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236 comments on Advice to Pres. Obama (#5): One Engineer's Advice for Energy Policy

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john milton on January 26, 2009 - 11:47am Permalink | Subthread | Parent | Comments top

Nate:

With regard to your citation regarding increased water consumption and withdrawal:
http://pubs.acs.org/doi/pdf/10.1021/es0716195?cookieSet=1
I think it would be helpful to put the increase in context.

In displacing gasoline miles with electric miles,approximately 3 times more water is consumed (0.32 versus 0.07–0.14 gallons/mile) and over 17 times more water is withdrawn (10.6 versus 0.6 gallons/mile) primarily due to increased water cooling of thermoelectric power plants to accommodate increased electricity generation.

But from earlier in the same paper at page 4310:

These increases in water
usage represent approximately 0.2–0.3% (28) and 3% (27),
respectively, of overall U.S. water consumption (100,000
Mgal/d freshwater in 1995) and withdrawal (408,000 Mgal/d
in 2000).

and from the "Conclusions", also at page 4310:

Overall, we conclude that the impact on water resources
from a widespread shift to grid-based transportation would
be substantial enough to warrant consideration for relevant
public policy decision-making. That is not to say that the
negative impacts on water resources make such a shift
undesirable, but rather such impacts should be quantified
ahead of time to avoid unnecessary conflicts due to potential
water shortages.

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Nate Hagens on January 26, 2009 - 12:02pm Permalink | Subthread | Parent | Comments top

The quote was directly from the papers abstract.

The tradeoffs and linkages between energy and water are among the most critical we face; but they are very complicated, and extend beyond just those two commodities (e.g. we could use more water for bioenergy if we used less for meat, etc.) I am painfully aware of how complicated the linkages are, as evidenced by my current 3rd rewrite on a paper on Energy and Water. As soon as possible I plan to devote an entire post to the issue -but my main point here is everytime we look to a supply side 'solution', we tend to make some other limiting input more limiting.

P.s. are you the TOD'er I went to U of Chicago with? I keep forgetting.

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Gail the Actuary on January 26, 2009 - 12:39pm Permalink | Subthread | Parent | Comments top

A large share of the publicized problems that the Atlanta area has had with water have to do with water usage for electricity.

One view now is that the estimates of water available for electric cooling were based on the assumption that Atlanta can be expected to get 50+ inches of rain a year--more than Seattle. Now there is some question whether the time period over which the 50+ inches assessment was taken was really representative. Maybe we should be expecting only about 40 inches of rain a year.

If this is the case, the problem is that the Southeast is already overbuilt for water-cooled electric power generating plants. All of the arguing about how much water Atlanta gets vs Florida vs Alabama basically has to do with how much water is available for electric power plants. It starts sounding a lot like the US Southwest.

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Cyril R. on January 26, 2009 - 12:58pm Permalink | Subthread | Parent | Comments top

Much thermal generation is near the sea/ocean since a lot of population is there as well. Once through cooling is no problem there, only a diffusor pipe system has to be built to mitigate local thermal pollution issues.

Anyways, for inland locations, dry cooling is a proven option available at a small increment in levelised cost. Historically little value was placed on water conservation, so most plants are wet cooled. Adding a tax on freshwater cooling (bigger in arid areas) is one solution.

However, a more productive option is to use CHP thermal desalination and water treatment plant, using the nuclear plant's turbine heat rejection. This both creates a lot of new freshwater supply (from eg sewage water, brakish water, salt water if a sea/ocean is nearby) while reducing cooling water use (because the CHP can be seen as increased efficiency). If no water supply is nearby (could be the case in arid areas where freshwater supply is most dire), other CHP uses can be devised, such as paper and pulp heat input, other chemical processes etc. etc. that can also greatly reduce cooling water use.

This is one problem that is easily solved, and that can actually reduce fuel use and imports even more in the process by reducing natural gas and oil that would otherwise be needed if CHP was not available. This all does require the right policy. So now that you're reading along, Barack... :)

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Gail the Actuary on January 26, 2009 - 10:25pm Permalink | Subthread | Parent | Comments top

Is it possible (reasonable) to go back and retrofit some existing power plants for dry cooling? It seems like both the Southwest and Southeast could use the additional water, if it were available.

