Evaluation Of Accident Frequency For

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It is estimated that over 600 million passenger cars travel the streets and roads of the world today which is around 87 of the total vehicles in the world; this gives an idea that there are about 1.12 billion vehicles in the world. It gives a ratio approximately 1:6 with the world's population which is nearing 7 billion [11]. Thus they produce serious hazards to the common public worldwide. This hazard converts to risk with accidents on road.

Canadian Institute for Health Information (CIHI) reports that 48 of the severe injuries are due to motor vehicle collisions [1]. As per data available by National Highway Safety Traffic Administration (NHTSA), in 2007, over 0.5 of the total vehicle crashes resulted in fatality and over 28 resulted in nonfatal injuries [2]. Transport Statistics - Great Britain: 2009 states from Department of Transport (DFT) 2,538 fatalities among 171,000 traffic accidents and 228,000 casualties in year 2008 due to it [3], while Australian Bureau of Statistics (ABS) in their report "Measures of Australia's Progress - transport" writes 1,621 fatalities in year 2003[4]. These fatalities are mainly due to crushed by, struck by, overturning and fire and explosion. Fire and explosion in the vehicle after accident causes maximum number of fatalities.

Introduction of new technology in transportation industry will have a great impact on the safety of people. Thus efforts are made by many countries and independent organizations to demonstrate safety of hydrogen technology vehicles. Special projects are undertaken all around the world to fulfil the need of necessary standards and legislations for safe introduction of hydrogen vehicles. For many years various pilot projects are accomplishing their tasks to test pros and cons of this clean technology.

When it comes to use hydrogen in vehicles, it is of utmost importance to quantify risk. Brief useful properties of hydrogen.. .. . . . . . .

There is a need to find out impact of technology change to society. To estimate this change accident severity and frequency is used as a tool and the project was selected under supervision of DNV, Aberdeen.

This report briefs important properties of hydrogen as vehicular fuel; tries to compare it with hydrocarbon fuels like diesel, petrol, CNG, LPG etc. and estimates the change in accident severity with hydrogen as vehicular fuel. I believe that this report will help, to readers, to give initial knowledge of hydrogen technology and its impact on common public.

This section provides briefs about project and describes outline of dissertation.

Background

Accident frequency and severity rate have been treated as measures of safety performance. Almost all government and many non-government bodies have dedicated staff to prepare and maintain these databases. They collect all the data related to accidents and provide an input to various studies for research.

This report uses data from various sources, namely Australian Bureau of Statistics (ABS), Department for Transport (DFT), National Highway Traffic Safety Administration (NHTSA) etc., to find present status of accidents in transportation industry, by calculating accident frequency and severity of accidents in vehicles. An effort has been made to estimate accident frequency and severity for use of hydrogen, as potential alternative fuel, with present automobile technologies. This will give a safety performance measure of hydrogen technology in current scenario. It is expected that it will provide a baseline measure for further improvements to use hydrogen as vehicular fuel.

Objectives and Scope of work

Objectives

Project will try to state and investigate issues with hydrogen stored as vehicular fuel instead of conventional fuels (Diesel, Petrol, CNG, LPG etc.). Objectives are:

To calculate and analyse accident frequency for existing conventional fuelled vehicles.

To study severity of impact when using Hydrogen as fuel.

To study consequences of fire/explosion resulting from Hydrogen leak due to impact, and compare with conventional fuelled vehicles.

To quantify the increase for Hydrogen as a vehicular fuel.

Scope of Work

The scope of this project is as follows:

Calculation of current vehicular accident frequency

This involves calculation of accident frequency and severity rate for light motor vehicles (e.g. Car, Matador, public road transport vehicles etc.). Collision of mentioned vehicles is considered. Individual leakage incidents are not considered for calculations.

Impact analysis for storage tanks containing Hydrogen.

Focus is to find the conditions (speed and weight of vehicle) of collision between two vehicles sufficient to damage fuel tank containing Hydrogen fuel

To find potential hydrogen release rate from fuel tank in case of collision and frequency of release.

To study incident scenario for fuel release and

To calculate severity and frequency of fire/explosion in hydrogen vehicle accidents.

Limitations and Assumptions of Study

To achieve realistic results availability of primary data is important. Approachability to detailed databases that can provide critical details of accident is difficult. Being a new industry data available publicly for hydrogen vehicle is negligible. Collision analysis data is also very difficult to obtain.

The hydrogen database is based on accident frequency for existing petrol/ diesel vehicles. . . . . .

Methodology

Demonstration of safety aspects of systems is very important. Accident severity is a negative indicator of safety performance. Positive indicators give an estimate of performance that can be achieved with certain safety measures, while accident frequency and severity rate gives actual status of how that particular system has performed in the past. It indirectly includes safety lapses, system failure, and human mistakes.

