On the thermal and mechanical response of sodium-cooled solar receivers

The growing impetus for clean energy contributions has inspired a resurgence in Concentrated Solar Power(CSP)interest. This has lead to significant technological advances, with almost 5.8GWe of installed capacity worldwide, and just under 4GWe in development. However, commercial CSP remains economically uncompetitive with established generation technologies. Consequently, it is necessary to reduce costs and maximise component performances in order to encourage large-scale deployment. For CSP tower systems, maximising receiver outlet temperatures and heat flux concentrations is key to enhancing competitiveness. Conventional working fluids such as water/steam and molten salt limit these values to < 600◦C and < 0.9MW/m2 respectively. The unique heat transfer properties of sodium offer an interesting alternative by facilitating outlet temperatures and incident heat fluxes that surpass these limits – thus rendering it an attractive working media in tubular concepts. This thesis aims to further explore the potential of sodium-cooled receivers using comprehensive modelling techniques, considering influences of design and operating conditions. Thermal and mechanical models are developed in order to study the response of sodium receiver concepts, whilst an optical model involves heliostat field effects. The thermal model is integral to the work, yielding receiver performance calculations, and providing a temperature profile for mechanical reliability studies. This model is semi-empirical, provides a high degree of accuracy and resolution, and is compatible with a range of design configurations and boundary conditions. Mechanical models identify component limits through extreme thermal loading scenarios that threaten damage via creep-fatigue, with two reliability models utilised. Finally, an optical model and aiming strategy generates realistic heat flux boundary conditions for the thermal model. Aseriesofdetailedinvestigationsrevealsomefundamentalcharacteristicsofsodiumasareceiver coolant, including: large thermal efficiencies (> 90%), permission of substantial incident heat fluxes (> 1MW/m2), and stable tube-side heat transfer performance (104 −105 W/m2K) across a range of operating conditions. As a liquid metal, sodium heat transfer exhibits a relative insensitivity to flow conditions in the operating range of solar receivers (104 ≤ Re ≤ 105), with large molecular conductivities (101 W/mK) dominating thermal energy transport well into the turbulent regime. This promotes excellent flexibility in design and operation, with consistently high efficiencies observed across large design spaces, and a relative de-coupling of mechanical reliability from internal flow velocities. In a comparative analysis, sodium yields a greater thermal power output than single-phase rivals molten salt and lead-bismuth, due to larger internal heat transfer coefficients and a moderate pressure drop. However, it falters as a direct storage media in comparison to molten salt due to high per-unit costs and a low volumetric heat capacity, with economic feasibility restricted to short term storage (< 3 hours). The involvement of mechanical and optical models is essential for exhaustive appraisals of sodium receiver performance, enabling practical considerations such as component reliability and heliostat field interactions. Thermal stresses increase with tube diameter and heat flux, diminishing reliability according to material creep-fatigue resistance at temperature. A key feature of the optical model is the necessary concession of incident power (spillage) to the aiming strategy in order to affect a safe heat flux condition for sustained operation(30+years), with absorber area critical for interception of concentrated sunlight. A parametric design study reconciles declining thermal efficiency with rising incident power levels to optimise absorber area, with small tube diameters identified as key towards maximising efficiency and reliability. A high temperature design investigation pursues improved solar-to-electric efficiencies via enlarged sodium temperatures and novel receiver concepts. Thermal efficiency and permissible heat flux levels decline with rising temperatures, however these losses may be sacrificed at the receiver for improved thermodynamic efficiency. Plant power output is enhanced by over 7% using Ni-based superalloys operating at 650−700◦C relative to more traditional heat exchanger materials at 550◦C. An integrated optical-thermal-mechanical model simulates receiver field operation through diurnal cycles,with significant variations in daily power output demonstrated for material and temperature combinations