Design and dynamic analysis of steam generators of concentrating solar power plants

Abstract

Commercial concentrating solar power (CSP) plants normally use an indirect steam generation system due to the advantages provided by the heat transfer fluid (HTF) which allow the installation of cost-effective storage systems. However, the pinch point temperature difference limitation imposed by the indirect steam generator systems has great influence on the overall plant performance because it sets the mass flow rate of the HTF. As a consequence, a trade-off is obtained between the investment cost of the heat exchangers and the operational pump cost of the heat transfer fluid. CSP plants need to increase the flexibility in order to be competitive in the current electricity markets. The steam generator (SG) has a great influence on the flexibility of CSP plants due to the thermal stresses on thick-walled components which limits the start-up and load changes ramps. Furthermore, the cycling operating conditions of CSP plants leads to fatigue damage which may eventually leads to material failure. For this reason, the dynamic analysis of SG is mandatory to assure its structural integrity. This PhD thesis consist of the design and dynamic analysis of SG for CSP plants. The section of the thesis corresponding to design begins with the presentation a methodology for the design of the SG and oil-to-salt heat exchangers of 50 MWe parabolic trough power plant (PTPP). The heat exchanger design is made following TEMA standards and ASME Pressure Vessel code. The global economic of SG is made using as main variables the evaporator pinch point and the HTF outlet temperature, in order to take into account the total operational HTF pump cost and the investment cost of the SG heat exchangers. The heat exchanger design is made by using genetic algorithms to obtain feasible and optimized results. Two design strategies are compared: the minimization of the total heat transfer area and the minimization of the total annualized cost. The results show that the second approach leads to substantial savings. A recirculation evaporator specially designed for PTPP is modeled and compared with kettle design. On the other hand, a methodology for heat exchanger design of solar power tower plant (SPTP) is also proposed. The special operating conditions with high fluid temperatures and the high heat duty make this issue a non-typical heat exchanger design problem. For this reason, heat transfer and thermal stress requirements are considered in the heat exchanger selection of superheater, reheater, evaporator and preheater. The economic analysis consist of the evaporator pinch point temperature difference optimization taking into account its impact on global plant performance. Two SG configurations are studied: with one or two parallel trains of heat exchangers. The results show an extremely low optimum pinch point values. A preliminary economic study is also made to compared forced and natural circulation evaporator designs The section of the thesis corresponding to dynamic study begins with the dynamic analysis of the proposed design of SG for SPTP. For that propose, a transient models are developed to the single phase heat exchangers and the recirculation evaporator with steam drum. Furthermore, different models are proposed to estimate the transient temperature field and stresses on critical parts of shell-and-tube heat exchangers such: tubesheets ligaments and tubesheet junction. Two SG start-up initial conditions are studied. The first considers a non-isothermal temperature profiles on the heat exchangers at the beginning of the start-up whereas the second considers isothermal initial conditions. For both scenarios are proposed a start-up procedure where are calculated the main allowable temperature fluid ramps to operate the SG on safety-side. Finally, it is presented a dynamic analysis of the SG proposed for PTPP. To estimate the dynamic response of the heat exchangers are proposed the transient models for TEMA F and TEMA H. The thermal stresses are calculated on critical zones such tubesheets, head-nozzle junctions, steam drum-downcomer junction and U-bend regions. In addition, the analytical stress models are validated by means of different finite element models. A SG start-up is performed using a temperature ramps to not overpass the ratcheting and magnetite protection stress limits. Lastly, a study is made to compare the dynamic behavior between kettle and recirculation evaporators.