In this report based on data, for UK and US road traffic accidents, collected from [2] and [3] accident frequency of light motor vehicle is calculated. Serious and slight injury rate is also calculated for the same. It can be assumed that frequency of slight injuries is equivalent to minor accident. Also serious injuries can represent major accident. This accident frequency and severity rate remains unchanged with the introduction of hydrogen as onboard fuel. Then an estimate of collision impact energy is made. "Accident Impact Calculator" [16] is used to calculate the impact force and dissipated impact energy. Based on speed and weight of vehicle impact energies are sorted to represent serious and slight injuries. Scenarios with higher impact energy then needed to damage hydrogen fuel storage tank are sorted. Based on injury rates frequency of fuel tank damage is calculated and then scene is escalated to fire and explosion. Based on these calculations and hydrogen properties expected fire and explosion frequency is estimated.

Outline (0.5) chapters described

Chapter 1 gives introduction to the subject under study

Chapter 2 reviews some literature related to the problem that mainly includes hydrogen hydrocarbon fires in vehicles, QRA for various accident scenarios (PhD Thesis),

Review of Literature

Recent Projects

In 1981, Peter Hoffman, in his book, The Forever Fuel, the story of Hydrogen, stated that, serious work on hydrogen vehicles began in 1930s. Rudolph Erren converted over 1000 vehicles to hydrogen and hydrogen/gasoline in England and Germany [12]. Now there are many development programs in Europe, UK, USA, Canada, Japan and Russia.

In Europe under European Union (EU) Framework Programme 5 and 6 (FP5 and FP6) many of the projects were aimed on hydrogen technology. These programmes were aimed to strengthen the scientific technologies and technological bases of Industry [23].

Based on recommendations from Vision report of the High Level Group on hydrogen fuel cells European Commission (EC) established the European Hydrogen and Fuel Cell Technology Platform. Its aim was to accelerate the development and deployment of hydrogen and fuel cell technologies in Europe. These technologies will be cost-competitive and world class and develop fuel cell based energy systems and component technologies for applications in transport, stationary and portable power [23]. Some of them are briefed below.

Safety of Hydrogen as an Energy Carrier (HySafe)

Under the European Union's 6th Framework Programme, a Network of Excellence project Safety of Hydrogen as an Energy Carrier (HySafe) was established and defined. It is one of the projects contributing to the safe transition to the more sustainable hydrogen technology and its application. The project is to develop methodologies and collecting data for risk management of hydrogen infrastructure [9].

For successful public acceptance of a new technology a trust worthy demonstration of safety is required. This demonstration has to be based on widely known facts and collection of data containing accident and incident information. This data can be utilised for risk assessment of hydrogen applications [9].

Risk assessment has been an important tool in many industries for decades. Introduction of risk assessment technologies for development of new industries is very important. It provides sound basis for creating awareness about existing potential hazards and risks to make decision for improvement and reduction in risk in the new technology. It is very important to demonstrate that all safety aspects related to new technology are controlled to avoid unacceptable risk to society. This requires adoption of existing methods and standards to the specific applications [9].

DNV has worked as a lead participant in Work Package 12 (WP 12) of the FP 6 that studies risk analysis methodology and acceptance criteria. The study concluded with harmonization of determination of hazardous zones. It suggests that "ATEX 199/92/EC" should be the basis for this and "EN 60070-10" shall be used to develop methodology which is another task under WP12. It also suggests that Seveso II 1996/82 will be a basis related to legal frame work for decision of safety distances [24].

Hydrogen Incident and Accident Database

In Hysafe, a specific Work Package was devoted to database development, namely WP5 - Hydrogen Incident and Accident Database (HIAD).This data base is aimed to serve as a data source for doing risk assessment and revel trends. The data base is not limited to accidents and incidents, but also includes hazardous situations and near-misses. It is expected that it should contain all hydrogen releases irrespective of size/volume even if not ignited [9].

HIAD is planned to serve as a common format for data collection, high quality information about accidents in hydrogen industry. When fully operable, it will be an important source for hazard identification, estimation of probabilities, consequences and to propose risk reduction measures [9].

European Integrated Hydrogen Project

It is one of the most important projects for building database of existing codes of practice, standards and regulations applicable to use of hydrogen in vehicles.

The project (EIHP - Phase I) has worked in eight phases. The phases were:

Phase 1: Survey and analysis of rules, regulations and licensing procedures.

Phase 2: Analysis of existing and planned Hydrogen safety concepts and technologies

Phase 3: Identify readiness of rules and regulations ready for harmonization

Phase 4: Identify deficiencies in rules and regulations

Phase 5: Identify safety concept and technological deficiencies

Phase 6: Propose investigations to create a basis for standardization

Phase 7: Safety concepts proposal

Phase 8: Proposal for pre-normative rules

The project was conducted over two years (1998 to 2000). The key results of this project are:

Two draft regulations for the approval of hydrogen fuelled road vehicles were finalised. These drafts were under discussion within ECE. Also promoted these drafts as construction codes and to improve them with technical development of LH2 systems.

