Performance Analysis and Optimization of Receivers for Solar Thermal Power Plants

English

Parabolic Trough Collector (PTC) technology is one of the most mature and widely deployed Concentrating Solar Thermal/Power (CST/CSP) technologies for medium- to high-temperature thermal applications. Improving the thermal efficiency, reliability, and dispatchability of PTC plants requires advancements at both the receiver and system levels, including effective thermal energy storage. This thesis presents a comprehensive numerical investigation into the thermal optimisation of parabolic trough receivers and preliminary modelling of thermochemical energy storage (TCES) systems, addressing key heat transfer mechanisms, heat loss pathways, and system integration challenges in CSP plants. Following a comprehensive literature review on heat transfer enhancement techniques and thermal loss mechanisms in parabolic trough receivers, the first part of the thesis focuses on enhancing the thermal performance of receiver tubes through internal heat transfer augmentation. Longitudinal fins are investigated as a simple and cost-effective modification to the absorber tube. Detailed Computational Fluid Dynamics (CFD) simulations are conducted to evaluate the influence of fin number, height, and circumferential placement under the inherently non-uniform solar heat flux distribution characteristic of PTC receivers. A novel performance evaluation metric is proposed to provide a more representative comparison than conventional methods. The numerical model is validated against experimental data from the literature. Results demonstrate that optimal configurations require shorter fins in high heat flux regions and longer fins in low heat flux regions, achieving up to a 45.6% enhancement in the overall Nusselt number and an 18.0% reduction in heat losses compared to smooth receivers. The second part of the thesis investigates heat loss mechanisms in smooth and longitudinally finned parabolic trough receivers under varying annulus vacuum levels. A coupled heat transfer model incorporating conduction, convection, and radiation is developed to quantify the effects of annulus pressure, ambient conditions, and heat transfer fluid inlet temperature. The results reveal that total heat loss increases significantly with rising annulus pressure and operating temperature, with conductive losses becoming dominant as vacuum degrades, while radiative losses remain relatively constant. The inclusion of longitudinal fins lowers the outer surface temperature of the receiver and reduces total heat loss by approximately 9%, highlighting their added benefit under non-ideal vacuum conditions. The third part examines the thermal behaviour of evacuated linear receivers under realistic operating scenarios, including hydrogen accumulation in the annulus and external wind effects. CFD simulations over a wide range of mass flow rates, inlet temperatures, solar irradiance levels, and wind speeds show that hydrogen presence can increase heat losses by up to three times compared to fully evacuated conditions, resulting in elevated glass envelope temperatures. Increasing the mass flow rate significantly reduces circumferential temperature gradients in the absorber tube, thereby mitigating thermal stresses. Wind effects are found to increase convective heat losses while simultaneously reducing glass temperatures. The final part of the thesis presents additional work on the numerical modelling of CaO/Ca(OH)2-based thermochemical energy storage to complement the receiver-focused investigations and address system-level dispatchability. A high-fidelity CFD–UDF framework is developed in ANSYS Fluent to simulate coupled heat transfer, mass transport, and reaction kinetics in a packed-bed TCES reactor. Overall, the thesis provides an integrated numerical framework for improving receiver performance and advancing thermochemical energy storage modelling, contributing to more efficient, reliable, and dispatchable solar thermal power systems.