# Virtual Power Plant Optimization in Smart Grids: A Narrative Review
## 2 Materials and Methods  
In this study, we will follow the guidelines for narrative reviews as described in the research literature in several highly cited articles [7]. We have adapted them for the article context and the addressed domain related to VPPs and their efficient integration into the smart grid. Even if narrative reviews do not feature a strict reporting guideline and methodology (such as the PRISMA methodology used for systematic reviews) [8], in this section, we will highlight the main methods used for the literature search process. The following points will be discussed:  
* Searching strategy: database, search key words/key phrases;
* The main inclusion/exclusion criteria: types of articles, language, time periods, visibility, and availability. The main objective of the search strategy was to carry out a review and analysis of the status of the literature for VPPs' concept and technology, as well as on their usage and operation, in the smart grid domain. We are interested in finding the most important VPP and smart grid integration scenarios, use cases, and solutions, and classifying them. As the main database, we have used the well-known Scopus articles repository. We carried out separate and aggregated keyword manual searches and finally extracted the papers of interest. The search keywords and key phrases to narrow the search results involve the following concepts: energy forecasting, energy modeling and optimization, energy management, renewable integration, local energy autonomy, energy community, and energy services provisioning. These key concepts were concatenated with VPP in the search process. Additionally, we have considered in the review eight online/magazine articles from the industry to capture the companies' opinions and advances about the researched topics. We have eliminated the duplicates from the results and defined an initial set of articles. In the search strategy for filtering the Scopus database search results, as a first inclusion criterion, we have targeted three of the most important publishers, namely MDPI, IEEE, and Elsevier. Additionally, we have considered high-quality articles from other databases such as Wiley, Springer, etc. Conference proceedings and journal articles, and review papers, were the kinds of studies selected. The English language for article redaction was also used as an inclusion criterion. As for the publication time, we have included works newer than 2015, with an emphasis on the past four years (2019-2022). The exclusion criteria used are mainly on the availability of the articles. If it was not possible to have access to the full text, they were excluded. Additionally, we have excluded papers that feature a low number of citations or the ones that do not cover well the search keywords or phrases. Table 1 presents an overview of the search strategy inclusion and exclusion criteria. We have filtered the search results and created a list of more than 95 articles that were analyzed in the current review. Figure 1 shows the distribution of the selected papers by using the publisher as discriminant, while Figure 2 shows the research works' year coverage. \begin{table}
\begin{tabular}{c c} \hline \hline
**Inclusion Criteria** & **Exclusion Criteria** \\ \hline MDPI, IEEE, Elsevier, or other highly & Not available as full text \\ rated databases & Low number of citations/views \\ Conference proceedings & Not connected to the research topic (VPP \\ Journal articles/review papers & applications for smart grid) \\ Usage of English language & Duplicate \\ Publication date 2015+ & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Criteria for including and excluding articles. We have grouped them into two categories: (1) VPP concept and technologies presenting the main models and techniques for VPP creation and operation, and (2) VPP application in smart energy grid management.