The choice between Shuttle-based systems and ASRS Stacker cranes for optimal storage efficiency

When selecting an automated storage method for pallets, two primary options commonly emerge: the ASRS stacker crane and the shuttle-based pallet storage system. While the ASRS is a well-established and proven technology, shuttle-based systems have also matured in recent years. Shuttle-based systems offer multi-deep storage with multiple shuttles and lifts, allowing for very high throughput levels. On the other hand, ASRS compensates for storage density by reaching heights of 40+ meters. However, if the decision must be solely based on storage efficiency, which option should be chosen and why?

Intuitively, one might assume that shuttle-based systems have higher storage efficiency, as they can accommodate depths of 25+ pallets in a lane. But is this truly the case? This article will explore three scenarios using line graphs to ascertain when one storage method becomes more efficient in terms of square meters per pallet (Sqm/Pallet) requirement compared to the other. Before delving into the scenarios, a few assumptions need to be established to prevent endless speculations.

Assumptions:

Pallet dimensions: 1x1.2x1.2 meters (Length x Width x Height)
Shuttle systems can accommodate a maximum of 25 pallet positions in depth
ASRS does not employ shuttles for achieving multi-depth storage and is limited to a maximum depth of 2 pallets
Throughput requirements do not influence the decision-making process
Shuttle systems are restricted by lifts, with a maximum height of 25 meters

Understanding the Graphs:

X-axis: Lane depth per track (Combining right and left sides of the track)
Y-axis: Sqm/Pallet

Shuttle based and Crane based system both at 25m high

The graph illustrates a fixed 2-depth configuration for ASRS, comparing it to a shuttle-based storage system with depths ranging from 1 to 25. This approach allows for identifying the point at which the Sqm/Pallet of the shuttle-based system intersects the Sqm/Pallet line of the ASRS. The depth per track (combined right and left sides) at which the shuttle system's Sqm/Pallet becomes lower than that of the ASRS signifies when the shuttle-based system becomes more efficient than the ASRS.

The three scenarios chosen for analysis are those frequently encountered during solution design. However, readers can apply this concept to devise scenarios tailored to their specific requirements.

Scenario 1: Shuttle-based system at 25 meters height and ASRS at 25 meters

n this scenario, the break-even point for the shuttle-based system is at a depth of 5 (achievable through combinations such as 3 and 2 depths). If the depth exceeds 3 on each side of the track, the shuttle-based system becomes more storage-efficient than the ASRS.

Shuttle based system at 25m and crane based system at 30m

Scenario 2: Shuttle-based system at 25 meters height and ASRS at 30 meters

In this scenario, the break-even point for the shuttle-based system is at a depth of 14 (achievable through 7 depths on each side). Although exceeding 7 depths on each side results in higher storage efficiency for the shuttle-based system, the marginal increase in rate of efficiency of shuttle based system with higher depths is lower than in Scenario 1. Overall, the improvement at greater depths is minimal, making either system a viable choice.

Shuttle based system at 25m and crane based system at 40m

Scenario 3: Shuttle-based system at 25 meters height and ASRS at 40 meters

In this scenario, the shuttle-based system never reaches a break-even point with the ASRS. This is due to ASRS's vertical space utilization becoming significantly more advantageous than the depths achievable by the shuttle-based system. Consequently, ASRS is the clear choice in this case.

In reality, designers must consider numerous factors beyond storage efficiency when making decisions, such as equipment specifications, throughput requirements, SKU batches, FIFO and LIFO needs, building limitations, and more. Nonetheless, prioritizing storage efficiency as the initial parameter can serve as a fundamental criterion for selecting the appropriate automated storage mode.

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