Abstract
The Internet of Things (IoT) infrastructure requires billions of devices that must ideally be self-powered. Ambient RF and thermal energy have great potential since they are both available continuously throughout the day. An RF harvester is a rectenna that is a combination of a receiving antenna and a rectifier. Thermal energy harvesters (TEH) are typically static type, with a fixed hot source at one end and a cold source at the other. Here, we present a transient type TEH that generates energy from diurnal cycle temperature fluctuations. Smart integration is achieved by designing the antenna to also act as the heatsink for the TEH. The antenna must be optimized while considering the electromagnetic radiation as well as the heat transfer performances. Thus, two simulators, Ansys HFSS and Ansys Fluent, were employed. The antenna operates at GSM900, GSM1800, and 3G bands simultaneously, with measured gains of 3.8, 4, and 5.3 dB, respectively, which have increased by ~3-4 dB (radiation efficiency doubled from ~40% to ~80%) compared to the flat antenna (with no heatsink fins). The TEH is in the form of a square box where two identical rectennas cover the four sides. Through RF field testing, ~250 mV is consistently collected at any instance ( 6.25 {\mu }\text{W} for a 10 \text{k}\Omega load). Without the heatsink antenna, the average power collected from the TEH is 13.6 {\mu }\text{W} , which increases by 2.3 times when the heatsink antenna is integrated, highlighting the utility of this co-design and monolithic integration, which enhances both RF and thermal harvested powers.