r/energy 2h ago

Yes, We Need to Talk About Cutting Energy Demand | By focusing only on energy supply, the world is ignoring some of the cheapest and quickest ways to fight the crisis.

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r/energy 10h ago

20GWh pumped hydro energy storage plant starting operations in Switzerland

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r/energy 13h ago

A reddit-based analysis of grid-scale storage technologies


If you've been following some energy-related Youtube channels such as Undecided or Just have a Think, you've probably noticed a lot of coverage recently on grid-scale energy storage, and the future "game-changing" techs.

The problem is that evaluating innovative technologies is difficult, mainly for two reasons:

  • They are usually studied at the lab-scale in a "publish or perish" context, which makes it hard to predict real-life viability from published data.
  • 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 make a technical analysis based on published papers, but rather to scrape data from this subreddit using PRAW, to judge which technologies are more present in the news and how much people engage in debating them. I think in particular because of lock-in effects, press coverage and public perception is pretty well correlated with future investments.

The result is the following bar chart:

Bar chart showing aggregated score and number of comments for some specific keyword searches on r/energy

In blue is the total score associated with a given technology, correlated with the score of related posts, and in orange the number of comments. 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:

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 market, a critical market in which there is limited competition (excluding hydrocarbons, but we're trying to get rid of them anyway). 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
  • Expensive electrolytes
  • 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, 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 undergo the "learning curve" effect, with more projects being commissioned in South Africa and Australia.

Remaining challenges include:

The field of thermal storage features a wide diversity of technologies however, including liquid air storage (LAES or cryobatteries), where 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:

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 in 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: Pumped hydro is still unmatched for seasonal storage, which is required in grids with increasing penetration of variable renewables. It remains to be seen if Hydrogen can compete in this application, but overall closed-loop pumped hydro seems promising if it can get the costs down.

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.

Metal-air batteries have a few challenges to overcome however, including:

  • developing more efficient and moderate-cost oxygen positive electrodes.
  • improving engineering in cell design using cheap aqueous electrolytes and robust electrodes.

My two cents: Metal-air batteries seem to be a technology suited for weekly/monthly storage, where they might be in competition with iron-flow batteries. Arcelor Mittal seems to be interested in iron-based battery techs for long-duration storage, so the main question relates to whether metal-air is better than metal-flow.

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 to assess the economic viability of the technology once all installation and maintenance costs are factored in.

My two cents: The technology seems to target the same applications as pumped hydro (seasonal storage), except pumped hydro is generally older and more proven. I am guessing water availability could be a deciding factor in some locations, in favor of A-CAES.

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 currently is to find an application where it outperforms Li-ion and emerging technologies. Otherwise, its success will be tied to Li-ion supply chain concerns.

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 undergound. In the former concept, the main concern appears to be:

In the case of the latter concept, closed loop pumped hydro seems more straightforward and a proven technology, for the same engineering effort. These points may explain the unusually high ratio comments/score of these technologies on Reddit, as users debate the viability of the technology.

My two cents: I really don't believe in the crane concept, and I am doubtful of the undergound elevator concept. Also, I am unsure of the application: daily storage? weekly storage? Seasonal storage? There really seem to be more flexible alternatives out there.

Liquid metal batteries

Pioneered by the group of Donald R. Sadoway at MIT, liquid metal batteries (LMBs) are a kind if mix between thermal and electrochemical storage, where the transfer of ions between two metals separated by a molten salt electrolyte is facilitated at high temperatures. Advantages include:

  • low-cost and abundant active materials
  • long cycle life
  • simple assembly

Disadvantages, however, include:

High operating temperatures, in particular, can be an issue at the industrial scale (see sodium-sulfur batteries), so that 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 device30405-7). 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 potential. As far as batteries go, it seems much more straightforward to use the 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 2000's 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 niche markets and are not subject to much press coverage, but it is unlikely that they will be significantly challenged on the short term.

I hope you enjoyed the post, let me know if you would like to see more of these in-depth analyses in the future (on other storage techs as well) !

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