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About this blog: Climate change, despite its outsized impact on the planet, is still an abstract concept to many of us. That needs to change. My hope is that readers of this blog will develop a better understanding of how our climate is evolving a...  (More)

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Flying: Coming soon to an airport near you...

Uploaded: Apr 12, 2020
This is the fourth post in an (intermittent) series on flying…

Flying is a growing component of greenhouse gas emissions, and one of the largest for many of us. Can we find a way to halve aviation emissions in the next 10-20 years that doesn’t involve a global pandemic (!) or rely on people volunteering to fly less? With planes on track to be 25% of global emissions by 2050 (1), we need to make some big changes to how we fly. Is there a technical solution? Are we close to building low-emission planes?

Many of you drive hybrids, EVs, or even hydrogen-powered cars. So you may be wondering if we can apply those technologies to planes. Unfortunately, it’s not easy. Fundamentally, batteries are heavy and hydrogen takes up a lot of space. A “good” fuel in the graph below is high up (has lots of energy per volume) and far to the right (has lots of energy per weight). You can see that lithium-ion batteries (purple) are heavy, hydrogen (blue) is light, and neither has close to the energy per volume as jet fuel (red).

Colored dots indicate jet fuel (red), compressed hydrogen (blue), and lithium-ion batteries (purple). Fuels that are higher and to the right are more powerful. Source:

Electric Planes
Batteries have very low energy by weight, with current lithium-ion batteries at around 250 Wh/kg. The technology is improving — various sources suggest we may have 350 Wh/kg in five years, and 500 by 2030. But 800 Wh/kg is needed for standard-sized jets to go even 600 miles, by some estimates. (2) Only small electric planes will be flying for the next 15-20 years, and even then will go only relatively short distances. Eviation’s Alice is one such example, and has been described by its CEO as “basically a huge battery with some plane painted on it.”

Eviation’s 9-passenger electric plane “Alice”. Source:

Small planes flying short hops are not likely to significantly reduce aviation emissions. Indeed, the focus of many of the electric aircraft companies seems to be more on convenience than on globally significant emissions reductions. Several are developing personal aircraft for commutes, representing a sort of uber-ization of the airspace. (3) The pioneers in this field are hoping to take advantage of the many under-utilized local airports across the country to transform the way that people commute today. Following a path similar to Tesla, these early products are targeted at the very wealthy. To the extent the new aircraft replace cars and trucks, they will add to air traffic but with little impact on emissions since we are electrifying our road transportation. While it’s important to start somewhere, I find it hard to get excited about having these small personal aircraft zipping around overhead to avoid congestion on our streets. When I look at how kindly (or not) we have managed our land and our oceans, I can’t help but worry when we start setting our sights on the skies.

Hybrid Planes
So, how do we clean up the larger jets that represent the lion’s share of our emissions? Hybrids are an option. Plane concepts are being developed in which batteries would be used for takeoff and landing, the flight segments that generate the most emissions. With batteries offloading the most intensive power needs, the jets used for cruising can be smaller and more efficient, which should further reduce the climate impact. Large planes with hybrid drivetrains could plausibly be flying in 30 years. The Airbus E-Fan X, NASA STARC-ABL, and Boeing Sugar Volt are examples of this. But because the advantages of these hybrids apply mainly to the takeoff and landing segments, it could be that they will show substantial emissions reductions only for shorter flights, which represent only about 30% of aviation emissions (see left axis in chart below).

Flights of 1500 km or less constitute around 30% of emissions. Source: ICCT

Hydrogen Planes
What about hydrogen-powered planes? Hydrogen has very different characteristics than batteries do. While batteries are heavy, hydrogen is big. You can see that in hydrogen-powered cars. The fuel tank on a hydrogen-powered Toyota Mirai is twice the size of that of a Camry (32 vs 16 gallons), but its range is only half as much (300 vs 600 miles). Large planes go much farther than a few hundred miles, so where are you going to put all of the fuel? Even when you account for the increased efficiency of the electric motor in these planes (hydrogen typically powers electric motors), and the lighter weight of the fuel, estimates suggest that 3x more space is needed for liquid hydrogen and 5x more if it is used as a compressed gas (this is more common).

