Hydrogen is abundant and can be used for combustion and in fuel cells, but there are significant challenges in energy storage and conversion.
Like many other transportation-related segments, the aircraft industry is looking to go “green” and reduce its use of traditional hydrocarbon-based fuels. Whatever the arguments and rationale for this goal, meeting it is a real challenge, as the power/weight ratio for aircraft and rockets is especially critical, more so than it is for ground or water transportation. As a result, we’re reading and seeing a lot about electric power for aircraft ranging from personal commuter taxis to small multi-passenger aircraft and even larger ones.
From an engineering perspective, there are two fundamental questions. First, presuming the use of electric motors, how the electrical energy is stored and carried on the aircraft. Batteries are likely the first thing that most people suggest, but the numbers make it a challenge. The second question is, from where does this energy for charging the batteries come? But that’s a question for another time.
The first thing to look at is the fuel’s energy density by weight and volume, as highlighted in comparisons shown in figures 1-3. There are tables, charts, and more, but it’s often hard to compare them because of the different ranges and measurement units. Still, they are useful enough to provide a general sense of the span between advanced battery chemistries, hydrocarbons, and hydrogen alone. Note that hydrogen is way at the top for density.
Figure 1 A comparison is shown of major energy combustibles. Source: The Geography of Transport Systems
Figure 2 The battery comparison chart illustrates the volumetric and gravimetric energy densities based on bare battery cells. Source: Epec LLC
Figure 3 This energy density comparison shows different transportation fuels. Source: U.S. Energy Information Administration
You have likely read the story about the hydrogen-powered drone from Doosan Mobility Innovation (DMI) of Korea, which uses a small, custom canister of fuel and proprietary fuel cells and standard DC/DC converters from Vicor (Figure 4). This drone, which is in commercial use, claims a flight time several times greater than a battery-powered drone of comparable size and weight. See the recent EE Times column, “Hydrogen Fuel Cell-Powered Drones.”
Figure 4 This is the hydrogen-based, fuel-cell power subsystem of the Doosan drone, with range and flying time far greater than a comparable battery-powered unit. Source: Doosan Mobility Innovation
Engineers and process specialists know that it’s one thing to build a small-scale prototype or model and it’s another to scale it to a commercially-viable size. Many potentially promising advances in battery chemistry, for example, never make it from pilot plant to full-scale production due to intractable issues and costs that escalate exponentially rather than modestly.
That may be the case with hydrogen as fuel. It may seem that the hydrogen and fuel cell combination is a better choice than even the best lithium-based batteries. Maybe, maybe not. The reason is that fuel energy-density alone is insufficient to make a viable source, especially as you scale up. There are issues of fuel storage, conversion of the raw energy source into the energy source, and subsequent power for the aircraft, which could be electric, but doesn’t have to be.
A recent article in Physics Today, titled “Hydrogen-powered aircraft may be getting a lift,” discussed the broad range of issues associated with hydrogen as a “dream” high-density fuel, including the net energy density when you factor in the weight and size of tanks and others subsystems needed to contain and use the hydrogen. It also pointed out that hydrogen can be used in two ways: for combustion or in fuel cells.
Today’s gas turbine-based jet engines, like those used in many commercial passenger aircrafts, could burn hydrogen with relatively few modifications. Phillip Ansell, director of the NASA-funded Center for High-Efficiency Electrical Technologies for Aircraft at the University of Illinois at Urbana-Champaign, is quoted saying, “You can almost just drop hydrogen into today’s engines.”
The alternative is to use the hydrogen in on-board fuel cells, an established approach that has been used for decades (think Apollo moon mission) and for which there have been significant advances in the technology and fuel-cell materials, as well as in their management and output electrical-power conversion.
Attractive as hydrogen seems at first for larger aircraft, it’s difficult to beat the power/weight ratio of hydrocarbon fuels when you see the system-level data of the fuel system and the engines and motors they power, not just the raw fuel itself. If passenger-carrying aircraft ranging from taxis to larger ones powered by hydrogen and fuel cells do become a reality—and that’s a very big “if”—it will be interesting to see what kinds of advances are needed and to what extent. Will it be the accumulation of many small/medium improvements? Or will there be some bigger breakthrough we just can’t see right now? If history tells us anything, it could be either one or both.
Even if hydrogen is the clean fuel for the future, there’s some irony here as well. While hydrogen is abundant and ubiquitous, extracting it to use as fuel is not trivial. For example, there are plans for a $5 billion-plus hydrogen plant in Saudi Arabia, using sunlight by day and wind power by night for “green” production. Construction has not yet started, but the goal is to have this facility operating by 2025 and producing 650 tons/day, about one-and-a-half orders of magnitude greater than the 9 tons/day of the largest existing green hydrogen-producing facility located in Quebec. For more, see “Green Hydrogen Plant in Saudi Desert Aims to Amp Up Clean Power,” published in The Wall Street Journal.
What’s your view of use of hydrogen, fuel cells, and electric propulsion in aircraft? Will the lessons of ground vehicles and small drones be applicable, and to what extent? Or is hydrogen just another “pipe dream” for commercial aircraft when all is said and done?
This article was originally published on EDN.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
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