Tuesday, March 13, 2012

Steven Chu to Scientific American on biofuels on 7 March 2012

In the interview, Energy Secretary Chu noted:

Another company is using photosynthetic bacteria and swapping whole genomes and metabolic pathways. It generates long alkane chains that are the immediate precursors to diesel fuel. It's 5 to 10 percent energy efficient where a typical plant is only 1 percent efficient. Eighty percent of the metabolism of this poor cyanobacteria has to be generating fuel. It's already worked. Now we have to get the metabolism up further and many synthetic biologists feel they can do that. They know how to ratchet down metabolic energy used in things like reproduction. They want to just keep those systems alive that do self-repair and make the fuel. This is a little weird bacteria or yeast. In the last 15 years or so, I've gotten into biology like this. I follow this with avid interest. It's really almost science fiction.

Chu also noted: The sweet spot at DoE has been on the front side, research and development

Of Joule Unlimited, see post Is Joule's Renewable "Liquid Energy" Far More Efficiently Produced Than Biofuel?

The paper by Joule Unlimited A new dawn for industrial photosynthesis provides information on photosynthetically active solar radiation hitting certain spots in the United States:

Table 2 Average annual total and photosynthetically active (PAR) ground horizontal radiation (PAR) at various US sites

Location Historical average total ground radiation MJ/m2/year Historical average PAR MJ/m2/year

El Paso, TX 7460 3460

Phoenix, AZ 7300 3400

Albuquerque, NM 6990 3240

Efficiency is based on enthalpy of combustion of produced product divided by energy of incoming solar radiation.
Within the Joule article: For example, the direct process, operating at the calculated 7.2% efficiency would yield 350 bble/acre/year. This equates to 15,000 gal alkane/acre/year where a C17 alkane has a heating value of 47.2 MJ/kg and density of 777 kg/m3.

Work through an example for C17 in Phoenix.

(enthalpy of combustion C17 in MJ/kg X kg-C17)/(3400 MJ/m2/year) = 0.072

C17, in kg per m2-year = 0.072 X 3400/47.2 =5.19 OR
C17, in m3 C17 per m2-year = 5.19 kg/(m2-year) /777 (kg/m3) = 0.00667 m3 C17 per (m2-year)
which is 0.00667 m3/(m2-year) X (1 m2/0.000247 acre) X (1 gallon/0.003785 m3)

or 7145 gallons C17/(acre-year), if the efficiency is based on 7.2% of PAR radiation.

[If based on TOTAL incoming radiation, with an efficiency of 7.2%, the areal yield of C17 would be higher. On the topic of efficiency, see Robert E. Blankenship, et al., Comparing Photosynthetic and Photovoltaic Efficiencies and Recognizing the Potential for Improvement, Science Vol. 332 no. 6031 pp. 805-809 (May 13, 2011).]

***As to enthalpy of combustion of n-C17, from Heptadecane
-11351.2 ± 2.2 kJ/mole
One mole weighs 240.4677 grams

Thus, 11351 kJ/ 240.4677g = 47.20 kJ/gram or 47.20 MJ/kg.


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