energy History of engineering & technology 15 Technology: general issues 14 sustainability 11 Research & information: general 9 renewable energy 9 energy efficiency 8 biomass 7 energy consumption 7 optimization 7 circular economy 6 climate change 5 efficiency 5 gasification 5 numerical simulation 5 biogas 4 clustering 4 data envelopment analysis 4 energy management 4 forecasting 4 genetic algorithm 4 life cycle assessment 4 machine learning 4 polyphenols 4 smart grid 4 sustainable development 4 Physics 3 Spain 3 antioxidant 3 artificial intelligence 3