Thermo-economic optimization of molten salt steam generators

Abstract This paper presents a methodology to guide the design of heat exchangers for a steam generator in a solar power tower plant. The low terminal temperature difference, the high fluid temperatures and the high heat duty, compared to other typical shell and tube heat exchanger applications, made the design of the steam generator for molten-salt solar power towers a challenge from the thermomechanical point of view. Both the heat transfer and the thermal stress problems are considered to size the preheater, evaporator, superheater and reheater according to the TEMA standards and ASME Pressure Vessel code. An integral cost analysis on the steam generator design effects on the power plant performance reveals an extremely low value for the optimum evaporator pinch point temperature difference. Furthermore, an optimization using genetic algorithms is performed for each heat exchanger, which leads to economical and feasible designs. A 110 MWe solar power tower plant is studied. Two configurations of the steam generator are proposed: with one or two trains of heat exchangers. The results show that the optimum pinch point temperature differences are very close to 2.6 °C and 3 °C for the steam generator with one and two trains, respectively. The proposed design of the steam generator consists of a U-shell type for superheater and reheater, a TEMA E shell forced circulation evaporator and a TEMA-F shell preheater. Also, the approach point temperature difference analysis is performed to avoid subcooled flow boiling in the preheater. An economic study to compare forced and natural circulation evaporator designs is carried out.

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