It gave guidelines for step by step development of regulations for hydrogen infrastructure.

A table of relevant regulation is reproduced as Appendix C

The European Thematic Network on Hydrogen Energy (HyNet)

HyNet was founded under the 5th Framework programme of the European Commission. A major task of this network was to develop roadmap for European Hydrogen Energy infrastructure. An assessment of socio-economic and political issues associated with development towards hydrogen based energy future was carried out. It proposed large demonstration activities and joint projects to act in response to the concerns of commission for bundling the European efforts on hydrogen energy [19].

HyNet was active from 2001 - 2004. Later many of its activities were continued under European Hydrogen and Fuel Cell Technology Platform knows as The Fuel Cell and Hydrogen Joint Undertaking (FCH JU) [19].

Project's approach was to consult Europe's key stakeholders over one year period through a hydrogen review work shop and gather their views. In the main workshop key data on hydrogen technologies was collect and harmonised. This data was produces as a matrix which is published in the referred article [25].

The Fuel Cell and Hydrogen Joint Undertaking (FCH JU)

It is a public-private partnership supporting research, development and other demonstration activities in fuel cell and hydrogen energy technologies in Europe. It aims to introduce, a low/negligible carbon emission technology, hydrogen technology. In this three members pool resources and plan activities for commercial deployment of hydrogen technologies. Its objectives are to reduce time to introduce these technologies, to deliver robust hydrogen supply and fuel cell technologies developed. In automotive sector it aims to enable industry to take the large scale commercialisation decision to substantial growth in the time frame 2015-20 [20]

An Integrated Project to Develop the European Hydrogen Energy Roadmap (HyWays)

HyWays was co-funded by research institutes, industry and by European Commission (EC) under the 6th Framework Programme. The project started in 2004 and completed in 2007. In this the members stated specific results for green house emissions, target hydrogen production and infrastructure technologies. Further supply technologies and end-use technologies are integrated into proposal for EU Hydrogen Energy Roadmap. These results were given for the timeframes 2020, 2030, and 2050.

Naturalhy

This project was also supported by EC. It is set up to investigate possibilities of delivery of hydrogen by using the existing natural gas network. This involved feasibility studies and the consequences and benefit analysis of using natural gas network for transport of hydrogen safely and efficiently. The project demonstrated capabilities of NG network for hydrogen delivery, which may be a major contribution towards development of hydrogen economy [26].

The project stated that depending of the type of steel to construct high pressure pipelines, a mixture of up to 50 hydrogen can be used. It suggests that a mixture of up to 20 hydrogen in natural gas will not significantly affect the safety during transport, and for 50 concentration feasibility must be assessed case by case [26].

Roads2HyCom

It is a partnership of 29 organizations supported by EC's FP 6. It studied technical and socio-economical aspects associated with fuel cell and hydrogen energy economy. This project provided support information and feedback to EC and HFP, and various other hydrogen programmes. The study aspects include -

Characterisation and mapping of European technology developers.

Creation of an online encyclopaedia combining numeric data of key technology metrics with recent advances.

Analyzing technical feedback from public demonstration projects,

Analysis and projection of future performance of technology applications, commercial viability analysis against a backdrop of rising energy and raw material prices

Deriving strategic recommendations for the research agenda etc

The final report says "Road transport is the most technically challenging application, but the latest generation vehicles are realising the efficiencies that the fuel cell has always promised. Sustained research effort on cost reduction, durability and on-board hydrogen storage remains vital to realise the great economic and environmental potential in this sector."[27]

Clean Urban Transport for Europe (CUTE)

It was a European project aimed to demonstrate feasibility of clean urban public transport. It was sponsored by EC FP6 an also involved several industrial, government and academic partners and transport operators. This transport is innovative and highly efficient. It used hydrogen as vehicle fuel to run 27 public transport fuel cell buses in nine cities across Europe. Different hydrogen production and refuelling infrastructure was established in each of the cities. It observed the practical application of renewable energy sources to transportation system.

It is expected that this transportation system will contribute to reduce overall CO2 emissions. It will also contribute for elimination of NOx SO2 and particulate emissions. It greatly improved public acceptance of hydrogen fuel cell transport system and contribute to diversifying guaranteeing more secure energy supply.

This project was officially closed on March 2006 with HyFLEET:CUTE as a ongoing project. Both projects have objective of developing and demonstrating hydrogen based public transport system.

Evaluation of Accident Frequency in Hydrogen Vehicles

Vehicles have an important place in our day to day life. One cannot imagine an easy life without them. Right from the invention of wheel and then vehicles we are using them for transportation. Since the age of bullock carts humans are developing vehicles for easy and fast transportation. Internal Combustion engines (ICE) played an important role in our development. However, every coin has two sides, on one side we developed and progressed; on the other side this development and race to progress created hazards and risks. Inventions gave us speed and comfort to travel. However, accidents cannot be prevented completely. Accident causes property damage, injuries and fatalities.

