Optimization Of Compressed Hydrogen Supply System In Hydrogen Refueling Station Using Thermodynamics And Queuing Theory

Hydrogen is considered to be one of the most promising clean energy in 21st century because of its cleaness,reproducibility,wide availability and high conversion efficiency.It will be an effective solution to air pollution and energy crisis.The development of hydrogen energy and fuel cell technology have been included not only in the national development strategy of USA,Japan and Germany,but also in the key fundamental research content of China.At present,compressed hydrogen storage is the most predomiant storage method for hydrogen fuel cell vehicles(HFCVs)and hydrogen refueling stations(HRSs)due to its technical simplicity,rapidness for fueling/defueling and low energy consumption compared with liquid hydrogen storage.Detailed study is required of all aspects of HRSs to achieve a smooth transition to HFCVs from existing gasoline-powered vehicles.However,there are few comprehensive studies about the configuration and design of the compressed hydrogen supply system,which is integrated by the compression system,storage system,dispensing system and precooling system.Optimization of compressed hydrogen supply system and its key compent is conducted in this paper,which is supported by the National High Technology Research and Development Program of China(863 Program)"Study of key technology and equipment for 70 MPa hydrogen supply system"(No.2009AA05Z118).The main content and conclusions are as follows:(1)A queuing model is developed to evaluate the queuing performance of vehicles at HRSs by incorporating the statistical and thermodynamic characteristics of refueling.An optimization framework is proposed to determine the minimal number of dispensers required to meet the upper limits imposed on two important performance measures:mean waiting time and mean queue length.The effects of dispenser allocation and station capacity on queuing performance are studied and the optimum number of dispensers are obtained for station capacities of 50-6000kg/d.Server(dispenser nozzle)utilization under the optimal dispenser allocation generally increases with the increase in station size and tends to exceed 50%for large stations.The proposed approach can offer significant performance improvements for small stations and considerable savings in the number of dispensers for large ones.(2)A thermodynamic model is developed to simulate the hydrogen supply process in tube-trailer supplied hydrogen refueling stations with tube-trailer bank-consolidation process by including the real gas behavior of hydrogen,the flow characteristics of diaphragm compressor,the fueling requirements of compressed hydrogen and the thermal effects during the charing/discharging process of tube-trailer,cascade storage vessels and vehicle tanks.The compressor capacity is optimized to achieve the lowest cost and the influencing factors of the optimum dimensionless compressor capacity are studied.At the optimum dimensionless compressor capacities,effects of tube-trailer operation strategy,the division of cascade storage system and the number of tube banks on cost and energy consumption have been studied.The results show that the tube-trailer operation strategy has a great effect on cost,but small effect on energy consumption.The lowest cost can be achieved with the following tube-trailer operation strategy:use the tube with the highest pressure to replenish the cascades if necessary;otherwise,use the tube with the lowest pressure to consolidate the tube with the highest pressure;always use the tube with the highest pressure to fill the vehicles initially.For a three-bank cascade storage system,the optimum volume ratios among the low pressure bank,mid pressure bank and high pressure bank are 1:1:1 and 3:3:2.Increasing the number of cascade banks can reduce the energy consumption and the total cost of compressor and cascade vessels,but it increases the piping cost,which should be considered when optimizing the number of cascade banks.The cost of HRS generally decreases at a dereasing rate with the increase in the number of tube banks.However,increasing the number of tube banks will not definitely reduce the cost when the number of tube banks is no less than four.The recommended number of tube banks is 4.According to the above conclusions,the optimum number of cascade banks and the optimum compressor and cascade storage capacity are obtained for various tube-trailer utilization rates and station capacities.(3)The hairpin heat exchanger is adopted as hydrogen pre-cooler and liquid ethanol is used as the secondary refrigerant.A simulation model is developed by considering the flow and convective heat transfer characteristics of the hot/cold fluids,the heat conduction in the pipe wall,the real gas behavior of hydrogen and the physical property variation of liquid ethanol and 316 stainless steel with temperature.Theheat transfer capacity of pre-cooler is characterized using entransy dissipation rate.The effects of fueling parameters on the required heat transfer rate and the mass flow of hydrogen are studied using the thermodynamic model of fueling process and the fueling condition with the highest heat transfer rate and pressure drop is obtained.The required heat transfer areas and the maximum pressure drops should be calculated at the obtained fueling condition.Theprecooling process is analyzed and theobtained hydrogen temperature,entransy dissipation rate and pressure drop of hot/cold fluid are compared with the quasi-steady results.The calculations of pressure dropand entransy dissipation rate are simplified.The effects of ambient temperature,initial pressure of vehicle tank,pressure drop coefficient for fueling line downstreamnozzle and onboard storage capacities are studied and fueling condition with highestprecooling temperature is obtained.At fueling condition with the highest precooling temperature,effects of annular ratio,inner-tube diameter,inlet velocity and temperature of coolant,the number cooling branches and the excess in heat transfer area on precooling temperature are studied.According to the above studies,the following conclusions are obtained:Both the maximum required heat transfer rate and the maximum hydrogen pressure drop during refueling peak at 30℃ with the minimum pressuredrop coefficient downstream nozzle and the minimum initial pressure of vehicletank;the precooling temperature at the start of filling peaks at the highest ambient temperature with the maximum pressure drop coeffient and the minimum initial pressure ofvehicle tank;at fueling condition with the highest precooling temperature,theannularratio,inlet velocity and excess in heat transfer area have little effects onprecooling temperature;the precooling temperature increases with the increase in inner-tube diamter,the number of cooling branches and the inlet temperature of coolant.A multi-objective optimization model is proposed by regarding entransy dissipation rateand hydrogen pressure drop in precooler as objective function with limitaionsonprecooling temperature and ethanol pressure drop.The parameters of precooler are optimized and the results are can be used for precooler design.