Powering up composites
Demand for small, electrically driven unmanned air vehicles (SEUAVs) in the military and civil sectors has led researchers at The Welding Institute (TWI) in Cambridge, UK, to explore the use of embedded fuel cells to help power lightweight composite structures, typically constructed from carbon or glass fibre-reinforced epoxy.
Composites Engineer at TWI Paul Burling says, ‘The problem with SEUAVs is that they tend to be battery driven. If they take damage to the lithium polymer battery, for example, by being shot at, then you probably lose your vehicle as the battery provides power for the control surfaces and propulsion. In our case, the battery still provides the forward thrust but the fuel cells will operate the control surface actuators independently. So you could still steer the vehicle to glide down to a safe area’.
Due to the smaller surface area of proton-exchange membrane fuel cells (PEMFCs), they possess a lower power density and about a dozen are required to provide sufficient power. These are embedded into the composite prepreg during layup, prior to curing, to make ‘multifunctional materials’. The composite is either a sandwich or laminate structure, typically comprising two plys and a honeycomb core.
Burling says, ‘Once the [PEMFCs] are encased into the composite, they are protected and the structure is robust’.
The main challenge, he explains, is storage and transportation of the fuels – oxygen and hydrogen.
The cells of the honeycomb can be pre-drilled and expanded to create discrete flow channels for transportation of the fuel to the PEMFCs, or, in the case of the laminate, slots can be machined using a five-axis router. The fuel cells are then placed on the laminate, connected in series or parallel depending upon the power requirements, and a mirror image of another laminate is fitted on top. The structure is bonded together using a platen press, or, for more complex structures, a vacuum bag.
The machining must ensure good fit-up and reduce gas leakage between the parts. The team uses refillable, filament-wound composite gas bottles for fuel storage.
A 3D perspective
The materials created so far have been tested and found to transmit loads effectively while allowing the fuel cells to operate. A single fuel cell has been shown to produce power equivalent to a 1.5V battery.
The next stage is to experiment with 3D composite lay-ups. The placement of fuel cells and flow paths can be tailored to suit particular applications and provide power to specific areas. This can help eliminate wiring, providing weight savings to the overall structure, and can be useful for powering self-contained or isolated plug and play devices.
Burling says, ‘It is possible to have dwells to isolate a particular fuel cell and run another fuel cell, if a structure is damaged by impact, using non-return valves. [This stops] the hydrogen or oxygen leaking from where the damage occurred. You have to build in a certain amount of redundancy within the structure [with back up fuel cells]’.
The team is currently working with industry partners within the military sector, and is interested in proposals to scale up the technology for other niche applications, such as a steering wheel that can operate an MP3 player or a vehicle door that can power its own electric window.
Further information: The Welding Institute