Safety Performance Indicators play an important role to identify deficiencies in the level of safety. They are the outcome of analysis of safety activities or safety breechings. They can guide progress of technology towards a safe system [5].

Accident frequency and severity rate are the two important safety performance indicators. They are the indicators of actual deficiencies in the system. These deficiencies may be related to human factors, environmental conditions, integrity of system or escalation of accident.

Vehicle accidents and severity

Accident frequency is often calculated as number of accidents per million man hours worked. While in transport industry it is calculate as number of accidents per km of vehicle travel. Severity rate can be calculated based on severity of injuries (sever / slight injury) per vehicle km. Following formula is used to calculate the accident frequency and severity rate.

. . . Equation Accident frequency

. . . Equation sever injury rate

As stated earlier, data was collected from DFT, UK about accidents over a range of 12 years (1997 to 2008). This data is reproduced in the following table.

Table : Reported Road Accidents,

Source [3]

Year

Number of Accidents

Number of Fatalities

Number of Serious Injuries

Number of Slight

Injuries

Number of casualties

Total vehicle km

1997

2,40,000

1,934

43,000

2,81,000

325934

4.54E+11

1998

2,39,000

1,859

41,000

2,81,000

323859

4.62E+11

1999

2,35,000

1,834

39,000

2,78,000

318834

4.71E+11

2000

2,34,000

1,820

38,000

2,79,000

318820

4.71E+11

2001

2,29,000

1,903

37,000

2,73,000

311903

4.79E+11

2002

2,22,000

1,917

36,000

2,63,000

300917

4.91E+11

2003

2,14,000

1,927

34,000

2,53,000

288927

4.95E+11

2004

2,07,000

1,831

31,000

2,46,000

278831

5.03E+11

2005

1,99,000

1,813

29,000

2,39,000

269813

5.04E+11

2006

1,89,000

1,752

29,000

2,27,000

257752

5.12E+11

2007

1,82,000

1,576

28,000

2,17,000

246576

5.17E+11

2008

1,71,000

1,358

26,000

2,02,000

229358

5.14E+11

It can be considered that, sever the accident serious will be the injuries hence it can be assumed that fatality and serious injuries corresponds to sever accidents. Similarly, slight injuries will correspond to slight accidents.

Graph : Accident and Injuries per vehicle km

The above graph shows that slight injury rate is more than the corresponding accident rate. It is because; an accident may produce more than one slight injury. It should be noted that although slight injury rate is more than accident rate, the ratio of slight to total accident rate is almost constant. Calculations to find probability of sever accidents slight injury rate can be used.

Above data suggests an accident frequency of 4.4E-07 accidents per vehicle km over the period of 12 years. It is found that ratio of sever to slight accidents ranges in between 1:7 to 1:8. It can be said that probability of sever accident is 0.13 when an accident takes place.

Now as the accidents largely depend on road conditions, traffic density, and driving habits, while, type of vehicle has very insignificant effect for accident to take place. It can be assumed that, for the same scenario, hydrogen vehicles will also have the same accident frequency for the baseline. However, type of fuel used will also have an effect on severity and frequency of accidents. This effect is discussed in section 4.3.

The above calculated accident frequency can be compared to a similar work done by Rosyid [17]. He calculated an accident frequency for three states in USA as 3.0E-06. Considering traffic density, volume of vehicles and historical data for USA and the above calculated vehicle density can be justified.

Probability of Hydrogen Leak

In Hydrogen vehicles additional safety precautions are designed. The only accident scenario causing hydrogen leakage is storage tank failure. According to existing manufacturer specifications for hydrogen fuelled vehicles, the fuel system includes hydrogen sensors. These sensors activate solenoids in the hydrogen tank and computer programming to shut off fuel supply. This happens if the fuel flow defers by a predetermined value. Swain says that, "For hydrogen to leak under a hydrogen-powered vehicle in amounts that would produce a severe accident four failures, must occur." [30]

They are:

Sealing failure in fuel system.

Failure of Hydrogen sensor shutoff system.

Excess flow valve on hydrogen tanks must fail.

Failure of hydrogen flow sensing computer programs analysing hydrogen flow to hydrogen consumption of the fuel cell.

Other failures will not cause system failure enough to produce a severe hydrogen leak in the vehicle [30].

Now we need to analyse cases in which there will be an impact on fuel tank.

Position of fuel tank

Fuel tanks are designed to mount on the rear or sides. In most of the hydrogen vehicles, storage tanks are positioned at rear of vehicle. For the example case a diagram of BMW is shown below.

Fuel_Tank_Possition.jpg

Figure : Fuel Tank Position in BMW 7 Series

Source: [29]

Impact to fuel tank may occur when a vehicle accident takes place. In next section different scenarios of vehicle collision are studied.

