As wind farms age, owners need to make decisions regarding either the extension of the operational life of its plants or their complete decommissioning and repowering. In addition to the commercial factors affecting these decisions, technical aspects must also be considered to ascertain the risk associated with the prolonged operation of an aging fleet.
Assuming a typical design life of 20 years, it is expected that 86 GW of wind generation capacity will be decommissioned across Europe by 2030 if operational lives are not extended1.
Source: L. Ziegler, E. Gonzalez, T. Rubert, U. Smolka and J. J. Melero,“Lifetime extension of onshore wind turbines: A review coveringGermany, Spain, Denmark, and the UK,” Renewable and Sustainable Energy Reviews,vol. 82, no. 1, pp. 1261-1271, 2018.
While repowering wind farms using novel technologies offers higher energy yields and financial revenues, the increasing cost of capital, difficulties in securing a land lease, planning permissions, grid licenses and a drop in the level of subsidy available to Onshore/Offshore projects all encourage owners to consider extending the operational life of their existing fleet.
One immediate benefit of extending the operational life is the reduction in levelized cost of energy (LCOE) and an increase in revenue over the plant’s lifetime; however, the life cycle cost remains approximately unchanged, with some variation in the operational expenditure (OPEX). This assumes that no major part of the plant requires replacement, and only minor refurbishments and maintenance are necessary.
The average cost of extending the operational life of an Onshore Wind farm is approximately 100 KEUR/MW, whereas the cost of repowering is approximately 1 MEUR/MW.
Conventional commercial wind turbines are typically designed and certified for 20 years of operation; these specifications include those for major structural components of the turbine such as blades, towers, foundations, yaw rings, pitch bearings and drive train components.
Typically, the structural components of turbines are subjected to cyclic fatigue loads; the reliability and failure probability of such components therefore increases as the turbine approaches the end of its design life. For instance, rotating bearings in turbines are designed based on 90% reliability, implying that the probability of bearing failure before the end of the designated 20-year design life is 10%2. Moreover, the blade exterior (including leading edges) requires ongoing and thorough life maintenance. However, the structural elements are designed to last for the intended design life (i.e. 20 years).
Towers are designed based on class loads on Onshore Wind turbines. However, Offshore Wind towers are designed based on the design of the offshore foundation and site-specific metocean loads, as well as according to an iterative process executed by the foundation designer and turbine original equipment manufacturer (OEM).
Onshore turbines can have gravity or pile-based foundations. Furthermore, they are normally designed based on the interpretation of the climatic class loads on the tower bottom and according to the EUROCODE design standard.
Offshore foundations are often monopile or jacket types, designed in conjunction with the turbine tower using site specific metocean loads. It is expected that a lesser degree of conservatism is considered in the design of the offshore foundation compared to older onshore foundations, due to accounting for site-specific loading in the design process.
To read more, please download the full article, below.
1 By Paul Dvorak, September 29, 2017, “An Owner’s guide to wind turbine life extension”, 10th Annual Wind O&M Europe 2018 Conference
2 John Coultate, Mike Hornemann, December 2017, “Why wind-turbine gearboxes fail to hit the 20-year mark”, WindPower Engineering & Development
|Wind power: extending beyond the design life