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73 Teledyne Energy Systems EDR fuel cell | Dossier gen fuel cell,” Dr Valdez says. “Smaller billets and amounts of cutting work have gone into this version; commercial users don’t have the US Navy’s budget so we wanted to make the second-gen more cost-effective as well as lighter.” Each of the three manifold channels contains a COTS solenoid valve. Each valve is larger than the next, so each one opens to enable a greater recirculation volume, corresponding to a higher current draw and power output. The topmost of these is always open so long as the fuel cell is operating, and is the only one open so long as 0.7- 1.5 kW is being generated. If 1.5-2.5 kW is required, the middle valve opens, and for power up to 8 kW, the third solenoid opens as well. Thus the solenoid- actuated valves allow the cell system to operate at maximum efficiency through its entire power range. The pressure regulators are typically installed next to the manifold channels, as is the pump for input of cooling water. A coolant filter is mounted above the pressure regulators for the water exiting the cell stack. Although unlikely, particles of plastic or epoxy might be picked up by the coolant (particularly on a freshly assembled system), which could clog the liquid’s inlet ports or cooling channels. Above the coolant filter, near the top of the cell enclosure, is a thermal regulator bypass valve, which opens to allow more coolant through if the cell gets too hot. “That system is passive, rather than electronic, like a car thermostat,” Dr Valdez explains. “It contains wax and a spring, and excess heat causes it to melt and open to allow water through. It’s another way of ensuring maximum efficiency.” End-users can also have an electromechanical valve though, if they prefer. Also, as gas leakages are a perennial concern for fuel cell powertrains, a purge system is typically installed in the pressure housing, Teledyne having recently patented such a system for use in cells operating in enclosed environments. This is essentially a very small PEMFC system that draws in any stray hydrogen and oxygen over time and consumes them as reactants. The molecules are dispersed as relatively harmless water vapour, and the power produced goes to a resistor to be dissipated. Control electronics The control electronics are typically mounted on a circuit board running Unmanned Systems Technology | December/January 2021 On the basis of received sensor data, the main controller board determines whether to close the electric contactors or open the purge valves for safety Dome-loaded pressure regulators enable passive maintenance of the cell stack’s internal pressure at 30 psi