Magnetic field energy harvesting for Smart Grid application

• Main Author: Yuan, S.
• Published: University of Liverpool 2016
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.706863

Real-time data of power infrastructures collected by wireless sensors are the foundation of many smart grid applications. However, the finite life span of the batteries which power the wireless sensor becomes a bottleneck problem as it is expensive to periodically replace these batteries. Energy harvesting can be an effective solution for autonomous, self-powered wireless sensors. In the vicinity of high voltage/current equipment, a strong magnetic field is generated, which could be a consistent energy source. The purpose of this thesis is to present a comprehensive study into a magnetic field energy harvesting system which mainly consists of coil and rectifying circuit with the ultimate goal to obtain the maximum energy as efficiently as possible for a given condition. The main contribution of this thesis is in two research areas: The first area is about the design of an energy harvesting coil. The ferromagnetic core is the most important part to determine the output power from the whole energy harvesting system. Thus, the precise knowledge of ferromagnetism is critical. As the harvesting coil may not fully enclosure the current conductor, the demagnetization factor which is closely related to the core geometry is carefully studied and minimized. Two new core shapes have been proposed and optimized to have much lower demagnetization factors (hence more power) than that of a conventional rod. 1. A bow-tie-shaped core is introduced and designed to have two large end surfaces. By making its two ends broader, more magnetic flux can be guided from the air into the ferromagnetic core. This intensifies the magnetization at the middle of the core where the wire is wound on. 2. A new helical core is introduced as a further improved version of the bow-tie core. It utilizes two big circular plates fitted at the both ends for the flux collection. In the middle, a helical-shaped core is introduced to increase the path of the magnetic flux. Therefore, the separation between the north pole and south pole is lengthened dramatically, which leads to a reduction in the demagnetization factor and an increase in the magnetic flux density. In addition to the core shape, the selection of the core material is studied and found that high permeability ferrite is the most suitable core material due to its high relative permeability and ultra-low conductivity. Thus, the eddy current losses in the ferromagnetic core can be significantly reduced. Experimental results show that the proposed helical coil with only 400 turns of wire can have a power density of 2.1 μW/cm3 when placed in a magnetic flux density of 7 μTrms. This value is 17 times greater than a previously reported design with 40,000 turns of wire (0.12 μW/cm3 ) placed in the same magnetic field. If a longer helical coil with 8,000 turns of wire is placed in a magnetic flux density of 11 μTrms, the produced power density is around 131.4 μW/cm 3 which is comparable to a solar panel working during a cloudy day. The second area is about the rectifying circuit design which utilizes the energy harvested from the coil to power a commercial wireless sensor. A voltage doubler is applied to provide full-wave rectification and simultaneously boost the output voltage. A transient analysis is carried out to calculate the input resistance of a charging capacitor as a function of time. The theoretical analysis indicates that the input resistance is highly related to the input and output voltages. Therefore, a conventional matching network which consists of linear components cannot work well. A switch mode power converter is introduced as a matching network so that the charging capacitor can be isolated from the harvesting coil. The emulated input resistance looking into the power converter is a constant and determined by the frequency and the duty cycle of the power width modulation applied on the switch. The system is designed and made. The experimental results demonstrate that the energy conversion efficiency from the harvesting coil to the storage capacitor is around 74.6% which is twice as large as a previously reported design. Finally, an energy management unit is developed and it effectively utilizes the energy stored in the capacitor to power a commercial wireless sensor. It is shown in this thesis that a highly efficient magnetic field energy harvesting system has been successfully demonstrated. A wireless sensor can be properly powered up by using a small coil (15 cm long) placed in a small magnetic flux density (7 μTrms). The proposed solution is a very efficient and attractive method for harvesting the magnetic field energy for a wide range of smart grid applications.