energies Article Estimation of the Performance Aging of the Vestas V52 Wind Turbine through Comparative Test Case Analysis Davide Astolﬁ 1, *, Raymond Byrne 2 and Francesco Castellani 1   Citation: Astolﬁ, D.; Byrne, R.; Castellani, F. Estimation of the Performance Aging of the Vestas V52 Wind Turbine through Comparative Test Case Analysis. Energies 2021, 14, 915. https://doi.org/ 10.3390/en14040915 Academic Editor: Juri Belikov Received: 28 January 2021 Accepted: 7 February 2021 Published: 9 February 2021 Publisher’s Note: MDPI stays neu- tral with regard to jurisdictional clai- ms in published maps and institutio- nal afﬁliations. Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con- ditions of the Creative Commons At- tribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Engineering, University of Perugia, Via G. Duranti 93, 06125 Perugia, Italy; francesco.castellani@unipg.it 2 Centre for Renewables and Energy-Dundalk Institute of Technology, Dublin Road, A91 V5XR Louth, Ireland; raymond.byrne@dkit.ie * Correspondence: davide.astolﬁ@studenti.unipg.it; Tel.: +39 075 585 3709 Abstract: It is a common sense expectation that the efﬁciency of wind turbines should decline with age, similarly to what happens with most technical systems. Due to the complexity of this kind of machine and the environmental conditions to which it is subjected, it is far from obvious how to reliably estimate the impact of aging. In this work, the aging of ﬁve Vestas V52 wind turbines is analyzed. The test cases belong to two different sites: one is at the Dundalk Institute of Technology in Ireland, and four are sited in an industrial wind farm in a mountainous area in Italy. Innovative data analysis techniques are employed: the general idea consists of considering appropriate operation curves depending on the working control region of the wind turbines. When the wind turbine operates at ﬁxed pitch and variable rotational speed, the generator speed-power curve is studied; for higher wind speed, when the rotational speed has saturated and the blade pitch is variable, the blade pitch-power curve is considered. The operation curves of interest are studied through the binning method and through a support vector regression with a Gaussian kernel. The wind turbine test cases are analyzed vertically (each in its own history) and horizontally, by comparing the behavior at the two sites for the given wind turbine age. The main result of this study is that an evident effect of aging is the worsening of generator efﬁciency: progressively, less power is extracted for the given generator rotational speed. Nevertheless, this effect is observed to be lower for the wind turbines in Italy (order of −1.5% at 12 years of age with respect to seven years of age) with respect to the Dundalk wind turbine, which shows a sharp decline at 12 years of age (−8.8%). One wind turbine sited in Italy underwent a generator replacement in 2018: through the use of the same kind of data analysis methods, it was possible to observe that an average performance recovery of the order of 2% occurs after the component replacement. It also arises that for all the test cases, a slight aging effect is visible for higher wind speed, which can likely be interpreted as due to declining gearbox efﬁciency. In general, it is conﬁrmed that the aging of wind turbines is strongly dependent on the history of each machine, and it is likely conﬁrmed that the technology development mitigates the effect of aging. Keywords: wind energy; wind turbines; technical systems aging; performance analysis; power curve 1. Introduction It is a well-known fact that machines and technical systems are affected by aging [1–6], but it is difﬁcult to theoretically estimate this kind of effect. In particular, the power production of a wind turbine has a very complex dependence on ambient conditions [7–10], on the stochastic nature of the source [11,12], on the working parameters [13,14], on the wake interactions [15,16], and on the health status and on the efﬁciency [17,18] of the sub-components. On these grounds, it is a common sense expectation that the efﬁciency of wind turbines declines with age, but there are no standards about how much and in how much time the performance should decline. Energies 2021, 14, 915. https://doi.org/10.3390/en14040915 https://www.mdpi.com/journal/energies