# New Tendencies in Wind Energy Operation and Maintenance
## 4 Discussion
### Maintenance Optimization (M_OPT)  
As mentioned at the beginning of this article, more than 27,000 papers in the Scopus database (around 43% of all papers in Table 1) include wind energy as the main topic (data collected in November 2020). However, if the different fields of O&M costs are analyzed, more than 38,000 papers exist that deal with wind energy and some of the keywords of each group of variables (Table 2), as reflected in Figure 3. It should be noted at this point that the same article can be counted in different groups, as it contains different keywords. Taking into account the different groups (Figure 3), M_COST is the group achieving the highest number of results, with more than 12,000 papers (approximately 30%). C_M and M_OPT are positioned in second place, with more than 8000 results (approximately 20%) each. On the contrary, M_STRA and O&M are the groups with the lowest number of results in the database, not reaching 600 results each (approximately 1.5%). Looking at each of the keyword groups (Table 2), in the O&M group (Figure 4a), the most widely used keyword is \"operation and maintenance\", which was included in over 90 papers in 2019. In the FAIL group (Figure 4b), the most widely used keyword is \"failure\", with over 500 results by 2019. Figure 3: Wind energy O&M cost fields: number of papers in Scopus. Figure 4: Wind energy O&M variables and number of papers in Scopus: (**a**) M_COST; (**b**) M_OPT; (**c**) C_M; **(d)** RELI; (**e**) FAIL; (**f**) M_STRA; (**g**) O&M. In the RELI group (Figure 4c), the most found keyword is \"reliability\", with almost 600 results in 2019. In the group C_M (Figure 4d), the main keyword is \"algorithms\", which goes from approximately 100 results in the year 2007 to almost 1200 results in the year 2019. The group M_STRA (Figure 4e) does not obtain remarkable results over this period of time. The most outstanding keyword in this group is \"maintenance strategies\", exceeding 30 results in 2019. With regard to the group M_STRA (Figure 4f), the progression of the keyword \"costs\" stands out, which grows throughout this period, exceeding 1100 results in 2019, while the rest of the keywords in this group have not exceeded 400 results per year. Finally, in regard to the group M_OPT (Figure 4g), the keyword \"optimization\" drastically exceeds the keyword \"maintenance optimization\". In 2019, \"optimization\" exceeded 1400 results. Thus, it can be seen that the most significant keywords in the O&M of wind energy research are: \"costs\", \"optimization\", \"algorithms\" and \"reliability\". Overall, the most widely used keywords over the period of time from 2007 to 2019 are: \"costs\" (M_COST group, Figure 4a) and \"optimization\" (M_OPT group, Figure 4b), with more than 8000 results each (appearing in 21% of the papers). More of the most found words are \"algorithms\" (C_M group, Figure 4c), with almost 7000 results (appears in 17% of the papers), and \"reliability\" (RELI group, Figure 4d), which appears in 10% of the papers. In contrast, the least found words in the database are: \"failure of gearboxes\" (FAIL group, Figure 4e), which appears in 0.01% of the papers; \"maintenance action\" and \"maintenance free\" (M_STRA group, Figure 4f), which appears in 0.05% of the papers. Among the research articles indexed in Scopus related to the operation and maintenance costs of wind energy from 2007 to 2019, only 12 keywords considered in this article (Table 2) appear in more than 1% of the results obtained. Only four keywords (\"costs\", \"optimization\", \"algorithms\" and \"reliability\") mentioned above appear in more than 10% of the articles in Scopus. The rest of the keywords searched for (20 keywords) do not appear in even 1% of the results obtained.