Comment: Protecting wind turbine gearboxes

The Global Wind Energy Council’s forecast of 1,221 GW of new renewable electricity capacity being built worldwide over the next seven years is a significant prospect. Additionally, the expected growth of the wind turbine sector from $24bn today, to $44.6bn by 2032[1] reflects the industry’s increasing momentum.

Unfortunately, mechanical wear in large multi-megawatt (MW) wind turbines has become an increasingly serious issue over the last decade. In-depth investigations have found widespread gearbox failures within five years of operation, meaning operators now consider them to be the single most expensive wind turbine component to replace.

As wind turbine technology has evolved, leading to larger, more powerful turbines operating in increasingly challenging conditions, the demands on gearbox lubrication have also grown. Sadly, some of the traditional lubricants cannot meet these performance demands, leading to decreased efficiency, increased maintenance, and in some cases, high-cost premature failure. However, newer, high-efficiency synthetic lubricants could be the answer as they are designed with long-term component maintenance in mind.

The task of effectively lubricating a wind turbine gearbox is challenging, fraught with difficulties arising from harsh operating conditions, stringent performance requirements, and high expectations for longevity and reliability.

Understanding gearbox corrosion

Research is ongoing to determine what causes such a high failure rate in wind turbine gearboxes. A key factor, that effects every instance when two surfaces meet and move against each other, is hydrogen wear. The science of which can be traced back to the 1960s, when Russian engineer Professor Dmitry N. Garkunov made two discoveries while investigating mechanical failure in aircraft landing gears.

Garkunov discovered that during friction, copper ions were being selectively transferred into the crystalline grid of iron forming a thin film. Then, he found that hydrogen was the primary cause of metal surface deterioration. Further research has found that hydrogen wear occurs during sliding and rolling contacts that are found in rotating machinery such as gears and hydraulics.

Taking hydrogen wear into consideration, another of the most significant challenges for wind turbine gearboxes is elemental.  The inconsistent and unpredictable nature of wind speed and direction has a major impact. These variations can result in fluctuating loads and torque on the gearbox, increasing wear and tear on the components. The lubricant must effectively protect the gearbox components under these dynamic operating conditions.

From what research has discovered so far, studies are concentrated on the amount of energy required to remove hydrogen ions from either water or from the base oil. The sheer size of turbines, along with the amount of torque that goes through their gearboxes, and the power that flows through their blades, results in a sizeable but destructive amount of energy.

Consequently, cracks appear on the inner ring of high-speed shafts (HSS) and high-speed intermediate shafts (HSIS) bearings. If left ignored, these cracks will spread and damage the ring, shafts, gears, and gearbox housing, leading to catastrophic premature failures.

When examining this cracking up close, researchers concluded that three factors appear to be working together: non-metallic inclusions, sliding, and hydrogen. However, as the cracks occur below the surface, it is impossible to detect with the naked eye, and by the time it reaches the surface the damage is done.

This phenomenon has been named White Etching Cracking, or WEC[2], and although researchers agree that its exact cause remains unclear, many suggest that lubricants may play a significant role. Interestingly, the description of WEC is nearly identical to the description of hydrogen wear.

Given the fact that many wind turbine gearboxes are not achieving their expected 20-year design life, and around 60% of the industry’s gearbox failures are caused by WEC, it is unsurprising that wind turbine operators and OEMs are conducting extensive investigations on how to solve this complex problem.

How to ensure longevity through lubrication

Wind turbines often operate in locations subject to extreme hot and cold temperatures. High temperatures can reduce lubricant viscosity (and therefore load-carrying), while extremely low temperatures can cause the lubricant to thicken, leading to inadequate lubrication. Therefore, the lubricant must have a broad operating temperature range to maintain its viscosity and performance under any temperature or adverse conditions.

Take offshore and coastal wind turbines as a prime example of where the weather and the natural environment can severely impact the durability of a gearbox. In these conditions, moisture and salt can penetrate the gearbox, leading to corrosion of components. As a wind turbine needs to be robust to stay operational, so does its gearbox. This sort of environment means a gearbox requires lubricants with excellent water separation capabilities and corrosion inhibitors to protect it.

High-efficiency lubricants can reduce the stress on bearings, ensuring an adequate lubrication film, thereby reducing metal-to-metal contact that may lead to WEC. Moreover, the introduction of certain performance additives into the lubricants that eliminate the hydrogen wear will protect against specific conditions leading to WEC.

Improving reliability through durability

There is an urgent need for more sophisticated lubrication solutions to meet the evolving requirements of wind turbines. As global reliance on wind energy increases, it’s crucial to maintain this renewable resource both reliable and cost-effective. High-efficiency synthetic lubricants could provide the solution, particularly as they are designed to safeguard gearboxes from the damaging WEC phenomenon affecting wind turbines.

If all wind turbine operators adopted high-efficiency synthetic lubricants, it would mark a significant advancement in self-sufficiency, engineering, and component upkeep. This technology could prevent wind turbine gearboxes from becoming landfill waste, instead ensuring these costly components remain functional for their full lifespan and contribute to the production of more dependable renewable energy.

Leyla Alieva, CEO and co-founder , NEOL Copper Technologies