My general rule of thumb is that if you read headlines “X will save the planet” or that biofuels will do anything at all it’s better to just flip the page. However a report in the Guardian last week informs that seaweed biofuels can do all kinds of magnificent things, and as the headline above suggest saving the planet may be among them.
Can biofuels save the planet? No, and this can be demonstrated by some very simple, but powerful calculations.
The fundamental problem is simple: biofuels low power density. In every country each unit of land, on average, consumes X units of energy. On the other hand each unit of land can produce on average Y units of energy in the form of biofuels. The problem for biofuels is this: Y quite often is less than X.
Discussions about energy are marred by the myriad number of confusing units. Exactly why anyone would wish to use British Thermal Units is a perpetual mystery. Here I’ll use the more common and sensible unit of watts per square metre (W/m2) to describe power density. This has a lot of advantages, in particular that you can very quickly calculate land requirements for renewable energy installations. David MacKay has produced a remarkably simple and informative graph to demonstrate this point:
Quite clearly South Korea isn’t going to be getting 100% of its energy from onshore wind farms any time soon, but I’ll leave that to another post.
You’ll also notice that the United Kingdom uses energy at the rate of about 1.2 W/m2. This does not compare very favourably with 0.5 W/m2. provided by energy crops, i.e. biofuels. The exact power densities of existing biofuels can be debated somewhat, but quite obviously not to the point where you would not need country sized biofuel plantations to provide a significant percentage of UK energy supply. The difficulty is thatthis is true for all existing biofuels, including those derived from seaweed.
These simple facts however are clearly not known, or are ignored, by Damian Carrington who in his Guardian piece informs us:
Many see huge potential, with the UK government already including up to 4,700 sq km of seaweed farming cultivation in its future energy scenarios and another study finding it could in theory supply the world’s needs several times over.
Consider the first statement. What exactly is this huge potential? For some reason Carrington tells us how much land we could cultivate, but does not bother telling us how much energy we could get from it. However, it’s a rather straightforward calculation so let’s do it.
The UK government document he links to give us the information we need: yields of macro algae could reach 20 dry tonnes per hectare by 2025 and 1 million dry tonnes of micro algae give up 3.9 TWh of energy. This works out at 0.9 W/m2 . Clearly, this power density is awful – just how much of the North Sea do people imagine is available for this stuff. And to put firmer numbers on this “huge” potential, consider what the UK government actually estimated we can get from 4,700 square kilometres worth of biofuels. A grand total of 5 TWh per year (Level 4 in the graph below represents 4735 km2 covered in algal biofuels by 2050).
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Instead of “huge” potential for seaweed biofuels this is in fact completely marginal. According to the latest statistics from BP, total UK energy consumption in 2012 was 203.6 million tonnes of oil equivalent. This converts over to approximately 2,400 TWh. So, this supposedly huge potential for seaweed biofuels amounts to a mere 1% of UK energy supply. As always, beware the hype.
Now, the above discussion of the low power density of seaweed biofuels ought to convince anyone that they won’t be able “to supply the world’s needs several times over.” But to make it even clearer why seaweed biofuels can’t save planet, let’s consider scale. Right now, we produce just over 4 billion tonnes of oil per year. According to the Guardian’s story “a Californian firm produced genetically modified bacterium that can produce about 1kg of ethanol from 3kg of dried seaweed.” So, to totally replace global oil production with seaweed biofuels we would need to harvest 12 billion tonnes a year of dried seaweed (and remember the stuff we take out of the sea is not dry). To put such a figure in perspective remember all of the coal, oil and gas produced on the planet comes to 11 billion tonnes.
And how much seaweed do we harvest as of today? 17.3 million tonnes. Converting all of that over to seaweed biofuels would provide us with less than half a day of global oil demand.
You can now remove that “Seaweed will save the world!” bumper sticker.