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 batterysupercapacitor
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 fieldprogrammable
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.
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