(373am) Thermodynamic Analysis of Hydrogen Refueling Station

Hydrogen is an environmentally friendly energy source that can replace gasoline or diesel in vehicles. Fuel cell electric vehicles (FCEV) have an advantage over driving distance and filling time compared to electric vehicles (EV). In order to have this advantage, FCEV uses the highly pressurized storage method because of the increased energy density per unit volume through pressurization. Furthermore, the higher injection rate is required to reduce the charging time.

However, when the high-pressure hydrogen is rapidly injected into the FCEV storage tank, the temperature of filling hydrogen increases due to two factors. First, as the hydrogen goes through the valves between the station and the FCEV storage tank, the hydrogen temperature rises as a result of the isenthalpic expansion. Second, when the hydrogen enters the storage tank, the fluid work of the hydrogen changes into the internal energy in the tank causing the temperature of hydrogen to rise, which is called heat of compression. At the same time, the heat energy of the hydrogen is transferred to the tank making the temperature of the tank increase, too.

Many studies have tried to identify factors to influence the temperature behavior during filling, and developed protocols like SAE J2601. Lots of researches have been conducted to design and optimize the hydrogen refueling system following these protocols. However, few optimum designs of a hydrogen station for the future have been developed fully. Therefore, this study mainly focused on the development of a thermodynamic model for the hydrogen filling simulation and analyzed the storage tank behavior during filling as a preliminary step to suggest the optimum design.

The pressure from the station (about 900 bar) to the FCEV tank during filling changes dramatically. This study used a transient model. In order to solve ordinary differential equations, an implicit Euler method was applied. In addition, a proper EOS for high-pressure hydrogen was determined. Heat transfer was also considered in the developed model. This study simulated the filling process basically using the station storage tank, the vehicle tank, and the valve. According to the protocol, when the hydrogen is injected into the vehicle tank, the pressure ramp-up rate should be constant. Thus, a controller for the dispenser valve was modeled so that the pressure of the vehicle tank is increased proportionally. The results of the simulation were compared with the published experimental data of 35MPa filling. In addition, hydrogen filling simulation for 70MPa which is a common case for the hydrogen refueling station was conducted following SAE J2601 protocol. It is recommended that the gas temperature should not exceed 85°C during filling in any scenarios. Accordingly, sensitivity analysis with respect to the cooler performance was conducted. This study showed that this model will enable optimizing the hydrogen refueling system in the future.