A mathematical model is formulated for solar optical and thermal processes in cellular arrays. Explicit computational results are presented on optimization of wall separation and aspect ratio of arrays with square and parallel slat cross sections. The arrays with wall spacings of 0.5–1.25 cm and aspect ratios of 10–17 are found to be suitable for solar-collector and storage applications. A performance comparison is described for square-cell honeycomb and parallel-slat devices. For a horizontal collection surface, the honeycomb is superior to the slat device. On the other hand for a surface inclined at more than 30 ° with respect to the horizontal, a slat device is preferable.