An earlier design from Airbus envisioned storing the fuel in a “hump” atop the fuselage, sort of like a camel.

Airbus Cryoplane design (2001). Source

NASA is considering a different “blended wing” aircraft body for these planes, which provides more room for the fuel.

N3-X design on the left and CHEETA design on the right

These designs have promise, but decades of research and testing are needed. Can hydrogen play a role in the meantime?

A local-ish company called ZeroAvia, with offices in Hollister and Cranfield (UK), is trying to thread the needle, pursuing a smaller hydrogen-powered design that can be ready for market much sooner while addressing some of the problems we will need to solve if hydrogen planes scale up. I had the opportunity to talk with Gabriel DeVault, Head of Drivetrain for ZeroAvia, about how his company is tackling this problem. DeVault previously worked on zero-emission motorcycles and learned to love the simplicity of electric engines. While flying an electric plane as a hobby, he became intrigued by the promise of hydrogen for clean aviation because of its light weight and the decreasing cost of hydrogen sourced from renewables. Excited by the prospect, he joined ZeroAvia about a year ago.

ZeroAvia’s near-term goal is to make commercially available, in just three years, a 10-or 20-passenger zero-emissions plane that can go up to 500 miles. DeVault makes it clear that “We are not a flying car company. We are not building autonomous planes. We build power plants for existing and new aircraft that fly 500+ mile routes. We have a near-term, achievable, beneficial program.” For this first step, the electric drivetrain they are building will work with existing aircraft designs, which speeds things up and removes regulatory hurdles. The fuel tanks are a new element but will be much like those in hydrogen-powered cars. They will likely be mounted externally on the wings, and will be made of a combination of carbon, kevlar, and fiberglass that has been well tested in the automotive space. The fuel cells and other components of the drivetrain do not require much innovation. The engine itself, DeVault says, is simple, with only two moving parts. In fact, he emphasizes that this is a big area for cost-savings, given that maintenance represents about half of the operational costs of aircraft. Combined with savings from cheaper fuel, ZeroAvia predicts that this hydrogen-powered plane will save a customer 40% in operational expenses over a comparable traditional plane.

So what are the hard parts of their plan? It’s not the technology. It’s not even regulation, since they are working largely with existing and approved components. “Honestly, in my opinion, the biggest challenge is the scaling, the bootstrapping of the process,” said DeVault. “To happen quickly, a lot of airports have to jump in simultaneously. It’s like what Tesla did, rolling out both the cars and the charging infrastructure at the same time. We have to do something similar, with hydrogen at the airports.” ZeroAvia is partnering with UK-based EMEC for hydrogen production and distribution in the UK. Though EMEC produces hydrogen from surplus tidal and wind energy, DeVault envisions a small electrolyzer at each airport to create the hydrogen locally. The hydrogen would be as green as the power grid at the time it is generated. Locally synthesized hydrogen, with a port in the tarmac (and mobile fuel trucks as needed) would enable sub-15-minute refueling.

You may be wondering about safety, since hydrogen can ignite much more easily than other fuels. (4) At relatively high concentrations, the tiniest spark can cause hydrogen to ignite (see third and fifth row below). Hydrogen will also ignite across a wide range of mixtures with air (first two rows).


Hydrogen also burns invisibly and without much heat (energy is emitted as UV radiation). So it needs to be handled very carefully, particularly in closed environments. Despite this, DeVault says he much prefers working with hydrogen to batteries. “From someone who has to wrench on these things on a daily basis, when the (hydrogen) valve is off you can touch anything. There is no high voltage. High-voltage systems are scary.” He also notes that the lower operating temperatures and simpler engine are meaningful safety improvements.

