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Page added on March 12, 2014
Flying Without Fossil Fuels: The Need For High Energy Density
There are few more remarkable machines than a Boeing 747. Four hundred people can be hurled half way across the planet with levels of comfort, efficiency and reliability that would have been deemed miraculous by those living a few centuries ago. A vision of the incredible technical proficiency humanity has gained since the Industrial Revolution, the Boeing 747 is also a remarkably potent symbol of what we can achieve with fossil fuels, and what we currently cannot achieve with their low carbon alternatives.
The Boeing 747: a vision of high power density
The impossibility of solar powered aviation
Last year an adventuring Swiss team managed to fly across the United States in a solar powered plane. This feat, which took a leisurely two months, was described by some as a symbol of what can be achieved with solar energy, a rather curious inversion of reality. It is a symbol of exactly the opposite.
Could commercial flight ever be powered by solar panels? The answer is an obvious no, and any engineer who suggests it is yes is likely to find themselves unemployed. However considering why the answer is no is illustrative of the multiple challenges faced by a transition to renewable sources of energy, which is principally a transition from high density fuels to diffuse energy sources.
So, why can a Boeing 747 not be powered by solar panels?
I will now reach for the back of an envelope and compare the energy consumption of a Boeing 747 with what you could possibly get from a solar powered plane. This calculation will tell us all we need to know about solar powered flight.
Mid-flight a Boeing 747 uses around 4 litres of jet fuel per second. Therefore given the energy density of jet fuel, approximately 35 MJ/litre, a Boeing 747 consumes energy at a rate of around 140 MW (million watts).
We can then convert this rate of energy consumption into power density, that is the rate of energy consumption per square metre. Typically this is measured in watts per square metre (W/m2 ). A Boeing 747 is 70 by 65 metres. So the power density over this 70 by 65 metre square is approximately 30,000 W/m2, and of course the power density over the surface area of the plane will be a few times higher, over 100,000 W/m2.
What can be delivered by solar energy? Solar panels essentially convert solar radiation into electricity, and average solar irradiance is no higher than 300 W/m2 on the planet. In the middle of the day this can be perhaps 4 or 5 times higher than the average. However solar panels are typically less than 20% efficient. So sticking solar panels on the roof of a Boeing 747 is unlikely to provide anything close to 1% of the flight’s energy consumption. Perhaps they can power the in-flight movie.
The power density of a Boeing 747 can further be compared with that of a wind farm.
140 MW. How big would a wind farm need to be to provide this in electricity on average? Probably bigger than Europe’s largest onshore wind farm.
Whitelee Wind Farm, outside Glasgow in Scotland, is a 140 turbine wind farm covering 55 square kilometres. It has a rated capacity of 322 MW, and given its average capacity factor of 23%, it has an average output of around 75 MW, almost two times lower than the rate of energy consumption of a Boeing 747. (Of course chemical and electrical energy are not strictly speaking completely comparable, but when I am trying to illustrate here is the orders of magnitude differences in power density.)
The obvious lesson here is that fossil fuels can deliver power densities orders of magnitude higher than wind or solar. And mobile sources of energy consumption such as Boeing 747s require power density at a level that is physically impossible from direct provision of wind or solar.
The limits of batteries
Perhaps we could store low carbon energy in batteries and use them to power planes. Here we move from the problem of low power density to the problem of low energy density. Despite one hundred years of technical progress batteries still offer very poor energy density compared with fossil fuels.
Consider the lithium-ion batteries that power that excessively hyped luxury car the Tesla S. They offer up just over 130 Wh/kg according to Tesla. So in conventional scientific units they provide an energy density of below 0.5 MJ/kg. In contrast jet fuel provides over 40 MJ/kg. This is a two order of magnitude difference.
Again, reaching for the back of an envelope. A fully loaded Boeing 747 weighs around 400 tonnes at take off, with around 200 tonnes of fuel. The Tesla lithium-ion batteries that could store the same amount of energy would weigh as much as about fifty Boeing 747s.
Lithium-oxygen batteries perhaps could reach close to 4 MJ/kg, an order of magnitude lower than jet fuel, after a couple of decades of future technical progress, according to a recent report in Nature.
So, this is where we are with batteries: a couple of decades from now they might reach energy densities of only 10% of that provided by the best fossil fuels. Clearly a solar energy and battery powered world has its limits.
