Heat Transfer In Liquid-Cooled Type Cooling System For Direct Drive Generator Of Wind Turbine
Direct drive generators for offshore multi-megawatt class wind turbine have evolved from synchronous machines permanent magnet excited. In the ways of magnetic flux direction,they are divided into AFPM(Axial Flux Permanent Magnet)- and RFPM(Radial Flux Permanent Magnet)-type machines. The AFPM-type machine with the topology of one single-rotor and double-stators arrangement is commonly used, where double iron-cored stator discs are installed and the rotor disc mounted by permanent magnets on its both sides is placed between the two iron-cored stator discs.The most unique feature of AFPM-type machine is their size, which look like large diameter, slenderrings. The large diameter allows high torque to be developed.The authors developed an AFPM-type machine for 2.5MW class wind turbine with outer diameter and width of 5,300 and 650mm, respectively, which provides a single-rotor and double-stators arrangement. A substantial structural reinforcement is required tosupport the high torque developing in high power AFPM-type machine. Larger diameter and the need for adequatestructural reinforcement leads to a massive structure. Therefore,one of the main problems facing the designers of large-power,large diameter of the AFPM-type machine is how tominimize its size and weight.Due to overheating on the surface of stator, the temperature of permanent magnets mounted on the rotor rises. Although there are conventional forced-air convection type cooling systems, they must be equipped by heavy parts such as air blowers when the generator’s capacity exceeds multi-megawatt class.Size reduction of AFPM-type machineis ultimatelylimited by the ability to remove heat from the stator windings. Theapplication of liquid-cooled type cooling system installed around windings enablesoperation of the electrical generators at a higher output for aparticular generator size as compared to an air cooled generator ofthe same size. Therefore, liquid cooled electrical generators may besmaller than forced air cooled electrical generators having the samerated power. The multi-megawatt class wind turbine is located in the coastal area with a temperature ranging from -35 to 40oC. The objective is to design a liquid-cooled type cooling system to meet the cooling demand of the wind turbine and to control its structural sizes to be most favorable for the durable operation of the wind turbine based on the giving ambient conditions and technical requirements. To meet the technical requirements of -35oC for the minimum ambient temperature in winter, the ethylene glycol aqueous solution with a concentration of 50% and a freezing point of -38oC. In this study, a newly developed liquid-cooled type system of the AFPM-type machine for 2.5MWclass directdrive generator was proposed. The liquid-cooled type system used heat pipe wound around the stator and heat-exchanger to decrease the temperature of overheated coolant in the cooling system. The location of the heat pipe was determined such that a heat-transfer should be occurred effectively, as well as electrical losses should be minimized. The stator was assembled by laminating thin stator plates with a thickness of 1mm, and then was fixed by an iron rod that penetrated through a center hole of the thin stator plates. A total of 600 assembled stators were manufactured, where the winding coils were wound around each stator at the middle- and bottom-parts. The heat pipe applied to the liquid-cooled type system was wound at top-side of the stator. These prevent overheating from occurring on the stator surface, and also avoid an increase in the temperature at the permanent magnet. The heat transfer occurring in the generator using the proposed cooling system was investigated theoretically, and the temperatures on the surface of stator and permanent magnet were estimated by an electro-magnetic and thermal fluid interaction analysis. The cooling performance of the proposed cooling system was evaluated and compared to conventional cooling system.Results obtained from an electro-magnetic and thermal fluid interaction analysis showed that the surface of the stator and permanent magnet were cooled to 69 and 66°C, respectively, which met the requirements for electric power production. From a light-weight design point of view, the weight of the equipment required for the proposed cooling system can be reduced by 20% compared to conventional system.
Index Terms: AFPM-type machine, Heat transfer, Liquid-cooled type system,Permanent magnet, Wind turbine