Don't Book Your Solar-Powered Flight Yet

• An around-the-world flight by a solar-powered airplane is a remarkable achievement, but it does not signal that solar passenger planes are the next big thing.
• Compared to other options, solar's low energy density makes it an especially challenging pathway for pursuing large cuts in the emissions from aircraft.

Earlier this week the pioneering solar-powered airplane, Solar Impulse 2, completed its record-setting circumnavigation of the Earth, returning to Abu Dhabi. Just a few hours earlier, the US Environmental Protection Agency announced its intention to regulate greenhouse gas emissions from aircraft engines under the Clean Air Act. Over the last dozen years I have written numerous posts linking stories like these together, but this is one case in which I sincerely hope these events were entirely unrelated. That requires a bit of explanation.

Let's start by acknowledging the engineering talent and sheer courage involved in the flight of the Solar Impulse 2 (Si2). The aircrew and designers deserve all the kudos they will receive; they have earned a place in aviation history. However, notwithstanding the prediction of pilot Bertrand Piccard that, "within 10 years, electric aircraft could be carrying up to 50 passengers on short to medium-haul flights," I am skeptical that this project will be the forerunner of solar-powered commercial flight in the way that Charles Lindbergh's transatlantic flight in 1927 led to the first non-stop commercial flight across the Atlantic in 1938.

There's no anti-solar bias involved in that statement, just an appreciation of the constraints that physics and geometry (e.g., the "square-cube law") impose on the amount of solar energy an aircraft can harvest during flight with anything like current technology. Energy density is an essential factor in the economics of commercial air travel.

According to the website for the Si2, the aircraft is approximately "the size of a 747 with the weight of a car." That should be our first hint that scaling up to the performance and capacity of today's jets would be an even bigger challenge than the one these folks have just completed. During the course of its journey, which entailed over 500 hours of flight spread across 17 months, the Si2 collected and consumed electrical energy equivalent to a little over 300 gallons of kerosene-based jet fuel. By comparison, a Boeing 777, which is capable of carrying up to 400 people, burns an average of around 2,000 gallons of jet fuel per hour.

If you covered a 777's wings with the same 22%-efficient SunPower solar cells used by the Si2, they would generate the fuel-equivalent of less than 3 gallons per hour at noon on a cloudless day. Even allowing for the higher efficiency of electric motors compared to gas turbines, that is still orders of magnitude less than the energy necessary to push a fully-loaded jetliner through the sky at 550 miles per hour. (The Si2 averaged 47 mph.)

As the Financial Times reported, the near-term applications of solar-powered flight are likely limited to surveillance drones and other specialized platforms for which long-range fuel-free flight confers a big advantage. I could also envision lightweight, high-efficiency solar cells being used on next-generation commercial aircraft to provide auxiliary (non-motive) power, saving both fuel and emissions.

That brings me back to the EPA. The agency's stated rationale for targeting aircraft engines now is that they expect these emissions to increase in the future, and that reductions would lead to climate and health benefits. There's no mention of solar-powered aircraft, and I must trust that had nothing to do with their announcement.

The EPA's latest greenhouse gas inventory reported that in 2014 commercial and other aircraft accounted for 8% of US transportation-related emissions, and about 2% of all US emissions of CO2 and other greenhouse gases. It also showed that aviation emissions have fallen 22% since 2005.

Perhaps the growth they are worried about is proportional, rather than absolute, as emissions from electricity generation and other sources decline faster. However, compared to cars and light trucks that account for over 60% of emissions from transportation, and for which many emission-reduction options are available, aviation is a small and rather challenging focus for further reductions.  Those will likely rely on advanced biofuels, along with additional gains in turbine efficiency and airframe weight reduction. 

The website for Solar Impulse 2 acknowledges that its flight was intended to highlight the earth-bound applications of renewable energy: "Behind Solar Impulse’s achievements, there is always the same goal: show that if an airplane can fly several days and nights in a row with no fuel, then clean technologies can be used on the ground to reduce our energy consumption, and create profit and jobs." Solar-powered air travel for the masses seems pretty far off, and certainly not something we can count on for cutting our emissions. 

Jet Fuel from Trees (or Almost Anything Else)

Out of the dozens of press releases that hit my email inbox in the last week, one that caught my eye was for a gathering of a group called the Northwest Advanced Renewables Alliance (NARA) in Missoula, Montana this Thursday.  Their agenda is focused on "challenges to develop a residual woody biomass to jet fuel and valuable co-products industry in the Pacific Northwest."  Somewhat more snappily, their website calls this "from wood to wing."  With oil prices (UK Brent) persistently over $100 despite the weak global economy, the appeal of such an effort is not hard to understand.  Whether it's feasible at an acceptable price remains to be seen.

