Carnot wrote: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).
There is a discrepancy with that efficiency number
and the number stated in this paper:Algenol aims to produce about 56 000 L of ethanol
per hectare per year using about 430 polyethylene photobioreactors
per hectare, each with about 4500 L of culture
medium containing about 0.5 g/L of cyanobacterial
biomass (10). This production target is within achieved
photosynthetic yields (2-4) and corresponds to 1.8% solar
energy conversion efficiency for average incident sunlight
energy levels in the United States (3).
56,000 L/ha yr equals to about 6000 gallons/acre yr. So rescaling to 8000 gallons/acre yr means
about 2.4% of solar conversion, still far away to the 5.7% you estimated.
By the way, that paper includes the LCA study of the Algenol process (well an old PBR design less productive) and it yields positive net energy.
Carnot wrote:Talk is cheap. Now do the economics and the EROEI and net energy gain. Oh, and by the way the circulation has to be maintained in a raceway ponds to avoid settling. A PBR could be drained down to a circulating tank I guess.
The thing is, it doesn't seem to me (from videos and photos I've seen, and articles read) that the Algenol's PBRs are circulating the growth medium, in fact its PBRs work in batch. That could be quite an advantage to minimize energy and OPEX costs.
But again, the big question mark are the CAPEX costs, will be low enough to make algae fuel to a reasonable cost?