A survey of wearable energy harvesting systems

Recently, we have witnessed a remarkable proliferation of wearables in various smart applications. These applications include smart healthcare, smart military, smart infotainment, and smart industries, to name a few. However, wearables suffer from significant energy limitations. To cope with this challenging issue, energy harvesting can be a viable solution. In this paper, various ambient sources like heat, vibration, radio waves, and solar energy are compared and critically analyzed based on size, voltage, frequency, power density, and power. Critical analysis of vibrational, solar, heat, radiofrequency, and hybrid show convincing output results, but a hybrid solar wearable energy harvester (SWEH) and thermoelectric wearable energy harvester (TWEH) give a maximum output power of 501 mW. However, piezoelectric clothes fabrication is growing and coming out to be a good competitor because of flexibility and comfort. Work has been done on the hybridization of wearable clothes to generate maximum power and is good enough for wearable applications. On the other side, solar individually is enough to power wearable devices but for a specific time. In the radiofrequency still, research is required because of very low power generation or a flexible array can be integrated into a dress or certain other technique maybe adapted for flexible yet more power generation capability. Overall sizes of the reported wearable energy harvesters are in the millimeter to centimeter scale, with resonant frequencies in the range of 1 to 1400 Hz, while rectenna wearable energy harvester (RWEH) exceeds the limit and is reported in the range of 1.8 to 3.2 GHz. A maximum energy conversion for a piezoelectric wearable energy harvester can potentially reach up to 29.7 mu W/cm(3) and 14.28 mu W/cm(2). The power produced by the reported hybrid energy harvesters (HEHs) is in the range of 0.00012 to 501 mW. Due to the combined solar-thermoelectric energy conversion in HEHs, these systems are capable of producing the highest power densities.