Questions: Several key questions arise in discussions of extracting energy from biomass.  What fraction of solar energy hitting the Earth is converted to biomass?  That is, with what efficiency does the Earth capture solar energy for plant production?  The plant material can be used either for food or for fuel.  If it is used for food, then an interesting question is the estimated capacity of the Earth for feeding human beings.  If it is used for fuel, then what fraction of our present energy use could be replaced by biomass?  [Ref: Ristinen & Kraushaar, Energy and the Environment, Chapter 5.

Biomass efficiency of the Earth:  We define the biomass efficiency as the ratio:

      e = [biomass energy/m²/day] / [solar energy/m²/day]

We can derive an estimate of the biomass energy/m²/day of 20 grams/m²/day from measurements of dry mass produced in a corn field in mid summer in Indiana.  The solar intensity in Bloomington in summer is 262 watts/m².  In the proper units we find that the

    Biomass Energy/m²/day = [20 gms/m²/day] * [674 Kcal]  =  74.9 Kcal/(m²*day)
                                                                           [180 gms]

Meanwhile, the solar energy/m²/day is given by:

       Solar Energy/m²/day = [262 W/m²] * [86,400 sec] * [1 Kcal]  = 5408 Kcal/(m²*day)
                                                                      [1 day]       [4186 J]

The summer corn-field efficiency is then: e = 74.9/5408 = ~1.5%.  Table 5.3, page 151 in R & K gives a number of solar efficiencies for other crops.  A pine forest, for example, has an efficiency of 0.5%.

Growing Glucose:  The ratio [674 Kcal]/[180 grams] is taken from an equation for the photosynthesis of glucose, C₆H₁₂O₆.

        6CO₂  +  6H₂O  +  Light (674 Kcal/mole)  -->  C₆H₁₂O₆  +  6 O₂.

Glucose is one of the carbohydrates, molecules of the general form: C_x(H₂O)_y.  Its molecular weight is ~180 grams.  Since 180 grams is a mole of glucose, 674 Kcal of energy is required for each 180 grams of produced glucose.

World Support Capability:  The total area of the Earth is A = 32 x 10⁹ acres, about 1/4 of which (8 x 10⁹ acres) is arable and 1/4 fit for grazing of animals.  At present, approximately 4 x 10⁹ acres are under cultivation and 5 x 10⁹ acres being used for grazing.  That is, we are using approximately half of the world's total usable area (the better half, no doubt).  Multiplying our 6 billion people by 2, the Earth could support roughly 12 billion people if all land were forced into service to grow food.  This assumes that the presently idle land produces food at the same level as the presently cultivated land.  At present, we support about 1.4 people/acre.