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 ºC and 3 ºC 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.

Transient thermo-mechanical analysis of steam generators for solar tower plants

“…Additionally, the SG includes, among others, feedwater pumps, hot salt pumps, the steam drum, supporting systems, etc. The SG train designed by González-Gómez et al [27] for nominal conditions is chosen for the dynamic analysis. The subsystems of the SPTP are represented in Figure 1 with the main design parameters used in the start-up procedure.…”

Dynamic performance and stress analysis of the steam generator of parabolic trough solar power plants

“…Normally the SG of CSP plants consists of shell-and-tube heat exchangers as was shown in Refs. [12,13]. Extensively used methods for the heat exchanger design are TEMA standards [14] and ASME Section VIII Div 1 [15], which consider continuous operation and do not consider fatigue due to cyclic operation [16].…”

“…In addition, these standards do not address specific guidelines for the stress estimation of typical loads of a SG, e.g. thermal loads on U-tube tubesheets [13] and temperature gradients on the tubesheet junctions [17]. In such cases it is necessary to carry out a finite element analysis in order to assure an appropriate SG design.…”

Dynamic simulation of steam generation system in solar tower power plant

“…To better understand the dynamic performances of the steam generator in PT, which act as a peaking plant in the multi-energy integration system to carry out peak-valley regulation, will be especially significant. It is noticed that during the starting, shutting down and varying working conditions of PT, the steam generator will suffer strong thermal stress [17][18][19][20]. In order to make continuous improvements in PT safety, reliability, and efficiency, it is necessary to analyze the limitation of the rate of temperature variation.…”

“…Since the variation of the solar irradiance is large, the variation of the molten salt mass flow rate is large as well. Therefore, the dynamic characteristic of the heat transfer feedback is obvious, such as response time and delay[18]. A detailed study for the purpose noted above (case E) was performed.…”