Historically, repowering a wind project has meant the complete removal of older turbines, including foundations and electrical collection systems, because the scale and generating capacity of modern turbines have grown to the point where new machines are five- to 30-times larger than the original turbines, rendering the original equipment effectively useless in terms of upgrades.[adright zone=’190′]
However, now that the industry has been deploying thousands of multi-megawatt-scale turbines since about 2000, there is a significant population of turbines that have similar physical characteristics such that they can be adapted (within limits) to accept current state-of-the-art nacelles, rotors, power electronics and controls. Turbine original equipment manufacturers (OEMs) have been actively taking technology improvements contained in their new models and offering them as upgrades for models sold five to 10 years ago that are suitable. This upgrade approach can be effective in achieving some improvement in power generation, increased reliability and the better adaptation of turbines to actual site conditions. However, there are limits to the magnitude of the performance improvement achievable in this upgrade approach: Roughly speaking, single-digit percent increases in annual energy production (AEP) are most common (but your mileage may vary).
Partial repowering involves leaving some portions of the existing turbine and replacing other portions with new equipment and technology. This could mean as few parts as the foundation, tower and balance-of-plant electrical systems remain, while all other elements are replaced. There are also a variety of other possible component replacement combinations, depending upon the turbine model under consideration. Given that wind turbines are fatigue-limited structures, reuse of the tower and foundation means partial repowering is best performed on machines that are able to retain a good portion of their remaining useful life such that the partially repowered turbines could operate for a sufficient period and achieve the economic benefits of repowering.
Why partially repower a 10-year-old turbine?
Ultimately, the partial repowering of a project must deliver improved economics, both for the project owner and, ultimately, the consumers of electricity. There are a number of scenarios by which this can be achieved. Projects where turbines are “under-sited” or certain phases or rows of turbines are significantly underperforming due to wind speeds being significantly lower than original projections are good candidates.[adleft zone=’190′]
The low wind speeds mean the turbines have not experienced structural load levels, and the rate of loading (fatigue accumulation) is less than expected in the design. Although the turbine may be 10 years old, analysis of historic wind and SCADA data demonstrates that it may have accumulated only five years of fatigue. Although updating software, operating the generator at higher power levels and installing certain aero devices on the blades are potential upgrade options, more dramatic increases in energy production could be achieved by replacing the entire rotor and nacelle with larger and more site-specific equipment. Depending on the wind resource, a 20% larger rotor (in terms of swept area) might be able to extract 10% to 20% more energy, given other characteristics such as hub height and rated power remaining the same.
Consider another scenario in which a project has turbine models that have been significantly less reliable than expected and the OEM is no longer active in the market or able to support that specific model. Due to accumulation of downtime, it’s possible the towers and foundations have experienced a lower level of fatigue accumulation – meaning they have remaining structural life that can be utilized after the replacement of the nacelle and rotor.
Much has been written and discussed this year about U.S. Internal Revenue Service guidance on the eligibility of repowered projects to qualify (or re-qualify) for production tax credits, provided the new investment value is at least four-times greater than the retained value of the unmodified equipment. Although this clarification and opportunity are welcomed, it’s important that obtaining tax credits does not become the main driver for performing partial repower projects. The combination of increased energy production, improved efficiency of energy capture, increased equipment reliability and a potentially longer operating life is expected to offer improved project economics and competitive cost of energy, which should be the primary drivers.
Are there limits?
There are some practical limits to the size of new rotors and nacelles in comparison with the existing towers and foundations. Although this is not absolute, rotors larger than about 20% could impart loads too high for the remainder of the original tower and foundation to withstand and achieve a desired 20-year operating life. There are also physical tolerance considerations related to dimensions of a new nacelle in comparison with yaw bearing and tower top diameter or blade tip-tower clearance to avoid tower strikes. Larger rotors will mean greater wake impacts on downwind turbines (and related load impacts, as well), potential infringement on setback requirements, and potential impact on avian and bat mortality, among other issues. Therefore, a partial repower effort takes on many characteristics and requirements of new project development but with more focused reviews and targeted actions.
