Tuesday, November 18, 2008

Oil From Algae - How Big Do the Tanks Need to Be?

The comment reproduced below to my article "Wednesday, April 09, 2008; Oil from Algae: Photosynthetic Efficiencies?" prompted some very salient points regarding the size of the tanks or tubular reactor vessels that would be required to grow algae on a large scale, and the quantity of the materials needed to make them from. I have therefore done a few sums, which I hope are illustrative:

I agree completely with Nic's figures which are very close to those I have deduced here previously, and I shall use them now. First I want to consider tube-reactors, and I am assuming that the tube is about a foot (30 cm) in diameter.

(.3 m x pi =) .94 m (circumference of tube) x 3.3 x 10^12 m (length of tube) = 3.1 x 10^12 m^2
as the area of plastic needed to fabricate 3.3 billion km of it! If we assume that the wall thickness is 2 mm (0.002 m), we have a volume of plastic = 3.1 x 10^12 m^2 x 0.002 m = 6.2 x 10^9 m^3.

Taking the density of the material as 0.85 (t/m^3), that amounts to 0.85 x 6.2 x 10^9 = 5.3 x 10^9 tonnes. If there are 7.3 barrels of oil to a tonne, that is the equivalent of 38.7 x 10^9 barrels of oil to make the plastic (this is about 3% of all the oil left in the ground). The world uses 30 x 10^9 barrels in total annually, and so this is more oil than the world gets through in 16 months all told.

I agree with Nic that it probably wouldn't be possible to make more than few million km of pipe per year and so it would take centuries to make it. Well before then we won't be recovering significant crude oil anyway, given the total of 1.2 x 10^10 barrels there is thought to be recoverable. On this basis, I think making biodiesel from algae on a large scale in tube-reactors is unlikely.

So, what about the open (raceway) ponds? If we need 1 million km^2 of them, that's 1 x 10^12 m^2. I am going to assume they are each 1 hectare in area. That's 100 million of them. I shall also assume that they are 1 metre deep. The area of the walls is 100 m x 1 m x 4 walls = 400 m^2. But the floor area is 10,000 m^2 (i.e. 1 ha). hence I shall forget the 4% of the total pond area made up by the walls and concentrate on the larger, floor area of 10,000 m^2 for each one.

The ponds could be lined with either concrete or plastic. Let's consider the concrete option first. If the floor is four inches, 10 cm thick = 0.1 m. So we need 10,000 m^2 x 0.1 m = 1,000 m^3 of concrete for each pond. So 100 million of them needs, 100 x 10^6 x 1,000 m^3 = 1 x 10^11 m^3 and at a density of sat 2.3 t/m^3, that's 2.3 x 10^11 tonnes of concrete, or 230 billion tonnes. In 2007, the world produced 2.6 billion tonnes of concrete, and again the job would take centuries assuming that an equivalent amount of additional concrete could be made to current consumption, or some of it diverted from existing purposes. The global warming effect would be pretty severe since making concrete releases huge amounts of CO2.

What about plastic lining? Well, the area is the same, at 10^12 m^2 and I shall assume that a 2mm thickness is O.K. again, so the volume is 10^12 m^2 x 0.002 = 2 x 10^9 m^3 of plastic.
This equals around 12.4 billion barrels of oil, which is about 1% of the world's total recoverable of 1.2 trillion barrels, or 1/3 that for the tubes, and it might be easier to make plastic sheets than tubes, but even if we could make say, a few thousand square kilometers (a few billion m^2) of plastic sheeting annually, the job would once again take centuries. We are dealing with a classic rate of flow (rate of conversion, rate of recovery) problem, and while not hampered necessarily by the resources per se, the engineering can't be done in time to save us from running out of cheap crude oil.

On Nic's final pint about instead using electric vehicles. The best figure I have is that you need a bare minimum of 0.08 g of lithium/Wh of power. But for real Li systems, it is more like 0.15 - 0.32 gLi/Wh. I shall take the mid-range of 0.23. An average PHEV needs around 9kWh. So we need 9,000 x 0.23 g = 2070 g = 2 kg = 0.002 tonnes per vehicle. Now there are supposed to be around 700 million vehicles on the roads. There are something like 30 thousand tonnes of Li recovered per year and so we would need to double production capacity to get something reasonable, i.e. that amount again to make the electric cars etc. [There is plenty of lithium in the ground, 28 million tonnes, I think, but it's the rate of recovery that is the limiting factor].

700 x 10^6 x 0.002 t = 1.4 x 10^6 tonnes to make that equivalent car fleet from PHEV's. Well at 30,000 t/year it's going to take 46 years. So, in principle this is the best bet. However it would mean making 15 million cars a year. We'd need a lot of new mining, processing and engineering
though. In all likelihood the world will have far fewer vehicles in the future, and we can supplant urban transport with trams and light railways so long as we can make enough electricity to run them and so need less anyway.

In conclusion I take your point about cars Nic, but still since aviation consumes more fuel per year than we can tool-up for with algal biofuel within decades (centuries) plane-flight does seem doomed in its present form and scale.

