Efficiency maximization in solar-thermochemical fuel production: challenging the concept of isothermal water splitting

Abstract

Widespread adoption of solar-thermochemical fuel production depends on its economic viability, largely driven by the efficiency of use of the available solar resource. Herein, we analyze the efficiency of two-step cycles for thermochemical hydrogen production, with emphasis on efficiency. Owing to water thermodynamics, isothermal H2 production is shown to be impractical and inefficient, irrespective of reactor design or reactive oxide properties, but an optimal temperature difference between cycle steps, for which efficiency is the highest, can be determined for a wide range of other operating parameters. A combination of well-targeted pressure and temperature swing, rather than either individually, emerges as the most efficient mode of operation of a two-step thermochemical cycle for solar fuel production.

Design and construction of a cascading pressure reactor prototype for solar-thermochemical hydrogen production

“…Because the steam heating input is a small fraction of the total heat requirement, the approximation effect on efficiency is negligible. Importantly, the latter is true only when the steam and oxide flows in the water splitting (WS) step are countercurrent, as described in 9,10 . FIGURE 1.…”

“…Furthermore, advanced oxides are expected be most efficient at a much lower T WS than ceria, making a high T WS design unwarranted. 9 …”

“…(b) Prototype diagram, with loose resemblance to reactor geometry. Solid arrows indicate particle flow, dashed ones gas flows (1)(2)(3)(4)(5)(6)(7)(8)(9). Dimensions are of the internal packed particle beds, not the exterior chambers and connecting elements.…”

“…An anticipated prototype efficiency calculation, shown in FIGURE 1, comprehensively accounts for the heat equivalents of all energy inputs, and is described in detail in 9,10 . Briefly, the reactor efficiency is the ratio of the H 2 output heat equivalent, and the sum of all required heat and heat equivalents of mechanical work.…”

“…Briefly, the reactor efficiency is the ratio of the H 2 output heat equivalent, and the sum of all required heat and heat equivalents of mechanical work. In addition to the energy inputs described in 9,10 , and considering the relatively small physical size of the prototype, a heat loss factor of 25% was included in the efficiency calculations. Furthermore, owing to the prototype design characteristics, all solid and gas heat recovery efficiencies were assumed to be zero.…”