An indoor wireless Wi-Fi energy harvesting system with super capacitor backup

Abstract

Currently, researchers are highly interested in finding alternative methods of powering lowpower wireless devices. One of the promising methods is radio frequency energy harvesting (RFEH). The harvesting system, which is a combination of a rectenna, a power processing circuit and an energy storage device, harvests available radio energy in the atmosphere of the wireless devices. This thesis focusses on the system design, analysis, and measurement of a RFEH system which operates in an indoor environment. The thesis is divided into three major parts. The first part is about approximating the amount of incident power received on the surface of a harvesting antenna. Previously, received power at a harvesting antenna location is determined using the Friis equation in which the equation considers antenna gains, the output power of a transmitting antenna, and the transmittingreceiving antenna distance. The distance between the antennae is related to the free space loss, and it is used in estimating the received power. For the thesis, the approximation of incident power on the harvesting antenna is investigated further by considering the antenna dimensions, its elevation from the reference plane, which is the ground level, its distance to the transmitter, and misalignments. These three factors contribute to the characterization of the amount of power that could be supplied at the output of the harvesting antenna. The ray-based model is the approach selected for determining the incident power, and the three factors are added into a general transmitter-receiver model to approximate the incident power characteristics. A full simulation study is conducted, and it is found that the results obtained from the analytical analysis and simulations are in good agreement. The second part of the thesis is about developing a new bridgeless rectification circuit for an indoor Wi-Fi energy harvesting system. The proposed circuit is dedicated for impedance matching and voltage boosting for an indoor Wi-Fi energy harvesting system. The impedance matching is achieved by an LCL T-network in parallel with an inductive branch; such an arrangement can boost the input voltage level entering the rectifier diode by the combined effect of a T-network and bipolarity charging of the output capacitor. The return loss equation and an estimated value of matching circuit parameters are obtained analytically. There are three different modes for the voltage boosting process at two different input cycles. The power converting process is modelled, analysed, and relevant equations are derived. For simulation purposes, the dimension of the bridgeless rectifier circuit input port is designed using a co-planar waveguide arrangement where from the measurement, the impedance is 52.5 Ω. Low return loss at 2.45 GHz is presented by proper circuit analysis and design. Practical measurement results based on an AC power source with an output impedance of 50 Ω also verified that the proposed circuit could function as an impedance matching and voltage boosting converter circuit with the aid of two additional low-voltage boost modules attached to the output of the bridgeless rectifier circuit as a load. It has demonstrated that the output voltage of the final energy buffering supercapacitor of 1800 μF can be approximately charged up to around 3 V for about an hour when the input power of the AC source is set at -30 dBm (1 μW). The third major part of this thesis is about a power processing circuit for an indoor Wi-Fi energy harvesting system. The complete power processing circuit is for an indoor energy harvesting system. The circuit is used to convert power from high-frequency AC to DC and stored in a supercapacitor bank for driving low power wireless sensors. The circuit is modelled using an equivalent circuit which is analysed mathematically. In the analysis, the need for matching the antenna and the rectifier input impedance is highlighted. The proposed circuit can work with an incident power as low as -50 dBm. The final RF to the dc conversion efficiency of the proposed system is consistently within 30% at -20 dBm input power. The practical results demonstrated that the efficiency of the harvesting system is higher than previous multi-band radio frequency energy harvesters. This research has created some outstanding methods and results for an indoor Wi-Fi radio frequency energy harvesting system, especially in approximating the incident power at a harvesting antenna, proposing a bridgeless rectification circuit, and developing a power processing circuit. The contribution will be useful for further research and application of the RF energy harvesting systems in the near future.