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In a solar power tower plant, the role of the heliostat aiming strategy is to control the radiative flux distribution at the receiver surface to avoid thermally induced damage, while minimising spillage losses and maximising the receiver thermal efficiency. Flux limitations arise from factors including the heat transfer fluid stability limits, and thermo-mechanical stress limits in receiver pipes. Maximised flux, as close as possible to local flux limits, facilitates an overall smaller receiver with lower thermal losses. Methods exist to sequentially optimise aiming points of single heliostats, using fast convolution-based optical simulations to evaluate individual flux maps. However, to accurately determine receiver flux distributions, ray-tracing is preferred. Ray-tracing is computationally expensive and determination of the aim-points for every heliostat independently potentially leads to impractical simulation times. In this study, a new parameterisation of heliostat aim-point locations is introduced that significantly simplifies the aiming problem. The new aiming model, named Modified Deviation-Based Aiming (MDBA), enables efficient use of ray-tracing to optimise the aiming strategy and, together with receiver thermal and mechanical models, is able to closely match the flux distribution to local values of allowable flux on the receiver. This new parameterisation enables accurate aiming strategy interpolation, used to dynamically predict full field aim-points at different sun positions and values of direct normal irradiance (DNI). A reference case with a surround field and a cylindrical external receiver compatible with the Gen3 Liquid Pathway project is presented to test the capability of the method developed in this study.

Artificial Intelligence (AI) is increasingly playing a significant role in the design and optimization of renewable energy systems. Many AI approaches and technologies are already widely deployed in the energy sector in applications such as generation forecasting, energy efficiency monitoring, energy storage, and overall design of energy systems. This paper provides a review of the applications of key AI techniques on the analysis, design, optimization, control, operation, and maintenance of Solar Tower systems, one of the most important types of Concentrating Solar Thermal (CST) systems. First, key AI techniques are briefly described and relevant examples of their application to CST systems in general are provided. Subsequently, a detailed review of how these AI techniques are being used to advance the state of the art of solar tower systems is presented. The review is structured around the different subsystems of a solar tower system.

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% NaNO3, 40% KNO3), liquid sodium, supercritical carbon dioxide (sCO2), 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 sCO2 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.

A cavity receiver geometry may represent a better option than an external receiver geometry for high temperature solar power systems because of reduced thermal losses. However, a limitation is that the small aperture necessary for an efficient (and affordable, practical) cavity receiver may lead to high spillage losses, negating the thermal efficiency gains. In this study, Monte Carlo ray tracing is coupled with heat transfer models to assess the benefit of secondary reflectors and spillage skirts on the performance of cavity receivers in the context of both dish and tower CSP systems. The heat transfer fluid is chosen as liquid sodium with inlet and outlet temperatures of 520 °C and 740 °C respectively. A spillage skirt and three different types of secondary reflectors, including conical, trumpet and compound parabolic concentrator (CPC) reflectors are investigated to decrease the energy loss. The results show that the spillage skirt helps in greatly decreasing the aperture size, while simultaneously reducing the spillage loss. The conical and trumpet reflectors increase the efficiency by reflecting rays outside the cavity inlet into the receiver, but the CPC does not achieve efficiency gains comparable to the other designs in both the dish and tower systems. For dish systems, highest efficiency is obtained with a combination of a spillage skirt and conical or trumpet reflectors, with a 1.0% increment in receiver efficiency compared to the best cylindrical receiver. For the tower system the efficiency gain is approximately 1.3% relative to the optimised cavity receiver alone.

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.

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.

