By First Officer Adam Crehan- Clark, BALPA member Could the aeroplanes of the future be powered by something other than jet fuel? POWER SHARING magine this: the ground handlers disconnect the external power and give their final checks. Push and start clearance is obtained, you say goodbye to the ground crew and reverse your electric passenger jet on to the taxi line with motors in the wheels. The main engines start up instantly, and the taxi to the holding point is almost in silence. The airline is happy with reduced costs, the environmentalists are happy with the reduction in carbon footprint and, hopefully, you’re happy with a much more comfortable and quieter cockpit. This may not be our reality, but could it be the reality for our children, who might decide to follow us in our careers as airline pilots? If so, how far away is it – and why do we not have it already? The short answer is that the gravimetric energy density of today’s batteries is too low. We’ve all seen the groundbreaking and industry-changing work of electric automotive manufacturer, Tesla. The problem starts as soon as we add a third dimension – air – to a vehicle. This massively increases the demand on an engine. We no longer have the ground to support the weight of an engine and fuel. For aviation, a propulsion system needs not only to provide an element of forward velocity, but also to counter its own weight through the generation of lift. The present generation of batteries cannot provide enough energy to be able to lift themselves, let alone provide any excess energy for payload or structural weight. This is the main barrier to producing a large, electric- powered commercial aviation transport vehicle. For the long answer, we can investigate what it would take to convert a standard THE CURRENT GENERATION OF BATTERIES DON’T HAVE ENOUGH ENERGY TO LIFT THEMSELVES level, dropping to 25kN in the cruise due to the drop in air density. This is equivalent to a power output of 6MW and the average jet engine weighs about 2.2 tonnes. The typical industrial electric fan on the market is around 1MW/tonne, so we would need six tonnes per engine to get the same output. A realistic advance in technology could get to 2MW/tonne, so only a three-tonne engine would be needed. But, again, any increase in mass requires a much higher increase in energy to keep it airborne. The big advantage from electric fans comes from not requiring the expelling of exhaust gases out of engine, like a traditional engine needs to. The engine, fairing and its mounting position could be radically re-designed, leading to all sorts of revolutionary aircraft designs that could shake up the entire industry. Lots of concept designs have been drawn up, which range from fairly familiar underslung designs to weird and wonderful prop-pusher delta wing combinations. Other advantages include very few moving parts, short- to medium-range passenger jet from a fossil fuel source to an electric power source, albeit with some simplistic maths and physics. This type of aircraft usually has around 55 tonnes of operating weight and payload, and a capacity of around 10 tonnes of jet fuel on top of that. For an aircraft of this type, the average amount of energy required per hour to maintain a constant altitude and speed in the cruise is around 100,000 mega joules (Mj). Jet fuel has an energy density of around 43Mj per kilogram, so it would burn slightly more than two tonnes of fuel per hour of cruising to keep aloft. With a capacity of around 10 tonnes, this gives us a usual endurance of around five hours. That’s enough to get around all of Europe, or even cover the distance between London and New York – with some modifications and restrictions. This means it takes about 1.6Mj per hour to keep a kilogram of mass airbourne. One of the most suitable and most advanced rechargeable batteries available today is lithium nickel cobalt aluminium oxide (LiNiCoAlO2 - NCA), which has an energy density of around 0.8Mj per kilogram. So to provide the energy per hour required for an hour of cruising, we need 125,000kg of charged batteries per hour. For just a four- hour hop with the electric aircraft, we would need to load on 500 tonnes of batteries, making the total weight with operating weight and payload 550 tonnes. This type of weight would be nearly impossible to operate, and the energy requirement would increase again because of the weight of the batteries and structural improvements. So, until batteries get to at least 1.6Mj/kg, they cannot lift themselves and the total weight required will increase in an unending and recursive fashion. Each excess iota of energy over the 1.6Mj/kg limit can then be converted into lift for the airframe itself, and further excess can be turned into payload. Another disadvantage to a battery powered aircraft is that a charged and a discharged battery weigh the same. With a conventional fuel engine, we can take off above maximum landing weight, knowing that we will land under the weight limit as the fuel burns off; with an electric aircraft, we need to take off below the maximum