## 3 Results  
### Bibliometric Analysis  
Figure 2 shows the annual scientific production when HIL is used related to energy systems. The first documents already appeared in 1992, but until 2004 the annual number of publications was exceptionally low (between 1 and 7 documents per year). There was a steady growth from 2004 until 2014 when there was a drop until 2016, when the number of publications started to grow at a rate of 50 to 100 documents per year. The first bibliometric analysis is the three-field plot, where the nine authors and the ten institutions with more publications are related to the nine keywords appearing in more documents (Figure 3). The authors, institutions, and keywords will be discussed in detail later on, but here it is interesting to highlight that while, as expected, the relation between author and institution with quite linear, the relation with the keywords is much more interlinked, showing that most authors/institutions work in several of the identified keywords. The sources most used to publish documents on HIL in energy systems (Figure 4) were Energies (with 64 documents), nearly doubling the rest of the sources, which were SAE Technical Papers (with 39 documents), IEEE Transactions on Industrial Electronics (with 39 documents), and Energy and IEEE Access (with 33 documents each). Of these ten sources, six of them are from IEEE (Institute of Electrical and Electronics Engineers), and two are from Elsevier. Figure 3: Three-field plot using R-studio software and bibliometrix library, relating the 9 authors and the 10 institutions with more publications with the 9 keywords appearing in more documents. Data obtained from Scopus in April 2023. Query: (TITLE-ABS-KEY(((”hardware in the loop” AND (energy)) ANDNOT “synthetic aperture”)). Figure 2: Annual scientific production of HIL in energy systems. Data obtained from Scopus in April 2023. Query: (TITLE-ABS-KEY(((”hardware in the loop” AND (energy)) ANDNOT “synthetic aperture”))). Figure 5a lists the authors with more documents retrieved with the query HIL in energy systems. The most prolific author had 40 documents, [PERSON] (Jilin University), followed by [PERSON] (China Electric Power Research Institute--CEPRI, State Key Laboratory of Control and Operation of Renewable Energy and Storage Systems) with 38 documents, [PERSON] (Hebei University of Technology and China Automotive Technology and Research Center) with 34 documents, and [PERSON] (Research and Development Center) with 30 documents. It should be highlighted that nine out of the ten authors for more documents are from China; the other author, [PERSON], is affiliated with the Center for Research on Microgrids (CROM) at Aalborg University in Denmark. Figure 5b shows that most authors from China work together and in close collaboration using VosViewer software. For example, the two authors with more publications, [PERSON] (Jilin University) and [PERSON] (China Electric Power Research Institute--CEPRI, State Key Laboratory of Control and Operation of Renewable Energy and Storage Systems), are together in the yellow cluster. Similarly, [PERSON] (Hebei University of Technology and China Automotive Technology and Research Center) and [PERSON] (Research and Development Center) are in the same cluster (green cluster), showing their close collaboration. On the other hand, the authors from Europe, including [PERSON] (Aalborg University), are in a cluster together (brown cluster) and at quite a long distance from the Chinese authors, although showing some collaborations. Figure 4: Most relevant sources 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”)). Figure 5: Most relevant authors of HIL in energy systems. (**a**) Number of documents—using R-studio software and bibliometrix library—and (**b**) collaboration network—using VOSviewer. Data obtained from Scopus in April 2023.