r/Energy on Grid Scale Storage
Marie-Curie PhD Fellow Gaël weighs in on hydrogen vs flow vs hydro vs gravity vs metal-air vs...
Gaël Mourouga gives us a peek into his first thesis chapter, on the overview of next-generation technologies for grid-scale storage. He did his Master's thesis on Lithium-Sulfur batteries at the University of Cambridge, before joining ETH Zürich for a Marie Curie PhD on modelling and simulation for organic redox-flow batteries!
An overview of next-generation technologies for grid-scale storage
Comparing innovative technologies is difficult.
First, they are usually studied at a relatively small scale, which makes it hard to find real-life operation data.
Second, even when some data is available, the absence of testing standards makes it hard to draw conclusions on the viability of different technologies in different contexts.
And finally, technological lock-in effects may result in technology A being chosen over technology B because of higher investments committed, even if technology B was the optimal choice to start with, from a technical point of view.
For these reasons, I have chosen not to provide you with yet another techno-economical analysis, but rather a quantification of the public perception of next-generation technologies for grid-scale storage.
To do so, I have scraped data from r/Energy, an energy-related forum on Reddit gathering more than 100k users. In short, it acts as a weighed news aggregators which gives a score and a number of comments to energy-related news articles. By looking for specific keywords, it is possible to sum the score and number of comments for a given technology. The result is the following bar chart:
In blue is the total score associated with a given technology, correlated with the popularity and number of related articles, and in orange the number of comments under the articles, correlated with the intensity of the debate around the technology. I will quickly go over each technology, in decreasing order of score (note: lithium-ion batteries were excluded from the analysis, as it is debatable to consider them "next-generation" since in 2020 they represented 93% of newly installed storage capacity):
Grid-scale hydrogen storage
So, hydrogen seems to be capturing most of the headlines as the next go-to grid-scale storage technology (mobility applications were excluded from the search).
But not always in a good way: in "Hydrogen (against)" I counted articles openly criticizing hydrogen storage. Also, the ratio comment/score is much higher than for most other technologies, which would indicate a more intense debate surrounding hydrogen.
Among the support, we can find:
The "hydrogen economy". Hydrogen can be used in many applications: ammonia production for agriculture, direct iron ore reduction for the steel industry, mobility applications, seasonal grid-scale storage, etc...
Hydrogen can be produced in a carbon-neutral way from renewable sources through electrolysis or other methods.
When stored at high pressures, it exhibits high energy density as a fuel and can be stored for large amounts of time.
Among the criticism, we can find:
Concerns about the flammability of hydrogen. Beyond the possibility of accidents, this results in added safety and storage costs.
0.03% of hydrogen is actually "green". To date, most hydrogen is produced through steam reforming of natural gas, which bears the question of the "grey price" (carbon emissions) of the technology, and which industries may benefit first from the hydrogen hype.
The demand may not follow the hype. The "hydrogen economy" idea rests on the assumption that hydrogen will be competitive with other technologies. What if it there are better alternatives in each separate application?
My two cents: I think "green" hydrogen should target the fertilizer and steel production markets, by far the largest. As far as grid-scale storage applications go, there might be better alternatives out there, especially in terms of round-trip efficiency.
🔋 Flow batteries
In short, these batteries store energy in large tanks of liquid electrolytes, pumping them into a porous electrode to enable the electrochemical reaction. Many combinations of electrolytes are possible, the most developed to date being the all-vanadium chemistry.
They present a range of advantages:
Long theoretical lifetime, the reactions being more reversible than in solid-state electrodes like Li-ion.
Independent power/energy sizing, which makes them more suitable for long-duration storage applications (e.g 8-12h, Li-ion being limited to 4h)
Non-flammability, due to the large volumes of water
But also a few drawbacks:
Low energy density
Higher maintenance costs than "black-box" systems
Very large industrial projects have propelled the Vanadium flow battery to the industrial scale and demonstrated its viability in grid-scale storage applications. The main issue, namely the cost of the battery when compared to lithium-ion, will highly depend on the "learning curve" effect (broadly speaking, costs being divided by 10 when installed capacity is multiplied by 1000) and the supply chain of Vanadium.
