

Electricity stored as temperature difference - simon_ks
http://www.windpowerengineering.com/featured/business-news-projects/didnt-think-electricity-stored-temperature-difference/

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aaron695
Europe has already looked at industrial freezers as batteries.

Take them to -40 when electricity is cheap let them get to -20 during peak.

[http://spectrum.ieee.org/energy/the-smarter-grid/swiss-
wareh...](http://spectrum.ieee.org/energy/the-smarter-grid/swiss-warehouse-
helps-buffer-the-grid)

~~~
majormajor
They call it a "virtual battery" but that seems misleading to me - I don't see
any way to get electricity back out, it sounds more like it just saves
electricity consumed at peak periods by using, but they couldn't store more
than they could use in the first place?

~~~
dredmorbius
One element of using intermittent or variable renewable energy is to take the
approach of dispatching demand to meet your available capacity rather than
dispatching capacity to meet demand.

Presently, electrical generation relies on base-load power -- plants which run
very nearly all the time, typically coal or nuclear -- with "peaking" and
matching capabilities -- generation which can be rapidly added or removed,
with response times typically measured in minutes (grids can change fairly
rapidly, but not _that_ rapidly, and much demand is highly predictable based
on weather, human behavior, and other patterns).

Some large power loads can effectively be banked, and heating and cooling
loads are key among these. By providing a large thermal storage mass (either
hot or cold), it's possible to "bank" energy when there's a surplus, or when
generation is low-cost, and draw down that bank when energy's scarce. Other
alternatives are running energy-intensive processes which can be cycled either
rapidly or predictably as needed. Aluminium smelting (very electrically
intensive), hydrogen electrolysis, and other industrial processes are
candidates for this. It's also effectively how pumped-storage hydro works.

You may not be getting back electricity directly, but by _offsetting_ a large
cooling load, you're shifting demand.

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mikeyouse
This is essentially the same system as Light Sail Energy:

[http://www.lightsail.com/](http://www.lightsail.com/)

Started by HN's own DaniFong
([https://news.ycombinator.com/user?id=DaniFong](https://news.ycombinator.com/user?id=DaniFong)).

~~~
dredmorbius
Yes in that it's utilizing compressed gas for energy storage, but no in that
in how they're addressing the issue of adibiatic heating and cooling of gas --
the fact that gasses heat when compressed and cool when expanded -- they're
actually using this heat itself as the energy storage mechanism, with argon as
the working fluid.

Light Sail's approach harvests the provided heat and cooling, but utilizes
those for what is effectively a coordinated heating/cooling service. The heat
itself isn't part of the energy storage/recovery process.

Both are interesting approaches to the heating/cooling problem.

If you _don 't_ allow for the heating and cooling, you run the risk of
explosions or of excessive thermal loss from your compressed gas. Explosions
are possible if you're storing large volumes of gas in underground former
natural gas reservoirs -- injecting large volumes of high-temperature N2 + O2
atmospheric gas risks igniting the residual methane. You also have the problem
of icing over your recovery apparatus. As a consequence many CAES (compressed
air energy storage) designs incorporate a natural gas burner on the exhaust
side which both heats the expanding gas and adds additional energy to the
process. The resulting systems are a hybrid of natural gas generation plus
energy storage.

~~~
lotsofmangos
The heat is certainly part of the energy storage in lightsail, if the heat
generated by the compression wasn't being stored in water and then fed back in
during expansion then that energy would be lost.

Is just that with lightsail it is a minor part of the storage, the majority
being pressure. They are two very different solutions that approach two
similar but differently structured thermodynamic problems, lightsail's system
being high pressure/low temperature, and these folk doing a low pressure/high
temperature store.

------
pmalynin
Thats actually an impressive efficiency as the maximum theoretical efficiency
of this "carnot engine" is 85% at these temperatures.

~~~
zackmorris
For anyone curious, Carnot efficiency is the maximum efficiency of extracting
work from a temperature differential:

Carnot efficiency = 1 - Tc/Th = (Th - Tc)/Th <\- I find the last form easier
to remember

Where temperatures are in Kelvin. So in this case 500 C and -160 C are 773 K
and 113 K so:

Carnot efficiency = (773 - 113)/773 = 85.4%

It’s a handy approximation for the most efficiency that could be expected from
other cycles. So for example an internal combustion (Otto cycle) engine
running below the temperature of boiling water would have an expected
efficiency at room temperature of about 68 F or 20 C or 293 K of:

Carnot efficiency = (373 - 293)/373 = 21.5%

Whereas jet (Brayton cycle) engines may have a temperature differential of
1000 C:

Carnot efficiency = (1293 - 293)/1293 = 77.3%

Things are a bit more complicated than this because with active cooling it’s
not just the temperature of the engine’s components, but the temperature of
the exhaust gasses, efficiencies of valves, compressors, turbines, etc. So
modern internal combustion engines may reach 25% efficiency and turbines may
reach 45% efficiency but I still find the Carnot cycle good for guestimation.

So for example, I remember research in the 90s for making ceramic internal
combustion engines lubricated with graphite or exhaust that would run at a
higher temperature and have an efficiency closer to jet engines. There were
also working Stirling engine cars that would have gotten significantly better
mileage because it’s more practical to approach the Carnot limit with the
Stirling cycle than the Otto cycle:

[https://www.youtube.com/watch?v=H_Vnxapd5fs](https://www.youtube.com/watch?v=H_Vnxapd5fs)

The main tradeoff is that there hasn’t been as much research in high
compression Stirling engines so they tend to have a higher volume than
internal combustion engines at the same power output. But since Stirling
engines have significantly fewer moving parts and use external combustion
(meaning they can run on any fuel), I could never quite figure out why they
were never mass produced. Perhaps if they had been, we would have seen
industrial sized Stirling engines with Argon as the working fluid decades ago.

Then again, before the web and Wikipedia it would have been hard to make these
kinds of points at a Thanksgiving dinner table.

------
padobson
Ok, so this makes a lot of sense for renewable sources of energy like wind and
solar - production facilities that are distant from bodies of water can store
surplus energy during peak production times (windy or sunny days) and then use
that stored surplus when production is stagnant (calm days or at night).

The current practice is to use surplus energy to pump water into a reservoir
during peak production, and then converting it to hydro-electric power later
when production is stagnant. The Isentropic advantage is that you don't need a
reservoir to store the energy, the storage system can be build regardless of
geography.

I have two questions:

1\. What kind of insulation does it take to keep the gravel hot or cold enough
to store the energy for a long time? Is the energy lost due to natural
thermodynamic processes comparable to energy lost due to evaporation in a
pumped-hydro storage system?

2\. If the storage system is geographically independent, could it be moved in
an energy efficient manner? The problem with almost every energy source we
have right now is that it has to be close to the population it serves. On the
other hand, if we could generate everyone's electricy as solar energy in
Nevada or nuclear energy hundreds of miles from populations, and then move it
to distribution plants, then we could answer a lot of energy questions.

