A record-breaking commercial-scale hydrogen plane has taken off in the UK, with more set to join it soon. How far can such planes go in cutting the aviation industry's emissions?

一架创纪录的商业级氢动力飞机在英国起飞，不久数量将越来越多。这种飞机能在多大程度上减少航空业的排放？

As the plane rose from the runway for what was to prove a smooth and uneventful flight, the team breathed a sigh of relief. The six-seater Piper M-Class had been fitted out at a research and development hub at Cranfield airport in the UK to run on hydrogen, and on this maiden flight in the late summer of 2020 everything worked perfectly. With that flight, ZeroAvia, the California-based start-up that had developed the aircraft with partners in Britain and elsewhere, was ready to move to the next stage in the journey towards zero carbon aviation.

随着飞机在跑道上起飞，证明这次试飞圆满完成，科研团队松了一口气。在英国克兰菲尔德机场的研发中心，这架派珀（Piper）M级六座飞机被组装完成，它采用氢动力。在这次2020年夏末的首次试飞中，一切进展顺利。总部位于美国加州的初创企业ZeroAvia与英国等国家的合作伙伴共同研发了这架飞机。有了这次试飞，该公司准备在零碳排放航空道路上迈向下一阶段。

A catchphrase for the transition to a low or zero carbon economy is "electrify everything" – that is, create a world in which most human activities, from manufacturing and construction to transport and tourism, run on electricity generated from low or zero carbon sources such as wind, solar and perhaps nuclear power. But there is a problem: some sectors look to be hard if not impossible to electrify in the near and medium term, and aviation is, perhaps, foremost among them.

向低碳或零碳经济转型的口号是“让一切电气化”——即在新开创的世界里，大部分人类活动，包括制造、建筑、运输、旅游，依靠低碳或零碳能源发电来维持，例如风能、太阳能、可能还有核能。但存在一个问题：某些行业看来难以做到，在近期和中期可能无法实现电气化，航空业可能首当其冲。

Before the pandemic grounded most flights, commercial aviation accounted for about 2.5% of global emissions of carbon dioxide. It sounds like a small proportion of the whole, but it is more than those of Germany (2.2%), and this is not the whole story. Carbon dioxide accounts for about half of aviation's contribution to what is known as its effective radiative forcing – that is, its total contribution to the factors that actually drive a rise in global average temperature. Contrails – water vapor trails from aircraft – are aviation's largest other factor.

在疫情迫使大多数航班停飞之前，商业航空约占全球二氧化碳排放量的2.5%。听起来好像仅占很小一部分，但高于德国排放量（2.2%），事情不止如此。二氧化碳约占航空业对“有效辐射强迫”贡献的一半——这才是航空业对全球平均气温升高的实际推动因素的总贡献。凝结尾迹——飞机造成的水汽凝结尾流——是航空业的另一最大因素。

The good news is that commercial aviation has an excellent track record in improving efficiency. Carbon dioxide emissions per passenger flight have fallen more than 50% since 1990 thanks to improved engines and operations. The bad news is that these gains have been overwhelmed by rising volumes of air traffic. This has increased by at least a fifth over the past five years, and is predicted to reach 10 billion passengers a year by 2050.

好消息是商业航空在提高效率方面成绩斐然。1990年以来，得益于发动机和运营的改善，每架次客运航班的二氧化碳排放量下降了50%以上。坏消息是这些进步敌不过日益增加的民航客运量。在过去的五年里，客运量至少增加了20%。预计到2050年，每年客运量将达100亿人次。
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At first glance, hydrogen looks to be a good solution to the challenge of flying without wrecking the climate. Whether hydrogen is used to power a fuel cell to generate electricity or directly combusted for motive power, the only waste product is clean water. Importantly in the context of flight, hydrogen packs a lot of energy per unit of mass – three times more than conventional jet fuel, and more than a hundred times that of lithium-ion batteries.

从表面来看，氢是很好的解决方案，避免飞机对气候造成破坏。无论氢被用于驱动燃料电池发电，还是通过直接燃烧来提供动力，产生的唯一废弃物是清水。重要的是，在飞行过程中，单位质量的氢储存的能量巨大——相当于传统航空燃料的三倍，锂电池的一百多倍。

Governments and companies are investing in this potential. ZeroAvia's 2020 hydrogen-powered flight, known as HyFlyer I, was supported by the UK Government, whose Jet Zero Council promises "a laser focus on UK production facilities for sustainable aviation fuels and the acceleration of the design, manufacture and commercial operation of zero-emission aircraft."