Collision Analysis

A collision can take place in two ways.

Vehicle hits a stationary object

In this case damage to fuel tank is very low in case. A vehicle - stationary object collision should be strong enough to damage the vehicle frame and than the fuel tank. This can happen in two ways.

Vehicle hits stationary object, or

Two vehicles collision when one of them is stationary.

Probability of fuel tank damage will be more if a moving vehicle hits a stationary vehicle in rear, however, a vehicle collision to a stationary object damaging fuel tank will be negligible.

Two vehicles can collide in three possible ways

Front - Front (Head on) collision

In this case the two vehicles collides front on front. The vehicles must be moving in opposite direction. It increases their relative speed and gives higher collision energy for a given set of mass and speeds. These type of collisions are highly destructive

Front - Rear Collision

In this type of collision, probability of damage to fuel tank will be lower than in the a(ii) above. As two vehicles will be moving in same direction, their relative velocities and thus energy of impact will be lower.

Front - side on collision

It's the most frequent type of vehicle collision. However, fuel tank damage can happen if the impact is very high in magnitude.

The following figure shows the crash zones of a BMW 7 series car as shown by Vernier [28].

Primary_Crash_Zones[1].jpg

Figure : Primary Crash Zones

Taken from [28]

In figure 2 the possible crash zones are shown with their probability. Position of fuel tank as in BMW 7 series hydrogen cars is clear from figure 2. Comparing above figures, it is clear that the said fuel tank is placed in a 1 to 5 zone. This suggests that probability of fuel tank damage can be taken as 0.05. the probability of leakages given damage to fuel tank can be taken as 0.5.

Hydrogen

Hydrogen is simplest atom and has three isotopes, namely Hydrogen, Deuterium, and Tritium. Hydrogen atoms exist in certain conditions. It is normally available in pure form as H2 molecule. It is lightest among all the gases. At NTP it is a colourless, odourless, test less, non poisonous, flammable gas. Its density is 70.6 kg/m3 at -2620C and remains in solid state. Its density is 0.898 kg/m3 at 00C and 1 atm and is gaseous [17].

It is known to be the most abundantly available element in the universe. From water (H2O) to hydrocarbons (CnHm) and almost everything contains some form of hydrogen associate in it; while there is hardly any hydrogen available in molecular form by any natural resource.

Fossil fuels are limited and their making needs hundreds of years. They will be exhausted after some years. To support economy and maintain a secure energy supply there is a need of an alternative fuel that can replace the fossil fuels over time. Using fossil fuel causes environmental damage thus there is a need of renewable and clean energy source. To be precise, we need an energy carrier that can carry the available renewable energy (solar, wind, hydro etc.) to the point of application or use.

In 1875, the famous French writer, Jules Vernes, in his book "L'Ile Mystérieuse", projected that water would replace coal as energy "source"; as water being split into its constituents hydrogen and oxygen to supply endless electricity and heat. A century later, in 1976, the famous Club of Rome in its 3rd report was considering that "at the eve of the 21st century, we should consider the combination of 2 energy carriers of equal importance, electricity and hydrogen, which is cheap and available in huge quantities from nuclear or solar energy" [13].

We are familiar with electricity as the energy carrier. For generations it has been used for daily activities (cooking, heating etc.). Now a days there are many vehicles as well that use electricity to run the traction motor. But use of electricity in automobile is limited. For last 50 years electricity has been the fastest growing source of energy. It is highly flexible, efficient and pollution less at the point of end use. Although during production it has serious concerns of pollution, and supply of primary energy source, if produced from fossils. In past, studies have shown that hydrogen can show the same characteristics of an energy carrier. [6]

Like electricity hydrogen is an energy carrier. It is clean and renewable. It is not a primary energy source as we have to produce it. It can be produced from fossils (solid, liquid or gases) and also from abundantly available and renewable water by various methods [6]. Some methods for production and important properties of hydrogen are mentioned in Appendix A.

In this section properties of hydrogen important in relation to fuel tank leakage are discussed. Other properties are discussed in relevant sections ahead and some introductory properties in Appendix B.

Hydrogen and hydrocarbon properties in relation to hydrogen leakage,

Table Comparison of hydrogen and Natural Gas properties

Property

Hydrogen (H2)

Natural Gas (CH4)

Specific Gravity (Air = 1)

0.0696

0.641

Relative Orifice Capacity (J)

0.93

1.0

Viscosity at 00C, 1 atm (Pa.S)

8.42x10-6

10-6

Molecular Speed at 00, m/s

1692

600

Specific Volume (lt/kg)

14.3

1.33

Hydrogen has very (probably lowest) density. Along with its high diffusivity and buoyancy it tends disperse more rapidly than natural gas. The low viscosity and small molecular size of hydrogen may give it a greater propensity to leak than natural gas. For a given pressure and hole size, hydrogen will leak approximately 2.8 times faster than natural gas on a volumetric basis. In low momentum hydrogen leaks buoyancy affect gas dispersion and mixing in air more significantly. While for high momentum leaks, direction of release will determine gas movement with least significance to buoyancy.