Aviation’s limited and unpromising low carbon options
Put simply getting a Boeing 747 off the ground requires the provision of high energy dense fuels. This clearly cannot be done with direct provision of renewable electricity, or by storing it in batteries. Nuclear energy is capable of providing extremely high power density, but try powering a plane with a nuclear reactor (or even more importantly try getting a few hundred people to sit in a nuclear powered plane).
There appear to only be two half-plausible low carbon options. The first is the use of biofuels. The second is the use of low carbon electricity to generate synthetic hydro-carbon fuel, so called renewable fuels. Neither of these options is particularly promising.
A growing consensus indicates that current biofuels offer little benefit either economically or environmentally. We have converted large amounts of cropland over to biofuel plantation, all so that we can burn a fuel that an increasing amount of scientific evidence indicates is not reducing carbon emissions. From an environmental and humanitarian perspective this has become indefensible.
Few people realise how dreadful the land use impacts of biofuels are. Consider this: 6% of Germany is used to produce liquid biofuels, yet they only provide around 1% of German energy consumption. Can you imagine a less efficient use of land? Next generation biofuels appear to offer more of the same. The fundamental problem of bio-energy’s low power density cannot be overcome any-time soon.
The only prospect for biofuel production that is actually low carbon and does not have a significant land use impact is to use synthetic biology and genetic engineering to radically alter plants so that they are far more photosynthetically efficient. However the results to date of the research by Craig Venter’s team suggest that this will be the work of a generation, and perhaps generations, of geneticists.
Renewable synthetic fuels are similarly many decades from being an economic reality, if they ever will be. In essence the idea is that you use renewable (or if you prefer nuclear) electricity to convert carbon dioxide into a hydro-carbon based fuel, such as methane or methanol.
However for this to be half-economical, there are no shortage of problems to be overcome. First we need to figure out a way to suck carbon dioxide out of the air on a billion tonne scale. This is obviously not going to happen tomorrow. The cost of this renewable fuel is also guaranteed to be at least two times more expensive than renewable electricity, because of the efficiencies of the conversion process. In other words you will pay for 1 kWh of renewable electricity and get less than 0.5 kWh of renewable fuel out the other end. These scale and cost barriers will be incredibly difficult to overcome, and will likely require either a drastic reduction in the cost of low carbon electricity, or increase in the price of oil.
Renewable fuels then don’t seem to be very promising, on a one or perhaps two generation timescale, as a replacement for jet fuel. This did not stop the German Environment Agency from recently putting forward a scenario where Germany can completely move away from fossil fuels by 2050, which depended heavily on renewable fuels. How heavily? Well, Germany would be sucking around 200 million tonnes of carbon dioxide out of the air by 2050 in this supposedly “technically achievable” future.
I will realise this is all rather pessmistic, but things are what they are. So I will close with a prediction. Aviation will still be powered by fossil fuels by the middle of the century, but this is put forward in the hope that someone proves me wrong.
62 Comments on "Flying Without Fossil Fuels: The Need For High Energy Density"
MSN fanboy on Wed, 12th Mar 2014 1:57 am
“Aviation will still be powered by fossil fuels by the middle of the century, but this is put forward in the hope that someone proves me wrong” Lol, really.
How about my prediction, the canary in the coal mine will have died long before 2050 😀
When air travel goes bust, the red carpet for peak oil is here and the time for prepping is over, the time for survival will have begun.
DC on Wed, 12th Mar 2014 2:09 am
Mass market jet travel is not economically now-in 2014. Same as 2004 and 1994. Airlines are perpetual wards of the state, always one 1/4 away from another bankruptcy or bailout. Just who will flying solar to disneyland in 2050? Flight is handy yes, but 99.5% of humans don’t require it. So if solar planes are available for critical needs-thats one thing. But mass, discretionary travel by solar planes, LoL!
PapaSmurf on Wed, 12th Mar 2014 2:11 am
Jets probably will still be powered by fossil fuels in the middle of the century. There will simply be fewer of them in the absence of there being a breakthrough in biofuels or hydrogen storage, and I am not betting on the hydrogen scam.
