Solar fuel from two step termochemical cycles

With the increasing need to reduce CO2 emissions to attenuate the effects of global warming there is a significant role of renewable energy sources to achieve this goal. However, issues of variability of energy sources and unpredictability are still barriers for integration of renewables. In this regards, energy storage and dispatchability issues prevent further penetration of renewable power sources (Bader et al., 2013).

One promising approach to solve this is the production of “solar fuels” (Hydrogen and syngas). This is, the use of solar energy to produce synthetic fuels via thermochemical cycles. This way solar heat can be stored in the chemical bonds of chemicals that can be used to produce fuels or other useful transportable and storable chemicals (Chueh et al., 2013). One advantage of this approach is that the fuels produced can be easily incorporated into current energy distribution system.

The main ways that have been explored to produce fuels from solar energy are the splitting of water and carbon dioxide to generate hydrogen, carbon monoxide or a mix of both (syngas)(Chueh et al., 2010). This offers the added benefit of possible CO2 sequestration. Splitting water and carbon dioxide can be done in a single high temperature step, however this requires temperatures over 3000 degrees and it poses the problem of separating oxygen from hydrogen and/or carbon monoxide at a high temperature (otherwise the resulting mix is explosive). 

A more sophisticated approach is the use of two step solar thermochemical cycles based on the reduction and oxidation of a metal oxide. In this cycles a metal oxide is reduced at a high temperature (provided by concentrated solar radiation) producing an oxygen deficient material. Then the metal oxide is re-oxidized with water or carbon dioxide capturing the oxygen from them and releasing hydrogen or carbon monoxide. This approach has the advantage of requiring lower temperatures and avoiding the problem of  separating oxygen from hydrogen and/or carbon monoxide at a high temperature (Furler et al., 2012). 

Diagram of a two step metal oxide-redox water/CO2 sppliting thermochemical cycle (Energy&
Environmental Science, 2012)

However there still several problems associated with this cycles that need to be overcome. So far the efficiencys achieved are the low, with record solar to fuel efficiency being 1.7% . During the reduction step a low oxygen partial pressure is required which is usually achieved by using and inert gas sweep. This reduces the efficiency due to the energy penalties of heating the gas and maintaining its flow. Another issue is that due to the high temperature required to drive the reactions, conduction and re-radiation heat losses become significant, which also penalizes the efficiency of the cycle (Ermanosky et al., 2013). Finally there also issues with the cyclability and stability of the metal oxide materials, some metal oxides tend to volatilize at high temperature and also tend to sinter which reduces the surface area slowing down the reaction kinetics (Furler et al. 2014).

Several solution are currently being tested to overcome these limitations. Different metal oxide compositions and morphologies have been developed in an attempt to improve reaction kinetics. The use of heat recovery from the inert gas sweep is also a promising solutions and thermodynamic analysis suggest that with heat recuperation efficiencys of up to 30% could be achieved. The use of a carbonaceous feedstock during reduction allows to reduce the temperature and increase gas yields, however it is a less sustainable option due to the use of the carbonaceous feedstock.

Summarizing, it can be said that two step metal oxide redox solar thermochemical cycles appear as a promising pathway to achieve a sustainable energy system in the future. However this technology is still very much in development and several issues need to be solved before its economic viability.

References 

1. Bader, R., Venstrom, L. J., Davidson, J. H., & Lipiński, W. (2013). Thermodynamic Analysis of Isothermal Redox Cycling of Ceria for Solar Fuel Production. Energy Fuels(27), 5533-5544.
2. Chueh, W. C., & Haile, S. M. (2010). A thermochemical study of ceria: exploiting an old material for new modes of energy conversion and CO2 mitigation. Philosophical Transactions of the Royal Society A, 368(1923), 3269-3294.
3. Chueh, W. C., Falter, C., Abbott, M., Scipio, D., & Furler, P. (2010). High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O Using Nonstoichiometric Ceria. Science(330), 1797-1801.
4. Chueh, W. C., McDaniel, A. H., Grass, M. E., Hao, Y., Jabeen, N., Liu, Z., . . . Gabaly, F. E. (2012). Highly Enhanced Concentration and Stability of Reactive Ce3+ on Doped CeO2 Surface Revealed In Operando. Chemistry of Materials, 24(10), 1876–1882.
5. da Silva, A. L., & Assaf, J. M. (2007). Activity of Cu/CeO2 and Cu/CeO2-ZrO2 for low temperature water-gas shift reaction. Natural Gas Conversion VIII, 167, pp. 213–218. Natal.
6. Ermanoski, I., Siegel, N. P., & Stechel, E. B. (2013). A New Reactor Concept for Efficient Solar-Thermochemical Fuel Production. Journal of Solar Energy Engineering(135).
7. Furler, P. (2014). Solar Thermochemial CO2 and H2O Splitting Via Ceria Redox Reactions. ETH Zurich. Zurich: Ph.D. Thesis, ETH Zurich.
8. Furler, P., Scheffe, J., Gorbar, M., Moes, L., Vogt, U., & Steinfeld, A. (2012). Solar Thermochemical CO2 Splitting Utilizing a Reticulated Porous Ceria Redox System. Energy Fuels(26), 7051-7059.
9. Furler, P., Scheffe, J., Marxer, D., Gorbar, M., Bonk, A., & Steinfeld, U. V. (2014). Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dualscale porosities. Physical Chemistry Chemical Physics, 16, 10503-10511.
10. Furler, P., Scheffea, J. R., & Steinfeld, A. (2012). Syngas production by simultaneous splitting of H2O and CO2 via ceria redox reactions in a high-temperature solar reactor. Energy & Enviromental Science, 5, 6098-6103.

  

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