As you know, I talk a lot on this channel about energy storage, especially the utility
scale energy storage that we'll need if we're going to reach the goal of 100% renewables on
our electricity grids. The predominant energy storage medium right now is of course lithium-ion
batteries, and I should probably take this opportunity to clarify the comments I made
about them in a recent video when I stated that, at utility scale, they're only good for
a discharge time of about four hours. Several viewers, quite reasonably, asked why that was.
Surely you can simply make the battery bigger or use more batteries to get a longer discharge time?
And yes you can in theory. But battery technology is always a bit of a trade-off
between various criteria like the amount of energy it can hold versus how quickly it can release that
energy or how long the battery lasts versus how safe it is to operate and of course how cheap it
is to produce. If you optimize your battery to hold more energy you tend to limit how quickly
that energy can be released and if you optimize for safety you tend to limit energy density.
Lithium ion batteries are brilliant at releasing energy instantaneously and that makes them great
for frequency regulation, which is the mechanism that prevents shortages or oversupply when spikes
or troughs occur in demand. That's where they're most cost effective. And that grid stabilization
cost curve generally works okay for time periods of between four and six hours. But for longer
duration storage providing predictable base load style electricity overnight or over a few days
where you don't really need instantaneous response times,
the economics of lithium-ion just don't stack up so well. There's no physical reason why it can't
be done, it's just prohibitively expensive. That's why so much development work is being conducted
on modular energy storage systems that use cheap abundant materials and predominantly off-the-shelf
components. They don't tend to have the instant discharge time or energy density of lithium-ion
but they can provide very long duration discharge times at low cost.
Often with very high cycle lifetimes and with extremely good levels of operational safety.
That's precisely the kind of combination that's been developed over the last few years
by the godfather of wind turbine technology Henrik Stiesdel and his team at Stiesdal storage
technologies in Denmark. Now they're ready to show the world what their technology can do,
and they've chosen a very apposite name for their energy storage system : Gridscale
Hello and welcome to Just Have a Think. Henrik Stiesdal is a Danish innovator
with more than 175 inventions and over a thousand patents to his name, mostly in
wind power technology. The most significant of those was the so-called Danish concept for wind
turbines that he launched in 1978 and which became the industry standard dominating the global wind
industry during the following decades. He also installed the world's first offshore wind farm
in 1991 and spent many years working as chief technical officer for Siemens wind power. So I
think we can confidently say he knows his stuff. Stiesdal Storage Technologies, or SST, was founded
several years ago to focus on what they call high impact solutions to climate change. The Gridscale
energy storage system is just one of those solutions, and I recently had the opportunity
to chat via Zoom with the Project Manager Bo Birkemose, to learn more about how it all works.
Bo explained that there are two processes in the system. Unsurprisingly the first process charges
the system up and the second process discharges it. The essential principle is the movement of
heat energy stored in simple crushed rocks via the movement of air from one large storage cylinder
to another and back again, with a method of intercepting the energy at some point as an output
that can do some useful work. So let's jump into the system at the start of the charging process.
What we've got here is two storage cylinders containing crushed rocks - usually basalt, but
other materials could be used depending on what's available in any given geographical location.
At this stage of the process the air and crushed rocks in the first cylinder are at a temperature
of 385 degrees Celsius and the air and crushed rocks in the second cylinder are at a temperature
of 75 degrees Celsius. The hot air comes out of the top of the first cylinder and goes through a
compressor which super heats it all the way up to 600 degrees Celsius. That superheated air is then
pumped into the second storage cylinder where it transfers its heat energy into the crushed rocks.
At the same time the cooler 75 degrees Celsius air is being drawn off from the bottom
of the second cylinder and run through a heat exchanger to reduce its temperature to about
25 degrees Celsius. That air then goes through a turbo expander which drops the temperature
right down to -30 degrees Celsius. Then that very cold air is fed into the bottom of the
first storage cylinder where it replaces the hotter air that's being pulled out of the top.
That push and pull of air in both cylinders produces very steep temperature gradients
between hot and cold that ensure energy losses are minimized in the system.