政府和企业正在对这一潜力进行投资。ZeroAvia公司在2020年研发的氢动力飞机HyFlyer I得到了英国政府的支持，“零排放航空理事会”曾诺： “注重发展英国可持续性航空燃料生产设施，加快零排放飞机的设计、制造、商业运营”。

The UK government, together with private investors and commercial partners are supporting ZeroAvia in the development of an aircraft with a hydrogen-electric (fuel cell) powertrain capable of carrying up to 20 passengers about 350 nautical miles (648km). ZeroAvia's founder and chief executive Val Miftakhov, says the company expects to offer commercial flights using such a plane as early as 2023, and that by 2026 it will be able to realize flights over a range of 500 nautical miles (926km) in aircraft with up to 80 seats. For 2030, Miftakhov has even bigger plans: "We will have single-aisle jets, 100-seat category," he says.

在英国政府、私人投资者、商业合作伙伴的联合支持下，ZeroAvia公司正在研发采用氢电（燃料电池）动力总成的飞机，载客量20人，最大航程约350海里（648公里）。ZeroAvia公司创始人、总裁瓦尔·米夫塔霍夫表示，公司希望在2023年利用这种飞机提供商业航班，2026年增至80个座位，最大航程达500海里（926公里）。

There is ambition in mainland Europe too. Hydrogen "is one of the most promising technology vectors to allow mobility to continue fulfilling the basic human need for mobility in better harmony with our environment", says Grazia Vitaldini, chief technology officer at Airbus, the world's largest aircraft manufacturer. In September 2020, Airbus announced that hydrogen-fuelled propulsion systems would be at the heart of a new generation of zero-emissions commercial aircraft. The project, named ZeroE, is a flagship of the European unx's multibillion-euro stimulus package, aimed at greening the bloc's economy.

欧洲大陆也充满了雄心壮志。氢是“最有发展前景的技术载体之一，能够使人类出行的基本需求继续得到满足，与我们所处的环境更和谐地相处”，世界最大的飞机制造商“空客”公司首席技术官格拉齐亚·维塔尔迪尼这样说道。2020年9月，“空客”公司宣布氢燃料推进系统将成为新一代零排放商业客机的核心。欧盟制定了数十亿欧元的刺激计划旨在实现绿色化经济，“零排放”是该计划的旗舰项目。
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Airbus has presented three concept planes which it says could be ready for deployment by 2035. The first is a turboprop (propeller) driven aircraft capable of carrying around 100 passengers about 1,000 nautical miles (1,850km). The second, a turbofan (jet), could carry 200 passengers twice as far. Both look similar to already existing planes, but ZeroE's third concept is a futuristic-looking blended-wing design that is a striking departure from commercial models today. Airbus says this third design could be capable of carrying more passengers over longer distances than the other two, but has not released more detail at this stage. All three designs are envisaged as hydrogen hybrids, which means they would be powered by gas-turbine engines that burn liquid hydrogen as fuel, and also generate electricity via hydrogen fuel cells.

“空客”公司已推出三款概念飞机，声称2035年可投入使用。第一款是涡轮螺旋桨飞机，载客量100人，最大航程约1000海里（1850公里）。第二款是涡轮风扇式飞机，载客量200人，最大航程增加一倍。两者的外形与当今飞机类似。第三款概念飞机采用极富未来感的翼身融合设计，外形明显不同于当今的商业机型。“空客”公司透露第三种设计的载客量更多，最大航程比前两者更远，但现阶段尚未公布更多细节。三款设计都被设想为氢混合动力飞机，也就是由燃气涡轮发动机提供动力，它以燃烧液氢为燃料，同时由氢燃料电池发电。

The work at ZeroAvia and Airbus has aroused a lot of interest, but not everyone in the aviation industry is convinced hydrogen will play a major role in a transition towards low or zero carbon flight.

ZeroAvia和“空客”的研发成果引发了许多关注，但并非所有的航空业人士都相信，在飞机向低碳或零碳转型的过程中，氢将发挥主要作用。

The disadvantages start with the physics and chemistry. Hydrogen has higher energy by mass than jet fuel, but it has lower energy by volume. This lower energy density is because it is a gas at typical atmospheric pressure and temperature. The gas needs to be compressed or turned into a liquid by cooling it to extremely low temperatures (-253C) if it is to be stored in sufficient quantities. "Storage tanks for the compressed gas or liquid are complex and heavy," says Finlay Asher, a former aircraft engine designer at Rolls-Royce and founder of Green Sky Thinking, a platform exploring sustainable aviation.