Ignition and Explosion

Ignition occurs when three elements of combustion, namely fuel, air, and ignition energy (e.g. Spark, flame, etc) come together in definite proportion. Combustion starts immediately and sustains until this proportion is disturbed or one of the elements is eliminated completely. Minimum ignition energy and flammable fuel - air mixture are two important elements. These elements are discussed below.

Ignition energy, flammability range and leakage rate can be considered as key parameters influencing the ignition probability in accidental release conditions. Furthermore, the low viscosity and small molecular size of hydrogen may give it a greater propensity to leak than natural gas.

Hydrogen - Air mixture is flammable over a wide range (4 to 75 by volume) of hydrogen concentration. Here 4 is the Lower Flammability Limit (LFL) [1] and 75 is the Upper Flammability Limit (UFL) [2] . Hydrogen burns with no visible flame in day light. It is difficult to identify hydrogen flame than natural gas flame which burns with blue-yellow flame. It can lead to deflagration and detonation as it nears stoichiometric mixture. Ignition energy of hydrogen near its stoichiometric mixture can be as low as 0.017 mJ while at lower concentrations it will be higher 10 mJ for 4 [9]. It is more vulnerable to catch fire or explode then other flammable gases. It has low ignition energy thus explosion sets off more easily. Moreover, with three time's greater escape velocity than natural gas, it reaches its lower flammability limit about four times faster than natural gas. Also it has very high upper flammability limit so it needs 1.6 times longer than natural gas. Depending on the flammable mixture hydrogen has 6-100 times more propagation velocity than natural gas which increases the probability of detonation in hydrogen mixture. When hydrogen escapes from orifice it gets hotter increasing probability of spontaneous ignition [15].

Table Ignition properties of Fuels

Properties

Hydrogen

Methane

Propane

Gasoline

LFL - UFL (vol )

4 - 75

5.3 - 15

2.1 - 9.5

1 - 7.8

Stoichiometric concentration with air (vol )

29.5

9.5

4.1

1.8

Minimum ignition energy (mJ)

0.019

0.29

0.26

0.24

LDL - UDL (vol )

11/18 - 59

6.3 - 13.5

3.1 - 7

1.1 - 3.3

On the other hand low energy per unit volume of hydrogen may compensate its high escape velocity by bringing with it about 0.93 times lesser energy per unit time as compared to natural gas. Also hydrogen explosion at its lower flammability limit (4 vol) gives one fourth of energy by methane explosion at 5 vol. Hydrogen has very low density, which, with its high diffusivity enables it to rise faster and higher in air than natural gas. Also hydrocarbons like petrol and propane are heavier than air. Their vapours remain near the site of release increasing likelihood of explosion while probability of hydrogen explosion is less as hydrogen dissipates quickly [15].

Flammability limits

Graph MIE of Methane and Hydrogen

Source [33]

Ignition of a combustible gas or vapour - air mixture may occur in two ways. First, the energy for ignition is supplied by a local source such as a spark or small flame at a point within the mixture as presented in Error: Reference source not found(a). Second, the bulk gas mixture is heated up to its auto - ignition temperature as shown in Error: Reference source not found (b).

Cloud heated to auto-gnition temperature

(b)

Ignition source

(spark /

small flame)

(a)

(a) (b)

Figure Ignition of flammable cloud (a) by Spark (b) by auto ignition

The probability (PI) of ignition of gas is dependent on the probability (Q) of a sufficiently strong ignition source being present and the probability (Pv) of this ignition source being exposed to the gas concentration within the flammable limits in the area of leak. Now the probability of ignition of gas can be given by:

The above formula is used by Funnemark [9] for analysis of 72 hydrogen leaks reported in EIGA database showed that 49 of the reported leaks were ignited and 43 of ignited leaks (21 of total leaks) resulted in explosion forming a blast wave [9]. A study by Rosyid, 2006 says that 30.7 of the accidents under study resulted in fire. 20 resulted in explosion. 4 resulted in both fire and explosion, rest of 45.3 accidents include un-ignited hydrogen release [17].

Based on the above two studies we can consider an ignition probability of 0.40. This can be stated as 40 of the hydrogen leaks will be ignited.

Event Tree Analysis

Petrol/diesel vehicles...

Accident Major Damage Fuel Fire /Explosion

Frequency Accident to fuel tank Leak

0.03

0.50

0.05 0.97

0.13 0.50

0.95

4.4E-07 0.03

0.50

0.05 0.97

0.87 0.50

0.95

In the above event tree Accident frequency is as calculated in section 4.1. Ratio of Major to minor accident is calculated and based on this probability of a major accident is calculated. Probability of damage to fuel tank is taken from section 4.2.2 and it is considered that damage to fuel tank, sufficient to affect its integrity take place with a probability of 50. Probability for fire or explosion is 3 for hydrocarbons [31].

