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%%(text-align:center; background-color:lightgrey; width:80%; left:10%; position:relative;) #Hydrogen Safety %%
%%(text-align:justify;) ##%%(text-decoration:underline; color:grey;)Abstract%%
This document reviews the literature available regarding the safety aspects of using hydrogen as a fuel for automobiles. Past experiences of hydrogen are reviewed and it is found that the Hindenburg accident would still have happened even if inert Helium was used for buoyancy. The track record of hydrogen usage is explored and it is found that hydrogen has been used safely for many years. The storage and use of hydrogen on the vehicle is found to be safer than a typical gasoline vehicle. In addition, the properties of hydrogen are reviewed against the properties of gasoline in an attempt to provide a comparison with an accepted vehicle fuel. It is found that the properties of hydrogen make it safer in the event of an accident and fuel leak scenario, with a possible exception being an unvented, fully enclosed space. Finally, the safety aspects of a FCEV and hydrogen filling stations are explored and recommendations made.
##%%(text-decoration:underline; color:grey;)Contents%% \

Introduction\

[Fear of the Unknown|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%202.%20Fear%20of%20the%20Unknown]\

[Common Misconceptions|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%203.%20Common%20Misconceptions]\

[Proven Track Record|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%204.%20Proven%20Track%20Record]\

[Gasoline vs. Hydrogen|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%205.%20Gasoline%20Vs.%20Hydrogen]\

[Safety Considerations|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%206.%20Safety%20Considerations]\

Main Considerations for FCEVs\

Filling Stations\

[Conclusions|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%207.%20Conclusions]\

[References|http://www.40fires.org/Wiki.jsp?page=Hydrogen%20Safety%20-%208.%20References]\



##%%(text-decoration:underline; color:grey;)1. Introduction%%
Hydrogen fuel cell electric vehicles (FCEV) are set to become a key part of the future for sustainable transport [1]. Riversimple are to produce a hydrogen powered FCEV in which the main energy storage will be compressed hydrogen gas.
In general, there is a lack of public understanding of hydrogen as a fuel, and of fuel cell technology [2]. This document will attempt to increase the visibility of the issues of hydrogen in a balanced and critical manner by summarising independent research on hydrogen safety. \


##%%(text-decoration:underline; color:grey; )2. Fear of the Unknown%%
Fear of change and fear of the unknown is a major issue for any new technology. It is interesting to note that the public perception of gasoline as a fuel in 1875 was negative in regards to safety. A quote from a Congressional Record statement illustrates this: \

%%(position:relative; left:5%; width:90%;) ‘‘“A new source of power… called gasoline has been produced by a Boston engineer. Instead of burning the fuel under a boiler, it is exploded inside the cylinder of an engine… \

“The dangers are obvious. Stores of gasoline in the hands of people interested primarily in profit would constitute a fire and explosive hazard of the first rank. Horseless carriages propelled by gasoline might attain speeds of 14, or even 20 miles per hour. The menace to our people of this type hurtling through our streets and along our roads and poisoning the atmosphere would call for prompt legislative action even if the military and economic implications were not so overwhelming … the cost of producing [gasoline] is far beyond the financial capacity of private industry…In addition, the development of this new power may displace the use of horses, which would wreck our agriculture.” ‘’ %%
The public perception of gasoline has markedly changed since this statement; people are content with filling their vehicles with gasoline fuel and subsequently driving around with a large store of the fuel. Therefore, in an attempt to provide a point of comparison, the characteristics of gasoline and hydrogen will be analysed later in this document.

