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Flying Without Fossil Fuels: The Need For High Energy Density

Robert Wilson's picture
University of Strathclyde

Robert Wilson is a PhD Student in Mathematical Ecology at the University of Strathclyde.

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  • Mar 11, 2014

Fossil fuels and 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.

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Keith Pickering's picture
Keith Pickering on Mar 11, 2014

Nice try, but completely wrong. It is indeed entirely possible to power an airplane — even a Boeing 747 — with non-fossil fuel. Here are a couple of ways to do that.

1. Biofuels. The US Navy and several airlines have already successfully experimented with biofuels for jet aircraft. The only barrier to using bio aviation fuel now is cost.

2. Synfuels. Once again, the US Navy is in the lead. In 2009, a US Navy study found that it was entirely feasible to manufacture jet fuel from seawater, using a nuclear-powered factory ship. This is possible because a liter of seawater holds 140 times as much CO2 as a liter of air — and since most of the CO2 in seawater is in the form of carbonate and bicarbonate ions, it’s fairly easy to remove. So you combine CO2 from seawater with electrolyzed H2 using the Fischer-Tropsch process and get hydrocarbons. Once the water is electrolyzed (using nuclear electricity) the rest of the chemical process is entirely exothermic.

The Navy study figured the cost of this process to be about $6/gallon, but considering that half the cost of the factory ship was in the ship (the other half being the reactor), presumably with a beached reactor the cost could be about half that, or roughly comparable with today’s jet A prices.


Ed Dodge's picture
Ed Dodge on Mar 11, 2014

The U.S. military has been wrestling with these issues for a long time and have come to similar conclusions.  From a competitive performance perpective, nothing beats hydrocarbons, and that is why militaries the world over rely on them and expect to continue relying on them indefinitely.

There is lots of great analysis coming out of the US Air Force who is the single largest consumer of petroleum products on the planet.  The Air Force has put a lot of effort into quantifying the technical and economic requirements for new fuel supplies and evaluating alternatives to current petroleum based products.  The US military understands better than anyone engineering performance, economics of oil, and the strategic nightmare of reliance on foreign supplies. 

This report from the Rand Corporation provides a pretty thorough review of the possible alternative fuels from a military performance perspective.



Robert Wilson's picture
Robert Wilson on Mar 11, 2014


I am talking about powering commercial planes such as Boeing 747s, not drones. So what if you can power a drone with a fuel cell? In no way does this mean “we can fly quite well without fossil fuels.”

Robert Wilson's picture
Robert Wilson on Mar 11, 2014


As you point out one of the big problems is that jet engines are phenomenal machines. The aviation industry will not want to move away from something that offers such reliability and performance anytime soon.

There are two problems with hydrogen. It has significantly lower energy density volumetrically than jet fuel, which poses major engineering challenges. A bigger problem is that hydrogen powered planes will have to fly at higher altitudes. This is a problem because at such altitudes water vapour becomes a very potent greenhouse gas. So hydrogen powered flight might not help at all with climate change, even if you can get it to work.

Robert Wilson's picture
Robert Wilson on Mar 11, 2014


Your response here is a bit annoying. I addressed both biofuels and synfuels in the piece. Did you not read that far before writing your comment? If not, then please don’t come out with condescending “nice try” remarks in future. Or at least bother to address my remarks about it.

Hops Gegangen's picture
Hops Gegangen on Mar 11, 2014


I think those people “living a few centuries ago” would also have been apalled at how we waste our wonderful resources and trash the whole planet.

I sometimes imagine people in an era when the only light at night was from whale oil, and how they would have cherished an electric light. And here we are, leaving the lights on carelessly.



Ed Dodge's picture
Ed Dodge on Mar 11, 2014


In both of these cases synthetic hydrocarbons are being created that are essentially identical to conventional jet fuel.  There is nothing new about this.  The Germans ran huge coal to liquids plants during WWII and these processes have been in continuous commercial operation ever since.  Coal to liquids is far more cost effective and scalable than any of the biofuels options commercially available.

The problem with today’s biofuels is not so much about cost, though that obviously matters, but scalability.  Conventional agriculture or forestry simply does not provide any where near the quantities required to make a dent in the petroleum markets.  This is the big fallacy of biofuels, we would ravage the landscape long before we succeeded in replacing oil.

I am a big believer in biotech and I think there is a lot of great potential in that space, but nothing is remotely proven yet.

Curious to learn more about the nuclear powered synfuels you mentioned.  Can you provide a link?

At the end of the day hydrogen and carbon are two of the most common elements and we can manufacture synthetic hydrocarbons all day long, there are many ways to do it.  We are not in any danger of running out of fuels, we just need to pay for them.

Roger Arnold's picture
Roger Arnold on Mar 11, 2014

Well, nothing really wrong that I can see with the basic facts, but the  interpretation and conclusions are a bit off (IMO).  The problem is a warped perspective stemming absence of quantification.

