Fuel cells are an important driver in the current energy system landscape with significant impact on the technology base and economic growth. Global fuel cell system shipments saw a 10% increase in 2020, totaling 1.3GW. The transport sector continues to lead with a growth of 25% on the number of units shipped globally.
The recent years have seen the launch of many projects aimed at the development of fuel cell systems for aviation powerplants. In this context, the effective integration of turbomachinery components is key in driving the overall performance and the economic viability of this technology. These aspects are the topic of this blog.
Fuel Cell Technology
Fuel cells are devices which convert the chemical energy of a fuel directly into electricity by electrochemical reactions. A fuel cell element has a matching pair of electrodes (anode and cathode) separated by an electrolyte. An appropriate flow of fuel (e.g. hydrogen) and oxidizer (frequently oxygen) is delivered to the electrodes: the resulting reaction produces electricity and water plus an amount of heat. The simplicity of this process is shown in Figure 1.
There are many advantages: efficiency, reliability, low noise, and compactness, all while implementing an environmentally progressive solution. The application potential is also very diversified, sometimes in very critical fields.
As early as 1965-66 the Gemini manned space capsules (Figure 2) included fuel cells featuring a “proton exchange membrane”, a thin permeable polymer sheet coated with a platinum catalyst that facilitated fuel-oxidizer reaction.
In 2008, Boeing flight-tested a modified Diamond DA20 lightweight airplane powered by a single proton exchange membrane fuel cell for 20 minutes (Figure 3).
As of 2021, several companies are active in the field of aviation hydrogen-powered fuel cells.
Fuel Cells Systems and Turbo-compressors: Requirements and Trends
A fuel cell element is integrated into any system that manages the flow of fuel and an oxidizer (Figure 4). A turbo-compressor is used to provide an appropriate flow of pressurized air to the cell and to support the cooling of parts.
And here is how fuel cells and turbo-compressors finally come together in applications.
For aviation propulsion applications, a turbo-compressor system provides the ideal weight, size, and efficiency when compared to other solutions. Additionally, the proven technologies employed in the design and manufacturing of these units support high levels of reliability.
Typical systems currently implemented in demonstrations or prototypes include centrifugal stages which guarantee high performance for the mass flow rates (often lower than 0,1 kg/s) and the rotational speeds (up to 8,0000 rpm) that characterize these systems. The Aeristech AeS801A, for instance, is a 2.5 kW, air-cooled unit weighing less than 2.6 kg with a pressure ratio of 1,6. Its rotor is equipped with rolling bearings in a sealed casing greased for life.
Aviation requirements include mechanical stresses (shock, vibration) and compliance to wide environmental temperature and pressure ranges. The turbo-compressor must be capable of handling a demanding operating envelope (Figure 6) while delivering prescribed flow conditions to the fuel cell. Discharge pressure currently considered for state-of-the-art systems are the order of 3 bar to enable optimal fuel cell design. The delivery temperature can be subject to constraints based on the fuel cell temperature and the need to avoid condensation inside the device. For a proton exchange membrane unit, the operational temperature can be up to 180 C.
As the aircraft operating conditions change during the flight, the turbo-compressor must handle a large variation in pressure ratio and volumetric inlet flow. This may be achieved by a variable compressor speed. Consideration has also been given to compressor systems equipped with two parallel units operating simultaneously, though, only in cruise condition.
Fast transient response of the compressor system may also be a requirement for aviation powerplants. The dynamic behavior of the system can be significantly improved by proper selection of light-weight rotor materials, redesign of rotating parts for a minimum moment of inertia, bearing selection, and configuration of the air ducting system. The Aeristech AeS801A shown above is quoted with a transient response of less than 0.28 s.
Air contamination must also be avoided to prevent damage of the membrane. Typical fuel cell requirements specify an oil content lower than 100 ppm. The design may therefore include air bearings and other specific design solutions to ensure air quality. The compressor must be capable of delivering a steady, pulsation-free air flow across the whole operating envelope as excessive pressure difference across the membrane may result in damage.
In general, a turbo-compressor system has the capability to fulfill all these requirements optimally, thanks primarily to the technology maturity acquired from the aero-engine design and related fields.
Further Development and Challenges: Role of Turbo-Compressors
With these advantages, one may ask, “What is preventing aviation companies from switching to this approach right now?” The answer requires us to look at the complete fuel cell powerplant, of which the compressor is only a sub-component.
While the overall technology approach for small aircraft can be considered proven, considerable development is still needed for effective integration on large commercial platforms. Figure 7 shows a comparison of the specific power and power density for several generation systems. As it can be seen, proton exchange membrane fuel cells still do not match the performance of today’s combustion engines.
It is generally recognized that closing the gap in specific power will be a key enabler for the implementation of fuel cells in large aircraft. Future requirements are expected to increase specific power up to 2 kW/kg for large-scale passenger/commercial aviation and drive the related development of advanced systems.
To achieve these demanding targets, the design of fuel cells will need to be strongly optimized for aircraft applications, including possibly the development of liquid-cooled units with a good balance of weight, cost, and durability.
The turbo-compressor system has a very important role to play in this context. The development of lightweight, high-pressure ratio designs, closely matched to the technology and operations of the fuel cell is expected to be a key driver in the push towards better specific power. In light of these remarks, aviation applications will be best satisfied by highly customized, advanced performance turbo-machinery equipment.
Current forecasts predict that aviation may be producing up to 24% of global CO2 emissions by 2050. Hydrogen fuel cell technology has the potential to deliver “true zero” solutions – in other terms reduce all gross emissions to zero. The environmental agenda is therefore giving a fresh impetus to develop these systems, as we can see from the increasing number of projects. Many players are currently working on powerplant solutions for small aircraft. Large-scale propulsion applications will require systems capable of significant improvement in specific power, dictating the need for enhanced technologies.
In this landscape, customized high-performance turbo-compressors will be a major driver. The design challenge for such units will require a careful balance of performance, weight, and compliance to operational requirements. The availability of software tools characterized by strong optimization capability and fast turn-around will be key for such designs. For more information on how SoftInWay can help you get the most out of your fuel cell system, check out the AxSTREAM platform or join our Tutorial of the Basics titled “Design of Fuel Cells-Based Power & Propulsion Systems for Different Applications” at Turbo Expo 2022!
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