##%%(text-decoration:underline; color:grey; )3. Common Misconceptions%%
Due to the fact that public awareness of hydrogen is limited, opinions tend to be drawn from major events and occurrences covered in the press; the events that have the most sway on public opinion tend to be negative. There are two main past events that the public tend to draw their evidence from: The Hindenburg accident and the Hydrogen Bomb.
###%%(text-decoration:underline; color:grey; )The Hindenburg%%
The Hindenburg disaster of 1937, in which a fire destroyed the German airship and claimed the lives of 35 of its 97 passengers, is often attributed to the hydrogen that provided the buoyancy. This belief is in part due to the fact that the company chairman Hugo Eckener publicly blamed the disaster on hydrogen, despite having evidence of the contrary. This blame was given in part due to the fact that the US reneged on their commitment to supply helium, of which the craft was designed to use.
Addison Bain from NASA carried out extensive literature research and investigation as to the true cause of the Hindenburg disaster [4]. It was concluded that the accident was attributed to the skin of the Zeppelin, which, in order to tauten and waterproof the fabric, and to reflect heat, was doped with materials now often associated with solid rocket fuel. The material then ignited due to electrostatic discharge upon landing. Other evidence to support this includes letters to the company from an electrical engineer concerning flammability tests of the material. It must be noted that the chairman had the reputation of the company to uphold, and blaming the hydrogen was deemed to be less harmful than blaming the design decisions leading up to the construction of the airship.
Furthermore, it has been shown that modern fuel cell vehicles cannot be compared to the Hindenburg for a number of reasons. In a report to the American Department of Energy [5], it is pointed out that the energy content of the hydrogen onboard the Hindenburg was equivalent to 19 GJ per passenger. Compare this to a modern FCEV which would hold on board approximately 0.2 GJ per passenger. In addition to this, note that in a FCEV the hydrogen is stored in crashed tested composite tanks, whereas on the Hindenburg it was essentially stored in a large cloth bag. It is concluded that due to the fact there is up to 100 times less energy stored in a FCEV, and that the storage method is much stronger, there is no comparison between the safety aspects of a FCEV and the Hindenburg.
###%%(text-decoration:underline; color:grey; )The Hydrogen Bomb%%
The hydrogen bomb has no association to that of hydrogen gas. The term “hydrogen bomb” comes from the use of isotopes of hydrogen to produce a fusion reaction. In a fusion reaction, isotopes of hydrogen are fused together at very high temperatures and pressures. These conditions are similar to those found at the centre of the sun. These conditions have been in the past achieved by first setting off an atomic or fission bomb. In fact, scientists have been trying to form and sustain fusion reactions for many years using powerful lasers and magnetic containment in order to produce large amounts of clean energy: so far without success, highlighting the difficulty of creating such conditions. It is noted that under conditions that would be found on a FCEV, there is zero probability of forming a fusion reaction. Therefore, there is no relationship between the hydrogen bomb and hydrogen FCEVs.
##%%(text-decoration:underline; color:grey; )4. Proven Track Record%%
Hydrogen has been used commercially for many years [5]. Some of the current uses include: \


