Modeling of a Hydrogen Refueling Station
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Gaseous hydrogen is one of several alternatives to fossil-based vehicle fuels. In order to establish hydrogen as a good alternative, it is necessary to implement hydrogen refueling stations that can compete with conventional refueling stations. That is, the hydrogen refueling stations must be able to meet the customer's demands and expectations. The Society of Automotive Engineers has therefore developed a protocol that provides performance requirements for gaseous hydrogen refueling stations. These requirements include average pressure ramp rates (APRR) which describe the desired pressure rise in the vehicle tank along with precooling temperatures of the hydrogen prior to entering the vehicle. The precooling is necessary due to two phenomena occurring during refueling: 1) reverse Joule-Thomson effect, and 2) heat of compression. In this thesis, a simple dynamic model of a hydrogen refueling station with a direct compression design has been developed and implemented in MATLAB. The direct compression design consists of two parallel process lines where one contains a reduction valve and the other a compressor. The pressure difference between the station and the vehicle decides which process line is utilized during the refueling. In order to comply with the precooling temperature set by the protocol, a heat exchanger has to be included after the compressor and reduction valve but prior to the nozzle connected to the vehicle. A production section where hydrogen is produced from electrolysis of water has also been included in the model. Two pressure controllers have been implemented; one for each process line. These controllers are of the PI type and control the pressure rise in the vehicle tank. Various conditions such as different initial vehicle tank pressures, ambient temperatures and APRRs have been tested. These conditions were all handled well by the model. Additionally, the implemented controllers were able to track the APRR tightly under all the various conditions. The waste heat generated in different components at the station was quantified to see whether an absorption refrigeration process can be utilized for cooling the hydrogen prior to the vehicle tank. In total, four different cases for making the absorption refrigeration process workable were considered. However, the process is only achievable in one of the cases for an ambient temperature of 15 degrees C and presumably below.