Cost and performance comparison for tier-captive and tier-to-tier SBS/RS warehouse configurations

Abstract

This paper compares two shuttle-based storage and retrieval system (SBS/RS) configurations developed on travel policy of shuttles in the systems. Specifically we compare two system configurations of SBS/RS: SBS/RS with tier-captive shuttles and SBS/RS with tier-to-tier shuttles. In a tier-captive SBS/RS warehouse design there is a dedicated shuttle in each tier that cannot travel between tiers. Due to a large number of shuttles in such a configuration lifts are always bottlenecks and the utilizations of shuttles are very low. In this study we propose an alternative SBS/RS design with tier-to-tier shuttles in which shuttles can travel between tiers by using a separate lifting mechanism dedicated for them. The advantage of this design would be reduction in investment cost due to the decreased number as well as higher utilizations of shuttles in the system. The two configurations were compared on the following performance metrics: initial investment costs throughput rates per hour and average energy consumption per transaction when there is a regeneration mechanism in the system. In a tier-to-tier SBS/RS design allowing shuttles to travel between tiers increases operational complexity of the system. Thus we simulate the two system designs and compare their performance metrics by conducting several rack design experiments based on the number of aisles tiers and bays. The results of this study suggest that there may be a better configuration in terms of total investment cost and throughput rate in a newly proposed tier-to-tier SBS/RS design that could be considered as an alternative to a tier-captive SBS/RS design.