Air-Breathing Aqueous Sulfur Flow Battery for breakthrough ultralow cost energy storage

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Joule Journal – Air-Breathing Aqueous Sulfur Flow Battery for Ultralow-Cost Long-Duration Electrical Storage

•Chemical cost analyzed for 40 rechargeable couples developed over the past 60 years
•Aqueous sulfur/sodium/air system identified with ultralow chemical cost of ∼US$1/kWh
•Air-breathing flow battery architecture demonstrated at laboratory scale
•Techno-economic analysis shows installed cost is comparable with PHS and CAES

Above – Curves for the present air-breathing aqueous sulfur flow battery approach using Na and Li chemistry are shown in green and gray, respectively. The chemical costs for Na and Li are shown as dashed lines. Curves of constant power cost show that the power stack dominates the system cost at short storage durations, whereas at long duration the cost asymptotically approaches the energy cost due to chemical constituents plus storage tank and related costs. 5 M concentrations of both Na and S are assumed, with cycling of the sulfur over the speciation range S22− to S42− corresponding to 25% of theoretical capacity. The peak power density of the stack ranges from 4.3 mW/cm2 at US$4,000/kW and US$2,000/kW to 28.6 mW/cm2 at US$150/kW. The projected costs for air-breathing aqueous sulfur compare favorably with those for pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES), while also having several-fold higher energy density and being free of the locational constraints faced by each.

Context and Scale
Wind and solar generation can displace carbon-intensive electricity if their intermittent output is cost-effectively re-shaped using electrical storage to meet user demand. Reductions in the cost of storage have lagged those for generation, with pumped hydroelectric storage (PHS) remaining today the lowest-cost and only form of electrical storage deployed at multi-gigawatt hour scale. Here, we propose and demonstrate an inherently scalable storage approach that uses sulfur, a virtually unlimited byproduct of fossil fuel production, and air, as the reactive components. Combined with sodium as an intermediary working species, the chemical cost of storage is the lowest of known batteries. While the electrical stacks extracting power can and should be improved, even at current performance, techno-economic analysis shows projected costs that are competitive with PHS, and of special interest for the long-duration storage that will be increasingly important as renewables penetration grows.

Summary
The intermittency of renewable electricity generation has created a pressing global need for low-cost, highly scalable energy storage. Although pumped hydroelectric storage (PHS) and underground compressed air energy storage (CAES) have the lowest costs today (∼US$100/kWh installed cost), each faces geographical and environmental constraints that may limit further deployment. Here, we demonstrate an ambient-temperature aqueous rechargeable flow battery that uses low-cost polysulfide anolytes in conjunction with lithium or sodium counter-ions, and an air- or oxygen-breathing cathode. The solution energy density, at 30–145 Wh/L depending on concentration and sulfur speciation range, exceeds current solution-based flow batteries, and the cost of active materials per stored energy is exceptionally low, ∼US$1/kWh when using sodium polysulfide. The projected storage economics parallel those for PHS and CAES but can be realized at higher energy density and with minimal locational constraints.

The Na chemical cost is US$1.7/kWh, and after accounting for tank cost (US$0.15/L) and other costs, the resulting energy cost for Na chemistry is a factor of 4–6 higher than the chemical cost alone. Thus, as with PHS and CAES, the energy-storing fluids have lower cost than the structures used to contain them.

The largest contributions to power cost come from the membrane and catalyst. Our current experiments use ceramic membranes, for which the US DOE has projected a cost, at high production volumes, of less than US$10/m2.39 In the cost model, we conservatively assume costs of US$10/m2 and US$100/m2. Alternative low-cost membranes such as polymer-ceramic composites40 could also be developed for this application. The total PGM catalyst loading is assumed to be 0.05 mg/cm2; the experimentally validated value in Figure 7 is 0.03 mg/cm2 for the Pt black alone.

