Long-duration storage can help clean up the electricity grid, but only if it's super cheap

Here at Volts, I recently spent a week … OK, a month writing
about batteries, which store energy for electronic devices,
electric vehicles, and, at least for short periods of time (four
to six hours), the power grid.


Lithium-ion batteries are extremely good at those tasks — and
they’re getting better, and cheaper, all the time.


But here’s the thing: a net-zero-carbon grid is going to need
storage that lasts a lot longer than six hours. It’s going to
need durations of up to 100, 300, 500 hours or more, and it’s
going to need them cheap. Lithium-ion batteries just aren’t going
to work for that.


What will work? Good question! No one really knows yet.


Whatever it is will require substantial research, development,
and scaling, and possibly some good geographical luck. We can’t
know yet what technology or technologies might win that race, but
we do have a good sense of what they need to accomplish, thanks
to some new research in Nature Energy from a team at MIT (along
with Jesse Jenkins, who used to be at MIT but is now at
Princeton).


Their findings on long-duration energy storage (LDES) are
daunting and somewhat deflationary. In a nutshell: LDES needs to
get extremely cheap before it will play a substantial role in a
clean grid — cheaper than almost any candidate technology today,
and cheaper than any geographically unconstrained technology is
likely to get any time soon.


Let’s start with some background on the need for LDES.


Renewables on the grid need firming


The cheapest large-scale renewable energy sources are wind and
solar, but wind and solar are variable. They come and go with the
sun and the wind, and as you may have heard, the sun is not
always shining and the wind is not always blowing. We cannot turn
them on or off, up or down.


The supply of wind and solar energy will not always match the
“demand curve,” i.e., the level of electricity demand throughout
the day. As more and more wind and solar are added to the grid,
there’s more and more need for flexible resources that can fill
the gaps when supply doesn’t match demand.


Today, those gaps are overwhelmingly filled by natural gas power
plants, which are 100 percent “firm” in that they can be turned
on at will and run as long as necessary. As the grid is
decarbonized, however, fossil fuel plants (at least those without
carbon capture) will be phased out of the electricity system and
wind and solar penetration will increase. As that happens, other
resources will be needed to firm the system.


There are four basic options.


* Transmission: connecting larger geographical
areas raises the chances that sun or wind will be available
somewhere within them.


* Low-carbon (“clean”) firm generation: the MIT
team’s paper lists “nuclear, fossil fuels with carbon capture and
storage (CCS), bioenergy, geothermal, or hydrogen and other fuels
produced from low-carbon processes.” (The first three items are
anathema to some climate activists, but they may end up being
necessary.)


* Negative emissions technologies (NETs):
technologies that permanently bury carbon dioxide can offset
emissions from firm fossil fuel plants.


* Long-duration energy storage (LDES):
technologies that can store enough energy, for long enough, to
displace firm generation.


(Note: natural gas power plants are much cheaper than any of
these options, save perhaps some transmission. There will be no
market or pressure for any of them unless there are policies that
require reduced greenhouse gases.)


NETs are likely to stay expensive and transmission can only do so
much, so the real fight is likely to be between clean firm
generation and LDES.


The new research is an extremely detailed modeling look at the
role LDES might play in a decarbonized energy grid — how much
clean firm generation it might displace and how much it might
reduce energy system costs.


The LDES “design space”


The researchers took an interesting and (to me at least) somewhat
novel approach. The problem with trying to study LDES is that a
bunch of incredibly heterogenous technologies are claiming that
mantle, with different mechanisms and different performance
characteristics, at different levels of development and
commercialization, competing for different market niches. It can
be difficult to compare them or to say anything meaningful about
them as a class.


So instead of focusing on particular technologies, the
researchers modeled different combinations of performance
characteristics. Specifically, they used a high-resolution model
to represent five separate LDES technology parameters:


1) energy storage capacity cost (using a bathtub as an analogy,
think of the cost of increasing the size of the tub); 2) charge
power capacity cost (cost of enlarging the faucet); 3) discharge
power capacity cost (cost of enlarging the drain); 4) charge
efficiency (how much water is lost when filling the tub), and 5)
discharge efficiency (how much water is lost when draining the
tub).


Here’s how they went about it:


We modelled a total of 1,280 discrete combinations of these cost
and efficiency parameters encompassing performance levels that
are consistent with projections for existing LDES technologies
found in academic peer-reviewed studies as well as domains that
are currently infeasible but that could be the focus of
technology development efforts in the future. Furthermore, we
evaluated the technology design space for LDES in multiple power
system contexts encompassing different wind, solar and demand
characteristics and different assumptions regarding the
availability of firm low-carbon technologies.


They present the results in two basic contexts: a Northern Grid
representing average New England conditions and a Southern Grid
representing Texas.


The methodology was complex but the goal was simple: to determine
which LDES performance characteristics would do the most to
displace clean firm generation and reduce system costs for a
net-zero-carbon grid.


First we’ll look at which parameters made the biggest difference
and the aggregate impact they could have. Then we’ll take a quick
tour through current LDES techs to see which might make the cut.