Take, for starters, the issue of CO2 sources for fuel synthesis.  Althouth I don’t feel that air capture is all that impractical, it would definitely not be necessary to resort to air capture to get enough CO2 to make jet fuel for commercial air travel.  Doing some rough figuring from data on  Wikipedia, world consumption of jet fuel appears to be about 0.6 Mt (megatonnes) / day, or 220 Mt / yr.  Synthesis would require a bit over 600 Mt of CO2 / yr.  CO2 produced from calcining of limestone for the cement industry is 1,500 Mt / yr. There’s another 1,200 Mt / yr of CO2 from firing the lime kilns, but that would go away if we switched to concentrated solar thermal for the kilns. The calcination output is an easily tapped, nearly 100% CO2 stream. No problem there.  There are plenty of other easily tapped, nearly pure CO2 streams as well, but you get the idea.  

Now, about energy for synthesis…  The figure of 2:1 for joules of electrical energy in to joules of chemical potential energy out is about right.  220 Mt / yr of jet fuel has a chemical energy potential of about 270 TWhr, so synthesis would need 540 TWh / yr, or about 1.5 TWh / day.  Worldwide electricity production for 2008 was about 55 TWh / day (2.3 TW average for 24 hrs), so we’re looking at roughly 3% of 2008 electrical energy production.  And that’s for pure synthesis, from CO2 and water, which is the most energy-expensive approach.

At the level needed for the airline industry, biofuels might conceivably scale adequately.  I agree that “conventional” biofuels can’t scale to anything like the current level of fossil fuel consumption, and that the environmental impact of even trying would be unacceptable.  But there’s a non-trivial level that can be sustainably maintained with little impact.  It would tap current waste streams and some specialized energy crops grown on marginal, non-irrigated lands.  The amount of fuel that can be produced that way can be doubled by using the biomass strictly as a carbon source, and not as an energy source.  The energy input for converting the solid biomass into liquid fuel would need to come from non-carbon energy resources that are much more efficient than photosynthesis.

Finally, on the matter of economic feasibility.  If you compare any alternative to the cost we have been accustomed to for fossil fuels, of course it’s going to look “infeasible”.  That’s a bogus comparison.  The world changes, and our current way of living has to change with it.  Unless or untill non-carbon energy gets really cheap, then high energy density liquid fuel will become a comparative luxury.  Air travel will be more expensive and less common.  But it won’t go away.

Eventually, it’s quite possible that zero-carbon electricity will become cheap enough to make synthetic fuels at lower cost than what we currently pay for petroleum.  After all, the “fuel” for zero-carbon energy is free (wind or sunlight) or extremely cheap (advanced nuclear).  It’s just a matter of reducing the capital cost of the systems needed to tap it.  That can be done through a combination of up-front cost reduction, lower financing costs, and longer working lifetime.  I can think of no fundamental reason why it can’t happen.

Keith Pickering's picture
Keith Pickering on Mar 11, 2014

Ed — see:

Willauer, H. D., Hardy, D. R., Schultz, K. R., & Williams, F. W. (2012). The feasibility and current estimated capital costs of producing jet fuel at sea using carbon dioxide and hydrogen. Journal of Renewable and Sustainable Energy,4(3), 033111.

pdf here:

This study came out in September 2010, and was (oddly) unreferenced by the RAND study in January 2011. In terms of scalability, Willauer was looking at 100,000 gallons per day as a design basis.

Regarding the scalability of biofuel, miscanthus (a C4, minimum tillage perennial) grows fast enough and contains enough carbon to replace all US gasoline usage for about the same amount of land we’re currently using for ethanol. Cool Planet claims to have a proprietary process that can turn biomass into gasoline for a cost input (including energy cost) of less than $1/gallon. Don’t know if that’s hype or not, but they are past the experimental stage and are currently building pilot plant. So we’ll see.

Keith Pickering's picture
Keith Pickering on Mar 11, 2014

I’d be happy to.

1. “ Next generation biofuels appear to offer more of the same.” Based on what source? As mentioned in my reply to Ed, miscanthus grows fast enough and contains enough carbon to replace all US gasoline use for about the same amount of land currently used for ethanol. And it also requires significantly less tillage and less fertilizer than corn. For example, at 25 tons per acre, assuming 50% of dry weight is carbon (typical for woody plants), 1 acre would produce 12.5 tons of carbon per year, which is the same amount of carbon in 4659 gallons of gasoline. With US gasoline consumption of 133 billion gallons per year, it would require 28.5 million acres for full replacement, or about 7% of US agricultural land. Cool Planet has a process that can turn miscanthus (or any other biomass) into gasoline, and do it cheaply. In other words: next generation biofuels are very, very promising, and far from being “more of the same”.

