Exergy Destruction as an Ecological Indicator

From eqns [1] and [3] we see that the exergy destruction in a process can be expressed as a function of the initial and final thermodynamic states of all participating media. Could E\ be used as an ecological indicator? It clearly possesses all of the required properties defined in the

Table 2 Calculated process parameters for the cogeneration plant of Figure 3

^1,plant

0.658

^2,plant

0.348

Table 1 Design data for the cogeneration plant of Figure 3

Tenv = 300 K

Penv = 0.1 MPa

Ap2-4 = 0.02 p2

Vc = 0.85

T1 = 300 K

p1 = 0.1 MPa

Ap5-9 = 0.08 p9

Vt = 0.9

T4 = 1300 K

p2 = 0.8 MPa

Ap11-12 = 0.1 MPa

rçel = 0.98

T9 = 353 K

p9 = 0.1 MPa

^mec = 0.95

T10 = 673 K

p10 = 0.1 MPa

cp = 1.1 J kg-1 K-1

■ncc = 0.98

p11 = 1 MPa

Rair = 287 J kg-1 K-1

■qP = 0.8

K = 1.4

^hrb = 0.9

Figure 4 A system that exchanges only thermal energy with its environment.

Figure 6 A system that exchanges only chemical species with its environment.

Figure 4 A system that exchanges only thermal energy with its environment.

Figure 6 A system that exchanges only chemical species with its environment.

Mechanical Dissipation (Lost Work)

Consider now a different situation, in which the system S exchanges only mechanical energy with the environment O (Figure 5). The power Pin enters the system (e.g., hydraulic or wind power), and the power Pout leaves the system (e.g., mechanical or electrical power). The system's internal conversion efficiency is known and equal to "qs. The respective specific exergies are now equal to the respective specific works, and the frictional heat ^f (proportional to 1 — ^S) is dissipated into the environment at a temperature so close to TO to be indistinguishable from it. For the exergy destruction we obtain in this case:

With "qS = 0.9, eqn [13] tells us that, for every kW of mechanical input, 0.1 kW (10%) are destroyed in this mechanical transformation chain.

We conclude that E\ is a correct indicator of the adverse ecological impact of mechanical dissipation.

Chemical 'Dissipation' (Lost Work)

Consider now a situation in which the system S exchanges only chemical species with the environment O (Figure 6).

Figure 5 A system with only work exchanges with its environment.

The two pure substances A and B enter the system (e.g., carbon C and oxygen O2), and the single product C leaves the system (CO2 in this case). The respective exergies are a function of both the enthalpy of formation of the reactants and of the product and of their respective temperatures. But, in this case, they also depend on the concentration of A, B, and C in the environment. Let us assume that the reaction A + B ) C is exothermic, that is, that it generates a heat flux Qr at a temperature Tr higher than T0. The exergy destruction is in this case e\ = (gC - gA - g

Figure 5 A system with only work exchanges with its environment.

With T0 = 293 K, Tr = 1000 K, and (cA*cB)/cC = 1, eqn [6] tells us that, for every kW of chemical input, 0.293 kW (or 29.3%) are destroyed in this chemical reaction. Equation [6], however, does not convey any information on another important property ofthe product C of reaction: whether C is toxic or not, and if it is, does not provide a measure of its toxicity level either. We are faced here with an extremely clear and profound limitation of our so-called 'environmental approach': from all point of views, except from the anthropocentric one, Ex is a perfectly acceptable EI, because it correctly quantifies the effective 'impact' on the environment of any natural or man-made process. Since though our real interest is of a more species-egoistic type, we cannot accept 'impacts' like sweetwater eutrophication or global warming, to which the biosphere would perfectly adjust, because these phenomena are certain to have a negative impact on the survival of our species. The choice is one of value, of course, and therefore cannot be judged on a thermo-dynamic basis: but the fact that a thermodynamic indicator like Ex that correctly measures irreversibility is not suitable as an EI because of our own anthropo-centric attitude is perhaps worth some reflection.

However, in line with the presently dominating perception of 'environmental impact', we must conclude that Ex is only an 'incomplete indicator' of the adverse ecological impact of chemical reactions.

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