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Third-generation concentrating solar-thermal power (CSP) systems are proposed which aim to halve the cost of electricity from CSP. This work demonstrates that a transition from state-of-the-art cylindrical molten nitrate salt receivers with a peak concentrated solar flux of 1 MW/m2 and a heliostat field optical error of 3 mrad to future cylindrical sodium receivers with a peak flux of 1.8 MW/m2 and a heliostat field optical error of 1 mrad could enable the exergy output of CSP receivers to be increased significantly, by 25% at design point, while keeping the general system design very similar. This comparison is conducted on a large dataset established with a detailed field and receiver energy balance model considering field aiming, peak receiver flux and hydrodynamics. It is based on design point simulations, and is independent from cost considerations. At high temperatures, the optimal configurations achieve their gains only with very accurate heliostats and when the peak flux is increased, while at low temperatures, there is nothing to be gained from very high peak flux limits on external tubular receivers. This underscores the critical importance of high heliostat accuracy for high-temperature CSP systems design. For the third-generation, high-temperature (740 °C) sodium receiver configuration considered, it is found that the upper limit to thermal–optical efficiency of optimised receivers at design-point is around 88 %. • An extensive thermo-optical parametric study of tower CSP systems is performed. • Performance limits, independent of cost considerations, are analysed in detail. • Liquid sodium receivers can enable a 25% exergy gain over molten salt systems. • Performance is strongly affected by field optical error and peak design flux values. • High accuracy heliostats appear critical for the next generation of CSP systems. [ABSTRACT FROM AUTHOR]

• Solar central receiver systems with a polar field and an optional CPC are studied. • Effects of geometrical parameters on energetic and economic performance are explored. • Optimal performance for selected receiver temperatures (600–1800 K) are obtained. • Overall minimum levelized cost of exergy is obtained by a no-CPC system at 900 K. • A CPC benefits energetic and economic performance for receiver temperatures >900 and 1200 K, respectively. Energetic and economic characteristics are studied for solar central receiver systems consisting of a polar heliostat field, a tower, a single-aperture cavity receiver, and an optional compound parabolic concentrator (CPC). System characterization and optimization are performed with a numerical model combining an in-house developed Monte-Carlo ray-tracing optical model, a simplified receiver heat transfer model, and a cost model based on the System Advisor Model (SAM). Based on the model, the effects of receiver temperature on the optical configuration of cost-optimal systems are elucidated, along with the benefits of using a CPC for improved energetic and economic performance. Under the assumptions made in this study, it is found that the overall minimum levelized cost of exergy is obtained by a non-CPC system with a receiver operated at approximately 900 K. A CPC benefits both the energetic and economic performance of systems only at elevated temperatures. The working temperature thresholds at which the energetic and economic performance benefit from the addition of a CPC are identified as 900 K and 1200 K, respectively. The general formulation of the model and broad range of values of the investigated parameters provide a universal predictive capability for studying techno-economic performance of concentrating solar thermal systems. [ABSTRACT FROM AUTHOR]

• Reflection losses from an operational receiver are directly measured using a camera. • Images captured from numerous positions are mapped onto a 3D receiver mesh model. • Directional and spatially resolved losses are obtained, even for a cavity receiver. • The method, applied to a full-scale on-sun test, is validated against ray tracing. A methodology for in-situ measurements of radiative reflection and emission losses from a solar thermal receiver under high-flux irradiation is demonstrated. It combines radiosity analysis with photogrammetry and image recognition techniques to obtain directional and spatial radiosity distributions over receiver surfaces with a simple setup, mainly consisting of a camera. A CCD camera can acquire the radiosity in the visible range, which predominantly captures reflected solar irradiation. A thermal infrared camera can acquire the radiosity in the infrared range, which predominantly captures emission losses from the hot receiver surfaces. A hyperspectral camera can be used to obtain spectrally resolved results across a range of wavelengths. Images are taken from different directions in front of the receiver, and processed in software to obtain a point cloud via three-dimensional reconstruction, allowing the image data to be mapped onto a receiver mesh model. The receiver can be any shape, including those with complex-shaped cavity-like geometries exhibiting surface occlusion and light-trapping effects. These camera-based non-contact measurements allow for the performance of a receiver to be evaluated without interrupting its normal operation. The feasibility of the method is tested by quantifying the reflection losses from a multi-cavity tubular receiver under ~850 kW/m2 concentrated solar irradiation. The proof of concept is established by comparing the measured results with those from Monte-Carlo ray-tracing simulations. [ABSTRACT FROM AUTHOR]