2. Regarding synfuels, you claim, “First we need to figure out a way to suck carbon dioxide out of the air on a billion tonne scale.” Not true. There is 140 times as more CO2 in a liter of seawater than in a liter of air, and in water it’s in the form of cabonate and bicarbonate ions, which are easily separated electrically. You state “you will pay for 1 kWh of renewable electricity and get less than 0.5 kWh of renewable fuel out the other end.” Doubtful, as the electrolysis of water is the energy bottleneck, and efficiencies there are at least 70% (and improving with improving catalysts). Once you’ve electrolyzed the water, the remaining part of the Fischer-Tropsch chemical process is exothermic, i.e., “downhill”. So the real barrier is cost, not feasability, and that hump could be overcome with a fossil carbon tax.

With a fossil carbon tax in place, there will be no fossil aviation fuel left by the end of this century. 

Robert Wilson's picture
Robert Wilson on Mar 11, 2014


Romantic illusions about previous societies have little appeal to me. Yes, socieities which gladly tolerated the likes of slavery would be appalled by how we behave today. Should we care?

Roger Arnold's picture
Roger Arnold on Mar 11, 2014

Hydrogen powered planes will have to fly at higher altitudes?  I haven’t heard that before, and I can’t think of any reason for it.  Can you elaborate?  I do agree that if they did fly significantly higher, water vapor would be a problem.  

Hydrogen-fueled aircraft are also of dubious value, given the huge size and insulation requirements of the fuel tanks.  Using liquid hydrogen vs. synthetic jet fuel gives at most a 2:1 energy advantage for producing the fuel; it seems more practical to just keep the current designs and pay the energy cost for high grade synthetic jet fuel.

Robert Wilson's picture
Robert Wilson on Mar 11, 2014
Roger Arnold's picture
Roger Arnold on Mar 11, 2014

The Naval Research Lab has done some investigation into producing synthetic jet fuel at sea.  They have proposed using CO2 obtained from sea water, and electricity from military nuclear reactors.  They’ve estimated a cost of $6.00 per gallon.  That’s actually $2.00 per gallon cheaper than what they figure it costs them now to purchase the fuel stateside, and ship it via military tankers to overseas carrier groups. The Navy’s primary motivation, however, isn’t cost savings but security.  They really like the idea of carrier groups that can operate without concern for long supply lines.

Roger Arnold's picture
Roger Arnold on Mar 12, 2014

Ah, thanks.  That’s a nice report to have as a reference.  

The cited reason is that the larger volume and lower weight for an LH2-fueled airliner give an *optimal* cruising altitude that is a bit higher than for current jets.  But it’s not a large difference, and the penalty for staying at current cruise altitudes is small. (That’s not in the report — just something I know from working around aircraft designers at Boeing in the late 1970’s.  Yes, they were looking at LH2-powered aircraft back then.)  So it would be feasible to fly such planes still within the troposphere, where convective mixing would render water vapor from hydrogen-fueled engines a non-issue.

It’s when you try to push to advanced supersonic aircraft that you’re more or less forced to move into the stratosphere — where a high volume of water vapor emissions definitely *would* be problematic. The lousy economics of supersonic flight make that an unlikely development anytime soon.  Could happen though, if flying becomes too expensive for the common folk, and reverts to its old status as a prerogative of the very wealthy.

BTW, the link above, copied and pasted, did not work.  I googled and found the report.  Turns out that the last two characters of “.pdf” were truncated in your answer window.

Robert Hargraves's picture
Robert Hargraves on Mar 12, 2014

From pp 378-9 of THORIUM: energy cheaper than coal…

Hydrogen can power airplanes.

With extensive development, hydrogen may become a possible commercial airplane fuel. For the same amount of energy, hydrogen fuel has only 1/3 the weight of petroleum jet fuel, very advantageous to aircraft performance. Containing compressed hydrogen at 350 atmospheres of pressure (5000 psi) is possible with lightweight carbon-fiber tanks, but higher densities would require heavy steel tanks. At this pressure hydrogen’s energy density of 2.8 MJ/liter compares unfavorably to jet fuel at 33 MJ/liter, so the volume occupied by hydrogen tanks will be 12 times more than jet fuel tanks, reducing cargo or shortening flights.

Experimental Tupolev TU-155

Russia demonstrated an airplane fueled by cryogenic, liquid hydrogen in 1989. Boeing used internal combustion engines on a hydrogen-fueled unmanned aircraft. 

Ed Dodge's picture
Ed Dodge on Mar 12, 2014


Great report, thanks.  I always like reading military analysis on these issues.

Biomass can certainly make a useful contribution but it is often oversold as a grand solution.

Nathan Wilson's picture
Nathan Wilson on Mar 12, 2014

The other things to keep in mind is that not all air travel routes are equal.  The 747 has an enormous range (around 8,000 miles!) which is much more than most routes need (New York -> Los Angeles is only 2,500 miles, or 31% of what the 747 offers; most US flights are even shorter, as they stop at hubs in the central US).

