A number of companies are developing electric power plants for aircraft, basically for powered gliders and small ultralight aircraft. In most of these designs the power plant is of secondary importance and the aircraft can fly, or better, glide considerable distances without an engine purely by its basic aerodynamic properties.
Developments are in progress to electrify some every day general aviation class aircraft or even new designs. Because of many limitations to overcome, large scale applications will not come any time soon. The main problem is energy storage: liquid fuel (Mogas, AVgas or Jet fuel) has the highest energy density per weight, which is very hard to beat, except for nuclear fission or fusion. This will limit the range of any aircraft trying to fly any considerable amount of time or distance with a worthwhile payload.
Off late some are developing systems which are supposed to use Hydrogen as a fuel for aircraft, claiming that it is a better fuel. On this page we delve a little deeper into this.
"Hydrogen is like electricity in being an energy carrier, a variety of manufactured fuel. It is made either by electrolysis, which uses electricity generated by other sources to disassociate the hydrogen from the oxygen in water, or steam methane reforming (SMR), in which energy, usually natural gas, is used to raise steam. The steam is then brought into contact with a catalyst and an additional supply of natural gas to extract hydrogen from both, generating carbon dioxide in the process."
Source: The GWPF report 44 (2019). Read the full paper here: Hydrogen, the once and future fuel? by John Constable, it is an eye opener!
This can be stored in a number of forms: under high pressure, cryogenics and with chemical compounds that release the hydrogen when heated. Liquid storage of hydrogen requires a cooled vessel (20 K or -250 °C) which must be well insulated resulting in added weight and cost. This might not be ideal for aircraft. Compressed hydrogen has a nice energy density per weight but very low energy density by volume. You will need a larger tank compared to normal liquid fuels. More info here on hydrogen (Wikipedia).
SkySpark performed a test in a Pioneer 300 with a 75 liter fuel tank at 350 bar containing some 26000 liters (!) of hydrogen (H). The aircraft has a fuel cell in the copilot seat, a 65 kW motor and an auxiliary lithium type backup battery.
Oct 2020: ZeroAvia is testing a hydrogen fuel cell powered with a Piper six place aircraft flying from Cranfield in the UK. Read more here in the article Zero Emission Hydrogen by CNN. Skip the alarmist part about the CO2 emitted by aviation, as this comes down to: 3% (aviation) of 3% (total human) of 0,04% (total = 415ppm, 2020) resulting in a total of 0.4 ppm or 0.00003735% for aviation as a whole.
Feb 2022: Airbus announced that they will partner with CFM (GE and Safran Engine) to develop a hydrogen powered aircraft turbofan engine. The engine will be a high bypass (5.6:1 ratio) twin-spool axial flow turbofan with a pressure ratio of 45:1. The engine will be tested on a A380 where it is mounted on the fuselage, see image. CFM will redesign the combustor, fuel system to accommodate for the hydrogen fuel. Image from AVflash.
This 'fuel' can also be used in internal combustion engines (Airbus) and the only byproduct coming from the exhaust is water (H2O). This would be the solution for our cars on the road if a transition is required from ordinary petroleum fuels. Laboratory test have proven that combining H2O and CO2 makes it possible in combination with the catalyst Ruthenium to store and release the hydrogen much quicker.
| Property | Hydrogen | Comparison |
| Density (gaseous) | 0.089 kg/m3 (0 °C, 1 bar) | 1/10 of natural gas |
| Density (liquid) | 70.8 kg/m3 (-253 °C, 1 bar) | 1/6 of natural gas |
| Boiling point | -252.76 °C (1bar) | 90 °C below LNG |
| Energy per unit mass (HHV) | 141 MJ/kg | >3 x that of gasoline |
| Energy per unit mass (LHV) | 120.1 MJ/kg | 3 x that of gasoline |
| Energy density (ambient, LHV) | 0.01 MJ/l | 1/3 of natural gas |
| Specific energy (liquefied, LHV) | 8.5 MJ/l | 1/3 of LNG |
| Flame velocity | 346 cm/s | 8 x of methane |
| Ignition range (% volume) | 4 - 77% | 6 x wider than methane |
| Autoignition temperature | 585 °C | 220 °C for gasoline |
| Ignition energy | 0.02 mJ | 1/10 of methane |
The table above describes the physical properties of hydrogen.
Hydrogen can be used in a fuel cell where electricity is produced and heat and water results as a byproduct. This technology has been around since 1959 in several cars and used during manned space missions. The cell consists of an anode (where hydrogen is fed in) and a cathode where oxygen is fed in. At the anode side, hydrogen molecules are split into electrons and protons. These protons pass through an electrolyte membrane and the electrons are generating current. At the cathode the protons, electrons and oxygen combine resulting in water molecules.
The dangers of widespread use of hydrogen as an energy carrier have been both under- and overstated. In common with other powerful energy sources and carriers, such as gasoline and electricity, hydrogen is intrinsically hazardous, more so in several respects, namely that it has a comparatively low ignition energy, a wide flammability range, and a strong tendency to proceed from deflagration (a simple fire) to detonation (an explosion with a flame frontier moving at speeds greater than sound and with an accompanying shock wave), GWPF.
From the manual: "While the hydrogen has a high energy density per kilogram, the density per unit of volume at normally ambient temperatures and pressures compares unfavorably with natural gas and other fuels. However, its very low boiling point, near absolute zero, makes the liquid state energetically costly to achieve and difficult to maintain. Compression is also energetically costly and, being small, molecular hydrogen is likely to leak, which, combined with its low ignition energy and wide range of concentrations at which it will ignite, raises questions about safety and ease of handling."
More information in this article at ScienceDirect: "Review on hydrogen safety issues: Incident statistics, hydrogen diffusion, and detonation process".
The higher heating value is 141 MJ/kg and lower heating value is 120 MJ/kg. You only get energy from the LHV but thermodynamics dictate that you have to put in the HHV in energy to make H2 from H2O. This results in a 15 % loss.
Electrolysis is about 70% efficient thermodynamically, so you need 56 kWh of energy to make 1 kg of H2. 1 Metric Ton (MT) of H2 requires 56 MWh of energy and produces 15 MWh thus ENERGY INVESTED is 3,7 x greater!
People talk about plants producing 1 million MT H2/yr or 114 kg/hr (continuously, not intermittently). This requires 6391 GWh of energy each hour or 6.39 GW of power. That is 3195, each 2 MW wind turbines, but multiply that by 4 for the capacity factor of 25 %. Now you have a lot of wind turbines but still do not have a continuous power supply. And at 4 million (and counting up) cost for each turbine, that is again a lot of money. A electrolysis cell cost about 1000 to 1500 USD/kWh and need to be replaced every 7 to 10 years. Do the math and let us know who can afford even this much H2. And they want 10 times that amount of H2.
Hydrogen power is another possibility but we need large amounts of very cheap electricity to convert water (not sea water) to obtain this energy carrier, pure hydrogen is not as abundant on the planet as hydrocarbons (almost nothing). As long as we need oil or gas to do that, instead of clean nuclear power like a Thorium MSR reactor (or LFTR, not Uranium), we would better use ordinary liquid petroleum based fuels and concentrate on building higher efficiency combustion engines with hybrid drives.
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