Hydrogen as a fuel: Why airships are a natural fit
Author: Dr Bob Boyd, COO, AT2 Aerospace
Hydrogen has been touted as key component of a decarbonized future. It’s clean, energy-efficient, and the most abundant element. But using hydrogen as fuel requires overcoming numerous challenges, including the significant hurdle of early adoption.
As with any new technology, investors fear failure. They’re afraid of making the wrong choice between similar alternatives, and concerned about cost overruns, disappearing suppliers, ineffective products, and promises not realized.
Hybrid airships, however, present a unique opportunity for hydrogen-as-fuel. Recognized as the future of aviation technology and cost-effective transportation, cargo airships combine all the elements necessary to reduce the risk of early adoption. They present a driving need, a sensible path to adoption, favorable cost benefits, and technological maturity.
The adoption of hydrogen-as-fuel faces key obstacles
Storage volume and tank size
One of the biggest drawbacks to using hydrogen-as-fuel relates to the volumetric nature of hydrogen as an energy source. Compared to existing hydrocarbon fuels—such as Jet A (a diesel fuel) which is liquid at room temperature and pressure—hydrogen is a low-density gas at standard temperature and pressure. At this density, a tank of hydrogen the size of a semi-trailer truck would be required to gain the energy ordinarily produced by a single gallon of diesel fuel.
To counter this effect, hydrogen is often pressurized to increase its energy density. But it does not compress well, so to achieve a practical tank volume, the pressures must go as high as many hundred atmospheres. Even at this higher density, the tanks are much larger than a comparable diesel tank.
Because of this need for high-volume hydrogen-as-fuel tanks, cars and trucks struggle to include a tank with enough volume to achieve an effective range between refueling. Fixed wing aircraft also suffer from this challenge. To achieve similar ranges to the Jet A wing tanks, aircraft would need high pressure hydrogen tanks at the same scale as the entire passenger fuselage—not a practical solution. On the other hand, trains and ships have the volume to house these large hydrogen tanks, making them viable adopters of hydrogen-as-fuel, but they are extremely cost sensitive.
Infrastructure requirements
A second serious concern for investors relates to infrastructure. Hydrogen-as-fuel must be provided at the correct locations and in the correct form to refuel vehicles. As we’ve seen from the rolling adoption of electric cars, refueling (or recharging) infrastructure is required at hundreds of thousands of locations to support the millions of cars operating in the US alone. This need for extensive infrastructure is a significant challenge for most forms of transportation wanting to adopt hydrogen-as-fuel.
Fuel and equipment costs
Like any new technology, hydrogen-as-fuel and the dedicated equipment it requires face cost pressures. And they will continue to do so until production rates increase, and economies of scale come into play. Although hydrogen is currently cheaper per unit of energy than Jet A fuel, the onboard equipment required to support it is substantially more expensive than a direct drive diesel system. That means the expense of hydrogen-as-fuel is currently too high for low-cost transports like ships, trains, and trucks.
Hybrid airships rise above these challenges
Large integrated tanks and energy efficiency
Hybrid airships are very large—often stadium sized—so integrating sizeable tanks for low-density hydrogen-as-fuel is relatively straightforward. Airships also provide a low vibration, low acceleration environment, which is well suited to both the tankage and hydrogen energy conversion systems such as fuel cells.
Despite their large size, airships do not require nearly as much thrust and thus electric power as fixed wing airplanes, which makes them very energy efficient. A typical cargo carrying airship will use less than a tenth the power of a fixed wing aircraft carrying the same load. So, while the physical volume available is much larger in an airship than a fixed wing aircraft, the required tank volume is more than ten times smaller than a fixed wing aircraft.
This lower demand for power also allows for much smaller equipment in the drive line, such as electric motors, switchgear, battery banks, and energy conversion systems. Furthermore, integrating hydrogen-as-fuel in cargo airships is also fairly low risk from a technical standpoint.
