Optimising hydrogen based energy storage systems using super capacitors
Abstract
One of the greatest challenges when it comes to the efficient energy use in unmanned aerial vehicles is that of the energy storage systems. As drones require minimal weight to have optimal mobility the capacity of the energy source is reduced to decrease the weight. This reduction decreases the flight time of a drone. Batteries overcome the weight issue but have a low power density and perform poorly at peak power demands. A larger power source is found in hydrogen fuel cells that possess a very high energy density, but this comes with an increase in weight and they too have a low power density. A solution to this problem is found in the hybrid combination of these two power sources. However, as they both have low power densities, they still perform poorly at peak power requirements. Super-capacitors are presented as a solution to this problem as they have high power densities and respond significantly better to peak power demands. Previously, super-capacitors have been combined with fuel cells, and in some cases batteries, to improve the functionality of hydrogen vehicles, with small improvements observed and suggestions made to incorporate all three power sources to supply the required load. The research presented in this dissertation requires the analysis of a hybrid system designed with drone application in mind. Due to the low energy density of super-capacitors many are required to deliver a sufficient voltage level and an increase in the quantity leads to an increase in weight – in some cases – preposterously. This research thus presents the solution of combining the super-capacitors with an appropriate DC-DC converter to assess the effect thereof on the super-capacitors’ capacity as well as the operation of the hybrid system as a whole. The proposed DC-DC converter was verified through simulation and validated alongside the entire system through laboratory implementation. A load profile obtained from an existing hydrogen fuel cell drone was used to assess the experimental operation of the system. Six tests were conducted using the different power sources and the load profile. The fuel cell system and super-capacitor bank were all tested individually for the first two tests; the next two tests consisted of combining the super-capacitor bank with the fuel cell and a switching module – these tests consisted of two rounds varying the order of connection; two additional tests were conducted with the inclusion of the DC-DC converter. From the results obtained through the experimental tests the fuel cell – super-capacitor combination utilizing a DC-DC converter was seen to provide the longest duration of 365 s, the highest energy and power density of 0.70 Wh/kg and 73.5 W/kg, respectively, and an overall efficiency of 96.25%. In obtaining these results the objectives and requirements of this research were met. The findings of this dissertation are not restricted to drone applications as they include outcomes pertaining to super-capacitors and their combination with a DC-DC converter.
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