Advanced Thermal Management for Hydrogen Fuel Cells in Aircraft

: (Alternative Format Thesis)

Abstract

The emissions contribution from civil aviation must be reduced for three primary reasons: first, passenger demand is growing, largely driven by the Asia-Pacific and Middle East regions; second, the relative contribution of other sectors to emissions, such as the automotive industry, is declining; and third, the global average surface temperature continues to rise with associated consequences for the climate. In the near-term, improvements in aircraft trajectory optimization and the continued development of next-generation engine technologies will help reduce emissions. However, to achieve substantial reductions in emissions, more disruptive solutions are required that remain competitive with conventional aircraft fueled by Jet A-1. In this respect, developing low-emission propulsion systems that can compete with in-service aircraft is critical. Proton Exchange Membrane Fuel Cell (PEMFC) stacks are a leading candidate for the powerplant of such low-emission propulsion systems because: first, under hydrogen and air operation their only byproduct is water; second, hydrogen offers a markedly higher gravimetric energy density than conventional hydrocarbons (despite lower volumetric density); and third, PEMFC technology has undergone sustained development since its aerospace debut as a non-propulsive powerplant on the US Gemini spacecraft. To make an appreciable impact on civil aviation emissions, regional PEMFC-powered aircraft must enter service within the next two decades, which will require the development of megawatt-class PEMFC powertrains. In such systems, irreversibilities in the electrochemical reaction generate megawatts of heat energy that must be dissipated to the atmosphere to maintain the prescribed operating temperature of the PEMFC stacks. This is particularly challenging in PEMFC-powered aircraft because the enthalpy flow rate of the exhaust is significantly lower than that of conventional jet engines. To this end, a dedicated Thermal Management System (TMS) is required to transfer sufficient heat energy from the PEMFC stacks to the atmosphere while minimizing its impact on power consumption, system weight, and net cooling drag throughout the aircraft's mission profile.

The aim of this research is to: examine the feasibility of Thermal Management Systems (TMSs) in PEMFC-powered aircraft. This thesis is presented in an alternative format and comprises four academic publications, each investigating distinct aspects of thermal management within PEMFC powertrains. The thesis primarily focused on liquid-cooled PEMFC powertrains owing to their greater technology maturity compared to other TMS configurations. The work includes the modeling, analysis, and evaluation of thermal management components, subsystems, and the aircraft-level system. A survey of the wider literature identified three fundamental areas governing the feasibility of thermal management: first, the impact of the coolant's properties on the sizing of the powertrain; second, dissipating megawatts of heat energy to the freestream at significantly lower operating temperatures than jet engines; and third, the aircraft-level impact of the TMS and methods for expediting their assessment.

The first publication is a review article that examines High Temperature (HT) PEMFCs operating between 100 and 250 C. The wider literature reveals several TMS architectures currently under active investigation for PEMFC applications. This publication presents a detailed review of the literature pertinent to TMS configurations, including their design principles and operational characteristics. The review is subsequently used to identify knowledge gaps across several critical thematic areas.

The second publication investigates the design of a liquid-cooled PEMFC powertrain and demonstrates the sensitivity of its performance to the choice of the liquid coolant. This finding highlights the need for informed coolant selection to minimize associated power consumption and system weight. A Figure of Merit (FoM) was developed to screen candidate coolants that minimize pumping power and weight while maximizing heat transfer. Furthermore, a TMS model of the liquid cooling system was employed to identify Pareto efficient coolants, and the results were compared with those obtained from the FoM analysis.

In the third publication, a fan-fed ducted radiator was investigated as a means of transferring megawatts of heat generated by the PEMFC stacks to the atmosphere. A thermodynamic analysis demonstrated that the cooling drag could be offset by accelerating the airstream through the radiator via heat addition. The fan-fed ducted radiator was sized for LT- (Low Temperature) and HT-PEMFCs to achieve zero internal drag using a multi-objective Genetic Algorithm (GA).

The final publication has been submitted to the AIAA Journal of Aircraft; it investigates the optimization of a liquid-cooled PEMFC-powered regional aircraft. The OpenConcept toolkit for conceptual design of aircraft was employed with LT- and HT-PEMFC technology benchmarks to optimize liquid-cooled PEMFC powertrains for minimal fuel burn. The payload capacity and associated fuel cost per Revenue Passenger Kilometers (RPK) were computed for comparison with conventional aircraft fueled by Jet A-1. The outputs from OpenConcept were used to assess the performance of PEMFC powertrains for different aircraft weights and mission ranges with a FoM derived from Breguet range theory.

The research presented in this thesis demonstrated that elevating the operating temperatures in PEMFC stacks provides aircraft-level benefits. The sizing of the TMS was sensitive to the thermophysical properties of the coolant, and the fan-fed ducted radiator concept provides a low net internal drag solution to heat rejection. Liquid-cooled HT-PEMFCs are the most promising alternative to conventional aircraft fueled by Jet A-1 by 2035. Innovations in HT-PEM electrochemistry and the development of novel HT-PEMFC propulsors must be accelerated to realize PEMFC-powered aircraft capable of achieving parity on Revenue Passenger Kilometer (RPK) with today's in-service aircraft.

Date of Award	24 Jun 2026
Original language	English
Awarding Institution	University of Bath
Sponsors	GKN Aerospace Services Limited
Supervisor	Oliver Pountney (Supervisor) & Carl Sangan (Supervisor)