Tuesday, October 29, 2013

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flawed because its modeling employs withdrawal, when once-through cooling systems return 99 percent of the water withdrawn,1  and the power sector as a whole returns 98 percent of water withdrawn, to the source water body.2 For example, according to EPRI, nuclear plants with once-through cooling withdraw between 25,000 and 60,000 gal/MWh, but consume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.


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


jimwg said...

Sharp insight!

Has MIT been directly and publicly contacted or corrected on this by NEI or any other nuclear professional organization?

James Greenidge
Queens NY

Joffan said...

Either way, it's still a 64% increase, so that's the wrong figure to attack.

Probably the best point of reference as to whether this is a lot of water or not is agriculture. The water used there is typically mostly consumption with no scheduled return (although the water cycle will eventually operate to return any water). Comparing the consumption (not withdrawal) to agricultural needs gives a true perspective on the very limited importance of that 64% increase.

OmegaPaladin said...


Respectfully, I believe you are missing the point. A large number of once-through cooling systems literally route water back into the water source, only warmer. The withdrawal is being immediately recirculated. The spray canals and cooling towers evaporate more water, but I believe they take less water to begin with.

Don Kosloff said...

It is not possible for me to believe that the people at MIT are too stupid to realize that they were presenting false information. In addition to returning the water, the water is returned cleaner.

Joffan said...

OmegaPaladin, I'm not missing the point at all. A 64% increase in water withdrawals is probably extremely closely matched to the increase in water consumption. It's simple nonsense to talk about the increase in consumption as a proportion of the withdrawals.

Both consumption and withdrawals go up by 64% or thereabouts under the proposed scenario.

So attacking the value of 64% is wrong. Putting the consumption into context to show that it is a rather minor player in the total water consumption of society is much more useful, relevant and convincing.

OmegaPaladin said...

Water withdrawal - water return = water consumption.

A fountain pump continuously withdraws water from its basin at a high rate. The water consumption is only a fraction of that, as the water is returned to the basin. It's like talking about how libraries are are going to be destroyed by people checking out books -as long as the books are returned, it is fine.

Cooling water does not become unusable or contaminated. Cooling water flows through a heat exchanger / condenser and then back into the environment. Most of the water is returned in liquid form, just warmer and cleaner.

Ernest said...

The MIT researchers aren't totally stupid. They know that almost all water withdrawals for energy use are returned to the source. But they say "withdrawal creates competition for water resources not completely mitigated by the fact that most is eventually returned." The only backup they give for that claim is a commentary in Nature magazine titled "Thirst for Energy."

Too bad for the MIT researchers that "Thirst for Energy" doesn't back up their claim at all. "Thirst for Energy" only says that utilities need to make sure their water sources won't dry up in the future because of global warming. It doesn't say anything about withdrawals for energy "creating competition."

Another thing the MIT study overlooks is that 1/3 of all water withdrawals for energy are saline. "Thirst for Energy" says use of seawater for energy should be encouraged where possible.

Another thing the MIT study overlooks is technology innovations. Their model looks to the year 2050 but it leaves out turbine technologies that are developing in labs right now. Like the supercritical Brayton-cycle turbine that can work with high-temp Gen IV nuclear reactors and can be dry cooled. Little or no water required.

Nuclear power in the desert, it's coming in your future.

Ernest said...

Three of the small modular reactors (at least) will be able to work with air cooled condensers- the Holtec SMR-160, B&W mPower and NuScale SMR.

The World Nuclear Association has this information page about cooling-