Sunlight is free, dilute and unevenly delivered, and a reactor for sunlight must be designed around that fact. The flat-plate photoreactor is the canonical answer: it compresses the liquid layer to millimeters so every part of the volume sits within reach of the photons, trading depth for area.
Solar irradiance peaks near 1000 W/m² under clear skies, but absorbance limits how deep photons penetrate a real suspension. A thin channel, a few millimeters, keeps the whole volume illuminated; the plate simply widens to deliver the volume. The design converts sunlight's low intensity from a liability into a geometry problem.
Channel depth matched to the suspension's attenuation length, measured, not guessed; flow distribution across the plate width, dead zones waste area; materials that survive UV, weather and the chemistry for years; and a transparent cover that resists soiling, dust is a performance loss that accumulates daily.
Sunlight heats the plate: dark absorbers push temperatures well above ambient, sometimes an advantage for thermally assisted catalysis, sometimes a selectivity problem. Design the thermal path deliberately, insulated backing to retain heat, or cooling contact to reject it, and measure liquid temperature, not just ambient.
Flat-plate productivity scales with area and insolation, and honest comparisons normalize by measured irradiance. Expect lab-to-field gaps: solar spectra, temperatures and diurnal cycles all differ from laboratory constants, the plate's job is to stay productive under all of it.
Perfectlight's outdoor solar application systems include flat-plate photoreactor configurations; see engineering solutions.
