The compound parabolic concentrator (CPC) is a highly interesting solar collector technology for different low-concentration applications due to no tracking requirement. The CPC with a tubular absorber is the most common type of CPC. Here, a comprehensive state-of-the-art review of this CPC type is presented, including design features, structure, applications, etc. Key design guidelines, structural improvements, and recent developments are also presented.

In this paper, a novel two-stage dish concentrator (TSD) with a rotary secondary mirror (SM) is presented for solar thermal water/CO₂ splitting. An in-house code for ray-tracing simulation of the concentrator was developed and validated. Among all feasible geometries, a hyperboloid with an upper sheet is the most popular option and is widely used as a secondary reflector, which is mainly discussed here. All para-hyperboloid geometric combinations can be categorized into three typical patterns (φ₁ < π/2, φ₁ = π/2, φ₁ > π/2, φ₁ = field angle of PM). The initial designs of the TSD, respective to different off-axis levels for each combination, were first designed. Then a new mathematical model was introduced to reshape the SM to reach optimal truncated designs. Finally, a new concept of an off-axis primary mirror (PM) combined with the truncated SM was evaluated by using the in-house ray-tracing code. The results include the optical efficiency, concentration ratio and intercepted radiant flux. The best solutions with the highest optical efficiency fall in the range π/2 ≤ φ₁ ≤ (π − arcsin 0.8) rads and 0.4 ≤ NA₂ ≤ 0.6 (NA₂ = sin φ₂, φ₂ = field angle of SM), which vary with the concentration ratio and inclination angle.

In this study, a solar-driven reduction process of nonstoichiometric cerium oxide in a fixed bed is optimized for efficient water splitting via metal-oxide-based redox cycling. Nitrogen is used as sweeping gas to scavenge oxygen from the beds during the reduction process. A transient lumped heat transfer model is developed for the simulation of the process. Parametric analysis and genetic algorithm are used to find the optimal N₂ flow rate and establish a novel N₂ feeding strategy with variable flow to maximize the thermal efficiency for water splitting. An efficiency close to 13% is estimated without solid-phase heat recovery, which is more than twice that of the best present experimental systems (∼5%). The results are regarded preliminary as a thermodynamic analysis.