From the article:

"Reactors that could power the hydrogen extraction are small enough to be transported by truck and would occupy a building one-10th the size of a nuclear power station."

They are talking about a specific type of small nuclear reactor tightly coupled with the synthetic fuel generation.

Just speculating, but the waste heat from the nuclear reactor itself could be fed into the fuel synthesis process, making it a sort of combined cycle system. That could be more efficient than trying to use renewables, which generate very little in the way of waste heat.

Fischer Tropsch is typically what people are talking about when they talk about synthetic fuels, and when that is the case, extra heat doesn't help, apart from starting the reaction. You need enough heat to get it to about 250-300C, but once the reaction has started, you need to aggressively remove heat from the reactor, as the reaction is highly exothermic.

Even so, seems like pricing would work out better with the reactors attached to the grid. Lock in a rate for providing baseline power, and then bid for renewable energy to power the facility when spot prices are low enough at times of peak production, and ramp up synthetic fuel production at the same time.

You can keep using process heat from the reactor since it's already operating anyway, but get much cheaper electricity (sometimes even negative prices!) from peak renewable generation and turn that into synthetic fuels for storage.

The challenge with many environmentally-powered renewables is specifically that they can't be used for spot production.

Nuclear tends to share this. Most nuclear reactor technology is not optimized for rapid swings in production. A reactor is a very expensive thing to not run most of the time.

The two technologies aren't terribly compatible. The first tend to be intermittent (not great for on demand needs) and the second are best suited for base load.

What you need is something that is relatively cheap to maintain, and can be ramped up quickly. This is where you typically see natural gas peakers in a fossil-fuel based system.

Wikipedia summarizes these well:

https://en.wikipedia.org/wiki/Peaking_power_plant

https://en.wikipedia.org/wiki/Base_load

For peaking demand you need something that can store energy and can adjust to load rapidly. Batteries do this (I include mechanical solutions as batteries, e.g. a flywheel or pumped hydroelectric). Natural gas plants tend to fill this role now. These are not the only options, just common examples.

Could the excess energy be fed into the local grid during non productive times, help balance out the cost of keeping it running?

In short, yes, but there are limits. A really (really) rough analogy would be filling a tank of water. If you fill it faster than it can drain, eventually it will overflow. If it is sealed, it may burst. The grid equivalents of an overflowing or bursting tank are not great scenarios.

And with intermittent energy sources, there are additional problems. We only get energy sometimes. We want energy always. It basically inverts the idea of base load and peak load sources. If your source is intermittent, then you do want to be able to over-produce at certain times, with an ability to capture the energy. Think batteries, but know that they need not be chemical batteries. Energy storage can then cover the valleys of production or the peaks of demand.

Add coordination to the challenges as well. Much (most?) solar is distributed. It's much easier to ramp generation up or down in a few centralized plants. If every home has a solar panel array, then you need to be able (either at the local or the grid level) be able to store excess energy or turn off production.

None of this is insurmountable, but there are many challenges that are non-trivial by nature and some that are difficult d/t the way energy grids are typically set up.

In an efficient market that's probably the way to go. But it depends partly if your production process can scale up and down easily, which is rarely true in general. It could be designed like that though.

And now your coslty electrolyser only run a few sunny or windy hours a day?

That depends on the capital expense of the fuel plant. If it's a high percentage of the fuel cost, then it needs power 24/7.

On the other hand if energy is the dominant cost, then it's best to take cheap low-cost energy and shut down fuel production other times. That would apply in a nuclear grid as well; you could keep your nukes running 24/7 and run the fuel plants when demand is low.

from the article:

> The plants, costing 1.8 billion pounds ($2.4 billion) apiece, would feed the national grid and come online from the 2030s, with all complete by 2050.

I agree, replace all the current coal/natural gas/etc factories with nuclear. It's here, it's well understood and it can be done. It will serve us well until fusion is perfected. It will end the global warming issues and we can even take older more fallible reactors offline.

Nuclear reactors must be periodically shut down for days to months at a time for maintenance. That fact often fails to be mentioned when the 'constant reliable' claim is raised. The Sun, on the other hand, rises each day.

