Potential system efficiencies for MEMS vibration energy harvesting

Reliable power sources are needed for portable micro-electromechanical systems (MEMS) devices such as wireless automobile tire pressure sensors. Vibration is an ubiquitous energy source that maybe 'harvested' as electrical energy at the site of the MEMS device. Existing vibration energy harvesting systems use either a piezoelectric or an electromagnetic transducer to convert vibrations into electrical energy. This electrical energy is then conditioned using a passive rectifier dc-dc converter circuit. Such vibration harvesting techniques have focused on optimising circuit efficiency and, hence, have ignored the system efficiency i.e. mechanical-to-electrical efficiency. Results obtained in the laboratory can be extrapolated to predict potential system efficiencies for MEMS vibration energy harvesting systems. Results to date, using a standard speaker as the electromagnetic transducer, have demonstrated system efficiencies of greater than 14%. Initial estimates suggest a MEMS system efficiency of more than 80% could be achieved with a high performance transducer. Research is continuing to demonstrate these higher system efficiencies with the experimental apparatus.