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,
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.
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?
The parent post happened to pick a solid material for a nice round molar mass ratio.
Focused sunlight is certainly capable of boiling water.
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).
Nitrates can of course be an oxidizing agent, so you'd like to keep them away from being e.g. carbothermically or aluminothermically reduced.
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 ),
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".
What about the use of fiberglass and carbon fiber tanks? Is heat tolerance a problem with those?
For a summary, this is an energy storage project, where the energy is stored in tanks of high and low temperature fluids. Temperature differentials can be generated by heat pumps, and can be converted back into useful energy by heat engines.
However, the theoretical maximum energy efficiency of this unit, assuming no heat losses from the tanks, will probably be poor compared to LiIon or pumped storage (of the gravity type), simply due to the fact that heat engines are strictly limited in efficiency to the Carnot efficiency (and are usually much less efficient than that).
Depending on the temperatures involved, I'd be surprised if it can return as much as half the energy that is put into it.
Also, conflation of energy and power in the article.
But Li-ion is orders of magnitude more expensive than what they're proposing (no matter how cheap the cells are getting), and couldn't possibly be deployed at this scale. It's more useful for stop-gap loads when you need instant discharge. Pumped hydro is only possible when the local geography is perfect for it and there is an abundance of water. The reality is that our future will require a host of different technologies like this working in synergy to provide grid scale energy storage. No single solution will become the standard.
5% of US daily grid storage is an excessive goal, but assuming 100$/kwh would only cost 3,500$ / person. Assuming they last ~15 years that's under 20$/month which is clearly a deal killer, but not exactly impossible.
PS: Hydro storage is generally a much better option, but as a supplemental option batteries could be very useful.
Mixed application installations (e.g. campus-scale air conditioning and hot water, generously overprovisioned to allow some paid grid stabilization work on the side) might be an economic sweet spot even with terrible cycle efficiency. In any case, for grid storage, price per effective capacity will remain the key metric as long as the storage market is not saturated. Tanks scale pretty good.
It's frustrating when some reporter writes an article about one of these companies with great PR (Google, FB, Apple, Tesla et al) writing that since the reporter's never heard of this before (or of a web search either, apparently) that the company must have come up with something amazingly new.
Yes, google could cut the marginal cost of storage, but the economics of thermal storage make sense in thermal plants, not direct grid storage (I worked in thermal storage myself, though using a different technology). This is the big point, and as cheap PV has driven funding for thermal plants away it's hard to see hw this can fit in. That should have been the thrust of the article instead of breathless boosterism, especially for a magazine ostensibly aimed at the finance industry.
I was working on supercritical (pressurized) water. Sand has also been considered.
What changed in the economics?
That doesn't sound right. According to  that is 2/3 of total electricity generated, from all sources, in the whole of US.
The key word is "curtailment" for renewables: https://electrek.co/2017/03/27/california-solar-wind-renewab...
For wind farms, this just involves turning the blades on their axes like "feathering" a propellor. This reduces efficiency and hence power output, potentially to zero. Turbines may also need to feather if the wind is too high to avoid exceeding max speed.
All larger systems - coal, CCGT etc - have some kind of system for throttling back input of steam or gas to turbines. Nuclear power is slightly different in that it takes a long time to cool down the reactor even once you slow the reaction, and heat is still generated from intermediate decay products. So they have "bypass" systems instead for dumping steam.
All power plants have to be able to survive the loss of their upstream output load without self-destructing. In case the wires go down.
Spot prices may also go negative for short periods, but that's a different thing. https://www.cleanenergywire.org/factsheets/why-power-prices-...
Edit: I also wonder how balanced heat and cold is in such system? One side effect could be cooling solar cells to improve efficiency of them. I had an idea few months back - why solar cells don't come with antifreeze that could be used to warm up your hot water in house?
However -- for this to work as described (hot air spins a turbine), you need to get a much higher temperature differential than would be typical for data center waste heat. You'd still need to concentrate that heat somehow into a high-temperature reservoir (think hundreds of degrees Celsius) to have it be space- and energy- efficient.
Construction costs for pumped hydro storage seem to be in the range of 500$/KW.
Using electricity to create hot and cold reservoirs, then using the thermal difference to make electricity again cannot ever exceed ~40% efficiency. There are existing fully developed technologies which exceed that by far!
Where do I sign up to receive lots of money to try out a fanciful idea which goes against the basic laws of physics?
The usual efficiency calculation for a Carnot cycle tells you what fraction of heat energy that's removed from the hot reservoir can be converted to mechanical work. This is always less than 100%.
However, when running the cycle in reverse as a heat pump, it is possible to transfer more than one Joule of energy from the cold reservoir to the hot reservoir per Joule of mechanical energy used. That's why the round trip efficiency from mechanical energy back to mechanical energy can be 100%.