%%(position:relative; left:5%; width:90%;) ‘‘“… placing the cylinder in a crash car, firing armor-piercing bullets at it, dropping the cylinder from six feet onto a concrete surface, placing it in a diesel fire, cycling it thousands of times, and subjecting the cylinder to extreme cold and to corrosive liquids encountered in automotive environments, such as battery acids, saltwater, brake oils, and methanols.”’’ %%
There are also strict standards for the design and manufacture of hydrogen cylinders, one of which is a permeation standard which limits the allowable diffusion of hydrogen from the container. This standard is required as the hydrogen molecule is so small, meaning that minute quantities do pass through seemingly impermeable materials. Currently, the standard means that an explosive mixture of hydrogen and air would not be reached in a fully airtight garage for 3 to 5 years [14]. In reality, no garage is fully airtight meaning the minute quantities of hydrogen that do diffuse out of the cylinder will escape quickly posing no risk. In addition, Quantum is working to produce near zero permeation from their cylinders.
Composite cylinders have been used in road transport applications for some time now, both for hydrogen, liquid petroleum gas (LPG), and natural gas powered vehicles. Some interesting events are discussed below:
A bus with roof mounted hydrogen cylinders travelling at approximately 70 km/h collided with a low bridge [9]. The front cylinder was severely damaged but did not rupture. It was subsequently burst tested and was found to have a burst pressure of 597 bar. This is exceptional when compared to the legal requirement of 559 bar.
Another example is a rear impact with a vehicle with a type 4 CNG tank mounted in the trunk [9]. The collision was with a fully loaded gasoline transport. The tank did not rupture and the crash site investigator stated that the driver survived due to the strength of the CNG tank.
##%%(text-decoration:underline; color:grey; )5. Gasoline vs. Hydrogen%%
Gasoline has been accepted widely as a vehicle fuel for many years. This section attempts to draw comparisons and contrasts to the characteristics of gasoline and hydrogen in an attempt to provide a firm basis from which to relate the familiar experience of using gasoline, to the unfamiliar experience of using hydrogen.
###%%(text-decoration:underline; color:grey; )Stored Energy%%
Table 1 - Stored energy comparison of Riversimple hydrogen car and gasoline car. Sources: [5,8,15] || ||Hydrogen (Riversimple) ||Gasoline |Energy Density (MJ/kg) |120 |44.4 |Onboard Fuel Stored (kg) |1 |30 (approx 40 litres) |Total Energy Stored (MJ) |120 |1332
From Table 1, it can be seen that the energy required onboard the Riversimple vehicle is approximately ten times less that of a standard gasoline vehicle. So if people are happy filling up their gasoline vehicle and driving around with that amount of energy onboard, then it appears to be reasonable to fill up the Riversimple vehicle with hydrogen. Furthermore, considering the strength of a composite hydrogen tank and the strength of a polyethylene petrol tank, the argument becomes more compelling, as the risk of rupture is considerably lower. ###%%(text-decoration:underline; color:grey; )Bouyancy%%
Indeed the energy content is not the only factor which needs to be taken into account. Table 2 compares the density and diffusion coefficients of hydrogen and gasoline.
Table 2 - Bouyancy characteristics of hydrogen and gasoline. Sources: [5,8,15]
|| ||Hydrogen (Riversimple) ||Gasoline |Density w.r.t air (air = 1) |0.07 |3.4 to 4.0 (vapour) |Diffusion coefficient (cm2/s) |0.61 |0.05
Due to the fact that hydrogen is approximately 14 times lighter than air, if it is to escape from the cylinder, it will tend to rise quickly. In addition, the relatively high diffusion coefficient means that the hydrogen will disperse quickly into the surrounding air. The compound effect of these two characteristics means that should a leak occur, the hydrogen escapes quickly into the atmosphere and poses little risk in an open environment. Compare this to gasoline vapour, which is approximately four times heavier than air and has a relatively low diffusion coefficient. These characteristics of gasoline vapour mean that if gasoline leaks, it will tend to pool up near the ground, posing a fire risk. Furthermore, the fact that gasoline itself is a liquid promotes a similar danger.
The above mentioned properties make for a highly contrasting situation in the event of a fire. Dr M. Swain carried out an experiment in which two vehicles, one containing hydrogen and one containing gasoline, were ignited [6]. The results were filmed and are quite spectacular.
[{Image src=’Hydrogen_Test.png’ align:center caption=’Figure 1 - Left: 3 seconds after ignition. Centre: 1 minute after ignition. Right: 1.5 minutes after ignition.
Left car – Hydrogen; Right car – Gasoline.’ }]
