Mass and Energy Flows Modelling: Part II – Exergy-Based Assessment of Forest Efficiency

Exergy represents a measure of the quality of energy; it denotes the thermodynamic distance of a system from equilibrium with its surrounding environment and serves as a quantitative and qualitative measure of the energy (primarily free energy in the context of ecosystems) embedded within a system. The initial application and development of the exergy concept in ecology are linked to the idea of structural exergy, or eco-exergy, determined by the relationship between the total exergy and the total biomass of the analyzed ecological system. The use of eco-exergy allows for measuring several aspects of an ecosystem, including the distance from thermodynamic equilibrium, structure and functions, survival capacity, energy use efficiency by the ecosystem's organisms, and the ability to regulate interactions among organisms or groups of them. However, eco-exergy diverges from the classical meaning of exergy and overlooks a fundamental aspect of systems: it is an indicator capable of reflecting pure thermodynamic measures of work and energy. Reflecting on this last point, the present work was conceived to define the exergy of a Mediterranean forest system in the context of climate change. CO2 and H2O fluxes of the forest system, along with local climatic data, were used to apply a simplified exergy model that assumes the entire canopy can be considered as a single large leaf (Big Leaf approach). The exergy analysis shows that the majority of incoming energy (Exin), which is then available or usable to perform useful work for the system, primarily comes from the energy produced by solar radiation absorption (approximately 95%), followed by the energy from liquid water in the leaf (4.45%), and finally, to a negligible extent, the energy from CO2 assimilated through the leaf (about 2.83*10-7%). The energy used to perform work (Exout) is utilized by the processes of sugar (Exsu) and water (ExwL) formation to produce biomass (Exsu + ExwL) (8.10%), for processes of water diffusion in the absence of active transpiration (Exvap) (4.44%), and to a negligible extent, for processes of heat convection (Exqk) (0.001%) and energy exchange (ExL) (1.21 *10-6%) between the leaf and the atmosphere. About 88% of the outgoing exergy is identified as exergy lost due to spontaneous irreversible processes. The efficiency exergy/energy ratio of the forest system quantified for the study year (2022) is 1.62. The use of thermodynamic measures in ecological systems allows for a better understanding of the energetic dynamics within the system. When adequately developed, these analyses can be integrated with classical ones and further support the development of sustainable management practices and policies.