A fluid pump has been developed for mechanically pumped fluid loops for spacecraft thermal control. Lynntech’s technology utilizes a proprietary electrochemically driven pumping mechanism. Conventional rotodynamic and displacement pumps typically do not meet the stringent power and operational reliability requirements of space applications. Lynntech’s developmental pump is a highly efficient solid-state pump with essentially no rotating or moving components (apart from metal bellows).
Figure 1 schematically illustrates the electrochemically-driven actuator. The conversion of electrical energy to mechanical work is achieved by transporting hydrogen across an electrochemical cell. Hydrogen is dissociated at the anode; protons that are formed from the dissociation are driven across the membrane by an applied potential, while electrons are conducted through an external circuit; protons and electrons recombine at the cathode to form hydrogen. The transport of hydrogen into and out of the attached bellows results in a pressure variation that is used to actuate the fluid pumping diaphragms. Each electrochemical cell is made up of a proton exchange membrane, typically Nafion®, with platinum catalyzed electrodes on either side, called a membrane electrode assembly (MEA). The use of hydrogen and its low oxidation and reduction potentials results in high electric-to-mechanical work conversion efficiency.
For size and convenience, a plurality of individual MEAs is used to transport the hydrogen, rather than a single electrochemical cell. The MEAs are connected electrically in series and fluidically in parallel. The stack of MEAs is sandwiched between two current collectors, and a voltage is applied across the stack, driving hydrogen gas from one bellows to the other. The hydrogen flow rate through the actuator is directly proportional to the applied current. The hydrogen- filled bellows are used to actuate a second set of bellows, which displace the fluid in the pump head, as shown schematically in Figure 2. To further reduce the pump power requirements, a stroke-volume multiplier is utilized wherein a smaller-volume hydrogen filled bellows actuates a larger-volume fluid bellows. The stroke volume multiplier also allows the pump frequency to be reduced below audible frequency while maintaining adequate flow.
The largest factor affecting the lifetime and reliability of the pump is expected to be loss of hydrogen from the electrochemical actuator. The electrochemical actuator is hermetically sealed; however, permeation of hydrogen is expected to eventually result in loss of hydrogen. The lifetime of the pump is extended by generating hydrogen onboard the pump. The onboard hydrogen generation also allows the hydrogen pressure and pump performance to be optimized for varying operating temperatures.
The prototype pump is expected to operate with a power consumption of 2.4 W at a flow rate of 0.76 L/min and pressure rise of 27.6 kPa. The pump will operate at temperatures between 0 and 100 °C and survive temperatures between –60 to 110 °C. The prototype occupies a volume of ≈600 cm3.