Desarrollo de receptores solares volumétricos activos de elevadas prestaciones para centrales termoeléctricas de concentración en torre
Authors:   , Rández Diago Xabier 
Published: May 2023

Electricity production from solar thermal power, also called concentrated solar power (CSP), is a renewable generation technology with great potential, as it directly uses solar radiation that reaches the earth's crust. Approximately this solar power provides a total of 85 Peta Watts (85x1015 W), which can be used by terrestrial solar collectors to generate electricity or energy for high-temperature processes. This amount of power is 5000 times higher than the current global energy consumption, which is more than 15 Tera Watts (15x1012 W). In addition to this, CSP technology is able to manage the energy it generates and discharges to the grid, which is an important advantage over other renewable technologies such as wind and photovoltaics, either through thermal storage or hybridization with photovoltaics. Concentrating solar power plants collect the direct solar radiation (direct normal irradiation or DNI) that reaches the earth's surface and redirect it by concentrating it on a receiving surface through which a fluid circulates, absorbing this energy. They transform the energy from solar radiation first into thermal energy and then into mechanical work and electricity through a power cycle. As the element that transforms radiation into thermal energy, the receiver is one of the crucial elements of the plant to achieve high performance and increase the profitability of this type of technology. Moreover, the designs used today do not differ much from those used in the early days as far as air receivers are concerned. Due to the key importance of this element and the little evolution in its development, this thesis has focused on the design and characterization of an air receiver that provides an evolution in this technology. In this work we have developed the design, analysis and experimentation of an innovative receiver geometry and design with the aim that this technology achieves improvements in the thermal efficiency of the receiver as well as improvements from the point of view of durability that make solar thermal plants more competitive in the current electricity market. Thus, the work of this thesis has been structured in 4 blocks corresponding to chapters 3 to 7. The first work of this thesis, contained in Chapter 3, is focused on the explanation of the concept of rotating disks on which the receiver is based and on the experimentation, by means of a prototype at laboratory scale, of this receiver in a test bench prepared for this thesis. The results obtained from this experimentation have served to validate the results obtained from the CFD model of the receiver developed in Chapter 4. Furthermore, with this experimentation, the operation and feasibility of the concept in a controlled, small-scale environment has also been demonstrated at operating temperatures of almost 700°C. As previously mentioned, a CFD model of the prototype tested on the test bench has been developed in Chapter 4. The objective of this model is to reproduce the flow and radiation conditions provided by the test rig in order to validate the results of the model. Once the model was validated, details of the thermal and fluid-dynamic behaviour of the receiver could be obtained. In addition, the validation of the CFD model allowed during Chapter 6 to reproduce the homogeneous radiation conditions present in a real CSP plant to estimate its thermal performance under nominal working conditions and its effect on a CSP plant. In addition to the thermal performance of the receiver, it is important to ensure its structural integrity under the operating conditions that it could face during its working life. To this end, throughout Chapter 5, the study of the thermo-mechanical behaviour of the receiver from the point of view of fracture mechanics has been developed. For this purpose, silicon carbide, the receiver material, has been characterized to feed a FEM model with which the expansion of the micro-cracks due to the stresses induced by the stress gradients produced by the temperature have been studied. This study has yielded promising results in terms of the lifetime of the receiver discs for all the disc sizes studied, ensuring their durability in plant use and demonstrating the structural feasibility of the concept. After demonstrating the concept thermally (in chapters 3 and 4) and mechanically (in Chapter 5), in order to check if this concept is an improvement over current volumetric air receiver technology, during Chapter 6 the thermal performance of the receiver under plant radiation conditions (homogeneous radiation up to 1000 kW/m2) and its effect on the energy produced and the cost of energy in two tower CSP plant configurations were studied using the previously validated CFD model. The results shown in this chapter demonstrate improvements in receiver thermal performance of more than 0.1, a 10%, at 750°C. Finally, Chapter 7 develops the overall conclusions of the thesis, which are very positive in terms of technology advancement, due to the innovation of the concept, and in terms of thermal performance of this new concept. In addition, the necessary future lines have also been developed until the final development of a receiver based on the concept of disks.