The end of the fossil fuel age is within sight. Whether it’s because we eventually run out of the stuff, find better alternatives, or wake up to the realities of climate change remains to be seen. But whatever the cause, how we fuel our cars, trucks and buses is ripe for a shakeup.

Truly sustainable fuels will need to come from a renewable source, and an often-cited problem with most forms of renewable energy is that they have low energy densities. Wind turbines, solar panels and fuel crops need to be spread over large areas to capture the same amount of energy that can be obtained from a barrel of oil or a tonne of coal.

The transport sector is a major user of energy, and much effort is going into the development of liquid biofuels that can easily be slotted into current refuelling infrastructure. However there are concerns about the impact of biofuels on both food production and water use, and along with efficiency in the use of energy, we also need to consider the efficiency of production. At the moment we think of vehicle fuel efficiency in terms of miles per gallon (mpg) or litres per 100km (l/100km), but to appreciate the efficiency of renewable transport energy sources maybe we’ll need a new unit of measurement…

Kilometres per square metre

How far can you go on the diesel, petrol, ethanol or electricity that can be harvested each year from a square metre of land, ocean or hydroelectric dam?

Obviously the result depends as much on the choice of vehicle as it does on the source of fuel. We don’t want to compare electric Smart cars with petrol engine Hummers, so for this exercise I’ve used the VW Golf. It’s an average sort of car that is conveniently available in petrol, diesel and electric versions with fuel economy figures available for each.

So how many kilometres per square metre will your renewable energy-fuelled Golf travel? Here are some rough estimates. I haven’t sought out the most efficient generators in all categories or taken into account all the possible energy losses that can occur between primary production and final distance travelled. I have also made a few conservative assumptions and it is important to note that some of the technologies have yet to be deployed at scale (or at all). Still, these calculations provide ‘ballpark’ figures that, along with other considerations such as cost, competition for food production and the availability of suitable sites for different types of technology, may point to the fuels that will be driving us in a renewable energy future.

And the results are…

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Clear contender

Rough as these calculations are one conclusion leaps out. Compared to biofuels, electricity from renewable sources looks like the big contender for sustainably driving our future.

This shouldn’t come as a surprise. In the case of solar, photovoltaic panels are significantly more efficient at harvesting solar energy than plants are, and electric motors convert more joules into kilometres than internal combustion engines do.

These are, as I mentioned, fairly rough calculations, so for the numerically interested I’ve included notes on the data sources and basic assumptions at the end of the article. One point is worth clarifying here: for wave power I used the figures for a single prototype device that has undergone sea trials. For wind and solar the figures come from operating, farm-scale power plants. Wind turbines in particular need to be spaced well apart which leads to a relatively low value for power produced per unit area. If the calculation is done for the area of land that sits under a single 5 MW wind turbine the figure is similar to that for wave power.

The e Golf could edge out its liquid-fuelled siblings in a renewable energy future | Image: MotorBlog via Flickr (CC BY 2.0)

Just one measure

The energy yield from a given unit of area is just one pointer to where we will be getting our sustainable transport fuel from in the future. Factory roofs aren’t much use for generating wave power but are ideal for solar panels. Miscanthus and switch grass won’t grow in the coastal deserts that may suit algae cultivation. Wave power is still to prove itself, hydroelectricity comes with a range of environmental problems, and hanging over everything is the matter of cost.

But even on this front, renewable electricity looks good. Wind and solar power are already much cheaper ‘fuels’ for cars than petrol or diesel. And a decent sized domestic solar power system can generate enough electricity to drive thousands of kilometres in a year.

Now we just need the cost of the cars themselves, and batteries in particular, to come down to more affordable levels, but plenty of people are working on that.

Sources and notes

  • Fuel use figures for petrol and diesel models of the VW Golf were taken from Figures for the electric version came from For ethanol, I assumed the mileage was around 25% less than for petrol.
  • The yields of biodiesel and ethanol from oil palm, soybean, maize, and sugar cane were taken from table 2 of this excerpt of a 2008 report by the FAO.
  • Solar power production per unit area was calculated from data for the Nyngan solar power station.
  • Switch grass is often touted as a high-yield source of cellulosic ethanol, but according to this reference, the tall grass Miscanthus is even more productive so it is included in the chart.
  • The wind power harvested for a given area of land varies widely, and I’ve ignored any potential to use the land between turbines to produce, say, energy crops. The 420 MW Macarthur Wind Farm is spread over 5,500 hectares, and I assumed a capacity factor of 30%.
  • Hydroelectricity production was calculated from the output of the Three Gorges Dam and the surface area of its reservoir. Figures from other hydro projects will no doubt be vastly different.
  • As pointed out in this article, algae could be the hot prospect in liquid fuel production. I used figures from Muradel (diesel and petrol) and Algenol (ethanol). Muradel uses algae grown in salty water to produce ‘green crude’ which fractionates into a similar range of products as fossil crude oil. For this exercise I assumed production of just petrol or just diesel.
  • Wave power is probably the most speculative. Many proposed systems are buoy-based, and I don’t have figures for how tightly they might be spaced when deployed at large scale. I used figures for the Wave Rider. It’s a rectangular wave energy converter, and a sizable (500kW) prototype has undergone successful sea trials. I assumed a capacity factor of 30%.