An Overview of Solar Fuel Technologies
By: William Meng
Tags: Oil And Gas, Sustainability, Aviation, Green Energy, Solar Energy

Solar Reactor

After capturing the CO₂ from the atmosphere, the next step to making solar fuel is to heat up CO₂ and H₂O in very high-temperature solar reactors, where CO₂ and H₂O are endothermically reduced to CO and/or H₂, which can then be fed to downstream processes for the synthesis of liquid fuel.

The basic chemistry

As mentioned previously, the basic idea is to reverse the combustion reaction and turn CO₂ and H₂O back into liquid hydrocarbon fuel. Therefore, the first step would be to remove the extra oxygen atoms from CO₂ and H₂O and create the more chemically unstable (i.e. having more potential energy) molecules of CO, H₂, and O₂.

CO₂ + H₂O → CO + H₂ + O₂ (∆H = 524.8kJ/mol)

Thermodynamically, this is obviously a very endothermic reaction, since we need to put in at least as much energy as was released during combustion. To facilitate this reaction and get an output stream of synthesis gas (syngas) consisting of CO and H₂, we need a catalyst that can strip the oxygen molecule from CO₂ and H₂O and then release the O₂ in an environment separate from the CO and H₂.

It’s worth noting that, while we are going to produce both CO and H₂ as intermediaries at some point in the process to make hydrocarbon fuel, they don’t necessarily have to be generated together in the solar reactor. Although ETH Zurich’s SOLARJET project used a reactor that could simultaneously generate both CO and H₂ from a CO₂/H2O mixture, it is also viable to generate either CO or H₂ exclusively in the solar reactors, and then use either Water Shift Reaction or Reverse Water Shift Reaction to create a CO/H₂ mixture.

The choice of catalyst and reaction pathway

Whether to generate syngas (CO/H₂ mixture) directly or to produce only H₂ or CO in the solar reactor affects the overall reaction pathway and the types of catalysts that may be used.

Over the past 10 years, a number of different catalysts have been proposed. Most of them are in the form of metal oxides, which can release oxygen when heated during their oxygen-rich stage and bind to oxygen when reacting with CO₂ and H₂O in their oxygen-poor stage.

While there are over 350 known thermochemical redox cycles capable of dissociating H₂O to produce H₂, only a minority of them occur in two discreet steps, which are easier to manage compared to processes requiring 3 or more steps. Some of the common redox materials that have been studied include volatile redox materials such as zinc oxide, non-volatile stochiometric redox materials such as iron oxides, ferrites, hercynite, perovskites, and non-volatile non-stoichiometric redox materials such as cerium oxide (ceria).

Volatile reactions refer to those where the catalyst undergoes a gas-solid phase change, while in non-volatile reactions the catalyst remains solid. Although volatile reactions tend to have better oxygen exchange rates, they require rapid quenching to avoid recombination, and this issue hasn’t been solved energy efficiently to date (Scheffe et al. 2014).

Among the non-volatile materials, ceria has gained a lot of attention among solar fuel researchers due to its high melting point and its ability to diffuse oxygen much faster compared to iron oxides or ferrites (Scheffe et al.). It started back in 2009 when a research group led by William Chueh proposed using nonstoichiometric reduction of ceria to split H₂O and CO₂ and then demonstrated the first ceria-based solar reactor under realistic conditions (Chueh et al. 2010). Later, the SOLARJET project in Europe used the same reaction pathway to demonstrate the end-to-end production of liquid solar fuel from CO₂ and H₂O. The chemistry of ceria-catalyzed reaction works as follows:

First, a high-temperature endothermic reaction at 1450–1600°C:

CeO₂ →CeO₂₋ₓ + (x/2) O₂

Then, the non-stoichiometrically reduced CeO₂₋ₓ gets oxidized by CO₂ and H₂O in these two exothermic reactions at the relatively lower temperature of 700–1200°C:

CeO₂₋ₓ + xH₂O → CeO₂ + xH₂ CeO₂₋ₓ + xCO₂ → CeO₂ + xCO

According to the experimental results from Marxer et al. (2015), the overall solar-to-fuel efficiency using ceria catalysts is currently at 1.72%. This is under a condition where no attempt was made to recover waste heat after heating CO₂ and H₂O. If they did recover the waste heat and used it to preheat the input gases, we can probably expect the efficiency to be an order of magnitude higher. A separate study done on methanol production using similar solar reactors produced a cost estimate of $4.24 per gallon of gasoline-equivalent (Kim et al. 2012).

