Capturing energy from wind and distributing it as electricity has become a reality. Rooted in small windmills for farms, utility-scale wind farms are developing at a fast pace evidenced by the new landscapes in Texas and Iowa with tall white structures with rotating blades on top of them. Several sources have reported that 4.7% of all U.S generated electricity was supplied by wind energy in 2015, and projections show that it will not stop here. AWEA (American Wind Energy Association) reported that 2016 saw an incremental 15% growth per quarter on wind project development and construction, which will add 20GW of generation capacity to the already existing 75GW capacity in the U.S. What drives this fast pace growth and what challenges lay ahead for what can be considered an industry in its infancy?
The pursuit of “green”, “clean” and renewable resources for electricity generation are definitely a big driver for wind energy. So much so that 19 U.S. states have adopted Renewable Portfolio Standards (RPS) requiring utilities to supply a percentage of electricity to come from renewable resources. An example is California with an RPS of 50% by 2030 or Hawaii with 100% by 2045. The RPS opens the market for wind (among other renewables) utility purchasers.
To make wind energy more cost appealing, a renewable energy Production Tax Credit (PTC) was created under the Energy Policy act of 1992 that allows for an income tax credit of 2.3 cents per kilowatt-hour produced. The PTC was a temporary credit that was to be phased out within a specified number of years. It has driven the development of wind projects such as the boom in 2012 and 2015. In December 2015, the PTC was extended for four years which has driven increased investments in new wind projects for the near future.
Added to these drivers is the fact that wind’s Levelized Cost of Electricity (LCOE) is projected in 2020 to be around $74/MWh placing it in direct competition with natural gas combined cycle (see Figure 1 from U.S. Energy Information Administration). This LCOE has made wind energy very attractive for non-utility purchasers such as large companies that own large distribution centers. In 2016, 33% of total capacity in development was contracted to non-utility purchasers.
In summary, demand for wind energy is increasing and new and existing projects will have to adapt to meet market expectations.
Building a wind project is not a trivial task. There are many steps in development that can delay or stop a wind project such as land permitting, environmental siting and local government policies/regulations. Often the biggest challenge is being interconnected to the grid. Some sites that are optimal for wind projects are in isolated rural areas, where specific substations and introduction to the grid are very challenging, with long lead times and great expense.
Wind itself can be a challenge to predict or measure for specific areas when planning a project. Wind turbines work across a specific range of wind speeds. If too fast, the wind turbine will trip offline. If too slow, there is no energy to extract. In some locations wind conditions are favorable at night when there is little demand for electricity. All of this intermittent energy production can create disruptions. Energy storage, such as batteries, would solve the variability issues, but the technology for this scale is still in development.
Last but not least is the cost of operations and maintenance. Wind Turbine O&M has been reported to account for 25-30% of a turbine’s life cycle cost. A current challenge is keeping turbines running while minimizing maintenance costs. Presently, there are approximately 40,000 wind turbines online in the U.S. at capacities greater than 1 MW. The fleet’s age varies with approximately 20% of the fleet at 10-16 years of age, 60% at 5-10 years and 20% under 5 years. This means that none of the modern utility scale wind turbines have experienced their 20-year design lifetime and historical O&M data is scarce. Hence, as wind turbines age the industry is learning what are the most common failure modes and their associated rates, and the appropriate maintenance strategies needed to address them.
To date statistics show more turbine failures and higher maintenance costs than expected, which has caused this to become a major concern to a lot of owners and operators. A wind turbine consists of various components as shown in Figure 2. In the last few years, blades, gearboxes and generators have been reported to have the highest rate of failures and/or repair/ replacement downtimes (See Figure 3).
Gearbox and drive train failures were experienced from the earliest turbine designs at high failure rates. These have been the focus of maintenance improvements including, adding condition-based maintenance systems and implementing appropriate inspection intervals. According to NREL the leading failure mechanism is axial cracking in the high or intermediate shaft bearings.
Blades failing within years 1 and 2 have been attributed to manufacturing defects or transportation damage. While lightening damage is regarded as one of the main causes for blade failures, many other causes exist such as, adhesive bonding issues, trailing edge separation, and de-bonded protective coatings. Unfortunately, condition-based maintenance is still challenging for blades and maintenance data comes from limited up-tower inspections.
In Shipurkar et al study of wind turbine generator failures it was found that stator and bearing damage were responsible for approximately 75% of the generator failures.
As a young industry, wind farm owners, operators and maintenance service providers do not have a lot of historical data and past experiences to learn from. At Structural Integrity we have applied our expertise with failure analysis, damage evaluations and NDE solutions to a number of wind turbine components. We have recently expanded our capabilities to include composite materials which will prove invaluable as blade damage assessments become a vital part of wind life management.
: American Wind Energy Association (2016 RPS and Wind Market Reports)
: U.S Energy Information Association
: National Renewable Energy Laboratory, Report on wind turbine reliability – A survey of various databases. June 2013 (NREL/PR-5000-59111)
 : Shipurkar, U., Ma, K., Polinder H., Blaabjerg, F. and Ferreira J.A. A review of failure mechanisms in wind turbine generator systems. 2011 Electrical Insulation Conference (EIC). IEEE, Jun. 2011,pp. 392–397.