applied sciences Review New Tendencies in Wind Energy Operation and Maintenance Ángel M. Costa 1 , José A. Orosa 1,* , Diego Vergara 2 and Pablo Fernández-Arias 2 1 Department of N. S. and Marine Engineering, Universidade da Coruña, Paseo de Ronda, 51, 15011 A Coruña, Spain; [email protected] 2 Department of Mechanical Engineering, Catholic University of Ávila, C/Canteros s/n, 05005 Avila, Spain; [email protected] (D.V.); [email protected] (P.F.-A.) * Correspondence: [email protected] Abstract: Both the reduction in operating and maintenance (O&M) costs and improved reliability have become top priorities in wind turbine maintenance strategies. O&M costs typically account for 20% to 25% of the total levelized cost of electricity (LCOE) of current wind power systems. This paper provides a general review of the state of the art of research conducted on wind farm maintenance in recent years. It shows the new methods and techniques, focusing on trends and future challenges. In addition to this, this work includes a review of the following items: (i) operation and maintenance, (ii) failure rate, (iii) reliability, (iv) condition monitoring, (v) maintenance strategies, (vi) maintenance and life cycle and (vii) maintenance optimization As for offshore wind turbines, it is crucial to limit the maximum faults, since the maintenance of these wind farms is more complex both technically and logistically. Research into wind farm maintenance increased by 87% between 2007 and 2019, with more than 38,000 papers (Scopus) including “wind energy” as the main topic and some keywords related to O&M costs. The LCOE in onshore wind projects has decreased by 45%, while in offshore projects it has decreased by 28%. The O&M costs of onshore wind projects fell 52%, while in the case of offshore projects, they have declined 45%. Thus, the results obtained in this paper suggest that there is a change in research on wind farm operation and maintenance, as in recent years, Citation: Costa, Á.M.; Orosa, J.A.; scientific interest in failure has been increasing, while interest in the various techniques of wind farm Vergara, D.; Fernández-Arias, P. New maintenance and operation has been decreasing. Tendencies in Wind Energy Operation and Maintenance. Appl. Sci. 2021, 11, Keywords: wind energy; new tendencies; review; maintenance; optimization 1386. https://doi.org/10.3390/ app11041386 Academic Editor: Frede Blaabjerg 1. Introduction Received: 20 January 2021 Renewable energies have become the greatest ally for the generation of electrical Accepted: 1 February 2021 energy worldwide that is free of CO2 emissions. In the first quarter (Q1) of 2020, renew- Published: 4 February 2021 able energies reached a worldwide share of electricity generation of nearly 28%, i.e., an in- crease of 2% compared to Q1 2019. Even with the outbreak of the COVID-19 pandemic, Publisher’s Note: MDPI stays neutral total global renewable electricity generation was estimated to increase by almost 5% in with regard to jurisdictional claims in 2020 [1]. Taking into account that hydroelectric technology was developed throughout published maps and institutional affil- the 20th century, wind power is, among the current technologies for generating electrical iations. energy from renewable sources, the most widely implemented renewable energy around the world. In 2018 (currently the last consolidated year, 31 December 2020), wind power had an installed capacity of more than 560 GW, assuming a worldwide production of more than 1.2 million GWh (Table1). Copyright: © 2021 by the authors. On the other hand, a simple query in Scopus on the number of articles including Licensee MDPI, Basel, Switzerland. “wind energy” in the title, abstract or keywords (Table1) shows that there are more than This article is an open access article 28,000 articles in the Scopus database going back to 2008 (data collected in November 2020). distributed under the terms and This result means an average annual scientific production related to wind energy of more conditions of the Creative Commons than 2000 articles (Scopus). Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ The levelized cost of energy (LCOE) is the primary metric for describing and compar- 4.0/). ing the underlying economics of power projects (Figure1). For wind power, the LCOE Appl. Sci. 2021, 11, 1386. https://doi.org/10.3390/app11041386 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 26 Appl. Sci. 2021, 11, 1386 2 of 26 Table 1. Wind energy data (data collected from [1] and Scopus database). Year representsMeasure the sum of all costs of a fully operational wind power system over the lifetime of the project with financial flows2008 discounted 2010 to a common2012 year.2014 The principal2016 components2018 of theCapacity LCOE of (MW) wind power systems1.20 × 105 include1.81 × 10 capital5 2.67 costs, × 105 operation3.49 × 105 and 4.67 maintenance × 105 5.63 × costs 105 and theProduction