首先存在物理和化学上的缺点。氢的质量能量密度相对喷气发动机燃料较高，但体积能量密度较低。这是因为氢在一般的大气压力和温度下是气体，要想将足够多的氢储存起来，需要降至极低温度（-253C）才能压缩或转化成液体。“压缩气体或液体的储存罐复杂而笨重”，芬莱·亚瑟说道。他曾在劳斯莱斯公司担任航空发动机设计师，也是Green Sky Thinking平台创始人，旨在探索可持续航空。

And there are other challenges. The energy density of liquid hydrogen is only about a quarter of that of jet fuel. This means that for the same amount of energy it needs a storage tank four times the size. As a consequence, aircraft may either have to carry fewer passengers to make space for the storage tanks, or become significantly larger. The first option, which applies to Airbus's first two concept planes, would mean a reduction in ticket revenue, other things being equal. The second option, embodied in Airbus's third concept, requires a bigger airfrx, which is subject to more drag. Further, an entire new infrastructure would need to be put in place to transport and store hydrogen at airports.

此外还有其他挑战。液氢的能量密度大概只有喷气发动机燃料的四分之一，也就是说储存同等的能量需要四倍大小的储存罐。这样一来，飞机可能只有减少客运量来为储存罐留出空间，或者显著增大飞机尺寸。“空客”公司的前两款概念机采用第一种办法，这意味着在其他因素不变的情况下，机票收入会减少。第三款概念机采用第二种办法，这需要增大机身尺寸，受到更大的空气阻力。另外，机场需要配备全新的基础设施来运输和储存氢燃料。
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In addition, there is the question of whether hydrogen can be produced at scale and at a competitive price without itself having a large carbon footprint. The great majority of hydrogen used in industry today is created using fossil fuel methane, releasing carbon dioxide as a waste product. Hydrogen can be produced from water through a process called electrolysis, driven by renewable power, but this process is currently expensive and requires large amounts of energy. Only about 1% of hydrogen is produced this way at present.

还有一个问题是，氢在不产生巨大碳足迹的情况下，能否以具有竞争力的价格实现大规模生产。当今工业中使用的大部分氢是利用化石燃料甲烷制造而成，释放出的废弃物是二氧化碳。在可再生能源的推动下，可通过电解水工艺来制造氢气，但该工艺造价昂贵，能量消耗巨大，目前只有1%的氢气是以这种方式制造的。
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As things stand, liquid hydrogen is more than four times as expensive as conventional jet fuel. Over the coming decades the price is expected to drop as infrastructure is scaled up and becomes more efficient. But according to Britain's Royal Society and the management consulting group McKinsey, it is likely to remain at least twice as expensive as fossil fuels for the next few decades.

就目前的情况来看，液氢的价格相当于传统喷气发动机燃料的四倍。在未来的几十年里，随着基础设施规模的扩大和效率的提高，价格有望下降。但据英国皇家学会和麦肯锡管理咨询集团透露，在未来的几十年里，液氢的价格仍可能相当于化石燃料的至少两倍。

These factors, and others, give pause for some major players in aviation. Sean Newsum, director of environmental strategy at Boeing Commercial, Airbus's main rival, recently told the Financial Times: "Our belief is that it will take a while for all the technology and elements of hydrogen propulsion to be worked out before we can get to… commercial use." And a key advisory body to the UK government also has doubts. "Switching to direct hydrogen adds decades into the transition," says David Joffe, of the Climate Change Committee, an advisory body to the UK government.

这些以及其他因素会使航空业里的某些大企业犹豫不决。波音民用飞机集团是“空客”的主要竞争对手，其环境策略总监肖恩·纽瑟姆最近向《金融时报》透露：“我们认为氢燃料推进系统的所有技术和组成部分需要时间来完成，然后才能实现……商业化用途”。英国政府一个主要咨询机构也对此存疑。“直接以氢为燃料会使转型期延长数十年”，英国政府的咨询机构“气候变化委员会”的戴维·杰夫说道。

So what are the alternatives? In September 2020, the Air Transport Action Group, a Geneva-based body that speaks on behalf the global aviation industry, published a set of scenarios which suggest that, even as the volume of air traffic increases, it will be possible for global aviation to reach zero emissions of carbon dioxide – but only a decade or so later than 2050. According to these scenarios, the direct use of hydrogen will play only a marginal role, but the game-changer will be what are termed "sustainable aviation fuels", or SAFs.

