Sunday Energy Post Is Here!
For people new to this, the premise is always the same: You just spend $10k on solar panels, and because you're not dumb, you bought your panels direct from China and you now have something like 30KW of installed capacity. This means you only need to buy power on a few of the darkest days of the winter, but it also means there's a good 3 months of the summer when your system is cranking out 100kWh per day. What do you do with all of that excess energy? (Ideally, you store it)...
Today we're going to talk about making your own batteries.
Last Sunday's post:
https://pkteerium.xyz/@cjd/posts/BAqT8uoMirk4hqxHZwIn case you missed it, a tangental topic: Nobody Understands Leyden Jars:
https://pkteerium.xyz/@cjd/posts/BAySnj8t1CerbFWADY@BigBirdPhoenix requested a tag.
You might wonder why I would suggest making your own battery when companies like CATL and EVE already make them really well and really cheaply. If this was a hobby chemistry post, then a legitimate answer would be "because it's fun". This is not a hobby chemistry post, so we're not talking about doing anything that's already being done more cheaply at a factory.
Commercial batteries absolutely have a role in a home energy storage system for spreading power generation from the sunny part of the day to the night and perhaps bridging over a cloudy day or two. However, commercial batteries cost at least about $50/kWh, so a 100kWh battery costs about $5000 (at the cheapest). We're talking about 90 days of energy in the summer that we need to store, and we're not interested in buying 1/2 million dollars worth of commercial batteries.
> What makes you think you can beat what's commercially available?
We have different requirements.
1. Commercial battery cells must be self-contained with no user-serviceable parts. Nobody wants a battery they might have to open up and clean once a year. For our needs, some amount of greasing, oiling, cleaning, and inspecting is not unreasonable.
2. Commercial batteries are usually designed with EVs in mind so they are expected to charge really quickly. If we have a battery for storing 90 days of summer energy, it's okay if it takes 90 days to charge.
3. A commercial battery cell must not require on-site assembly. We're assembling the cell already, so this restriction has zero value to us.
4. Commercial batteries need to have (at least) good enough energy per weight that they're worth shipping over from China. As they're typically designed for EVs or other portable applications, the energy density expectation is usually far higher than that. We care far more about the up-front cost per kWh than the weight.
5. Commercial batteries typically have a round-trip efficiency in the 90+% range. A battery chemistry that loses 20% of its energy charging and discharging might be dismissed as commercially infeasible, but when we compare it with pumped hydro at 70% efficiency, and various hydrogen storage models with 10-50% efficiency, we see that fairly lossy batteries can still be quite practical.
On our side, we have three requirements that commercial battery companies don't have:
1. We need our battery components to be *readily available*. Something like LiPF₆ electrolyte salt might be "cheap" if you buy it by the train car load, but you won't find it at the corner store, and buying it in small quantities might lead you to lab supply companies with very high markups.
2. We need a chemistry which is tolerant of some contamination. We don't have a factory where we can exclude water and oxygen down to parts-per-million levels, so we'd rather avoid chemicals that react with them rather than having to build one.
3. We'd like our chemistry to be more or less human friendly and realistic to dispose of. We can be expected to act like adults and use PPE, but it would be nice not to have mustard gas coming out of our cell when we open it for inspection. Moreover, if we have 100 tons of acid, we would like to have a plan to neutralize it rather than just: Truck it all to a toxic waste disposal company at massive expense, or dumping it in the storm gutter which is a crime and is gonna leave a LOT of evidence when it kills absolutely everything.
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The simplest kind of battery is basically an electrolysis cell. You put salt water in a cell with anode, cathode and ion exchange membrane, the salt splits into NaOH and HCl. Discharging it turns the NaOH and HCl back into salt. You can store about 0.1 kWh per kilogram of salt this way. Your salt concentrations should be somewhere between 1% and 11%, so lets say you are able to convert 1kg of salt per 10kg of water. Figure 1, from this fabulous YT:
https://www.youtube.com/watch?v=eq7fR9ISuCw (but note this is not the iron battery he describes)
This is obviously challenged in terms of energy density. You're looking at 10kwh per ton (cubic meter) of water. But just because you're limited to 10% salt in water doesn't mean you're required to keep that water indefinitely.
The same chemistry works in acid or alkaline as it does in neutral PH, so after you've finished making your salt into HCl and NaOH, you can then take each component and electrolyze them in isolation, turning even more salt into even more HCl and NaOH.
It turns out that you don't need to pump the electrolyte from one cell to another, all you need to do is add more ion exchange membranes, and you raise the voltage since now it is a series cell stack. The same rules apply - one side becomes acid and the other side becomes base, it's just that they become far more acid and base than they would.
Now that brings us to our extremes:
At 37% HCl in water, the HCl begins leaving as a gas.
At 50% NaOH, the NaOH begins settling out as a solid.
There are two storage choices, either store enriched water, or store solids. 1kg of NaCl makes 0.684kg of NaOH and 0.624kg of HCl.
If we choose to store 33% HCl in water rather than letting it get to 37% and offgas, we can store our 0.624kg of HCl in 1.63L.
On the NaOH side, we'd be better off driving it to crystalize out and then using settling ponds and spiral classifier to get it out of the liquid and store it in a semi-dry storage. It's more mechanical hardware, but the resulting material to store is a solid which is much safer. Our 0.684kg of NaOH comes out to 0.32L of solid material.
So this way, 1kg of salt turns into 1.63 liters of liquid, giving us 100 watt-hours of storage. 1 tonne of salt into 1.63 cubic meters of acid plus 0.32 cubic meters of lye gives us 100 kWh of storage.
This is "okay" but not great, because we're tying up a lot of acid and if it spills, that's a serious environmental disaster. It's kind of obligatory to put the acid in a double-walled container with water on the outside so that if the inner wall fails, water goes in rather than acid coming out. But 163 cubic meters is "a couple swimming pools" and would store 10,000 kWh - or 500mn$ worth of batteries.
Batteries are a massive area of research so there's a lot of other stuff I can write. Eliminating the storage of large amounts acid is possible by playing with the chemistry - and I may or may not continue in a part 2...