Wei-Wei Zhou，Chuan-Xiang Zheng，Rong Li，Liang Wang

(Institute of Chemical Machinery and Process Equipment，Zhejiang University，Hangzhou 310027，China)

1 Introduction

The energy derived from the combustion of hydrogen is considered clean energy as only pure water is produced and there is no harm or pollution to the environment.With a rich source of supply，hydrogen can be recycled making it a renewable energy resource.Therefore，the research in hydrogen energy has drawn a great dealofinterestin many countriesamongst researchers.The key technologies of hydrogen energy are hydrogen effective conversion and utilization，hydrogen storage and transportation and hydrogen production technology. Amongst all these， the hydrogen storage and transportation is at present the most expensive and difficult to achieve.The method of hydrogen storage and transportation so far include metal hydride storage，low temperature liquefaction storage，adsorption storage，and high pressure storage，and so on.High pressure storage is used more widely because it is simple，direct and easy to manufacture industrially. However， high pressure storage is dangerous to use and necessary precautions have to be taken.

There are many factorsthatcan affectthe performance of onboard high pressure hydrogen storage vessel，such as fatigue，hydrogen corrosion，impact，and high temperature，and so on.The fatigue resulting from the frequent hydrogen charging and discharging is the main cause of failure.The traditional fatigue test is usually replaced by hydraulic test owning to its better experimental equipment and cheap cost， however，hydrogen is corrosive to materials contiguous with it;moreover，the inner wall is suffered from high stress.Therefore，the fatigue behavior is rather complicated to monitororobserve under hydrogen environment.Therefore，itis necessary to conducta fatigue experimental study for hydrogen vessels under hydrogen environment.Before that，it is necessary to set up such a testing system［1－5］.

2 FatigueTestingSystem D evelopmentwith Hydrogen Surroundings under 70 MPa

2.1 Integral Design of the System

The pressure of onboard high pressure hydrogen storage vessel reaches up to 70 MPa at present.This is also the critical pressure of high-pressure hydrogen storage because it is very difficult to enlarge the hydrogen storage capacity with higher pressure.So in this paper，the whole testing system is designed at 70 MPa.As shown in Fig.1，its working principle is as follows:a 80 MPa high pressure hydrogen source chargeshydrogen into an onboard high pressure hydrogen storage vessel(called sample vessel)through a pneumatic ball valve and orifice plate flowmeter.It will stop after the pressure of the sample vessel has reached to the working pressure.Then the pressure release valve of sample vessel will open to release the hydrogen into a low pressure buffer tank until the pressure of sample vessel is reduced to less than 1 MPa.The low pressure hydrogen is compressed into the 80MPa high pressure hydrogen storage vessel.This is a complete cycle.The next fatigue cycle will follow the same step.It is the only hydrogen environmental fatigue testing system under high pressure in China.Several key problems should be solved to develop this system:the development of a 80 MPa high pressure vessel，monitoring of hydrogen safety and automatic control system［6－7］.

Fig.1 Principle sketch of the system

2.2 Development of a 80 MPa Flat Steel Ribbon WoundHighPressureHydrogenStorage Vessel

2.2.1 Anti-hydrogen corrosion and anti-hydrogen penetration design

Hydrogen is a gas with strong permeability and it has some brittleness effects on mild steel under certain conditions.That is hydrogen brittleness.In order to solve this problem，0Cr18Ni9 is adopted as the lining material and Q345R as the outer layer material to make a double layered cylinder.The overall dimension of the vessel is shown in Fig.2.This double layered cylinder forms a liner of 80 MPa hydrogen vessel to prevent instability caused by prestressed stress of ribbons.Prestressed optimized flat steel ribbons are wound according to the even strength cylinder design.Two layers forged semi-spherical head is adopted.An inclined plane is machined for welding both ends of flat steel ribbons on both semi-spherical head.

This structure has the characteristics of hydrogen resistance and anti bust explosion.High strength steel，which is sensitive to hydrogen，is separated by the stainless steel which is insensitive to hydrogen.Even though for some reasons，leakage of hydrogen appears，hydrogen will flee through the gap in the flat steel ribbon wound layer.So it is very convenient to detect whether a leakage occurs through the hydrogen sensor or the pressure sensor in the protective shell.On-line real-time safety monitoring can be realized in this structure［8－9］.

2.2.2 Optimization design of even strength cylinder

Reasonable prestressdistribution ofthe high pressure vessel can be achieved in the flat steel ribbon wound high pressure vessel because the prestress and wound angle ofsteelribbons can be regulated according to the needs.

Thus we can design an ideal high pressure vessel with reasonable operating stress distribution in the vessel wall.That is low stress in the area contiguous with medium and uniform stress distribution in the rest area of the vessel wall.This is an ideal stress distribution and can not be achieved in monolayer thick wall vessels.