In this regard, organic flow batteries (with large companies such as Lockheed Martin developing the technology) or iron-flow batteries appear as a promising alternative, but it remains to be seen how much they can benefit from spillovers from Vanadium batteries in terms of system design and operation, and more importantly, if their electrolytes can be produced cheaply, at scale.
My two cents: Flow batteries appear as a promising technology for peak-shaving and intra-day trading applications, complementary to Li-ion for frequency regulation. The big question mark relates to costs, the economy of scale and the right chemistry.
♨ Thermal storage
In good correlation with the size of the biggest concentrated solar powerplants installed in Spain and the US, some of the most discussed articles on thermal storage relate to the molten salt technology, which has reached industrial maturity and should see its cost decrease as it will undergoes the "learning curve" effect, with more projects being commissioned in South Africa and Australia.
The remaining challenges include:
Elucidating structure-property relationships for the formulation and fabrication of optimised composite materials
Limiting high-temperature corrosion
The field of thermal storage features a wide diversity of technologies, however, including liquid air storage (LAES or cryobatteries), where the air is cooled down during charge, stored in low-pressure tanks and expanded during discharge to drive a turbine. UK-funded Projects should reach the demonstrator scale in 2023, giving a better overview of operating performance and economic viability of the concept.
Other solutions exist in the form of waste heat recovery and storage, which include:
using water in large underground reservoirs below cities in Finland for seasonal heat storage
composite concrete materials for mid-size systems
organic molecules for customer applications
My two cents: I think thermal storage will target the conversion heat->electricity rather than the two-way conversion electricity->heat->electricity, so in a way it should be regarded as a generation technology.
🌊 Pumped hydro
In 2018, pumped hydro accounted for 97% of storage capacity worldwide. It is therefore the most mature technology on the list, but also present the issue of requiring pretty specific geographic conditions, leading to little remaining growth potential in Europe.
The closed-loop technology holds the advantage of not requiring specific geographic conditions, at the cost of higher engineering and installation costs due to the creation of the upper and lower reservoirs from scratch. Some possible solutions include using old coal mines as reservoirs.
My two cents: In a context of decentralisation of the energy sector towards microgrids, especially in developing economies, investors will favour smaller-scale storage systems with low investment costs. I am unsure how closed-loop hydro would perform in this context.
🔋 Metal-air batteries
Based on the same general principle as rusting (but in a reversible way), metal-air batteries target the market of long-duration storage (up to 150h) and use cheap active materials, the most common to date being zinc and iron. The main selling point of metal-air batteries is their low predicted LCOS (between 20 and 45 $/kWh), although these costs haven't yet been demonstrated in large operating systems.
Furthermore, metal-air batteries have a few challenges to overcome, including:
developing more efficient and moderate-cost bifunctional oxygen positive electrodes.
using technologies that are readily scaled for manufacturing negative electrodes.
improving engineering in cell design and materials using cheap aqueous electrolytes and robust electrodes.
My two cents: Metal-air batteries seem to be a technology very well-suited for weekly/monthly storage, complementary to flow batteries for intra-day trading and Li-ion for frequency regulation. The costs announced, however, seem overly optimistic and should be taken with a grain of salt until large-scale demonstrator data is available.
💨 Compressed Air Energy Storage
Next-generation compressed air technologies include Advanced Compressed Air Energy Storage (A-CAES) systems which, similarly to the concept of closed-loop pumped hydro, can be built anywhere and don't require specific geographic conditions such as large underground salt caverns. Gigawatt-scale projects have been announced in the US, which should allow assessing the economic viability of the technology once all installation and maintenance costs are factored in.
My two cents: Same as pumped hydro.