What LDES needs to be able to do


The basic question is how much clean firm generation LDES can
displace in a model optimizing for total system costs. Here are a
few basic findings:


1. Of the five modeled technology parameters,
the two that enable LDES to have the biggest impact are energy
storage capacity costs (the cost of increasing the size of the
tub) and discharge efficiency (how much water is lost draining
the tub). The other three parameters don’t matter nearly as much.


A good benchmark for energy storage capacity costs are
lithium-ion batteries (LIBs). Last year, BNEF’s annual battery
price report had the average capacity costs of LIB battery packs
at $137 per kilowatt-hour (down 89 percent from 2010), projected
to hit $100 by 2023. Dan Steingart, a materials scientist and
co-director of Columbia University’s Electrochemical Energy
Center, told me he thinks LIBs are eventually going to get down
“somewhere between $45 and $60 per kilowatt-hour.” (To be clear:
that’s extremely aggressive and optimistic.)


2. That gives some context to the next finding:
not until it hits $50/kWh will LDES even begin to see meaningful
deployment or declining costs. Not until $20/kWh will they
reduce system costs by 10 percent. And “to deliver more
significant savings in electricity costs (>10%), storage
technologies must exhibit costs in the $1-10/kWh range and
discharge efficiencies greater than 60%.”


That pretty much rules out LIBs. It also, as we’ll see later,
rules out quite a few of the technologies currently claiming the
mantle of LDES.


3. What’s more, across the full design space
modeled, the very best LDES could hope for is to reduce system
costs 50 percent; the best existing LDES techs could do is 40
percent. And in a system with more clean firm options available,
the max is 20 to 30 percent.


4. What’s even more, not until LDES capacity
costs get down to $10/kWh could it displace all firm generation —
if firm generation includes only nuclear. If it includes other
clean firm generation with lower capital costs and higher
operating costs (like natural gas with CCS or hydrogen combustion
turbines), then LDES would have to get down to $1/kWh to displace
it all.


5. What’s even more more, the modeled cases
where LDES displaces the most clean firm generation involve
storage durations of 100 hours or more, up as high as 650 hours
(concentrated in 100 to 400 hours).


6. As No. 4 suggests, aside from energy capacity
costs, the single factor that most influences LDES’ ultimate
deployment has nothing to do with LDES tech at all — it’s the
cost and availability of other clean firm options. If only or
mainly nuclear is available, LDES has a much better shot. If
other forms mature and get cheaper and more widely available,
LDES will have a harder time.


7. LDES has a harder time on the Northern Grid,
and an even harder time on the Northern Grid under scenarios of
high electrification. (Electrification in the cold northern
winters means a huge winter demand peak, which renewables
struggle to meet.)


So, what have we learned? LDES needs to store huge amounts of
energy for cheap. Though it will run only intermittently, it
needs to discharge energy efficiently. And it needs to be capable
of durations of well over 100 hours. Even if it meets those
requirements, it will likely displace less than half the clean
firm generation required to run a clean grid.


That said, reducing systems costs by even 10 percent represents
billions of dollars in savings, so it’s nothing to sniff at.


Let’s take a quick tour through the technologies competing in the
LDES space.


The varieties of LDES and their chances of success


Energy wonks are aware of the need for more and better options in
the LDES space. ARPA-E has a DAYS (Duration Addition to
electricitY Storage) program intended to spur development of
energy storage technologies capable of anywhere from 10 to
approximately 100 hours duration.


But this new research reveals that LDES will need to run much
longer than that, and for dirt cheap. Can any of today’s
technologies measure up? Let’s run through the four broad
categories of competitors.


Electrochemical storage


There are two big electrochemical contenders. The first is flow
batteries, which I wrote about in a previous post. They have the
advantage of being able to scale energy storage and power
capacity separately, which theoretically opens the door to very
high energy capacity for fairly cheap.


Unfortunately, the most common varieties of flow batteries
(vanadium redox and zinc bromine) have energy storage capacity
costs in the hundreds of dollars per kWh, which means they
probably won’t cut it as LDES. (As I wrote, flow batteries are
aiming for that awkward mid-duration storage space and getting
squeezed on both sides.)


Another electrochemical option (an alternative flow battery tech)
is liquid metal batteries, which I also wrote about previously. A
company called Ambri is building a 250 MWh demonstration project
with liquid metal batteries in Reno, Nevada. In this paper,
researchers demonstrated an “air-breathing aqueous sulfur flow
battery” with $10-$20/kWh energy capacity costs at 100+ hours
duration. That wouldn’t entirely displace firm generation, but it
could theoretically get to 10 or 20 percent system cost
reductions, which is no small thing.


Chemical storage


Chemical candidates mainly include hydrogen and hydrogen-derived
fuels like ammonia and syngas (e.g., synthetic methane). This is
something of an odd category, since the result of all these
processes is a fuel, which operates just like natural gas, as
firm generation. You can see chemical storage as a direct
substitute for, or a form of, clean firm generation.


The cheapest forms of chemical storage rely on specific
geological features for storage, like compressed hydrogen in
caverns and porous rock formations. Energy capacity costs range
from $1 to $5/kWh for hard rock caverns all the way down to
around $0.5/kWh for some depleted gas or oil fields.


