Observing the world of renewable energy and sustainable living

EROI Threshold / Net Energy Cliff

Article that discusses the EROI (EROEI) threshold idea. According to Charles Hall our current society would probably not be able to function if the EROI for the entire society slipped below five. The graph is produced as follows: A society needs to invest energy in order to ‘harvest’ energy:
Net Energy = Eout – Ein
Definition of EROI: EROI = Eout/Ein
Substitution leads to: Net Energy = Eout*((EROI-1)/EROI) (see graph).
The net energy cliff figure relates the percent of energy delivered as net energy (y-axis, green) and the percentage of energy used to produce energy (y-axis, brown) as a function of EROI (x-axis). EROI is a confusing concept. There is hardly a difference between EROI 100 (oil from Saudi-Arabia in the sixties) and EROI 10 (solar panel). Here some examples:

EROI 100 means that if you put 1 potato in the ground you can harvest 100 next season. EROI 10 means that you must put 10 potatoes in the ground to harvest 100 next season.

To put it differently: if your starting capital is 100 potatoes and EROI is 10, then you need to plant 10 potatoes and you can eat 90 in order to harvest 100 again and the cycle can start all over again in a sustainable way. In case of EROI 100 you only have to plant 1 potato and you can eat 99 to have 100 potatoes again in the next season. Comparing EROI 10 and 100 means incomes resp. of 90 and 99 potatoes, a difference of only 10% and not a factor of 10! EROI applied to energy same story. In case of EROI ratio 15:5 the income difference is 14:12 or ca. 16%. Not that spectacular either.


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