Collect

1. Introduction

One of the main limitations of battery electric vehicles (BEVs), especially those that use large battery packs to achieve a respectable range, is the time it takes to recharge the battery. This forces vehicle operators to stop for long periods of time to recharge on long journeys. In the case of construction equipment, such as forklifts or excavators, this puts the equipment on standby until the battery is recharged. Integrating fuel cells into electric vehicles may bring us a better solution for clean mobility. To recharge the vehicle, you just need to fill the fuel tank with hydrogen fuel. For passenger cars, this operation takes no more than 5 minutes, and for long-distance trucks, it takes no more than 10 to 15 minutes. For example, Toyota claims that the Mirai fuel tank can be filled with hydrogen in less than 5 minutes, meeting a range of 650 kilometers.

Below we use Simcenter Amesim to compare different system architectures and evaluate the refueling time for a fuel cell electric vehicle (FCEV). The model can also be used to predict the energy consumed in compressing the hydrogen and determine the selection of a cooling system to keep the gas at the appropriate temperature level during the compression process.

2. Hydrogen refueling station system modeling

Figure 1 System model

2.1 Hydrogen Storage System

For example, using a combination of solar panels and electrolyzers, hydrogen refueling stations can produce hydrogen locally. Another alternative is to use a larger electrolyzer system to produce hydrogen at a lower cost. This can then be transported to the hydrogen refueling station using a tube trailer. We will consider this scenario later in the model using the following predetermined parameters:

Trailer hydrogen tank capacity: 5 m3
Initial pressure: 200 bar
Initial temperature: 20°C

Under the above assumptions, the hydrogen storage capacity on the trailer is approximately 76kg

2.2 On-board hydrogen storage tank

We selected passenger cars as the research object, and we used the following parameters to analyze the hydrogen storage tank:

Hydrogen storage tank volume: 150 L
Initial pressure: 50 bar
Final pressure: 700 bar
Initial temperature: 20°C

At a pressure of 700 bar, the hydrogen storage tank will hold approximately 6 kg of hydrogen.

2.3 Hydrogen booster system

In order to fill hydrogen from the 200 bar hydrogen tank of the hydrogen refueling vehicle into the hydrogen tank of the passenger vehicle and reach the final target pressure of 700 bar, the hydrogen storage system uses 2 positive displacement compressors and operates in parallel under the following assumptions:

Compressor displacement: 150 cm3/rev
Compressor speed: 120 rpm

2.4 Thermal Management System

To ensure the safety of the system, while increasing the amount of hydrogen in the tank, it is necessary to avoid high temperatures that would damage the lining of the vehicle's hydrogen tank, so several heat exchangers are integrated into the system. In our case, we assume that the heat-integrated exchangers need to meet the following conditions:

Ensure that the hydrogen at the compressor outlet reaches 30°C after cooling
At -50°C, the upstream hydrogen tank is guaranteed to supply hydrogen to the passenger car hydrogen storage tank

2.5 Buffer tank

To meet system requirements, low-pressure, medium-pressure and high-pressure hydrogen storage tanks are integrated into the entire system:

Improving the speed of vehicle hydrogen refueling
Use a smaller compressor or reduce the operating speed
Reduce preheating during hydrogenation and reduce cooling system power

The buffer tank is set up as follows:

Low pressure tank

Initial pressure: 380 bar
Volume: 500 L

Medium pressure tank

Initial pressure: 580 bar
Volume: 300 L

High pressure tank

Initial pressure: 850 bar
Volume: 300 L

2.6 Control

To ensure the stable operation of the integrated system, a special control strategy was developed based on the state machine. The switching between the various control states is mainly determined by the hydrogen pressure at different locations in the system.

Generally speaking, when the vehicle hydrogen storage tank is connected to the hydrogen refueling system, the corresponding valve is opened to smoothly balance the pressure between the vehicle hydrogen storage system and the hydrogen refueling system. After that, the compressor can be started or the buffer tank can be used.

Figure 5 Control module of valve and compressor

2.7 Hydrogen Model

Throughout the system, a true gas model (Peng-Robinson) is used to predict the flow, pressure, temperature, and thermodynamic properties of hydrogen.

3. Solution Analysis

3.1 Filling system does not use the buffer tank function

Using our system model, we can simulate the hydrogen refueling process using hydrogen from the hydrogen refueling trailer tank and 2 compressors. During this process, the valve of the buffer tank is always closed. Once the vehicle tank pressure reaches the expected pressure (700 bar), the simulation will stop.

