In heavily constrained environments requiring very high density storage, traditional aisle-based warehousing may not provide viable options. One feasible manifestation of high density storage systems is the ‘puzzle-based’ system, in which unit loads are moved through the system via manipulation of empty (escort) locations to retrieve desired items. Another option would be the use of movable concentric rings, with escorts being utilized to enable lateral movements of unit loads between the rings. In this paper, the authors present analytical results to compare retrieval time performance for these two types of high density storage systems for randomly demanded items under various assumptions regarding the initial placement of escorts. The paper concludes that the use of movable concentric rings results in significant improvement to retrieval time performance in comparison to rectangular, puzzle-based systems, and further concludes that additional research is warranted
