Using wireless transmitters to create an ocean of radio frequency (RF) energy and to power remote devices have been a dream since Nikola Tesla invented wireless communications. Recently, wireless power technology has been embedded into a plethora of consumer electronics to improve device reliability and extend battery life. Soon enough, miniature wireless sensors adapting microwave energy harvesting modules for power will be located at spots where it would be otherwise inconvenient, such as, to change their batteries. These spots include locations inside human body, within the steel or concrete of buildings, and in the dangerous innards of chemical plants. However today, even the most robust nodes can be counted on to last only a few years due to the existence of a battery. Ideally, engineers need a sensor that can last forever without external power sources or battery changes. According to research presented in this dissertation that dream is now within reach.
While solar energy harvesting has been widely used for years to power remote devices, several other types of energy-harvesting approaches have also emerged for micropower applications including vibration, thermal, mechanical, and RF. Of these technologies, RF energy is the only one that can provide either intentional or ambient power source for batteryless applications. Thanks to mobile and Wi-Fi networks, ambient RF energy is ubiquitous. Note that more than a million smartphones phones are activated every day, representing a large source of transmitters for RF energy harvesting. Moreover, when more power or more predictable energy is needed than what is available from ambient sources, RF energy can be broadcasted in unlicensed frequency bands.
Of particular interest is the efficient harvesting of low power RF signals, which would possibly mean range improvements for dedicated microwave power transmission and considerably more DC power from ambient RF energy harvesting. Hence, in this dissertation, we propose a new class of microwave energy harvesting system which exhibits substantially improved conversion efficiency than the ones available off-the-shelf or in literature. The enhancement is due to novel rectifier circuits that can approach theoretical efficiency bounds by handling the best components on the market.
Although the developed energy harvester offers game-changing efficiency performance, maximum power supplied by a single module is usually not enough to energize most consumer electronics. Accordingly, a mathematical tool is presented to predict the optimal way of interconnecting multiple microwave energy harvesters. Subsequently, an energy harvesting array with nine elements is designed to power up a commercial thermometer and its LCD display using nothing more than ambient Wi-Fi signals in an office environment. In the end, the operational bandwidth of the designed ambient energy harvester is widened to include all cell phone bands and Wi-Fi to harvest more RF power from the environment.