
Malta – Alphabet X’s latest grid-scale energy storage technology - Osiris30
https://www.bloomberg.com/news/articles/2017-07-31/alphabet-wants-to-fix-renewable-energy-s-storage-problem-with-salt
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phasetransition
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
This 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?

~~~
phasetransition
The 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
It 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
3X 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
Ah, thanks!

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mnw21cam
Nice idea.

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.

~~~
URSpider94
You're forgetting something on the Carnot cycle efficiency, which is that the
efficiency of the storage cycle can be GREATER than unity. Heat pumps (of
which this is an example) can move more than one joule of heat per joule of
input power. I agree that the output efficiency (Joules of power : Joule of
heat) is going to be low, given the relatively small temperature differential
between the reservoirs, but the round-trip efficiency might not be that bad.

~~~
URSpider94
To correct my own comment, Joules are of course units of energy, not power...

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gumby
Molten salt is one of the standard solar reservoir techniques. In fact there
is such a system already deployed in California today in Crescent Dunes. It
burns natural gas to keep the salt molten when there isn't adequate solar
resource. There are some in operation in Spain as well.

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.

~~~
phasetransition
What is the current status of solar thermal plants in terms of commercial
readiness? Are there any to pay attention to other than Ivanpah? What thermal
storage technique were you working on? I may have some chemistry in this
thread, but energy storage is not my day job.

~~~
gumby
There Are a few in operation in Spain and some in India that I suspect will
never be operational. The economics have changed substantially just over the
last five years.

I was working on supercritical (pressurized) water. Sand has also been
considered.

~~~
phasetransition
Were you using inconel for containment? Sand would be an interesting nod to
the heating towers of blast furnaces.

What changed in the economics?

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logingone
If Carnot efficiency is so well known that numerous hn'ers mention it, how
likely is it that a team of alphabet people wouldn't be aware of it?

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kees99
> In the first half of this year, California tossed out more than 300,000
> megawatts

That doesn't sound right. According to [0] that is 2/3 of total electricity
generated, from all sources, in the whole of US.

[0]
[http://www.wolframalpha.com/input/?i=average+electricity+gen...](http://www.wolframalpha.com/input/?i=average+electricity+generation+in+us+in+megawatt)

~~~
scblock
I have to assume that they meant megawatt-hours, a unit of energy, rather than
megawatts, a unit of power.

~~~
schainks
I also think that's what they meant. Painful to read that sentence from such a
large media outlet. You'd think they'd have some people on staff to help
double check incorrect vocab usage...

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dzhiurgis
There are better alternatives than just lithium batteries - for example liquid
batteries by company with a weird name - RedFlow:
[https://www.youtube.com/watch?v=4OHstY_kKUY](https://www.youtube.com/watch?v=4OHstY_kKUY).

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?

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yoz-y
I thought they did. Well, at least I know of installations that use solar
power to heat water. Maybe there are combined panels that do both heat and
electricity?

~~~
jdmichal
Apparently those are a thing!

[https://en.wikipedia.org/wiki/Photovoltaic_thermal_hybrid_so...](https://en.wikipedia.org/wiki/Photovoltaic_thermal_hybrid_solar_collector)

~~~
tekknolagi
Fun fact, that's actually what we had in my last house. My dad wanted to heat
the pool like that, so he Googled around, found a guy with a patent, and the
guy helped him figure out how to build it for the house.

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chongli
I see a lot of objections based on Carnot's heat cycle. What about using waste
heat as the energy input? This would make the system an effectively free
source of energy. Existing data centres could be retrofitted with this
technology. Instead of dumping the waste heat outside with air conditioning,
you store it in a fluid tank.

~~~
URSpider94
It's definitely possible to combine this with some sort of "co-generation"
scheme, where either the heating or cooling are combined with e.g. energy
flowing off of a data center.

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.

~~~
planteen
Right. Just to elaborate, there is a difference between quality and quantity
of heat. A red hot nail has high quality but low quantity. A hot bath has low
quality but high quantity. Data center waste heat is like bath water.

