Egregoros

Okay, it is time for another longpost on one of my favorite topics: Energy storage.

In case you haven't seen the previous posts, here's a short rundown on the situation:
1. If you install solar, you end up with a TON of energy in the summer and very little in the winter.
2. A battery the size of an EV will store about a day of energy, there really isn't anything that makes sense for storing a whole year of energy. There are a LOT of options, but they all suck.
3. The best option seems to be generating hydrogen and converting that into methanol, but for methanol you need carbon as well as hydrogen. This must be supplied as carbon monoxide or carbon dioxide.

Once you have methanol, you can burn it, run an engine off of it, convert it into gasoline, or use it with vegetable oil to make biodiesel. It's a highly versatile form of liquid stored energy.

The process to make it would look like this:

Sun -> Electricity (+ water) -> Hydrogen

Wood / Woodchips / Hay / ... -> Carbon -> ???? -> CO / CO2

Hydrogen + CO / CO2 + pressure -> Methanol + heat

The methanol process requires about 100bar (1500 PSI) pressure and 250C (500F) temperature, but the process makes heat, so once it's running the only thing you need is compressed inputs.

And the big question is how to get the CO / CO2. You can of course burn the fuel, but it poses two problems: 1. burning is very dirty, and the methanol reactor requires very clean inputs. 2. All of the heat you make from burning the fuels is totally wasted. The methanol process makes heat so you don't need any more heat. To add insult to injury, this is happening in the middle of the summer when heat is the LAST thing you need, so basically any heat made by burning carbon is going to be wasted.

NOTE: You cannot pass hot gasses into the methanol reactor because they must be pressurized, and to compress them with any kind of efficiency, you need to cool them down. So if the creation of the gasses involves high temperatures, you either need a massive heat exchanger to re-capture the heat, or else you need to tolerate a lot of wasted energy (or both).

Enter Carbon Assisted Water Electrolysis (CAWE):

So, this is tricky, but in THEORY, you can use carbon in the normal (room temperature) hydrogen electrolysis process, and instead of making oxygen, it will bond with the carbon and make CO2, AND it will reduce the electricity needed to make the hydrogen because the energy released by oxidizing the carbon is assisting the electrolysis process. See picture.

There's a paper (attached) on various efforts to do CAWE. It's challenging because the carbon doesn't want to break down at room temperature.

Best results were seen electrolyzing in a hydrochloric acid bath, with added potassium ferricyanide. The iron in the potassium ferricyanide works as an oxygen shuttle, picking up oxygen in solution, oxidizing the carbon, and then going back to pick up another.

One open question is whether there's anything slightly easier to make/get and less toxic than a cyanide. The key workhorse is the iron, not the cyanide, so it's plausible.

Another question is whether this can be implemented as a Bipolar Fluidized Bed Electrode - meaning you pump a slurry of carbon particles, and each particle acts as a tiny "neutral plate" in the electrolysis cell - whilst also being consumed.

The last point I'll make is that if CO2 + H2 can be generated from carbon/water slurry, it is possible to run the entire electrolyzer under pressure. This eliminates the need for a physical compressor between the H2 + CO2 generation stage and the methanol stage.

I got GPT to make a fairly reasonable diagram of the process.

The inputs to the main process are water, electricity, and charcoal powder.

Step 3 makes both CO2 and H2, but not in the right balance for step 5. Therefore we need a second electrolyzer (4) which is the classic style, making H2 and O2. This electrolyzer needs to make roughly 1/3 as much hydrogen as the first.

Because step 5 needs to run at 100 BAR (1500 PSI), it's attractive to put (3) and (4) under pressure as well, because it's a lot easier to pump 1500 PSI water than it is to compress 1500 PSI gasses.

1500 PSI is about half of SCUBA tank pressure, so it's no joke. Any parts that are under that much pressure need to be thick walled metal, NOT welded sheet metal like a propane tank.

Step 5 needs some initial heating to around 250C / 500F, but once it starts working, then it needs cooling because the methanol reaction is exothermic. Steps 3 and 4 would benefit from some higher temperature, particularly step 3 which has the difficult task of cracking solid carbon, so waste heat could be recycled.

However, this reaction actually produces quite a lot of heat and it just needs to be wasted.

Step 3 reaction is: C + 2H20 → CO2 + 2H2

Step 4 reaction is of course: 2H2O -> 2H2 + O2

Step 5 reaction is: CO2 + 3H2 → CH3OH + H2O

For 1KG of carbon, Step 3 makes 1/3 KG of H2 and requires an additional 1/3 of that amount (110 grams) of additional H2 made from step 4.

Typical H2 generation (60% efficient cell) is 66kWh/KG, but (re the previous paper), step 3 generation should cut that number in half. So expect 11kWh in step 3 + 7.26kWh in step 4.

The final result is 2.67 KG of methanol with a total of 14.77 kWh of chemical energy. The energy inputs are electricity: 11kWh in step 3 + 7.26kWh in step 4, and chemical (9.2kWh per KG of carbon). A total of 27.46kWh of energy input. 12.69kWh of energy is lost to heat, mostly in the hydrogen generation step.

2.67KG of methanol is 3.36 liters, or 0.9 gallons, and the electrical input needed to make it is 18.26kWh.