> I was recently presented with a supposed "scientific paper" that was
> meant to prove that algae based biofuels were unscientific hokum and
> snake oil. I was, to say the least, rather upset when I read this paper.
> I am not a scientist and all the stuff about PAR's and joules and the
> like was really confusing. But I think I managed to deal with all of
> that and still find the false framing.
>
>
http://www.nanostring.net/Algae/CaseStudy.pdf
>
> In the "study" we find the following pronouncement:
>
> E is the Energy in the biomass
> PAR is the energy from the sun
> Q is the efficiencey of the conversion of sun energy to biomass energy
>
> W = PAR * Q
>
> All of this is well and good and reasonable. And then we have:
>
> Photosynthetic organisms use 8 photons to capture one molecule of CO2
> carbohydrate (CH2O). Given that one mole of CH2O has a heating value of
> 486kJ (486 kilo jules) and that the mean energy of a mole of PAR photons
> is 217.4kJ then the maximum theoretical conversion efficiency of PAR
> energy into CH2O is:
>
> 468kJ/(8 * 217.4kJ) = 27%%
I'm mildly surprised it's that high. Conventional
biodiesel has the pitifully tiny efficiency of about 3-6%%
-- which is logical given a delta of about 10 degrees at
300 degrees Kelvin.
http://en.wikipedia.org/wiki/Biodiesel#Efficiency_and_economic_arguments
Carnot predicts (10)/(300) = 3.33%% in that case.
I have no idea what the actual operating temperatures of
a leaf or bit of algae are; these are admittedly total guesses,
though 300 K = 27 C is a nice summer day.
This is not to say algae-based photosynthesis can't be
more efficient than a thermal heat engine, of course;
if nothing else, one can cool the incoming water (somehow)
and increase insolation, if the algae can handle it.
http://en.wikipedia.org/wiki/Biodiesel#Yield
suggests an estimated 1800 gallons per acre for algae
(one assumes per year but the article doesn't state),
which compares very favorably to palm oil (508 g/a) or
soybeans (59.2-98.6 g/a). Since a gallon of diesel oil
is about 120 MJ and an acre 4046.85642 m^2, that translates
into (very roughly) 100 GJ/day if one assumes 8 hours/day of
usable sunlight, or 36 TJ/year -- for 21.6 GJ worth of oil.
Granted, these figures are extremely rough, and one has
to further factor in insolation angle (latitude + season)
and weather estimations, but even assuming 15000 g/a,
one still only has an efficiency of about 7.5%%.
No doubt someone out there has better figures.
Contrast this to a good thermosolar plant, which could
probably generate 35%% efficiency easily by just heating the
oil to 600 degrees Kelvin (327 Celsius, 620 Fahrenheit) and
rejecting boiling water (373 K): (600-373)/600 = 37.8%%.
Even cheap photovoltaics beat biodiesel.
Admittedly, Google is being reticent again regarding thermosolar
efficiencies, and of course there are the usual battery weight
issues were one to charge electric cars therefrom. I can tell
you that a 50 MW plant in Aragon, Spain will cost 200 million euros,
or about $293M, to build.
http://www.reuters.com/article/rbssIndustryMaterialsUtilitiesNews/idUSL183635172...
