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Energy Risk: Why Seaweed Biofuels Cannot Save the Planet

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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  • Jul 9, 2013

Seaweed biofuels: a green alternative that might just save the planet

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 that this 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 km covered in algal biofuels by 2050).

https government uploads system uploads attachment_data file 47880 216 2050 pathways analysis report.pdf

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.

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John Miller's picture
John Miller on Jul 9, 2013

There is a certain irony or contradiction when individuals advocate that fossil fuels can be totally replaced by biofuels such as ethanol produced from seaweed.  As you have done a good job of covering the sheer volume and size of seabed areas required, there is the issue of adverse sea life-environmental impacts.  Not unlike land cultivation needed to feed the world’s 7 billion and growing population that has led to the destruction of multi-millions of acres of forest lands, massively cultivating algae or seaweed will lead to similar destructive impacts of coastal areas.

Another issue with Y (net biofuel energy yield) vs. X (cultivation-production-distribution energy consumption) is that the preliminary, rough yield approximations made at the laboratory-research level often omit substantial energy consumed in the overall biofuel ‘lifecycle’.  As I am sure you are aware, each time one form of energy (bio, chemical, heat, etc.) is converted to another form of energy there is a conversion efficiency loss or penalty.  The more steps required to physically handle, process, purify, etc. a given biofuel, the greater X becomes and the less the ‘net energy value’ (Y-X) becomes. 

Nathan Wilson's picture
Nathan Wilson on Jul 10, 2013

Well, the ocean is huge.  But what the Guardian article does not make clear, is that seaweed does not grow  in very much of the huge ocean.  The seaweed in question is kelp, which likes to grow only where the water is 30-90 feet deep, i.e. along the coasts (as I recall from a former life as a California SCUBA diver).  It is a giant plant, which anchors to a rock, and extends to the surface, forming “kelp forests” (which can be a menace to small boats because of propeller fouling).  Some comments following the article speculate on the construction of farm-sized structures that could support kelp in the open ocean.

The Guardian article does mention that free-floating micro-algae grows in the open ocean, but suggests that there aren’t any good ways to harvest it (short of grow it in giant plastic bags, tubes, or ponds).

Not very persuasive.

Of course I must mention that there is one practical fuel that can be made from any sustainable energy source, air, and water: ammonia.  With a formula NH3, it does not involve carbon and when burned produces only water and nitrogen.  It is liquid at room temperature under moderate pressure (like propane), which gives it an energy density slightly better than that of compressed natural gas (more than 2x that of 10,000 psi H2).

It can be burned in certain fuel cells almost as efficiently as H2, but can also be used in a modified internal combustion engine (with better energy efficiency than gasoline), so the large expense of fuel cells is not required.  That means that it will complement battery electric cars in the marketplace; and will be a better solution for large trucks, trains, ships, and mobile off-grid power.

Ammonia is more toxic than gasoline or methanol, so new spill-resistant refueling systems will be required.  Of course the pressurized storage will mean than no evaporative emissions would occur.   It is readily detected by smell at 100x lower concentration than is dangerous.  Ammonia is a natural part of cell biology in all animals, so chronic exposure to low doses is completely harmless.  Overall, the safety is predicted to be similar to that of gasoline (the toxicity is offset by greatly reduced risk of fire and explosions).

When it’s made from hydrogen, only 15% of the energy of the H2 is released as heat (at high temperature, so it can be recycled), so it’s actually more efficiency than making liquid H2 at factories or making compressed H2 at small distributed retail stations.  Also a fuel cell that can efficiently synthesize ammonia directly from N2 and H2O has been demonstrated in the lab.  This means that ammonia is now and likely always will be the cheapest fuel that can be made from solar, wind, OTEC, or nuclear energy.  It is  made from fossil fuel today (with carbon capture at some plants) for a cost which is competitive with gasoline.  

When chilled to -33C, ammonia can be stored in unpressurized tanks.  This means that ammonia can be stored (in warehouse-sized insulated tanks) for seasonal demand leveling. 

The US already has a few thousand miles of ammonia pipeline, as it is a commonly used agricultural fertilizer.


Robert Wilson's picture
Robert Wilson on Jul 10, 2013


That’s a typo. 

Nichol Brummer's picture
Nichol Brummer on Jul 10, 2013

This is indeed a great plot. But is a mistake to ignore energy efficiency: every country can move down along a vertical line without giving up any of the energy services we currently enjoy.

The other important point is that liquid fuels are most important for transport, where batteries are not yet the universal solution. So liquid biofuels may still be a very important part of the solution for that particular part of the problem. Especially for airplanes.

