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74 up one of the narrow sides of the cell stack. Given the plethora of passive systems on the fuel cell, Teledyne has aimed to keep these electronics as simple as possible. The embedded control systems, as with other fuel cell systems, include processors for a number of parameters, such as a digital voltage monitor for measuring each of the cells’ voltages. The data collected from these goes to a controller unit, which also measures information from other onboard sensors including stack pressure, current output and coolant circulation. Based on the information it receives, the controller unit can send a signal to open the electrical contactors on the fuel cell to cut off the current and shut down the proton exchange reaction as well as the gas supplies to the stack. It will also actuate the reactant solenoid valves to maintain Dossier | Teledyne Energy Systems EDR fuel cell The individual membrane electrode assemblies (MEAs) in the EDR cell are connected to (but physically separated from) one another in series by carbon bipolar plates (BPPs). Teledyne’s BPPs are constructed from a carbon-polyvinyl fluoride composite as rectangular shapes with six identical square-shaped ports – three on either end – each roughly 0.67 cm in diameter. The BPPs and MEAs are installed atop one another, so the ports line up precisely to form vertically oriented channels. As oxygen and hydrogen enter the balance of plant on one side, each gas flows up its own separate channel in the corners of that side, before dispersing lengthways across the MEA via the channels in the BPPs. Cooling water also flows up the BPPs’ central channels on that side of the stack, from an input port installed at the top of the water management unit. The ports and channels on the opposite side serve to deliver the heated water out of the stack, as well as the waste water and any excess oxygen (largely dissolved into the water) coming from the MEAs. BPPs using surrounding air as a reactant source can now reach a mean time between failures of 50,000 hours. The figure for the EDR fuel cell is typically 7000 hours, but there is no direct and fair comparison between the two, as an experiment testing the lifespan of fuel cells incorporating metallic, stamped BPPs (with anti-corrosion coatings) using pure oxygen rather than atmospheric air has yet to be run. While it is fairly common (especially in automotive fuel cells) for MEAs to be enclosed by circumferential gaskets to prevent the hydrogen, oxygen and water from leaking out of the stack, Teledyne’s seals are built directly onto the BPPs. A form-in-place epoxy is extruded around the six ports and outer border of each plate. Each BPP features two additional holes that are smaller than the channels for input gases and output water. They are located at opposing corners of the plates, which act as inserts for alignment pins. Each EDR cell therefore uses two of these pins, running the vertical length of the stack to help keep them aligned and absorb structural loads, with each BPP typically being hand-guided onto the pins and slid downwards. In addition to maintaining BPP and MEA alignment, consistent pressure must be kept on the cell stack for preventing expansions or leakages. Teledyne achieves this by using six tie rods that run the vertical length around the stacked plates, torqued to the required pressure and fixed into mounting plates above and below. Belleville washers – conical shells that can be loaded statically or dynamically along their axis – are installed atop each of them to hold the torque while still allowing some movement due to thermal expansion. At the top and bottom of the stack are end-plates for maintaining even cell compression, which are made from a thermoplastic called Ultem. The water management section is CNC-machined from a block of that material, before being fastened to the cell stack by six screws. Previous versions were made from stainless steel, but Ultem is lighter and transparent, the latter quality enabling visual checks of how thoroughly water is being evacuated. Assembling the overall system is a largely manual process with a few points of automation, and takes two people roughly 6 hours to complete. Anatomy As well as the integrated balance of plant, other key parts such as a coolant filter and an inert concentrator are installed around the fuel cell December/January 2021 | Unmanned Systems Technology