Hydrogen

• Enthalpy of combustion - 286kJ/mol - [1].
• Heat of combustion (per mass) - Compare to gasoline or diesel - enthalpy of combustion of hydrogen is about 3x as large - [2]
• Hydrogen has about 1/8 and 1/3 the volumetric density, resepctively, compared to propane and natural gas.
• Let's look at basic calculations.
• Compare to energy fuel efficiency of gasoline engin - 200 g/kWh, best marine diesel - 150 g/kWhr - [3]

Pressurized storage of hydrogen: molar mass of hydrogen molecules is 2 g/mol. 1 mole is about 5 gallons. Pressurized to 100 atm (under 1500 PSI) - readily doable with high pressure electrolyzer - you get 200 grams per 5 gallon container. Max practical for volumetric reasons would be say up to 100 gallons of 20 cu ft volume in cylindrical tanks- say 2 meters by 1 meter wide (base of a single person car). Thus, 100 gallons volume at 100 atm gets you 4 kg of hydrogen. For combustion purposes - based on enthalpy of combustion - that is equivalent to about 12 kg of gasoline or diesel. That is about 3 gallons of fuel.

In practical terms - based on our Briggs and Stratton 27 hp gas engine - that is about 75 hp hours. Sufficient for a very small personal vehicle, and arguably more efficient than electric cars from the scalability and Decentralization Perspective.

For comparison to propane - a 20 lb tank of propane (5 gallons about) - is only 200 grams of hydrogen at 100 atmospheres. So 10,000 grams/200 grams = 50x less volume efficiency. Marginal - but just sufficient for microcars with combustion engines.

Based on efficiency measures - heat engines being about 20% efficient - can we show that enthalpy of combustion gives a reliable measure of hp-hr? Gasoline is 50MJ/kg enthalpy of combustion. 50 MJ 50 MWs - or 14 kWh. This is 56 kWh of enthalpy for one gallon of fuel. Or 75 hp-h. A 27 hp engine running for 1 hour is about 30% efficiency - about right.

Another check - H2 is 150 MJ/kg. Thus, 4 kg in the 100 gallon / 100 atm storage have 600 MJ. Divide by 5 for 20% efficiency of internal combustion engines - or 120 MJ. That is 33kWhr of energy, or 44 hp-hr. About 2 gallons of fuel equivalent. Say we do an average of 5 hp requirement for a lightweight vehicle - 200 kg vehicle - that might get us from a to be in a reliable way. But, marginal. Definitely run into storage limits - unless extremely efficient design is involved. High tech.