Researchers around the world are pursuing the goal of harnessing the vast amounts of energy available from the sun to address climate change and other impacts of the world’s growing dependence on carbon-based fuels. Myriad technologies for capturing and storing the sun’s power are maturing — but large-scale commercially viable and economically competitive processes based on these technologies are likely still years away.
Nonetheless, by building on other efforts, notably the findings of Sandia’s Sunshine to Petrol (S2P) Grand Challenge Laboratory Directed Research and Development (LDRD) project, a Sandia team of S2P veterans that includes principal investigator Tony McDaniel (8367) and Ivan Ermanoski (6124), has taken significant steps toward an intriguing possibility: creating hydrogen fuel using a two-step thermochemical process powered by the sun. Hydrogen fuel for transportation is widely
viewed as an environmentally friendly alternative to gasoline, natural gas, and other carbon-based fossil fuels — especially if the hydrogen can be generated without using fossil fuels in the process.
Broad outline of concept is simple
“If the concept can be validated and scaled up, it could lead to an economically viable means of creating hydrogen from water and sunlight — two very abundant resources,” says Tony.
Collaborators from the University of Colorado, Colorado School of Mines, and Bucknell University are also contributing to this project, which is funded by DOE’s Fuel Cell Technology Office under the Solar ThermoChemical Hydrogen Production (STCH) program.
The broad outline of the team’s concept is fairly simple: use sunlight to split water into hydrogen and oxygen. The concept uses a second-generation version of the
Sandia-developed CR5 receiver reactor for converting solar energy into an easily storable form: a chemical fuel, such as hydrogen. The reactor is placed at the top of a tower centered within a large field of heliostats (flat mirrors that track the sun). Together, the tower and heliostats form a concentrating solar power plant. Such plants — which commonly store solar energy in the form of molten salt that can be used to generate steam, and thus electricity — already exist in many parts of the world, including Spain and the United States.
The reactor being developed by Sandia uses metal-oxide particles sized a few tens of microns in diameter as the working “fluid.” The particles are transported between two isolated reaction zones: an upper chamber illuminated and heated by concentrated solar energy and a lower chamber exposed to steam. Using gravity feed and a unique particle elevator concept patented by the team (based on an Olds Elevator™), particles are lifted from the lower to the upper chamber.
Here, concentrated sunlight heats the particles to temperatures as high as 1,600 C, providing sufficient energy to remove oxygen from the oxide particle. The oxygen is continuously pumped away, and the oxygen-reduced particles (designed not to melt or sinter at such high temperatures) flow to the lower chamber.
In the lower chamber, the particles are exposed to water in the form of steam. Strongly attracted to the oxygen-reduced particle, the oxygen breaks away from the water molecule to deposit in the metal oxide, creating hydrogen in the process. The re-oxidized particles are then ready to be elevated to the upper chamber and repeat the cycle. It is expected that these particles will be cycled hundreds of thousands of times before replacement.
Project goes farther
“In part, this concept draws on past work to generate hydrogen by splitting water,” says Tony. “But this project goes farther by exploring several novel aspects that hold a lot of promise.”
For example, meeting the project goal of developing a process that requires only two steps, as opposed to the numerous steps (up to nine) required for many of the other thermochemical processes under development, will allow for greater process efficiencies. Efficiency is further enhanced by incorporating key features of the earlier Sandia CR5 reactor — such as recuperation of thermal energy, continuous on-sun operation, and direct absorption of sunlight by the working metal-oxide — coupled with new features developed in the second-generation design. The most significant of these is the intrinsic gas and pressure separation between the two chambers made possible by the moving packed bed of particles.
“From what we’ve seen in the literature thus far, one or more of these attributes is missing from other systems. Yet all are critical to achieving economic viability because of the high capital costs associated with building solar concentrators and the direct tie between efficiency and cost. Using particles as the working fluid enables the high efficiency, mechanical simplicity, scalability, and material and operational flexibility of the Sandia reactor concept,” says Tony.
Equally important, the team has identified a novel material chemistry for the metal oxide particles. Researchers to date have focused on two chemistries, ferrite and ceria, the current state of the art. Both ferrite and ceria have issues and, at best, have demonstrated efficiencies of less than a few percent.
“By leveraging Sandia efforts to understand these materials and their limitations, and working closely with a separate Sandia project led by Andrea Ambrosini (6124) that is developing new thermochemical materials, we have identified a different chemistry based on perovskite materials that opens the door to some interesting possibilities,” Tony says.
As reported in a recent paper (“Sr- and Mn-doped LaAlO3−δ for solar thermochemical H2 and CO production,” Energy and Environmental Science), the perovskite chemistry produces significantly more hydrogen per reduction/oxidation cycle than ceria while maintaining rapid reaction kinetics, a principal advantage of ceria. In addition, the perovskites can be cycled effectively at temperatures below 1,350 C, as opposed to the 1,500 C minimum temperature required for ceria. Operating at a lower temperature range allows for use of less exotic, and therefore less expensive, materials for constructing the reactor.
In fact, the team believes perovskites have the potential to meet or exceed the 26 percent solar conversion efficiency targeted by the DOE STCH program, whereas it is almost certain that ceria and ferrite chemistries will not. In addition, because perovskites have highly tunable properties and because an overwhelmingly large number of compounds can be formed in the perovskite crystal structure, it’s highly likely that a metal-oxide material can be discovered that will efficiently and economically produce hydrogen fuel.
To evaluate and refine their reactor design concept, the project team is building a small (1 kW) engineering test stand. At the moment, the team is testing the particle conveyance and pressure separation concepts at room temperature and plans in the near future to retrofit the stand to test these features simultaneously and at high temperature.
“By studying how this unit operates and doing extensive modeling, we’re working to gain a deeper understanding of reactor function and how to create a viable system at a much larger operating scale,” says Tony. “In particular, we’re looking hard at how the particle elevator works and at how to improve efficiencies.” In this endeavor, the STCH program will benefit from work being conducted in Sandia’s Materials, Devices, and Energy Technologies Dept. 6124 and supported by an early career LDRD that is examining the complexities of solid-solid heat exchange for particle systems.
Though aware of the work ahead, the team is optimistic about the prospects for their reactor design. “Ultimately, this effort could contribute to a new transportation infrastructure based on hydrogen or enable carbon-neutral, renewable-based synthetic liquid fuels that could be inserted directly into the existing infrastructure. Either way, it could smooth the transition away from fossil fuels,” Tony states.