Green Aviation

Those who know me will know of my life long-love of aviation. I wanted to be a test pilot even before wanting to be an astronaut but ended up getting into energy.  My under-graduate dissertation combined my two interests as it was on the use of hydrogen as an aviation fuel, an idea that was being heavily promoted in the late 1970s by Lockheed and others with a proposal to build hydrogen fuelled Tristars ferrying between the US, Europe and the Middle East.  (This document is now online here).  I gave up flying myself a few years ago and still miss it.  As everyone knows aviation is a major problem in terms of emissions, with 500 million tonnes CO₂ a year expected to triple by 2050, and I do sometimes struggle to square the desire to minimise emissions and environmental impact and the need and desire to travel, especially with the amount of air travel I have done in the last five years.

To start with I don’t think we can ever (or should try) suppress the basic human desire to travel. I actually think this is a reflection of the exploration drive, without which we would still be the arboreal primates or even the ocean dwelling mammals we are descended from.  Likewise we should not constrain space exploration, in fact we should be doing more, it is just a fundamental human drive.  Given that, plus all the conventional economic trends (i.e. increasing wealth leads to increasing travel), we will continue to see a growth in air travel with an increased environmental impact – if we don’t change the technology.  The question is how to change the technology and how quickly can we do it, especially in the safety driven culture and regulatory environment of aviation.

A decade ago the emphasis was on bio-fuels and much capital was invested in biofuel production and trialling them in aircraft.  Safety is of course critical in aviation and I always said I would rather not fly in a bio-fuelled aircraft for the first five years of its use (the same would apply to electric planes) – although that would have been different if I had become a test pilot!  Clearly bio- and synthetic fuels will have a major role but the ultimate dream is electric power. Even a decade ago the idea of electric aircraft was science fiction but since then the advances in battery technology, coupled with the work of entrepreneurs and larger companies, has made the dream of electric aviation seem much closer.

A recent article in AirSpaceMag.com described some of the developments including Eviation’s nine passenger regional commuter plane that is supposed to fly in 2019 (which has the odd name of Alice).  It shouldn’t be a surprise, as we have seen the same thing in electric cars, but the choice of electric propulsion leads to significant changes in the way the rest of the aircraft is designed, both in terms of structure and layout.  A lot of the structure of aircraft is designed to cope with the stresses of relatively heavy, vibrating engines. Electric motors are lighter but of course there is the huge weight of batteries which will account for 60% of Alices’s total weight.  For comparison fuel makes up c.48% of the weight of a fully laden Boeing 747.  The range of the Alice is estimated at 650 miles at 275mph.

Interestingly enough the projected operating costs are low enough that the cost to passengers could be reduced by 30-60% compared to a conventional aircraft, savings being made in fuel and maintenance.  Bonny Simi, President of JetBlue Technology Ventures, is quoted in the article as saying on short trips regional turboprops have an Available Seat Mile (ASM) cost of $0.15 to $0.20 with spikes above $0.40.  Larger capacity, long haul jets have ASMs in the range of $0.08 to $0.12 as they fly higher where jets are more efficient and proportionately less time is spent in take-offs and climb.  Simi goes onto say “Forecasts for electric aircraft [flying] 300 to 700 miles estimate 10 to 12 cents” ($0.10 to $0.12 per ASM).  If that sort of cost advantage can actually be delivered the economic driver is clear.

As everyone knows battery technology is improving rapidly and costs are falling. There is, however, a long way to go before larger aircraft could be electric.  The “magic number” where long-distance flight can become viable is cited at 1,000 Wh/kg of battery weight whereas existing batteries are in the range of 270-300 Wh/kg.  The battery in a Tesla S stores 85 kWh and weighs 540 kg – a specific energy of 157 Wh/kg.  The target 1,000 Wh/kg for viable aircraft does not seem to take into account possible improvements in drag reduction (and possibly further advances in light weight structures).  Professor Viswanathan from Carnegie Mellon University asserts that a battery producing 400-500 Wh/kg could propel an airplane 200 to 400 miles on a single charge.

EasyJet has set a target to begin operating electric routes within 10 years and Norway has proposed making all flights less than 1.5 hours all electric by 2040.  EasyJet has partnered with Wright Electric who are working on an aircraft that could carry 120 people on flights of 300 miles or less. Although these targets are exciting we should never forget both the hype cycle and the length of time (& huge amount of money) it takes to get new aircraft certified for public operations.  There is a long way, and lots of capital, between announcements and glossy websites and computer generated images and a flying, certified aircraft.  As I highlighted in my under-graduate dissertation, it is not only the aircraft where you need to innovate and invest, the ground infrastructure would need to change considerably.  When Terminal 5 was constructed it was designed with higher capacity ground power connections as the A-380 was coming into service, imagine the extra power capacity needed at airports for electric aircraft re-charging, as well as the operations impact because of the required charging time.

At the larger, long-range end of the spectrum the direction of travel is towards hybrids.  In 2008 Boeing introduced the concept SUGAR (Subsonic Ultra-Green Aircraft Research) Volt which has not been built. NASA is also working on hybrid concepts. At the Glenn Research Center the focus is on concepts that could carry 150 people long distances.  As well as propulsion systems designs concepts include the highly efficient blended wing designs, a big departure from the tube designs we are familiar with. (I think it was low cost aviation pioneer Freddy Laker who said he was in the “aluminium tubes” business.) Blended wing designs can save 50% of fuel usage and NASA is moving towards funding a flying large-scale X-plane by 2021.  In July 2018 at the Farnborough Air Show the UK Business Secretary Greg Clark announced a £343 million government and industry R&D drive including £58m for electric flight.

Aviation has always been incredibly innovative.  Thirty three years separated the Wright Flyer and the DC-3, the first really effective air transport aircraft, thirty seven years separated the DC-3 and the Boeing 747 which significantly reduced cost and enabled the boom in international travel, forty years separate the Boeing 747 and the Boeing 787 which has a c.50% lower fuel burn.   With all the research on innovations in aircraft design, engine design, electric propulsion and batteries it is clear that aircraft can continue to become much more efficient and ultimately much cleaner for the environment than they are today.  The race is on between reducing emissions through higher efficiency and new propulsion technologies and increasing demand for air travel.