Friday, October 01, 2010

Do powerplants use too much water?

Coal and nuclear powerplants make heat, convert some of that to electriciy, and reject the rest. They use water, and lots of it, to reject the heat.

The USGS says that thermoelectric powerplants (nearly all coal and nuclear) use 49% of the water withdrawn in the US. That sounds like a lot, and it is. It's also misleading.

92% of the water used by powerplants is used for once-through cooling. That means they suck water from the river, use it to cool their condensers, then pump it back the river at somewhat higher temperature. There are legal limits to the temperature they can send back out, and as the intake water temperature rises closer to those limits, they have to pump more water, and eventually shut down the powerplant. This has happened, famously, in France during a heat wave, right when everyone wanted to run their air conditioners.

The other 8% of the water used by powerplants is used in recirculating cooling. In these systems water is used to cool the condensers, but then some of that water is evaporated in those familiar hyperbolic cooling towers, which cools the rest, and the water is cycled around. These systems use a lot less water because they only need to make up the water that evaporates. Of that 8%, about 70% is evaporated and 30% returned to the lake or river it came from.

Since 1990, the US has mostly built gas-turbine powerplants. These reject heat in the form of incredibly hot jet exhaust, and don't need water. But they burn natural gas which has caused us to send our plastics industry to China. I don't think many people appreciate how dumb that was.

New nuclear plants in the US will be either on the coastline or evaporatively cooled, because there is no appetite for increasing the amount of once-through freshwater cooling. And I don't think there will be many evaporatively cooled plants at either greenfield or brownfield sites: Greenfield evaporatively cooled plants require new water rights which are very difficult to secure. Brownfield replacements of older coal fired powerplants will be difficult because nuclear plants are much bigger than older coal plants, reject a lot more heat, and so need a lot more water, getting back to the new water rights problem. That leaves new PWR development for areas with a lot of water (US southeast) and coastlines. [Edit: And any new coastline PWR developments are going to face new hurdles as a result of Fukushima.]

Water rights are one reason why I'm so interested in molten salt reactors. MSR cores and turbines run at higher temperatures than those of pressurized water reactor cores, so they can be air cooled without killing their efficiency (and thus jacking up their costs a lot). Air cooling is a good thing because it removes an entire class of regulatory problems, and thus an entire kind of project risk.


  1. It takes about 360,000 gallons of water to reject one Gigawatt hour of heat by heating the water and evaporating it. That sounds like a lot. A 1 GW electric power plant that is 33% efficient will consume 44,000 acre-feet of water per year. Is that a lot? Well, about 1850 cubic miles of rain falls in the US each year. If the US produced 8 kw per person (enough for all US energy consumption) and we used evaporative cooling for all of this, it would consume 0.5% of the rainfall on the US. Realistically, we'll use a lot less than this because we can use ocean cooling, dry air cooling, and some of that evaporated water will fall again as rain. But all this concern about not enough water is crazy. look at Palo Verde nuclear power plant in the Arizona desert. It is cooled by evaporating treated sewage. We have lots of options. And lots of water too.

  2. Chris,

    All that looks about right. You are saying "there is plenty of water", and I'm saying "it's owned by other people."

    My guess is that securing additional water rights for new nuke plants will continue to be extremely difficult. Coal replacement will probably work when the nuke goes in the same place as the coal plant, but that won't happen much. When the nuke goes in an existing nuke part, new water will be hard, and air cooling will be much easier to license.

  3. It is sad that the media are so deliberately obtuse about the difference between withdrawals and consumption. It is such as simple concept, anyone with more than elementary school credentials should have no problem understanding it. But journalists typically have axes to grind. They generally don't like centralized powerplants of any type, and prefer the more sexy but inefficient and generally retarded altogether, 'distributed generation'. It just shows the level of intelligence most journalists have, and/or the level of honesty they have in portraying a representative story. Frustrating that the people who inform the broad public are denser than uranium.

    US thermo-electric water consumption is about 3 percent of total. Agriculture is the big water hog, with a whopping 80% of total freshwater consumption.

    Don't bother telling most journalists though. Their craniums are fully depleted.

    Cheers, Cyril.

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