The weekend read: Guilt-free solar flying?


From pv magazine 12/2019

The hardest flights to pass up are the “love miles” – those important visits to loved ones. No one needs a shopping trip to London with a budget airline; many holiday destinations can be easily reached by other modes of transport, and for business, a telephone conference can often suffice. But the birthday of a far-flung sister, or weekend visits to a partner in a distant city – do we really want to abstain from these flights to protect the climate? “Love miles” make things a bit more difficult.

Global air traffic accounts for only about 2.5% of total global CO2 emissions. Yet, for many people, flights are by far the largest item in the individual climate footprint, and thus the most important factor in reducing your carbon footprint. Frequent flyers often feel pressure to justify themselves, and travelers feel themselves to be “flight shamed.” But the gap between awareness and action is huge. The International Air Transport Association (IATA) expects the number of air passengers worldwide to double over the next 20 years. Policymakers are calling for these flights to be more climate friendly. The EU has set itself a target of reducing CO2 emissions from aviation by 75% by 2050. That falls short of the Paris climate targets, but it is enough to present the aviation industry with major challenges.

The industry is pinning its hopes on kerosene that is produced synthetically from hydrogen and CO2. The aviation industry argues that if the hydrogen it used is produced by electrolysis with solar or wind power, the fuel is climate neutral. In the town of Heide in northern Germany, a consortium consisting of partners from the industrial and scientific communities plans to build a plant that will produce methanol from the air using water, wind power and CO2 (Power to X). The fuel will then be processed into synthetic kerosene in a local refinery using the Fischer-Tropsch process, which has been used in the chemical industry for decades.

The fuel would be sold to Lufthansa. “Starting in around 2024, we want to cover part of our fuel requirements at Hamburg Airport with synthetic kerosene from the Heide refinery,” says Steffen Milchsack, a sustainability expert at Lufthansa. Since it has roughly the same characteristics as fossil kerosene, the aircraft can easily handle it, says Milchsack.

Solar kerosene

While such concepts are primarily based on using electricity from renewable sources, the international “Sun to Liquid” research project is currently testing the production of kerosene using solar radiation, in a process similar to solar-thermal power generation. At the center of its research facility near Madrid are huge mirrors that reflect the sun’s rays to the top of a tower. The heliostat array, which tracks the sun, concentrates sunlight by a factor of 2,500. The resulting heat of up to 1,500 degrees Celsius is used to convert water and carbon dioxide into a synthetic gas of hydrogen and carbon monoxide in a thermochemical redox reaction. Synthetic kerosene can then be produced from the gas using the Fischer-Tropsch process.

The advantage of this process over electrolysis is that fewer energy conversion steps are required, explains Christian Sattler from the Institute for Solar Research at the German Aerospace Center (DLR), which is participating in Sun to Liquid.

That holds the potential for higher efficiencies, “but it needs sites with very good solar conditions,” Sattler explains, which is why both methods are needed. “The choice of technology depends on the size of the facility, the location, the product, and other factors.”

The Sun to Liquid research partners expect it to take at least another 10 years before industrial production of solar kerosene is conceivable. But the electricity-methanol route, as is now being pursued in Heide, is different: The individual components of the process have been tried and tested, and the technologies are mature.

This alternative fuel will initially be three to five times more expensive than its fossil-fuel equivalent. How the costs of synthetic kerosene will develop in the medium to long term cannot be stated authoritatively at such an early stage of market development – this depends, among other things, on the political and regulatory framework, on the speed with which production plants are scaled up, on the establishment of an international hydrogen economy and, not least, on the further development of the technologies.

Milchsack advocates increased support for research and putting the process to use. “Airlines in Germany currently pay more than €1.2 billion a year in air traffic taxes, which end up in the federal budget,” he says. “These funds could be invested in the research and production of synthetic kerosene.”

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But even at today’s production costs, the price premium for the CO2 neutral fuel would still be bearable for passengers in many cases. This is because kerosene currently accounts for 25 to 30% of Lufthansa’s operating costs. The complete substitution of fossil kerosene with synthetic fuel would roughly double operating costs. In view of current, often very-low ticket prices, the added cost would likely be acceptable for many customers.

