Davep,
Grame was correct. He is such a genius with his "fill the gap" point.
I have explained the thermodynamic position before but few people read it and even fewer understood it. I even gave references to several papers, one by David Walker, who was one of the pioneers on photosythesis research, who worked with Hill. Sadly his paper, Biofuels: facts, fantasy and feasibility is now behind a paywall, but if anyone wants a copy send me a pm and I will send it to them.
I think we can all agree that there will be rate limiting steps in any process, and that energy cannot be created from nothing.
For photosynthesis the absolute limit will be a function of the incident insolation, which varies throughout the year and latitude.
Photosynthesis generally absorbs radiation in two bandwidths 440-450nm and at 680 nm which drives the z scheme of the Calvin Benson cycle. Theoretically 8 photons of light energy are required to produce one CH2O carbohydrate unit; in reality the number is between 9-10.
Now assuming that the other factors, listed below are within range it is this reaction that sets the theoretical maximum yield of carbohydrate units. The other factors are:
1. Light intensity (high and low)
2. CO2 concentration
3. Temperature
4. NPK levels
5. pH for aquatic systems
This is not the complete list. Assuming all of the above are optimum then the supply of energy is the rate limiting step. Any no-optimisation of the above (unavoidable) will reduce the yield. Selectively breeding and tinkering with the algae species is not going to have any effect on the theoretical yield, though it might (a big might) potential allow some leeway in the parameters above. Playing with the antenna is not likely to achieve much in my opinion. Reducing the antenna size will have an adverse impact in low light conditions, particularly shading which results in aquatic systems. There might be some scope in the chlorophyll absorption range, but I doubt it.
Isgota has provided an example form Algenol. Algenol have made many claims and have been plying their process for years. I doubt if it can be made to work commercially. But let us examine their claim, provided by the Isgota post, of an ethanol yield of 8000 gallons per acre. As ever the devil is in the detail and misleading units are always used to mask the truth. For simplicity I will use the following conversions:
1 acre as 4000 square metres.
Average PAR lower US 100 watts/square metre (8760 hrs/year) = (8760 x 100 x 3600) = 3.153 GJ per square metre
(For simplicity- I have not calculated photon density or number of photons which is more correct)
PAR source:
http://www.atmos.umd.edu/~srb/par/Figure01.htm8000 UG galls ethanol per acre = 2 US galls per square metre
2 US galls ethanol = 6 kg ethanol
1 kg ethanol = 30 MJ HHV
Energy of Ethanol produced = 180 MJ (6 x 30)
Efficiency on ethanol yield = 180/3153 x 100 = 5.7%
Now for a C3 plant, which algae is, to produce a 5.7 % yield of ethanol (+ other products which are required by the algae cell) this would be exceptional
(some might say impossible, which is where I sit).
The believer types on this board would republish the Algenol claims as fact, invest their life savings in Algenol shares, sit back and wait for the world is about to change to biofuels. The slightly sharper types might scratch their heads and start calculating and doubting. The even sharper types will start shouting SCAM and words like it exceeds the the theoretical limit, or why is it not commercially viable then?
The believer types will then come back with evidence from some "Bionews" publication (take your pick) of the impending explosive growth of such a process and shout that the sceptics are wrong, because it is in "Bionews", that they do not know what they are talking about, that they are lackeys of the fossil fuel industry, and even worse, they might even be the sinner of sins, a self confessed petroleum chemist. If you are really lucky you might even be maligned as an engineer with 10 minutes of experience working for a consultant. The believers are even more vociferous if the SCAM artist has won a biofuel award and his company is haemorraging money like there was no tomorrow.
Yes, that is the reality when you objectively reply to some of the believers - just read the posts.
So going back to the start of the post. In my opinion the failure of biofuels to be commercially viable, now and most likely in the future, is down to thermodynamic limitations. None has yet shown a significant net energy gain and an EROEI than would allow them to be competitive against existing fossil fuels, now and in the future. The cost of these biofuel products is linked to the cost of existing fossil energy and will not decouple any time soon. None has shown any evidence that in the absence of fossil energy inputs that they can operate in a stand alone cycle, either singly or in mixed mode. Trying to replicate what nature has done of millions of years processing fossil fuels is simply not possible. That means we will be reliant on fossil fuels until they are depleted and then who knows. Some biofuels might be possible for limited applications but they are not going to power the world as it is today for the masses. Get used to the idea.