Geothermal may beat batteries for energy storage

The reservoirs we already have are rated in the thousands of MW. There is no storage problem, we already have large enough reservoirs where we can practically store years of energy indefinitely; especially now in drought conditions where reservoirs are regularly well below historical levels. So if there's ever a need to store solar energy from summer for use in winter, pumped hydro is the only energy storage solution that works.

They also don't have issue with storing energy quickly, they can all store energy at a significantly faster rate than they can discharge. We can run pumps as quickly as possible and install as many as you'd like, but the discharge has to be controlled (thus limited) because releasing massive amounts of water at once. So their main use case today is storing massive amounts of energy generated in a short amount of time and releasing slowly across a long period of time.

What the grid actually needs is faster discharge than charging, because that more accurately matches summer energy use patterns. This is what chemical batteries excel at which pumped hydro cannot easily do.

So it's unlikely we'll be able to make them 100-1000x bigger, but they're already 100-1000x bigger than other battery solutions. We should be able to make 100x more of them because the reservoirs already exist and very few of them currently are used as both power sources and energy storage, we simply need to add pumping capability to them in most cases.

> the question really comes down to what should we be spending money on to get "better storage"? What are the best solutions for long-term local storage?

The solution I'm proposing is basically 'the best place to spend your money on storage is to not spend it on storage yet'

If the goal is to reduce emissions asap, then focusing on the strategy that removes x% of 100% of the emissions rather than 100% of y% of the emissions makes sense unless there are enough resources/money that y% is more than x%. And storage is currently expensive enough that you need many times as much money for this to be true to 99.9% confidence.

Getting a wind + solar system that has at least y watts at least eg. 90% of the time is remarkably affordable already and still going down.

In excellent climates new solar costs less per MWh than fuel for a gas turbine (and is not far off fuel for a nuclear reactor). Wind is not much more. Distribution, dealing with less than ideal sites and oversupply increase the cost, but an ideal mix has very little storage (4-12 hours) which can be delivered by lithium batteries.

By relying on the existing fossil fuel/hydro/nuclear/whatever to pick up the last 10% for now, you can replace more coal/oil more quickly than other strategies. During this build all storage technologies where they make the most sense so that when that last 10% is needed, prices will have dropped. I'm fairly sure some mix of green hydrogen and green ammonia burning in those same turbines will be one of the winners (ammonia in particular has negligible marginal cost of capacity allowing for a strategic reserve, and will be needed to replace fossil fuel derived fertilizer anyway).

In the unlikely case that there's an overnight $2 trillion investment in new wind/solar/powerlines and production capacity to match in the US then choosing a dispatchable power source from some or all of: expensive green hydrogen, expensive abundant existing batteries, expensive pumped hydro, and expensive nuclear or immediately going all in on commercialising every vaguely promising electrolyser tech becomes the priority.

Completely agree with the hybrid approach wrt reducing emissions. I am talking more towards work that would be done concurrently with that.

> During this build all storage technologies where they make the most sense so that when that last 10% is needed, prices will have dropped

this is kind of the point of what I'm getting at. Without any investment, none of the storage technologies are going to make much progress. If not financial investment, then at least a time investment from research/science teams. then again, maybe opportunism/free market will take care of this and we can assume any progress that can be made will be made by people trying to make a name for themselves or be first to market. I'm still curious to size up what that progress might look like for discussion/entertainment purposes in any case

Good storage solutions would immediately pay dividends through arbitrage, which would keep electric prices stable, and then anywhere renewable energy generation is more than demand and storage is sufficient, that stable price point could come down below the cost of using coal/oil as well as any other continuous production method. We would be able to consolidate power generation over time, not just space, and realize gains from that. As in, use massive bursts of energy production to top off storage and use them to exactly meet demand. Maybe this opens the door for more alternative energy production methods as well (that are better suited for burst than steady)

In terms of promising technologies, they're broadly categorisable as thermal, kinetic, battery/fuel cell, and thermochemical. Most of the promising ones are far enough along the learning curve that other markets (such as green hydrogen/ammonia for fertiliser driving electrolysers and small scale/more efficient chemical reactors) will drive the learning curve.

Thermal storage concepts include:

Molten salt thermal. short/medium for high grade heat. Most high grade heat is dispatchable (fire) and so doesn't make sense to store, or expensive (solar thermal, nuclear) and so isn't worth pursuing.

Sand thermal batteries. Low grade heat for medium/long term. Only useful for heating and some industrial purposes. Has a minimum size (neighborhood). Literally dirt cheap.

Thermochemical. I guess this is kind of a fuel? Use case is for low grade heat so it can go here. Phase change materials like sodium acetate or reversible solution like NaOH seem really appealing for heating. Back of envelope says it's close to competitive with electric heating, so I'd expect more attention as it's cheaper than any technology that stores work. No idea why it isn't being rolled out. You could even charge it with heat pumps for extremely high efficiency if needed.

