The Molten Salt Solution to Storing Solar Energy
by Matthew McDermott, Brooklyn, NY on 06.30.08
image source: United Technologies
The perennial complaint by renewable energy skeptics about wind and solar power is, that despite the fact that they produce clean power, they are intermittent in nature and require some sort of storage technology to fully exploit the power generated. A recent article at Renewable Energy World spotlights one solution to this very real issue: Molten Salt.
Solar Thermal Power with a Difference
Basically the set-up works like this: Heliostats (large concentrating mirrors) focus the sun onto a central tower, which heats a liquid inside. The liquid in question is molten salt—in this case a combination of sodium and potassium nitrate—which is heated, transferred to a storage tank, and then fed through a steam generator to turn a turbine and generate electricity. The cooled salt is fed back into the tower to repeat the process. The difference between the molten salt and other liquids that are used in this sort of system is that the molten salt retains heat for a long enough time that it can effectively time-shift the stored solar energy from when it is most efficiently generated to when it is most needed.
The original article quotes Terry Murphy, CEO of SolarReserve who helped develop the technology.
Molten salt is a heat storage medium that that retains thermal energy very effectively over time and operates at temperatures greater than 1000°F, which matches well with the most efficient steam turbines. Second, it remains in a liquid state throughout the plant's operating regime, which will improve long-term reliability and reduce O&M costs. And third, it's totally 'green,' molten salt is a non-toxic, readily available material, similar to commercial fertilizers.
Personally, I wouldn’t go around touting commercial fertilizers or anything similar to them as being “totally ‘green’”, but as a way to time-shift renewably generated energy—this really isn’t long term storage in the sense of a conventional battery or fuel cell—it certainly is a solution.
via :: Renewable Energy World and :: Clean Technica
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Tongue firmly in cheek: it's certainly *not* a solution, it's a molten liquid mixture! B^>
Rgds
Damon
It may be a molten liquid, but the good thing is any leak or release almost instantly will freeze to a solid. Any leaks will be easily contained this way and ground contamination would be easy to manage as it won't flow and be carried away in groundwater streams.
The hot material is a safety concern, but one that can be managed with the appropriate engineering controls.
Jim
Compared with some phase-change-materials being proposed for thermal storage, molten salt is almost saintly! I always shudder when I read about molten SODIUM as a thermal storage.
Thanks for the chem joke, Damon.
While this seems to be a good means of storing energy overnight, I think a better solution would be use excess power produced during the day to do large-scale hydrolysis to produce hydrogen, which can be stored for days or weeks, if not longer. Then we can have large fuel cells to convert back into electricity.
If we ever want our power plants to be more than 40% efficient like they are today at their best, we are going to need to move beyond the heat engines that we have been using for 3 centuries. Chemical processes often have the potential to be vastly more efficient because they have different thermodynamic (read: entropic) constraints. Electrolysis and formation of water can each be over 80 efficient, and even today we can make each of those processes 60%.
If you don't like hydrogen, we can just as easily use the energy to make alcohols or other organic compounds from water and atmospheric CO2. The chemistry is more complex, perhaps, but the product is easier to store.
@Anthony:
Electricity storage by producing hydrogen is very inefficient (n
Good comment, Anthony. There are some great ways they're coming up with for storing hydrogen as well -- metal storage for example. So the danger of having some explosive hydrogen gas in high-pressure tanks can be avoided by storing it in metal.
All good ideas. I like. I like.
Electrolysis wastes 50% of the energy, heat loss might be less than that in a thermos style storage tank.
I read about this a long time ago. Molten salts are used for other processes. I was reading about solar furnaces because I wanted to know what materials they had in the target.
In case it's news to anyone, you can store energy in flywheels and compressed air fairly easily too.
Modern large scale thermal storage is 99% efficient even over several days.
That's better than even the highest claims of the hydrogen people and way better than the 25% that they've delivered so far. Don't forget that hydrogen leaks though solid metal ruining it over time. If you need a gas fuel then methane is much easier to handle and can be injected into the existing natural gas pipelines, used in existing gas turbines, stoves and buses.
I was recently looking at the Scientific American December 1991 article (page 102) on thermo-chemical heatpipes as a method of storing and transporting solar energy in chemical form. I wonder what the relative efficiency would be versus molten salt. Any thoughts?
Anyone know how molten salts behave and what they can be stored in? From their chemical origins they seem pretty benign.
It would be great if the generation of the nitrate salts could be made in some method through collection of nitrate from contaminated water or soil. Then perhaps this process could have an environmental remediation impact, as well.