EROI of Offshore Wind
There is a lot of confusion about the real value of the EROI of wind energy. Time for a back-of-an-envelope calculation.
By far the largest energy input in the construction of a wind turbine from iron ore is realized in the steel mill. How much energy does it take to produce 1 ton of steel? Here is an old Dutch study (1986) from the Energie Centrum Nederland:
[ecn.nl] – Energy Consumption For Steel Production
On p20 we see that the lowest value for steel production is ca 20 GJ/ton or 5555 kWh/ton. We can safely assume that in 2017 the amount of energy required is lower, but we will accept this figure anyway to be on the prudent conservative side.
A 6 MW offshore wind turbine has roughly the following weight distribution:
| Part | Weight [ton] |
|---|---|
| monopile | 2200 |
| tower | 650 |
| nacelle | 350 |
| Total | 3300 |
Energy cost for the steel production: 3300 * 5555 = 18331500 kWh
In the North Sea this 6 MW wind turbine on average produces 144,000 kWh/day, see:
[deepresource] – Gold Mine North Sea
Payback time in energy terms therefore is: 18331500/144000 = 127 days
There are of course extra energy costs. Say you need to get the iron ore from Australia, the worst case in transport energy terms, from a Dutch perspective.
[withouthotair.com] – Sustainable Energy – without the hot air, David JC MacKay
This source claims a shipping transport cost for dry cargo: 0.08 kWh/ton-km
Australia-Rotterdam = 20,000 km, in other words: 1600 kWh/ton. Or 3300 * 1600 = 5280000 kWh for the entire wind turbine. Divide it again by the daily energy production of 144,000 kWh of our 6 MW turbine to arrive at 37 days extra work for the wind turbine to earn itself back.
Total energy payback time: 127 + 37 = 164 days.
In other words: the offshore wind turbine must work for less than half a year to “earn” itself back in energy terms.
The remaining items like rotor (3 * 25 ton), construction of the gear, generator, maritime handling, installation, etc, have far smaller energy cost. Add a 22 days (wet finger in the air) to conveniently arrive at exactly half a year.
Things get even better if it is realized that these days about 1/3 of the world’s steel production comes from scrap metal, which requires far less energy to turn into new steel than iron ore. If in the future the windturbine has arrived at end-of-life, you can reuse the steel of the old turbine. You don’t have to get the iron from Australia anymore and recycling of steel costs far less in energy terms than producing steel from ore.
Summarizing: assuming a very conservative [*] life cycle of 30 years for the turbine, EROI for our offshore 6 MW wind turbine is therefore 30/0.5 = 60 or higher.
[*] The Eiffel tower is around since 1887 or 130 years. Experts estimate that the tower can easily survive another 300 years. Likewise it is absurd to assume that an offshore wind tower will fall over after 30 years.
P.S. After writing this post we discovered this calculation by Jan-Pieter den Hollander:
[duurzaamgebouwd.nl] – Energiebalans van een windmolen
Embodied energy
Onshore wind turbine: 0,6MW, height = 50 m, rotor diameter = 40 m
| Part | Energy in GJ |
|---|---|
| Production | 1900 |
| Installation | 495 |
| Maintenance (20 year) | 774 |
| Total | 3169 |
Generated energy
Energy yield per year: 5015 GJ
Energy payback time: 7-8 months
In essence a comparable value to ours. In the example of Jan-Pieter den Hollander we are dealing with an onshore machine with less yield than offshore. Although it must be said that in his calculation there are counter-intuitive high values for installation and maintenance. Perhaps we will update this post in the future to have look at those.
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