So a low performing fuel like ammonia might be adequate for the majority of civilian air routes, inspite of the fact that it has only 44% of the energy per unit weight of jet fuel, and 31% of the energy per unit volume (at 80F), but about 4x the energy per unit volume of 5,000 psi hydrogen and 30% more than liquid hydrogen.  As discussed in this presentation, ammonia as a fuel for gas turbine engines has been tested and found acceptable.

Ammonia can be stored at ambient temperatures using only 200 psi (even in the summer sun), or with cooling to -33C, can be stored at sea-level pressure, so it will have much less impact on the aircraft design than trying to switch to hydrogen (20C cryo-liquid, or 5,000 psi gas).  Wing-tanks will still work, and the proposed blended wing-body airframes would be well suited to ammonia.

Since ammonia (NH3) production requires only hydrogen and nitrogen (which is 80% of the air), ammonia is the second easiest syn-fuel to make (after hydrogen).  Even an “easy” carbon source like cement production requires co-ordination of two very different businesses.

We should remember that NASA, European space agency, and the Japanese space agency all concluded that hydrogen was the “best” fuel choice, but in fact there have been about as many ammonia powered X-15 rocket plane flights as hydrogen power Space Shuttle Flights.  Similarly, the US military decided hydrogen is a terrible fuel for a rocket that has to be easy to store and operate (i.e. a missile).  Newcomer SpaceX has also found that for low cost systems, hydrogen is a poor choice: the cost and handling challenges out weigh the weight advantage (they use kerosene).

Sure, sustainable ammonia will cost more than fossil fuel, but the increase will be a small percentage of the total cost of travelling.  Plus the fact that any nation can make it provides security benefits. 

So the situation with aviation parallels other fossil fuel applications: non-fossil solutions are possible in which solar, wind, and nuclear provide all primary energy; the increased cost and decreased convenience of them are still overwhelmed by the benefits of living energy rich lifestyles.

Nathan Wilson's picture
Nathan Wilson on Mar 12, 2014

All good points.  

Regarding biofuel for long distance aviation, I would add that it becomes a lot more feasible if we are not also using biofuel for electricity (e.g. load following and regulation), combined heat and power, land transportation, and short distance aviation (all suitable applications for ammonia).

Bas Gresnigt's picture
Bas Gresnigt on Mar 12, 2014


Thanks for the link to the GHG neutral study by the Umwelt Bundesamt!

Assume that Denmark must have done similar much earlier, as they have accepted to reach the explicit target for 100% renewable regarding all energy in 2050.

Thomas Garven's picture
Thomas Garven on Mar 12, 2014

Gentlemen; Do We Really Need to Fly at 550 mph?  

With all of the technological advances we have made in telecommunications over the last 10 years, does it really make any difference if we get somewhere by 10:00 am instead of 11:00 am?  And do we really need to fly as frequently as we do?  Because like it or not; a planet filled with 8 billion people all trying to live the “American Dream” is probably going to look quite a bit different than what we see today don’t you think?   

instead of the path we are currently on; maybe one of the first steps we should take is to establish a set of measurable goals and objectives for “we the people”.  You know, stuff we the people can do to “improve the quality of the air we breath and the water we drink”.  

Do you think that might be more successful than trying to teach everyone what 400 ppm of CO2 on a mountaintop in Hawaii means?  Outside of The Energy Collective website most of the American people don’t even know what “ppm” means.  If you have ever watched J. Leno’s man on the street episodes before he retired, you will understand why I say this.  Most Americans don’t even know who the Vice President is when shown his picture.   

There is nothing more dynamic and powerful in my opinion than a team of people working together to meet a common set of goals and objectives.  As a team leader, I have personally witnessed the power of teams to stimulate action and American business wouldn’t be where it is today without people working together.  

It is my belief that a nation filled with people striving to reach the same goals and objectives would probably become an almost unstoppable force.  What if everyday everyone went to work, school or during their normal daily activities looked for ways to improve the efficiency of something or ways to clean up our air and water. Now multiple that effect by 200,000,000 kids and adults.  Could that be thousands or even millions of times more effective than another climate study?  It is not that climate studies and technological achievements aren’t needed; they certainly are.  But right NOW; we have everything we need to start living better lives than we currently are; and if we don’t act SOON that may change and it may not be for the better. 

So in the end what do you think – do we really need to fly at 550 mph? 

Hops Gegangen's picture
Hops Gegangen on Mar 12, 2014


Future generations may likewise be appalled by our waste of resources. Should we care? 

I do, because my kids are among them.



Robert Wilson's picture
Robert Wilson on Mar 12, 2014


You should put your comments on a Hallmark card.

Paul O's picture
Paul O on Mar 12, 2014

Hops, We all have Kids and Grandkids and There are ways to carry this to extremes.

Would it be fine to keep using Whale oil if we farmed the whales?

Should we turn off wastefull electric fans in an empty room, even if the fans are running directly off PV and there was no other need for the energy?

Should we not use street lights at all, or ban airconditioning while temperatures are lower than 90 deg?