Kinetic:

Lifting stuff. Only really works for water without large subsidies and only if you already have at least one handy reservoir like a watershed or cavern. No reason to expect it would suddenly get cheaper as digging holes and moving big things is already something lots of industries try to do cheaply. Great addition to existing hydro.

Sinking stuff (using buoys to store energy). I can't comprehend how this can be viable. I have seen it espoused, but it doesn't pass back of the envelope test unless I did a dumb.

Squashing stuff. Compressed air energy storage. Tanks are just barely competitive with last gen batteries capacity-wise, efficiency isn't great. There are concepts for underwater bladders (let the watter do the holding) or cavern based storage that seem viable at current rates. Achievable with abundant materials so worst case scenario we nut up and spend$500/kWh. Key word CAES, cavern or underwater energy storage

Battery/fuel cell:

Lithium ion: One of the best options currently. Will be heavily subsidised by car buyers. Has hit limits of current mining production which puts a floor on price and is ecologically devistating.

X ion where x is probably sodium: Great slot in replacement. Barring large surprises will expect it to replace LiFePO4 very soon for most uses. Expect the learning rate of lithium ion manufacturing to continue resulting in a sharp jump to $60/kWh in 2021 dollars and eventual batteries around $30/kWh. Key word natron (have just brought their first product to market and are working with other parts of the supply chain to scale up)

Flow batteries, air batteries and fuel cells. These are almost the same concept. You have a chemical reaction that makes electricity with a circular resource like hydrogen, methane, ammonia, or electrolyte. Downside is most versions require a prohibitive amount of some metal like rutheneum or vanadium or something. Not a fundamental limit, but not sure it will be a great avenue as research goes back a fair ways. Aluminum-air batteries are one interesting concept. Essentially turning Al smelters into fuel production facilities. Keywords iron-air aluminum-air, redox-flow, direct methane fuel cell, ammonia fuel cell, ammonia cracking, nickel fuel cell.

Molten salt batteries. Incredibly simple, cheap and scalable concept that has no problems with dendrites (and so theoretically no cycle limit) with one limitation on portability (they must be hot, sloshing is bad) and one as yet insurmountable deal breaking flaw (incredibly corrosive material next to an airtight insulating seal). Look up Ambri for details of an attempt which has presumably failed by now. There is a more recent attempt using a much lower temperature salt and sodium sulfur which shows promise. Keywords ambri, sodium sulfur battery.

Thermochemical:

Any variation on burning stuff you didn't dig up.

Hydrogen is hard to store more than a few days worth, but underground caverns could help. I expect a massive scandal about fugitive hydrogen, toxicity and greenhouse effect in the 2030s sometime. It's borderline competitive to make now. Main limitation is cost of energy (solved by more wind and solar and more 4 hour storage) and cost of capital (platinum/palladium/rutheneum/nickel are usually required). Lots of work going on to reduce the latter and to increase power density and efficiency. If you were directing a billion dollars of public funds this would probably be the place to put it. Keywords $200/kw electrolyser, hysata 95% efficient.

Methane, ammonia, dimethyl ether, methanol, etc. These are all far easier to store than hydrogen. Production needs large scale but is borderline viable already if you have cheap hydrogen. Keywords ammonia energy storage, synthetic fuels, efuels, green ammonia, direct ammonia electrolysis.

Then there's virtual batteries.

Many loads like aluminum smelting can be much more variable than they are now. Rearranging workflows such that they can scale up or down by 50% and change worker tasks to suit has the same function as storage during any period where consumption isn't zero. EV's can kinda fit here too and kinda fit actual storage (especially if they power other things)

Biofuels. Not technically storage, more dispatchable, but it serves a similarfunction. Bagasse is an option for a few percent of power. Waste stream methane is a possibility for a couple % of power. Limited by the extremely low efficiency of photosynthesis so something PV based will likely be a better way of making hydrocarbons from air and sunlight. Most other 'biofuels' are either fossil fuels with extra steps or ways of getting paid green energy credits for burning native forests. Some grad student might surprise us by creating a super-algae that's 10% efficient and doesn't all get eaten if there's a single bacterium in the room. Detangling it all is hard, but I wouldn't be surprised if wind + solar + biofuels + reigning in the waste was enough -- it certainly works for some people doing off grid.

I'd expect a system based on sodium ion (or even lithium) batteries and synthetic fuels to render any fossil fuel mix unviable in the next decade or two. More scalable batteries or scalable fuel cells would hasten this somewhat.

Thanks a lot for the detailed reply. Old radiators operate in high temperature but from what I understand, the fan coil heaters operate in lower temperature more equivalent to radiant floor heating.

For cost reasons, installing undefloor heating is not an option, that's why I opted for low temp radiators on top of floor. However, it sounds like the heat pump system will not be cheap either. According to BTU calculator I found online, seems like I need 28K BTUs to heat the home. I thought about pellet burners as well, but here in Europe, the cost of pellets has sky rocketed this year. I don't want to be dependent on gas either, also electricity is not cheap... So I am not sure exactly what i'll go with. I'll do the calculation, but your input on the effectiveness of the theromosiphon system is valuable, thank you.