The problem with chemical storage is that, while energy capacity
costs are low, there’s lots of infrastructure and conversion
processes involved in making hydrogen, storing it, capturing CO2,
combining hydrogen and CO2, and then burning the resulting fuel
in combustion turbines. This gives chemical storage high power
capacity costs and low round-trip efficiency. Lots of work needs
to be done to bring down costs, particularly of electrolysis and
fuel cells, for making and burning hydrogen respectively.


Mechanical storage


The oldest and still most common form of large-scale energy
storage is pumped hydroelectric energy storage (PHES), whereby
water is pumped from a lower reservoir to a higher one and then
run back down through turbines to recapture the energy. (PHES
accounts for about 99 percent of the US grid energy storage
market.)


Most PHES installations today are built for diurnal cycling
(every six to 24 hours) and have energy capacity costs in the
hundreds of dollars per kWh, which makes them unsuitable for
LDES.


Some PHES projects with particularly large reservoirs can get
over 100 hours of duration at energy capacity costs in the $20 to
$30/kWh range, which combined with their relatively high
round-trip efficiency means they can probably eat into some clean
firm generation at the margins. But those sites are even more
geographically limited than PHES generally.


The other viable mechanical LDES technology is compressed air
energy storage (CAES), which is just what it sounds like — use
energy to compress air, then use the pressure of the compressed
air to run a turbine and generate electricity. In the best
locations, with access to large saline aquifers, CAES can get
down in the $1/kWh range with hundreds of hours of capacity;
costs rise in less ideal sites (e.g. salt caverns). Like PHES,
CAES is geographically limited (and might compete in some sites
with compressed hydrogen storage).


There are many other forms of mechanical energy storage out
there, everything from pushing rocks uphill on a train to lifting
rocks with a crane to spinning a flywheel, but none of them yet
have energy capacity costs low enough to make them eligible as
true LDES.


Thermal storage


There are numerous ways to store electricity as heat. The most
familiar is concentrated solar power using molten salt, but
capacity costs for that technology remain prohibitively high and
round-trip efficiency prohibitively low for it to serve as LDES.


There are also (less developed) proposals to store heat in
ceramic “firebricks” for electricity or heat applications, with
energy capacity costs potentially as low as $5 to $10/kWh and
round-trip efficiency over 50 percent. A process called pumped
thermal energy storage using reciprocating heat pumps also shows
great promise. Thermal storage remains an intriguing area of
study, in part due to its usefulness as both direct heat and
electricity.


Here’s a roundup from the MIT team’s paper of all the LDES
technologies and their projected future costs:


The ultimate potential of LDES


As I said, the research is not explicitly framed this way, but
the results strike me as fairly deflationary toward LDES. The low
costs and long durations required rule out several existing
technologies and set incredibly ambitious targets for others. And
even if LDES meets those targets, it won’t erase the need for
clean firm generation — at best it will take a good chunk out of
it and reduce overall system costs a few dozen percent. That’s
not nothing, but it’s not a silver bullet either.


Here’s a visual from a seminar Jenkins gave at Stanford:


Just a few things to note about this graph. The colored boxes are
different LDES technologies — the solid lines indicate
geographically unconstrained options and dotted lines indicate
those that are geographically constrained. You’ll notice that all
the options in the $1/kWh box have dotted lines, and aside from
CAES, they’re all chemical. There are currently no geographically
unconstrained LDES technologies that have even the potential to
knock out clean firm generation.


Labels identify a few of the technologies. CAES and hydrogen have
huge potential; some thermal technologies and metal-air batteries
are intriguing; PHES and reciprocating heat pumps can trim a
little off the top.


In reality, there will likely be a mix of LDES technologies in
the market, targeting various performance or geographical niches.
If I were a betting man — which I emphatically am not — my money
would be on hydrogen over the mid- to long-term. It has an
advantage similar to LIBs’ advantage in the short-duration
market: it can draft off of other, bigger markets.


LIBs for energy storage can draft off of the much larger
electric-vehicle market, which is driving their costs down.
Similarly, hydrogen and hydrogen-derived fuels for energy storage
can draft off of much larger hydrogen markets: industrial
applications, possibly airplane or shipping fuel, possibly mixing
with natural gas in existing pipelines. Relative to those
markets, the market for hydrogen as LDES is likely to be
marginal, but it will benefit from the cost reductions driven by
scale in those other markets.


Thermal storage might also benefit from being of use in both heat
or electricity applications. As I’ve written before, there’s a
huge potential market for clean heat.


The practical implication of this modeling is that research and
development in LDES needs to focus primarily on driving energy
capacity costs as low as possible, while targeting durations of
100 hours or more (beyond the 10 to 100 hours the DAYS program
targets) and discharge efficiencies of at least 60 percent.


It’s a tough set of aspirations and there’s no guarantee any
technology will get there any time soon, so another practical
implication of the research is that we need to start thinking
hard about where to find lots of clean firm generation. We
probably can’t store enough energy to get around it.


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