The main results, as seen in Figure 1, are that initially the hydrogen burns vertically upwards and the gasoline pools underneath the vehicle and ignites. After one and a half minutes, the hydrogen supply is burnt off and the fire is extinguished with no damage to the vehicle. On the other hand, the gasoline vehicle is aflame and poses a very dangerous situation.
###%%(text-decoration:underline; color:grey; )Flame Characteristics%%
So we have seen so far that the amount of energy stored onboard a hydrogen vehicle is less than that on a gasoline vehicle, and that the buoyancy characteristics make the hydrogen safer than gasoline in the event of a leak. We have also seen visually the effects of a hydrogen and gasoline vehicle fire. The properties of hydrogen and gasoline that affect the flammability and safety in the event of a fire are shown in Table 3 below.
Table 3 - Flame characteristics of hydrogen and gasoline. Sources: [5,8,15]
|| ||Hydrogen (Riversimple) ||Gasoline |Flammability Limits In Air (by volume) |4% to 75% |1% to 7.8% |Minimum Ignition Energy (mJ) |0.017 |0.24 |Flame Colour |Colourless if burning with pure oxygen |Sooty, visible orange flame |Autoignition Temperature (ᵒC) |520 |228 to 470
Hydrogen can be seen to have broader flammability limits and lower ignition energy than gasoline, however, in practical situations, it is the lower flammability limit which takes precedence. In a small fuel leak scenario, a mixture of hydrogen and air must reach 4% before ignition, whereas a mixture of gasoline and air must reach only 1%. As regards the low ignition energy, the value of 0.017 mJ is the minimum ignition energy (at 25 – 30% mixture). In fact at the lower flammability limit, the value is approximately 10 mj [5]. It is also pointed out that an approaching human can produce a static spark of upto 10 mj. This is enough to ignite most fuels, so the low ignition energy isn’t of much relevance. The autoignition temperature, at which the gas or vapour will ignite when in contact with a hot surface, is slightly higher for hydrogen.
In an enclosed space, hydrogen may become more dangerous than gasoline. The wide flammability limits and buoyancy characteristics mean that the hydrogen can rise quickly and reach concentrations high enough for ignition in the event of a large leak. However, in the event of a small leak, the hydrogen will generally disperse and escape before it reaches the lower flammability limit. It is however advised to ensure adequate ventilation in places that hydrogen is to be stored.
It must be noted that in the event of hydrogen being ignited, the resulting flame tends to be colourless and so cannot generally be seen. In Figure 1 the flame can be seen due the air containing naturally occurring particles of sodium and other particulate matter [6]. Care must therefore be taken when coming close to a possible hydrogen fire. Some tests include holding a broom out in front so that the broom will ignite when in contact with the fire. Of further note is that the radiated energy from a hydrogen fire is much less than from a gasoline fire.
###%%(text-decoration:underline; color:grey; )Other Factors%%
Other factors that affect the relative safety of hydrogen and gasoline are shown in Table 4 below.
Table 4 - Other characteristics of hydrogen and gasoline. Sources: [5,8,15]
|| ||Hydrogen (Riversimple) ||Gasoline |Odour |Odourless |Strong odour |Emissions |Water vapour |Carbon dioxide, carbon monoxide, nitrogen oxides |Toxicity |Non-toxic but simple asphyxiant. |Some additives carcinogenic.
The fact that hydrogen is odourless means that it is difficult to detect and this can lead to safety issues. In practice however, electronic leak detectors will be used to limit this. The other factors in the table are self explanatory.
###%%(text-decoration:underline; color:grey; )Further Comments%%
As the FCEV is similar to the gasoline powered car when it comes to performance characteristics, it is fair to assume that there will be a similar number of accidents in terms of collisions. Therefore, the safety aspects of the FCEV only differ in terms of fire safety. NASA has determined that in incidents related to the transportation of hydrogen, 71% of hydrogen releases did not lead to an ignition [8]. Based on this and the above, hydrogen can be considered safer than gasoline as the burning characteristics tend to promote less damage to the vehicle as a whole. This is confirmed by the report to the U.S. Department of Energy who summarise that:
“In normal operation, a hydrogen-powered FCEV and dispensing system, with proper engineering, should be as safe as a gasoline, natural gas, or propane vehicle system.” [5]
And also by K.A. Adamson and P. Pearson who conclude that hydrogen and methanol are both safer than petrol, although hydrogen may in some cases be a higher risk than methanol [7].
Also of interest is the storage method of hydrogen and gasoline. Because gasoline is a liquid, air must enter the tank and so the flammability limits can be reached within the tank itself. However, with hydrogen being a gas, air cannot enter the tank and an explosive or flammable mixture can never develop in the hydrogen tank.
##%%(text-decoration:underline; color:grey; )6. Safety Considerations%%