~~~
ghshephard
re: #2 - the challenge with power systems and their locality to the population
is the cost of Transmission System. The closer they are, the less real-estate
you have to dedicate to massive transmission towers, as well as less lost
energy due to heat in the transmission lines.

Placing the Isentropic center near a population center wouldn't give you any
advantage there.

~~~
jonsen
I read 2. as about moving the storage unit itself. Charge the unit at on place
then move the unit and generate power at another place.

~~~
padobson
Yes, this was what I meant.

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exch
As a completely non-technically-inclined person, can someone explain how this
is intended to yield electricity as output?

From what little I do understand, the enormous surface area of the crushed
gravel acts as a very efficient heat transfer mechanism, so it can cool(or
heat) the Argon, depending on which chamber one is looking at.

The pistons are there to cycle the Argon between high and low pressure..
Presumably to keep the gas flow going. But can't this be done with just the
pistons and without the pressure gradient? (edit: nvm this part; the heat
energy is obviously generated by this pressure gradient).

Then comes the time to get the heat energy out for actual use. The article
mentions that the gas flow can be reversed. So presumably the gravel that was
previously being cooled, will now be heated. Isn't this just exchanging the
heat between the gravel and gas? How does this yield net energy output?

~~~
al_bundling
Isentropic PHES Technology Explained [https://youtube.com/watch?v=sIxt6nMf-
IQ](https://youtube.com/watch?v=sIxt6nMf-IQ)

~~~
exch
Thank you! It seems my confusion arose from assuming this is a power
generator, while it is a storage device.

------
pkulak
Sounds like the molton salt systems that are already deployed. I assume this
is better?

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marcosdumay
If that pumped gravel heat exchanger works that well, why don't they create
phase change reservoirs and save most of their capital costs?

~~~
lotsofmangos
Phase change systems like steam engines follow the Rankine cycle, which cannot
reach as high an efficiency as systems like this which follow the Carnot
cycle.

~~~
marcosdumay
Not a phase change heat engine, but a phase change heat reservoir. Yes,
they'll lose some efficiency, but if they can exchange heat as well as it's
implied (from their efficiency figures), they may lose very little.

------
jjoonathan
> a per hour storage cost of $103/kWh

Ouch!

~~~
mikeyouse
I think the confusion is that the $103/kWh is the _capital cost_ to build the
storage rather than the _operating cost_ to actually recover a kWh of energy.
So to provide 10kW for 10 hours (for instance, overnight storage for a large
house), they'd need a 100kWh system at a cost of ~$10,000, where they're
claiming that a pumped hydro system would cost 20% more. The benefit of course
to pumped hydro is that you can have hundreds or thousands of MW in storage.

These units would be consistent with other reporting for grid-scale battery
systems costing ~$250/kWh to construct:

[http://cleantechnica.com/2013/12/18/utility-scale-battery-
st...](http://cleantechnica.com/2013/12/18/utility-scale-battery-storage-
costs-dropping/)

As for operating costs, I'm assuming that the nameplate storage capacity is
the output. So if you have a 100kWh system with ~60% throughput efficiency,
you'd need 160kWh of input at your standard energy rates and then the energy
coming out would be 'free' \-- of course there'd be costs for maintenance and
depreciation too though.

~~~
gonzo
> So if you have a 100kWh system with ~60% throughput efficiency, you'd need
> 160kWh of input at your standard energy rates and then the energy coming out
> would be 'free'

I think the idea is that this is storage for renewables. As such, you wouldn't
need more "input at your standard energy rates" than what it might take to
charge the system for overnight use.

~~~
mikeyouse
Right, I guess I was getting at the 'cost' of energy would be he opportunity
cost of not selling it back to the grid which is typically the retail price in
areas with net metering. Although in off-the-grid situations, you're right
that there wouldn't be any incremental cost to charging the storage.