But whatever we do it's got to be done against that backdrop of cheap oil running out - yes, I know its cheap now because the world financial system is melting down - but it wont be for much longer; also the economic turndown means that there is little fiscal incentive and little money to be borrowed from the banks in their nervous condition, to build new PHEV factories and mining and processing plants for Li. PHEV's are expensive to make too and that is also bad is the current state of credit-crunch. There are other resource issues to PHEV's too, since they use various other metals, cobalt etc. depending on the exact battery design

Thanks very much,


Hi Chris

I was checking some background info on Algae after reading an article “Algae on the Move: The 2008 Algae Biomass Summit Wrap-up” in Renewable Energy World < id="54033"> when I came across your site.

You’re right 14.7%+ is too high for PSE, its limited by the nature of PS process. I can’t remember the exact figures but the maximum practical yield, under ideal conditions, is between 5 and 10%. High concentrations do not improve the situation as only the algae near the surface will photosynthesise and if the light’s too strong they can’t use it all so high agitation is also required.

I attended the International Society for Applied Phycology (algae) conference this summer in Galway and picked up some interesting data on algae, the best algae brains in the world were there so I am fairly confident that their figure of just over 10% absolute maximum for photosynthetic efficiency is correct. If I recall correctly, figures for open (raceway) pond reactors of a few percent would be about right.

I was mulling over the figures for replacing all our oil with algae biomass rather than just diesel, thought you may find my workings of interest:

World oil consumption is ~= 85Mbboe/day @ 6.1GJ/bboe (bboe = barrels of oil?)
= 5.185e17J/day = 12.345Mtoe/day (@42GJ/t)
> Most of world’s oil energy is used for fuel.
> Use total algal biomass is for fuel manufacture e.g. via hydrothermal liquefaction (a form of pyrolysis).
> Crude oil to fuel (well to tank) losses ~10%
> Algal biomass to fuel (well to tank) losses ~25%

Realistic practical PSE for algae is about 5%, possibly slightly higher for enclosed reactors and probably about half for raceway ponds, assuming they run 24/7:
,’, @ 5KWh/day energy yield/capture by algae daily yield is
5,000 * 0.05 * 3600 = 900,000J/sq-m

Production space:
Therefore algal biomass energy content needs to be ~15% more than crude production then
Required algal biomass production per day = 1.15 * 5.185e17 / 900,000
= 662,528 sq-Km of bioreactor area
Add to this access roads, space between reactors and space for processing and storage then the total area required is of the order of 1 million square kilometres, or about 10% of the Sahara desert.

Production mass:
Micro algae energy density (HHV) depends very much on relative oil, protein and carbohydrate content but would typically be around 20MJ/Kg (~half of oil)

,’, total algal mass required = 1.15 * 5.185e17 / 20e9 ~= 30 Million tonnes/day

The burning questions are:
 How fast could we build this quantity of algae production and processing facilities?
 How much will it cost?, is it economically viable etc.
 Can we build this amount of infrastructure? Algae ponds will probably need to be concrete or plastic lined over an area of 1million sq-Km plus, closed reactors would probably require (using AlgaeLink system) about 3.3Billion kilometres!!! of bioreactor tubing (my guestimate is that at best we could produce a few million Km of tubing per year so we a talking of a multi century project).
 Is there a better solution?

The answer to last question is yes, change to electric technology for land transport, save biofuels for air transport. Electric vehicles far more efficient (~80% tank to wheel) than other technologies, best hybrid might ultimately achieve about 50% and renewable electricity easier to generate than biofuels, best PV nearly 40% conversion efficiency.


Anonymous said...

Unwarranted assumption: Algal oil production requires lined ponds.


Professor Chris Rhodes said...

So what are you going to make them out of then?

Just holes in the ground, steel sheet, fully standing plastic sheets?

Of course they have to be fabricated or lined in some way, unless you are proposing to grow algae on the open ocean?

Incidentally, the idea of growing the algae within sheets of plastic also has similar resource implications and rate-of-conversion criteria.


Anonymous said...

As a boy, I used to go waterskiing on lake Windermere with the family of a colleague of my father. Pray tell, is that particular lake lined with concrete or plastic?

If you'll forgive my sarcasm above, Chris, you could click on this link to find out more:

"The basic design of a large-scale (400 ha of ponds; 500 ha, total area) algal biomass production process is rather similar to that of current commercial algal production systems, such as the Spirulina producing Earthrise Co. plant located on the South shore of the Salton Sea, only larger. Instead of the typical 0.3 to 0.5 hectare ponds used for Spirulina culture, the individual growth ponds would be about twenty-fold larger, some 8 hectares, and would not be lined with plastic, but have dirt bottoms. Unlined ponds have been demonstrated in both the PAS process and in pilot plant-scale algal biomass production systems."

"Large (>4 ha) unlined ponds are also used in same Spirulina production and wastewater treatment, suggesting that this technology can, indeed, be scaled-up. Large unlined, paddlewheel-mixed, ponds are of simple design and low cost, with some $5,000 per hectare of growth pond area for the site clearing and pond levees, and another $5,000/ha for paddle wheels."