The Reactor Design

Direct solar radiation under ideal circumstances on earth can typically reach about 1kW/m² in “sunbelt” regions such as the American southwest, Australia, and North Africa.

In general, there are four types of ways to concentrate solar energy: a) parabolic troughs, b) linear fresnel reflectors, c) parabolic dish collectors, and d) heliostats.

Illustrations of four types of solar collectors (Onur and Halil 2013 )

The linear concentrator designs such as the parabolic troughs or the linear fresnel reflectors tend to have much lower concentration factors compared to point concentrators such as the parabolic dish reflector and heliostats, which can reach maximum thermal concentrations of 5000 kW/m² and 1500 kW/m² respectively (Kodama et al. 2017).

The high temperature required for the solar-thermal reaction means that only the point concentrators are viable. Furthermore, since in actual industrial-scale production, we would need to have equipment in place for downstream processing after syngas is generated from the solar reactor, a centralized system based on large reactors with high throughput would be more preferable than a decentralized system of many small reactors. For this reason, the reactor designs we are going to explore are based on heliostat collectors instead of parabolic dish collectors.

As we discussed in the previous section, the reaction using ceria takes place at over 1400°C. At such temperatures, containing the heat of the reactor while also efficiently absorbing incoming solar radiation becomes a difficult engineering challenge. Unlike solar thermal power plants that merely need to heat their boilers to 550°C (BrightSource) and can therefore use indirect heating — where sunlight is concentrated to the outer walls of the boiler to indirectly heat the water inside — the reactors for solar fuel would require the direct heating of reactants. This means that our reactors would need a window through which concentrated solar radiation can be shined directly onto the reactants.

Ivanpah Solar Electric Generating System. An example of indirect heating.

Now, this leads to an obvious question: if the incoming rays from the heliostat fields are from a wide range of angles similar to the photo above, how would we be able to point all that sunlight through a small window onto the reactants? This issue can be solved by having a secondary reflector mounted on top of the solar tower, which then redirects the beams down to a compound parabolic concentrator (CPC) that concentrates the beams further before the beams enter the reactor.

Solar tower reflector concept (Yogev et al. 1997)

This type of top-down design where the beams enter the reactor from a top window enables many possible applications. For example, there are designs where concentrated solar radiations irradiate a fluidized bed so that the reactant gas and solid particles can react with maximal surface area and even temperature distribution. In this case, a top-down design holds the unique advantage of using gravity to keep the reactant passively separated from the window.

Schematics of a solar reactor using an internally-circulating fluidized bed (Kodama et al. 2017)

For the solar-thermal reaction using ceria, the mechanism as shown below is actually relatively simple — instead of having moving particles like in a fluidized bed, a ceria reactor uses a porous ceria cylinder to let the input gases (H₂O and CO₂) come in contact with the ceria catalyst during the exothermic oxidation step, where non-stoichiometrically reduced CeO₂₋ₓ strips oxygen atoms from H₂O and CO₂ to produce H₂ and CO. Then, in the endothermic reduction step, a purge gas, such as N₂ or Ar, enters the reactor and carries the released O₂ from the reactor.

The geometry of the porous ceria material in the reactor has been subject to many optimizations. In the original monolithic design, a peak solar-to-fuel efficiency of only 0.8% was achieved. Later, people have experimented with porous ceria felt, which yielded even less efficiency at 0.16% due to the poor thermal conductivity of the felt material. Finally, researchers settled upon a type of CeO₂ reticulated porous ceramic (RPC) foam, which yielded a more promising solar-to-fuel efficiency of 3.53% (Yadav et al. 2015). (Note that the efficiencies here are without any heat recycling, so the overall efficiency still has substantial room to improve.)