Scientists from the University of Modena and Reggio Emilia in Italy and the University of Technology of Compiègne in France jointly with engineers from HYDAC Technology, Faber Industrie and Hydrogen Refueling Solutions have developed an original method of hydrogen compression for refueling heavy vehicles using hydraulics. This solution was developed as part of the European project H2REF-DEMO, which aims to create more reliable and energy-efficient hydrogen stations for buses, trucks and trains.
The biggest challenge of hydrogen refueling is that hydrogen has a very low density, and it has to be compressed to extremely high pressures of up to 70 MPa in order to be stored in sufficient quantity on board a vehicle. Conventional mechanical compressors that are used for this purpose are often expensive, energy-consuming, high-maintenance and subject to wear and tear. The scientists have proposed an alternative: using hydropneumatic accumulators in which hydrogen is compressed not by a piston but by the pressure of hydraulic oil, which deforms an elastic membrane containing gas.
This solution is based on a modular two-stage compression system. On the first stage, hydrogen pressure is raised from the pressure of the source (i.e., 5 MPa) to an intermediate level of about 14 MPa; on the second stage, it is raised to 35–40 MPa required for refueling heavy machinery. The key engineering solution was the ratio of accumulators: three membrane accumulators on the first stage serve one accumulator on the second stage, which ensures the consistency of their operation and preserves a safe compression ratio of no more than 3. This makes it possible to avoid overheating and membrane degradation. The system supports three modes of operation: “Bypass” (when the pressure in the source is sufficient for direct feeding), “Parallel” (fast filling using both stages) and “Consecutive” (staged compression with low pressure at the entrance).
In order to design and optimize the system, the researchers created a detailed model in a Simcenter Amesim environment, which took into account hydraulic, gas-dynamic and thermal processes, material properties and control algorithms. The simulation made it possible to choose the optimal parameters for the equipment, including hydraulic pumps with the most suitable characteristics, and to determine the location of the heat exchangers that provide effective cooling of hydrogen between the stages.
Calculations showed that such a station can inject 25 kg of hydrogen into a tank with a volume of 1,250 liters in about ten minutes, which complies with the requirements for commercial hydrogen filling stations. The specific energy consumption stood at 1.11–1.54 kW·h per kilogram of hydrogen, which is comparable to the best modern technologies. At the same time, the temperature of hydrogen in the process did not exceed 120°C, which guarantees the safety of the equipment.
Even today, the newly-developed architecture makes it possible to add a third stage for operation with even lower initial pressures. In the future, a transition to an adaptive algorithm for switching modes could be made, which would also increase overall efficiency. Meanwhile, the researchers seem to be working to create a physical test sample for field testing.
