BLOG | 29 October 2021
THERMAL AND WATER MANAGEMENT OF THE PROTON-EXCHANGE MEMBRANE FUEL CELL
“Flownex gives you several options and capabilities for modelling the system components and fuel cell stack of the PEM fuel cell.”
HYDROGEN AS A PATHWAY TO DECARBONISATION
As climate change has become a rising concern, more and more countries have dedicated themselves to a zero-emissions goal. Many have now joined the Paris Agreement on climate change and are aiming for net-zero by 2050.
“When using hydrogen within fuel cells, no CO2, or other hazardous emissions such as SOx’s or NOx’s are produced.”
Green hydrogen (hydrogen produced from renewable energy) has been one of the main topics of discussion as the world is shifting efforts to a zero-emissions solution. The combustion of hydrogen with pure oxygen will release only heat and water as by-products – without any direct emissions. Combustion of hydrogen in air at high temperatures will, however, result in NOx emissions: a dangerous group of gasses that can cause respiratory problems, headaches, eye irritation, and other impacts on human and animal life. When using hydrogen within fuel cells, no CO2, or other dangerous emissions such as NOx are produced. Due to this, fuel cell technology has been getting a lot of attention in the past few years.
THE PROTON-EXCHANGE MEMBRANE FUEL CELL
Fuel cells can be used in a wide variety of applications, from transportation to electrical systems on a space craft. Fuel cells also have several advantages when compared to conventional combustion-based technologies. One being that it has a much higher efficiency, even above 60%. Another, as mentioned, no harmful emissions are produced during the operation of the fuel cell.
“This highlights the reason why so much focus is put on fuel cells: it is an energy source with the only emission being water and heat.”
The Proton-exchange membrane (PEM) fuel cell has the advantage that it operates at lower temperatures (at around 80oC) when compared to other fuel cells. This means that it has a quick start up time because less warmup time is needed. It also has a high power density and is low in weight. The fuel cells used in electric vehicles are most commonly the PEM fuel cell.
- Hydrogen is supplied from a high-pressure tank into the anode side of the PEM fuel cell where it comes into contact with the electrolyte and splits up into Hydrogen ions (protons) and electrons.
- The electrons cannot move through the membrane and is forced move through a conductor.
- Air is suppled at the cathode side using a compressor.
- When the electrons arrive at the cathode side through the conductor, the oxygen reacts with the hydrogen ions to form water and heat.
This highlights the reason why so much focus is put on fuel cells: it is an energy source with the only emission being water and heat. The fuel cell can be connected to a load, such as an electric motor in a hydrogen vehicle, and can be used in conjunction with a battery to accommodate for certain high demands.
PEM FUEL CELL SYSTEM COMPONENTS
Even though the PEM fuel cell has several advantages, compared to other conventional fuel cells. There are some technical challenges.
“So, to ensure that high performance is achieved, proper thermal and water management is required for the PEM fuel cell.”
- The membrane used as an electrolyser must be kept at a specific humidity to allow for adequate hydrogen ion transfer.
- If the membrane dries out, performance will be decreased, whereas when there is too much water content in the air, flooding will occur blocking the channels and preventing proton transfer.
- To keep the humidity within a certain range has proven to be quite challenging. A humidifier is needed on the air supply line to increase the humidity as required.
- A thermal management system is also added to extract the heat generated due to the chemical process at the cathode side.
- If this heat is not removed, the fuel cell will heat up and damage the membrane, reducing the performance significantly.
So, to ensure that high performance is achieved, proper thermal and water management is required for the PEM fuel cell.
CAPABILITIES INCLUDED IN FLOWNEX
Flownex gives you several options and capabilities in modelling the system components and fuel cell stack of the PEM fuel cell.
Flownex includes compressible gasses, two phase fluids, mixtures, etc. Custom fluids can be created from properties defined in literature. Higher incremented fluids or properties at different operating conditions can also be imported from NIST. The mass or mole fraction of mixtures can also specified and Flownex allows for changes in these fractions throughout the network. More important for the air side: Flownex allows for property calculation of humid air with detailed psychrometric charts available.
Chemical reaction models for the fuel cell stack
The complex physics associated with the fuel cell stack can be included in the Flownex network. This allows for the analysis of how the entire fuel cell will behave at different conditions. Custom stack models can be implemented using scripting languages such as C#, Phyton and EES. Flownex can also be integrated with software packages such as Cantera and Matlab to include external stack models into Flownex. Reduced order models (ROMS) can also be imported using the Flownex FMI capabilities.
