Major differences in net photosynthetic assimilation of C02 are apparent be­tween C3, C4, and CAM biomass species. Biomass species that fix C02 by the C4 pathway usually exhibit higher rates of photosynthesis at warm tempera­tures. At cool temperatures, C3 biomass species usually have higher rates of photosynthesis. Plants that grow well in the early spring such as forage grasses and wheat, are all C3 species, whereas many desert plants, tropical species, and species originating in the tropics such as sugarcane are C4 plants. One of the major reasons for the generally lower yields of C3 biomass is its higher rate of photorespiration. If the photorespiration rate could be reduced, the net yield of biomass would increase. Considerable research has been done to achieve this rate reduction by chemical and genetic methods, but only limited yield improvements have been made. Such an achievement if broadly applicable to C3 biomass would be expected to be very beneficial for both the production of foodstuffs and biomass energy. Another advancement that will probably evolve from research concerns increasing the yields of the secondary derivatives such as the liquid terpene hydrocarbons and triglycerides produced by certain biomass species to make direct fuel production from biomass practical and economically competitive. Detailed study and manipulation of the biochemical pathways involved will undoubtedly be neccessary to achieve some of these improvements, particularly since most of the advancements that have been made by controlling growth conditions and trying to select improved strains of biomass have not been very successful in increasing the natural production of liquid fuels.

The C02-fixing pathways used by a specific biomass species will affect the efficiency of photosynthesis, so from a biomass energy standpoint, it is desirable to choose species that exhibit high photosynthesis rates to maximize the yields of biomass in the shortest possible time. Obviously, however, there are numerous factors that affect the efficiency of photosynthesis other than the carbon dioxide-fixing pathway. Insolation; the amounts of available water and macronutrients and micronutrients; the C02 in the surrounding environment; the atmospheric concentration of C02, which is normally about 0.03 mol %; the temperature; and the transmission, reflection, and biochemical energy losses within or near the plant affect the efficiency of photosynthesis. For lower plants such as the green algae, many of these parameters can be controlled, but for conventional biomass growth that is subjected to the natural elements, it is not feasible to control all of them.

The maximum efficiency with which photosynthesis can occur has been estimated by several methods. The upper limit has been projected to range from about 8 to 15%, depending on the assumptions made, that is, the maxi­mum amount of solar energy trapped as chemical energy in the biomass is 8 to 15% of the energy content of the incident solar radiation. It is worthwhile to examine the rationale in support of this efficiency limitation because it will help to point out some aspects of biomass production as they relate to energy applications.

The relationship of the energy and frequency of a photon is given by

e = (he)/A,

where e = energy content of one photon, J; h = Planck’s constant, 6.626 X 10-34 J • s; c = velocity of light, 3.00 X 108 m/s; and A = wavelength of light, nm. Assume that the wavelength of the light absorbed is 575 nm and is equivalent to the light absorbed between the blue (400 nm) and red (700 nm) ends of the visible spectrum. This assumption has been made for green plants by several investigators to calculate the upper limit of photosynthesis efficiency. The energy absorbed in the fixation of 1 mol C02, which requires 8 photons per molecule, is then given by

Energy absorbed = (6.626 X 10_34)(3.00 X 108)(575 X 10~9)-1(8) (6.023 X 1023)

= 1.67 MJ (399 kcal).

Since 0.47 MJ of solar energy is trapped as chemical energy in this process, the maximum efficiency for total white light absorption is 28.1%. Further adjustments are usually made to account for the percentages of photosyntheti­cally active radiation in white light that can actually be absorbed, and respira­tion. The fraction of photosynthetically active radiation in solar radiation that reaches the earth is estimated to be about 43%. The fraction of the incident light absorbed is a function of many factors such as leaf size, canopy shape, and reflectance of the plant; it is estimated to have an upper limit of 80%. This effectively corresponds to the utilization of 8 photons out of every 10 in the active incident radiation. The third factor results from biomass respiration. A portion of the stored energy is used by the plant, the amount of which

depends on the properties of the particular biomass species and the environ­ment. For purposes of calculation, assume that about 25% of the solar energy trapped as chemical energy is used by the plant, thereby resulting in an upper limit for retention of the nonrespired energy of 75%. The upper limit for the efficiency of photosynthetic fixation of biomass can then be estimated to be 7.2% (0.281 X 0.43 X 0.80 X 0.75). For the case where little or no en­ergy is lost by respiration, the upper limit is estimated to be 9.7% (0.281 X 0.43 X 0.80). The low-efficiency limit might correspond to terrestrial biomass, while the higher efficiency limit might be closer to the efficiency of aquatic biomass such as unicellular algae. These figures can be converted to dry biomass yields by assuming that all of the C02 fixed is contained in the biomass as cellulose, -(C6H10O5)x -, from the equation

Y = (CIE)/F,

where Y = yield of dry biomass, t/ha-year; C = constant, 3.1536; I = average insolation, W/m2; E = solar energy capture efficiency, %; and F = energy content of dry biomass, MJ/kg.

