5.2 Energy Return On Investment (EROI)
Energy Returned on Energy Investment (EROI or EROEI) is an expression of energy payback – how much energy we get out of the energy we put into a system. (Source : http://energytransition.de/2014/09/renewables-ko-by-eroi/
Links to an external site.) The amount of energy that has to be expended in order to produce a certain amount of energy. Hence, EROI is a unitless ratio of the energy returned to the society to the energy required to make that energy. If the EROI is less than 1:1, the energy source is not considered viable. The minimum EROI required to maintain current rich-world industrial societies is set as 3:1. (Source : http://www.mdpi.com/1996-1073/2/1/25
Links to an external site.) The energy return on investment (EROI) is a key determinant of the price of energy, as sources of energy that can be tapped relatively cheaply will allow the price to remain low. The ratio decreases when energy becomes scarcer and more difficult to extract or produce. (Source : http://www.investopedia.com/terms/e/energy-return-on-investment.asp
Links to an external site.)
EROI studies for most energy resources show a decline, indicating that depletion has been more important than technological improvements over time.
EROI=EnergyDeliveredEnergyRequiredtoDeliverthatEnergy=LifetimeEnergyoutputEnergyInput(EnergyInvested)=LifetimeoftheSystemEPBT
(Source : http://www.sciencedirect.com/science/article/pii/S0301421513003856
Links to an external site.)
(Source : https://www.bnl.gov/pv/files/pdf/241_Raugei_EROI_EP_revised_II_2012-03_VMF.pdf
Links to an external site.)
(Source : http://astro1.panet.utoledo.edu/~relling2/PDF/pubs/life_cycle_assesment_ellingson_apul_(2015)_ren_and_sustain._energy_revs.pdf
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However, there is more than one type of EROI measurement. As is often the case when comparing energy sources, deciding which is best depends what you include in the calculations. One key difference between EROI calculations is what is considered as an energy input. (Source : https://www.carbonbrief.org/energy-return-on-investment-which-fuels-win Links to an external site.)
EROI of Photovoltaics
The EROI of conventional thermal electricity from fossil fuels has been viewed as being much higher than those of renewable energy life-cycles, and specifically of photovoltaics (PVs). However, some suggest that this is largely a misconception fostered by the use of outdated data and, often, a lack of consistency among calculation methods. (Source : https://www.bnl.gov/pv/files/pdf/241_Raugei_EROI_EP_revised_II_2012-03_VMF.pdf
Links to an external site.) In addition, the calculations don’t include the environmental costs of the different energy sources – like those related to greenhouse gas emissions.
(Source : https://www.carbonbrief.org/energy-return-on-investment-which-fuels-win
Links to an external site.) The traditional way of calculating the EROI of PV is EROI = lifetime / EPBT, thus, an EPBT of one year and life expectancy of 30 years corresponds to an EROI of 30:1.
EROI of Different Energy Sources
The world's most important fuels, oil and gas, have declining EROI values. As oil and gas provide roughly 60- 65 percent of the world's energy this will likely have enormous economic consequences for many national economies.
Although most alternative renewable energy sources appear, at this time, to have a lower EROI values than any of the nonrenewable fossil fuels. But wind and photovoltaic energy are touted as having environmental benefits which may be substantial. These benefits may in fact have larger initial carbon footprints than originally suggested.