Thermoeconomic Analysis of Steam Turbines: An Approach to Marginal Cost Allocation

Abstract

Abstract One of the main problems arising with the thermoeconomic optimization of energy plants, is marginal cost allocation in components containing a flow bifurcation. A typical example is a cogeneration steam turbine. The analysis of the marginal cost allocation of this type of turbine is rigourously studied in this paper. Structural Theory (Valero et al., 1992; 1993) takes under consideration that marginal cost assessment depends on the marginal consumptions of the inlet flow for each outlet energy flow. Supposing that the behaviour of the turbine would not be affected by the quantity of mass flowing in the process, these marginal consumptions could easily be calculated (Royo and Valero, 1995). Nevertheless, the calculation of marginal consumptions under this supposition is only an initial approach to the problem. The accurate values of marginal consumptions (and therefore of marginal cost) may be calculated if one knows the actual behaviour of the steam turbine. This is the process followed in this paper to obtain the values of the marginal consumptions of several cogeneration steam turbines. The analysis offers a complete knowledge of the marginal consumptions for different steam turbine characteristics that work under on-design and off-design conditions. The results obtained are compared with respect to the above initial approach. It is shown that the errors appearing when assessing marginal costs in this way could be relatively important. Nonetheless, a series of correction terms is proposed, by means of which it is possible to considerably decrease these errors.