Systems

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Goal

Ensure alignment between energy research and systemic factors which determine the feasibility of new solar energy technologies.

Strategy

Contour plot of the levelized cost of hydrogen (LCH) produced by a plant using photovoltaic cells wired to separate electrolysis units to split water. For comparison, the current LCH of hydrogen produced from fossil energy is appx $1.39 kg-1.1

We use modeling and simulation tools to assess constraints and requirements – both physical and technoeconomic – which govern the technical and commercial feasibility of new energy technologies and of renewable energy systems.

We collaborate with Chengxiang Xiang, a Lewis Group alumnus, to model and simulate systems that make sunlight from fuels.  This work allows us to understand the physical constraints of such systems.

We collaborate with Prof. Eric McFarland from the University of California at Santa Barbara to analyze the technoeconomic potential of new renewable energy technologies, and to understand the constraints and requirements for commercial feasible renewable energy systems.

These efforts keep our research aligned with the big-picture context of global energy systems.

Highlights

Operational Constraints and Strategies for Systems that Reduce CO2 from the Atmosphere

CO2_transport_limitations_chen

Schematics for (top) the five regions for which CO2 transport was modeled and (center) the model used for CO2 transport near the surface of a cathode in an electrochemical CO2-reduction cell, showing solution species included in the model. (bottom) Steady-state limiting current densities as a function of pH for a six-electron/six-proton reduction of CO2 fed by air containing CO2 at a concentration of 400 ppm.2

Sunlight-driven systems that reduce carbon dioxide are still in the research and development stage and face challenges with regard to the discovery of catalysts and the design of system architectures.  However, modeling and simulation allow an analysis of the upper limits of attainable efficiencies for systems that capture and convert CO2 from the atmosphere to fuels.   Specifically, upper limits of efficiencies can be estimated based on the rates of mass transport of CO2 through the atmosphere to the sites of chemical reduction to fuels.

In collaboration with Yikai Chen and Chengxiang Xiang, we used a multiphysics model to evaluate the mass-transport limitations for CO2 at the current concentration in the atmosphere (400 ppm) through five different atmospheric regions: the troposphere, atmospheric boundary layer, canopy layer, membrane layer, and the aqueous electrolyte layer of an electrochemical CO2-reduction cell.2  We additionally considered the concentration overpotential by accounting for diffusion, migration, convection, and the bulk electrochemical reactions in such a cell.

The low diffusion coefficient, combined with the low solubility of CO2 in aqueous solutions constrains the steady-state limiting current density of CO2-reduction systems fed with CO2 at atmospheric concentration (400 ppm) to < 0.1 A cm-2, regardless of the efficiency of the CO2-reduction catalysts. This result is consistent with estimates of oceanic CO2 uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in atmosphere-ocean circulation models.  Hence, the CO2 capture area must be 100- to 1000-fold larger than the solar collection area to enable a > 10% efficient sunlight-driven CO2-reduction system.

We also evaluated strategies to improve the steady-state efficiency of CO2-reduction devices, including accelerating the transport of CO2 in solution using as-yet-undiscovered catalysts that enhance the rate of the acid-base reactions in the bicarbonate buffer system and a cell architecture based on the use of an ultrathin polymeric membrane electrolyte.  Although these strategies could allow the CO2 mass-transport limitations associated with flux to and within the electrolyte of laboratory-scale CO2 reduction to be overcome, atmospheric flux limitations will constrain the operation of systems intended to reduce atmospheric CO2 at a regional or global scale.

References

  1. Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W., A Comparative Technoeconomic Analysis of Renewable Hydrogen Production Using Solar Energy. Energy Environ. Sci. 2016, Advance Article.
  2. Chen, Y.; Lewis, N. S.; Xiang, C., Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO2. Energy Environ. Sci. 2015, 8, 3663-3674.

by Kimberly Papadantonakis, June 2016.