It is not easy to transform our energy systems, and it is particularly tricky overhauling aircraft. There are tradeoffs in all of these designs. There is general agreement that batteries alone won’t be enough to clean up our transportation emissions, at least without radical changes in battery design. (5) Planes are a problem, but so are heavy-duty trucks and shipping. The number and variety of green hydrogen testbeds is growing, and I expect we will learn a lot about the production, storage, and distribution of hydrogen in the next few years. I am glad to see ZeroAvia exploring its near-term potential for aviation, and bigger players like those involved in the CHEETA project looking into more impactful but longer-term concepts. But it is hard to imagine that the planes most of us fly in will change much in the next 20 years. If technology is coming up short, what role can policies play in decreasing aviation emissions? I will cover that in the final blog post on flying.

Notes and References
1. This recent report from the International Council on Clean Transportation is a great overview of aviation emissions.

2. Vox has a nice overview of electric planes, which includes this statistic from a 2018 Nature article.

3. Lilium and Joby are examples of companies making flying electric “cars”. This report from the International Civil Aviation Organization has a list on page 3 of many of the companies making electric aircraft. Some of these companies are also working towards autonomous flights, for example for cargo.

4. You can find some basic information on hydrogen fuel here.

5. Lithium-air batteries and lithium-sulfur batteries can have about 10x the energy per unit weight as lithium-ion, but they are a ways out.

Current Climate Data (February/March 2020)
Global impacts, US impacts, CO2 metric, Climate dashboard (updated annually)

You may have seen pictures of places like LA or Delhi with clear, unpolluted skies that they haven’t seen in decades. Or maybe you’ve seen a spectacular view on a nearby hike. Turns out skies over our area are clearing up too! Here is a snapshot from PurpleAir showing air quality near downtown Palo Alto since January 1. You can see the lower values in March. Will you be sad to go back to smog? Thanks to my neighbor Sharad for pointing this out to me...

Air quality near downtown Palo Alto in 2020. Source: PurpleAir

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- Avoid disrespectful, disparaging, snide, angry, or ad hominem comments.
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Posted by Anon, a resident of Another Palo Alto neighborhood,
on Apr 12, 2020 at 11:45 am

We've had more cool, slightly foggy, "naturally air-conditioned" weather lately. It could be just random weather. But, is anyone looking at whether the currently lower high-altitude H2O emissions from aircraft is cooling us off a little right now at ground level?

On the electric airplane question, is there a data source for what the fuel consumption is currently for short-hop commercial flights? We don't see too much of that here, but, it some locations, e.g. coastal British Columbia/Vancouver/Victoria/etc., there is a lot of short-range seaplane air-travel among islands and to less-accessible coastal locations. I've never seen any kind of data roll-up on how much these puddle-jumpers consume globally. In BC, Harbour Air is looking to go electric for some of these short hops: Web Link

Posted by changes in airline functionality, a resident of Charleston Gardens,
on Apr 12, 2020 at 12:11 pm

When airlines start to ramp back up in 6 months, there will be a lot less connecting flights than in our previous 'era'.

Let technology solve the jet emission problem in due course. We have much bigger climate-related 'shovel ready' projects to address when the GND passes ( putting folks back to work retrofitting existing commercial buildings, for example.)

The GND is a win-win-win (efficiency, jobs, attacking climate change.) Perfect for getting the economy back on track in 2021.

Posted by neighbor, a resident of Midtown,
on Apr 12, 2020 at 3:35 pm

hopefully, there will be a long lag time before "non-essential" flying returns.

thanks again sherry!

Posted by Paul G, a resident of Stanford,
on Apr 12, 2020 at 5:26 pm

1) The author - like 99% of journalists - makes no mention of next-gen energy-dense supercapacitors. As everywhere else only "batteries and fuel cells" are considered. Why is this? Despite the fact - for instance - that Rolls Royce plc(not cars!) has joined forces with breakthrough UK supercap company Superdielectrics - and despite Superdielectrics' independently verified claim that their energy-dense hydrophilic polymer-based supercaps(quote):
"..could theoretically deliver 50-500(sic) times the energy-density of lithium ion".
2) Re: lithium air. The author repeats the same old "long way off" dismissal of
lithium air that we've been hearing for circa 25 years! More than 20 years ago we were told that "lithium air is at least 5-10 years away". We're still being fed the same narrative.
Paul G

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Apr 12, 2020 at 9:29 pm

Sherry Listgarten is a registered user.

