency of around 95%. Energy storage technologies have lifecycle efficiencies in the range of 45% to 75%.”

We also created some interesting graphics based on the data. Firstly we adapted the bubble charts idea used by Tom Konrad, the Clean Energy Wonk. His idea is to calculate the energy internal rate of return (EIRR) for technologies. This is the percentage of the energy invested that is returned each year, analogous to IRR for financial investments.

This gave the chart below.

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Technology Average EROEI Average EIRR Average lifespan Number found
Coal no CCS 5.5 17% 31 11
Coal w. CCS 1.5 8% 23 2
Solar thermal elec. 9.9 40% 25 7
Gas no CCS 3.5 11% 32 5
Gas w. CCS 2.2 13% 23 2
Nuclear 10.9 36% 29 50
PV 8.3 34% 24 46
Tidal range 115.9 97% 120 1
Tidal stream 14.9 74% 20 2
Wind 25.0 125% 21 108
Wave 12.0 60% 20 2

What’s interesting about this, particularly for the Offshore Valuation Group, is that the offshore resources (and on-shore wind) have high EROEIs and very high EIRRs. There is a clear demarcation between wind, wave, and tidal at the top of the EIRR tree, solar and nuclear in the middle, and coal and gas at the bottom.

Even more interesting (although not very reliable in statistical terms) is the trajectory of historic estimates of the EROEI of some of these technologies. The graph below is projected out to 2020.

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This graph is based on some very small data-sets for gas and coal. The justification for including them is that there are sound technological reasons why we can expect the EROEI of gas- and coal-fired generation to fall. Coal- and gas-fired power plants are becoming less efficient. This is primarily due to legislation passed to minimise the environmental impacts. In the past, flue scrubbers were mandated in order to minimise the emissions of sulphur dioxide, a key cause of acid rain. In the future carbon capture and storage is very likely to be required. Both of these impose additional parasitic loads on power plants which make the plant less efficient and so reduce the EROEI.

Additionally, although the peak for coal is not expected for some time, the energy required to mine and transport coal is increasing. Over time high quality black coal will run short and dirtier brown coal will be substituted. This will carry an energy penalty as brown coal is less energy-dense and takes more energy to avoid sulphur dioxide emissions. Peak gas is expected before peak coal. The energy inputs to North Sea oil and gas have been increasing over the years as more advanced oil extraction techniques have been applied. Another major future source of gas in the UK in liquefied natural gas which is chilled, compressed and imported by tanker with a high energy cost.

On the other hand, wind energy is becoming more efficient. With wind energy, bigger is better for energy return. Energy return increases with the square of rotor diameter. If the rotor is twice as big, it produces four times the power. Turbine size has been increasing for many years, which is the key reason for the steep rise in EROEI.

What about PV? The problem with PV is that the improvements in EROEI are incremental, not geometric as with wind (and tidal) turbines. They don’t have the same power law increase in energy return when scaled up. Despite this fact, the error margin in the data and the ongoing development of thin-film PV which has higher EROEI than traditional crystalline PV mean that the true trajectory could well be upwards.

And nuclear. This was the real surprise for me in this analysis. It appears that there are currently two main types of enrichment used, gaseous diffusion and gas centrifuge enrichment. Gaseous diffusion uses more energy and is being replaced by gas centrifuge which is one of the main influences which has led to a higher nuclear EROEI. The peak for fissile materials is not expected for some time, at least at current rates of extraction. Nevertheless, the purest and most convenient supplies will inevitably be used up first causing a downward pressure on EROEI. Some companies are even currently researching producing yellow cake (Uranium ore) from the ash from coal-fired power stations which is, I suppose, a good example of cradle to cradle thinking.

All of this goes to show that for the UK to get over the looming energy gap there is little better in energy terms than to go for the on-shore wind, a suite of offshore renewables, and much as I hate to say it, probably a wedge of nuclear too. Clean coal is a dead end.