The reuse of foundations has several potential advantages, including the avoidance of soil excavation and related new permitting. If the tower is being reused, or being replaced with a similar configuration, existing foundation anchor bolts or mounting pieces could be reused with replacement or rehabilitation of the grout bed or mounting surface. However, the foundation for a partially repowered turbine will experience different ultimate and fatigue loading than the original turbine, and the capacity of the foundation and surrounding soils must be reviewed. Wind turbine foundations designed more than 10 years ago may not have been designed for fatigue loading at all and would need to be brought up to current standards – if even possible.
Soil conditions should be assessed for settlement, excessive deformation or other signs of load-related effects. Finally, the loading history of the turbine must be reviewed to estimate the amount of remaining life in the structure. Rebuilding or increasing the capacity of an existing foundation (where possible) is a very challenging task and could reduce economic benefits. Depending upon the findings, the capability of the foundation can pose clear limitations on the magnitude of possible repowering.
Repowering a project and increasing generating capacity will result in having to renegotiate the power purchase agreement (PPA), and prices will likely go down – so why should we do this?
This is a fair point, and partial repowering does not always make economic sense for every project. As noted earlier, partial repower projects necessitate performing many activities that are required as part of developing a new project, including evaluating various contracts. Certainly, a potential risk that must be evaluated is the impact on power sale price triggered by renegotiating the PPA to enable high production and/or generating capacity levels. Although current power prices may be lower than originally negotiated, the impact of using turbines with higher performance levels and current pricing has the potential to result in acceptable project economics. The same turbine technology deployed at new projects is resulting in economically viable projects with lower PPA prices that are competitive in current markets. [adright zone=’190′]
Where partial repowering is performed and a new calculation of future energy production is completed, use of historic operating data and wind data, as well as the analysis of windflow, can be used to improve energy calculation accuracy. Unlike pre-construction energy assessments, energy assessments based on operation data can more accurately account for long-term effects, environmental losses, electrical collection system losses, wake effects and terrain effects of windflow. We estimate that the uncertainty of long-term AEP is reduced by about 2%-3%, and the one-year P99 energy value can increase 4%-7% (but your mileage may vary).
Partial repowering has likely been performed at projects with very old, kilowatt-scale technology to one degree or another. But looking at modern megawatt-scale turbines, there are some early examples of partial repowering. At Medicine Bow Wind, Gamesa was able to acquire and modify two V42 and seven V47 turbines via its turbine life extension upgrades, increasing generation capacity by 10%. This process retained the original blades and nacelle, thus limiting the magnitude of performance increases, and the company was able to successfully demonstrate the process. General Electric completed a demonstration project by adding a seven-meter extension onto turbines at Noble’s Clinton Wind Park. The blade extensions increased the swept area by 40% and reportedly increased production by about 20%. Although this did not involve changes to the nacelle and drivetrain, the rotor size was increased and an assessment of the associated implications in loading was performed. Finally, NextEra has announced it is evaluating options to implement partial repowering in its fleet and appears to have selected a couple of initial sites in Texas to begin the effort.
As the wind industry matures, full and partial repowering will continue to play a role, along with new project development, in meeting society’s need for low-cost, carbon-free electricity. Areas that have experienced the economic benefits of wind energy will likely continue to be strong supporters of the industry. It’s difficult to imagine scenarios in which large projects in the Midwest are simply decommissioned at the end of their 20- to 30-year operating life, and we don’t take further advantage of the known good wind resource in these areas. Although it’s not currently a significant contributor to increasing generation capacity today, full and partial repowering will become a critical industry capability in the near future.
Kevin J. Smith is director of asset management and operating services for the Americas at DNV GL, and Alex Byrne is senior engineer of turbine technology at the company. They can be reached at firstname.lastname@example.org and email@example.com, respectively.