More kelp forests in certains parts of our seas could be a great way to preserve and grow our marine ecosystem: fish. Could windfarms at sea be a good base for artificial reefs, and kelp? So if people can find a way to create kelp farms at sea that actually enrich the ecosystem .. then that would be great. It doesn’t need to solve all problems. It only needs to be profitable. If it has positive side-effects on fish, it may be worth subsidising.

John Miller's picture
John Miller on Jul 10, 2013

Willem, the problem with algae based biofuels is that current state-of-technology(s) has significantly negative ‘net energy values’ (NEV) for the full-lifecycles.  This condition often yields total carbon emissions similar to the petroleum fuels being displaced; of course the NEV and associate carbon emissions are contingent on the actual fossil fuels energy mix consumed in ‘cultivation-to-wheel’ full-lifecycle.  One very significant factor that directionally indicates that this negative net energy value/high actual carbon emission factor may exist is the price of the finished biofuel.  Since algae based biofuels production can be very energy intensive (transportation/equipment fuels, electric power, process heating/cooling, energy to produce required chemicals/enzymes, etc.) and cost is usually a function of production (fossil fuels) energy consumption, biofuels that cost on the order of 2-3 times alternative petroleum fuels very likely have carbon emissions similar (and possibly greater) than the fossil fuel being displaced.

Robert Wilson's picture
Robert Wilson on Jul 10, 2013

Thanks Willem

You do realise I calculated what percentage it was in the piece, and also referred to it as marginal.

Rick Engebretson's picture
Rick Engebretson on Jul 10, 2013

Without having read the described article, it is non the less easy to debunk this one dimensional arithmetic analysis.

The Gulf Stream already pipelines seaweed to the UK from the Gulf of Mexico. The Gulf of Mexico is already super heated and super saturated with critical nutrients like phosphorus, growing too much biomass and creating “dead zones.” If you want to complain the gift of gold is not the gift of platinum by dumping some numbers, it has nothing to do with creating wealth from abundant bioresources.

So take the free air, free nutrients, and free carbon reagents and try some imagination. About the same time the Brits were bragging about their “Titanic” and battleships, others were tying fabric to sticks to fly circles around them.

With all the talk about problems storing intermittent solar energy, free reactive carbon can store some TWhs for you if you think beyond one dimension.

Robert Wilson's picture
Robert Wilson on Jul 10, 2013


You call this a debunking! This is more like an evidence free helping of white noise. Please return with evidence and argument.

Rick Engebretson's picture
Rick Engebretson on Jul 11, 2013

Robert, I try pretty hard not to be specific and revealing while plastering stuff on the internet.

Perhaps it is sufficient to say I just had a nice visit from 2 Silicon Valley engineers, and biofuels were considered in ways not reflected in your unimpressive arithmetic. Both work at leading innovation companies, one with MIT the other with Oxford credentials.

One has family in Calcutta, and requested to try a riding lawnmower. We discussed other technologies like the history of digital and internet now microcontroller electronics. But the glitter in his eyes told me he understood how a simple riding lawnmower harvesting food and fuel in south Asia was worth considering.

People selling arithmetic and colored charts calling mid-American agricultural technology a “boondoggle,” with no constructive alternative to offer, must be challenged. I gain nothing from it, and likely lose. Personally, I prefer turning farmland over to “Pheasants Forever'” and “Ducks Unlimited” and forget about too many people with attitudes.

Robert Wilson's picture
Robert Wilson on Jul 11, 2013


Please stop taking up the time of people on this site with your rambling comments.

Paul O's picture
Paul O on Jul 11, 2013


Let me add that you underestimate the problem of replacing Oil with bio fuel Methanol in you math.

1) If 1 Kg of Methanol requires 3kg of dried sea weed,  the WET Haarvested Sea weed to produce 3Kg of the dried stuff could be as much as 8kg before it is dried (an energy concuming act.

2) The sea weed is converted to Methanol. Methanol has a lower energy content than gasoline. In other words, you’d need at least Twice the volume of methanol, to go as far as Gasoline.

3) Ikg of Methanol requires 8 (or more) Kg of seaweed. For Methanol to produce an equivalent amount of energy as gasoline, you’d need Twice the volume of Methanol. (I understand that you are using Mass and I am using volume).

Obviously, to produce enough biofuels (wet) to replace 4 billion tonnes of Oil, weed need  at least 24 billion tonnes of Wet sea weed, and factoring in the Mass energy densities, weed need to multiply this figure by 2 to account for energy densities, seeing that the Specific Gravityis of both Methanol and Gasoline are close.

Okay, These are rather rough estimates, and I am intermingling the ter “OIL” with “Gasoline”, but  you get the picture. 

If anything I think you understate the problem.

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