Once the process is complete we've got a fully charged hot storage cylinder full of
crushed rocks storing heat energy at 600 degrees Celsius and a cold storage cylinder sitting at
-30 degrees Celsius. Internal insulation ensures that the temperature of the rocks is maintained
with very minimal thermal transfer. In fact even when the rocks inside are at 600 degrees Celsius
you could place your hand on the outside wall of the cylinder and you'd find it was at more or less
the same temperature as the surrounding ambient air. To discharge the system air is pumped out
of the hot cylinder and passed through another turbo expander. That works a bit like a modern
hybrid turbocharger in a car engine that converts the hot exhaust gases into mechanical rotation
of a generator to produce electrical energy. That energy transfer reduces the heat of the
air from 600 degrees Celsius down to 385 degrees Celsius, at which point it gets sent into the top
of the cold storage cylinder. At the same time the minus 30 degrees air is being pulled out
of the base of the cold cylinder producing that steep temperature gradient I mentioned earlier.
The cold air goes through another compressor which is mechanically driven by the turbo expander.
The compressor heats the air up to 75 degrees Celsius and pushes it into the bottom of the
hot storage cylinder as the very hot air is coming out of the top, again maintaining the
steep temperature gradient that the system needs to optimize efficiency. So when the system is
fully discharged we're back to our original state with the first cylinder containing air and rocks
at 385 degrees Celsius and the second cylinder containing air and rocks at 75 degrees Celsius.
Bo told me that one of the most important developments they've been working on for some
years is in producing an internal insulation method that can flex with the expansion and
contraction of the rocks as they get heated and cooled. If you didn't have that flexibility then
as the rocks cooled and contracted they'd settle towards the bottom of the tank and then as they
expanded with heat they'd cause a very unwelcome deformation of the cylinder walls. The method also
had to be a solution that could be industrialized and installed in any location around the world.
The exact description of that method is proprietary information so I can't tell you
what it is, but it's one of the main elements that makes the storage modules of this system viable.
The motor that drives the system will be powered by renewables like wind and solar
and the overall round trip efficiency of the system is targeted at 60 percent. Gridscale
are aiming to cover both the 12 to 18 hour discharge duration and the three to seven day
discharge duration depending on the number of storage tanks in any given configuration.
That modularity is what keeps the cost right down compared to lithium-ion batteries
because the storage tanks are the least expensive component in the system. That means that the
higher the capacity of the system the more cost effective it becomes. So you won't be seeing these
things in your backyard or basement folks, they're very much a utility scale energy storage solution.
It's also important to point out here the Gridscale are not attempting to compete with
lithium-ion batteries in the four to six hour discharge period. Lithium-ion batteries already
do a brilliant job in that space and their instant response time is crucial for coping with spikes in
energy demand on the grid. So the two systems will sit very happily alongside each other with lithium
ion kicking in instantaneously and allowing the grid scale system a few minutes to get going.
Then once the grid scale system is at full speed it can provide that longer duration
steady discharge which provides energy for the grid and also recharges the lithium-ion batteries.
A very neat solution! The system will be rated and certified for more than ten thousand
cycles but in theory there's no reason why it couldn't keep going for much longer than that.
In Autumn 2021 Stiesdal will commence installation work on their first operational demo plant at a
place called Rodby on an island called Lolland in the Baltic sea where their system will be
charged up using surplus power from the island's wind and solar farms. As early as 2006 Lolland
was already producing 50% more power from wind than it needed, and today it's producing twice
as much as its residents can use. But without a high cost investment in power infrastructure
to export it to the mainland, much of the produced power goes to waste. And that's where the
Gridscale installation comes in. The Rodby site is also right next to a district heating plant
so there's a possibility to connect the expelled heat from the charging process of the grid scale
system into that district heating network bringing even greater efficiencies to the overall setup.
The site is scheduled for completion in the first quarter of 2022.
Operations will commence immediately after that and test results will be available for assessment
by the middle of the year. As any regular viewer of this channel will know,
Stiesdal are entering an already very competitive market. We've looked at several long duration
storage solutions over recent months and there are more coming to market all the time. That level of
competition is of course a very good thing, not only for end consumers like you and me who will
get the benefit of less expensive electricity, but more importantly it's great for accelerating the
move away from fossil fuels and towards national grids powered by 100% renewable technologies.
If you've got views on these types of energy storage systems or if you work in the industry
and have direct experience of them then why not jump down to the comment section below and leave
your thoughts there. And if you want to learn more about the Gridscale system itself then you can
follow the link up there somewhere to jump over to the Engineering With Rosie channel to watch
Rosie's site visit to Stiesdal's test facility. That's it for this week though. As always a
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