PhD Thesis - Cranfield University

Chapter 2
system. Theoretically, the switching of the ultracapacitors to closely match the terminal
voltage is similar to synthesising a current pump by controlling a voltage source. Based on
this concept, Takara et al [86, 87] simulated series/parallel bank switching on the premise
that the usable energy that the ultracapacitors are able to provide is increased. This was done
without the use of a designated DC-DC converter. Following this, Rosario, Economou and
Luk [88] reported that since the peak power demands of propulsion load in an electric
vehicle are relatively of short intervals, sequential switching of ultracapacitors could be
exploited. By coordinating the switching topology, the effective energy that can be extracted
out of the ultracapacitor network showed an increase whilst the terminal voltage constraints
were satisfied by sequentially changing the connections within the ultracapacitor network.
Miller and Everett [85] studied the effects of ultracapacitor time constant in relation to the
specific demands of non-propulsion loads. They introduced the concept of distributing
banks of ultracapacitors throughout the vehicle power network. By matching the capacitance
to the load power and demand frequency, they demonstrated an increased utilisation of the
energy content in smaller, matched capacity ultracapacitor banks. Further to this, distributing
the ultracapacitors also eliminates the single point of failure that can occur in a single
ultracapacitor bank configuration. The results of their report are of particular interest, since
it supports the concept of adapting the capacitance of an ultracapacitor network to the load
profile, this concept that was also investigated by Rosario, Economou, Luk and El-Hasan
[89] in a publication on pulse power management.
49