The readers of "The Oil Drum" are familiar with the concept of "Energy Return of Energy Invested" (EROI or EROEI). It is the ratio of the energy produced by an energy plant during its life cycle to the amount of energy needed to build, operate and dismantle the plant.


EROEI remains one of the most useful parameters that can be used for evaluating an energy technology, but it is not the only one. Another element is the need of water. Water is needed for irrigation of plants to be used as fuel and all large plants using thermal engines need water cooling. We can speak, then, of Energy return of Water Invested (EROWI). It is a concept much more recent than that of EROEI, but which is rapidly gaining attention and may be not less important.