Renewables 'on a large scale' become less reliant on storage. Their 'viability' depends on the grid connecting them. On a continent-sized grid the wind is very likely to be blowing. Coupled with full-scale decentralized storage, the viability question blows away.

An electrolyser is CAPEX expansive based on power usage, so you wouldn't want to run it at 10% to 40% charge factor.

Clean energy seems wildly impractical for airplanes. There is no battery tech approaching kerosene for power density, and unlike fuel, the battery doesn't get lighter as it's used up.

The article suggests compressed hydrogen. Although that has obvious problems (at some point there is probably a tradeoff between increased density and the weight of the storage tank required to safely hold that density, which directly related to energy density) and probably more subtle problems, it would be clean-burning.

Edit: Random wikipedia page[1] gives energy density of "jet fuel" as 35 MJ/L and liquid (i.e., about as compressed as you're going to get) hydrogen as ... 8.5 MJ/L. Yeah, that's not even close, nevermind. LNG gets you to 22 MJ/L, which is still 2/3 of the density and requires storage at -160°C, which costs more energy, etc, and isn't clean-burning (but maybe slightly better than kerosene).

[1]: https://en.wikipedia.org/wiki/Energy_density

But a liter of compressed hydrogen weighs 38 grams, while a liter of kerosene weighs 810 grams. So the energy density by gram, rather than liter, is in favor of compressed hydrogen by a factor of more than 5x.

I don't know what the tradeoff is between volume and weight in aviation, but it's worth noting that we can measure density by both volume and weight, and by weight compressed hydrogen defeats kerosene.

If you are going to compare hydrogen and kerosene by weight you need to include the weight of the containment vessel. For kerosene that is a thin piece of aluminum. For hydrogen it is...not.

In aviation weight is more important. Volume is important, but weight (and distribution) is more important.

So, for sleek jets, heavy but compact jetfuel will rule. But for zeppelins, the weight-to energy ratio is much more important, as they have plenty of volume to spare..

That's a good point I hadn't considered, thanks.

They are not suggesting hydrogen. The article is specifically about _electrofuel_, which is extracted from a mix of hydrogen and carbon dioxide in a very power-hungry process; it’s a transparent liquid similar to diesel, and energy density is not too far off.

As jemfinch points out the value that is important is the Wh per kg, or MJ per 1000 kg, not the liter volume. Though certainly the carbon fiber composites tanks required to store hydrogen at very high pressures are not light in weight.

One of the reasons why electric aircraft are currently very short range is that the best li ion batteries top out at 250Wh/kg.

This was largely the point I was making

Liquid ammonia contains 15.6 MJ/L (half of diesel). Ethanol - 24MJ/L. Methanol - 15.6NJ/L. Mix of ethanol or methanol with ammonia can replace gasoline[1][2]. Both ethanol and ammonia can be generated from air using solar energy.

[1]: https://nh3fuelassociation.org/wp-content/uploads/2012/05/le... [2]: https://asmedigitalcollection.asme.org/IMECE/proceedings-abs...

Hydrogen is much lighter though. You wind up with big ugly planes that are lighter than the planes we have now. Apparently they’ll be able to use shorter runways as well.

You're forgetting the tank. There are a few hydrogen powered cars on the road, their tank weights about 125kg and stores a whopping 5kg of hydrogen.

The article mentions hydrogen at the start but then says the hydrogen would be used to produce synthetic jet fuel with the properties of regular fuel.

So the idea being: if you pull the carbon out of the air to make your kerosene, burning it again is carbon neutral? That makes sense.

"Synthetic fuels" means jet fuel. There are various ways to make synthetic kerosene with atmospheric CO2.

https://en.wikipedia.org/wiki/Jet_fuel#Synthetic_jet_fuel

I believe the idea is that you can use hydrogen to synthesize kerosene, which assuming you use clean energy inputs, is carbon neutral. There was a similar sort of startup that showed up on HN a few months ago.

It is already taking over the light plane market. Short haul flights will all be electric in less ten years.

I think you're being wildly optimistic here about the state and future of the technology