Why wouldn’t it be good for cars? It seems a fluid exchange could recharge the car, limited by how fast you can pump out the discharged fluid and pump in charged fluid.

I have no idea whether there are containment or environmental challenges that make this especially hard.

That sounds like moving to a non solid state situation, right? One of the big benefits, imo, is removing as many moving parts from a car, not reintroducing them.

Surely it is more complicated. It may be able to serve the goal of eliminating fossil fuels from the car while not getting the full simplicity of (not really) solid state batteries we have today.

It would seemingly also allow you to carry around 50 miles of juice when you are running around town and bump up to 400 miles of juice when you are taking a road trip. This really only makes sense if the car is able to recharge the juice.

Maybe a new hybrid model (not gas electric hybrid) could emerge. The base (say 50 mile) capacity is solidish state like we have today and a flow battery range extender could be filled with fluid when the extended range is needed. This would make it so the car wouldn’t need to charge the fluid, if that helps.

I could see a hierarchy of power in a car, or truck/rv.

motor <-> lithium <-> flow battery

the low current from the flow battery could be continuously augmenting and replenishing the lithium ion battery

or even:

supercapacitor <-> lithium <-> flow battery

Being able to "refuel" quickly is a pretty big plus, so in the balance it could still be better to accept some moving parts in exchange for that.

And it's easy to imagine retrofitting a gas station to all of a sudden have charged electrolyte instead of gas, but I'm sure that has all sorts of additional complexities that I know nothing about... ;)

Probably adds two low-pressure low-volume pumps. Which is more, but not a large increase in complexity for a car. The driver's seat probably has more moving parts.

It’s going to depend on the details. In a power substation space and weight are fairly flexible. In a vehicle they matter quite a bit. If you can build a flow battery that rivals solid state batteries for weight and volume, you’ve made an improvement because the fluid is swappable in ways that batteries are not. Batteries meant to move will move in a crash. Fixed batteries tend to stay put but complicate road trips.

In a UPS the tanks and valves and pumps are new points of failure, as the other responder states. In a car they are many times more complicated than electric vehicles, yes, but still less complicated than ICE vehicles by far. Your heater has more moving parts. Hell, the emissions control gear is probably more complex than the entire drivetrain of the electric vehicle.

If one can actually make a useful economy out of replacing flow battery fluid at a station, sure. But I’m making a different point:

A Li-Ion battery can charge and discharge in something like an hour. If you double the number of cells in a battery or installation, you can store twice as much energy and you can draw twice as much power. (You don’t have to provision twice as much power circuitry, but you do need to purchase that extra power worth of electrodes in the cells.)

If you’re building something for which a roughly predetermined power to energy ratio (i.e. hours of use at a reasonable rate of discharge for the technology), this is fine. Similarly, if overprovisioning power or energy is not a problem for cost or weight, also fine.

But for grid use, wide differences in the duration of energy storage for different purposes can make sense. And a flow battery can separately provision power and energy. This gives a possible cost benefit to flow batteries.

Flow batteries have way less current capacity so it could plainly not be powerful enough.

It also have lower energy density so you pumping 600l of new electrolyte on the station might still give you only 200km of range

Could it be good for a long-haul truck?

or maybe a locomotive, which is less sensitive to weight and cargo capacity?

It kinda depends to what power density it gets. Trading say 30% range for ability to refill the truck within minutes might just make enough sense to work. But if, say, electric truck gets enough range to run the full 9 hours (max allowed driving time per day in EU for truck drivers), then it doesn't matter as trucker can do "100% of their work", get to stop and leave it to charge for rest of the day.

For locomotives ? I'd imagine the simplest solution would be to have "battery wagon" ; get on the station, replace the battery wagon, and be on your merry way while it is recharging in the station waiting for next train to pick it up, at least for the long haul stuff. But capacity density still matters, if flow batteries are being close and cheaper then it makes sense, but if you need to pay slighty more per KWh but get 2x the capactity... thats 2x the range and less maintenance.

Or, you know, just electric rail. It already works fine in many places. I wonder if making some hybrid solution with electric rails also being covered by solar panels (basically so same maintenance crew can manage both) would make sense

So it all depends to what level of density it gets. If it is same capacity per kg but lower power there are still many places it can be used, if it is much lower it stops being sensible for moving stuff. And the price would need to be significantly below the normal batteries for anyone to even bother.

Trucks are weight limited with LFP, costs already favour electric even with NMC but the tradeoff is payload.

Trains maybe, but surely the money is better spent on overhead wire? Even if tunnels/bridges/etc. can't take it you're better with just the power-buffer battery and no flow battery.

Isn't the costs and recurring costs of railroad infrastructure orders of magnitude larger?

Full electrification is expensive, but overhead wires where it doesn't require rebuilding anything isn't.

And rail is far cheaper to operate once built per unit capacity than trucking. If you don't inflate it with endless feasibility studies and contracts that get paid regardless of output the initial building costs are about double a 4 lane highway, but it carries a lot more and maintenance is lower.

> for grid use, where a discharge time of days to months is useful.

Not sure I know the use case for a duration of that length. It's possible today with pumped hydro, but I don't think it's used in that mode.

The DoE's "long duration storage earthshot" effort is loking for 10+ hours, which makes more sense: https://www.energy.gov/eere/long-duration-storage-shot

A zero-carbon northern Europe that replaced all gas heating with heat pumps would benefit from being able to store electricity during the warm summer months, to power heating in the cold winter months.