Less complex fueling infrastructure than other transport solutions
Refueling infrastructure is also much less of a concern for hybrid airships using hydrogen-as-fuel. Large cargo airships must operate from pre-defined places like large container ships or aircraft do. This drastically reduces the number of fixed locations where hydrogen will need to be available for fueling to perhaps a few hundred worldwide. Early in the adoption path, hydrogen could even be provided by pressurized truck transports, which are already in use.
Another important factor will be the availability and reliability of the equipment needed to use hydrogen-as-fuel in practice. Thankfully, fuel cells and electric drive motors have been used for decades, including in trucks and other vehicles. Large tanks have also been developed for many applications at pressures that meet practical requirements and emerging hydrogen system standards, so these will not be obstacles for airships.
Favorable long-term operating costs
When used in a hybrid airship, a hydrogen fuel cell and electric motor drive system should offer a substantially longer operating life than current diesel systems, as well as operating at a reasonable cost per hour. This makes hydrogen-as-fuel an excellent choice economically for cargo airships operating for hundreds or thousands of hours each year.
As noted above, the operating environment of the cargo airship—low vibration, low acceleration, constant power demand, and stable temperatures—also favor long life for the hydrogen equipment and thus cost savings. Obviously, we will need to wait to see these long-term benefits, but as the technology stands today, the outlook is positive.
Hybrid airships also gain the advantage of mass-balancing
Hybrid airships can meet the common challenges of using hydrogen as a fuel. At the same time, hydrogen-as-fuel offers airships significant operating advantages, particularly in terms of mass balancing.
Mass balancing refers to the balancing of materials entering and leaving a system.
All aircraft rely on lift to keep them airborne, but fixed wing aircraft and helicopters can effectively turn that lift off. When helicopter rotors stop or planes cease to fly forward, the vertical lift goes away. Airships are different because they rely on buoyant lift. This is a more efficient lift mechanism, but it requires mass balancing. If mass comes off the airship, mass must go on, otherwise the airship will float upward.
On the ground this can be handled easily, but in mid-flight it is not as simple. As fuel is consumed, it steadily reduces the mass on board the airship. In the case of an airship fueled with Jet A fuel, if enough fuel is burned, the airship will become neutrally buoyant and burning any more fuel will cause it to rise and not be able to land. For this reason, Jet A fueled cargo airships are limited in range by this mass-balancing effect.
Hydrogen-as-fuel, however, can overcome this operating challenge. When hydrogen is converted to electricity in a fuel cell, the only chemical product is water in the form of steam. That steam is ported overboard, which causes a slow reduction in mass as fuel is burned. But unlike Jet A exhaust, the hydrogen system steam can be cooled and condensed into liquid water that can be captured onboard with no extra energy. The outflow steam is eight times heavier than the inflow hydrogen, so to apply mass balancing, only a fraction of the steam needs to be captured to offset the hydrogen being consumed.
This simple system will eliminate the current cargo airship range limitation and presents a driving need for the adoption of hydrogen-as-fuel. But hydrogen also offers additional benefits, such as lower air drag, plus plenty of hot clean water for showers or to make coffee—as well as cargo transportation with zero emissions.
Bringing the hydrogen economy to life
Hydrogen-as-fuel has been facing obstacles for decades. Even though it provides a recognized clean energy system, the challenges of cost, integration, and practical operations have plagued efforts to bring it into the mainstream.
Fortunately, hybrid airships offer a unique set of benefits that lower the risks of adoption. They can easily be adapted to integrate large fuel tanks, they don’t require a vast new infrastructure, and they offer the potential for favorable operating costs. Additionally, the use of hydrogen-as-fuel could solve the mass-balancing issue that currently limits the range of cargo airships—and with zero emissions.
The only real barrier today is that hydrogen-as-fuel has not yet been approved for use in commercial aircraft. However, we expect regulations will become available in time to make this clean fuel a reality. The regulatory process could even be accelerated with the added impetus of a strongly backed cargo airship application that has a driving need for these regulations.
AT2 Aerospace plans to make that happen.