@Anon -- good questions! I would hope that scientists are looking at local impacts of the reduced emissions over various urban areas but I don't know specifics. It will be interesting to see what they find. Is there an impact on health (e.g., asthma)? On temperature or weather more generally (e.g., rainfall)? On plant/crop growth? Hopefully we'll learn more in the coming year.

Re short hops, the graph in the middle of this blog post shows that flights of less than 300 miles (500 km) are responsible for less than 6% of aviation's CO2 emissions. Sounds like that is close to what you are asking in your second question?

@changes -- Yes, there are so many ways to put people to work to make our lives more sustainable. On the aviation side of things, it looks to me like it will be a long wait for tech to solve the aviation emissions problem. Do you think people should reduce their air travel in the meantime?

@Paul -- It would help if you would include references that will help people understand why you are excited about superdielectrics for cleaning up aviation, and why it is more promising than lithium air batteries (which is indeed a long slog).

Thanks all for the comments!

Posted by Close PAO, a resident of Crescent Park,
on Apr 13, 2020 at 12:14 pm

Despited the government shut down I still hear planes at PAO doing touchdown/takeoff routines essentially flying circles. Is this "essential"? I doubt it , I'm disappointed that the City of Palo Alto and PAO allow this to continue.

Posted by RP, a resident of Community Center,
on Apr 13, 2020 at 12:53 pm

Very interesting and well-presented!

A question: do lighter-than-air vehicles (blimp-like) fit into the picture?

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Apr 13, 2020 at 2:46 pm

Sherry Listgarten is a registered user.

@RP -- Airships! That's a great question. From what I can tell, they are mostly considered for cargo (because they are so slow and so big). But one problem is that recently they are fueled with helium, which is very expensive and not renewable (and some say we are running out of it). They used to be built with hydrogen, but then the Hindenburg happened. The Week has an interesting perspective on that, saying it was blown out of proportion (no pun intended :) ). Overall, I'm not sure airships will play a big role in reducing our cargo transportation emissions, particularly if they continue to rely on helium.

@Close -- The City of Palo Alto is asking for feedback on its so-called "S/CAP" goals and actions -- its plans for becoming more sustainable and addressing climate change. You can find the presentations here, as well as some videos (which are largely of someone reading the presentations). You can send your feedback to When I was giving feedback on the "Natural Environment" section, one question I asked is whether we are making the best use of the land we have dedicated to the golf course and airport. Are those uses compatible with our vision of a sustainable city? (The answer is not obvious to me, since that land has limited uses and existing contracts may bind us. But I thought it worth asking.)

More generally, the city is asking for feedback on what are the most important things we should be doing to make a big dent in our transportation and heating emissions in the next ten or so years, and to improve our resiliency to the coming climate changes? I know it can be hard to think about this with all of the coronavirus stuff happening, but if you have time and interest, it's a good time to chime in. City Council is also discussing this at 8:30pm tonight (Apr 13).

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Apr 14, 2020 at 9:12 am

Sherry Listgarten is a registered user.

@Anon, regarding your question about whether people are using this opportunity to look at the climate impacts of reduced aviation, here is a relevant article on that. (Yes, they are!)

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Apr 14, 2020 at 11:35 am

Posted by Sherry Listgarten, a Palo Alto Online blogger,

>> Re short hops, the graph in the middle of this blog post shows that flights of less than 300 miles (500 km) are responsible for less than 6% of aviation's CO2 emissions. Sounds like that is close to what you are asking in your second question?

Yes, thanks! I posted in haste.