Why don’t we force foot or bicycle travel on all trips less than 10 miles, or make every home use only one LED bulb per room?

Robert Wilson's picture
Robert Wilson on Mar 12, 2014


The key thing almost everyone seems to miss here is time and scale. It’s very easy to sit at a keyboard and dream up “solutions” to these problems. But please tell me when you imagine we will have the excess nuclear, wind or solar electricity to produce this ammonia in the first place. How can we possibly even think about starting this ammonia industry you keep talking about in the next two decades? What you are proposing is a scheme that will take at least one generation to even get started. And how do we possibly know how much these things will cost a few decades from now? Care to show me your crystal ball?

Nathan Wilson's picture
Nathan Wilson on Mar 13, 2014

So are you saying fossil fuels are so irreplaceable that modern humans will go extinct as soon as peak oil hits, with said disaster occurring in a few decades?  Or are you recommending that we focus on business as usual until someone invents a technology that is cheaper than fossil fuel and can be implemented completely in a 4 year election cycle?

I completely agree that a transition to non-fossil energy will take many decades, but the clock does not start until we begin using the new technology; never starting means never finishing.  I don’t agree that things that take many decades are not worth doing or talking about or planning.

I believe that the purpose of a energy/environmental web forum should be not just to entertain people with interesting articles, but to actually educate people about the technologies that can or cannot make a big difference in the future.  Remember it is the job of government to not just do the right thing, but to strike a balance between the right thing for incumbent businesses and the right thing as perceived by the (hopefully educated) people/environment movement.

But in terms of a road map to non-fossil energy, it is important to remember that the time it takes does not depend on how fast we can build solar farms and nuclear plants.  It mainly depends on how fast we decommission existing fossil fuel infrastructure; a realist rate is “after it reaches end of life”, because faster transitions involve very high cost to society (even though it may appear that only investors are losing from early plant closures).  So there is no real need to fully convert the electricity industry before moving to fuel; they can be done in parallel, with their own 60 transitions.

For syn-fuel (assuming it is not drop-in compatible with gasoline or diesel), I would expect that following large field trials, government would use policy incentives to drive market share to critical mass of 5% market share (about 1 in 4 refueling stations selling the fuel, and said fuel comprising 20% of sales per station).  After that, society could decide to keep the plan simmering until a future commitment to grow it further.

I would expect the fuel itself to be made from around 80% fossil fuel and 20% sustainable energy initially, with the sustainable component growing as society decided to increase its commitment.  This blend would be achieved with mandates, so it is not a very free-market solution.

Note that once the technology is deployed at scale in developed nations, it would likely become cost advantageous in developing nations with low labor costs and little or and undesired dependence on imported fuels.

Nathan Wilson's picture
Nathan Wilson on Mar 13, 2014

Keith, I’m not sure whether CO2 from sea water is really CO2 neutral.  Sure what little carbonic acid is present got there easily from the atmosphere.  But isn’t the carbonate sequestered there?  I though it resulted from slow rock weathering?  And wouldn’t localized depleated areas make it hard for sea creatures to form shells?  If so, it wouldn’t be sustainable unless someone started dumping powdered rocks in the sea.  (I’m just asking, I’m no chemist).

Nathan Wilson's picture
Nathan Wilson on Mar 13, 2014

Scientists (like those at the IAEA) keep telling use that we should reduce CO2 emissions, using the available tools:  a combination of energy efficiency, renewables, nuclear power, and CC&S.

If it were really easy, we could probably just pick one of two of these, and ignore the rest.  But as Robert’s columns have repeatedly show, it will not be easy.  Therefore, suggestions to focus on only one strategy strike me as not taking the problem seriously enough.

Just because you don’t wish to travel at 550 mph does not mean that you’ll succeed in convince the rest of the world.  Just consider California’s problems trying to build a high speed rail system… painfully slow progress on a very heavily travelled route.

Ed Dodge's picture
Ed Dodge on Mar 13, 2014

We will never run out of hydrocarbons.  We already hit peak oil right on schedule around 2004 when oil prices shot up above $100 a barrel.  Rather than go into decline, production has only gone up. The lesson learned is that even if petroleum becomes increasingly expensive there is no limit to hydrocarbons in general.  There are endless supplies and until you can outperform them from a military perspective we will continue to use them.

We can manufacture synthetic jet fuel and diesel that is superior to todays using long proven technologies for reasonable costs.  Using gasification technologies diverse carbon resources ranging from lignite and other low grades of coal, garbage, biomass, petcoke, peat and more can be converted into a wide variety of fuels, chemicals and power.

Among the advantages of gasification includes the ability to separate out all the pollutants and convert them into commodities for sale.  Pure fuels, for which methane is the gold standard, impart tremendous benefits on society by not polluting the air, soil and water with toxins that are killing us every day.  Every step the EPA takes to ratchet down on sulfur levels helps push the entire industry towards synfuels and natural gas, which is great.