There are numerous travel applications that will be able to switch easily to electric power. Cars, trains, motorcycles, etc. Mining
The type that seem more questionable are large trucks, but moderate improvements in lithium battery energy densities might change that math. The recharging speed of direct DC (135 kW currently and rising) seems to make long distance EV travel viable.
But I don’t see much chance for Boeing or Airbus still being in production at their current rates in the year 2050. But who knows, engineers can be clever bastards. Maybe they will surprise us. But I suspect we have a better bet on the Hyperloop or Bullet Trains.
ghung on Wed, 12th Mar 2014 2:11 am
“If man was meant to fly ……”
I expect flying will become more simple and local like everything else. As with many other technologies, it won’t be abandoned because it’s no longer possible at the scales we take for granted. The Gyro Captain in Mad Max 2 (‘Jedediah’ in Thunderdome), may become the model for bush pilots in the future. Fun to think about.
Got wings?
Davy, Hermann, MO on Wed, 12th Mar 2014 2:16 am
Flying will in the near future be like the past for the rich, military, and government. Aviation for the masses will end when globalism ends. Globalism days are numbered.
PapaSmurf on Wed, 12th Mar 2014 2:27 am
Davy,
I doubt it within your lifetime. Most global trade is not done by flying 747 cargo jets. The bulk of global trade is handled by trains or sea and that is far more efficient than anything else around.
andya on Wed, 12th Mar 2014 2:52 am
You can still get the energy density from biofuels. Null problemo’.
It will be market dynamics that determine the future of aviation. Anything is possible, but in the end society decides what is worth keeping, and what is not. Eventually aviation will not be worth keeping, like concords and trips to the moon. Though as a hobby some people will still play around with it.
Steve on Wed, 12th Mar 2014 3:00 am
Hybrid trucks debut: http://www.altenergystocks.com/archives/2012/12/epowers_series_hybrid_electric_drive_unmatched_fuel_economy_for_heavy_trucks.html
Tom S on Wed, 12th Mar 2014 4:35 am
It’s entirely possible to power airplanes using other chemical fuels. There are chemical fuels which are not hydrocarbons and which would serve perfectly well for aviation purposes.
One example is anhydrous ammonia. Ammonia can be produced right now from wind power and stored in tanks. Ammonia is flammable, is suitable as a jet turbine fuel, is liquid under only very modest pressures, and has energy densities only modestly lower than hydrocarbons. Furthermore, ammonia can be made out of nitrogen taken from the atmosphere, which requires far less energy than removing carbon from the atmosphere, since nitrogen makes up ~80% of the atmosphere and carbon only 0.04%. There are already pilot plants for producing ammonia from wind power and nitrogen taken from the air. This is unsurprising since the technology is straightforward.
The transition to ammonia for air transportation is something which won’t happen anytime soon. Air travel will probably be one of the last sectors of the economy to transition away from fossil fuels and on to something else. I wouldn’t be surprised if airplanes still used hydrocarbons as fuel even a century from now, when almost everything else had transitioned to other fuels.
However, there is certainly a substitute for fossil fuels in aviation. In fact, there is a reasonable substitute for every single usage of fossil fuels. Fossil fuels are not strictly necessary for any purpose whatsoever. It will take time to transition to other fuels, but there is vastly more time than would be required. In my opinion, what people should expect is modestly higher prices for some things, such as electricity and air travel, as fossil fuel supplies gradually decline over centuries.
-Tom S
PapaSmurf on Wed, 12th Mar 2014 5:00 am
Tom S,
I am not so sure there will even be higher electricity prices. Grid parity for solar energy is practically here or very close.
With the advent of projects like Tesla’s Gigafactory expected to reduce battery prices by 30% to 40%, home storage of energy becomes affordable.
I used to be somewhat in the pessimistic crowd circa 2004-2009. Since then, not so much. The crazies on PO.com seem about 5 years out of date and still operating on stale info.
GregT on Wed, 12th Mar 2014 5:23 am
^^^^^ Multiple personality disorder ^^^^^
Papasmurf on Wed, 12th Mar 2014 6:06 am
^^^^^^^^ dipshit disorder
Makati1 on Wed, 12th Mar 2014 7:44 am
Local airline travel will be the first casualty. The biggest fuel use is getting it into the air. Cruising is easy. It will be the flights between airports a few hundred miles apart that will end. Then it will be intra-continental flights and then intercontinental flights and then only the wealthy and military will fly.