As discussed above, there are many characteristics which introduce new challenges for the safe use of hydrogen as a fuel. Described in the literature are various ways of managing these challenges.
###%%(text-decoration:underline; color:grey; )6.1. Main Considerations for FCEVs%%
In the event of an accident, the high pressure hydrogen should be vented in a safe manner to prevent a fire risk or an explosion. The vented hydrogen should be directed upwards and away from the vehicle. A. R. Carpenter and P. C. Hinze [12] suggest sounding an audible alarm when venting due to the fact that the hydrogen and/or hydrogen flames are difficult to see and can present a danger. It is noted that this could be designed into the relief valve or vent so that when venting the gas causes a loud whistle sound.
The FCEV should be designed, as is similar to a gasoline vehicle, so that the fuel tank is not in a position at which it would be compromised in the event of an accident.
In the event of an accident or leak, the fuel system should be designed to isolate the tank. This is done with excess flow valves and blow-off valves; in the event of a problem, solenoids are triggered to seal off the tank preventing the hydrogen from escaping. The author suggests using the same sensors that trigger airbags to trigger shut-off valves. Furthermore, leak detectors in the passenger area and at other positions should detect high concentrations of hydrogen.
###%%(text-decoration:underline; color:grey; )6.2. Filling Stations%%
For consumers, a hydrogen filling station will work in a similar way to the familiar petrol station. The car will be parked next to a pump and a hose will be attached to the vehicle. Filling of the hydrogen will then commence. Note that the hose will be attached, not just inserted into the filler as on a petrol or diesel vehicle.
###%%(text-decoration:underline; color:grey; )Break-away device%%
One issue with a hydrogen filling station is that if the vehicle was to accidentally move away during refilling, the dispenser hose would become detached and hydrogen would leak. It is noted that similar accidents have occurred in gasoline filling stations. In a report about hydrogen refilling, S. Kikukawa et al. [10] state that a break-away device should be used to prevent such accidents. Also considered is a built-in function for FCEVs that prevents movement during refilling.
###%%(text-decoration:underline; color:grey; )Excess flow valve %%
An excess flow valve would close automatically if a pipe would rupture and a large amount of hydrogen would leak. Note that the probability of this event happening is very low.
###%%(text-decoration:underline; color:grey; )Cooling the hydrogen%%
Another issue for hydrogen filling is that the compressed hydrogen gas at the station must be cooled to about -30 degrees to the hydrogen station. This is to prevent a rise in temperature in the tank beyond the operability limits. A pre-cooler can perform this task. In fact, due to the fact that the Riversimple vehicle requires only 1 kg of hydrogen, the fill rate can be lowered without extending the fill time beyond reasonable limits. This means that the gas does not need to be cooled and so energy is not expended in the cooling process.
###%%(text-decoration:underline; color:grey; )Emergency shutdown%%
M. Casamirra, et al. [11] discuss the use of strategically placed “panic buttons” that would shutdown all hydrogen lines; essentially a manual emergency shutdown. These buttons would be placed inside and outside of the filling station. A backup generator is also to be used to ensure power for all emergency operations, including the emergency shutdown, emergency lighting, and fire pumps.
###%%(text-decoration:underline; color:grey; )Layout%%
In terms of the layout of the filling station itself, NASA state that a weather shelter or canopy should not be enclosed by more than two walls set at right angles and should have vent space provided between the walls and the canopy [8].
##%%(text-decoration:underline; color:grey; )7. Conclusions%%
There is a large amount of information available concerning hydrogen as a fuel for vehicles. This report has attempted to draw together the available information and draw conclusions as to hydrogen safety in a fair, critical and transparent manner.
The main conclusions are as follows:
• Hydrogen, like any other fuel, is safe if handled correctly. Safety systems should be implemented for both FCEVs and hydrogen filling stations.
• Evidence suggests that hydrogen can be as safe as, if not safer than, gasoline. Hydrogen has been used extensively for other tasks and has displayed an excellent safety record.
• Hydrogen storage onboard a vehicle is safer than gasoline storage due to the fact that air is not allowed into the hydrogen tank, meaning a combustible mix cannot develop.
• The public image of hydrogen as a dangerous fuel is unfounded, and can be attributed to fear of the unknown, similar to when gasoline powered vehicles were first introduced in the 1800s.
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##%%(text-decoration:underline; color:grey; )8. References%% \