This work demonstrates a new electrochemical storage approach that uses an aqueous polysulfide anolyte in conjunction with an air-breathing catholyte to reach exceptionally low chemical cost of storage (∼US$1/kWh) while providing moderately high energy density (29–121 Wh/L at the solution level). The chemical cost of stored energy is one of, if not the, lowest among known rechargeable batteries. Implemented in a flow battery architecture, this approach could offer the cost/performance characteristics of PHS and CAES, today the lowest-cost and most widely scaled energy storage technologies, while being free of geographical and environmental constraints and having up to a 1,000 times higher energy density at system level. Techno-economic modeling shows that at the current stage of development, stack power cost is the limiting cost factor. A modest reduction in stack resistance over current laboratory results would allow power cost of US$1,000– 2,000/kW to be reached, competitive with PHS and CAES. This is achievable while using ceramic membranes and PGM catalysts as in the current experiments. Nonetheless, the scalability and cost of the flow battery could be significantly improved through development of low-cost low-resistance membranes, such as ceramic/polymer composite membranes, and non-PGM OER and ORR catalysts. With further development, a new ultralow-cost electrochemical storage option may become available to support the growth of intermittent renewable generation and decarbonization of the world’s energy systems.

21 thoughts on “Air-Breathing Aqueous Sulfur Flow Battery for breakthrough ultralow cost energy storage”

  1. The numbers are a bit suspect. A couple times they mention chemical cost of $1/kWh, but then in the details they say “The Na chemical cost is US$1.7/kWh, and after accounting for tank cost (US$0.15/L) and other costs, the resulting energy cost for Na chemistry is a factor of 4–6 higher than the chemical cost alone. ”

    So that tells me it may be up to $10/kWh. That’s a big difference from $1.

  2. Interesting it is that either buyer or seller of energy anywhere will be able to use less than $2,000/KwHr for storage economics. No longer will the argument be about intermittency alone but about who profits by owing the storage market.

    • The graph claims that some existing systems are already at $10/kw.h

      Why do you say it can’t go below $2000/kw.h?

    • Goat and friends are talking about the “Underground CAES” that appears to be shown on the top graph to be better than all the other techs (except a fortuitous pumped hydro scheme, but that needs a dam in the right spot).

      Underground CAES is Compressed Air Energy Storage, which relies on having huge caverns deep underground that you pump pressurized air into to store energy.

  5. On September 2017, a 700 MW installed capacity Concentrated Solar Power project was awarded at a cost of US$0.073/Kw-hr at Dubai – This solar project includes 12 hours of molten salt storage capacity. It is a game-changer baseload solar power plant which can generate electricity all night long. It beats by far any currently available storage technology and the cost is competitive with mainstream baseload thermal options such as coal. The market is changing way faster than most anticipated…

    • One number quotations kind of lose the picture. What color is the sky presently? Blue! Was that all blue, or blue with clouds, or hazy blue, or barely blue, or deep blue.

      7.3¢/kWh is great sounding. However, was that including the amortized land lease cost? Including maintenance? Including backup generation for strings of shîtty days? Was that 7.3¢/kWh for a 5, 10, 15 or 20 year full-pay-off lease? Did the 7.3¢/kWh include the PRE-cost of evaluating proposals, of hiring experts and engineers? Does the cost include the price of ongoing labor too? Is there a big ol’ balloon payment at the end?

      Just saying.
      GoatGuy

      • The Dubai solar project is significant on its own merits. It is the largest CSP project on a global basis. The investment is $3.9 billion. The leverized tariff includes no subsidies other than it appears to be exclusive of land lease costs. The tariff is inclusive of development, construction, financing operation and maintenance costs. IMHO, it sets a relevant bench-mark for all future large scale base-load CSP tenders. It also sets a bench-mark for comparing PV + battery solutions. The next couple of utility scale CSP tenders will, in any case, confirm if CSP + molten salt energy storage is now a competitive option for baseload capacity. The market will quickly provide confirmation.

        • Perhaps. (“market will quickly provide confirmation”) Perhaps not. It all depends on how effective the plant gets built and whether it satisfies the operations-and-maintenance criteria upon which its economics was judged.

          As long as the Dubai conventional generating capacity remains potentially in excess of its CSP, then it can take up the slack when a run of bad weather or accumulated dusts takes the bulk of a given plant’s (or all of them in agregate) off the grid. After that tho, then “peaker capacity” needs to be increased.

          Nothing hard to engineer, actually. The use-curves by time of day, of week, of month, of year, with holidays and notable exceptions from the general curve are well known. Computer Monte-Carlo sims do a good job playing millions of scenarios of various whole-system supply:demand realities. And in turn can give 95% survivability to a given spectrum of power generation technologies.

