6 ms·
Most of the comments below have focused on the thermodynamics in play for this. Let me instead speak a little on the materials chemistry: Materials that are st
by phasetransition 9y ago
Most of the comments below have focused on the thermodynamics in play for this. Let me instead speak a little on the materials chemistry:
Materials that are stable liquids at relatively high temperatures and that exhibit low vapor pressure have been investigated for quite some time as heat transfer fluids. See, for instance, the Molten Salt Reactor Experiment at Oak Ridge where a lithium fluoride-beryllium fluoride mixture was used.
The size of the phase diagram space to investigate comes from the phase rule, and the conditions for equilibrium from the Gibb's criteria. The classic materials science approach (how I learned) is to try and map the phase diagram space experimentally. This can become unwieldy for many component systems.
Common chloride salts and/or nitrate salts have been investigated as potential heat transfer/storage fluids, usually in eutectic compositions to lower the melting point. The actual ionic compositions of the molten fluids can be rather involved.
The widely used austenitic stainless steels (e.g. 304, 316) are susceptible to corrosion attack from chloride ions, so understanding of coordination chemistry around chlorine is important to determine the likely stability of a containment vessel.
It would be ideal to find salts comprised of common materials that formed moderate melting temperature eutectics, have no free chlorides (or no chlorine at all), low vapor pressure, and long term stability in the presence of common stainless (or even mild) steel. Potential candidates could be nitrates or the so called deep eutectic solvents.
Hopefully this is helpful detail,
- jpfed 9y agoThis is probably a dumb question, but do we need to use a liquid to do the salt's job? Might we just sidestep the corrosion question by using big-ass (solid) sheets of tungsten in lieu of a fluid?
- deleted 9y ago[deleted]
- phasetransition 9y agoThe short version is that solids have merely ok heat capacity. The molar heat capacity of many solids at high temperatures is approximately 3R or approx. 25 J/molK. Google "Dulong-Petit Law" and "Debye Model" if you'd like to see why. By comparison, the polar molecule water, which has extraordinarily good heat capacity, due in part to hydrogen bonding, is 75 J/molK at room temperature. So water has almost 31x the heat capacity of tungsten by weight.
- tene 9y agoIt may be I'm missing some obvious implication of something you've written that I've missed due to negligible background here, but I'm confused by the numbers you've cited here. You list many solids as having a molar heat capacity of "25 J/molK", water as having a molar heat capacity of "75 J/molK", and conclude that water has almost 31x the heat capacity, but naively comparing the numbers cited only gets you 3x. Where does the other 10x factor come from?
- phasetransition 9y ago3X molar heat capacity difference, yes. But water is 18 g/mole, and W is about 183 g/mole. So tungsten is about 10x heavier per mole than water. The parent post happened to pick a solid material for a nice round molar mass ratio.
- tene 9y agoAh, thanks!
- thanhquanky 9y ago10x comes from weight. 1 mol of Tungsten weights 10x 1 mol of water.
- aisofteng 9y agoMass, not weight.
- pjc50 9y agoSolids are notoriously hard to pump.
- bassman9000 9y agounderrated
- ams6110 9y agoNaive question maybe but why not use steam instead of salt? Well insulated pressure tank can hold a lot of steam, heat capacity is good, it's non-toxic, and it is ideal for driving turbines. Focused sunlight is certainly capable of boiling water.
- pjc50 9y agoRequires pressure vessels and has low heat capacity per unit volume.
- deleted 9y ago[deleted]
- deleted 9y ago[deleted]
- scythe 9y agoDo bis(oxalato)borates and tris(oxalato)phosphates have the requisite thermal stability? Uncommon anions, but made from common precursors. It's my understanding that nitrates are potentially explosive, but I haven't heard of any other anions being used except for fluoride (which has its own corrosion/toxicity issues).
- phasetransition 9y agoI don't really know anything about the oxaloborates, sorry. Can you synthesize them from the one of the borax hydrates? If so, what is the synthesis path? Nitrates can of course be an oxidizing agent, so you'd like to keep them away from being e.g. carbothermically or aluminothermically reduced.
- scythe 9y agoIt's my understanding that alkali metal bisoxalatoborates can be prepared by heating the corresponding metaborates with oxalic acid ( https://www.google.com/patents/US7674911 https://www.google.com/patents/US7674911 ). I know the lithium salt was of interest as a more thermally stable battery electrolyte (relative to LiPF6), although interest eventually shifted to lithium difluorooxalatoborate (two fluorines and one oxalate attached to boron tetrahedrally) which is still stable enough for batteries but not as stable as LiBOB. LiBOB is reported to decompose at about 290 C ( http://ma.ecsdl.org/content/MA2010-02/9/597.full.pdf http://ma.ecsdl.org/content/MA2010-02/9/597.full.pdf ), so I would expect the heavier analogs to go somewhat higher, but I've never heard of these salts being considered in the case of thermal storage. I do not know the thermal limit of trisoxalatophosphate salts, only that they look like oxalatoborates and share with nitrates the property of being "non-coordinating".
- stcredzero 9y agoThe widely used austenitic stainless steels (e.g. 304, 316) are susceptible to corrosion attack from chloride ions What about the use of fiberglass and carbon fiber tanks? Is heat tolerance a problem with those?
- phasetransition 9y agoBoth GFRP and CFRP have polymer matrix phases, which creates a limitation on upper temperature. And both have their own corrosion problems.