Thus, for high-cellulose dry biomass, an average isolation of 184 W/m2 (1404 Btu/ft2-day), which is the average insolation for the continental United States, a solar energy capture efficiency of 7.2%, and a higher heat of combustion of 17.51 MJ/kg for cellulose, the yield of dry biomass is 239 t/ha-year (107 ton/ac-year). The corresponding value for an energy capture efficiency of 9.7% is 321 t/ha-year (143 ton/ac-year). These yields of organic matter can be viewed as an approximation of the theoretical upper limits for land — and water-based biomass. Some estimates of maximum yield reported by others are higher and some are lower than these figures, depending on the values used for I, E, and F, but they serve as a guideline to indicate the highest theoretical yields of a biomass production system. Unfortunately, real biomass yields rarely approach these limits. Sugarcane, for example, which is one of the high-yielding biomass species, typically produces total dry plant matter at yields of about 80 t/ha- year (36 ton/ac-year).

Yield is plotted against solar energy capture efficiency in Fig. 3.5 for insol­ation values of 150 and 250 W/m2 (1142 and 1904 Btu/ft2-day), which span the range commonly encountered in the United States, and for dry biomass energy values of 12 and 19 MJ/kg (5160 and 8170 Btu/lb). The higher the efficiency of photosynthesis, the higher the biomass yield. But it is interesting to note that for a given solar energy capture efficiency and incident solar radiation, the yield is projected to be lower at the higher biomass energy values (curves A and C, curves В and D). From a gross energy production standpoint, this simply means that a higher-energy-content biomass could be harvested at lower yield levels and still compete with higher-yielding but lower-energy — content biomass species. It is also apparent that for a given solar energy capture

efficiency, yields similar to those obtained with higher-energy-content species should be possible with a lower-energy-content species even when it is grown at lower insolation (curves В and C). Finally, at the solar energy capture efficiency usually encountered in the field, the spread in yields is much less than at the higher-energy-capture efficiencies. It is important to emphasize that this interpretation of biomass yield as functions of insolation, energy content, and energy capture efficiency, although based on sound principles, is still a theoretical analysis of living systems that can exhibit unexpected be­havior.

Because of the many uncontrollable factors, such as climatic changes and the fact that the atmosphere only contains 0.03 mol% C02, biomass production outdoors generally corresponds to photosynthesis efficiencies in the 0.1 to 1.0% range. Significant departures from the norm can be obtained, however, with certain plants such as sugarcane, napier grass, algae, maize, and water hyacinth (Tables 3.1 and 3.2). The average insolation values at the locations corresponding to the biomass growth areas listed in Table 3.1 were used to calculate solar energy capture efficiency at the reported annual dry yields. Other than insolation, all environmental factors and the nonfuel components in the biomass were ignored for the solar energy capture efficiency estimates listed in Table 3.1 and it was assumed that all dry matter is organic and has an energy content of 18.6 MJ/dry kg (16 million Btu/dry ton). There is still a reasonably good correlation between dry biomass yield and solar energy capture efficiency in Table 3.1. The estimates of the efficiencies are only approximations and most are probably higher than the actual values. They indicate, however, that C4 biomass species are usually better photosynthesizers than C3 biomass species and that high insolation alone does not necessarily correlate with high biomass yield and solar energy capture efficiency. The biomass production data shown in Table 3.2 are some of the high daily rates of biomass photosynthesis reported for the indicated species. It has been estimated that water hyacinth could be produced at rates up to about 150 t/ha-year (67 ton/ac-year) if the plant were grown in a good climate, the young plants always predomi­nated, and the water surface was always completely covered (Westlake, 1963). Some evidence has been obtained to support these estimates (McGarry, 1971; Yount and Grossman, 1970). Unicellular algae, such as the species Chlorella and Scenedesmus, have been produced by continuous processes in outdoor light at high photosynthesis efficiencies (Burlew, 1953; Enebo, 1969; Kok and Van Oorschot, 1954; Oswald, 1969). Growth rates as high as 1.10 dry t/ha-day have been reported for Chlorella (Retovsky, 1966). In tropical climates, this rate might be sustainable over most of the year, in which case the annual yield might be expected to approach 401 dry t/ha — year. This yield range is beyond the theoretical upper limit estimated here

“Dry matter yield data from Berguson et al. (1990), Bransby and Sladden (1991), Burlew (1953), Cooper (1970), Loomis et al. (1963,1971), Lipinsky (1978), Rodin and Brazilevich (1967), Sachs et al. (1981), Sanderson et al. (1995), Schneider (1973), Westlake (1963).

from the basic chemistry of photosynthesis. It will be shown in later chapters that there are many species of biomass that can be grown at sufficiently high yields in moderate climates to make them promising candidates as biomass energy crops.

As indicated in the discussion of the chemistry of photosynthesis, PS II alone instead of both PS II and I is sufficient for photosynthesis of a green algal mutant under certain conditions (Greenbaum et al, 1995). The investigators suggested that the maximum thermodynamic conversion efficiency of light energy into chemical energy can be potentially doubled because a single photon rather than two is required to span the potential difference between water oxidation/oxygen evolution and proton reduction/hydrogen evolution. Com­parison of the experimental results from the wild strain that contained both PS II and I and the mutant indicated this did not occur because the quantum efficiencies are similar. Also, the phenomenon was not observed under aerobic conditions. But this research still suggests that yields can be improved if the single-photon system can be incorporated into other biomass that grows under atmospheric conditions. For example, such biomass might exhibit higher pro­ductivity not only because of more efficient usage of the solar energy that is absorbed, but also because more C02 could be fixed at lower insolation values due to longer equivalent growth times during the growing season. However, it has been suggested that since both PS II and PS I are required for photosynthe­sis under normal aerobic conditions, the validity of the Z-scheme remains secure (Barber, 1995).