# Renewable Energy and Sustainability from the Supply Side: A Critical Review and Analysis
## 3 Method and Data
### Network Analysis--Keywords Co-Occurrence  
The \"hot topics\" in a specific field can be defined by high-frequency keywords in an appropriate database. The keywords summarize the content of a research article, and serve to focus and refine the main ideas of the research [68]. For bibliometric application, Figure 4 shows the top keywords in renewable energies and sustainability papers. In this figure, each term is represented by a circle and its size indicates the number of publications that include that term in the title or abstract. Terms that often co-occur tend to be located close together in the visualization. The terms are grouped into five clusters by using VOSviewer, of which four are of significant size. To obtain an unbiased result, keywords that somehow were identical or already used in the search in the Scopus database were excluded (the keywords excluded were: \"sustainability\", \"sustainable development\", \"renewable resource\", \"b biofuel\", \"biomass\", \"biofuels\", \"renewable energy resources\", \"bioenergy\", \"alternative energy\", \"environmental sustainability\", \"renewable energies\", \"renewable energy\", \"fossil fuels\", \"biodiesel\", \"bio-energy\", \"biomass power\", \"fossil fuel\", \"industry\", \"solar energy\", \"wind power\", \"ethanol\", \"renewable energy source\", \"sustainable energy\", \"solar power\", \"biogas\", \"sustainability assessment\", \"photovoltaic system\", \"hydroelectric  
Figure 4: Network of co-occurrence keywords. Figure 3: Top active subjects in renewable energies and sustainability publication output. power\", \"fuels\", \"environmental performance\", \"biofuel production\", \"algae\", \"photovoltaic cells\", \"economic sustainability\", \"renewable resources\", \"energy\", and \"geothermal energy\"). After all the requirements were satisfied, VOSviewer generated the network with the next five clusters, which were carefully analyzed and defined with different classifications: \"Pollution effects and control\" (yellow), \"Policies and development\" (red), \"Innovation and production\" (purple), \"Environment\" (green), and \"Economic efficiency\" (blue). The main keywords considered as making up part of the cluster \"Pollution effects and control\" are: emission control, carbon emission, greenhouse gas, global warming, gas emissions; energy conservation, economic effects, and social effects. Grouped into the cluster \"Policies and development\" are the main keywords: energy policy, energy market, Eurasia, Europe, energy use, and developing world. Looking at the cluster named \"Innovation and Production\", some of the keywords that can be found are: investment, LCA, competition, innovation, and cost effectiveness. In the group named \"Environment\", the following main keywords can be found: environmental sustainability, decision making, Brazil, electricity generation, biotechnology. Finally, in the cluster \"Economic efficiency\", the main keywords are: environmental impact, energy efficiency, bio-refineries, economic analysis, supply chains, profitability, and efficiency. The network analysis using the keywords occurrence, visualized in Figure 4, presents the focus and trends of the selected studies on renewable energies and sustainability. There are 2864 keywords in total. Of these keywords, 632 repeated twice, 333 repeated at least three times, 213 keywords at least four times, and 147 keywords appeared five times. The network suggests that the biggest node is \"environmental impact\", which occurs 38 times and is connected to every single cluster, with 79 different links. It is followed by \"LCA\" (life cycle assessment) with 28 occurrences; \"climate change\", \"energy policy\", \"greenhouse gas\", and \"investment\" with 25 occurrences; and \"supply chains\" and \"carbon emission\" with 24 occurrences. According to the network analysis, it can be concluded that most works on renewable energies and sustainability focus mainly on environmental problems and attempt to find solutions regarding policies in the supply chain and investments of the economy.