(389f) On the Potential for Enhancement of Solar Thermochemical Synthesis Processes By Nonequilibrium Plasma

The sustainable synthesis of fuels and chemicals from abundant low-value feedstock, such as carbon dioxide, methane, and water, is a key technological and global challenge. Solar thermochemical synthesis processes directly utilize the most abundant form of renewable energy, but face severe challenges due to the low reactivity of most feedstock gases to solar radiation and the intermittency of the solar resource. These challenges often lead to limited process efficiency and resiliency, which translate into systems with higher capital and operating costs. The combination of a solar thermochemical process with a non-solar co-process can be an effective option to achieve continuous operation, improve resiliency, and reduce costs. Moreover, if the concurrent operation of the solar and non-solar processes leads to greater overall efficiency, then the combined approach could have greater viability than either process alone. This talk discusses the potential of the use of nonequilibrium plasma as a co-process to solar thermochemical synthesis. The combined solar-plasma approach, denoted as Plasma-Enhanced Solar Energy (PESE), has the potential to enhance the efficiency and resiliency, and consequently improve the viability, of solar thermochemical synthesis processes.

Nonequilibrium plasmas are partially ionized gases in which the temperature of the free electrons is significantly higher than that of the heavy-species (molecules, atoms, ions). The electron temperature in atmospheric-pressure nonequilibrium plasmas are typically on the order of 1 to 2 eV, commensurate with the threshold energies in molecular bonds, whereas the temperature of the heavy-species range between room temperature to a few thousand degrees. Therefore, electrons and heavy-species are not in state of Local Thermodynamic Equilibrium (LTE), i.e., they are in a non-LTE (NLTE) state. Plasma-based chemical synthesis processes use electricity to directly modify gas chemistry and produce activated species (radicals, excited states, ions). These processes can achieve high energy efficiencies by selectively transferring energy to different channels. For example, microwave plasmas have shown over 90% energy efficiency (i.e. electrical to chemical) for the decomposition of carbon dioxide thanks to effective step-wise vibrational excitation, the most effective channel for dissociation [1]. Moreover, the reliance of plasma processes on electrical energy makes them ideal for continuous operation, but also limits their economic viability and their net sustainability benefit, even if the required electricity is obtained from renewable energy sources, such as wind and solar.

PESE chemical synthesis processes seek to combine the advantages of solar thermochemical and plasma processes. The potential advantage of PESE to improve the efficiency of solar thermochemical synthesis is due to the fact that the molecular excitation produced by free electrons in non-equilibrium plasmas can significantly increase solar photon absorption leading to the enhancement of chemical reaction kinetics. Moreover, PESE chemical synthesis processes could rely on electrical energy to compensate for fluctuations in the solar energy input (during daytime) or even substitute it (at nighttime), and therefore ensure continuous operation.

Preliminary computational analyses of solar radiation absorption through a carbon-dioxide (CO₂) mixture in a state of NLTE reveal over two orders of magnitude increase in absorption compared to the same mixture in a state of LTE, achieving 50 to 80% radiative energy absorption efficiency. The calculations were performed with the line-by-line nonequilibrium radiation code SPARTAN [2, 3] using 32 types of atomic and molecular transitions. The results show that CO₂ in LTE absorbs negligible solar irradiance (mostly UV and far IR), whereas CO₂ in NLTE displays significantly more absorption, especially in the visible range, but also emission, which diminishes the overall amount of absorbed radiation. Both findings are consistent with established understanding of radiative transport; however, the novelty is finding that the net effect is a significant increase in solar energy absorption. The results also show that CO₂ tends to behave as a perfect absorber of solar energy for very high degrees of NLTE (i.e. larger differences between electron and heavy-species temperatures). This fact, although not intuitive given that plasmas are known to be good emitters (e.g., radiation from arc lamps), can be understood by considering that a medium is prone to absorb the same type of radiation it emits. Furthermore, it is to be noted that the concept of a blackbody, a perfect absorber and emitter or radiation, corresponds to a medium in LTE. In fact, the approximation of the solar spectrum as the emission from a blackbody at ~ 5780 K is a consequence that the sun’s photosphere is largely regarded as being in LTE. Therefore, a NLTE medium provides the possibility to control its radiative properties by tuning its degree of thermodynamic nonequilibrium.

To evaluate the potential of PESE, a solar receiver-reactor system equipped with two types of nonequilibrium plasmas, namely a gliding arc discharge and a microwave plasma, is being built at UMass Lowell. The PESE chemical synthesis reactor system is designed to evaluate comparable solar and electrical energy inputs at the 1 kW level, spanning the whole range of combined solar-plasma operation, i.e. from 100% solar to 100% electrical energy inputs. The microwave plasma has the potential for the greatest energy efficiency, whereas the gliding arc discharge has simpler operation, more stability, and greater potential for scalability. Design challenges being addressed include the establishment of a stable plasma volume within the focal point of the concentrated solar radiation from a high-flux solar simulator [4], as well as the maximization of the plasma volume and the gas-plasma interaction region, the incorporation of a downstream catalytic monolith, among others.

Acknowledgements

The authors gratefully acknowledge support from the U.S. National Science Foundation through award CBET-1552037.

References

[1] A. Fridman, Plasma chemistry, Cambridge University Press, New York (2008)

[2] M. Lino da Silva, SPARTAN - Simulation of PlasmA Radiation in ThermdynAmic Nonequilibrium, http://esther.ist.utl.pt/spartan/

[3] M. Lino da Silva, An adaptive line-by-line—statistical model for fast and accurate spectral simulations in low-pressure plasmas, J. Quant. Spectrosc. and Rad. Transfer 108 (2007) 106–125

[4] S. Bhatta, D. Nagassou, J. P. Trelles, Solar photo-thermochemical reactor design for carbon dioxide processing, Solar Energy 142 (2017) 253–266