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tomswift on January 27, 2009 - 6:47am Permalink | Subthread | Parent | Comments top

Is it possible (reasonable) to go back and retrofit some existing power plants for dry cooling?

Retrofitting is possible, however the plants electrical output would suffer. Thermodynamics of heat engines, of which steam turbines, diesel engines, and gas turbines are examples, state that the output is directly proportional to the heat flux times the difference between the input temperature and output temperature. For water it is difficult go higher than about 350º C on the hot end because of corrosion, high pressure and other engineering problems. On the low side we can go down to about 50º C if we can use cold sea water for cooling. For PWRs, (Pressurized Water Reactor) this limits us to about 34% efficiency. With a dry heat exchanger it would be difficult to get the low end temperature lower that about 110º C which would lower system efficiency to maybe 30%.

One large advantage of the LFTR and gas turbine is it can operate at between 650º- to 900º C on the hot end. With an air cooled heat exchanger on the low end running at 110º C we have an efficiency of over 50%, and lower cost.

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AlanfromBigEasy on January 27, 2009 - 10:35am Permalink | Subthread | Parent | Comments top

The Lower Mississippi River has no foreseeable heat sink issues. The volume of water is incredibly vast.

Just the shipping channel at New Orleans is 900' wide and 100' deep moving at several knots at the summer minimum. Add the water underneath the shipping channel and to the sides.

Alan

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Luke H on January 29, 2009 - 8:55pm Permalink | Subthread | Parent | Comments top

I was pretty sure 'frombigeasy' was refering to your hometown and not some techy organization that feels big things come easy (you do paint bright pictures with a broad brush). Always good to see someone give good information about a place they know well, thanks. :)

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Luke H on January 29, 2009 - 8:22pm Permalink | Subthread | Parent | Comments top

hit the key twice

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lengould on January 26, 2009 - 1:21pm Permalink | Subthread | Parent | Comments top

Overall, excellent article. Kudos.

I tend to discount the difficulties of "water for power generation cooling" on the following grounds:

a) Much of the present water "used" for power plant cooling is not really used, simply heated slightly then returned to source immediately available for other uses. Distinction needs to be made.

b) For only a relatively small (approx. 2% for added blower input, 2% reduced overall plant efficiency) hit plants can switch to air cooling and eliminate water use almost entirely.

c) Most population concentrations live near enough to "essentially unlimited" ocean water sources to ignore the issue.

d) Coming technologies such as the "Vortex Engine" technology have a clear potential with relatively small R&D to both eliminate the cooling tower requirement altogether, and also to contribute an added 20% to the overall plant efficiency. Probably very cost-effective, esp. in dry areas. See Atmospheric Vortex Engine (AVE)

I realize I'm somewhat over-simplifying, but so is above (necessarily, as myself) in the limited space and time available.

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Nate Hagens on January 26, 2009 - 2:22pm Permalink | Subthread | Parent | Comments top

The linked to paper (and indeed all papers addressing water limitation issues) distinguish between water consumption and withdrawals. Withdrawals are typically considered the diversion of freshwater from its natural hydrologic cycle, either at the surface or from below the ground, for anthropogenic purposes. Water consumed usually refers to water used in the energy production process that is either lost to a given watershed as steam or contaminated beyond cost-effective remediation.

Though energy from biomass is far more worrisome in its water impacts, the closures in Europe in 2006 and 2007 of nuclear plants due to heat/water limitations suggest this is not a trivial issue. Globally the current water stock in rivers is about 2000 km3; the anthropogenic water withdrawals from these rivers is 3800 km3/yr; and the global river discharge is 45,500 km3/yr -so clearly we can reuse this water many times over - (unlike energy) - but, as with energy, there are limits.

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Engineer-Poet on January 26, 2009 - 4:18pm Permalink | Subthread | Parent | Comments top

One of the advantages of molten-salt (and perhaps also metal-cooled) nuclear plants is that they can run hot enough to use gas turbines instead of steam turbines as the heat engines.  If the designers are willing to accept lower efficiency1, open-cycle gas turbines using air are possible.  This eliminates water as a coolant, and also eliminates the capital cost of condensers and cooling towers.  (It also makes the design suitable for sites where there is no available water.)