We first used the sketch animation function of Simcenter Amesim to analyze the entire process. Sketch animation can visualize the dynamic evolution of pressure or temperature in different parts of the system during the entire working condition on the model sketch (Figures 6 and 7).

Figure 6 Dynamic display of system temperature

Figure 7 Dynamic display of system pressure

By analyzing the simulation results in depth, we can also find that:

The time taken to refuel the vehicle is 8 minutes and 40 seconds (Figure 8)

Figure 8 Changes in pressure and hydrogen quality of hydrogen storage tanks

The maximum power consumed by the two compressors is approximately 17.3kW, and the energy consumed by these compressors is close to 1.1kW.h (Figure 9).

Figure 9 Compressor power and energy consumption

When the heat exchanger integrated upstream of the vehicle's hydrogen tank must have a heat dissipation capacity of 26 kW, the two heat exchangers integrated downstream of the compressor must have a heat dissipation capacity equivalent to 6.4 kW (Figure 10).

Figure 10 Heat exchange power of heat exchanger

From the above results, we can know that FCEV can complete hydrogen refueling in about 8 minutes. This is already much faster than the battery charging of pure electric vehicles. However, this is not really satisfactory. Because the purpose is to be able to complete hydrogen refueling in less than 5 minutes.

To achieve the above goals, possible solutions include:

For example, we could modify our model and analyze whether a system with 4 compressors and their own heat exchanger would meet the requirements. But this approach would mean a more complex and more expensive system.
Through the analysis of this model, it is found that increasing the compressor displacement to 320 cm3/rev can also complete hydrogen refueling in less than 5 minutes. However, we still need to use a larger heat exchanger (14kW) to remove more heat, and a larger compressor and heat exchanger will be more expensive and will definitely generate more noise.
Another solution is to use a buffer tank. This is the solution we will study below.

3.2 Filling system using buffer tank function

Using the same system model as before, I will integrate the use of a buffer tank through the control of a solenoid valve.

Once the vehicle tank pressure approaches the refueling vehicle system pressure (200 bar), the control strategy opens the valve connected to the low-pressure buffer tank (380 bar). Once the vehicle tank pressure is balanced with the pressure of one of the buffer tanks, the valve closes again and the valve that switches to the medium-pressure buffer tank (580 bar) opens. Finally, when the vehicle tank pressure is balanced with the pressure of the medium-pressure buffer tank, the valve closes again and the valve of the high-pressure tank (850 bar) opens.

When the vehicle's hydrogen storage tank is full at 700 bar, the compressor actuates, eventually re-establishing the buffer tank pressure to its initial level.

Figure 11 Dynamic changes in system temperature

Again, we can use sketch animation to visualize the dynamic changes in pressure or temperature in different parts of the system throughout the operating conditions (Figure 11).

We can also determine that, using the buffer tank, the system is now able to complete the vehicle's hydrogen refueling in less than five minutes. So, goal achieved: FCEVs can be refueled as efficiently as conventional gasoline vehicles in a short time! The system then uses another three minutes to refill the hydrogen in the buffer tank to its initial state.

Figure 12: System temperature changes

However, we can note that as a disadvantage of this case, the compressor now requires more power (up to 20 kW) and energy (1.5 kWh). This can mainly be explained by the need to compress the hydrogen to 830 bar in the high-pressure buffer tank (Figure 13).

Figure 13 Compression power and energy consumption using buffer tank

On the other hand, the selection of the cooling system has little influence, since the maximum heat power to be transferred is very similar (Figure 14).

Figure 14 Heat exchanger power using a buffer tank

4. Conclusion

System simulation in Simcenter Amesim facilitates the system design of hydrogen refueling stations. With fast simulation times (less than 1 second CPU time for the above simulation using a standard laptop), different technical solutions regarding system architecture, subsystem matching or control strategies can be easily and quickly evaluated. Using advanced post-processing tools, it is also easy to analyze the simulation results and the physical behavior of the refueling station, especially regarding the dynamic evolution of hydrogen flow, pressure and temperature at different locations. Therefore, this really helps in the optimal design of the system to refill the vehicle tank as quickly as possible, while also controlling the hydrogen temperature, gas compression and the power consumed by thermal management.