It is very interesting that the value Wch formally cOincides with the "chemical" exergy (see Chapter 5).
This chapter analyses the photosynthesis thermodynamically. Schneider and Kay (1994) have interpreted the energy balance of an active surface of vegetation: the more complex the structure of a vegetation system, the more effectively it dissipates the gradients and transfers the exergy to an exergy in the form of heat by the temperature of the environment, which seems in contrast to Prigogine's theorem and the interpretation by exergy (J0rgensen, 1997).
First of all, when the total biomass of the system stops increasing, the amount of exergy captured has reached the maximum (< 80% of the solar radiation), but we know that ecosystems can continue the development beyond this limit. Ecosystems have three growth forms: growth by increase of the biomass, growth by increasing the network and growth by increase of the information content. Only the first growth form is associated with more exergy destruction, because more biomass requires more exergy for maintenance, but the exergy destruction per unit of biomass is the same. A more complex network, on the other hand, is able to utilise the exergy of the solar radiation better, and increased information means increased exergy but not increased exergy consumption for maintenance.
The thermodynamic examination of photosynthesis in this chapter has introduced two important efficiency coefficients: the radiation efficiency is defined as the ratio of the amount of energy used for the photosynthesis to the radiation, and the exergy efficiency is the increment of information exergy relative to the total amount of energy coming from the solar radiation. Three teleological hypotheses are formulated as a result of the thermodynamic analysis of the photosynthesis:
(1) Vegetation works as an information machine—the exergy efficiency > the radiation efficiency.
(2) The exergy balance, the energy balance and the increment of information (Kullback's measure of information) have all maxima when the productivity of the vegetation is also at the maximum.
(3) The exergy efficiency coefficient is minimised with respect to the radiation efficiency (approaches the same value of the exergy efficiency as the value of the radiation efficiency) and maximised with respect to increment of information (Kullback's measure of information, K). In other words, the photosynthesis attempts to move the system as far away as possible under the given constraints.
Maintenance of the biological structure requires a major exergy input, including the coverage of evapotranspiration. It implies a high production of entropy, but the entropy is transferred continuously from the ecosystem to the environment, whereby the temperature is maintained within the range suitable for life processes. As discussed in Chapter 3, life not only requires an energy source, but also a transfer of the produced heat (entropy) to the environment. Expressed differently, ecosystems have an effective entropy pump sucking the entire entropy out of the system.
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