• Several heat transfer fluids were compared in optimised tubular receivers. • Sodium receivers reached highest solar-to-fluid exergy efficiency of 61% (540–740 °C) • Nitrate salt receivers were next best after sodium, then water, air and sCO₂. Central tower concentrating solar power (CSP) systems typically focus solar radiation upon a tubular solar receiver where radiation is absorbed and then transferred, by conduction and convection, into a heat transfer fluid. In this paper, a range of heat transfer fluids are compared, using energy and exergy analysis, and varying the tube diameter, tube wall thickness, and tube-bank flow configuration. The model optimises exergy efficiency including pumping work, assuming uniform flux, and neglecting the effects of thermal stresses, circumferential tube temperature variations and cost. Suitable temperature and pressure conditions are chosen for each fluid, based on a realistic configuration of an applicable thermal energy storage (TES) and power block (PB). The examined heat transfer fluids are molten salt (60% NaNO 3 , 40% KNO 3), liquid sodium, supercritical carbon dioxide (sCO 2), air, and water/steam. Results showed that liquid sodium at an elevated (540–740 °C) temperature range performed best, with a solar-to-fluid exergy efficiency of 61%. At a low temperature range (290–565 °C), sodium was still marginally superior to molten salt, even after allowing for some exergy destruction in a sodium-to-salt heat exchanger. Water/steam also performs relatively well in the receiver, although the difficulties of integrating it with large-scale storage make it a challenging heat transfer fluid for an integrated system. Using sCO 2 as the heat transfer fluid appears infeasible due to excessively-high pressure stresses on the tubes. Air also appears unsuitable for simple tubular receivers, since poor heat internal transfer results in high losses due to much hotter external surfaces. [ABSTRACT FROM AUTHOR]

• Cavity temperature non-uniformity significantly affects convective heat loss. • Three different cylindrical cavities were tested with varying temperature profiles. • A new cavity heat loss correlation includes the temperature non-uniformity effect. • Correlation agrees well with independently reported data and is easy to use. Correlations for natural convection heat loss from solar cavity receivers are widely based on isothermal surface temperature assumptions, which do not occur in practice due to the local heat balance varying with position. An open question thus exists regarding the suitability of such correlations for non-isothermal conditions. This paper addresses this issue by presenting a new Nusselt correlation developed from an experimental investigation of natural convection heat loss from a non-isothermal scale-model cylindrical cavity receiver. Cavities that are considered in this work have length-to-diameter ratios of 1 and 2, are operated at peak temperatures ranging from 355 °C to 650 °C, and exhibit temperature differences along the cavity wall between 40 °C and 342 °C. Stagnation and convection zones, as well as view factor profiles, are observed to contribute to the wall temperature distribution as the cavity is inclined downwards. An energy balance undertaken for steady state provides insight into the effects of non-uniform surface temperature distribution and inclination-dependent surface areas on radiative and convective losses. Natural convection heat loss results from this work are compared with widely-used correlations from the literature that assume isothermal wall conditions, and systematic discrepancies are observed. The proposed Nusselt correlation which accounts for the temperature non-uniformity, cavity inclination and geometric aspect ratio is evaluated against experimental data from this and other studies. It is found to produce excellent predictions of Nusselt numbers for cylindrical cavity receivers in the Grashof number range of 2.6 × 105 to 1.4 × 107. [ABSTRACT FROM AUTHOR]