Hydrogen Vehicles

Accident Major Damage Fuel Fire /Explosion

Frequency Accident to fuel tank Leak

0.40

0.5

0.05 0.60

0.13 0.50

0.95

4.4E-07 0.40

0.50

0.05 0.60

0.87 0.50

0.95

As discussed earlier, accident frequency for hydrogen vehicles is considered same as that of present vehicles. Also ratio of Major to minor accidents, probability of damage to fuel tank, and leakage from tank is same. How ever due to MIE of hydrogen and based on historical data studied by [9, 27] this probability is estimated to be 40.

Event tree calculations are detailed in Table 4 and 5

Table Event tree output for Petrol /Diesel Vehicles

Accident Frequency

Major

damage

Leak

Fire/Explosion

Output

yes

yes

Yes

yes

4.40E-07

0.13

0.05

0.5

0.03

4.29E-11

yes

yes

yes

No

4.40E-07

0.13

0.05

0.5

0.97

1.38E-09

yes

Yes

No

-

4.40E-07

0.13

0.05

0.5

-

1.43E-09

Yes

No

-

-

4.40E-07

0.13

0.95

-

-

5.43E-08

No

yes

Yes

yes

4.40E-07

0.87

0.05

0.5

0.03

2.871E-10

No

yes

yes

No

4.40E-07

0.87

0.05

0.5

0.97

9.28E-09

No

Yes

No

-

4.40E-07

0.87

0.05

0.5

-

9.57E-09

No

No

-

-

4.40E-07

0.87

0.95

-

-

3.64E-07

From above calculations it is clear that frequency of fire and explosion after accident of a vehicle using diesel, petrol or CNG is about 3.3E-10. It is the sum of fire/explosions resulting from Major and minor accidents. In comparison to accidents, frequency of fire and explosion in diesel / petrol fuelled vehicles is very low. These fuels are liquid at NTP. In the event of an accident the likely case after fuel tank damage is leakage and then formation of pool. As these fuels are stored almost at atmospheric pressure (may be slightly above it, due to presence of vapours inside the tank) there will be a negligible chance of rupture of fuel tank, after impact. This eliminates generation of extra energy, and hence extra ignition sources at the accident site. This reduces chances of fire /explosion further.

Table Event Tree Out put for Hydrogen vehicles

Accident Frequency

Major

Damage

Leak

Fire/Explosion

Output

Yes

yes

Yes

yes

4.40E-07

0.13

0.05

0.5

0.4

5.72E-10

Yes

yes

yes

No

4.40E-07

0.13

0.05

0.5

0.6

8.58E-10

yes

Yes

No

-

4.40E-07

0.13

0.05

0.5

-

1.43E-09

Yes

No

-

-

4.40E-07

0.13

0.95

-

-

5.43E-08

No

yes

Yes

yes

4.40E-07

0.87

0.05

0.5

0.4

3.82E-09

No

yes

Yes

No

4.40E-07

0.87

0.05

0.5

0.6

5.74E-09

No

Yes

No

-

4.40E-07

0.87

0.05

0.5

-

9.57E-09

No

No

-

-

4.40E-07

0.87

0.95

-

-

3.64E-07

For Hydrogen vehicles the final frequency of fire and explosions is found to be as 4.4E-09. It is much higher than found for diesel fuelled vehicles. We know that hydrogen stored on board in pressurised (up to 700 bars) cylinders or tanks. The pressure inside the tank makes it more vulnerable to bursting and rupture of tank in the event of impact due to collision. This imposes additional hazard apart from spillage and fire. Pressurised tanks if heated may explode due to over pressure. This generates additional ignition sources that increase probability of ignition of hydrogen.

Result (accident frequency and severity for use of hydrogen as vehicle fuel)

Hydrogen Accidents

AMC Jeep Tanker accident.

