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Wind Turbine Gearbox : Cooling

Heat is generated during the operation of the gearbox due to friction between the solid surfaces of the moving parts. Most of the heat is generated by the friction between the gear teeth, and between the rollers and rings of the bearings.

Depending on the type of the engaging gears, the input torque and speed, and the designed speed and torque ratios, the energy converting efficiency of gearboxes may be as low as 35% or as high as 90%.

Therefore, somewhere between 10% and 65% of the power applied to the gearbox is turned into heat. This may cause a rapid rise in gearbox temperature, resulting in a breakdown of the lubricating properties of the gear oil. Unless a high rate of cooling is maintained during operation, this loss of lubrication may lead to damage which can result in the failure of the gearbox.

Conduction, thermal radiation, and convection are important physics concepts concerning gearbox cooling or heat transfer.

Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. During operation, the heat generated by friction between the solid surfaces of the moving parts is transferred through conduction to the low-temperature portions of the moving parts, and then further to the gearbox housing.

Conduction heat transfer slows down the heat build up at the solid surfaces of the moving parts, and is part of the cooling process of the gearbox

Thermal radiation is the emission of electromagnetic waves from all matter that has a temperature greater than absolute zero. During gearbox operation, the emission of electromagnetic waves or radiation exists in the form of infrared light emission from the gearbox and its moving components, because the temperature at any part of the gearbox components is above absolute zero. This form of heat transfer also contributes the cooling of the gearbox.

While conduction and thermal radiation contribute to gearbox cooling, the rate of heat transfer through these processes is too low to cope with the rapid heat build that occurs as power is applied. Power input to a gearbox is defined as the input torque multiplied by the input speed. The amount of heat produced rises as input power goes up.

Convection is the most significant mode of heat transfer in gearbox cooling. Convection is the transfer of heat by the mass motion of a fluid when the heated fluid is caused to move away from the source of heat, carrying heat with it.

Two convection processes coexist simultaneously in the cooling of a gearbox in operation. They are convection of the lubricant and convection of the air. Both may occur naturally even without a pump system for the lubricant or an air to liquid heat exchanger.

First, the circulation of lubricant caused by the moving parts in a gearbox carries heat from the frictional surfaces of the gears, shafts, and bearings to the cooler portions of those parts, as well as to the gearbox housing. Second, as the air surrounding the heated gearbox becomes warmer and then lighter, the air starts to rise, forming a secondary convection heat transfer, into the atmosphere.

Because the amount of heat generated increases as power input rises, most large wind turbine gearboxes use a pump to circulate the lubricant, along with an air-to-liquid heat exchanger, or radiator. The pump moves lubricant through a filter, and out to the heat exchanger. In the heat exchanger, fans force air past tubes containing the hot lubricating oil. The oil gives up its heat to the cooler air, and is typically pumped to a holding tank before returning to the gearbox.

The rate of circulation and the size of the heat exchanger combine to produce a rate of cooling that matches the rate of heating, resulting in a thermal equilibrium at a working temperature that sustains the function and life of the gearbox. In extremely cold temperatures, however, the oil may require heating in order for it to flow properly. Electric heaters in the oil tank maintain the minimum oil temperature needed for good lubrication

Heat management in a wind turbine gearbox requires the proper selection, application, and maintenance of the lubricant, which is the medium for convection heat transfer. In addition, the lubricant circulation system, the heat exchanger, and the cooling fan must be properly designed, installed, and maintained to ensure long gearbox life.

Highland Community College as part of WindTechTV.org

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