Abstract: A thermal storage device interacting with a circulating coolant in an air conditioning system is disclosed. The thermal storage device includes a hollow ball containing a liquid to be frozen by the circulating coolant. A generall ball-shaped flexible air pocket is disposed in the hollow ball. An elongated weight member has one end connected to the air pocket and the other end extended to the internal face of the hollow ball, so that the center of the air pocket can be kept in the geometrical center of the hollow ball. The elongated weight member has a predetermined weight so that the total weight of the hollow ball, the air pocket and the elongated weight member is greater than the buoyancy force exerted on the hollow ball by the recirculating coolant. Therefore, the thermal storage device can always be submerged in the recirculating coolant and thus achieve a good heat transfer effect.