PROBLEM TO BE SOLVED: To provide an air conditioning system allowing heat source water returned to a heat storage tank to flow without generating cavitation in a water returning pipe, efficiently converting potential energy of the heat source water into electric power, and recovering the same. SOLUTION: In this air conditioning system for circulating and supplying the heat source water (a) to an air conditioning load 2 in a building from the heat storage tank 1 disposed at a lower part of the building, the water return pipe 10 for returning the heat source water (a) from the air conditioning load 2 to the heat storage tank 1 is provided with a curved portion 10a converting the flow of the heat source water (a) falling from the air conditioning load 2 into the horizontal direction, and a horizontal portion 10b for allowing the heat source water (a) to flow in the horizontal direction, a water wheel 11 for recovering kinetic energy of the heat source water (a) and a pressure regulating valve 22 are disposed in the horizontal portion 10b successively in the flow of the heat source water (a), a radius of curvature ρ of the curved portion 10a is 1.5 times or more of a pipe diameter D of the water return pipe 10, and a distance L from a starting end portion of the horizontal portion 10b to the water wheel 11 is six times or more of the pipe diameter D of the water return pipe. COPYRIGHT: (C)2008,JPO&INPIT