Posted by Sherry Listgarten, a Palo Alto Online blogger,

>> here is a relevant article on that. (Yes, they are!)

Thanks again. Fog along the west coast has declined over the years. Web Link This might be one of the reasons.

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Apr 14, 2020 at 2:54 pm

Posted by Paul G, a resident of Stanford,, on Apr 12, 2020 at 5:26 pm

>> 1) The author - like 99% of journalists - makes no mention of next-gen energy-dense supercapacitors.

Until the recent past, supercapacitors were heavy compared with batteries, and the cost was high, even if they had other useful properties. Lately, it seems that there are battery/supercapacitor graphene-based hybrids possible/proposed that have much better specific energy. Like these: Web Link I would like to see them operating in, say, the propulsion unit of a semi- truck first, before I flew in an airplane powered by them. (Maybe I've been too close to a couple of exploding large electrolytics to really trust these things without a proven track record.) In any case, the specific energy looks to be about the same as Li-ion batteries, which will work for some short-hop airplanes, but, not long-haul aircraft. Still, it is an interesting technology. Cost will be the determining factor for whether they can be used for time-of-day load shifting and grid stability. Existing supercapacitors are generally too expensive for any application other than those requiring large short-term current.

Keep us posted.

Posted by ASR, a resident of College Terrace,
on Apr 14, 2020 at 4:46 pm

Any means that does not enable you to reach to safety quickly as a backup is scary to me.

Let's walk more. Let's not fly more.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Apr 14, 2020 at 9:56 pm

Sherry Listgarten is a registered user.

@Anon: Keep in mind that high clouds and low clouds (fog) have opposite effects on climate. High clouds (e.g., contrails) have a warming effect. Low clouds (and smog) have a cooling effect.

Pollution from cars and similar often leads to low-lying clouds and smog, which cool the area underneath. Scientists are concerned that when the air gets cleaner over urban areas such as those in India, regional temperatures will increase. Recently Berkeley scientists found that the reason the tule fog in the Central Valley has decreased over time is because of the reduction in pollution. This has presumably caused a warming of the Central Valley.

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Apr 15, 2020 at 3:07 pm

Posted by Sherry Listgarten, a Palo Alto Online blogger, 16 hours ago

>> @Anon: Keep in mind that high clouds and low clouds (fog) have opposite effects on climate. High clouds (e.g., contrails) have a warming effect. Low clouds (and smog) have a cooling effect.

Not to divert attention from your main point, but, global computer models that do a pretty good job with the averages can have a tough time with odd special cases. Coastal fog like that on the California coast is an odd special case. It is a different type than ground fog triggered by radiative ground-level cooling, because the coastal fog depends on mixing of parcels of air with both different temperatures and humidities, and, air circulation to move the air and initiate mixing. Coastal fog depends on both wind and local topography, and the wind depends on the temperature difference between inland and coast. But, while it may be difficult to model, it has been observed to have decreased significantly over recent decades. Web Link

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Apr 15, 2020 at 6:26 pm

Sherry Listgarten is a registered user.

I've taken the rare step of removing two comments because of racist remarks and/or names. I have no tolerance for that. The comments were also not on-topic and were not fact-based, which is also in the guidelines.

To paraphrase the two comments that I removed, the writer says that the covid fatality projections were exaggerated by the models, and the climate models are similarly exaggerated, and the exaggeration is done intentionally in both cases for political purposes.

No evidence was cited to back up these claims. The covid models are based on constantly changing information about the underlying virus and disease, and need to be used with that in mind. We have huge gaps in our understanding of how the virus propagates and how the disease acts.

The climate models, in contrast, have been developed over decades and have lots of science behind them. They should be pretty accurate, and from everything I read, they are pretty good. Here is just one example, from our own NASA. But things like clouds are still tricky. As to the comment that the models for either covid or climate are politically motivated, I'd be interested to see the evidence behind that claim.

I don't want the comments on this blog to be an echo chamber. There are plenty of places we can disagree. But I do ask that you adhere to the guidelines, which include being respectful, staying fact-based, and staying on topic. The comments that were removed did none of that.