Example: I can fly from Manila to Philly for about $1,700.00 RT. 1,000 miles.
Makati1 on Wed, 12th Mar 2014 7:47 am
Example: I can fly from Manila to Philly for about $1,700.00 RT. 1,000 miles.
Something ate the line inbetween $1,700.00 RT. and 1,000 miles.
It should read:
Example: I can fly from Manila to Philly for about $1,700.00 RT. >25,000 miles.
A trip between Harrisburg, PA and Myrtle Beach,SC is about $400.00 RT. <1,000 miles.
???
simonr on Wed, 12th Mar 2014 10:15 am
Dont know the mathematics, but could you not use a helium blimp and thin film solar cells ?
meld on Wed, 12th Mar 2014 11:01 am
Dirigibles! I’m sure the safety side of things can be worked out with a little thought.
Arthur on Wed, 12th Mar 2014 11:25 am
The pessimists of the Energy Collective are at it again. The solution is simple: every plane should have a nuclear reactor, enabling the tourists to reach their destination of choice, any destination, like the Antarctic Rivièra, within 30 minutes. In fact, airports should provide for swimming pool-like facilities, like changing booths and towel handouts, so that the tourists can begin swimming at the very moment of arrival. It’s impossible to stop progress.
meld on Wed, 12th Mar 2014 11:46 am
@Arthur – not sure if you’re joking or not with your current track record. 😀
J-Gav on Wed, 12th Mar 2014 12:40 pm
Haven’t flown in 5 years – and don’t miss it …
Kenz300 on Wed, 12th Mar 2014 1:01 pm
Energy Collective — a shill for the fossil fuel and nuclear industries……
Davy, Hermann, MO on Wed, 12th Mar 2014 1:02 pm
The era of flight for the masses and generally all here are the masses, will end with the great financial correction. The airline business is about economies of scale. The airlines need volume to maintain the prices the masses need to afford air travel. The whole airline infrastructure is vastly complex and energy intensive. It is not just the fuel to get these huge machine off the ground. Maintenance and construction have huge embedded energy and complexity. We will still have flight as long as we have oil and a functioning economy. Yet, when complexity and prosperity give way in the coming contraction mass air travel will be history. I used to be a pilot. I flew a very simple high wing 4 cylinder Cessna 150. I used to use car gas back when it was $1 gallon in the early 90’s. The plane was simple, inexpensive and safe. This is a far cry from a 747 and all her back up!
GregT on Wed, 12th Mar 2014 1:03 pm
With all of this new-found tech, maybe we could have swimming pools on the planes?
GregT on Wed, 12th Mar 2014 1:09 pm
Davy,
I still am a pilot. Sold my last aircraft a few weeks ago. 1956 C-172, 1100H TT. General aviation here is a small shadow of it’s former self of 5 years ago. 100 LL doubled in the last few years. Disposable income has taken a big hit since the last oil price shock. People have been tightening their belts across the board.
Davy, Hermann, MO on Wed, 12th Mar 2014 1:29 pm
I have an grass strip here on my farm. My brother has a Cessna 206 he brings in on occasion. I hear you on the changes to general aviation. It is a far cry from the 90’s. It was a freedom hard to find anymore. BTW Greg, nice plane. I had 2 different 150’s it hurt both times I sold them.
rollin on Wed, 12th Mar 2014 2:59 pm
The airline industry is well on it’s way to doubling fuel efficiency.
How about using solar energy to produce liquid fuels. We do have the chemistry knowledge to do that, no need for petro liquids.
Steve on Wed, 12th Mar 2014 3:30 pm
Giga obsolescence: http://seekingalpha.com/article/2062263-3-billion-reasons-why-teslas-gigafactory-will-be-a-bloodbath
Steve on Wed, 12th Mar 2014 3:37 pm
rollin, we COULD use solar to produce liquid fuels, but we won’t in any quantity. We don’t have the infrastructure at scale and don’t have the capital to build it out (using fossil fuel by the way) There’s also the issue of existing infrastructure, including the planes themselves. Unless the proposed fuel is a direct drop-in replacement, those engines and systems will be obsolete long before their expected financial useful life. Cost to replace early??