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nerthus
Reading this, a question comes to my mind: how cheap does an energy storage
solution has to become, to finance it through speculation on energy/power
exchanges (buying in times of overproduction and selling at high consumption
times) ?

~~~
usrusr
Cheaper than fuel plus investment for peaker plants and cheaper than the
storage of your competition. (Sorry, I'm as lost for numbers as you are)

~~~
ChuckMcM
That is the mathematical answer but Energy, in California at least, is
regulated and that creates systems where we build plants we don't need. See
[http://www.latimes.com/projects/la-fi-electricity-
capacity/](http://www.latimes.com/projects/la-fi-electricity-capacity/) for
details.

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londons_explore
Whoever in GoogleX approved this investment wasn't aware of Carnot cycle
efficiency...

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?

~~~
jwmerrill
This is a misunderstanding of Carnot cycle efficiency. The thing that makes
the Carnot cycle special is that it is reversible. So in principle, you can
get 100% of the mechanical energy that you put in back out again, by running
the process in reverse.

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.

~~~
snovv_crash
Indeed. The Carnot Cycle has nothing to do with the efficiency, since it can
be reversed in what is typically referred to as a "Coefficient of
Performance", which is always bigger than 1 (assuming you aren't using
straight resistive/friction heating). I seem to recall from Thermos lecturers
that a CoP for consumer fridges is often in the 4 - 6 range, and it increases
the smaller the difference in temperature between sink and source is,
inversely to the Carnot efficiency.

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...

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basicplus2
Centrifugal storage systems would be more efficient, AND last 100 years with
only needing new bearings every 20years.

~~~
ridgeguy
I assume you mean flywheels. Flywheel energy storage seemed appealing until I
worked on it in an energy storage startup.

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.

~~~
basicplus2
In-ground installations protect against failure

This mob seem to have it all worked out
"[http://beaconpower.com/"](http://beaconpower.com/")

~~~
ridgeguy
In-ground gives additional protection, but things can still get away.

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
[1]. 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: [2].

[1] [http://eastwickpress.com/news/2011/07/a-mishap-at-the-
beacon...](http://eastwickpress.com/news/2011/07/a-mishap-at-the-beacon-power-
frequency-flywheel-plant/)

[2]
[http://www.sandia.gov/ess/docs/pr_conferences/2014/Thursday/...](http://www.sandia.gov/ess/docs/pr_conferences/2014/Thursday/Session7/02_Areseneaux_Jim_20MW_Flywheel_Energy_Storage_Plant_140918.pdf)

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OscarTheGrinch
Good luck searching for "google Malta" and getting decent product help. I just
spent the last two nights battling to find online solutions for a login
permissions error in their new "google backup and sync" product.

You would think Google would recognise the benifits of owning a namespace.

~~~
dragonwriter
> Good luck searching for "google Malta" and getting decent product help.

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.

~~~
bpicolo
> X isn't Google, though they share the same corporate parent.

To be fair, that used to be Google, with corresponding "Google X".

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cjbenedikt
Along the same line of thought: [https://www.energy-storage.news/news/german-
utility-ewe-want...](https://www.energy-storage.news/news/german-utility-ewe-
wants-to-build-giant-redox-flow-system-in-underground-ca)

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olegkikin
Any data on the energy density and the price per kWh?

Would be interesting to know how it compares to flow batteries.

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euroclydon
Hot and cold stores can be used to heat or cool a building as well as provide
hot water. So, the efficiency could be better by not converting back to
electricity.

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matt_wulfeck
This isn't really a climate change issue like the article suggest (and which
doesn't sell well commercially) but an arbitrage idea. Store energy when it's
cheap and use it when it's expensive.

~~~
0xffff2
It is a climate change issue in the sense that every green energy source we
have is intermittent (except nuclear, whose non-viability has been discussed
on HN today). If we want to run the whole world on sustainable energy sources,
large scale energy storage must be an integral part of our grid.

