energy harvesting History of engineering & technology 10 Technology: general issues 10 piezoelectric 6 IoT 5 graphene 5 optimization 5 triboelectric nanogenerator 5 Research & information: general 4 electromagnetic 4 piezoelectricity 4 structural health monitoring 4 triboelectric nanogenerators 4 5G 3 Energy industries & utilities 3 NOMA 3 carbon nanotubes 3 efficiency 3 energy efficiency 3 energy harvester 3 low frequency 3 machine learning 3 piezoelectric energy harvester 3 piezoelectric material 3 self-powered 3 vibration energy harvesting 3 wind energy 3 wireless power transfer 3 CO2 capture 2 MIMO 2