Tech Talk: the Ejector
In the Forze IX custom balance of plant – the umbrella term for all systems conditioning the various mass flows to the fuel cell – one of the major advancements in comparison to our previous vehicle is the inclusion of a fixed-geometry ejector. This is a passive device used to sustain hydrogen recirculation to the fuel cell, specifically on the anode side of the fuel cell.
What is the Purpose of an Ejector?
The ejector, in essence, can be viewed as a pump: a device that increases the pressure of a fluid to overcome the frictional losses it associated with mass transport. Within our balance of plant, an ejector is used to maintain the flow of hydrogen on the anode side of the fuel cell. Typically, this function is fulfilled by pumps, such as the recirculation pump used in the Forze VIII. The major disadvantage of such pumps is that such require large amounts of power, usually in the order of several kilowatts, to achieve the required pressure lift. This power, produced by the fuel cell system, is directly consumed by the systems supporting its operation, and are therefore referred to as parasitic losses.
The ejector aims to reduce the parasitic losses of the balance of plant by tapping into another energy source: the potential energy stored as pressure within the hydrogen storage tanks. The hydrogen in the vehicle is stored under 700 bar, which must be brought back to near atmospheric pressure before it can be used in the fuel cell. Ordinarily, this throttling process is not used to produce useful work; however, ejector systems are designed to use this potential energy to increase the pressure of the hydrogen in the anode recirculation loop. This allows the anode side of the fuel cell to be supplied with an excess amount of hydrogen, ensuring proper operation.
How Does an Ejector Work?
The ejector increases the pressure of the gases in the anode recirculation loop of the fuel cell by throttling the hydrogen to a pressure several bar above the final desired pressure. Using a convergent-nozzle geometry, the hydrogen coming from the storage system is accelerated, which consequently decreases the fluid’s static pressure. If the ejector geometry is designed correctly, the pressure of the fluid leaving the nozzle is lower than the pressure of the fluid already present in the recirculation loop: as a result, the hydrogen in the recirculation loop is entrained because of the negative pressure gradient. The gases in the anode loop are thus accelerated and mix with the hydrogen coming from the storage system at a high velocity. At this point, a significant amount of the fluid’s energy is in the form of kinetic energy. To transfer this kinetic energy back into potential energy, in the form of pressure, the flow is fed through a diffuser. If designed correctly, the ejector can thus increase the pressure of the fluid relative to the entrained flow.
How do you Design an Ejector Geometry?
The design of ejector geometry is no easy task. Ensuring that the maximum pressure lift the ejector is capable of creating is large enough for the desired recirculation mass flows requires extensive simulation of the ejector geometry. The main technique used in this process was computational fluid dynamics (CFD), which aims to accurately model the hydrogen flows.
To validate these simulations and confirm the performance of our design, we have developed a custom ejector test bench. With this knowledge we hope to confirm the feasibility of our design and gather the first data we will need to develop the control systems that will regulate the hydrogen flow coming from the hydrogen storage system.
Written by Alvaro Detailleur, Chief Simulation & Control, team XIV.
Published: the 4th of May, 2021, 13:30