In solar power tower plants, fast start-ups and/or load changes are mandatory to increase the power plant dispatchability. The high temperatures of the working fluids and the partial-load operation will reduce the lifetime of the thick-walled components at the steam generator.Therefore, a proper heat exchanger design should consider the stress evolution during the transient operation of the plant. This work addresses, for the first time, a methodology to determine the dynamic behavior of all heat exchangers of a steam generator train. The methodology proposed here is a powerful tool for the design of solar power plants. The stress analysis model identifies the most important components of the steam generation train during transient operation. The methodology consists of the combination of analytical models to obtain the coupled response of the steam generation train from the following dynamic variables: temperature, pressure and stress.An example of this methodology is presented for two start-up initial conditions: the assumption of non-isothermal and isothermal temperature profiles of the heat exchangers. A steam generator train based on conventional shell and tube heat exchangers is analyzed. The analysis shows that the non-isothermal condition takes approximately 50 min to reach nominal conditions, whereas the isothermal condition takes approximately 110 min, requiring 600 tons and 716 tons of hot salt to perform the start-up procedure, respectively.

This paper addresses two important concerns of the design of steam generators of parabolic trough power plants: cost minimization and component reliability. A thorough economic analysis of the heat exchangers of the steam generator and oil-to-salt heat exchangers of a 50 MWe parabolic trough power plant is presented. The heat exchanger design is realized following TEMA standards and optimized using a genetic algorithm. Two design strategies are compared: the minimization of the total heat transfer area and the minimization of the total annualized cost. It is seen that the second approach provides substantial savings over the lifetime of the plant.The economic analysis reveals a global optimum with an outlet temperature of the heat transfer fluid of 293 ºC and an evaporator pinch point of 4.85 ºC. The best design of the steam generator consists of a TEMA-H shell superheater and preheater and a TEMA-F shell reheater. The best design of the oil-to-salt heat exchangers includes six TEMA-F shell heat exchangers in series, with a log mean temperature difference of 7ºC and the molten salt on the shell-side. Lastly, a TEMA-X recirculation evaporator is proposed with a considerably reduced wall thickness when compared to a kettle evaporator.

Two-phase heat transfer model of a beam-down gas-solid fluidized bed solar particle receiver.

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.

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Solar power tower plants differ from conventional power plants in the steam generator design due to the higher heat duty. In this work, the influence of the steam generator heat exchangers (preheater, evaporator, superheater and reheater) on a solar power plant with molten salt receiver and thermal storage is studied for the first time. Energy, exergy and exergoeconomic analyses give a complete view of the cost flows within the system. The pinch point temperature difference in the evaporator is used as the main variable as it changes the steam generator design and the operating conditions of the plant, such as the inlet temperature of the receiver and the salt mass flow. All heat exchangers are designed and optimized at minimum cost for each pinch point to fulfill the thermomechanical limitations of TEMA standards and Pressure Vessel code. The field of heliostats, molten-salt receiver and the power-block (110 MWe) designs are kept constant throughout the paper. A low pinch point should be used to minimize the plant exergy destruction while the exergoeconomic approach obtains an optimum pinch point around 2-3ºC. Furthermore, the low exergoeconomic factor values show that the heat exchangers of the SG are crucial for the plant operation.

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