Caltech have found a way to use Cerium Oxide to break down atmospheric H2O and CO2. Article here. They expect to be able to use this to synthesise methane. Hopefully later they’ll manage more complex hydrocarbons.
At 1% efficiency, it’s not exactly currently useful right now. But they say they can get it to 15%.
But I’m wondering why they don’t just skip using sunlight and hook the thing to a nuclear reactor?
Because nuclear reactors don’t produce CO2 and H2O?
I totally don’t get your question. Are you asking why do they use the energy from the sun and not a nuclear reactor?
Because sunlight is free, and the chemical reaction might be activated by light? (I didn’t read your link; I’m just confused by your question)
If you read the article, you would not be confused.
If you’ve already got a nuclear reactor or other large power source, you can easily generate hydrogen via electrolysis, with something like 50% efficiency. There’s little need for exotic new processes that turn electricity into fuel.
Right now there’s all sorts of research into converting solar energy into other useful forms of energy. We can turn light directly into electricity with solar panels – that’s nicely scalable but not very cheap or efficient. You could easily run your electrolysis rig with a solar panel, but that’s even less efficient (and an already solved engineering problem). But wouldn’t it be great to efficiently convert sunlight directly into some sort of chemical energy? Imagine one of these reactors in a remote village or research station – it just sits around all day, generating fuel for when you need it.
It would be great, except it’s not terribly efficient. They’re hoping for efficiencies of up to 15%; you’d have to have a huge solar furnace to convert a useful amount of CO2/H2O into methane. Photovoltaic cells are already available with efficiencies near 25%; if you’re looking for a means of energy storage in your remote village, you’d do far better with a solar panel powering a pump that moves water to an elevated storage tank, and then use that elevated water to drive a pelton wheel/alternator. If you’re looking for a means to reduce greenhouse gases in the atmosphere, this probably isn’t it: you’d have to have absolutely tremendous furnaces to make a dent in the grand total quantity of atmospheric CO2.
It’s not so much the hydrogen they’re generating that interests me but the carbon monoxide which they use to make methane. If they can make methane, then hopefully more complex hydrocarbons will follow.
That’ll be pricey on a large scale.
I agree that it’s not yet a world-changing technology (though still worthwhile research). But – assuming this is developed to the point where it’s practical – why isn’t it part of a solution for climate change? Every Watt-hour we get from solar furnaces is one Watt-hour we don’t need to get by burning fossil fuels. Even if you don’t think solar can possibly supply every single bit of our energy needs, it still helps.
The point of this research seems not to generate electricity but to create transportable fuel. Hydrogen and methane are transportable with effort; that doesn’t apply to more complex hydrocarbons like methanol, alcohol, and ethane which are liquid at STP. This is very much a first step.
Use a nuclear reactor and you generate little CO2, the converter can operate 24/7, and be much more efficient because the heat will be more effectively trapped - no quartz glass panels.
It depends what you’re comparing it with. As a means of producing carbon-neutral, liquid fuels from solar power, their hoped-for 15% efficiency probably beats the hell out of fermenting grain to ethanol, for example. However, as a means of cutting greenhouse emissions as a whole, it’s pretty poor. Using photovoltaics to supplement fossil-fuel power plants and then using the fossil-fuels saved to synthesise liquid fuels saves more carbon dioxide per square meter of solar energy.
Given a cheap source of electricity, then ammonia would be a practical vehicle fuel and a proper combustion cycle produces only nitrogen and water, so it doesn’t release any GHGs. The reason that CNG is more discussed as a fuel is because we have to make ammonia, while natural gas we just pump out of the ground.
I believe there have been some studies at the University of Minnesota about using the electricity of wind turbines to produce ammonia instead of trying to put the electricity on the grid.
Because a nuclear fission reactor generates heat energy via a steam or pressurized fluid cycle, which is limited by the thermal allowables of the materials used to contain and transfer the fluid. Even the Very High Temperature Reactor (VHTR) only gets to around 2000 deg F, which is well below the critical temperate for catalyst reactions using ceria. And of course, there is the inherent cost in the nuclear fuel and energy production cycle, which even aside from the regulatory cost of licensing and oversight is relatively expensive. On the other hand, sunlight itself is free, both in fiscal and thermodynamic terms. Once you have met the capital costs of installing a solar collection facility, you have only the maintenance costs to consider (although depending on the technology and type of facility these can be significant).
The benefit of this system is the direct conversion of solar energy into storable chemical potential energy without going through a heat engine cycle. Even at 1%, it is still essentially free energy without loss; sunlight will fall regardless. At 15%, if that is in fact plausible, it will very likely be favorable to PV solar which is then converted (with attendant losses) into battery storage or chemical energy.
Stranger
So this is a practical limitation. Can it be overcome? Either directly by using a Thorium reactor which could get up to 1750C before the Thorium melts? Or indirectly, by using electricity generated by the fission reactor in the normal way for a normal electric furnace? Or something half-way between the two?
Why would you bother? Aside from the practical problem of dealing with a heat conversion cycle with temperatures that are way beyond that of normal structural and plumbing materials, the ceria reactor is purely an energy conversion device, transforming thermal energy to chemical potential energy. That means that efficiencies in the energy production process are compounded on top of the energy conversion process. With solar energy, we don’t really care what the efficiency of the energy production process is, as it doesn’t cost us anything. Solar energy is “free” in the sense that we don’t have to support any fuel production or disposal cycle; the only “costs” are those associated with converting incident solar energy flux to electrical or thermal energy, and thence to a working cycle or storage media.
Using a nuclear fission cycle as the energy source requires multiplying the efficiencies of the fuel production cycle, the nuclear fission cycle, and the Rankine heat conversion cycle. You definitely don’t want to operate a fission reactor close to the fuel melting temperature, as this is literally inviting catastrophe. If you add conversion of thermal energy to electricity which then powers an electric furnace (which is phenomenally inefficient) I doubt you’d even get a composite efficiency that would exceed the 1% currently capable by direct solar heating.
Stranger
Plants do this every day. Photosynthesis produces tons of chemical energy.
Reliability. Solar power only works when the sun is shining.
The Sun is always shining; it is just the case that some places on the Earth’s surface are concealed by cloud cover, a problem that can be readily ameliorated by locating the fuel production facilities in places that have low levels of cloud coverage and high angles of solar altitude to maximize solar flux.
Stranger
Alternatively, there’s this story about bioengineered cyanobacteria which positively invites scepticism.