**4.1 Direct external heat recovery**

Curves HI and JK represent the performance obtained for external heat recovery through a bottoming steam cycle (combined cycle). In particular, the curve HI refers to the case of a combined cycle with one pressure level at the HRSG, whereas curve JK to a combined cycle with three pressure levels and reheat. Points H and J correspond to the case of combined cycle without supplementary firing, the primary energy supplied from outside (*χ* = 0) remaining the same. Points I and K correspond instead to the case of supplementary firing using exhaust gases exiting the turbine and setting the limit temperature at the duct burner exit to 800 ° C.

Without supplementary firing, the combined cycle with three pressure levels HRSG and reheat (point J) is the leading solution in power generation, providing efficiencies of 54.6%- 56.8% in the explored range of *β* and *TIT*, higher by 2-3 percentage points than those obtained with one pressure level HRSG (point H). With supplementary firing, the further power increase is obtained at the expense of efficiency (I and K). Only in the case of one pressure level HRSG, with modest additional combustion and low values of *TIT*, a slight improvement in performance is achieved, as a result of the greater recovery feasible at low temperatures.
