Definition Eco Exergy

In ecology, technological exergy is not so useful because the reference state, the environment, would be the adjacent ecosystem and we would like to find an expression that can measure how developed an ecosystem is, that is, how far it is from thermodynamic equilibrium. For a reservoir or reference state, it is therefore advantageous in ecology to select the same system but at thermodynamic equilibrium, that is, that all components are inorganic and at the highest oxidation state, if sufficient oxygen is present (nitrogen as nitrate, sulfur as sulfate, etc.). The reference state will in this case correspond to the ecosystem without life forms and with all chemical energy utilized or as an 'inorganic soup'. Usually, it implies that we also consider T = To, and p=po, which means that the exergy becomes equal to the difference of Gibb's free energy of the system and the same system at thermodynamic equilibrium, or the chemical energy content included the thermodynamic information (see below) of the system. Gibb's free energy is defined according to the following equation:

where dV is the change in volume and dS is the change in entropy. T and p are the temperature and pressure, respectively. The exergy becomes by this definition clearly a measure of how far the ecosystem is from thermo-dynamic equilibrium, that is, how much (complex) organization the ecosystem has build up in the form of organisms, complex biochemical compounds, and complex ecological network. Here, we use the available work, that is, the exergy, as a measure of the distance from thermodynamic equilibrium.

This description of exergy development in an ecosystem makes it pertinent to assess the exergy of ecosystems. It is not possible to measure exergy directly - but it is possible to compute it by eqn [4]. Figure 2 illustrates the definition of 'eco-exergy'. As the chemical energy embodied in the organic components and the biological structure contributes far most to the exergy content of the system, there seem to be no reason to assume a (minor) temperature and pressure difference between the system and the reference environment. Under these circumstances we can calculate the exergy content of the system as coming entirely from the chemical energy:

This represent the nonflow chemical eco-exergy. The difference in chemical potential (pc - between the ecosystem and the same system at thermodynamic equilibrium determines the eco-exergy. This difference is determined by the concentrations of the considered components in the system and in the reference state (thermodynamic equilibrium), as it is the case for all chemical processes.

Displacement work, not useful

Displacement work, not useful

Work (exergy)

Figure 2 The exergy content of the system is calculated in the text for the system relatively to a reference environment of the same system at the same temperature and pressure, as an inorganic soup with no life, biological structure, information, or organic molecules.

Work (exergy)

Figure 2 The exergy content of the system is calculated in the text for the system relatively to a reference environment of the same system at the same temperature and pressure, as an inorganic soup with no life, biological structure, information, or organic molecules.

We can measure the concentrations in the ecosystem, but the concentrations in the reference state (thermodynamic equilibrium) can only be based on the usual use of chemical equilibrium constants. If we have the process

Component A $ inorganic decomposition products It has a chemical equilibrium constant, K K — [inorganic decomposition products]/[Component A] [6]

The concentration of component A at thermodynamic equilibrium is difficult to find, but we can find the concentration of component A at thermodynamic equilibrium from the probability of forming A from the inorganic components.

We find by these calculations the exergy of the system compared with the same system at the same temperature and pressure but in form of an inorganic soup without any life, biological structure, information, or organic molecules. As (pc - pcJ can be found from the definition of the chemical potential replacing activities by concentrations, we get the following expressions for eco-exergy:

where R is the gas constant (8.314J K-1 molT — 0.08207l atm. K-1 mol-1), T is the temperature of the environment (and the system; see Figure 2), while Cj is the concentration of the ith component expressed in a suitable unit, for example, for phytoplankton in a lake Ci could be expressed as mgl- or as mgl- of a focal nutrient. ci,o is the concentration of the ith component at thermodynamic equilibrium and n is the number of components. ci,o is of course a very small concentration (except for i — 0, which is considered to cover the inorganic compounds), corresponding to a very low probability of forming complex organic compounds spontaneously in an inorganic soup at thermodynamic equilibrium. ci,o is even lower for the various organisms because the probability of forming the organisms is very low with their embodied information, here represented by the genetic code.

By using this particular exergy based on the same system at the thermodynamic, chemical equilibrium as reference, the eco-exergy depends only on the chemical potential of the numerous biochemical components that are characteristic for life. It is consistent with Boltzmann's statement that life is a struggle for free energy. Eco-exergy has a definition close to the free energy, but unlike free energy, eco-exergy is not a state variable. It will depend on the reference state that will vary from ecosystem to ecosystem. Furthermore, it is difficult to apply the classic state variables in thermodynamics far from thermodynamic chemical equilibrium. Classic thermodynamics presumes that the system is close to equilibrium, which makes it possible to show that for instance free energy is a state variable that gives the same result independent on the pathway. We want to use eco-exergy far from thermodynamic equilibrium and can therefore not use free energy in this context.

As we know that ecosystems due to the throughflow of energy have the tendency to move away from thermodynamic equilibrium losing entropy or gaining exergy and information, we can at this stage formulate the following proposition of relevance for ecosystems: 'ecosystems attempt to develop toward a higher level of exergy'.

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment