When you can produce more energy than you put in .. Happens all the time with coal mines, oil and gas wells, nuclear plants, hydroelectric dams.
With corn ethanol, some studies claim a small net positive energy gain ... others say no. Brazil uses sugar cane based ethanol, which a different energy profile.
Whether or not ethanol production from corn is efficient is debatable. Proponents of corn-derived ethanol point to studies emphasizing an overall net positive energy gain, whereas others claim that when the complete production costs of farming, seed, fertilizer, pesticides, fuel, ethanol distillation, etc., are taken into consideration, ethanol requires 30% more energy to produce than it creates.[4][5] Ethanol proponents say that ethanol simply puts back the same carbon dioxide that the plants from which it is made absorbed while growing, and hence does not add to the world's balance of greenhouse gases. A 2006 University of Minnesota study shows a positive energy balance for ethanol of around 25%, but also highlights many environmental and economic limitations affecting the viability of corn ethanol.[6]
en.wikipedia.org
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As for Rio, if they do have ocean water problems (I wouldn't take your word for it) its not because of oil production but waste from the city. |
