Hydrogen

• Hydrogen is the lightest and most abundant of all elements. About 90 % of all atoms in the universe are hydrogen.
• In normal temperatures, hydrogen is a colorless and odorless flammable gas.
• Energy content per kilogram of hydrogen is three times larger than for gasoline. However, energy content per volume is low, due to low density of gaseous state.
• Hydrogen burns cleanly - The product of combustion is pure water.

• Hydrogen does not occur free in nature. Thus, hydrogen is an energy carrier, not a primary energy source. Before hydrogen can be used, it must be produced using energy from a primary energy source.
• Hydrogen can be used in two different ways. Burning in gas engines or gas turbines is an established and well known technique, and can be applied almost everywhere, where liquid or gaseous fuels are used. Fuel cells are a new and innovative way to use hydrogen. They provide a very efficient way to produce electricity, and have a very wide range of potential applications.

Physical properties of hydrogen

Melting and boiling point (1 atm)
T_m = -259.34 °C
T_b = -252.87 °C

Densities of gas and liquid (1 atm)
rho_g = 0.089 87 kg/m3
rho_l = 70.99 kg/m3

Diffusion coefficient in air
D_H2/air = 0.610 cm2/s

Flammability concentration in air
4.1 - 72.5 %-vol

Ignition energy in air
E_ig = 0.02 mJ

Autoignition temperature
T_ai = 530 °C

Flame temperature in air
T_f = 2045 °C

Specific heat at constant pressure
C_p = 14.89 J/kg

Heat of fusion and vaporization
H_f = 58.61 kJ/kg
H_v = 447.99 kJ/kg

Higher and lower heat values
HHV = 141.79 MJ/kg = 39.39 kWh/kg
LHV = 119.96 MJ/kg = 33.39 kWh/kg

• Hydrogen can be used to replace all gaseous and liquid energy carriers, for example gasoline, fuel oil and natural gas.
• Hydrogen makes an emissionless fuel cycle possible. If hydrogen is produced from biomass or by electrolysis using electricity from clean sources, there will be no emissions of greenhouse or toxic gases. Electricity for electrolysis can be produced using solar, wind, hydro, geothermal, or nuclear power.
• The hydrogen economy requires constructing an extensive hydrogen distribution infrastructure, converting existing fuel systems for hydrogen and introducing new, hydrogen-using applications. Realizing hydrogen economy will require political will, huge financial investments, and will take at least decades.
• The principle of an emissionless fuel cycle is presented in Figure 1.

Figure 1: Emissionless hydrogen fuel cycle. Hydrogen is produced from water using electrolysis. Electricity required by the electrolysis process is produced using solar panels. The only emissions are oxygen at the electrolysis plant and water vapor at the site of consumption. (Image: US DoE/EREN)

• Gaseous state and low density make distributing hydrogen more difficult than distributing liquid fuels.
• Gaseous hydrogen can be transported in pressurized containers, or by pipelines.
• Liquefaction of hydrogen improves energy density per volume drastically, but cooling hydrogen down to -253 °C consumes about one third of the energy content of the hydrogen, and requires well insulated containers.
• A promising alternative for distributing gaseous hydrogen is to distribute a hydrogen-rich compound, such as methanol. Methanol can be distributed utilizing the existing infrastructure for distribution of gasoline, thus removing the need for a new hydrogen distribution network. Hydrogen is extracted from methanol on-site before use.

H₂ + 1/2 O₂ = H₂O -reaction

Liquid product H₂0 (l)
deltaG = -237.1 kJ/mol = -13.16 MJ/kg
deltaH = -258.8 kJ/mol = -15.87 MJ/kg

Gaseous product H₂0 (g)
deltaG = -228.6 kJ/mol = -12.69 MJ/kg
deltaH = -241.8 kJ/mol = -13.42 MJ/kg

• Hydrogen is a much safer fuel than is generally believed. It can be used as safely as other gaseous and liquid fuels.
• In case of an accident, hydrogen has several very advantageous properties in comparison to conventional fuels. Hydrogen disperses very quickly into the surrounding atmosphere, whereas liquid fuels spread on level surfaces and burn much longer. In addition, hydrogen is non-toxic, hence wrecking of a hydrogen tanker does not cause an environmental catastrophe.
• Although hydrogen is very flammable, quick dispersion makes it very rare for hydrogen to reach combustible concentration outdoors or in well-ventilated indoor spaces.
• A common misconception is that hydrogen caused the Hindenburg disaster. In reality, Hindenburg caught fire from a static discharge spark that ignited the highly flammable impregnant of the fabric covering.

• Cheap district heating and lack of an extensive gas pipeline network make hydrogen a less attractive fuel for heating and local co-generation of power and heat.
• The potential for hydrogen use in Finland lie in the transportation sector. Ground vehicles consume about 50 % of all imported oil and is a significant source of carbon dioxide emissions.
• Replacing gasoline-using vehicles with hydrogen-fueled fuel cell vehicles would reduce CO2-emissions significantly.
• Wide-spread introduction of hydrogen cars requires an adequate hydrogen distribution network. First hydrogen vehicles will likely appear in the public transport, where vehicles are refueled at a central depot.