Ceph on Wed, 12th Mar 2014 4:25 pm
use the electric train for smaller distances.
use biofuels (Germany used 8,8 mio t of kerosine in 2012 and produced 3,8 mio t of biofuels in 2012)
use liquid hydrogen. The main problem of hydrogen is the lack of infrastructure, but if you just need it at airports you don’t need to built a large new pipeline system.
Yes, airplanes are the fruits hanging at the highest points, but even this should be doable with renewables and todays technologies.
Davy, Hermann, MO on Wed, 12th Mar 2014 5:16 pm
Ceph, yes, maybe keep a small amount of planes going for rich, military, and government with renewable produced liquid fuels. “But” you missed my point commercial aviation today needs volume to survive. The general public cannot afford expensive plane tickets period. The commercial aviation business cannot afford expensive fuel because they cannot charge high price airfares or few can fly. If few fly then their economic game plan does not fly. No volume equals no commercial aviation period! I might mention there is no way over the long term the airport infrastructure will be maintainable let alone the planes. This will become more critical when the commercial aviation business falls apart.
kervennic on Wed, 12th Mar 2014 5:20 pm
The only environementally friendly plane is the one that crashes in the ocean.
Steve on Wed, 12th Mar 2014 5:30 pm
Ceph, “Use liquid hydrogen”. Won’t get off the ground. Ha ha ha! http://ts3.mm.bing.net/th?id=HN.608022061852590819&w=227&h=185&c=7&rs=1&pid=1.7
Davy, Hermann, MO on Wed, 12th Mar 2014 5:35 pm
Steve, bio diesel type jet fuel will!
cephalotus on Wed, 12th Mar 2014 6:11 pm
Davy,
airplanes need heavy subsidies, at least in the EU, to operate. They don’t pay taxes on the fuels, they don’t buy for CO2 depletion, they don’t pay for the health problems people get when subjeted to the noise of starting planes.
They even get huge amounts of tax moneyf to built airports, bacause local politicans like to poase in front of them. ( at least until some years ago).
I don’t see any need for cheap subsidised flights. A passenger should pay the real prize for transportation and if it gets 3 times as expensive more will take the train. Ordinary people will stop doing their subsidised weekend trip to London for some shopping and a coffee.
That’s a perverted system anyway.
In China trains already seem to win over planes.
cephalotus on Wed, 12th Mar 2014 6:14 pm
Steve,
it doesn’t matter if you crash a plane with kerosine or with liquid hydrogen. Both will burn.
But liquid hydrogen will evaporate very quickly and move to the outer atmosphere doing no harm while kerosine is liquid and keeps an ecological and security threat.
Steve on Wed, 12th Mar 2014 6:39 pm
Davy, agreed, biodiesel COULD work, but won’t be scaled out due to costs, EROEI, and very low energy density of the cropland.
Ceph, that “will evaporate very quickly” is what makes it explosive. There’s also a net energy loss in producing it (its an energy CARRIER, not source, after all) and tends to leak out of, and make brittle metal containers and plumbing holding it. Most hydrogen today is made from relatively energy-dense natural gas. As I understand it, you propose making it using wind/solar electricity. Any idea on the number of acres of each required to produce enough hydrogen to fuel the traffic at a typical international airport??
PapaSmurf on Wed, 12th Mar 2014 7:21 pm
The Hyperloop concept is even faster (700+ mph) than a plane. Makes more sense. I wish California would build one between San Francisco and Los Angeles.
Steve on Wed, 12th Mar 2014 7:31 pm
Papa, Who pays? Is there a breakeven? Personally, I’d be happy if “they” would just restore ubiquitous conventional rail.
Ceph, Why aren’t there any natural gas fueled planes? Much easier than hydrogen, but the energy density issue applies to both.
Davey on Wed, 12th Mar 2014 8:11 pm
Papa, can you spell boondoggle ? Hyper loop is a joke when you consider a return on investment. With everyone broke now it will never happen
Stephen on Wed, 12th Mar 2014 8:13 pm
Algae Fuel? This one shows some hope. I also think that Nuclear fuel might also work too.
Outcast_Searcher on Wed, 12th Mar 2014 8:50 pm
If we can’t come up with good green energy solutions for certain forms of transport (like aviation), then people will simply need to do with less, and it will become far more expensive.
With all the improvements in communication via electronics, it’s not like that has to be the end of the world.