          Just don’t forget Mr. Smoke, that any clouds whether precipitators or whispy cirrus can significantly diminish CSP mirror arrays. This kind of power depends entirely on “specular sunlight”, not diffuse light. And even in a desert you can have a LOT of diffuse light days. Storms from elsewhere drifting by with nary a drop of rain landed. Diffuse clouds. Same goes for the legendary Mideast dust storms. And all the mirrors need to be kept pretty well smart-and-shiny to keep output up.

          GoatGuy

          PS: you didn’t answer most of my questions, friend. Go over the list and try each one.

      • The Dubai solar project is significant on its own merits. It is the largest CSP project on a global basis. The investment is $3.9 billion. The leverized tariff includes no subsidies other than it appears to be exclusive of land lease costs. The tariff is inclusive of development, construction, financing operation and maintenance costs. IMHO, it sets a relevant bench-mark for all future large scale base-load CSP tenders. It also sets a bench-mark for comparing PV + battery solutions. The next couple of utility scale CSP tenders will, in any case, confirm if CSP + molten salt energy storage is now a competitive option for baseload capacity. The market will quickly provide confirmation.

      • Well, that’s Dubai, they don’t have strings of cloudy days. 12 hours backup (as the OP said) should be sufficient, if solar is part of the overall energy market (if it’s sole-source, then yes, they will need more backup).

    • Actually, “not really”. After awhile the compression-decompression becomes what is called adiabatic. Compression heats the air; the heated air at first warms the rock; that heat-loss is “lossy”. But after a while, the heat loss is low, as the insulating value of rock prevents more than 10 or 20 meters of rock to significantly heat. Then the compression-decompression cycles keep all the entrained energy and release it efficiently.

      GoatGuy

  7. So… CAES is the present-and-future winner then? Who knew! The article kind of hones and polishes its own ipuku sword, then falls on it. Compressed Air Electric Storage wins. So long as there are either nice big left-over salt mines or other underground cavities that are the storage space. Like in California, Louisiana and Pennsylvania.

    One does though have to be careful: 1,000 PSI = 700,000 kg/m². The amount of rock above the storage cavern is therefor (at a density of ‘3’ say) about 234 m thick. Your storage location has to be at least 300 m below surface. Cracking-fracking on steroids? People will simply find it unacceptable if CAES fields erupt in huge “cold volcanoes” without much warning.

    GoatGuy

    • I’ve long wondered whether you couldn’t just have a loop of cable dragging bags of air down to the sea bed, and then let them supply power on their way back up. Isothermal rather than adiabatic, given good coupling to the water.

      Kind of an upside down hydropower plant.

      • Its been proposed a number of times. Mostly though, usually the football length gas bags are attached to a large diameter air hose going to the surface. The bags inflate and deflate, which while less efficient than your rising-and-lowering, have a subtle but potent advantage: All one’s off-shore bags reside permanently anchored near the bottom (or at a convenient deep level) in the pool / sea / harbor where they’re positioned. This greatly simplifies anchoring for storms and such. Free floating bags are a pain.

        Also ALL the bags can be “paralleled” as needed to increase capacity of the overall system trivially. More bags, more anchors, more air hose. The turbine-generator setup is also working at constant pressure. A powerful advantage, that. Both directions, since the depth is a near-constant.

        Bags, wrapped in a fine mesh (1 cm), then a heavier one (10 cm) and an even larger, heaver one (100 cm) can work to allow the bags to be substantially over-filled if the economics are good. Kind of like how the original 18th century hot-air balloons had 2 layers of rope fishing net. The cloth of the 17th century was so weak that it needed the support of the mesh just to contain 0.5 PSI air!!!

        GoatGuy

    • This new technology, if it works as projected, it seems even better. Downwards/upwards scalability, cheap, relative compact, 3D storage allowed, long live, efficient, etc.
      I see a future electricity offgrid like a three in one technologies. Supercapacitors to allow boots huge electricity peaks for some seconds. Ion-Lithium or similar batteries to have peak power for hours, and this technology to storage a lot of energy for weeks of storage.
      Only 150% over average power of this technology is needed to refill lithium batteries for peaks.

      Of course this is not needed on grid, as any technology could be connected in any place. Huge batteries will be located on industrial sites near big consumption sites.

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