As for carbon dioxide, obviously there is an important role for renewables, efficiency, nuclear, micro-grids, etc.  But we will continue to rely on hydrocarbons and so it is imperative to deal with the CO2 directly.  We need to capture every bit of CO2 we can, move it by pipelines, find marketable uses for as much as possible and sequester whats left.  Beyond that we need to use the soil as a carbon sink, grow plants and restore soil on every square inch of God’s green earth.  Its a greenhouse effect right?  Embrace it, grow plants, go green.  We are not doomed, there are solutions.

Thomas Garven's picture
Thomas Garven on Mar 13, 2014

Hello Nathan:

As someone who believes in an all of the above approach; I fully support this sentence from your posting.  “a combination of energy efficiency, renewables, nuclear power, and CC&S.”.  The point of my posting was to raise awareness that at some point in time some social behaviors may have to change.  I have serious reservations about our ability to meet the needs of 8 billion people given our current practices.


Roger Arnold's picture
Roger Arnold on Mar 13, 2014

It’s carbon neutral.  Atmospheric CO2 levels and CO2 dissolved in ocean surface waters are in quasi-equilibrium (per Henry’s law).  CO2 is constantly moving from one medium to the other.  A molecule removed from sea water and taken out of circulation will promptly be replaced by another taken up by the sea from the air.  

There’s a complex mess of other equilibrium reactions going on among carbonate and bicarbonate ions and other ionic species in sea water, but they don’t alter the result that — to a first approximation — a ton of CO2 removed from ocean surface waters will be replaced by a ton of CO2 taken from the atmosphere.  It’s not instaneous and not exact, since the more alkaline sea water that is discharged from the CO2 extraction units won’t all remain in the surface mixing zone where it can take up CO2 and regain equilibrium with atmospheric CO2 levels. But most of it will. The ocean is pretty stratified.

Robert Wilson's picture
Robert Wilson on Mar 13, 2014


My implication was that what you are advocating on ammonia is highly dubious, and your comments here don’t dissuade me from that view.

Governments should support using fossil fuels to produce synfuels? This is a borderline insane suggestion. Low carbon sources aren’t even close to keeping up with current growth of primary energy consumption from fossil fuels, and you want us to use even more fossil fuels to promote an ammonia industry. This will simply increase emissions and the cost of energy in the medium term. Not a good idea, and totally impractical.

Bas Gresnigt's picture
Bas Gresnigt on Mar 13, 2014

… quite possible that zero-carbon electricity will become cheap enough to make synthetic fuels at lower cost than what we currently pay for petroleum…
Many experts confirm!

Economics synthetic fuels
Variable costs for wind and solar are near zero, production will continue even if the price is only €1/MWh. Prices of ~€10/MWh may make synthetic fuel/gas production from CO2 economic (depending on fuel/gas price).

Even with the present low share of wind+solar (~14%, will rise to 70-85%), the electricity price in Germany is often below €5/MWh. As wind+solar increase, those low prices become regular.

E.g. In 2020 Denmark’s wind turbines will produce 50% of its electricity. They estimate that those will produce more than 100% of the needed electricity during 100 days / year = 2,400 hrs/year.

That imply that automated synthetic fuel plants that start the moment the whole sale price is low (e.g. <€10/MWh) will become economic.

So we see many pilot plants for synthetic natural gas and synthetic (car) fuel in Germany. And of course developments to decrease the costs of those plants.
Those plants can also produce plane fuel.

So, in a 100% renewable society flying around with air planes may cost somewhat more than now. However I do not think that the costs will rise to the levels of the eighties (compared with average income & earnings per hour worked).
Of course developing & building the huge automatic electricitiy-to-fuel production will take decades.

Nathan Wilson's picture
Nathan Wilson on Mar 13, 2014

Thanks Roger, that makes sense, especially for mobile naval applications like aircraft carriers which produce fuel, and perhaps OTEC systems (which also must be mobile to avoid being stuck in thermally depleted zones).

Roger Arnold's picture
Roger Arnold on Mar 13, 2014

It’s certainly true that spot prices on the wholesale electricity virtual markets are often low enough to make production of synthetic fuels economic.  That’s especially true in areas where there is a lot of wind power capacity on the grid.  Due to production subsidies, the  price even goes negative at times — “we’ll pay you to take this power”.  That’s the basis for David and Glen Doty’s ‘Windfuels’ proposals.

There are two problems with that model.  One is that, extrapolating from statistics on the Midwest ISO market, suitably cheap prices are only available, on average, about six hours a day.  That means that the electrolysis banks are only run at an average 25% duty cycle.  That makes the capital cost per daily unit of hydrogen 4x higher than it would be if the equipment were utilized and close to 100% duty cycle.  If the equipment were cheap enough — say $250 per kilowatt or so —  that wouldn’t matter much.  But the capital cost of electrolysis equipment hasn’t fallen that much.  I believe that for industrial orders, it’s still in the $800 to $1000 per kilowatt range.  And presently there are no big economic drivers that would push it lower.  