The receiver in a concentrating solar thermal system is a costly component, and its contribution to system-level techno-economics must be carefully evaluated. This study examined the strategy of adding 'blades' to conventional tubular receivers, in the form of additional tube-banks perpendicular to the primary aperture surface, to improve light trapping and reduce size. An integrated optical–thermal model was used, incorporating geometrical configuration, flow path design and aiming strategy, and optical and thermal performance assessed followed by an economic analysis of its dynamic annual performance. A 65 MW˙th polar heliostat field and molten nitrate salt working fluid were assumed, and design iterations conducted to maximise overall optical–thermal receiver efficiency. The final design achieved a 30% reduction in optical and thermal losses, equivalent to 2.5% increased efficiency. Even though the final design can sustain higher peak flux, its size could not be reduced without incurring excessive spillage loss. Meanwhile, the added blades did not increase the intercepted radiation. As a result, in the techno-economic evaluation, the approximately doubled tube mass increased the receiver cost by 16%–25%, and only 1–3 USD/MWh˙e savings were achieved in the levelised cost of electricity (LCOE). The LCOE of bladed receivers was evaluated for the first time in this study, confirming that bladed receivers in their current form are not promising. However, the study provides insight into the benefits of the related 'STAR' concept, where vertical blades are added to cylindrical receivers, which increase the receiver aperture while decreasing the tube cost per aperture area. • Bladed receivers offer lower losses via increased light trapping between tube banks. • An integrated optical and thermal model found ∼ 30% lower losses could be gained. • Trade-offs with spillage losses limit the size reduction of the receiver. • The higher tube mass in the bladed receiver undermines the techno-economic gains. [ABSTRACT FROM AUTHOR]

This paper presents FluxTracer, an advanced open source computer tool to assist in the analysis, design, and optimization of solar concentrators and receivers. FluxTracer is a postprocessor for Monte Carlo ray tracers used to simulate the optical behavior of solar concentrating systems. By postprocessing the rays generated by the ray tracer, FluxTracer can partition into volumetric pixels (voxels) a region of interest in three-dimensional (3D) space defined by the user and compute for each voxel the radiant power density of the concentrated solar radiation. Depending upon the set of rays provided by the ray tracer, it may be able to integrate the radiant power density in every voxel over time. The radiant energy density analysis described is just one of the analyses that FluxTracer can carry out on the set of rays generated by the ray tracer. This paper presents the main analyses that FluxTracer can provide. It also presents examples of how the information provided by FluxTracer can be used to assist in the analysis, design, and optimization of solar concentrators and receivers. FluxTracer is the first of a series of components of an open-source computational framework for the analysis, design, and optimization of solar concentrators and receiver, being developed by The Cyprus Institute (CyI) and the Australian National University (ANU). [ABSTRACT FROM AUTHOR]

In a concentrated solar power (CSP) tower plant, it is essential to understand the performance of the subsystem formed by the heliostat field and the receiver, operated with an optimal aiming strategy that guarantees the safety and lifetime of the receiver while maximising performance. State-of-the-art studies optimise the heliostat field, aiming strategy and the receiver independently. However, the field and the receiver are interdependent and co-optimisation of the field-receiver subsystem is necessary to obtain the optimal configuration. Fast and accurate annual performance assessments of the subsystem are needed to calculate the annual energy output and the Levelised Cost of Energy (LCOE) of the full CSP system. In this study, a co-optimisation method is proposed based on coupled instantaneous optical, thermal and mechanical models integrated into a system-level model for annual simulation of full system and economics. The resulting system-model is then used for design optimisation based on a genetic algorithm. Several techniques are implemented to make this complex and computationally expensive problem tractable. The proposed method is used to optimise the design of two systems composed of a surround field and a liquid sodium-cooled cylindrical external receiver for first the annual performance and then the LCOE. The good behaviour of the method is confirmed by a sensitivity study. The LCOE-based optimisation leads to a less efficient system than the efficiency-based optimisation but a higher capacity factor. The methods presented are applicable to other CSP plant configurations, including state-of-art molten salt power tower plants. • A new method for co-optimising the heliostat field and the receiver is proposed. • The co-optimisation is implemented based on annual performance. • A receiver performance reduced-order model is proposed for annual simulation. • An aiming strategy is included enabling thermo-mechanical flux limits to be considered. [ABSTRACT FROM AUTHOR]