The Hydrogen Economy – some realities
Hydrogen as aviation fuel
As an aviation fuel, liquid hydrogen has merit because of its high energy per unit mass and zero CO2 emission from combustion. In contrast conventional jet fuel incurs a significant parasitic payload mass with a mass heating value of 37% of liquid hydrogen.
However, the very low actual density of LH2 (~10% of jet fuel) increases the fuel tank bulk. Thus, there is an optimisation exercise in hydrogen fuelled aircraft design.
Another factor is the efficiency in converting the energy of the fuel into propulsive effort. For a conventional simple cycle gas turbine, the LHV efficiency is 31-38% (GPSA). For a fuel cell generating electric power it is in the region of 50%. But electric drive appears to dictate propeller driven planes.
We then should consider the lift/drag ratio. Aircraft are designed to maximise this parameter for the desired size and range. If we switch from fanjets to turboprop, the goal posts move. How quickly do we (or stuff ) being air-transported really need to get from A to B.
If we take things a step further, airships or more correctly Lighter Than Air (LTA) craft primarily need fuel for forward propulsion, not for lift, so it becomes a simple question of energy for overcoming the form drag at the operating speed. Airships are much bulkier than planes (high form drag), but travel at ~1/5 of the speed. Drag is related to speed squared.
This issue is slightly more complicated for hybrid airships which have a body shape designed to obtain some aerodynamic lift. The engines can be pivoted for VTOL. The hybrid craft is likely to have more control flexibility than a ship which must rely on ballast and ballonets for altitude control.
Liquid hydrogen feeding fuel cells for electric drive may make sense for LTAs and propeller driven planes, except when journey speed is paramount. It should be mentioned that although there are no CO2 emissions, the fuel cell or engine effluent contains a lot of water (hence the contrails). Water is a significant greenhouse gas, and the jury is still out on whether this is a serious drawback for H2 fuelled air transport.
Energy Efficiency of Liquid Hydrogen as fuel.
As stated above, liquid hydrogen has very high energy per unit mass and its use may make sense to reduce emissions for certain air transport applications. However, the production and liquefaction of hydrogen, itself consumes a large amount of energy.
The lowest cost route to hydrogen is steam reforming of fossil fuels and this requires carbon capture and sequestration to have an environmental benefit. This is nowadays called blue hydrogen.
A cleaner alternative is by electrolysis of water, but this requires renewable or nuclear produced electricity. If the hydrogen is then treated and liquefied, it can be shown that the conversion of renewable electricity to liquid hydrogen fuel energy is about 76% efficient. Then if the fuel cell is 50% efficient, then the net efficiency of using electrical energy to produce H2 (electrolysis), liquefy it and convert it back to electrical energy for a motor is only 38%.
If we now include the electrolysis efficiency of 80-90%, only 32-35% of the renewably produced electricity is finally delivered to the aircraft’s motors.
For land or sea transport, HP GH2 can be used and the energy penalty for liquefaction is removed.
Energy Efficiency of HP Compressed gaseous Hydrogen as fuel.
HP (300 bar) gaseous hydrogen has only about 36% of the mass density of LH2 and it would be further penalised as an aircraft fuel source by the weight of the pressure vessel containing the H2 which would be several times greater than the mass of its contents.
Therefore, for practical purposes HP GH2 is ruled out as an aviation fuel.
However, for land-based energy storage and transport use, these factors have less significance. Moreover, the energy required to compress hydrogen into the storage container is much less (about 20% of the energy for liquefaction).
Note: the physical (as opposed to chemical) energy of both very cold liquid H2 and compressed HP gaseous hydrogen may merit recovery which will reduce the above inefficiencies to some extent.
Therefore, for applications outside the aviation sector, the energy storage medium of compressed HP hydrogen can be considered.
It is a potential competitor of liquid ammonia.
Both can derive H2 source from electrolysis using renewable electricity.
Liquid Ammonia
Ammonia has about 42% of the mass energy density of hydrocarbon fuels such as gasoline and 13% of that of H2 (gas or liquid). However, its volumetric energy density is far better than Liquid H2.
Studies have shown that liquid ammonia can be effective as an energy storage medium for peak shaving the day to night demand swings for power. Ammonia could be used in a gas turbine generator possibly in co-combustion with some H2. The hydrogen can be produced at a steady rate and the ammonia plant can also run at a substantially constant rate. The demand swings are managed by either passing ammonia into storage at times of low demand or withdrawing from storage to meet peak demand.
The only drawback is Ammonia’s toxicity. Its main advantage is its ease of transportation as a liquid.
Batteries
Batteries have a role in delivering high power for a short duration, but with currently available technology become heavy and expensive for significant amounts of energy stored – so for long range journeys are impractical.
The Way Forward – summary
The move away from fossil fuels is finally gaining momentum. Several of the oil majors have changed direction quite dramatically in favour of becoming Energy Companies and not hydrocarbon oil and gas companies. The share price has suffered for those that haven’t.
The following energy-related issues need to be analysed objectively in terms of practicality and economics:
Transportation –
to what extent is a given journey necessary?
How fast does it really have to be?
Different solutions may be suitable for land, sea and air, and for human versus goods transport.
A new exciting but challenging solution is methane or natural gas pyrolysis. If you have got this far, well done…
Natural gas pyrolysis produces pure hydrogen and carbon a valuable by-product. The challenges are a high temperature up to 1000 degrees C to split the methane into its elements and then separating the hydrogen from the carbon. Many groups are working on ways to address these challenges within economic reality.
Watch this space as it will be a game changer for the world's energy crisis and for global warming.