Long range transport aircraft using hydrogen fuel

Highlights

• Hydrogen and kerosene fuelled large long range transport aircraft are compared.

• Hydrogen long range transport aircraft reduce energy utilisation by around 11%.

• Direct operating costs are about 3% lower for hydrogen than for kerosene.

• Hydrogen aircraft benefit slightly more from a reduction in operating empty weight.

Abstract

Hydrogen is since long seen as an outstanding candidate for an environmentally acceptable, future aviation fuel. Given that most comprehensive studies on its use in aviation were performed over two decades ago, the current article evaluates its potential as a fuel for long range transport aircraft at current and future technology levels. The investigations show that hydrogen has the potential to reduce the energy utilisation of long range transport aircraft by approximately 11%. The use of hydrogen namely allows a much smaller wing area and span since the wing size is not restricted by its fuel storage capacity. At a given price per unit energy content, the smaller wings lead to a reduction of around 30% in take-off gross weight and 3% in direct operating costs for a given fuel price per energy content. The hydrogen-fuelled aircraft are furthermore slightly more sensitive to a possible reduction in operating empty weight in the future and 20% less sensitive to further improvements in engine thrust specific fuel consumption.

Introduction

Civil aviation faces a mounting conflict to sustain growth in a way that meets the needs of society while aiding to protect the environment. Aviation is projected to be the fastest growing industry for the next two to three decades with global growth spurred by emerging-market economies [1], [2]. An average annual increase in revenue passenger miles of around 5% is foreseen, and the fleet size will double over the next 20 years [1], [2], [3]. There is no realistic prospect that gains in aircraft energy efficiency will continue to be sufficient to compensate for the increase in emissions of greenhouse gases due to this anticipated growth [4]. The aviation industry has additionally set itself an aggressive target to reduce aviation emissions by at least 50% by 2050 [1]. As a consequence, the forecasted growth might be undermined by restrictions imposed to limit greenhouse gas emissions.

Hydrogen (H₂) offers the potential to alleviate or maybe even completely avoid such restrictions. When entirely produced through electrolysis powered from renewable or nuclear energy, the use of H₂ namely strongly reduces both airborne as well as complete “well to wing” emissions, with contrails and contrail cirrus as the only remaining significant contributor to climate change [5], [6], [7], [8], [9], [10], [11], [12]. As a versatile energy carrier that can be produced from a wide range of primary energy sources, H₂ can additionally improve the reliability of the fuel supply for aviation [8] as it could remove the geopolitical tensions associated with the concentration of fossil fuel resources in a small geographical region.

This energy security perspective has historically been the primary impetus for investigations into the use of hydrogen [13]. Hydrogen has been considered as an aviation fuel from as early as 1918 [14]. Whereas the bulk of the studies were theoretical [13], [15], [16], [17], [18], [19], [20], [21], [22], flight tests were conducted in the fifties using a B-57 airplane [23] and in the eighties using an experimental Tupolev Tu-155 aircraft, modified from a Tu-154 [24]. In both cases, only one of the engines was converted to run on cryogenic liquid hydrogen (LH₂). In 2000, the European Commission funded the Cryoplane study for the system analysis of hydrogen-fuelled aircraft. Different aircraft configurations were studied using a minimal change approach where the wing platform and engine design were unaltered when converting from kerosene to hydrogen [19]. The study concluded that, due to the excessive tank volume required for LH₂, energy consumption would increase with 9–14% depending on the aircraft type [19]. This is in contrast with earlier studies [13], [17], [18], [20], [21] where energy utilisation for hydrogen is lower than for kerosene and is most likely related to the minimal change approach. Recent work additionally explored possible synergies with the current shift towards more electric aircraft, where hydraulic and pneumatic systems are replaced with electrical systems. The use of fuel cells as a replacement for the aircraft's auxiliary power unit has received a growing interest [25], [26], [27] and the use of hydrogen as fuel for the aircraft's main engines and its on-board storage in large quantities could benefit the development of such technologies.

Given that most studies of the LH₂ fuelled aircraft are over 2 decades old, and that the Cryoplane study adopted a minimal change approach which might bias the outcomes, the current work investigates the technological potential of hydrogen as a fuel for long range transport aircraft. The main aim of this study is to identify the technology areas where further research should focus on if the transition to a truly sustainable aviation fuel would come forth. Kerosene and hydrogen long-range aircraft are therefore compared at current and future aircraft technology levels. Large long-range aircraft are selected as their large fuel loads represents an upper limit to potential performance improvements from the adoption of hydrogen. The first section of this work details the methodology used for the aircraft designs for both fuels. The main focus of this section lies on the modifications required to model hydrogen-fuelled aircraft and engines. The results of wing sizing studies for large long range transport aircraft at current technology levels are given next. The wing is independently sized for both fuels to identify the optimum wing area and aspect ratio. The resulting designs are used as a baseline to assess the impact of technological progress in section 4.