The other problem is that model doesn’t scale.  The low prices on the spot market reflect temporary excess of supply.  If you try to build synthesis capacity specifically to utilize that excess supply, you find that it doesn’t take much to use it up and erase the low prices on which your operations are predicated.  

That doesn’t mean that production of synthetic fuels doesn’t work, just that it’s currently confined to niche situations.  E.g., very cheap baseload power due to geographic circumstances (the George Olah renewable methanol plant in Iceland), locations where abundant wind and / or hydro resources have few competing uses (northwest British Columbia — ‘Blue Fuels’ project), or high priority logistical drivers (the U.S. Navy proposals).

A serious fossil carbon tax would, of course, change the balance,  It would make it much easier for synthetic fuels to compete.

Nathan Wilson's picture
Nathan Wilson on Mar 14, 2014

Governments should support using fossil fuels to produce synfuels?”

Oh, I forgot to say that the syn-fuel process should include carbon capture and storage (CC&S).

For coal-to-liquids, changing from a liquid syn-fuel like methanol to ammonia and adding CC&S would be no worse for efficiency or cost than adding CC&S to power plants.  

Since natural gas is already a fluid fuel, obviously there is a big efficiency penalty in converting it to ammonia (or any other syn-fuel).  However, the conversion can be done near the oil & gas field, and the captured CO2 can be injected into the formation for enhanced oil recovery (EOR), with little need for vast CO2 pipeline infrastructure; this would also provide a source of CO2 for EOR for countries who export most of their oil & gas.

Use of fossil-derived ammonia is clearly the simplest way to add CC&S to portable users like vehicles, as well as small users like CH&P or building heating.  Ammonia is also truck-transportable and tank-storable for off-grid use like propane (which is more expensive/valuable than fossil gas).  For very low capacity factor electrical peaking plants (a renewable-rich grid will use a lot of these), ammonia fuel could be the cheapest CC&S route.

Because of these advantages, governments (other than early adopters) don’t necessarily need a specific ammonia policy, since any carbon tax will push the fossl fuel industry in this direction.

Mark Goldes's picture
Mark Goldes on Mar 14, 2014

A jet engine has been invented that needs no fuel. It is designed to run on atmospheric heat and pressure.

These are untapped sources of solar and gravitational energy never before tapped to run engines. The atmosphere contains thousands of times the potential energy than all the earth’s fossil fuels.

A piston engine designed to run on atmospheric heat is being prototyped. Once validated by independent labs a pair of turbine engines will follow.

The first will provide unlimited range to hybrid cars and allow vehicles so equipped to sell power to utilities when suitably parked. It is expected to also power propellor driven aircraft.

The pure jet turbine will then be prototyped. If it performs as expected it opens a new era in aviation.

These engines circumvent The Second Law of Thermodynamics. They are therefore hard to believe possible. Prototypes will tell the tale.

My work is atacked as dishonest and fraudulant by a cowardly troll who hides behine pseudonyms. The prototype program will prove him to be incompetent.

See to learn more.


Geoffrey Styles's picture
Geoffrey Styles on Mar 14, 2014


Post and comments excellent and thought-provoking. A couple of additional points occurred to me that I didn’t see reflected here, though I might have missed them. Apologies if this is repititious.

First, in aviation fuels we have an extreme case of the current divide between transportation and electricity. The latter includes many viable non-fossil sources, and we’re seeing the share of fossil fuels fall–dramatically for oil–although gas and coal continue to compete effectively on cost. For transportation. while we don’t yet see non-fossil alternatives that can match gasoline and diesel fuel for cost, convenience and performance, an improvement of less than an order of magnitude in traction battery cost, storage density, and recharging times could result in EVs that outcompete conventional cars. Similarly with fuel cells and H2, though with the added challenge of infrastrcuture and storage. High-speed air travel is clearly a much tougher nut for non-fossil energy to crack than ground or even sea transportation.

However, I’d suggest this isn’t as serious a problem as it might appear, requiring complex techincal solutions as numerous comments have suggested. Consider that total global jet fuel consumption accounts for just 8% of OECD petroleum consumption and probably a lower share of global demand. It contributes <2% of global GHG emissions. On that basis it’s probably sustainable indefinitely, particularly if you factor in the non-oil pathways for synthetic jet fuel described by Edward below. Since resources of all kinds for addressing climate change are scarce, why not focus them on easier problems than this one, that are more material to the outcomes?

As for Mr. Garven’s comment concerning the necessity of all of us being able to travel at 550 mph, I can recall when air travel was a luxury and only wealthy “jetsetters” traveled the way that hundreds of millions now do routinely. I don’t envision many people voting to return to that world.

Robert Wilson's picture
Robert Wilson on Mar 14, 2014

I am not anonymous or a troll Mr. Goldes, but I can be pretty sure that your engine requires that the basic laws of physics need to take a holiday the second your plane takes off the ground.

