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72 at the anode, leaving hydrogen ions (protons) to bond with the oxygen at the cathode, resulting in water (and heat) that has to be extracted lest the fuel cell performance be reduced by flooding. To keep that water from reaching the inlet area, a water management unit is installed in the recirculation loop between the outlet of the cell stack and the BoP, which separates the water from the unused reactants. The gases continue onwards into their respective manifolds in the BoP, as impelled to by the ejector, while the water flows out of the unit and the overall cell system. Because the system operates without the need for mechanical pumps, then so long as current is drawn, hydrogen and oxygen are consumed, and more gas is released into the stack by the pressure regulators to maintain the operating internal pressure at 30 psi (206.8 kPa). A pressure sensor in the stack measures any changes. When current is no longer drawn, gases are not consumed or moved, the internal pressure ceases to drop, the pressure regulators close the supply of reactants, and the Venturi effect stops. “This technology is 200 years old, but Art Vasquez was the first to think of applying it to fuel cells, and to understand that it could work so long as you constantly communicate the cell stack’s internal pressure to the pressure regulators,” Dr Valdez explains. “That’s the core thing that separates our PEMFC’s operation from those of Toyota, Intelligent Energy and so on.” Although a mechanical pump for circulating coolant is integrated outside the fuel cell – as well as a pump for removing water produced by the reaction in the fuel cells, if the ambient pressure is greater than that in the water management unit – by and large Teledyne has opted for passive componentry wherever possible. Balance of plant As indicated, the upper two-thirds of the system comprises the cell stack, underneath which is a water management unit. Teledyne calls this an auto-generated vortex separator, for its ability to separate water and unused oxygen without needing a motor. The BoP is installed below and arranged around the cells and water management, with all parts typically integrated and housed in a cylindrical pressure vessel. The system has two pressure regulators, one each for the hydrogen and oxygen, which ensure an input of the reactants at about 150 psig (gauge pressure). The pressure regulators are Tescom units from Emerson, which use springs to passively open and close their internal valves. The springs are set in-house to open when stack internal pressure falls below 30 psi. The high-pressure manifold for reactant input in the BoP is CNC-cut from stainless steel, for corrosion resistance, while the low-pressure manifold for recirculating unused reactants is cut from a thermoplastic called Ultem. The two are fastened together by a total of 12 screws, and their joining forms three channels that can act as nozzles for the Venturi effect. “There’s a lot less stainless steel in the manifold than there was in the first- A cell monitor board contains key sensors such as digital voltage monitors for measuring each of the cells’ voltages The bypass valve is a passive system, like a car thermostat. It contains wax and a spring, and excess heat causes it to open December/January 2021 | Unmanned Systems Technology