The EU decided to declare gas along with nuclear to be green. It's easy to see why one might see nuclear energy as a green energy source, it's harder to see the case for gas. 

As I previously pointed out, we currently have days in Germany where energy is free and companies can get paid to use up energy. The continuously lowering cost of solar power suggests that those days will increase. Rolling out enough solar and wind to be able to power everything on most days means massive overproduction in electricity on summer days that can be used for applications that need a lot of cheap energy but that are okay with only running when energy production is high. One option for using the energy is to turn CO2 into Synthetic Natural Gas (SNG):

The overall efficiency of energy storage and reuse (power-to-power) is quite high. In the case of SNG synthesis, the energy stored in methane accounts for 53.2% of the input energy. The maximum recoverable energy in the form of heat and electricity is 37.3%

If we predict a future where we will use surplus solar energy to produce SNG, it today makes sense to build gas plants to provide us with energy on winter days with little wind. It’s building plants that will actually provide green energy in the future, which justifies seeing them as green in the EU energy plan.

It also suggests that moving a huge sector like shipping to using natural gas is a valid strategy for making the sector completely green. 

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The overall efficiency of energy storage and reuse (power-to-power) is quite high. In the case of SNG synthesis, the energy stored in methane accounts for 53.2% of the input energy. The maximum recoverable energy in the form of heat and electricity is 37.3%

This would possibly be fine if methane was an end in itself. However, in this case it's just using methane as a scalable but grossly inefficient battery. There are tons of cheap storage technologies that are far more efficient than that and scale just as well or better.

There are tons of cheap storage technologies 

What do you consider better storage technologies?

Latest results from hydrogen electrolysis research show 95% efficiency on theoretical limit converting electricity to hydrogen.

The current mass market fuel cell conversion from hydrogen to electricity is about 70% efficiency.

I am under the impression that hydrogen storage and transport for static usages are not that significantly different from natural gas. Or natural gas facilities can be converted to manage hydrogen with relatively small cost.

I am under the impression that hydrogen storage and transport for static usages are not that significantly different from natural gas.

Hydrogen takes up around 3.3x more volume for the same amount of energy storage as SNG. If you have a lot of energy that costs you essentially zero in some parts of the summer storage costs can become more important than efficiency. 

according to wikipedia, pipeline storage of Germany is currently capable of several months

https://en.wikipedia.org/wiki/Hydrogen_storage#Power_to_gas

 

I confess I do not know what they mean by that precisely, but I am under the impression that hydrogen storage as low pressure gas is very viable

In fact I if the pipeline can handle town gas, I don't see why both hydrogen and natural gas can not be produced at the same time, to hit the pareto efficiency. Natural gas more heat storage per volume, hydrogen better heat storage efficiency, depending on storage capacity, projected energy production and usage a theoretically optimal mix can be calculated.

Gravity batteries can be pretty large, and have up to about 90% efficiency. They require infrastructure, but so does SNG.

If you have a lot of very cheap energy in the summer efficiency is not central. The storage costs for the energy become more important. I have the impression that storing liquid gas is vastly cheaper than storing energy in gravity batteries. 

Efficiency is still pretty central in that the summer stored energy is worth at least what it would return in the winter, and often more. Methane synthesis is still pretty expensive and definitely not very efficient, so a better alternative would be to run particularly energy-hungry industries at full capacity in the summer and slow down in the winter. This wouldn't be any more wasteful of capacity than having a major methane-producing industry that runs at full capacity in the summer and not at all the rest of the year.

If methane synthesis was substantially cheaper, then it could be useful to provide methane as a feedstock to various other chemical processes, which would also be more valuable than just burning it.

Holding energy in methane is cheap, as is holding energy in towers of dirt-cheap blocks or dams full of water. What's expensive - for all of these processes - is the equipment to transfer the energy into stored form. On the large scale there isn't really any known way to put a gigawatt-season of electrical energy into storage more cheaply than just building the capacity to produce an extra gigawatt in the winter. Methane doesn't come close. Gravity storage doesn't come close. Pumped hydro doesn't either, and nor does hydrogen.

That's why grid-scale energy storage systems focus on the short term: typically up to a few hours, with more speculative projects looking at a week or so.