Making fuels from waste or non-food crops is an attractive idea, and aviation fuels look like an especially promising market for bio- and synthetic fuels, for several reasons.  Unlike the markets for motor fuels--gasoline and diesel--you wouldn't have to convince millions of customers of the efficacy of using a new fuel.  You'd only have to convince the fuel buyers and chief engineers of a handful of airlines and aircraft leasing companies, along with the even smaller universe of engine suppliers.  Certifying that your fuel meets all relevant specifications is a key step in that process, though in some respects that should also be easier than for gasoline and diesel engines. If you doubt that, just consider the current fuss over increasing the ethanol content of gasoline from 10 to 15%.  Of course, having your car engine fail on the interstate is a very different proposition than having both engines shut down at 40.000 ft--or during take-off.

Fortunately, turbine engines are very reliable and fairly flexible.  The best proof of the latter is that the turbine at the heart of a natural-gas-fired power plant is essentially just a bigger version of the ones hanging under the wings of a Boeing or Airbus aircraft, which burn a close cousin of kerosene, a simple distillate refined from a wide variety of crude oils. Turbines on ships burn a fuel similar to diesel. Many of the specifications that jet fuel must meet have more to do with the conditions under which aircraft operate than the specific sensitivities of jet engines.  One example of that is the temperature at which a jet fuel becomes difficult to flow, just before it freezes solid.  That's one reason that many oilseed-based biojet fuels require essentially oil-refinery levels of processing.  Stepping back from such details, however, I'm skeptical that crop-based biofuels are the long-term solution to the fuel-diversification needs of aviation, for many of the same reasons we see playing out with regard to corn ethanol during the current drought.

Supporters of various biojet efforts often focus on two main benefits of renewable jet fuel.  The first is the reduction of greenhouse gas emissions, since the principal alternative available to airlines or military aviation is further efficiency improvements, which face diminishing returns, or reduced operations.  The other benefit that I often see cited is potential cost savings versus petroleum, though I regard this as largely illusory, at least on the level of the fuel customer.  As I've described at length, the output of even a captive biojet facility is worth its price in the market--set by petroleum jet fuel--not its cost of production.  That argument should also hold true for airlines buying oil refineries.  However, to the extent that biojet could be scaled up enough to apply competitive pressure on the 6 million barrel per day global jet fuel market, or in  isolated regional markets, that would benefit both airlines and consumers. Production at that scale will require feedstocks that are readily available in large quantities.

Many companies and researchers are pursuing renewable jet fuel pathways that don't rely on food- or food-competitive crops.  The RenewableJetFuels.org website of the Carbon War Room provides a portal into some of these efforts, including fuels based on factory waste gases, algae, and various other approaches.  Some of these have progressed to demonstration-stage production and fleet certification, though as we've seen with cellulosic motor fuels, scaling up to truly commercial production represents a much higher hurdle that could shake out many of these contenders.  For that reason, it's encouraging to see the NARA effort, nor should they worry about being too late to the party.

Prius in the Sky

A lot has changed since I last commented on the competing air travel visions of Airbus and Boeing two years ago. The former has stumbled, with its flagship A380 plagued by delays and manufacturing problems. Meanwhile Boeing rolled out the avatar of its new vision this week to much fanfare. Paralleling this change in fortunes, the implications of these two technologies now look different, as well. In 2005 I was concerned about the potential of thousands of A380s to put billions of new travelers in the air, consuming enormous incremental quantities of jet fuel in the process. But with the world increasingly worried about climate change and the means of managing it economically, Boeing's Dreamliner looks like the aircraft equivalent of Toyota's Prius hybrid car: the first real demonstration of a set of technologies that could dramatically reduce both fuel consumption and greenhouse gas emissions in the aviation sector, at least compared to their status quo trends.

A recent Economist article looked at this in some detail. It cited figures from the UK's Stern Report on climate change indicating that emissions from air travel, though only around 3% of the total today, are growing at a faster rate than those from other sectors. Saving 20% of fuel and emissions with the 787's better engines and lighter construction may not sound as dramatic as the doubling of fuel economy in hybrid cars, but aircraft don't offer similar opportunities to recapture braking energy, which is where hybrids derive most of their gains.

Economic growth is intertwined with mobility, and as long as the global economy keeps growing, more and more people will be flying. While planes like the 787 represent a hardware solution for minimizing the energy and environmental impacts of that growth, a broader range of strategies will be needed. Travel booking websites like Expedia already connect green consumers with the means of offsetting the emissions from their air travel, but airlines could provide this service on all their tickets at a lower cost; in the not-too-distant future, they may be required to do so.