OTOH, if we ONLY burn fossil fuels to do the things we can’t do viably with batteries, and we are willing to control the population size and live more modestly, that might work out for a long time. (The earth is impressively resiliant — it just has limits).
However, given peoples’ apparent unwillingness to compromise and to face scientific fact if it interferes with their world view — coming to such a solution before we destroy the environment on our current course seems like a pipe dream.
PapaSmurf on Wed, 12th Mar 2014 9:26 pm
In terms of credibility…. Elon Musk vs internet doomers from PO.com?
My money is on Elon Musk (see SpaceX and Tesla and Solar City and Paypal).
Steve on Wed, 12th Mar 2014 9:58 pm
Papa, Virtually ALL our money is on Elon Musk. Huge government (taxpayer) subsidies. Got one of those Tesla things? Too pricey for the 90%ers, although we enjoy? subsidizing the rich fellers with $7500+ rebates.
Davey on Wed, 12th Mar 2014 10:08 pm
Papasmurf credibility is not based on how big your net worth is!
DC on Wed, 12th Mar 2014 10:35 pm
The future of casual, jet-powered mass-market travel.
http://photoshare.shaw.ca/image/6/1/f/302878/1350211494549-0.jpg?rev=0
cephalotus on Wed, 12th Mar 2014 10:40 pm
Steve,
liquid hydrogen + tank should weight less/kWh than kerosine + tank and this weights less than LNG + tank.
Volume is another storry but volume isn’t the big problem.
Maybe you could use hydrogen in combination with fuel cells and drive electric motors for smaller / slower, but more efficient airplanes?
I’m not an expert on this.
There are HUGE amounts of oil left and you can liquify coal if you want and this should last thousands of years if just used for planes.
Our problem ist, that tehre are very easy solutins or the low hanging fruits (like domestic heating) which we don’t use so there is little reason to talk about technical possibilities of getting to the highest hanging fruit.
GregT on Wed, 12th Mar 2014 11:00 pm
Outcast_Searcher SAID:
“However, given peoples’ apparent unwillingness to compromise and to face scientific fact if it interferes with their world view — coming to such a solution before we destroy the environment on our current course seems like a pipe dream.”
Correct.
Our oceans are dying, our air, land and water are polluted, our climate is changing, our populations are in overshoot, and we are causing a mass extinction event on our planet. Everyone is talking about some new energy source that will allow us to continue on with BAU, when it is excess energy to begin with, that is causing most, if not all of our largest problems.
We either learn how to live within the confines of the natural environment, or life as we know it will be over. Nature is in charge here, we are not, and nature isn’t going to negotiate with us.
Steve on Wed, 12th Mar 2014 11:07 pm
http://www.udel.edu/johnmack/frec324/pix/limits_to_growth_chart.png
Steve on Wed, 12th Mar 2014 11:16 pm
Papa, couldn’t help myself. Just came across this regarding Tesla: “Tesla: Valuation Lunacy Straight From the Goldman IPO Hatchery
Indeed, as during the prior two Fed-inspired bubbles of this century, the stock market is riddled with white-hot mo-mo plays which amount to lunatic speculations. Tesla, for example, has sold exactly 27,000 cars since its 2010 birth in Goldman’s IPO hatchery and has generated $1 billion in cumulative losses over the last six years—–a flood of red ink that would actually be far greater without the book income from its huge “green” tax credits which, of course, are completely unrelated to making cars. Yet it is valued at $31 billion or more than the born-again General Motors, which sells about 27,000 autos every day counting weekends.”
GregT on Wed, 12th Mar 2014 11:26 pm
Steve,
That chart is pretty much what I predict IF we get off of fossil fuels soon enough. I do see population crashing sooner though. Kind of difficult to sustain population growth for 30 years after our food supply crashes.
If we keep burning fossil fuels, all of those lines will be close to zero long before 2100 though. Even if we do stop now, it may already be too late.
GregT on Wed, 12th Mar 2014 11:45 pm
20 years from now, people won’t be thinking about Sunday drives in the country, they’ll be too busy trying to grow food for their own survival. Once our roads are no longer maintained, it won’t take long before there is nowhere to drive to anyways.
Cars are not in our future. I CAN see some kind of all terrain vehicles for a very few. I suspect that they will look much more like armoured personnel carriers, than sports cars.