*2.4. Exergy Profit and Exergy Footprint*

The exergy assets and liabilities for a stream are both calculated using the equations in Section 2.3. They assign exergy extraction and utilisation potentials to the assets, and the exergy demands to the liabilities. Establishing the balance of the total exergy assets (EXasset) (Equation (2)) and the total exergy liabilities (EXliability) (Equation (3)) produces the exergy profit (EXprofit) of the process system:

$$\rm E\mathcal{X}\_{\rm profit} = \rm E\mathcal{X}\_{\rm assset} - \rm E\mathcal{X}\_{\rm liabbility} \tag{8}$$

The opposite difference (the negation) of the exergy profit is termed the exergy footprint (EXfootprint):

$$\rm E\chi\_{\rm footprint} = -E\chi\_{\rm profit} = E\chi\_{\rm liability} - E\chi\_{\rm asset} \tag{9}$$

In this way, a positive value for the footprint means an adverse impact on the environment by imposing the equivalent demand to be supplied from outside sources. With this criterion, the sustainability contribution of the evaluated process system can be clearly measured. A higher exergy profit, meaning a lower exergy footprint, also translates to a better sustainability contribution of the system.

All exergy components can be used in the general case. However, in the current study, only the thermo-mechanical and the chemical components are evaluated, since they are the most typical for chemical and waste processing.