I have no idea what the practical efficiency of this system can be. But it's wrong to immediately reject it on theoretical grounds.
That said, due to heat capacity of pump components and conduction losses, I would be surprised if the total cycle efficiency was above 80%, putting this well below gravity and battery storage schemes. Maybe it is just incredibly cheap? Water does have a high heat capacity...
An extremely cheap solution that can store energy well might be just fine in the face of huge amounts of surplus energy.
The Federal government is subsidizing solar production to displace fossil fuel generation.
Which, unlike the headlines about 'negative prices' and being 'forced to pay other states to take their excess' sounds boring and sensible.
One of the best ways for long-term energy storage is to take something (like water) and move it to a higher spot.
We can take advantage of the decay of an atom to generate electricity, but if we want to use that energy later, we push water up a hill.
Although, depending on your local geography, you could just dig a deep hole.
Also, you can use existing deep holes which were dug for other reasons - for instance, open-cut mine pits tend to end up flooded once they're no longer mined.
Not sure why they use those balloons though, seems like a maintenance nightmare. A diving bell would work just as well and be much simpler.
It's pretty huge, able to produce more than a gigawatt for ~8 hours.
Does it make sense to store energy as heat differentials for long term? Insulation can only work for so long, eventually it will come back to environment temp.
Coal generated energy traditionally has not been very efficient, but it's cheap cheap cheap! Likewise with your home automobile: fuel is cheap enough for us to afford continuing to drive despite the "low efficiency" of an internal combustion engine.
Lithium batteries might be coming down in price, but their cost per KwH scales linearly with the battery size. You want double the capacity or halve the price? Have fun building something twice as big.
If this design can do better than linear cost scaling and bring energy storage at household electricity prices, while also using cheap materials, it's safe to say it will show up soon at a datacenter near you.
But even so, there are an awful lot of cheap energy storage schemas that are not limited by Carnot efficiency, so it's a bit baffling why anyone would try to play Maxwell's daemon with their storage facility, when they could be dumping energy into reversible chemical reactions, as one would expect inside a chemical battery. Or maybe even a flywheel system. We're getting better at materials science all the time, and we could almost certainly dump lots of energy into flywheels without exploding them.
And the article sounds a lot like they aren't just sticking a Stirling engine between a hot well and a cold well and pairing it with a motor/generator. That's about as far as I'd go in the thermal energy storage game.
You certainly deserve to receive lots of money for your scrupulous critique of this team.
Flywheels require expensive protection against catastrophic failure. The costly part isn't fragment containment, that's affordable.
But angular momentum is conserved. That means when a flywheel crashes, it will exert a very large torque on its containment structure. If the containment breaks loose, it can do a lot of damage in moments. Containment that's safe against torque failure is very expensive.
Flywheels also have to run in a vacuum. If the rotor is s composite material, there has to be an active vacuum pump because composites outgas forever. Additional system cost and failure point.
If you don't use composite rotors, the energy storage density goes down because tensile strength of metals is so low compared to composite fiber materials. Low energy density increases overall cost/MWHr.
Rotor life is limited by static fatigue and by high cycle fatigue, which have different failure mechanisms. For carbon, glass, or aramid fibers, you can count on a rotor MTTF of ~20 years if the rotor is operated so it never exceeds ~35% of the fiber's ultimate tensile strength. But that cuts into energy storage efficiency, driving the cost up more.
There are some niches that flywheels can serve with competitive economics (grid intersection phase matching, for example). But overall, it's hard to see how flywheel economics can beat the declining cost curve of electrochemistry. The foreseeable cost floor, set by some of the system issues I listed, is too high.
This mob seem to have it all worked out "http://beaconpower.com/"
Beacon Power is AFAICT the leader in deployed flywheel electrical energy storage. Their units are all underground.
One of their failures dislodged a heavy concrete and steel lid on the unit . Part of their emergency shutdown routine is a water dump into the flywheel chamber to dissipate energy. The water flashed to steam and blew a multi-ton lid partly off the unit. The spinning rotor did not escape, so all was well.
More about Beacon Power technology: .
You would think Google would recognise the benifits of owning a namespace.
Why would you search for “Google Malta” for product help when Malta is from X, not Google (searching for “x malta” gets more relevant results up front than using the wrong company name) and is a project, which has not yet produced a product (and any product may well have completely different branding.)
Plus, when it does result in a product, it'll be a utility-scale energy storage system, so I'd hope anyone needing product help isn't just doing a web search, anyway.
> You would think Google would recognise the benifits of owning a namespace.
X isn't Google, though they share the same corporate parent.
To be fair, that used to be Google, with corresponding "Google X".
Would be interesting to know how it compares to flow batteries.