Posted by Sherry Listgarten, a Mountain View Online blogger,
on Apr 15, 2020 at 6:36 pm

Sherry Listgarten is a registered user.

@Anon -- I had understood that you were drawing a relationship between fewer flights ("lower high-altitude H2O emissions from aircraft") and increased low-lying clouds (fog). I pointed out that these are pretty different.

I have no reason to disbelieve that coastal fog is going away, and I don't know why that might be. I think you are suggesting it is because there are more flights, but I find that doubtful. In the Central Valley, the decrease in fog is due to decreased pollution. Coastal fog is different, but I am doubtful any change is due to a change in flights. There just aren't that many of them in that area, and cloud/contrail impacts tend to be regional. Instead, I would expect to see areas with a significant drop in overhead flights to have slightly cooler temps.

BTW, just a note, but anyone commenting in these forums can claim to be from Stanford. I would caution you not to read too much into that.

Posted by Anon, a resident of Another Palo Alto neighborhood,
on Apr 16, 2020 at 12:32 pm

This is probably too far afield anyway (sorry) but both the decreasing temperature delta coast/inland and the decreasing coastal fog are directly based on actual observations of the temperature differences, observed fog, and, impact on the redwoods. (See the above link.) My random comment about the impact of aircraft flights on the temperature delta/coastal fog was speculation-- and an expressed hope that someone will try to measure that impact.

Thank you for your article, and, it is interesting that the impact of local/regional flights < 500 km is less than I thought I recalled.

Posted by Robyn, a resident of another community,
on Apr 16, 2020 at 3:13 pm

I look forward to resuming international travel as soon as possible.
The overarching problem is overpopulation. People living and working in compressed spaces breathing the same air will most likely exacerbate all airborne illnesses.

Posted by Martin Engel, a resident of Menlo Park: Park Forest,
on Apr 18, 2020 at 1:06 pm

Without getting into any substantive details, I just need to say that this is one of the most impressive blog/comment/dialog entries on the web aimed at our local news and information junkies.

Content-intensive science and technology discussions for lay-people are rare, as are facts and solid information. Unsubstantiated opinion on the other hand appears to be plentiful.

Thank you, Sherry Listgarten, for this thoughtful, rigorous, blog.

PS: Having actually seen the Hindenburg in flight in Germany in my youth (1937), and flown on the zeppelin Eureka out of Moffett Field with my wife in 2009, I too have wondered about the future of lighter-than-air cost/benefit ratios.

Posted by Jetman, a resident of Another Palo Alto neighborhood,
on Apr 19, 2020 at 12:04 am

Technology doesn't matter in the airline industry.

Technology doesn't matter in an industry that spends >90% of its cash on stock buy-backs that enrich corporate executives and wall street speculators. It doesn't matter what technology is available if there is no will or cash available to deploy it.

Airports and airlines were the primary vector through which the Corona Virus spread throughout the globe. Airports and airlines need to make a massive investment in public health systems to prevent the next global pandemic, but they will probably go right back to spending their profits on executive compensation and stock buybacks.

In interviews and emails with TIME, more than a dozen flight attendants describe a continuing shortage of basic protection and a confounding lack of guidance over how to do their jobs without spreading the disease. Their gravest concern: that after weeks of working without proper supplies, they have been exposed to thousands of cases and in turn become primary transmitters to the hundreds of thousands of Americans who continue to fly every day. “It's awful, because we know we're definitely spreading it, seat to seat, city to city, person to person, hotel to hotel."

"We're Spreading It. Flight Attendants Fear Losing the Fight Against Coronavirus"
Time ~ April 3, 2020 Web Link

"More Qantas flights revealed to have been crewed by staff with Covid-19"
The Guardian ~ April 8, 2020 Web Link

Posted by aviation review, a resident of Old Mountain View,
on Apr 19, 2020 at 2:43 pm

Some reading
Web Link

Part 23 certification.

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