🧂 Sodium-ion batteries
Sodium-ion batteries are very similar in design to their lithium-ion counterparts, which allows spillovers in terms of cell design and manufacturing, while sodium is much more accessible and abundant than lithium. However, sodium batteries present their own challenges, including typically lower energy density, higher costs and slower charging. Cost and performance projections will highly depend on the installed capacity in the years to come, with major projects being commissioned in China and startups emerging in the UK, France and China.
My two cents: The main challenge of Na-ion is to find an application where it outperforms Li-ion and emerging technologies. Otherwise, it may remain a worst alternative to Li-ion.
🌌 Gravity storage (excluding pumped hydro)
Currently, the main alternative to pumped hydro in the field of gravity storage is to lift heavy materials, either using cranes to stack concrete blocks above ground or using shafts to bring weights up and down underground. In the former concept, the main concern appears to be:
wind conditions leading to heavy mechanical constraints on the cranes, rendering them either very expensive to manufacture or unsafe to operate.
Additionally, while the technology does not require specific geographic conditions on paper, it is hard to picture public acceptance of a concrete tower being perpetually under construction for decades.
In the case of the latter concept, closed-loop pumped hydro seems more straightforward and proven technology, for the same engineering effort. These points may explain the unusually high ratio comments/score of these technologies on Reddit, as users of the forum debate the viability of the technology.
My two cents: I really don't believe in the crane concept, and I am doubtful of the underground elevator concept.
🔋 Liquid metal batteries
Pioneered by the group of Donald R. Sadoway at MIT, liquid metal batteries (LMBs) are a kind of mix between thermal and electrochemical storage, where the transfer of ions between two metals is separated by a molten salt electrolyte is facilitated at high temperatures. Advantages include:
low-cost and abundant active materials
long cycle life
Disadvantages, however, include:
high operating temperature resulting in a loss in round-trip efficiency and potential parasitic reactions
low energy and power density
High operating temperatures, in particular, can be an issue at the industrial scale (see sodium-sulfur batteries), so a compromise should be found between added costs and operational safety.
My two cents: LMBs seem comparable to flow and metal-air batteries, and would target similar applications. While they may have a low floor in terms of cost, costs primarily go down due to the "learning curve" effect when more and more capacity is installed. The main issue to me is that the research community is much less active on LMBs, which makes me pessimistic about the future rate of improvement of the technology.
💨 CO2 batteries
Carbon dioxide making much of the headlines related to climate change, it is only natural that researchers would look into how to turn it into a directly usable material after capture.
Lithium-carbon dioxide batteries were first developed as a proof-of-concept at MIT, showing that the technology opens up a possible route to combine CO2 capture and electrochemical conversion in a single device. The technology, despite showing promises at the lab scale is still in its infancy, as shown by the low exposure on Reddit.
My two cents: If the process can be tuned to improve the synthesis of carbon-based materials from CO2 capture, I could see the potential. As far as batteries go, it seems much more straightforward to use lithium for Li-ion batteries.
🔋 Sodium-sulfur batteries
In a somewhat similar fashion as liquid metal batteries, sodium-sulfur batteries make use of molten sodium and sulfur at high temperatures to produce power. While a great deal of research in the early 2000s led to fast scaling-up of the technology and commercial applications, technical difficulties such as corrosion, high operation cost and more importantly accidents in Japan in 2011 leading to explosions slowed down investments and press coverage of the technology.
My two cents: Improvements in battery design regarding safety and efforts on lowering the operating temperature may revitalise the technology, but it is unsure how it will compete with emerging battery chemistries in the near future.
As for lead-acid batteries and flywheels, these are tried and tested technologies, which are well implanted in their respective markets and are not subject to much press coverage, but it is unlikely that they will be significantly challenged in the short term.
I hope you enjoyed reading the post, which is part of the first chapter of my PhD thesis "Modelling and simulation of organic redox-flow batteries". Let me know if you would like a deeper dive into this specific technology!
🌞 Thanks for reading!
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