There will be initial residual stresses on the cylinder after steel ribbon-winding prestressed on the pressure vessel.The final stress will be the summation of initial residual stresses and the stress caused by internal pressure.The final stress distribution state can be selected according to the need of operating condition of the vessels.For example，the stress on the inner cylinder of the vessel is zero and the stress of the ribbon wound layeris uniform，orthe stress is uniformly distributed along the thickness of the whole cylinder walland so on.Once the finalstress distribution is selected，prestress of the steel ribbon of the vessel can be determined.In this case，low stress distribution for the inner cylinder and even stress in steel ribbon wound layer is adopted.Its realization method is listed as follows［10－12］:where σθ，σr，σzare hoop stress，radial stress and axial stress of the vessel，respectively，MPa.σ，σpre，σpare final stress，prestress and the stress under internal operating pressure of the vessel，respectively，MPa.

Fig.2 Overall dimension of the vessel

Final stress expressions in the wall of vessel can be calculated through matrix transformation:

where σr，σθ，σzare final radial stress，hoop stress and axial stress of the cylinder，respectively，MPa.

σr，pre，σθ，pre，σz，preare radial stress，hoop stress and axial stress resulted from steel ribbon winding，respectively，MPa.

σr，p，σθ，p，σz，pare radial stress，hoop stress and axial stress under internal operating pressure，respectively，MPa.

If we want to achieve low stress on the inner wall and even stress distribution on the ribbon wound layer，we can regulate the prestress of the steel ribbon because the stress under internal operating pressure is constant.

2.2.3 Stress controllable design process of the flat steel ribbon wound pressure vessel

According to the theoretical calculation above，the required stresses distributing state of steel ribbon wound cylinder wall can be achieved by adjusting the prestress and wound angle of the steel ribbon.With the final expected stress distribution of the flat steel ribbon wound pressure vessel as target value，we can calculate the initially required prestress and wound angle of the steel ribbon by contrary step of the calculation above.

σpis determined by internal operating pressure P and can be obtained by Lame formulae.If σθis the final required hoop stress，then σprecan be obtained from Eq.(2)

If σθis the final expected hoop stress，then hoop stress can be obtained from Eq.(2)

The required steel ribbon wound prestress T can be obtained from the above equations according to the mechanical relation between tension of the steel ribbon and stress of the vessel［11］.

There is a relationship between hoop stress and axial stress for steel ribbon wound vessel and its value is variable by changing the winding angle α.We usually make axial strength a little stronger than hoop strength when designing，so the target value of axial strength can be selected.For example，we can make the value of axial stress 0.9 times of the value of hoop stress，and then we can approach the target value by constantly changing the winding angle α until it meets the demanded requirements.Because the radial stress value is relatively small compared to other stresses，its value is not controlled in this design and it is automatically coordinated by three dimensional stresses.More details on the method and process of how to control stress distribution of the vessel by controlling prestressofthe flatsteelribbon are described in Ref.［10］.

The volume of 80 MPa high pressure vessel is set to 0.9 m3so that the charging and discharging of hydrogen can be realized for a 70 MPa and 100 liter sample vessel，which considers the requirement of the presently biggest onboard hydrogen storage vessel.Due to the requirements of quickly charging and discharging of hydrogen，the pressure of high pressure hydrogen storage vessel is reduced from 80 MPa to 72MPa after the 100 liter cylinder is fully charged.From the design result，the liner is a bilayer structure:one is 8 mm thick with material 0Cr18Ni9 and the other is 25 mm thick with material Q345R.There are 36 layers in total and the thickness of each layer is 4 mm.The prestress of each steel ribbon wound layer T can be carried out as follows in accordance with the design requirementlow stress on the liner and even strength for the ribbon wound layer:

T=［110 108 97 90 85 79 75 72 69 67 65 63 61.5 62 61 59 58.5 58.5 57 56 56 55 54.5 54 54 54 54 53 53 53 53 52 52 52 52 52］

The finalstress distribution underoperating pressure 80 MPa is as follows:

Hoop stress σθ= ［39.51 129.54 129.14 129.18 129.70 129.40 129.13 129.93 129.53 129.50 129.76 129.60 129.71 129.75 129.40 129.61 129.57 129.38 129.54 129.92 129.40 129.26 129.65 129.89 129.01 129.20 129.05 129.19 129.33 129.46 129.25 129.35 129.45 129.55 129.65］.

Axial stress

σr= ［21.45 40.19 43.10 42.65 42.52 42.64 42.38 42.40 42.50 42.52 42.63 42.70 42.73 44.28 44.66 44.79 44.74 45.61 45.16 45.12 45.20 45.33 45.37 45.41 45.49 46.75 46.85 46.94 47.07 47.19 47.30 47.21 47.30 47.38 47.46］.

It can be seen that low stress on the liner and almost even strength on the ribbon wound layer are realized on the vessel.The stresses are shown in Fig.3.

Fig.3 Hoop stress distribution

2.2.4 Manufacturing process and test

This device is at present the highest steel ribbon wound pressure vessel in China and there are great improvements in both optimization and manufacturing.Supervision and test from the beginning to the end are carried out during the manufacturing process to make sure that the research target is achieved.Hydrostatic tests are carried outon the liner after itis manufactured.The steel ribbons are wound on the outside of the liner after it meets the requirements.The vessel with winding finished is shown in Fig.4.The ribbon wound machine is improved because the tension of every steel ribbon is strictly controlled to make sure the stress in the vessel distributed as design.

