Hybridisation of energy storage for multiple input DC-DC converters

Abstract

One of the biggest challenges towards achieving efficient and effective energy use in electric vehicles is the energy storage systems. Batteries, though being improved with newer technologies, are still not capable of meeting the load requirement while retaining their lifespan. Hybrid energy storage systems have been proposed in literature to solve this problem. It has been proposed previously to combine high power dense energy storage alternatives with batteries which are high energy dense. Supercapacitors (SCs) and hybrid capacitors (HCs) are quite similar energy storage devices as they are both double layer capacitors. However, SCs have a higher power density and a lower energy density than HCs. In battery and SC hybrid system, there has been a reported downside due to the poor energy density of the SCs while in battery and hybrid capacitor system, the low power density of the HCs have been reported to pose a challenge. The research presented in this work sought to address these shortcomings of both battery-supercapacitor and battery-hybrid capacitor energy storage systems by proposing a hybrid energy storage system that combines both supercapacitors and hybrid capacitors with a battery through a multiple input DC-DC converter. The proposal was verified in simulation and validated by implementing a laboratory prototype. A new hybridisation topology which reduces the amount of resource requirement when compared to the conventional hybridisation topology is introduced. An electric vehicle current profile from previous research was used to test the performance of the proposed topology. A new method of pulse width modulated switching of the gates of the multiple input DC-DC converter power switches using field-programmable gate array technology was also introduced and verified experimentally, this facilitated the switching of the multiple input DC-DC converter in a less complex way when compared to the conventional topologies. From the results obtained, the hybridisation topology proposed in this research had the lowest cost per unit power at 14.81 $/kW, the lowest cost per unit power to energy ratio at 1:1.3 and also the lowest available power to energy ratio at 1:1.3 thus making it a more attractive hybridisation topology than the two conventional alternatives. The multiple input converter built had efficiency values in excess of 80%. With these results, the objectives of the research were met. The application of the proposed hybrid energy storage system is not limited to only electric vehicles, but is applicable in other renewable energy systems such as photo-voltaic systems, wind turbines and also in applications like electric ships, micro-grids and even electric aircrafts like the more-electric aircraft.