那么还有别的办法吗？2020年9月，总部位于日内瓦、代表全球航空业利益的“航空运输行动组织”公布的一系列场景表明，即使航空交通量增加，全球航空业也可能实现二氧化碳零排放——但是比2050年晚十年左右。根据这些场景，直接采用氢燃料只能起到次要作用，起到变革作用的将是所谓“可持续性航空燃料”。

This catch-all term covers a range of products, such as biofuels, that result in low net emissions of carbon dioxide and other pollutants associated with conventional jet fuel. The advocates of SAFs argue they have a number of clear advantages over pure hydrogen. Because they are chemically identical to existing jet fuel, they can in principle be "dropped in" to existing systems with little or no redesign, without delay, and without the substantial ancillary investments required for hydrogen-powered airfrxs and their supporting infrastructure. Paul Stein, chief technology officer at the engine makers Rolls Royce, argues they are the key to a more sustainable future. "If SAF production can be scaled up – and aviation needs 500 million tonnes a year by 2050 – we can make a huge contribution for our planet," he says.

这一笼统词汇涵盖各种生成物，例如生物燃料，能够减少二氧化碳及其他与传统喷气发动机燃料相关的污染物净排放量。“可持续性航空燃料”的支持者认为，它们与纯氢相比有许多显著优势。由于化学性质与现有的喷气发动机燃料相同，理论上可“投入”现有装置，几乎不需要重新设计，不耽误时间，不必对氢动力机身和配套基础设施进行大量的辅助投资。发动机制造商“劳斯莱斯”的首席技术官保罗·斯坦认为，“可持续性航空燃料”是可持续未来的关键。“如果实现大规模生产——2050年航空业每年需要5亿吨——我们可以为自己的星球做出巨大贡献”，他说道。

SAFs can be divided into two categories. The first are biofuels made through the chemical or thermal treatment of biomass such as agricultural residues and other wastes. A second category is electro fuels, or "E fuels". Through these fuels, which are also known as "power to liquid", hydrogen could end up playing a key role in aviation after all.

“可持续性航空燃料”可分两种，第一种是生物燃料，通过对农业废弃物等生物质进行化学或热处理而生成。第二种是电燃料，也称为“电转液”，最终可能使氢在航空业里发挥关键作用。

E fuels are made by reacting hydrogen with carbon dioxide to make "syngas". This is then converted through what is known as Fischer-Tropsch process into "e-crude" – a crude oil substitute that can be refined to jet fuel and other fuels. If the large amount of energy required at each stage of manufacture is sourced from zero carbon sources, then the whole process can be carbon neutral, with no more carbon dioxide in the atmosphere after the flight than before the fuel was made.

电燃料的制造过程是让氢气与二氧化碳反应生成“合成气”，再通过“费托合成”工艺将“合成气”转化为“电原油”——这种原油代替物可被精炼成喷气发动机燃料及其他燃料。如果每个生产阶段所需要的大量能源取自“零碳源”，那么整个生产工艺就能实现碳中和，飞机向大气中排放的二氧化碳不会比制造燃料之前有所增加。
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Using direct air capture technology developed by the Swiss company Climeworks to source CO2, and hydrogen produced from water with renewable energy, the Oslo-based company Norsk e-Fuel aims to open what may be the world's first E fuel industrial plant Herøya, Norway, in 2023, producing 10 million litres of fuel per year for the Norwegian and European markets. The next step in 2026 would be a plant capable of 100 million litres a year. A full-sized plant could provide half the fuel for the top five most frequently serviced flight routes within Norway, cutting their emissions by half, says Karl Hauptmeier, Norsk e-Fuel's managing director.

采用瑞士Climeworks公司研发的“直接空气捕获”技术，利用可再生能源捕获水产生的二氧化碳和氢气，总部位于奥斯陆的Norsk e-Fuel公司计划于2023年在挪威哈略建造世界上有可能第一座电燃料工厂，每年为挪威和欧洲市场生产1000万升燃料，下一步是在2026年建造一座年产量达到1亿升的工厂。一座全尺寸工厂能够为挪威最繁忙的五条航线提供一半燃料，使排放量减半，Norsk e-Fuel公司总裁卡尔·豪普特迈耶说道。

For now, one thing remains almost certain: hydrogen and E fuels are likely to continue to be substantially more expensive than conventional jet fuel for years or decades to come, limiting their role in greening aviation – unless the other costs of aviation come to be weighed differently. Campaigners such as Leo Murray of the campaign group Possible argue that conventionally-fuelled aviation must be priced to reflect the cost of the damage it causes to the climate. This, he argues, might mean higher ticket prices, but it could also give us aviation that does not cost the Earth.

目前，有一件事情几乎可以肯定：在未来几年甚至几十年里，氢气和电燃料的价格仍将远高于传统喷气发动机燃料，使它们在航空绿色化中发挥的作用受限——除非换个角度衡量其余的航空成本。活动人士诸如来自活动组织“可能”的里奥·默里认为，使用传统燃料的航空在被定价时，必须反映出对气候造成破坏的成本。他认为这可能意味着飞机票涨价，但也可能避免航空让地球付出代价。