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.

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.

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.

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.

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.

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.

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.

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