With the ever-growing proliferation of renewable energy sources like wind and solar, combined with the ongoing refinement of photovoltaic and wind turbine technology, achieving the ultimate goal of round-the-clock green energy feels tantalizingly close.
But the question of how to store all that clean energy still remains a crucial factor. As engineers the world over seek the holy grail of batteries storage, they leave no stone unturned. It’s not like we can just make batteries out of thin air… or can we?
One alluring option for grid energy storage is cryogenic energy storage — basically storing air as energy!
Could this storage method pave the way for storing off-peak power to use for later? And how does it stack up to other storage options like pumped-hydro or lithium-ion battery storage?
We thought these questions deserved a deeper dive today on Two Bit Da Vinci.
We cover all kinds of energy storage technology on this channel — from EV batteries with various chemistries, to mechanical batteries like flywheels and Stirling engines. One notion that always crops up is that every technology has a tradeoff.
Pumped hydro is cheap with high energy efficiency, but a low energy density [https://youtu.be/kaCCozY8fx0?t=29 00:29 – 00:36] This means it requires a huge amount of space to store more energy — making it incompatible with certain regions.
There’s battery technology like Lithium-Ion which has high efficiency and density but requires a lot of resources, some of which are in short supply or difficult to acquire, meaning we have to make decisions about how best to allocate those resources.
The best way forward is to have a variety of options available to fit ever-changing factors like cost, size, and environmental context.
Over in the UK, Technology innovators, Highview Power has developed a proprietary cryogenic energy storage system that’s really starting to turn the heads of investors and environmentalists alike.
Now — when we think of cryogenics, you might imagine pictures of Han Solo frozen in carbonite or the myth that Walt Disney’s frozen body is stored in secret labs under Disneyland… which we can neither confirm nor deny…
But cryo storage is nowhere near as elusive… or creepy…
The company’s technology actually derives from a liquid air engine invented by engineer Peter Dearman in the early 2000s. Over the last two decades, Highview Power, together with researchers at the University of Leeds, built upon and refined Dearman’s engine to produce something that could realistically become a viable replacement for the UK’s current power infrastructure. (Hall, 2019) (Patel, 2021)
Like all energy storage systems, Highview Power’s cryo battery consists of three main processes — a charging system, a storage phase, and a discharge phase.
The charging process begins by using off-peak electricity — for instance when a wind turbine spins in the afternoon when no one is at home watching TV or warming their tea kettle. That energy powers two centrifugal integrally geared compressors — one main air compressor that pulls air out of the atmosphere, and a second recycle compressor which drives the refrigeration cycle. These compressors super-cool the surrounding air. We’re talking colder than cold. [https://youtu.be/PWgvGjAhvIw?t=209 3:29-3:32] Actually — it’s even colder than that — we’re talking -196 degrees C or -320.8 F! Cold enough for the air, which typiclly exists in a gaseous state, to become a liquid! (Hall, 2019) (Patel, 2021)
That cold, compressed air, now in liquid form, gets stored in heat-insulated, low-pressure vessels until all those Brits come back from work and start turning on all their lights and flipping on the tele. The cryogenic battery storage system takes advantage of the relationship between temperature, volume, and pressure, described by the ideal gas law.
Think of boiling water. Water unlike air, is liquid at room temperature, and if sufficient heat is applied, or pressure is dropped, say by putting water in a vacuum, it’ll turn to gas, and expand in volume. This complex relationship is how the refrigeration cycle in air conditioners and fridges work, and it’s also at the core of this energy storage technology.
When power is needed, the liquid air is heated back up to ambient temperature and drawn from the vessels, where it undergoes rapid regasification — yes that’s a real word. During this stage, the gas expands up to 700 times in volume, going from the low pressure in the tanks to incredibly high pressure — think of pressing the button on a can of compressed air you might use to clean your keyboard — only on a much larger scale. That high-pressure air is used to drive a turbine which then generates electricity. (Hall, 2019) (Patel, 2021)
The process uses clever precesses to recycle the heat and cold during the charge and discharge process. As the air is compressed, the heat that escapes gets stored — remember, there technically is no such thing as cold, only an absence of heat. So instead of letting that heat just escape into the atmosphere, the system includes a method of storing that heat while charging, then applying it using heat exchangers and an intermediate heat transfer fluid during discharge. (Hall, 2019) (Patel, 2021)
Also during discharge, the very cold air released during this process is exhausted and captured by a proprietary high-grade cold store where it can be used later to enhance the battery’s overall efficiency. More on that in a sec.
One main benefit of cryo storage over other complementary storage technology is its flexibility and scalability. Unlike pumped hydro, as we mentioned up top, which requires large amounts of space, cryo storage can be located anywhere with no size limitations or geographic constraints. It’s particularly well suited to niche applications for decentralized power. In fact, Highview plans to locate a 50MW/500MWh Liquid Air Energy Storage Facility in the Atacama Region of Chile.
But let’s talk about efficiency — how do these Cryobatteries stack up to other energy storage technology on the market today?
In terms of overall efficiency — Lithium-Ion batteries still reign supreme at between 85 and 95% efficiency. While Lithium-Ion can and is being used for grid-level storage, they tend to excel in things like computers, phones, and even electric cars. With demand for lithium and other EV battery materials soaring, it would be nice to have other options for grid storage,
Okay, so what about other forms of storage?
Right now in the US, pumped-storage hydropower accounts for about 95% of utility-scale energy storage, making it by far the most popular — and for good reason.
