Modeling the Performance of an Integrated Photoelectrolysis System with 10 × Solar Concentrators

Abstract

Two designs for an integrated photoelectrolysis system that uses a 10× concentrating solar collector have been investigated in detail. The system performance was evaluated using a multi-physics model that accounted for the properties of the tandem photoabsorbers, mass transport, and the electrocatalytic performance of the oxygen-evolution and hydrogen-evolution reactions (OER and HER, respectively). The solar-to-hydrogen (STH) conversion efficiencies and the ohmic losses associated with proton transport in the solution electrolyte and through the membrane of the photoelectrolysis system were evaluated systematically as a function of the cell dimensions, the operating temperatures, the bandgap combinations of the tandem cell, and the performance of both the photoabsorbers and electrocatalysts. Relative to designs of optimized systems that would operate without a solar concentrator, the optimized 10× solar concentrator designs possessed larger ohmic losses and exhibited less uniformity in the distribution of the current density along the width of the photoelectrode. To minimize resistive losses while maximizing the solar-to-hydrogen conversion efficiency, η STH , both of the designs, a two-dimensional "trough" design and a three-dimensional "bubble wrap" design, required that the electrode width or diameter, respectively, was no larger than a few millimeters. As the size of the electrodes increased beyond this limiting dimension, the η STH became more sensitive to the performance of the photoabsorbers and catalysts. At a fixed electrode dimension, increases in the operating temperature reduced the efficiency of cells with smaller electrodes, due to degradation in the performance of the photoabsorber with increasing temperature. In contrast, cells with larger electrode dimensions showed increases in efficiency as the temperature increased, due to increases in the rates of electrocatalysis and due to enhanced mass transport. The simulations indicted that cells that contained 10% photoabsorber area, and minimal amounts of Nafion or other permselective membranes (i.e. areal coverages and volumetric fractions of only a few percent of the cell), with the remaining area comprised of a suitable, low-cost inert, non porous material (flexible polymers, inert inorganic materials, etc.) should be able to produce high values of η STH , with η STH = 29.8% for an optimized design with a bandgap combination of 1.6 eV/0.9 eV in a tandem photoabsorber system at 350 K. Artificial photosynthesis could provide a promising route to largescale solar energy conversion and storage.1-4 Recent techno-economic studies have evaluated various designs for integrated photoelectrolysis systems, including a very promising system that makes use of concentrated illumination. 5,6 A discrete III-V photovoltaic cell connected electrically in series with a discrete polymer-electrolyte membrane (PEM) electrolyzer has demonstrated a solar-to-hydrogen (STH) conversion efficiency of 18% under 500 Suns.7 Although concentrated photovoltaics (CPV) typically i...

A Computational Study of Optically Concentrating, Solar-Fuels Generators from Annual Thermal- and Fuel-Production Efficiency Perspectives

“…The η STH,Avg is 11% or better in Barstow, CA and Albuquerque, NM (locations with good direct-solar resource), and declines in areas with lower direct-solar insolation resource to 9.1% in Quillayute, WA. These values are well below the instantaneous η STH values modeled by Chen et al, 17 where η STH in excess of 25% were reported for a tandem-junction CPEC with no optical losses and a trough-like lens with a CR of 10. It is somewhat problematic to make a comparison between these modeled CPECs, however, as the system modeled in Chen et al 17 will exhibit higher attainable efficiency than that modeled herein due to the higher attainable j SC of the tandem junction PV cells modeled in Chen et al 17 Furthermore, Chen et al 17 do not consider some of the optical losses modeled in this work.…”

“…These values are well below the instantaneous η STH values modeled by Chen et al, 17 where η STH in excess of 25% were reported for a tandem-junction CPEC with no optical losses and a trough-like lens with a CR of 10. It is somewhat problematic to make a comparison between these modeled CPECs, however, as the system modeled in Chen et al 17 will exhibit higher attainable efficiency than that modeled herein due to the higher attainable j SC of the tandem junction PV cells modeled in Chen et al 17 Furthermore, Chen et al 17 do not consider some of the optical losses modeled in this work. The effect of varying direct-solar resource is clearly evident in the SFP of the CPEC modeled herein, because regions with poor resource have a commensurate drop in annual SFP (ranging from 8.8 kg H 2 /m 2 -yr in Barstow, to 2.91 kg H 2 /m 2 -yr in Quillayute).…”

“…The relationship for the electrical conductivity of the membrane, κ Membrane (T ), the electrical conductivity of the electrolyte, κ Electrolyte (T ), and j O,r (T ), can be found in Chen et al 17 and the relationship for FF(T), the open-circuit voltage as a function of temperature, and j SC (T ) are given in Section S.2 of the supporting information. Note that all properties are a function of temperature.…”

A sensitivity analysis to assess the relative importance of improvements in electrocatalysts, light absorbers, and system geometry on the efficiency of solar-fuels generators

“…[16][17][18][19][20] We report herein a "one factor at a time" sensitivity analysis [23][24][25] for several types of generic designs of solar-fuels generators. In this study, the sensitivity of the efficiency of the generation of solar fuels to the properties of the various system components was evaluated as a function of the total overpotential of the electrocatalysts for a variety of light absorbers having a range of band gaps and having varying materials quality.…”

“…The voltage losses were not xed relative to the band gap regardless of the actual system operating conditions, 14,44 but instead, as given by eqn (1)- (10), the voltage losses were explicitly a function of the operating voltage and current, in conjunction with the current-dependent ohmicbased voltage losses and kinetically based electrocatalyst overpotential losses, in the specic system being evaluated. The sensitivity analysis investigated herein is applicable to several specic device designs, including a "closed-sandwich" design in which the photocathode and photoanode are assembled back-to-back, 18 an "open-sandwich" design with suitable spectral-splitting in which the photocathode and photoanode are assembled side-by-side, 18 a one-dimensional "trough" design and two-dimensional "bubble-wrap" design for the photoelectrochemical cells coupled to solar concentrators, 16 and a membrane-enclosed, vapor-feed design. 20 The analysis is not applicable to device designs in which the geometric area of the electrolyzing components is signicantly different from that of the light absorbers.…”