[#1] G.J. Offer, D. Howeyb, M. Contestabile, R. Clague, N.P. Brandon., 2009. Comparative analysis of battery electric, hydrogen fuel cell and hybrid vehicles in a future sustainable road transport system. Energy Policy, Volume 38, Issue 1, January 2010, Pages 24-29.
[#2] M. Ricci, P. Bellaby, R. Flynn, 2008. What do we know about public perceptions and acceptance of hydrogen? A critical review and new case study evidence. International journal of hydrogen energy, 33, (2008), Pages 5868-5880.
[#3] Walter F. Stewart,. 1986. Hydrogen as a Vehicular Fuel. Chapter 3 of K.D. Williamson, Jr. and Frederick J. Edeshty, Recent Developments in Hydrogen Technology, Vol. II, CRC Press, 1986, p.132.
[#4] Addison Bain, Wm. D. Van Vorst., 1999. The Hindenburg tragedy revisited: the fatal flaw found. International Journal of Hydrogen Energy, 24, (1999), Pages 399-403.
[#5] C. E. Thomas. 1997. Direct Hydrogen En-Fueled Proton-Exchange-Membrane Fuel Cell System for Transportation Applications: Hydrogen Vehicle Safety Report. Prepared for: U.S. Department of Energy: Office of Transportation Technologies.
[#6] Dr. Michael R. Swain. Fuel Leak simulation.
[#7] K.A. Adamson, P. Pearson. 1999. Hydrogen and methanol: a comparison of safety, economics, efficiencies and emissions. Journal of Power Sources, 86, (2000). Pages 548–555.
[#8] NASA. 1997. Safety Standard for Hydrogen and Hydrogen Systems.
[#9] Raufoss Fuel Systems. 2008. High Pressure storage systems for vehicle, stationary and transport applications [[Presentation]].
[#10] S. Kikukawa, F. Yamaga, H. Mitsuhashi. 2008. Risk assessment of Hydrogen fueling stations for 70 MPa FCVs. International journal of hydrogen energy, 33, (2008), Pages 7129-7136.
[#11] M. Casamirra, F. Castiglia, M. Giardina, C. Lombardo. 2009. Safety studies of a hydrogen refuelling station: Determination of the occurrence frequency of the accidental scenarios. International journal of hydrogen energy, 34, (2009), Pages 5846-5854.
[#12] A. R. Carpenter, P. C. Hinze. 2004. System Safety Analysis of Hydrogen and Methanol Vehicle Fuels. [[Online]] [http://www.interscience.wiley.com].
[#13] Mechanical Engineering Magazine. 2002. Fill’er up - with hydrogen. [[Online]] Available at: [http://www.memagazine.org/backissues/membersonly/feb02/features/fillerup/fillerup.html] [[Accessed 27th July 2010]].
[#14] Quantum. Type III and Type IV differences “At a Glance”.
[#15] George Thomas. 2000. Overview of Storage Development - DOE Hydrogen Program. US DOE Hydrogen Program 2000 Annual Review.
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Written by Simon Vaughan (August 2010) %% %%(position:relative; height:5000px;) %%

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