A Vertically Stacked Thin-Layer Cascade Photobioreactor for Space-Efficient Indoor Microalgae Cultivation: Design, Geometric Characterization, and Operational Baseline Across Scales

Indoor microalgae production under artificial illumination is constrained by facility footprint and energy demand, making biomass output per area the central design objective. Single-layer thin-layer cascade (TLC) systems are geometrically limited to a surface-to-footprint ratio (SF) of at most 1.0 m2 m−2. This study presents a stacked thin-layer cascade (STLC) photobioreactor validated at two scales sharing the same six-channel architecture: a laboratory-scale unit (2 L) and a pilot-scale unit (250 L). Both units achieved an SF well above the single-layer ceiling (3.65 and 4.2 m2 m−2), demonstrating that vertical stacking is an inherent structural solution to the geometric limitation of open thin-layer cascade systems, independent of scale. Both units operated reliably for 11 to 13 days across a 125-fold difference in working volume, converging to comparable transitional Reynolds numbers and confirming that the film hydrodynamics central to the STLC concept remain consistent by design across scale. Footprint-normalized productivity reached 3.41 g DW m−2 d−1 at laboratory scale and 6.11 g DW m−2 d−1 at pilot scale, with pilot-scale protein reaching 42.1% DW prior to nitrogen depletion. Growth kinetics differed between scales, reflecting light/dark cycling regime, initial biomass density, and retention tank geometry as distinct, tunable operational variables. Closing the residual productivity gap requires jointly increasing photon flux density and standing biomass density, since low-density operation causes a large fraction of incident light to pass through the culture unabsorbed regardless of irradiance.