Why an MIT Study on Energy, Water Use and Carbon Emissions is Seriously Flawed

The following post was submitted by William Skaff, NEI's Director of Policy Development.

The MIT study, “Water-CO2 Trade-Offs in Electricity Generation Planning,” that was recently published in Nature Climate Change Letters indicates that power sector water use increases as carbon emissions are reduced. The measure employed for water use is withdrawal. A closer look at the study indicates that this approach is seriously flawed and could lead to erroneous conclusions about nuclear power plants and cooling water.

Climate change eliminates water from watersheds. It does not take water out and then put it back again. Therefore, the appropriate measurement of power plant water use in this context is consumption. This study is seriously flawed because its modeling employs withdrawal, when once-through cooling systems return 99 percent of the water withdrawn,¹  and the power sector as a whole returns 98 percent of water withdrawn, to the source water body.² For example, according to EPRI, nuclear plants with once-through cooling withdraw between 25,000 and 60,000 gal/MWh, but consume only 400 gal/MWh.³

Thus, when the study says, “The water withdrawal under the CO2 limit is 64% greater than under the no-limit case, owing to the additional water withdrawals for nuclear energy” (p. 2), that percent would be substantially reduced to a very small percent increase if consumption were considered. Specifically, according to the statistics above, power sector withdrawal is 100 percent of water use, and 2 percent of this amount is consumed. If, according to the study, there is an increase in withdrawal of 64 percent, roughly 2 percent of which is consumption, then the additional increase in consumption is approximately a little over one percent, at 1.28 percent.

The MIT study demonstrates that selecting an inappropriate water use measure for computer modeling will yield deceptive results that, in turn, may lead to erroneous, if not detrimental, choices for electricity generation portfolio mix made for the purpose of climate change.

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¹ Electric Power Research Institute, Water & Sustainability, Vol. 3 U.S. Water Consumption for Power Production, 2002, p. 3-1.
² U.S. Geological Survey (Wayne B. Solley, et al.), Estimated Use of Water in the United States in 1995, 1998, p. 48-9.
³ Electric Power Research Institute, Water & Sustainability, Vol. 3 U.S. Water Consumption for Power Production, 2002, p. viii.