Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermal Energy Harvesting <s> Abstract The thermoelectric effects in metals and semiconductors are reviewed and particular attention is devoted to the Seebeck effect in silicon and its possible exploitation for thermal sensing. For this purpose, an analysis of the performance of integrated silicon thermopiles is presented. Several thermal sensors that measure magnetic, mechanical, radiation and chemical signals, as well as electrical converters, are reviewed. Many of the devices described are fabricated by integrated circuit (IC) technology, which makes them very attractive, since they can be batch-fabricated and on-chip signal conditioning-electronics added. <s> BIB001 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Thermal Energy Harvesting <s> Abstract Studies concerning the use of thermoelectric effect for the conversion of thermal energy into electricity have become increasingly numerous since applications of the thermoelectric phenomenon can be limitless. Thermoelectric Generators (TEG) have shown promising results in combustion vehicles, with an overall efficiency increment from 33% to 57%, in addition to an increase of 6% in the available power in an automobile. In the aerospace industry, thermoelectric devices have been employed due to the need of making the most of the available energy. Thermoelectric modules are also used in the harvest of waste heat in wood stoves, furnace walls, and industrial chimneys. In Bioengineering, there are applications for small-scale generation including the use of heat from the human body to feed microelectronic devices and autonomous sensors. Therefore, the article shows a review of the use of thermoelectricity for the energy harvesting, beginning with the main concepts and definitions on the subject, followed by the state of the art containing the main methods and advances in science and technology for the development and application of solid-state micro-generators to capture residual energies. <s> BIB002
In general, heat energy coming from temperature variation in the environment can be scavenged into electricity via some key thermal conversion techniques based on the Seebeck effect  BIB001  BIB002 . Thermoelectric energy harvesting and pyroelectric energy harvesting, relying on the temperature changing over distance and time, respectively, are the two most commonly used thermal energy harvesting techniques.