In 1975, AMC-Jeep (the UCLA vehicle) met with an accident. The vehicle was carrying liquid hydrogen in a spherical shaped fuel tank (VLH-50G-dewar), which was manufactured by Minnesota Valley Engineering. Designed for automotive application, this liquid hydrogen carrying vehicle was the first of its kinds. 3003 Aluminum was used to make the inner and outer spheres. This type of Aluminum contains 1.2 Mn and 98.8 Al. A neck made up of Gll glassfiber epoxy was used to suspend the inner sphere from the outer one. The bottom has support in the form of two G11 cylinders. One cylinder was attached to the outer sphere and the other to the inner one. The cylinders were designed in a way that could only come in contact with each other when horizontal loading was taking place in the vehicle. The supports were able to resist 18-20g of load in either direction. Horizontal loading could fail due to cracking of neck support and vertical loading due to the inner shell sinking at the point of attachment of the neck. When the accident occurred the vehicle was chained at one place and loaded on a trailer (tandem-axle). 190 1 of liquid hydrogen was filled in the vehicle at a bar pressure of 2. There was blowout in one of the vehicle's tires and as a result the vehicle went out of phase at 55mph and so did the trailer. The trailer tongue broke down when the trailer took a side turn at 20mph. As a result towing vehicle, LH2 and trailer went upside down. However no one was injured and no damage was observed in the fuel system as well. There was a sharp increase in pressure due to liquid nitrogen coming in contact with the top warm part of the vessel. The manual valve was opened repeatedly in order to bring down the pressure. The liquid hydrogen car was turned back again, in the upright position after the trailer was unchained from it. To reduce the pressure in the Dewar vessel, it was vented for 30-45s. A thorough inspection was done and it was decided that the vehicle would run on hydrogen and taken back to ULCA. The hydrogen Dewar vessel was not majorly damaged though the body of the car was damaged at many places. The repairing required a new roof, right door, left backup lens and right front fender. The evidence shows that the first liquid hydrogen vehicle accident didn't cause any serious damage or injuries. The accident was less severe and dangerous than what would have happened had a gasoline fueled vehicle was involved.

The Stockholm hydrogen accident in 1983

The Ostermalm district in Stockholm, Sweden witnessed a hydrogen gas explosion on 3 March 1983. The accident occurred in early morning when industrial gases were being delivered by an open truck in Stockholm. The truck was delivering argon bottles to a nearby laboratory when the explosion took place. A hissing sound was first noticed by the truck driver while offloading the argon bottles. The driver took immediate action and went on to check the source of the sound. However it was too late and that is when the accident occurred. The truck carried 18 interconnected vessels which had 180Nm3 of hydrogen stored in them. The industrial pressure in the vessels was 50 l and the explosion occurred due to leakage from two adjacent vessels. The cylinders in the rack were not secured by using the shut-off valves. The report mentioned the leakage of only one connection however the photograph clearly showed that there were two broken connections. The hydrogen source after the accident was consisted of 18 interconnected vessels at a pressure of 200 bars. The 2 line breakage resulted in 4 jets and that has been demonstrated with the help of arrows. One jet is parallel to the Z-axis and the remaining three to the Y-axis. The residential area in Stockholm where the accident took place has the usual 5-6 floors buildings. The explosion site was near one such office building. Brahegatan has 2.0m wide pavements with a 10m wide carriageway on either side. The street had parking spaces on either side.

The report highlighted the consequences resulting from the explosion. 16 people were injured and it also damaged 10 vehicles. The nearby building's windows were broken and it was severely affected. The damage took place due to an overpressure of 5 kPa which was generated during the accident and whose effects were felt till 90m from the explosion site. The explosion resulted in burning of 10 flammable mixture. The following figures from report give an idea of hydrogen cloud formed with regions of particular flammable mixture present at the site.

Figure Wind and Gas cloud profile over ground

Figure 4 shows arrangement of buildings at the site and wind velocity (1 m/s) in figure 5 dispersion pattern on surface is shown.

Figure H2 concentration fields over ground

Three distinct regions in figure 5 show regions with different flammable mixtures. The outer most region is relatively large in the original wind direction. It is clear form the figure that a potentially flammable mixture of 10 to 4 by volume is present is very close to the cylinder carrying vehicle. Figure 6 and 7 show dispersion of hydrogen in XZ and YZ Planes respectively.

Figure Dispersion of H2 in XZ Plane Figure Dispersion of H2 in YZ Plane

This flammable mixture caused explosion and created a spherical shockwave. These findings were in accordance with what is expected to occur due to leakage of 180Nm3 of hydrogen. Most probably this was estimated by using TNT equivalence method, though the report does not say anything about the type of method used.

NASA challenger Shuttle Explosion

Hindburg airplane accident

Discussion (Comparison of hydrogen and hydrocarbon vehicle accident severity)

Include text from Swain (Fuel Leak Simulation)

Conclusion

Recommendation

From Mathurkar

As hydrogen combustion will not result in CO2 formation there is some environmental benefit and this is often referred to as the "greening of gas". Hence the greening of gas is a secondary objective of the Naturalhy project and will result in Environmental benefits towards a reduction in CO2 emissions. In 2004, the total primary energy consumption in the EU15 countries amounted to 6.54 x 104 kJ of which 24 was provided by natural gas. If 1 (equivalent to 3 by volume) of the energy content of the natural gas were replaced by hydrogen (produced through CO2 free production technique), the total CO2 emission of the EU15 would be reduced by about 7.4 Million-tons/year (in the case of oil 11 Million-tons/year and in the case of coal 13 Million-tons/year). This is about 0.2 of the total annual CO2 emission in the EU15 countries. Accordingly, the potential of hydrogen-natural gas mixtures for the reduction of CO2 emissions could be significant

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