Hydrogen Refueling Station
This example models a hydrogen refueling station. Hydrogen is stored in low-pressure storage tanks at 200 bar at the station. A 3-stage intercooled compressor maintains the necessary pressure in a cascade buffer storage system so that the station is ready to dispatch hydrogen to any connected vehicles. The buffer is divided into high-pressure tanks at 950 bar, medium-pressure tanks at 650 bar, and low-pressure tanks at 450 bar. To avoid wasting compression energy, the lowest pressure buffer that is greater than the vehicle tank pressure is used to dispatch hydrogen. Priority valves switches between the different buffer tanks to control which buffer tanks to fill and discharge from.
When dispensing, a reduction valve controls the flow between the cascade buffer storage and the vehicle tank. A precooler chills the hydrogen before it is dispatched to the vehicle to avoid excessive temperature rise in the vehicle tank. This example is modeled after a type A70 refueling station from the SAE J2601 fueling protocols, which means that hydrogen is delivered at -40 degC and up to 700 bar.
A Stateflow® chart is used to model the logic needed to control the operations of the station. It determines when to turn on the compressor to recharge the buffer, when to dispatch hydrogen to the vehicle, and the switching logic of the cascade buffer priority valves. When dispensing, the Valve Controller and the Precooler Controller subsystems are enabled. They contain PI controllers that maintain the flow rate and temperature of the hydrogen fuel being dispatched.
Multistage Compressor Subsystem
The compressor is comprised of three positive-displacement compressors with intercooler. The cooling water is assumed to be maintained near environment temperature by a radiator or another cooling system (not modeled).
Stage 1 Subsystem
Stages 2 and 3 have identical components, but the compressor displacement is reduced.
Cascade Buffer Subsystem
The medium-pressure and low-pressure buffer tanks are identical to the high-pressure buffer tank.
Priority Valves In Subsystem
Reduction Valve Subsystem
A silicone heat transfer fluid is used to cool the hydrogen before it is dispatched into the vehicle. The heat transfer fluid is assumed to be maintained at -60 degC by a chiller (not modeled).
Control Logic Subsystem
The Stateflow chart is divided into 3 subcharts. The BufferStatus subchart reports the readiness of high-pressure, medium-pressure, and low-pressure buffer tanks depending on whether they have sufficient pressure for dispatching. The ChargeBuffer subcharts turns on the compressor to refill the buffer whenever it is not ready. It prioritizes filling the higher pressure tank over the lower pressure tank.
When the dispenser trigger is on, the Simulink Function is called to obtain the Average Pressure Ramp Rate (APRR) and the target pressure based on the tabulated data from SAE J2601. The ChargeVehicle subchart then turns on the reduction valve controller and the precooler to dispense hydrogen. It prioritizes dispatching from the lower pressure buffer tank before switching to the higher pressure buffer tank.
Precooler Controller Subsystem
Valve Controller Subsystem
Simulation Results from Scopes
The state of the hydrogen fuel in the vehicle tank is shown in the left column of the scope plots. At t = 2400 s, the dispenser is triggered and filling starts. The increase in pressure is maintained at a constant slope, called the Average Pressure Ramp Rate (APRR), by the Valve Controller. Filling stops once the target pressure is reached. The APRR and target pressure come from the SAE J2601 fueling protocols and depend on the environment temperature and initial pressure in the vehicle. In this example, they are 18.5 MPa/min and 74.5 MPa, respectively.
While filling, the temperature increases significantly due to the amount of compression that occurs in the vehicle tank. SAE J2601 sets a maximum temperature limit of 85 degC. In this example, the temperature reaches 68.6 degC when full. This is achieved by precooling the hydrogen to -40 degC before it enters the vehicle, as shown in the bottom right plot.
The bottom left plots shows the flow rate of hydrogen into the vehicle tank. Because the controller maintains an APRR, the mass flow rate decreases as the pressure increases. The two spikes in the flow rate occur when it switches from the low-pressure buffer to the medium-pressure buffer and from the medium-pressure buffer to the high-pressure buffer. Further adjustments of the valve timing while switching buffers may reduce these spikes. Nevertheless, the flow rate is below the maximum limit of 0.06 kg/s set by SAE J2601.
The top two plots on the right column shows the state of the cascade buffer storage. At the start of the simulation, the compressor is turned on to fill the buffer in order from the high-pressure tank to the low-pressure tank. Dispatching hydrogen to the vehicle causes a drop in the buffer pressure. Therefore, the compressor turns on again to refill the buffer tanks and maintain buffer pressure.
Simulation Results from Simscape Logging
This plot shows a closer view of the state of the hydrogen fuel in the vehicle tank while it is being filled. It also shows the state of charge (SOC) and the total mass of hydrogen in the vehicle. The SOC is defined as the density of hydrogen in the vehicle divided by the density of hydrogen at nominal working pressure (NWP) and 15 degC. The SOC must never exceed 100%.
This plot shows the pressures and flow rates in the cascade buffer storage tanks. At the start of the simulation, the high-pressure tank, medium-pressure tank, and low-pressure tank are filled at a rate of about 0.034 kg/s. While switching from one buffer tank to another, some hydrogen from the higher pressure tank leaks to the lower pressure tank, resulting in a spike in flow rate. This can be reduced by adjusting the timing of the valve switching.
At t = 2400 s, hydrogen is dispatched to the vehicle at a rate that varies from 0.035 kg/s to 0.018 kg/s. The hydrogen initially comes from the low-pressure buffer tank. As the vehicle tank pressure increases, it switches to the medium-pressure buffer tank, and then finally the high-pressure buffer tank. As the vehicle tank is filled, the buffer pressure drops, so it needs to be refilled for the next vehicle. A larger buffer will allow more vehicles to be filled before the buffer needs to be refiled.
This plot shows pressure drop and temperature rise across the reduction valve while the vehicle is being filled. Because the buffer is divided into high-pressure, medium-pressure, and low-pressure tanks, the pressure drop curve is divided into three segments and the maximum pressure drop seen by the valve is reduced.
The temperature increases across the valve because the hydrogen is above the Joule-Thomson inversion temperature. The bottom plot approximates the Joule-Thomson coefficient by taking the ratio of the temperature difference to the pressure difference across the valve. The coefficient is negative, meaning that the gas is warmed by the pressure drop.