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.