Fig.4 Photograph of the vessel after steel ribbon winding is finished

After the completion of manufacturing，a 100 MPa hydrostatic test is carried out to test the strength ensuring that the vessel is safe and reliable to use.The vessel with manufacturing completely done is shown in Fig.5.

Hydrogen resistance stainless steel material is adopted for liner and high strength and low price steel is selected for the remainder of the vessel as it cuts down on cost.Meanwhile，the cost of the vessel is greatly reduced because the weld is reduced by 50%.The structure has the function of anti burst explosion，and on-line safety monitoring is easily realized.The gaps between steel ribbons provide the leakage passage for hydrogen releasing in case of failure of the liner.The released hydrogen will be detected immediately by the sensor installed on the external protective layer，so there is enough time to take some safety measures.As aresult，explosion resistance and on-line safety monitoring are realized. Finally， the finalstress distribution of the vessel is optimized by adjusting the prestress and winding angle of the steel ribbons.The liner which is exposed to hydrogen is in a low stress states and the steel ribbon wound layer is under even strength state.This is reasonable for improving fatigue life and alleviating hydrogen stress corrosion of the vessel.It is difficult to realize these functions for other kind of structures of high pressure hydrogen storage vessel.

Fig.5 Photograph of the vessel after the manufacturing is finished

2.2.5 Realization of the control system

A pneumatic ball valve is adopted to control the charging and discharging process for the measurement control system of the 70 MPa hydrogen environmental fatigue test system.An industrial computer system is used to collect the data of the charging and discharging process.The leakage of hydrogen is monitored with the aid of a hydrogen concentration probe.A camera is used to monitor the situations of the testing field，and acoustic emission sensorisused to monitorthe performance of the cylinder during the test［13－15］.

Temperature data collected includes air source temperature of the high pressure hydrogen storage vessel，gas temperature in the hydrogen storage vessel，outer wall temperature of the liner and the hydrogen storage vessel and pressure data includes air source pressure of the high pressure hydrogen storage vessel，front and back pressure of charging and discharging orifice plate，gas pressure in the hydrogen storage vessel.Data collection of the hydrogen concentration probe is also included.Individual collectingand processing devices are adopted forthe acoustic emission detection instruments and endoscope.The data measurement and control system for test are managed with the aid of a commercial software package LabView as shown in Fig.6.The control system has a great relevance to the quick charging and discharging fatigue test system.It can be used in the measure and control system for hydrogen fueling station in the future after modification.It has important significance for developing our own hydrogenation equipment.

Fig.6 Control system window

2.2.6 Technical parameters of the hydrogen environmental fatigue test system

The 70 MPa hydrogen environmental fatigue test system is installed according to the working principle sketch as shown in Fig.1.This fatigue test system is installed at the Institute of Beijing Aeronautic and Astronautic Testing Technology.Several70 MPa sample vesselshave been tested underhydrogen environment.One is shown in Fig.7.The parameters of this system are as follows:

Fig.7 Installation photograph of the system

①This system can provide 5 cylinders to do the hydrogen environmental fatigue test at the same time as long as the total volume of the 5 cylinders is less than 100 liter.The charging and discharging of hydrogen cycles are done every 10 min and the leakage of hydrogen is less than 0.5%.

②The highest testing pressure of the hydrogen environmental fatigue test system is 70 MPa and the highest speed of quick charging and discharging of hydrogen is greater than 3.24 kg/min.

③ Automatic controland manualcontrolis realized in this fatigue test system.The hydrogen surrounding area is separated from man operating area by thick wall.Safety protections measure against explosion is done at the hydrogen surrounded area to safeguard workers and facilities.

3 Fatigue Test under Hydrogen Environment

The 70 MPa Fatigue test is done on the hydrogen environmental fatigue test system and the test process is as shown in Fig.8.The frequency of the fatigue test is shown in Fig.9.The maximum gas filling speed and the allowable rising temperature within 100℃for the sample vessel are taken into consideration.The cycle number is 500 times.This number is determined according to the hydrogen application code proposed by experts of the science and technology ministry in China.The performance of the sample vessel is checked after 500 times of cycles by Zhejiang Special Equipment Inspection and Research Institute.The check report shows that its pressure bearing capacity still reached good quality according to the code of China after the hydrogen environmental fatigue cycle［5］.

Fig.8 Fatigue test of transportable hydrogen storage vessel

Fig.9 Frequency plot of the fatigue test

4 Conclusions

Hydraulic test is used in traditional pressure vessel because of its safety and low cost.With the research need of the hydrogen energy and safety need of the onboard high pressure hydrogen storage vessel，the real hydrogen environmental fatigue test system is very necessary，so the 70 MPa hydrogen environment fatigue test system is developed with the support of National High Technology Development 863 Program of China.

Through the development of 70 MPa hydrogen environmental fatigue test system，a fatigue test system is finally established，which can be used for any kind of cylinders so long as their volume is less than 100 liter.An experiment of 30 liter，70 MPa composite hydrogen pressure vessel is done on this fatigue test system.The performance test of the sample vessel after 500 times cycles shows that it still meets the codes of China.

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