Or you could just do a google search for microalgae and unlined ponds and see what comes up. Either way, if we can agree that neither concrete nor plastic linings are absolutely necessary, then we should be as one in concluding that the algal biofuel production is not limited by the rate of their supply.


Professor Chris Rhodes said...


the bottom of lake Windermere is based-out with rock! Any cracks etc. are coated with organic detritus etc.

What we seem to agree on is that if it is necessary to build a load of ponds lined qith wither plastic or concrete it can;t be done.

Now, unlined ponds. O.K. they will need to be dug-out artificially. Right? That would certainly provide a "rate of conversion " problem... and we have not even considered the amount of engineering needed to build the oil-biodiesel processing plants.

However, I am a fan of algae and you have cheered me up that at least lining may not be necessary. Is the idea that they are effectively self-sealing once they are filled, dug and maybe after they have been used for a while?

But it still means an awful lot of digging doesn't it?


Unknown said...


As usual, your calculations regarding scaling give significant pause for reflection. I note that you assumed tubes of approximately 30cm in diameter and 2mm thick. I don't assume that it will be a gamechanger, but Valcent's (www.valcent.net) design uses 2 layers of sealed (hopefully highly UV resistant) plastic sheeting to form a vertical hanging courseway with diameters of some 10cm or so, but still uses PVC pipework to connect the courseway to the necessary venting tank.

For this to scale, it seems UV resistant bioplastics would need to be produced and perhaps developed to create the HDVB bio-reactors. Hopefully some of the algae byproduct could be used ...

CarbonZero Foundation

Professor Chris Rhodes said...

Hi Mark,

yes, the figures are guesstimates, but as you say, amendments to them will not change the conclusions that should be drawn very much.

A good point that UV-resistant plastic is necessary otherwise the reactors would simply degrade after a few years (or less).

I think it is all very interesting and indeed, that there might be very useful quantities of chemicals produced from algae grown in vertical systems etc., but it's that 20 billion fuel barrels that is the challenge.

Mark J makes a good point too, about the fact that open ponds don't necessarily need to be lined, i.e. they can just be dug out of dirt.

As far as the sums seem to stack-up, this is perhaps the only way algae/algal oil can be produced on the vast scale necessary to replace petroleum as a fuel source... but the engineering (digging) and final oil-to-fuel processing would need to be done on a huge scale, and by when I wonder??

How long would it take.

There are many issues attendant to growing algae per se, which need to be cracked before the technology becomes a full-scale reality, but if it can be done, then probably the greatest threat to humankind would be averted.

I am assuming that the loss of enough cheap oil to keep the world running will hit us well before CO2 emissions and climate change do their worst.



Anonymous said...

I just did a quick and dirty calculation. About 5 million tons of polyethylene film are produced each year in europe alone. At this rate, it would take about 30 years to cover an area equal to 10% of the Sahara desert.
As plastic bags fall out of favour, more film making capacity will become idle. As well, it is easy to add capacity in this well established, mature industry.

Algae farming should not be discredited because materials are not available .

Professor Chris Rhodes said...

I'm a fan of algae but the lead-in time for a full scale production is still likely to be long.

10% of the Sahara desert is about 500,000 km^2. On that you could maybe make 5 x 10^7 ha x 100 tonnes x 7.3 = 36.5 billion barrels equivalent of oil/year.

But it's going to take at least a a couple of decades to get there and then there is the processing either to biodiesel or via hydrothermal liquefaction, and none of those plants have been built yet.

The plastic is also made from conventional oil and will probably need to be since it is more resistant than the bio-plastics. Pressure on oil will become severe during the next two to three decades.

What I suggest in a later article is that algae production be done as part of smaller communities, as a fuel source but which have turned down their heavy dependence on transportation and fuel.

I have pushed algae and continue to do so, but I am aware of the huge engineering that will be needed to replace conventional oil by it.


Chris Rhodes.

Georg said...

All what you are saing about too much material for Algae production in ponds or tubes may be right as long energy is the only target.
But if you combine several targets e.g. [wastewater treatmentand CO2 solving and energy] it looks very different - to clear the wastewater one must anyway build an equipment - with only little more effort you have more effeicency with the same and an Algae-Production and energy; means wastewater treatment allon brings only costs for the township - in connection with Algae/Energy - if well designed - it brings win instead cost for the town - so you hit two or three bird with one stone
- that will bring really efficiency

Professor Chris Rhodes said...

Hi George,

you make a good point. It is the use of algae to kill several birds with one stone that is likely to bring forward the technology.

The rewards are considerable: making fuel/chemicals in a biorefinery; cleaning CO2 from smokestacks; cleaning wastewater or at least not using-up freshwater etc. and in combination at that.

We need to get building though and I still suspect the led-in time could be long. I think that the use of algae as a fuel/biomass source might work best on the small, local scale. The "village pond" I have called it!



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Plastic Pyrolysis Reactor

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