| Wind turbine technology Why lubricants matter A new wind turbine lubricant from TotalEnergies promises significant efficiency improvements
Mickael Ponsardin, Lubricant Specialist and Gear Oils Product Manager, TotalEnergies Lubrifiants, and Grégoire Roux, Senior Lubricant and Mechanical Engineer, TotalEnergies Lubrifiants
Wind turbines play a pivotal role in sustainable energy generation and the significance of their energy efficiency cannot be overstated. Maximising the efficiency of wind turbines and their associated processes not only reduces operational costs but also enhances the overall environmental impact.
Although gearboxes are well optimised today, one of the components is still too often considered as a necessary evil rather than an asset that could unlock higher energy efficiency: the lubricant.
Energy efficiency and viscosity index
Energy efficiency in lubricants has typically been closely linked to the concept of viscosity index (VI). The viscosity index is a measure of the change in lubricant viscosity with temperature variations. It quantifies the ability of a lubricant to maintain its viscosity and provide consistent lubrication performance across a wide range of operating temperatures.
The viscosity index plays a crucial role in minimising energy losses due to friction within machinery and equipment. When lubricants are subjected to temperature changes, their viscosity can vary significantly. Higher temperatures tend to thin out lubricants, reducing their viscosity and potentially leading to inadequate lubrication, increased friction, and higher energy consumption. Conversely, lower temperatures can cause lubricants to thicken, increasing resistance and energy losses.
A lubricant with a high viscosity index exhibits Figure 2. FZG test rig
Loss torque meter
Load torque meter
Test gear box MTM, 40°C, SRR 20% (slide-roll ratio)
0.08 0.07 0.06 0.05 0.04 0.03
0
500
1000
1500 Rolling speed (mm/s) ISO VG 320 Mineral PAO mPAO Above: Figure 1. MTM results
minimal changes in viscosity across temperature fluctuations. It maintains its lubricating properties and provides a stable and consistent protective film between moving parts, irrespective of the operating temperature.
Therefore, high-viscosity-index lubricants allow lower viscosities to be used. At low temperature, they minimise friction losses, when compared to conventional lubricants, resulting in lower energy consumption. Low-viscosity lubricants are particularly beneficial for high-speed applications, where it is critical to minimise drag and energy losses.
Nevertheless, it is important to note that the viscosity index is not the sole indicator of lubricant performance. Other first-order factors
are the film strength and the coefficient of friction of the lubricant chemistry itself, independent of the viscosity.
Energy efficiency and synthetic lubricants
Synthetic lubricants are engineered lubricants that offer superior performance characteristics compared to conventional mineral-based oils. They are synthesised from chemically modified base oils and often exhibit a high viscosity index, providing excellent stability across a wide temperature range and an extremely low pour point.
The most used synthetic base stock has been polyalphaolefin (PAO), which extends draining intervals thanks to its long lifetime compared with standard mineral base oils. In 2013, metallocene polyalphaolefin (mPAO) was introduced, with improved viscosity index and cold properties. Another synthetic lubricant, with even higher viscosity index is polyalkylene glycol (PAG). This is often characterised as a lubricant for high energy efficiency. However, PAG based lubricants are quite aggressive towards some materials, such as paint and internal coatings, are not miscible with mineral oils and are very sensitive to water. Because of these drawbacks, the uses of PAG remain limited.
Electric motor Inertia Test gear Test pinion Load clutch
Slave gear stage
Oil type (ISO VG 320) Typical viscosity index Mineral based PAO
98
mPAO PAG
155 188 238
The table below summarises viscosity index values for mineral based and synthetic oils:
NEW formula
2000
2500
3000
www.modernpowersystems.com | October 2023 | 37
Coefficient of friction (traction coefficient)
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