landing weight. Worth its weight So, weight is a limiting factor. But suppose we could design a battery tomorrow that has the same energy density as jet fuel? What about electric motors? A standard high- bypass turbofan produces between 100-150 kN (kilonewton) of static thrust at sea which would drive down maintenance costs. Electric engines would also not produce anywhere near as much heat as a combustion engine, so different materials could be introduced into the core of the design. The batteries would also be rechargeable, so could potentially – depending on the initial source of the electricity – be cheaper to operate per unit nautical mile, and be more environmentally friendly. So, does it look likely that batteries will improve enough to surmount the energy density problem? A study in 2010 concluded that technological advances mean that the energy density of batteries increases about 0.1Mj/kg per year. By this prediction, we may have batteries with 2Mj/kg in 2030. However, without any large leap or exponential increase in battery technology, we could only match jet fuel for energy density in 2400. So maybe our great-great-great-grandchildren will be flying electric aircraft, or be flown by autonomous electric aircraft while people talk nostalgically about the golden era of aviation when aircraft were flown by people. Lighter than air Another renewable energy option being researched is hydrogen-powered aircraft. Hydrogen has the opposite problem to batteries: with an energy density around 134Mj/ kg, it has about three times the energy per kilogram of jet fuel. This would either reduce the take-off weight required for a normal cruise or keep the weight the same, but vastly increase the range. Also, the only exhaust by-product of a hydrogen engine is water, potentially leaving a very small carbon footprint. So hydrogen seems perfect, right? Unfortunately, just like the batteries, there are a few physical properties of hydrogen that stop it being the aircraft fuel of the future. First, while hydrogen has a much higher energy density by weight, because it is stable as a gas, it has a much lower energy density by volume, so much larger fuel tanks are required. Even though a fully laden hydrogen aircraft would weigh less, it would have a lot more drag as it would require a much larger surface area, making the shape of the aircraft more like an airship than a fixed wing aircraft. In fact, airships were filled with hydrogen until the Hindenburg disaster, but they used hydrogen as a ‘lighter than air’ lifting source rather than a fuel source. The second problem is that hydrogen in any form is highly volatile and needs to be held under high pressure in strong, reinforced fuel tanks. They also need to be held close to the centre of gravity in the fuselage area of the aircraft. This runs counter to our current way of carrying fuel in the wings and payload in the fuselage. So again, a full redesign of how we construct and imagine our aircraft would be required. Hydrogen in a stable, fuel-ready state also needs to be produced, usually by separating the hydrogen and oxygen in water. It can possibly be produced by low- carbon energy sources like wind, water or nuclear, but is currently made by inefficient fossil-fuel processes. This process of making fuel-ready hydrogen is both more expensive and more environmentally damaging than the current fossil fuels we already use. Potentially, a rise in demand for hydrogen fuel would also raise more investment in the production of hydrogen, which could spur on more efficient sources of the fuel. But until hydrogen’s other problems are surmounted, it remains in a Catch-22 situation. Staying power Barring any unpredicted and significant technological leap in the near future, for now it seems jet fuel is here to stay. With aircraft manufacturers refining current jet engines and making efficiency a high priority, we are seeing more and more environmentally friendly aircraft taking to the skies. Bigger leaps in efficiency will come from restructuring airspace, so routing between destinations can be made shorter and aircraft can fly at optimum levels more regularly. A small change on a long-winded SID or STAR can make a huge difference when tens of thousands of aircraft routinely take identical flight paths. Removing step climbs and having a bilateral airline agreement to refrain from fuel tankering will also make appreciable differences. While it’s highly unlikely any of us will be operating in a fully electric passenger jet any time soon, it’s highly likely we’ll manage to make improvements in the carbon footprint of our industry through innovation and administration. Many airlines have projects currently under way, and they are investing a lot of time and energy into finding solutions to the problems listed in this article. Depending on the outcomes of these labours, you might be getting into a renewable energy-powered aircraft sooner than you think. TECH LOG POWER SHARING TECH LOG Could the aeroplanes of the future be powered by something other than jet fuel? POWER SHARING TECH LOG