A new micro ejector weighing just 0.2 grams replaced the electrically powered air pump inside a compact hydrogen fuel cell, thus fully eliminating the need for electricity to feed air to the cathode. The solution was presented by researchers from Science Tokyo, Japan, and specialists from the Japanese company Futaba Industrial. The device takes advantage of the gas flow from the anode part of the fuel cell to suck in ambient air and feed it to the cathode. In the test setting, a fuel cell with such a system installed developed power output of up to 0.28 W and operated steadily for 12 hours without external water pumping.
Inside the fuel cell, hydrogen is fed to the anode and the air carrying oxygen, to the cathode. The air is normally propelled by electric pumps and ventilators which add to the unit’s size and weight and expend some of the electricity generated. In compact systems, power consumption of such pump can be comparable to, or even exceed, the power output of the fuel cell itself. Giving it up is therefore especially important for portable electronics, sensors, drones and mobile robots.
Instead of the pump, the research team proposed using gas which leaves the fuel cell on the anode side. It travels to a micro ejector and flows through a narrow nozzle accelerating greatly. This creates an area of reduced pressure that sucks in ambient air and channels it to the cathode. The ejector has no moving parts and is not electrically powered.
The micro ejector was 3D-printed from photopolymer resin. It was less than 20 mm long. The researchers tested modifications with nozzle diameters of 0.02, 0.14 and 0.23 mm. The device performance was first observed in computer simulation and then tested experimentally. Estimates demonstrated that nozzle exit gas velocity can be as high as 231 m/s. Air suction was also detected using smoke, laser beam and a high-speed camera.
During the experiments, under a certain set of conditions the micro ejector would suck in air in quantities that were approximately 12 times the volume of driving gas flowing though it. It was important also to maintain a moderate pressure differential between the anode and the cathode: excessive differential can render the fuel cell inoperable or damage it. The most appropriate modification for the tested cell was the one with nozzle diameter of 0.23 mm and pressure differentials between 20 and 30 kPa.
The micro ejector was then connected to the fuel cell. Without forced air supply its maximum power output was around 0.02 W due to oxygen deficiency. With the micro ejector installed, it rose to 0.28 W. For reference: compressor-fed air generated a peak power output of 0.33 W. It means therefore that the ejector supplied sufficient air to the fuel cell with almost no power lost compared to the forced feed option.
The system was also tested for 12 hours under a constant current rate of 50 mA. Voltage fluctuations never exceeded 5 mV, and the gradual voltage reduction pattern was almost the same as with a conventional air pump. In a separate experiment, the fuel cell with a micro ejector successfully propelled a small toy car.
The micro ejector ended up approximately 75 times as light as the miniature pump used for the sake of comparison: 0.2 g against 15.1 g. The pump consumed between 0.37 and 1.35 W feeding up to 358 cm³ of air per minute. In contrast, the micro ejector operated in a variety of modes sucked in up to 1,628 cm³ per minute consuming no electricity at all. This allows for reduction of weight, size and own energy consumption of compact fuel systems.
The solution remains an experimental development. During the testing phase, hydrogen was supplied in quantities that greatly exceeded the actual demand, and only some 3.5% of incoming gas was used in the fuel cell. Such feed rate was used to generate a sufficient gas flow through the micro ejector, and it is yet far from optimal in terms of fuel consumption. More research is needed to make hydrogen use more efficient and to ensure safe removal of anode gas which can contain residual unreacted hydrogen.