There are other options, of course - new nuclear plants, importing renewable power from countries with better weather, huge numbers of wind turbines, and so on - but cost-effective long-term power storage would address some issues if it was available.

The times when there is insufficient wind and insufficient solar somewhere within transmission range of Europe are so small (hours) that this long term energy storage isn't something anyone is trying to build.

Better long distance transmission infrastructure is a much better solution to this.

Winter happens at same time in Europe. Unless we're talking shipping all the way from Africa

Offshore wind is already at large scale in N. Europe and will continue to add much more capacity.

Winds are stronger in winter time.

Winter is excellent for wind power.

Until something adversarial happens to your transmission infrastructure similar to what happened to Nord Stream 1 and 2 a few months ago.

Yes, black swan events happen for all kind of power sources.

Depending on your risk tolerance you overbuild to reduce that. Same as with all types of energy production.

There is a tradeoff between overprovisioning renewables, improving transmission networks and storage. Where the cheapest solution lies is not clear yet.

> A zero-carbon northern Europe that replaced all gas heating with heat pumps would benefit from being able to store electricity during the warm summer months, to power heating in the cold winter months.

Why store winter heat as work when you could just store it as heat or chemical energy?

All you need is a hole, some plastic sheeting, an element, and some pipes to store many GWh. Alternatively one of many cheap phase change materials, thermochemical stores, or some low grade sand unsuitable for construction.

How feasible is that at all? "According to Ofgem, the average household in the UK has 2.4 people living in it, and uses 8 kWh of electricity and 33 kWh of gas respectively, per day."

Lump those together, halve it for the magic of heat pumps but then add some back because winter will be above average use, call it 30kWh/day. Three months of winter is ~90 days, and ~30 million households, is storage of 30,000 x 90 x 30million = 81,000,000,000,000; eighty one trillion Watt-hours.

A classic (non-EV) car battery stores ~1kWh, so eighty one billion of them to store that kind of power. Thousands per household. Even to store ten percent of it is hundreds of car batteries per household which is still unfeasible, and not going to get you through winter.

That's exactly the appeal of flow batteries - for gigantic capacities, all you need is big tanks.

Working your example - google tells me that zinc-iodine electrolyte gets you about 200 Wh per liter. Therefore you need about 400 million cubic meters of storage for the capacity you suggest. To estimate the cost of this, I looked at reservoirs. The largest drinking reservoirs in the world are in Qatar, where they have built 5 tanks of 436,000 cubic meters each. Therefore we'd need about 200 of those facilities. The cost was around 5 billion dollars, so our total cost would be around a trillion dollars.

This is obviously a lot! But not unimaginable - the government borrowed over 300 billion pounds in the 2020 fiscal year alone just to pay for Covid. In practice you'd need considerably less than 90 days of continuous power, because the wind does blow and the sun does shine even in winter. All you really need is tanks to buffer the difference between winter and summer, which is certainly not 100%. And the system can be built up incrementally over a long period of time, and still yield value - we don't need to spend a trillion dollars all at once.

Obviously take all these numbers with a grain of salt, this is a back-of-the-envelope calculation built on another back-of-the-envelope calculation. The point is merely to show that it's not orders of magnitude outside our ability.

But you did say Northern Europe, where our estimates are UK only; add another trillion for France, one for Germany, one for say Ireland+BeNeLux, another for Poland, another for NorthEast-Lithuania+Estonia+Belarus, one or two for Norway, Sweden, Finland... double it for the cost of the electrolyte, maintenance, the fact that building in Europe without slaves and with heavy regulation and expensive land is more expensive than Qatar...

World GDP is only around a hundred trillion, and we're talking of maybe ten trillion of it to build a heating system for Western Europe, for very little financial return for any investors.

Practically, we've been struggling to build one nuclear power plant (Hinkley C) for twenty years. I think it sounds like it is orders of magnitude outside our ability.

The standard reply to any sort of gravity storage is "geography and land availability". I don't find that argument compelling enough to dismiss gravity storage outright, but I'll admit Oklahoma is pretty darn flat.

Chemical storage could be a big win when a mountain isn't readily available. Or the land is too expensive to use for power storage.

Decommissioned mines. Saw an article about it somewhere recently.

Really, you just need any elevation difference. I would imagine Oklahoma already builds water towers.

What about a couple days of overcast/rain (solar) or a couple days of stagnant air (wind) where storage would bridge the gap?

If the numbers work out (which, I suspect they don't), putting batteries at substations could provide continuous power to customers when the substation is isolated from the grid by a wiring fault. That shouldn't take months to fix, but sometimes it takes days (in my area, it's usually trees falling during storms, and the roads are dangerous during such times, so it takes some time for repairs). Might be nice anyway, and help with grid stabilization when the wiring is intact.

Form Energy seems to be landing new pilot projects with US utilities. Form Energy is an American energy storage company focused on developing a new class of cost-effective, multi-day energy storage systems that will enable a reliable and fully-renewable electric grid year-round. Form Energy's first commercial product is a rechargeable iron-air battery capable of storing electricity for 100 hours at system costs competitive with legacy power plants. [1]

[1] https://en.wikipedia.org/wiki/Form_Energy