(Invited) Life-Cycle Net Energy Assessment of Large-Scale Hydrogen Production Via Photoelectrochemical Water Splitting

Abstract

The Joint Center for Artificial Photosynthesis (JCAP) aims to find a cost-effective method to produce fuels for transportation and feedstocks using only sunlight, water, and carbon dioxide as inputs. Development of such a technology offers the possibility of widespread sources of environmentally benign, energy rich chemicals across the globe. Although still at an early stage, solar-driven water splitting to produce hydrogen is the most mature artificial solar-to-fuel conversion approach. Moreover, there is sufficient information from photoelectrochemical (PEC) water-splitting prototypes designed and developed by JCAP and others to perform an assessment of whether large scale application of this approach would be advantageous from a net energy production point of view. Here we report a prospective life-cycle net energy assessment of a hypothetical large-scale PEC-based hydrogen production facility, assuming energy output equivalent to 1 GW continuous annual average (610 metric tons of H2per day) [1]. We determine essential mass and energy flows required to support such production [2], and use heuristic methods to conduct a preliminary engineering design of the facility. We then develop and apply a parametric model describing system-wide energy flows associated with the production, utilization, and decommissioning of the facility. Based on these flows, we calculate and interpret life-cycle net energy metrics for the facility. We find that under base-case conditions (10% solar-to-hydrogen (STH) conversion and 10 year PEC cell life span) the energy payback time is 8.1 years, the energy return on energy invested (EROEI) is 1.7, and the life-cycle primary energy balance over the 40 years projected service life of the facility is +500 PJ. The most important model parameters affecting the net energy metrics are the STH conversion efficiency, the life span of the PEC cells, and the embodied energy of several components of the PEC cell. We discuss the sensitivities of these and other parameters, and highlight recent work indicating promising ways to improve the net energy metrics. Results suggest that a strong program of research into improved materials and methods to process the materials will have a high impact on the feasibility of this technology.