Probably depends where you are. in NZ, we already HAVE a bunch of hydro dams, so tweaking things to storage is probably more practical. In a country which were say... substantially dryer or flatter, Synthetic Gas might be the way to go.

You can't just release most of the water stored in a hydro dam whenever you want. If you would do that you would flood the land after the dam. 

On a  country level I would imagine there are not that big of spikes in demand. Maybe in winter you have 3 to 4 times more flow than without storage configurations. I have trouble imagining what kind of use case would call to throttle between no water level loss to thousandfold flow.

This is predicated on the claim that solar energy will be so cheap in summer there'll be no point paying for efficient storage, and a roughly 1/3rd storage efficiency is fine.

Let's do some quick numbers on this.

Let's make the simplifying assumption that energy requirements are constant throughout the year, solar energy availability follows a sin wave, with peaks at the summer and troughs in the winter. Let's also assume that solar power is twice as powerful in summer as winter.

Then if storage efficiency was 100%, you'd need peak summer solar capacity to be 1/3rd more than requirements.

If storage efficiency is 1/3, then peak summer solar capacity has to be 4/5ths more than requirements. (Sorry can't be bothered showing my working).

In other words using storage with 1/3rd efficiency instead of 100% efficiency requires installing 40% more solar capacity.

Somebody would have to check the relative price of solar Vs batteries to see whether it's cheaper to have excess solar capacity, or to have more expensive batteries.

solar energy availability follows a sin wave, with peaks at the summer and troughs in the winter. Let's also assume that solar power is twice as powerful in summer as winter.

Those are bad assumptions. Solar efficiency in winter is lower than that compared to summer.

Additionally, it's no sin wave but highly irregular and you want to still have energy when you hit one of those irregular spots where there are multiple days after each other and clouds block most of the sunlight.

Right now with the amount of solar capacity we have, there are days when energy is free and you even get paid to use up energy in Germany. 

If you have three times the energy production in summer with solar cells that you need in winter, you don't have reliable energy in winter without energy storage. But you do have a massive energy surplus in summer that you can put to use. 

Either way, the point still stands.

You're never going to have so much peak capacity that you can store it all as methane and not need to use any other energy sources during the night/winter. Or at least doing so would be very expensive.

So you're going to have to use other energy sources at solar trough times. If we can dirty energy sources, energy at such times is likely to be expensive.

So the question stops being about how cheap energy is at peak times. It's about whether it's worth storing 3 times as much energy to sell when it's most expensive, but using more expensive energy storage solutions.

At the moment levelised cost of storing a kWh in a gravity battery costs roughly the same as producing it does (highly dependent on country). However if due to high energy prices, or future improvement in technology, levelised cost of storage is more than 1/3 cheaper than producing electricity at the most expensive time in some particular region, it will always be worth using a gravity battery over SNG.

At the moment levelised cost of storing a kWh in a gravity battery costs roughly the same as producing it does (highly dependent on country). 

That depends a lot on how long you store the energy. As far as I understand most of the projects expect to use the energy within less than 24 hours to reach the cost efficiency you are talking about. Storing energy from summer to use in winter is two orders of magnitude more costly with gravity batteries.

I'm working at a startup trying to rapidly scale CO2 --> SNG right now. So if anyone has questions about the technical challenges & limitations, let me know.

Do you expect to be cost competitive with natural gas. If so, by when?

I would have expected that at the moment the tech is not cost competitive and thus can't be scaled up via the usual startup pathway.

Apologies for the delayed response, I hadn't seen where the notifications were until now!

We expect to be cost competitive with natural gas in Arizona sometime in late 2024. That area expands to much of the Western US by 2026. We are planning to expand by a more-or-less traditional startup pathway.

We're using a setup with very low capital costs to allow us to scale rapidly as soon as it's profitable.

I don't love gas but one point that hasn't been brought up explicitly is that gas is "dispatchable". Since the grid needs to be exactly balanced at all times, you need something that can ramp up and down easily. Obviously storage can smooth this out too but gas is often seen as a complement to solar and wind because you can turn the "gas -> electricity" dial up and down pretty much at will. this is much harder to do with coal and nuclear (although nuclear is also carbon-free in it's own right).

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