GE's recent F-series intercooled gas turbines produce several hundred megawatts from a rather small package.  A few similar units, with regenerators instead of heat-recovery steam generators and supplied heat from a molten salt or metal loop instead of combustion, would make a compact and innocuous generator system for a 1-GWe class reactor.

1 Nuclear fuel is so cheap that capital cost should probably be a greater concern.  Just being able to eliminate both the sulfur/mercury emissions and water consumption of plants in the Southwest would be a major selling point.

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Cyril R. on January 26, 2009 - 4:55pm Permalink | Subthread | Parent | Comments top

One of the advantages of molten-salt (and perhaps also metal-cooled) nuclear plants is that they can run hot enough to use gas turbines instead of steam turbines as the heat engines.

There are potential advantages to that, as you mention. But don't get ahead of the facts now. For a given temperature, advanced steam cycles are more efficient than gas turbines. This is a big advantage too. Sure, regenerators increase efficiency, but they cost $$$ and adding more has diminishing returns. Sure, ultracritical can't be taken to really high temps, but it's still increasing incrementally, and anyway we may find that very high reactor temps are not optimal from a total cost and durability viewpoint.

Gas turbines excel at power density. Great for airplanes. Not hugely important for a stationary utility generator.

Gas turbines could be safer since the pressures in/around the reactor can be lowered, and possibly yielding a bigger power density. That could be a big advantage. But it's a matter of good design, and it's not like there would be a meltdown in the event of a major failure.

Nuclear fuel is so cheap that capital cost should probably be a greater concern.

There is a difference between capital cost per unit of thermal output and capital cost per unit electrical output. Sometimes, increasing the net electrical efficiency can increase the cost per unit of thermal output, but decrease the cost per unit of electrical output. That last thing is what we're looking for. I think we may find that a rather high efficiency is optimal.

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Roger K on January 26, 2009 - 4:54pm Permalink | Subthread | Parent | Comments top

One of the advantages of molten-salt (and perhaps also metal-cooled) nuclear plants is that they can run hot enough to use gas turbines instead of steam turbines as the heat engines.

How high a temperature can they run at?

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Engineer-Poet on January 26, 2009 - 5:47pm Permalink | Subthread | Parent | Comments top

The Molten Salt Reactor Experiment ran at up to 650° C.  If you look at the pictures, you'll see that it didn't take much of a radiator to dissipate 7.4 megawatts of heat at that temperature!  I expect that the temperature was limited by the properties of the Hastelloy-N used to make the vessels, pipes and pumps, and increasing the temperature to the ~850° C needed to run a sulfur-iodine cycle for thermochemical hydrogen production would require some new materials science.

GE's latest gas turbines are running at ~1300° C turbine inlet temperature, so an air-cycle turbine fed from an MSR would be rather unchallenging from a technical standpoint.

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Cyril R. on January 26, 2009 - 5:59pm Permalink | Subthread | Parent | Comments top

The nuclear part is somewhat challenging at higher temperatures, especially with regards to durability over decades of use.

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Roger K on January 26, 2009 - 7:40pm Permalink | Subthread | Parent | Comments top

650°C is only 100 degrees above the typical operating temperature of a steam turbine. I am guessing that you could run a closed cycle gas turbine with this inlet temperature using coal as the energy source. I am not promoting more coal fired generation; I am just saying that it could be done if it was necessary. Presumably it is not done in practice because there is some kind of performance/cost penalty.

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Cyril R. on January 27, 2009 - 12:45pm Permalink | Subthread | Parent | Comments top

At the time of the experiment, no heat engines were available in the 650 degrees celcius range. This no doubt lessened the commercial interest in the MSR, along with little political backing it's no surprise that the experiment didn't lead to real reactor systems.

State of the art steam turbines operating at around 600 degrees C are more efficient than gas turbines at similar temperature. A huge amount of power is required to run the compressor in the gas turbine. Natural gas burning gas turbines are still very efficient because they are burning extremely hot, so as to get a high delta T. This is not optimal for a nuclear heat source, IMHO.

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236 comments on Advice to Pres. Obama (#5): One Engineer's Advice for Energy Policy

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