Storage system for transport units (T), with an exchange system and is operable to
- use at least one first storage rack level (3) for dedicated storage of fast mover transport units (FT), which is serviced by storage-entry and removal-from-storage apparatus (5) present on each such level and each such level is directly connected to the at least one removal-from-storage lift (8A) and the at least one supply-to-storage lift (8B), both removal-from-storage lift (8A) and supply-to-storage lift (8B) having interfacing rack conveyors (9) arranged within the footprint of the racking;
and is operable to
- use at least one second storage rack level (3) for dedicated storage of slow mover transport units (ST) or reserve transport units (RT) within the same aisle as the first storage rack level (3) for dedicated storage of fast mover transport units (FT), which is serviced by storage-entry and removal-from-storage apparatus (5) that change levels via the exchange systems;
wherein the storage-entry and removal-from-storage apparatus (5) of the first storage rack level (3) for dedicated storage of fast mover transport units (FT) are controlled to have a high probability of being in a first storage rack level (3) for dedicated storage of fast mover transport units (FT) and only leave such first storage rack level (3) temporarily to enter second storage rack level (3) for dedicated storage of slow mover transport units (ST) or reserve transport units (RT) to retrieve a slow mover transport units (ST) or reserve transport units (RT) and transport such to a first storage rack level (3) via the exchange systems.