Inside the engine of a car, gasoline reacts directly with oxygen from the air to cause combustion, thus enabling a car to drive. This is the conventional way that engines work. In doing so, chemical energy converts into useful mechanical energy. However, using a fuel cell is an alternative for combustion engines that produces a cleaner and more promising technology. In these processes, oxygen and a fuel, such as hydrogen, feed to fuel cells where it electromagnetically reacts and forms water as a byproduct.
As a result, electrons are able to flow through an external circuit to power an electric motor. The chemical energy from the fuel converts into electrical energy.
Essentially, fuel cells consist of a stack of two electrodes with an electrolyte membrane in between. The electrolyte allows ionic species to conduct and generate power. In a polymer electrolyte membrane fuel cell (PEMFC), electrolytes need to be in a hydrated state. This ensures maintenance of a high ionic (proton) conductivity and optimal performance. Humidification of such fuel cells is essential to this process. Moreover, this is a typical process for automotive applications.
For test benches in the automotive industry, a German research institute asks Bronkhorst for a solution to provide the reactants hydrogen and oxygen with certain relative humidity.
The user requires a solution to set an accurate gas flow. In addition, they need accurate control for the humidification of the gas flow to each side of the fuel cell. Since the role of critical input parameters such as gas flow, water vapor content, and type of fuel cell is experimental, a broad range of gas flows is necessary. This includes small flows where the working point needs to be able to change quickly.
Basically, the solution consists of a temperature-controlled mixing and evaporation device (CEM-system). This device can generate a highly controllable flow of water vapor to supply to fuel cells. Hydrogen and oxygen (or air) act as carrier gases for the water vapor flow. In this specific CEM system, an EL-FLOW Select or EL-FLOW Prestige thermal mass flow controllers control the gas flows of hydrogen, oxygen, and/or air. In addition, a mini CORI-FLOW or LIQUI-FLOW liquid flow controllers provide the water flow. The flows of these instruments enter the CEM device where the vapor/gas mixture generates.
The aim of the R&D in the fuel cell test benches is to optimize the individual components (electrolyte membrane, number of stacks, etc.) and the process conditions of fuel cells. Hydrogen, oxygen, air, and water flows, and their ratios, are input parameters. Furthermore, the performance of fuel cells in the form of cell voltage and current are measured as output parameters.
When considering variables from a research position, what happens when adding hydrogen or air in excess? What influence does the moisture degree have? What is the role of different membrane types?
The gas flows of the EL-FLOW Select or EL-FLOW Prestige can adapt for the specific testing use. High accuracy, with the lowest possible pressure differential, is necessary for these research processes. And these thermal gas flow meters are suitable for this task. Regarding the water vapor supply, we can generate a very high level of control stability, which relies mainly on the mixing valve in the CEM.
For single stack research, the typical water flow range is 100 to 1000 grams per hour to the fuel cell. In addition, it requires matching amounts of oxygen and hydrogen carrier gas so that the relative humidity is in the 5-95% range. Besides that, controlling water flows up to 1 gram per hour is technically possible.
Some fuel cells will eventually operate at elevated pressures, with the lowest possible pressure difference over the membrane, thus with equal pressure at both electrode sides. The current setup for single stack research is such that a measuring range up to 100 bar is possible. This enables specific types of membranes to perform better at higher pressures. The Bronkhorst IN-PRESS can control the absolute pressure and pressure difference in these processes. The CEM Vapor Generation System can provide accurate low water flows for use in single stacks, up to 1000 g/h water.
With respect to automation and communication for mass flow controllers and vaporizing systems, there are several control bus systems available that are integrated into the measurement setup: analog signal, Profibus, and FLOW-BUS, as well as systems like Profinet or EtherCat. LabVIEW is also often useful in these research environments. This way, all process parameters are available immediately, making these values controllable and allowing easy monitoring and quality assurance.
Due to its working principle, the working point of the CEM vaporization system can change quickly. This is a big advantage compared to traditional bubblers, especially at higher gas flows. We offer more than 20 years of experience in fuel cell technology. If you need advice on evaporation systems for the humidification of fuel cells, we are here to help.
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