expected (GWh) annual energy2.18 production× 105 3.43 × [102].5 Assessing5.26 × 105 the7.13 cost × 10 of5 a wind9.56 × power105 1.26 system × 105 Numberrequires of a Articles careful in evaluation Scopus 1014 of all of these1558 components2255 over2448 the life of2597 the project3167 [3]. The capital costs, which account for 10–15% of the total project cost, include all expenses incurredThe levelized in the purchase cost of ofenergy land, (LCOE) buildings, is the construction primary metric and equipment. for describing Equipment and com- is paringworth betweenthe underlying 70% and economics 80% of the of totalpower project projects cost, (Figure mainly 1). due For to wind the costpower, of the the turbine. LCOE representsConstruction the is sum worth of all around costs 5%of a to fully 20% oper of theational total projectwind power investment. system The over operation the lifetime and ofmaintenance the project (O&M)with financial cost is theflows cost discounted associated to with a common the operation year. The and principal maintenance compo- of a nentswind of farm. the LCOE of wind power systems include capital costs, operation and mainte- nance costs and the expected annual energy production [2]. Assessing the cost of a wind Tablepower 1. Wind system energy requires data (data a careful collected evaluation from [1] and of all Scopus of these database). components over the life of the project [3]. The capital costs, which accountYear for 10–15% of the total project cost, include all Measure expenses incurred in the purchase of land, buildings, construction and equipment. Equip- ment is2008 worth between 2010 70% and 80% 2012 of the total project 2014 cost, mainly 2016 due to the cost 2018 of the Capacity (MW) turbine.1.20 × Construction105 1.81 ×is 10worth5 around2.67 × 105%5 to 20%3.49 of× the105 total project4.67 × 10 investment.5 5.63 ×The10 5op- 5 5 5 5 5 5 Production (GWh) eration2.18 ×and10 maintenance3.43 × 10 (O&M) 5.26cost× is10 the cost7.13 associated× 10 with9.56 the× operation10 and1.26 ×mainte-10 Number of Articles in Scopusnance 1014of a wind farm. 1558 2255 2448 2597 3167 Figure 1. The economics of wind energy. Figure 1. The economics of wind energy. The fixed and variable O&M costs are a significant part of the overall LCOE of windThe power. fixed O&Mand variable costs can O&M account costs for are between a significant 11% part and 30%of the of overall onshore LCOE wind of LCOE, wind power.and typically O&M accountcosts can for account 20% to 25%for between of the total 11% LCOE and of 30% current of onshore wind power wind systems LCOE,[ 3and–5] . typicallyWhen, years account ago, for wind 20% farm to 25% promoters of the total signed LCOE full of O&M current contracts wind power with manufacturerssystems [3–5]. When,of wind years turbines, ago, wind they farm trusted promoters that technologists signed full wouldO&M contracts provide thewith highest manufacturers level of de-of windvelopment turbines, in thethey maintenance trusted that of technologi their installations.sts would Timeprovide has the shown highest that level in most of devel- cases opmentthis has in not the happened maintenance and thatof their gradually installation the ownerss. Time of has the shown installations that in have most been cases losing this responsiveness and knowledge of their own facilities, while perceiving that the interests Appl. Sci. 2021, 11, 1386 3 of 26 of the technologist had more relevance than the park and its owners. For this reason, in recent times, the O&M of wind farms has resulted in different concepts, which some- times were not the most appropriate depending on the casuistry and the owners of the installations [6]. Currently, there is no clear procedure showing researchers new research fields to which to devote their attention [7–10]. Accidents involving wind turbines increase year by year, which is why the cost of manufacturing, logistics, installation, grid control and maintenance of offshore wind turbines remains high [11]. For decades, much effort has been made in developing wind turbine condition-monitoring systems and inventing dedicated condition-monitoring technologies. However, the high cost and the various capability limitations of available achievements have delayed their extensive use [12]. A considerable percentage of the maintenance cost is caused by unexpected drive train failures [13]. Today, the availability of wind turbines usually approaches 98% [14]. Taking into account the relevance that wind energy has acquired throughout the 21st century in terms of scientific interest, as well as the influence that O&M costs have on the determination of the LCOE of the technology, the subject of this study is based on the maintenance of wind farms. The first objective of this paper is to offer an overview of the research from 2007 to 2020 on the maintenance of wind farms and equipment in wind farms, both onshore and offshore.