<Passive Solar Design Overview: Part 3 – Thermal Storage Mass The Oil Drum DrumBeat: January 27, 2009>

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

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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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