With the ability of coupling Flownex to Ansys Fluent, the advanced fuel cell models in Fluent can be directly coupled to a network in Flownex. Ansys Fluent has specific addon modules to include the complex physics of the proton exchange membrane fuel cell, the solid oxide fuel cell and electrolysis. This means that the geometry of a fuel cell can be created, then imported into Ansys Fluent and the performance will then be calculated with these advanced fuel cell modules. The performance can then be coupled to Flownex using the Ansys Fluent coupling components or by importing a ROM exported from Fluent.
Pump and compressor models
Flownex includes a large library of turbos and pumps – including centrifugal and positive displacement compressors, centrifugal and positive displacement pumps, turbines, etc. Detailed compressor maps and pump charts can be included in these components.
Discretised heat exchange models
Flownex includes different heat exchange models, such as common geometries – finned tube heat exchangers, plate heat exchangers and so on. Flownex also allows for custom correlations for heat transfer to be included. Flownex also gives you the capability of coupling to Ansys Mechanical, allowing the user to integrate complex conduction problems into Flownex.
Typical pressure drop models
Pressure drop models are available such as pipes, valves, etc. The pressure drop models ranges from basic components to more detailed components to replicate real life components.
Steady state and transient solver
Flownex also allows for steady state as well as transient solving. Including an implicit transient solver which allows for very large timesteps for long transients.
Control models for transient simulations
Lastly, Flownex includes a large control library with analogue and digital controls such as PIDs, filters, switches, etc.
MODELLING THE PEM FUEL CELL
Flownex has a comprehensive list of standard components found in typical applications. Flownex also gives you the ability to create and use custom components.
Before modelling a network, it is custom to apply a background image on which the components can be placed. This will simplify the network building process and give the user a more structured approach.
Modelling the network
Flownex uses a drag and drop approach in the design of a thermal fluid systems. This user-friendly approach makes using Flownex easy for beginners.
Custom components can be designed and used in Flownex. This allows the user to include non-standard components into the network.
One of the components that can be created using a custom component is the fuel cell stack. As mentioned previously, the physics of the fuel cell stack can be included using several methods. For this instance, a scripting component utilizing C# will be used in this example.
The script in the above fuel cell stack is used to calculate the hydrogen ion transfer rate from the anode (left) to the cathode (right), the stack current and voltage, heat generation, etc. The CEA Gibbs reactor calculates the chemical reaction of the hydrogen passed through the membrane and the oxygen in the air at the cathode side.
Anode and cathode side components
Custom components can be created for the humidifier as well as the condenser. The anode and cathode side components can then be dragged and dropped into the window.
The humidifier will ensure that the water content of the air stream is sufficient to prevent dry out of the membrane. The condenser is used to extract some of the water out of the stream which can then be introduced at the humidifier.
Adding thermal management
The heat being generated within the fuel cell can be extracted using a thermal management system. This can be quickly implemented in Flownex to ensure that this system is properly designed, preventing any damages to the membrane.
A control system can be added to the network using components such as the PID controller. These components can be added to
- Keep the fuel cell stack temperature consistent by changing the speed of the thermal management system’s fan.
- Ensure the correct hydrogen flow rate according to the required power by changing the valve fraction opening.
- Keeping the correct air to fuel ratio by changing the compressor speed at the cathode side.
In summary, Flownex is a tool that can be used by engineers to design, optimise, and evaluate thermal fluid systems. In the case of the PEM fuel cell where the thermal and water management of the stack is extremely important, Flownex provides a way of understanding the behaviour of these systems.
“It can be used to simulate the chemical reaction within a PEM fuel cell using different methods and allows for coupling with external software.”
Flownex gives you the ability to complete a full system design. It can handle any complex fluids and gives you the ability to include custom fluids. It can be used to simulate the chemical reaction within a PEM fuel cell using different methods and allows for coupling with external software. It includes a large library of components which ranges from basic to more complex models. It also gives you the ability to create your own custom components. Flownex also has numerous heat exchange models from basic to more advanced. It also includes various pressure drop models such as pipes, valves and so on. Flownex allows for steady state and transient solving allowing you to understand the behaviour of the system in its entirety.
Flownex is a useful tool for engineers in the designing and analysis process. As more emphasis is put on a cleaner zero emissions world, the design of greener energy solutions has become imperative. Using a systems simulation software such as Flownex will allow for rapid design of such systems, saving on cost and time.