Optical analysis of a solar thermochemical system with a rotating tower reflector and a receiver–reactor array

Abstract

We propose a concept of a rotating tower reflector (TR) in a beam-down optical system to alternate concentrated solar irradiation of an array of solar receiver–reactors, realizing multi-step solar thermochemical redox cycles. Optical and radiative characteristics of the proposed system are explored analytically and numerically by Monte-Carlo ray-tracing simulations. We study the effects of the system geometrical and optical parameters on the optical and radiative performance. TR axis is required to be tilted for accommodating the receiver–reactor array, resulting in reduced optical efficiency. We demonstrate that the annual optical efficiency of a baseline system with the receiver–reactor located south of the tower decreases from 46% to 37% for the axis tilt angle of TR increasing from 2° to 20°. The optical analysis conducted in this study provides a general formulation to enable predictions of required gain of thermal-to-chemical efficiency of the receiver–reactor array for obtaining improved overall solar-to-chemical efficiency of the solar thermochemical plant.

Thermodynamic Analysis of a Conceptual Fixed-Bed Solar Thermochemical Cavity Receiver–Reactor Array for Water Splitting Via Ceria Redox Cycling

“…where Δδ δ red − δ ox is the change in non-stoichiometry. Figure 1 depicts the schematic of the proposed beam-down thermochemical system featuring a heliostat field, a tower, a rotating hyperboloidal tower reflector (TR), and a stationary cavity receiver-reactor (CRR) array coupled with compound parabolic concentrators (CPCs) (Li et al, 2020b). The rotating TR directs the concentrated solar irradiation alternately to each CRR through the aperture of the CPCs.…”

“…The FIGURE 1 | Schematic of the proposed solar thermochemical system consisting of a heliostat field, a beam-down reflector, and a cavity receiver-reactor array. This figure is adapted from (Li et al, 2020b). mixture gas then flows continuously through the pre-cooling part from #1 to #N to cool the RPC material in each CRR.…”

“…The gas-phase heat exchangers and the related connecting devices are not shown in the figure. This figure is adapted from (Li et al, 2020b).…”

“…The improvement magnitude is relatively larger when N is increased from 2 to 4 that ε solid and η th are increased by 17% and 33%, respectively. ε solid and η th increase with N at diminishing rates, as limited by other factors such as T red , p O 2 , ε, and C. On the other hand, a large N also leads to a large required CRR disk diameter, and thereby a large tilt angle of the tower reflector axis, decreasing the system optical efficiency (Li et al, 2020b). Here, N is set to 10 in the baseline receiver-reactor system.…”

“…In our previous work, we proposed a modified solar beam-down system for realizing solid-phase heat recovery for the fixed-bed reactor configuration, where the analysis of the optical system was presented in detail (Li et al, 2020b). In the present work, we introduce the design of the array of receiver-reactors, coupled to the beamdown system, together with the operating strategy for realizing solidphase heat recovery.…”