Another challenge

Is kerosene produced with solar or wind power the miracle cure that will allow air traffic to grow as forecast without ruining the climate? For Volker Quaschning of the University of Applied Sciences (HTW Berlin), the answer is no. “In the case of electricity-derived kerosene and biofuels, people usually forget that, due to the formation of condensation trails at high altitudes, they have a much greater effect on the climate than on the ground,” Volker says. Condensation trails form when particulates from water vapor and exhaust hit the cold air at high altitudes, causing ice crystals to form. They prevent heat from radiating from the earth into space, thus contributing to the greenhouse effect.

“A climate-neutral aircraft fuel is not the same thing as a climate-neutral fuel,” says Ulrike Burkhardt of the DLR, who published a study on this subject with her colleague Lisa Bock. However, it is still unclear to what extent condensation trails contribute to global warming. One thing is certain: The more water vapor and soot the aircraft emit, the more condensation trails form. Synthetic kerosene could have a slight advantage over fossil kerosene in this respect, since fewer soot particles tend to form during combustion. According to Burkhardt, however, it is not possible to make sweeping statements about this, as the climate effect of alternative fuels depends on their chemical composition. She warns against ignoring the effect of contrails in the design of the aviation emissions trading scheme. “A system that looks only at CO2 emissions can overlook much of the climate impact.”

Add a battery or fuel cell

In the fight against global warming, alternative fuels such as synthetic kerosene are thus not a solution. Climate-neutral air traffic is possible only with electric propulsion – either battery-powered or with fuel cells, provided that the water produced during the conversion of hydrogen is collected to prevent it from escaping into the atmosphere and forming contrails there.

Possible applications for battery-powered electric drives are extremely limited due to the low energy density of the batteries. Milchsack calculates that to replace a single liter of kerosene, you would need batteries weighing 60 to 70 kilograms. According to the federal German Aviation Association (BDL), airlines currently require, on average, some 3.6 liters of kerosene per passenger per 100 kilometers. If we do the math, we find that an airplane with 150 seats on the route from Munich to Stockholm would require batteries weighing nearly 500 tons – many times heavier than the weight of the airplane itself. Therefore, the potential for battery-powered propulsion is limited to air taxis and small aircraft for short-haul flights.

Fuel cells are more promising. “Aircraft with 30 or 40 seats and a range of 1,000 to 2,000 kilometres are technically feasible without any problems,” says Josef Kallo of the DLR. With his team and their partners, Kallo has developed the four-seater HY4, which has already completed 30 test flights. The researchers are currently working on optimizing the way all of its components interact. Kallo expects the four-seater to be commercially viable in five to six years, provided his research budget stays at the current level. A 30- or 40-seater could take a few more years.

“The challenge here is not the technology, because an enormous amount of work has been done in recent years with the materials and the fuel cells themselves,” says Kallo. “Certification is likely to be a much greater hurdle.” And cost? Based on current hydrogen production prices, the DLR researcher assumes a net cost of €4.50-€5/100 km per passenger for a fully occupied 40-seater aircraft. This is about triple the cost of fossil kerosene at today’s prices, based on BDL figures. This is just the fuel cost; other costs, such as the development of the hydrogen infrastructure at the airports, would incur additional costs. Nevertheless, green hydrogen will become cheaper with the scaling-up of production in the coming years, and fossil kerosene will become more expensive with the introduction of CO2 trading, which should narrow the price gap.

Can the fuel cell reconcile air traffic and climate protection? Volker Quaschning points out that even if hydrogen-powered aircraft are ready for the market by 2030, it will still take several years before they are available in large numbers. “But we don’t have that much time,” he says. “After all, we have to become climate neutral within the next 15 to 20 years. So, the inescapable conclusion has to be, no more flying!” Quaschning no longer travels by plane.

As an optimist, however, you can also put a positive spin on all this: As research progresses, at least our grandchildren should be able to travel by plane without completely ruining the climate. And even the current generation may one day be able to enjoy their “love miles” in a climate-neutral way – if the airlines are prepared to switch to the new engines as soon as the fuel-cell aircraft are ready for the market.

Ultimately, it is probably up to travelers themselves. If they are unwilling or unable to discontinue their air travel, they must decide whether they are prepared today to pay more for CO2-free kerosene. And in 10 years’ time, they must make the switch to climate-neutral aircraft, or push policymakers in that direction.

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