Inside the engine of your car, gasoline reacts directly with oxygen from the air to cause combustion which enables your car to drive. This is the conventional way. Chemical energy is converted into useful mechanical energy. An alternative for this is combustion in an indirect way: a more clean and promising manner using a fuel cell. Oxygen and fuel, such as hydrogen, are fed to each side of the fuel cell, to electromagnetically react inside the cell forming water.
As a result, electrons flow through an external circuit powering an electric motor. The chemical energy from the fuel is converted into electrical energy.
Fuel cells consist essentially of a stack of two electrodes with an electrolyte membrane in between. This 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 to maintain a high ionic (proton) conductivity and, hence, optimal performance. Humidification of such a fuel cell is essential. This is a typical process used for automotive applications.
For test benches in the automotive industry, a German research institute asked Bronkhorst’s distributor, Wagner Mess- und Regeltechnik, for a solution to provide the reactants hydrogen and oxygen with certain relative humidity.
The user needed a solution to set an accurate gas flow and additionally humidify the gas flow in a controlled manner to each side of the fuel cell. Since the role of many input parameters such as gas flow, water vapor content, and type of fuel cell is investigated, a broad range of gas flows is necessary – including small flows – and the working point needs to be changed quickly.
- Quick change of working point
- Constant humidification possible, even at small flows
- Accurate control and measurement of media
- Quality assurance through all parameters
The Bronkhorst solution consists basically of temperature-controlled mixing and evaporation device (CEM-system) that is used to generate a controlled flow of water vapor, which is supplied to the fuel cell. Hydrogen and oxygen (or air) act as carrier gases for the water vapor flow. In this CEM system, EL-FLOW Select or EL-FLOW Prestige thermal mass flow controllers control the gas flows of hydrogen, oxygen/air, whereas mini CORI-FLOW or LIQUI-FLOW liquid flow controllers provide the water flow. The flows of these instruments enter the CEM unit where the vapor/gas mixture is generated.
The aim of the R&D performed in the fuel cell test benches is to optimize the individual components (electrolyte membrane, number of stacks) and the process conditions of the fuel cell. Hydrogen, oxygen, air, and water flows – and their ratios – are input parameters, and the performance of the fuel cell in the form of cell voltage and current are measured as output parameters.
Questions to be answered are for example: what happens if hydrogen or air are added 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 devices are adapted for the specific use. High accuracy with the lowest possible pressure differential is required, and these thermal gas flow meters are suitable for this job. 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.
In this application, for single stack research, the typical water flow range is 100 to 1000 grams per hour supplied to the fuel cell, with matching amounts of oxygen and hydrogen carrier gas so that the relative humidity is in the 5 to 95% range. Besides that, controlling water flows up to 1 gram an hour is technically possible as well.
Some fuel cells will eventually be operated in practice at elevated pressures, and with the lowest possible pressure difference over the membrane, so 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 – for specific types of membranes that will perform better at higher pressures. The absolute pressure and pressure difference are controlled by Bronkhorst IN-PRESS instruments. The Bronkhorst CEM system is typically used for accurate low water flows as used 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, Flowbus as well as systems like Profinet or EtherCat. LabVIEW is often used in these research environments. This way all process parameters are available immediately, making these values controllable and allowing good monitoring and quality assurance.
Due to its working principle, the working point of the CEM based vaporization system can be changed quickly. This is a big advantage compared to traditional bubblers, especially at higher gas flows.
We have more than 20 years of experience in the field of fuel cells. If you need advice on evaporation systems for the humidification of fuel cells, ask our advice.