## 3 Results
### Keywords Analysis  
Figure 8 presents the keywords found when applying HIL in energy systems as a query. The cloud representation (Figure 8a) shows that the keywords more used are \"energy management\" and \"microgrid\", followed by \"real-time simulation\", \"model predictive control\", \"energy management strategy\", and \"hardware in the loop simulation\". The number of occurrences (Figure 8b) is 83 for \"energy management\" and 68 for \"microgrid\". The other keywords with occurrences were \"smart grid\", \"energy storage\", \"electric vehicle\", and \"wind energy\". The other listed keywords had an occurrence between 51 (for \"real-time simulation\") and 29 (for \"wind energy\"). An interesting figure is Figure 9a, where the keywords more used are presented in frequency over time. Among the most recurrent keywords, the first one is \"hybrid electric vehicles\", which appeared in 2006 and disappeared in 2016, with a maximum in 2010 and a term frequency of 6. \"Energy conversion\" appeared in 2010 but disappeared in 2012, being the keyword with the shortest duration but with the earliest appearance. Among the terms with more frequency, \"hardware in the loop simulation\" is the earliest one, lasting from 2012 to 2020, with a maximum in 2016 and a term frequency of 168. \"Microgrid\" and \"energy management strategy\" had their maximum frequency in 2020 and a term frequency of 131 and 126, respectively, starting only in 2018. Finally, the newest keywords are \"cybersecurity\" and \"voltage control. Figure 7: Scientific production of HIL in energy systems by country using R-studio software and bibliometric library. Countries with most scientific production and relation between countries. Data obtained from Scopus in April 2023. Query: (TITLE-ABS-KEY(((”hardware in the loop” AND (energy)) ANDNOT “synthetic aperture”))). Figure 8: Most frequent keywords of HIL in energy systems using R-studio software and bibliometric library. (**a**) Cloud representation, and (**b**) number of occurrences. Data obtained from Scopus in April 2023. Query: (TITLE-ABS-KEY(((“hardware in the loop” AND (energy)) ANDNOT “synthetic aperture”)). Figure 9: **(a) Trending topics and (b) thematic map of HIL in energy systems using R-studio software and bibliometrix library. Data obtained from Scopus in April 2023. Query: (TITLE-ABS-KEY(((”hardware in the loop” AND (energy)) ANDNOT “synthetic aperture”)). **  
The thematic map of the results of the query HIL in energy systems (Figure 9b) shows that the basic keywords are \"energy management strategy\", \"model predictive control\", and \"hybrid electric vehicle\", while the motor keywords are \"microgrid\", \"smart grid\", and \"energy storage\". All these keywords appeared after 2012 and are still used in recent publications (Figure 9a). It is interesting to see that there is only one niche keyword, \"maximum power point tracking\". Finally, basic newer keywords are \"hardware in the loop simulation\" and \"wind energy\", and emerging keywords are \"distributed energy resources\", \"particle swarm optimization\", and \"power hardware in the loop\". A factorial analysis of the keywords helps in understanding the main topics found in the research (Figure 10). There is a big cluster (red cluster) that includes most keywords related to the power system. For example, the terms \"wind energy\", \"power quality\", \"smart grid\", and \"distributed generation\" are included here, but also terms related to control, such as \"control\" and \"model predictive control\", and terms related to energy management and energy storage, such as \"energy management strategy\", \"demand response\", \"electric vehicle\", and \"energy storage system\", can be found. A second cluster (blue cluster) includes the keywords related to renewable energies, such as \"renewable energy sources\", \"distributed energy resources\", \"microgrids\", and \"voltage regulation\". The third cluster (purple cluster) is related to energy storage, with \"hybrid energy storage system\", \"battery\", \"supercapacitor\", and \"fuel cell\".