Hydrogen Fuelling Stations : A Thermodynamic Analysis of Fuelling Hydrogen Vehicles for Personal Transportation

Abstract

This thesis concerns hydrogen fuelling stations from an overall system perspective. The study investigates thermodynamics and energy consumption of hydrogen fuelling stations for fuelling vehicles for personal transportation. For the study a library concerning the components in a hydrogen fuelling station has been developed in Dymola. The models include the fuelling protocol (J2601) for hydrogen vehicles made by Society of Automotive Engineers (SAE) and the thermodynamic property library CoolProp is used for retrieving state point. The components in the hydrogen fuelling library are building up following the same procedure for each component. This enables components to be connected in any random order when building systems. The systems are made in a graphical interface, were components from the library can be directly dragged in and connected by dragging a line from their input or output port. A system consisting of one high pressure storage tank is used to investigate the thermodynamics of fuelling a hydrogen vehicle. The results show that the decisive parameter for how the fuelling proceeds is the pressure loss in the vehicle. The single tank fuelling system is compared to a cascade fuelling system. This shows that the mass flow and thermodynamic development in the vehicle are independent of the station design, when fuelling according to the protocol made by SAE. Further the study showed that a cascade system is preferable compared to a single tank system, considering the energy consumption of the fuelling. Models of cascade systems consisting of between 1 and 8 tanks have been used to analyse the effect of the number of tanks at the station with respect to energy consumption. An optimisation using a parameter variation of the tanks volumes and pressures is performed, to reduce the energy consumption further. The study showed that the energy consumption at the station approach an exponential function of the number of tanks. The energy saving was highest going from one to two tanks in the cascade system and the saving levelled out when more than four tanks were used. Decreasing the tanks volumes to a minimum or decreasing the pressure to a minimum contributed to the overall saving with app. 4-5 %. Two alternative system designs to the cascade fuelling system have been suggested and analysed with respect to thermodynamics, energy consumption and exergy destruction. The first system uses a compressor to fuel the hydrogen vehicle. The second system uses a compressor followed by a small buffer tank. The system fuelling directly from a compressor does not follow the fuelling protocol, though it does not exceed the safety limits. The system using a compressor and a buffer tank is fuelling in accordance with the fuelling protocol. The analysis showed that it is possible to eliminate all the high pressure storage tanks from the cascade system, using one of the other fuelling systems. The energy consumption of the direct compression system was 18 % lower than for the two other systems. The exergy analysis showed that the largest exergy destruction was in the vehicle tank, due to compression. The compressor and heat exchanger at the outlet of the compressor also had high exergy destructions in all three systems. The reduction valve, which is eliminated in the direct compression system, had an exergy destruction corresponding to more than 0.75 kWh which was 11-17 % of the total exergy destruction in the two other systems. The direct compression system was the least energy consuming and had the lowest exergy destruction. The cascade system had the highest exergy destruction while the energy consumption was app. the same as for the direct compression system with a buffer.