Energy Return On Energy Invested
Calculating what we get, compared to what we put in…
This ratio, abbreviated to EROEI, has become quite a hot topic lately in the movement to replace fossil fuels with forms of energy that will not further contribute to global warming. The energy return of a power technology is the energy output of the system – nuclear, wind, hydro, etc. – over its expected lifetime. That, divided by the energy that must be invested to bring that system on-line and keep it functioning, is the EROEI.
Explanations I have read note that until about the year 1650 we humans gathered energy from animal and other biological sources – most notably firewood. Over that time, the EROEI is said to have been in a range of 3.5 to 5. I remember reading that by that same date – 1650 – the Royal Navy’s need for spars and timbers for shipbuilding had exceeded the capacity of in-country sources, compelling the import of timber from Scandinavia and elsewhere.
I grew up in the Adirondack region of New York State in the US – a wooded area that was clear-cut three times. The first time was for Royal Navy spars; the second time to make charcoal for the iron industry; and the third time for the construction of East Coast cities.
In England, that date also more or less marked the time by which fuel wood within convenient reach of cities had been cut, forcing a switch to another more compact and more readily available source of heating – coal. Its use, and the power soon derived from the coal-fired steam engine, brought a rise of EROEI to a value of roughly 10. The Age of Steam later gave way, with the development of petroleum extraction, to a yet more energy-dense Age of Oil, raising the EROEI as high as 30. Each change made possible widespread increases in human living standard.
After the second great war of the 20th century, the revelation of nuclear energy brought an even more dramatic rise in energy return – fission-based nuclear powerplants, able to deliver an EROEI of 100.
Bear in mind that this ratio involves no moral or political judgement and reveals nothing about an energy system’s influence on the environment. It speaks only of the energy investment cost required to realise an energy output by various means.
The surprise of the moment is that the energy return on the favoured renewables wind and solar is quite low. Wind systems require large amounts of buried reinforced concrete, and the tall turbine pylons are made of steel – both materials requiring large amounts of energy for their production. The materials must in general be transported over some distance to the point of use, and the presently estimated lifetime of such plants is estimated at 20-25 years. For the wind machines, an EROEI as high as nine or 10 to one is estimated, with a similar value for solar concentrators – many steered mirrors heating a central boiler to raise steam. Lowest of all came the familiar roof panels – photovoltaics (PV) at values of two to five.
This is a return to the lower energy returns of nowdiscarded systems. If we like, we can ignore that in favour of noting that wind and solar, in their operation if not in their construction, contribute no carbon dioxide or other greenhouse gas to the atmosphere, thereby rendering them harmless to climate and human survival.
Yet we also know that wind and solar as at present practiced must be backed by substantial capacity in fast-starting electricity-generating systems that can deliver base load power at night and during times of atmospheric calm. Those systems are in use now – often natural-gas-fired simple-cycle gas turbine plants that can start and be on-line as quickly as aircraft jet engines can be started, taxied to the runway threshold, and brought to full power for take-off.
Purists correctly will point out that huge batteries, charged by wind and solar capacity beyond what is needed for daytime operation, could deliver such backup power. Such batteries do not at present exist in this form, although relatively tiny systems in Australia and California are in use for load smoothing as varying wind and solar sources come on- and off-line.
Should we ignore the energy cost of energy production, and focus exclusively on ideal, 100 per cent carbon dioxide-free power sources? We probably can’t afford to, because replacing the entire energy economy of human society won’t be cheap – especially when it is also proposed that all existing ground vehicles, rail, ocean shipping and aviation, plus residential and business heating/cooling (and even future wars), be repowered from renewable sources.
I hope it all goes as planned, with none of the disruption of supply chains, shortages of critical materials, and economic inflation we are experiencing at the moment, which are being blamed on the Covid pandemic and the Russo-Ukraine war.