However if you wish to make a fool out of yourself by publicly stating that the second law of thermodynamics can be circumvented then please go ahead.

Mark Goldes's picture
Mark Goldes on Mar 14, 2014

“Over the last 15 years the absolute status of the second law of thermodynamics has come under increased scrutiny. More than two dozen distinct challenges have appeared in the refereed scientific literature—more than the sum total over the previous 150 years—raising the possibility that the second law might soon be shown violable in laboratory experiments.”  

Since he wrote those words, the Second Law has now been proven violable in experimental work by Dr. Daniel Sheehan of the University of San Diego en-route to publication in a refereed journal.  

Clifford Goudey's picture
Clifford Goudey on Mar 14, 2014

 “The obvious lesson here is that fossil fuels can deliver power densities orders of magnitude higher than wind or solar.”  No Robert, the obvious lesson is you can’t compare the two.  Consider instead bio-fuel power density?  It seems the Navy sees it as their answer to future supplies of aviation fuel.

Clifford Goudey's picture
Clifford Goudey on Mar 14, 2014

If it’s time and scale you want, how can you be championing the squandering of jet fuel considering it is a non-renewable resource.  How long do you think you can keep your 747 in flight?  Another 20 years?  Another 50 years?  How expensive will jet fuel be in 2025? 

Robert, you seem to lack a vision for the future and instead want to extrapolate from 20th century practices.  I’m not at all comforted by your confidence that we needn’t re-tool. 

Robert Wilson's picture
Robert Wilson on Mar 14, 2014


Whether aviation can be sustained indefinitely is a question of political judgement. Some would argue that meeting internationally agreed climate targets will require that aviation’s emissions must be lower. And the only credible way to do this is to fly less. Here I am not advocating either way, just pointing out the engineering problems.

Getting people to fly less in rich countries may or may not be possible. It certainly should be possible to switch a significant portion of air traffic to high speed rail. This should actually be a matter of national embarrassment in many countries, including the US and Britain. Japan and France have high speed rail systems in place. We don’t.

Large numbers of flights in the US could easily be replaced by high speed rail, and should have been replaced long ago. For example 3 million Americans fly between New York and Boston each year. Do Americans really prefer flying from New York to Boston to a 2 hour downtown to downtown high speed train journey? I think not. It’s mind boggling that there is no significant political pressure to get such things in place.

Robert Wilson's picture
Robert Wilson on Mar 14, 2014

Once again Clifford you put views into my mouth. Please stop commenting on my pieces if you are going to mis-represent me.

Clifford Goudey's picture
Clifford Goudey on Mar 14, 2014

Robert, I knew it had to be you when I read the title “Flying without Fossil Fuels: The Need for High Energy Density.”  This it typical of your stream of concocted arguments against renewable energy.  What surprises me is you failed to somhow make your usual case for coal.

You fail to grasp the reason we fly.  It is not because we enjoy life at 35,000 feet or because we covet airport parking or total-body X-rays.  It’s simply to get from point A to point B in a reasonable amount of time. 

The right way to frame the question would have been “Is High-speed Travel Possible Using Renewables?” The obvious answer is yes and any engineer who suggests otherwise is likely to find themselves unemployed. The suggestions for alternate fuels to continue flying are coming in faster than you can swat them away.  However, if travel is the issue, then rail is the obvious near-term alternative and for many routes the door-to-door time is less than flying.  High-speed rail exists elsewhere and higher-speed transport in evacuated tubes is already technically feasible. 

The consistency of your mindless fossil energy advocacy is appreciated, as it helps us all understand what the real problem is.


Robert Wilson's picture
Robert Wilson on Mar 14, 2014


I think calling you mindless would be much too polite. Please go to another part of the internet. Other people need to be mis-represented.

Thomas Garven's picture
Thomas Garven on Mar 14, 2014

Hi Geoffrey:

I am quite sure you are aware that in aviation its all about lift vs drag. The higher the speed the higher the drag. For myself, the idea of flying at 450 mph in a blended wing aircraft would be just as effective as flying at 550 mph in a typical aircraft. Someday, we may need to take a much more rigorous approach when determining cost vs results or convenience vs need.

Have a great day.

Thomas Gerke's picture
Thomas Gerke on Mar 15, 2014


Clayton Handleman's picture
Clayton Handleman on Mar 29, 2014

“For example 3 million Americans fly between New York and Boston each year. Do Americans really prefer flying from New York to Boston to a 2 hour downtown to downtown high speed train journey?”

Some do some don’t

High speed rail exists between Boston and NY.

To your larger point, maybe we curb air travel and add more rail transport.  At the end of the day, I think that air travel is one of the few places FF make sense and it is the last thing people should be focussing on at this stage of the game.  Lots of bigger fish to fry as they say.

Robert Wilson's picture
Robert Wilson on Apr 4, 2014

Please tell the Japanese or French that Acela is a “high speed” train. They will laugh at this curious example of American Exceptionalism.


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