MIT engineers make converting CO2 into useful products more practical

Perhaps someone with more knowledge can comment on why solutions like these can't be used to solve the energy storage problem. Is it just economics?

That is, renewables are now the cheapest form of energy by a significant margin, but they are unreliable with respect to timing, so a storage solution is necessary in order to provide electricity on cloudy days when the wind isn't blowing, at night, etc. Most of the research I've seen into solving the storage issue involves batteries or things like pumped hydro. If things like solar and wind were "overbuilt", could a solution like this be used to create hydrocarbons when there is excess electricity? Power prices already go negative in some places when it's particularly sunny/windy. If the excess energy at that time could be used to make gas that could then be utilized by gas plants, well then there is your net 0 storage solution.

I'm assuming solutions like this are uneconomic (and similarly with hydrogen plants, e.g. by using the excess renewable energy to generate green hydrogen by electrolysis for storage and later use), but I'd like to understand better why.

yep it should be possible. check this out: https://terraformindustries.com/

There actually have been several solutions and some proofs of concept offered. However except for things like batteries, the purists object to all of them as green washing. Why, I don’t know.

Off the top of my head, I believe someone demonstrated you can add thermocouples to your water to generate electricity. The idea was that during excess electricity generation during the day by a homeowner’s solar panels, use that to heat up the existing water tank. At night, use the thermocouple to generate electricity from the hot water. Granted the efficiency is abysmal. But 5% of something is better than 0% which is what happens when the electricity is thrown away.

> I'm assuming solutions like this are uneconomic

You kinda answered your own question already, I feel. The energy efficiency of cycling a battery (70-90% for grid scale) or pumped hydro (70-85%) is simply much, much higher than chemical storage. Here's a pretty recent one [1] showing 23% efficiency even at lab scale, and as described in the article scale is a big drain on efficiency.

[1] https://www.nature.com/articles/s41467-022-29428-9

It's essentially just another form of energy storage. I don't think there's any deep reason why it is worse than the other methods currently available, it's just not cost competitive.

My understanding is that creating hydrocarbons is quite difficult and that you lose a lot of energy in the process. Otherwise, it would be a very compelling way of storing energy.

I guess for one, you have to get the carbon from somewhere, which means either taking sequestered carbon (which is counter productive) or capturing it from the air (expensive).

We need vast amounts of energy storage is the problem and that won't be cheap no matter how you look at it. 20 years ago I saw an annalists that suggested the US needs the equivalent of lake Superior to get enough hydro storage - that is that much water, plus the ability to drain it all in just one day (to where!), and then pump it back up the next. Pumped hydro where we can use it should be used, but there isn't any place we can put it left (and we want to take some of what we have out because it is an ecological disaster). Batteries work but are expensive. This would probably work as well, but again be very expensive.

Remember you are competing with something we can pump/dig out of the ground for nothing anytime you propose storage. Renewables when the wind is blowing or the sun is shining are very cheap, but as soon as you need storage the costs go way up.

> Is it just economics?

It's ~75% economics, 25% learning curve.

For hydrogen, you need an electrolyzer, a hydrogen fuel cell (or turbine), and storage. The electrolyzer is the main capital cost, and it is only running for a fraction of the day (either whenever there is curtailed solar/wind, up to 40% of the time you have your own captive plant). It needs to be sized for peak usage. The storage optimum depends on whether or not there is a nearby salt dome, but if not it is extremely expensive per kWh, and so days and days of storage are untenable (going directly to CH4 changes some of this). Existing fuel cells and H2 turbines have not yet walked down the learning curve in the same way that an NGCC plant has for CH4, but those are running 24/7 so the amortization is not as bad.

With a salt dome and captive PV plant, you end up with (optimistically) system capital cost that roughly doubles the PV capital cost (using US pricing). That means your amortized $/kWh rate is about 2x the PV rate. Since PV and NGCC are roughly the same $/kWh at the plant, it makes H2 extremely uncompetitive unless there is a carbon price or H2 subsidy. At $3/kg hydrogen, it's almost just barely within reach assuming everything works well. If the cost of electrolysis came down, or if H2 were easy to ship globally from high insolation regions, that would substantially help the problem.

We need massive amounts of medium-term seasonal (3-6 months) stable energy storage, and liquid synthetic hydrocarbons are not a bad solution. Low efficiency isn’t a dealbreaker when the inputs are free.

> Perhaps someone with more knowledge can comment on why solutions like these can't be used to solve the energy storage problem. Is it just economics?

Yes. If you round-trip energy through hydrocarbons, then you have to pay the "Carnot tax". Your heat engine will be at best around 50% efficient at transforming hydrocarbons into energy. This is then compounded with the inefficiency of reducing carbon dioxide to get maaaaybe 20% round-trip efficiency.

And all of this with a huge capital cost.

It's not quite feasible with solar or wind.

If we could get controlled fusion though, we are going to see a massive surge of what you're suggesting.

The tax is fine _as long as_ it doesn't have to be transported, assuming the energy would otherwise be wasted.

Which is why hydrogen solutions for stationary storage could be interesting, but the moment you start transporting them around they become less useful.

I'm not seeing that. Hydrogen requires a ton of very expensive infrastructure for storage. Its density is impractically low for storage in tanks, it can't be liquified under reasonable conditions, and reversible hydrogen-binding materials so far have all been duds.

If you happen to have an underground geological storage available, then it might be reasonable. Right now, there's a demonstrator project for that ongoing in Germany. I guess this qualifies as "local"?

So yeah, if you need storage for 3-12 hours of runtime, then batteries are fine. Sodium batteries are probably going to fit this niche once they become cheaper. Anything more than that is a big gaping hole in the renewable story with no good solutions.