# Renewable Energy and Sustainability from the Supply Side: A Critical Review and Analysis
## 4 The Most Important Topics and Analysis  
Level of Countries' Development Focused on in the Papers and the Type of Renewable Energies Analyzed  
This section investigates whether there is a relationship between the level of countries' development and the type of renewable energies that are addressed in the research articles (RQ\({}_{1}\)). First, however, the most explored sources of renewable energies focused on in the sample articles are analyzed. Several types of renewable energies, with many of the energy sources being approached either simultaneously or alone are investigated in the research articles, such as biomass, biofuel, multiple (several renewable energies are focused on simultaneously in the articles), solar, biodiesel, ethanol, and hydroelectric. Biopolymer and hydrogen are also identified in the sample, but only barely. Table 2 shows the subsample of articles focusing on only one source of renewable energy. As shown in Table 2, biofuel is the type of renewable energy most focused on in the research articles, followed by biomass. After biomass, solar, hydroelectric, ethanol, and biodiesel are the sources of energy most focused on in the sample articles. The level of a country's development focused on in the papers and the type of renewable energies are also reported in Figure 7.  
As shown in Figure 7, the main sources of RE focused on in articles developed in high-income countries (Austria, Canada, Finland, Italy, USA, etc.) are biomass [111; 112; 113; 114; 115], wind-solar [116; 117; 118], and biodiesel [104; 110; 119]. Although the same renewable energies are focused on in countries with the same level of development, the research objectives are quite different. For example, in [116], wind-solar is focused on with the objective of suggesting an energy model with the main concern of improving economic and environmental performance; in [117], an approach is suggested to evaluate the profitability of the solar energy system in a context of self-consumption; and, in [118], the competitive market of North Carolina between large solar power producers and utility companies to finance, install, and operate solar generating systems is analyzed. Considering biomass, [PERSON] et al. [113] focused on policy and legislation conditions as mandatory to establish stable conditions and provided planned security for investment decisions on biomass exploitation, and [PERSON] et al. [114] identified pitfalls impeding a broad implementation of wood energy supply in forestry. Considering the works about biodiesel, while [PERSON] et al. [104] focused on policy mechanisms to facilitate more socially and environmentally sustainable energy production, [PERSON] and [PERSON] [119] explored sustainability and improvement options for biodiesel supply chains. In papers focusing on upper-middle-income countries (China, Brazil, Turkey, Malaysia, Colombia, etc. ), the type of renewable energy most studied is biodiesel [109; 120; 121], and the least studied is biogas [122; 123], but with different aims. In studies about biodiesel, supply chain analysis seems to be a good choice for the renewable energy. \begin{table}
\begin{tabular}{l l l l l l} \hline \hline  & **Biomass** & **Biofuel** & **Solar** & **Biodiesel** & **Ethanol** & **Hydroelectric** \\ \hline  
& & \\ \hline Total & 25 & 29 & 15 & 9 & 10 & 11 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main renewable energies focused on in the sample articles. Figure 7: Type of renewable energies by level of countries’ development analyzed in the research sample. be a critical issue and the idea that firms focusing on individual sustainable development elements independently are unlikely to find satisfactory solutions to their sustainability problems associated to biodiesel's SC seems to be common [109; 120]. In addition, [PERSON] et al. [121] proposed a model to assess the sustainability of the conditions in the baseline scenario of biodiesel production.