In terms of efficiency, it falls not far behind Lithium Ion at a range between 70 and 85% efficiency. It’s also the cheapest energy storage technology in the world in terms of cost per installed kilowatt-hour of capacity — ranging between $100 and $200 per kilowatt-hour compared to between $390 and $580 for lithium-ion batteries and around $400 per kilowatt-hour for Natural Gas.
How do Crybatteries stack up?
Right now, base efficiency is relatively low — around only 25%. But, developers have found several ways to get the most out of these systems.
A third-party review from DNV-GL, an international accredited registrar and classification society in Norway, suggests that without the use of external thermal energy, the charge-discharge cycle of cryobatteries can reach efficiencies of a minimum of 55%. Configured to receive thermal energy from an external source, say thermal exhaust from another power plant, could achieve round trip efficiencies of between 65 and 75% — which would make them far more competitive. (Patel, 2021)
By using waste heat from nearby industrial facilities, Highview has been successful in achieving up to 70% efficiency in their pilot facilities in the UK. And by efficiently recycling their own heat in from the charging station, these facilities could become even more efficient in theory.
Right now, a 10-hour, 200MW/1.2GWh cryobattery system offers a levelized cost of storage of $140/MWh. But, unlike chemical storage, with cryobatteries, the bigger the facility, the lower the cost becomes.
According to Dr. Javier Cavada, Highview’s CEO, a 50 MW, six-hour duration installation hits the “point of inflection” to get costs below Lithium-Ion. But, he says, with duration installations at 10, 12, or even 16 hours, the costs drop below Lithium-Ion significantly. (Patel, 2021)
So while the technology may trail behind other storage methods in terms of efficiency, the cost and flexibility of cryo energy storage systems may be an attractive solution, especially in places where options like pumped hydro storage may not be as feasible.
The best part about this technology is that, unlike so much of what we cover on this channel, these systems exist today. Highview has already begun construction of their first 50MW/250MWh project in the UK with a planned 4GWh pipeline of projects including some in the US and, as we mentioned above, Latin America.
I do like energy storage approaches that utilize very abundant materials, and what’s more abundant than air? Plus there is no mining or supply chains to contend with. But here’s the thing, at the end of the day, every form of energy storage comes down to efficincy. If I have 100 kwh of energy and I need to store it, how much actually gets stored? An efficiency of 90% for lithium-ion batteries means 90 kWh actually makes it to storage. Most energy storage converts energy in the charge cycle. For pumped hydro, you run big pumps to move water up into a higher resevoir. But to get that energy back, all you need to do is open a gate, and let gravity do the rest. Unfortunately for cyrogenic energy storage, this isn’t the case. Energy is converted from excess electricity into potential energy in compressed liquified gas. But to get this energy back out, the luquid gas needs to be heated, which also requires energy.
As we outlined, the key to make this sort of storage viable, will be in the synergy of housing alongside other industrial processes. Anything that produces excess waste heat, think anything with a heat pump, or smelters, could make good sites for such a technology. The more waste that can be harnessed, the less extra energy is needed for this discharge process. Even then, this technology will never be as efficient as lithium ion batteries. But if we could get closeto 60-70%, that might actually be good enough. Especially as producing wind and solar energy gets cheaper and cheaper. Like everything in engineering, you can draw a spectrum of tradeoffs, and try to find the ideal solution. More exotic materials, complex supply chains and more efficiency, or utter abundance and lower efficiency. But with research into this sort of technology, at least we’ll have every possible option available to us in the coming years.
But let us know what you think. What are some regions or contexts that could benefit from something like a Cryo Battery? Does the lower cost make up for the sub-par efficiency? Sound off in the comments below.
References
- Colthorpe, A. (2021, February 9). Cryogenic long-duration storage company Highview Power raises US$70m to take on baseload fossil fuel. Energy Storage News. Retrieved November 11, 2021, from https://www.energy-storage.news/cryogenic-long-duration-storage-company-highview-power-raises-us70m-to-take-on-baseload-fossil-fuel/
- Hall, M. (2019, December 19). Highview Power plans 50 MW/400 MWh-plus cryo energy storage plant in Vermont. PV Magazine.Com. https://www.pv-magazine.com/2019/12/19/highview-power-plans-50-mw-400-mwh-plus-cryo-energy-storage-plant-in-vermont/
- Hogberg, T., & Tholander, M. (2018). Evaluation of Liquid Air as an Alternative Storage Alternative. EXAMENSARBETE INOM TEKNIK, GRUNDNIVÅ,. http://www.diva-portal.org/smash/get/diva2:1216213/FULLTEXT01.pdf
- Patel, S. (2021, June 1). Market Prospects Heating Up for Cryogenic Energy Storage. Power Magazine.Com. https://www.powermag.com/market-prospects-heating-up-for-cryogenic-energy-storage/
- Spector, J. (2019, October 21). Highview Power to Develop First Major Cryogenic Storage Plant in UK. Green Tech Media. Retrieved November 11, 2021, from https://www.greentechmedia.com/articles/read/highview-power-is-developing-first-major-cryo-storage-plant-250mwh-in-uk
- https://www.rechargenews.com/transition/gwh-scale-liquid-air-battery-offers-storage-at-half-cost-of-lithium/2-1-629164
- https://www.power-technology.com/features/feature-reliable-renewables-cryogenic-energy-storage/
- http://www.wvic.com/content/facts_about_hydropower.cfm
- https://www.greentechmedia.com/articles/read/pumped-hydro-moves-to-retain-storage-market-leadership#:~:text=Pumped%20hydro%20is%20already%20the,batteries%2C%20World%20Bank%20figures%20show.
- https://www.eesi.org/papers/view/energy-storage-2019