Hydrogen Powered Transportation

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Technology Review (Nov. 2020)

A recent announcement of the successful first test flight of a hydrogen fuel cell powered plane prompted a review of how the technology is being used in different forms of transportation. Hydrogen fuel on its own has many benefits as an energy storage medium.

General Aviation

There appear to be several ways hydrogen can be used to power aviation. One is to use hydrogen fuel cells to generate electricity that drives electric motors. Another is more direct and involves modifying gas-turbine engines to run on hydrogen instead of jet fuel. Such a gas-turbine can be used in a conventional jet engine or to power a turboprop engine. Companies such as the European Airbus are exploring concept aircraft based on direct use of hydrogen fuel.

Rail

Hydrogen fuel cell technology has a number of advantages in rail transportation. The Austrian rail company ÖBB is planning to test a hydrogen fuel cell train through November of this year, with passengers, on challenging grades. The train, the Coradia iLint built by the French company Alstom is capable of speeds up to 87 mph, can carry 150 seated passengers and 150 standing passengers with a range of 625 miles in a two car configuration. The train has already been tested in the Netherlands (1) and demonstrated that it had fully zero emissions, was quick and easy to refuel, and fitted within the commercial service’s current time table. The train is designed around a converted diesel train. Alstom claims it to be the first passenger train in the world to run on hydrogen fuel cells. Alstom is also known for supplying Amtrak with trainsets for the US north east corridor in 2015. Alstom sees the US market for hydrogen fuel cell technology in the long term to be in converting diesel freight trains.

The BBC:Future Planet website (2) also brings us a story of Hydroflex, a hydrogen fuel cell powered train tested in 2019 in the UK. The set under test had a range of 50-70 miles. Again by retrofitting diesel trains, train service can be provided where electrification is just too expensive to install.

Meanwhile in the US, San Bernardino County Transit Authority has ordered a multiple-unit hydrogen fuel cell powered train from Stadler Rail. The train, a Flirt H2, is expected to enter service in 2024.(3). Somewhat like the Coradia iLint it will have 108 seats with lots of standing room and a top speed of 79 mph.

Busses & Trucks

The California Fuel Cell Partnership (4) reports on a number of projects designed to bring hydrogen fuel cell technology to busses and trucks. Some of these projects have now been running for several years.

AC Transit (The Alameda-Contra Costa Transit District) with 13 busses has put in more than 1.3 million miles and carried 5 million passengers in the Bay Area. In 2019, AC Transit added 10 New Flyer fuel cell electric buses. As part of the program they are comparing these vehicles in real world service against battery only powered busses. Results should be interesting. Joining AC Transit are the counties of Coachella Valley and Orange County Transit Authority, the latter with the OCBus above.

How do Hydrogen Fuel Cells Work? (5)

We are all probably familiar with the electrolysis of water. Put an anode and a cathode in water with a little bit of salt, connect to a supply of direct current and pretty soon you will see bubbles form on both electrodes. The two elements that make up water (H2O) are hydrogen and oxygen. On the anode will be oxygen while hydrogen appears on the cathode. The two elements that make up water. The process consumes energy to run.

That reaction is reversed in fuel cells and thus generates energy and the anode and cathode are switched. It does get tricky because if you take the power source out of the electrolysis cell it doesn’t automatically reverse to generate electricity. Fuel cells can generate electricity for as long as there’s a supply of hydrogen and oxygen, usually from the air. There’s no discharge-recharge cycle as in batteries. Special catalysts on both electrodes allow the electrochemical reactions to proceed and a special electrically isolating membrane between them forces the released electrons to follow an external circuit to deliver the electrical power.

What is this about Energy Density?

One of the advantages of hydrogen-electric powered transportation over battery-electric powered transportation is in the energy to weight ratio.

It’s complicated. Gravimetric energy densities are measured in energy per unit mass of fuel. For example, coal has an energy density of 34 MJ/kg, diesel 43 MJ/kg and hydrogen 140 MJ/kg. By comparison Lithium Ion batteries are currently running up to 0.4-0.9 MJ/kg and this must necessarily include the weight of the container, i.e. the battery. Theoretical rechargeable battery energy densities have been calculated as high as 3.6 MJ/kg. Note for context a mega-Joule (MJ) is about 278 watt-hr or the amount of electricity you might consume by leaving a 10W LED light on for 27.8 hours. So once contained in a gas cylinder, hydrogen has a much lower net energy density, more of the order of 2.1 MJ/kg (4) However, in practice compressed hydrogen occupies more space than other energy sources, it has a lower volumetric energy density and this affects how vehicles have to be designed - larger vehicles can accommodate hydrogen fuel cells more easily.

Where does the hydrogen come from?

None of this means much unless the hydrogen can be generated without fossil fuels. Hydrogen is currently found as a by-product from other processes, manufactured via steam reforming of natural gas at high temperatures, and via electrolysis – where hydrogen can be generated with excess renewable electricity, a.k.a. 'green hydrogen'.

The cheapest and most common method at present uses the steam reforming method. The direction of developments is to use renewable energy and electrolysis.

How is it stored?

Generally hydrogen is most commonly stored in its pure form as compressed gas (5,000-10,000 psi), or sometimes cryogenically as liquid hydrogen at around -252.8 deg C (-322 deg F). Another approach is to store hydrogen in another substance either adsorbed or chemically combined but easily released - research continues.

At ‘hydrogen fuel stations’ vehicles can fill up much like current gasoline stations. Light weight composites are being used to develop light weight storage options. California has dozens of hydrogen fuel stations in the Bay Area and around Los Angeles for road vehicles.

How safe is it?

Well, all fuels are flammable and must be handled carefully. However, in many ways, hydrogen is safer to use than conventional fossil fuels. If a leak occurs, lighter-than-air hydrogen gas rises up and disperses rapidly. It is also a non-toxic gas and therefore safe to breathe!

More importantly, the only emission is from the fuel cell operation. Even any unconverted hydrogen is recycled.

But about that plane…

CNBC brought us a report (6), that a six-seater Piper M-class aircraft successfully completed its maiden flight this past September. It was powered by a hydrogen fuel-cell as it took off from Cranfield Airfield just 50 miles north of London, UK. It completed a ‘full pattern circuit’ before landing back at the airfield.

ZeroAvia developed the plane as part of their HyFlier Project. They claim it to be the first commercial-scale hydrogen powered aircraft. According to their press release (7) the company plans to develop 10-20 seat planes within three years. More immediately they hope to develop planes capable of flying between the Orkney Islands and the Scottish mainland, some 250-300 nautical miles, by the end of this year.

Ref:

  1. "Trial runs of Alstom’s hydrogen train in the Netherlands deemed officially successful"
  2. "Next stop, hydrogen-powered trains"
  3. "US hydrogen train contract awarded"
  4. "California Fuel Cell Partnership"
  5. "How does the Fuel Cell Work"
  6. Hydrogen-powered passenger plane completes maiden flight in ‘world first’"
  7. "ZeroAvia Conducts UK’s First Commercial-Scale Electric Flight"

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