Storage system and a method of operating the storage system

A storage system comprising:

• a plurality of vertical stacks of storage containers, wherein the stacks are arranged in a horizontal first direction and a horizontal second direction which is perpendicular to the first direction,
• a transport unit for transporting at least one container to or from a selected stack in the first and/or second horizontal direction,
• a container handling device for transferring at least an upper container from a selected stack to the transport unit before the transport unit including the container is transported from said selected stack and for transferring at least a container from the transport unit to the top of a selected stack after the transport unit including the container is transported to said selected stack,
• wherein:
• the transport unit is movable above the stacks and the transport unit is provided with a first set of wheels at opposite sides of the transport unit for travelling in the first direction and a second set of wheels at opposite sides of the transport unit for travelling in the second direction, wherein at least one of the first and second sets of wheels is liftable with respect to the remainder of the transport unit so as to selectively run either the first set of wheels or the second set of second wheels on top of the stacks or on rails in a grid pattern across the top of the stacks,
• the container handling device comprises a lifting member for displacing a selected stack in vertical direction with respect to adjacent stacks,
• the lifting member is movable below the stacks in a non-lifting condition thereof, and
• the container handling device comprises an engaging member that is provided at the transport unit and suitable for engaging at least an upper container of a lifted stack so as to retrieve said container or for releasing an engaged container so as to place said container on top of a selected stack, and
• wherein the stacks rest on a support, the lifting member is suitable for displacing a selected stack in vertical direction with respect to the support, and the lifting member is movable below said support in the non-lifting condition thereof.