Time and energy optimal unit-load assignment for automatic S/R warehouses

Effectiveness in warehouse design and operations is crucial for the industrial companies to be competitive in the market arena by reducing the response time and inbound costs, increasing their global service level. Storage assignment deals with the definition of effective strategies to organise items into industrial warehouses to achieve high performances. This paper enhances the conventional approaches on storage assignment proposing a time and energy bi-objective model for single-deep rack automatic warehouses served by aisle captive automated storage/retrieval systems (AS/RSs). The model is based on the joint minimisation of the travel time and the energy required by the cranes to S/R the unit-loads. The analytic models to compute the expected single-command cycle time and energy are integrated into a bi-objective model, optimising the load assignment. An application, taken from the beverage industry, is, finally, discussed. The different perspectives of adopting time and energy to drive the load assignment are stressed proposing a practical trade-off rule. Results highlight the possibility to balance the time cycle and the energy intensity. Within the case application, an energy saving of 12.66% respect to the time model occurs with a time efficiency decrease of about 2.52%.