Order-picking system having a rack transport unit and method for operating such a system

An order-picking system comprising:

• a pallet warehouse, which is used as a supply warehouse for a plurality of different packing units, which are stored on pallets in the pallet warehouse wherein each of the pallets holds packing units of one sort only;
• a depalletizing device adjacent to the pallet warehouse to rehandle respectively one pallet layer of the packing units on empty trays, each tray being approximately pallet-sized in area;
• a tray warehouse, which is used as a picking buffer in order to feed connected order-picking stations with the packing units, wherein the trays which have been loaded with the respective one pallet layer of packing units before are stored in the tray warehouse;
• a conveyor for transporting the trays, which are loaded with the respective one pallet layer of packing units, across longer distances from the pallet warehouse to the tray warehouse;
• at least one multiple-level rack-transport unit for transporting the approximately pallet-sized trays between the pallet warehouse and tray warehouse and adapted to receive a plurality of trays each of which is loaded with the respective one pallet layer of packing units, wherein the at least one rack-transport unit is conveyed on the conveyor to the tray warehouse;
• and a shifting device which is arranged between the depalletizing device and the conveyor such that the trays, which have just been loaded with the respective one pallet layer, can be shifted directly in the at least one rack-transport unit, when the at least one rack-transport unit on the conveyor passes the shifting device, wherein the shifting device is a conveyor displaceable in a vertical direction.