Specific exergy

Ludovisi A, Poletti A. 2003. Use of thermodynamic indices as ecological indicators of the development state of lake ecosystems 2. Exergy and specific exergy indices. Ecol Model 159 223-238. 

In this book we shall use the symbol epsilon, E and e, in Greek to express total exergy and molar exergy (or specific exergy), respectively. 

The specific enthalpy and the specific exergy are calculated from the following simplified formulas. 

Specific exergy ex based on a unit mass is given by... 

In analyzing the production of materials and energy flows, the values of r can be compared with the exergy of the product. The ratio of the specific exergy of the product ex to is called the cumulative degree of thermodynamic perfection 17 for a certain production network... 

Equality method this method assumes that alt products have the same specific (exergy-based) costs, so the investment (including steam turbine s cost) is distributed equally into the products (steam and mechanical/electric energy). This method is applied here for the evaluation of the base case (actual plant situation). Because the plant chosen for this study nowadays sells some surplus to the grid and some bagasse to other plants, the revenues from these sales are included in the exergy-cost balance. 

Applying the equality method from Thermoeconomic Analysis to the equations (1) and (2), the actual specific (exergy-based) energy costs are evaluated for each of the above mentioned base cases I and II. 

Two hypotheses are assumed in each case the first one assumes specific (exergy-based) cost for boiler-feeding water equal to zero the second one considers this cost equal to process steam cost (exergy-based) [1]. Results are shown in Table 1. Table 1 Energy costs for the chosen sugar/alcohol plant... 

The total chemical exergy of the resomce is finally calculated as boh,res = Abf+ X ch,i+ X rs- In so doing, tables of exergies per unit mass or per mole (specific exergy) can be constracted for each resource. 

The subscript sys distinguishes the total exergy and physical exergy of a system from other exergy quantities, including transfers associated with streams of matter. The total specific exergy on a mass basis esys is... 

Where E, is the exergy destruction due to the system irreversibilities. In the absence of electrical, surface tension effects and the magnitude of the specific exergy in every state are determined from the following equation [12] ... 

Exergy flows are identical to power in the case of the change of potential, kinetic, or electric energies. To evaluate the flow i of a fluid mixture, specific exergies e, in kj/kg for every mass flow w, in kg/s must be calculated. 

The specific exergy of gaseous and liquid streams is calculated on a molar basis taking the molar flow in kmol/s into account ... 

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