To achieve further substantial reductions in the cost of concentrating solar-thermal power systems, recent efforts have been directed towards changing the working fluid in the receiver, adopting new approaches to thermal energy storage, and incorporating higher-efficiency power cycles. In this study, we present a novel CSP system configuration under development in ASTRI research program in Australia, incorporating a tubular sodium receiver, a high-temperature phase-change material storage unit, and a supercritical CO2 power block. The model is implemented using the SolarTherm framework in the Modelica language, and simulated using Dymola software. The novel system was compared to a ‘reference’ system with a molten salt receiver, two-tank storage, and subcritical steam Rankine cycle. For verification, the reference system was also modelled using the System Advisor Model software, and differences of 1% in annual receiver output and 2% in net electrical output were achieved when comparing Modelica and SAM simulations, despite difference operating strategies and time resolution. The results of the analysis of the novel system showed an improvement of annual solar-to-electricity efficiency to 19.6%, up from 15.6% for the reference system. This was achieved as a result of higher power cycle efficiency, despite the reduction in optical and receiver efficiencies resulting from the higher working fluid temperature. The ASTRI system was economically optimised for financial performance using 2012 as the basis year, and reached a minimum levelised cost of electricity of 11.42 ¢AUD/kWh for a solar multiple of 2.2 and storage capacity of 8–9 h at the location of Alice Springs, Australia.

Temperature distribution in nonaxisymmetrically concentrating solar thermal receivers tubes is calculated for the steady-state solution with a Gauss-Seidel iteration in cylindrical coordinates. The classical plane-biharmonic thermoelastic approach to nonaxisymmetrically heated tube stress is applied. Calculation of the dominant axial thermal stress component is included. Validation is obtained with the linear-elastic thermal stress OpenFOAM® solver. Thermoelastic stress in stainless steel 316 Schedule 5S DN25 (1″) tubes containing liquid sodium is found to be 35% lower than in tubes containing molten salt. The difference is due to the higher conductivity of liquid sodium which maintains a smaller temperature difference between the front and back tube sides. A simplified thermal stress formula is shown to be erroneous if not implemented as originally documented. The sensitivity of tube thermoelastic stress to tube material and flow properties is illustrated with parameter variation, and the impact of solar concentrated heat flux is explored with some typical and ideal tube circumference flux profiles.

The present article addresses the design, mathematical modeling and analysis of a high temperature solar thermal energy storage unit based on molten soda-lime silica glass. The 126 kW hth storage unit is aimed to be used as one of the main components of a novel solar power-generation system intended for a continuous operation. The proposed design for the unit, as well as the restrictions imposed by its intended operation inside the power generation system, are thoroughly discussed. The development of the mathematical model used to calculate the efficiency and performance of the thermal storage unit during the different stages of the work cycle as well as the assumptions and simplifications made are comprehensively explained. The results obtained through the model are exhaustively analyzed. Special attention is paid to the assessment of the behavior of the storage unit to guarantee that the functional requirements are met; the performance of the unit is not evaluated exclusively from an energy standpoint, but an in-depth exergy analysis is also presented. The overall performance of the TES unit is satisfactory; the unit is capable of supplying the required 4 kWth output throughout the 16-h discharge while it reaches its fully charged state during the subsequent 8-h recharge. The proposed designed for the TES unit exhibits a round trip exergy-efficiency of 59%.

This paper examines the potential of sodium receivers to increase the overall solar-to-electricity efficiency of central receiver solar power plants, also known as solar tower systems. It re-visits some of the key outcomes and conclusions from past sodium receiver experiments, in particular those at Sandia National Laboratories and Plataforma Solar de Almeria in the 1980s, and discusses some current development activities in the area. It also discusses research in sodium receivers with a liquid–vapour phase change (heat pipes and pool boilers), to explore whether technologies developed for dish-Stirling systems have applicability for solar tower systems. Lessons learnt from experience in the nuclear industry with liquid sodium systems are discussed in the context of safety risks.

We optimise steam network trees that connect Big Dish paraboloidal collectors to a central power block. Exergy costs, pipe material costs, and installation costs are estimated using an exergoeconomic model and used to optimise pipe links in network trees. The optimal network tree is then found for a 10 MW e collector field using a genetic algorithm. An optimised tree is found for a 20 MW e network. The optimised 20 MWe network has additional east–west branches not seen in the optimal 10 MWe network, that reduce steam transport costs from network extremities. Exergy costs from heat loss and pressure drop account for approximately 60% of total network cost in both cases. Total network costs are, respectively, 8.9% and 9.5% of potential plant revenue for the 10 MW e and the 20 MW e networks.

We present a new methodology for defining regularly-spaced collector layouts for the purpose of simulating annual system cost in two-axis sun-tracking collector arrays, such as those comprising solar-thermal dish collectors or two-axis tracking PV. We thoroughly consider all layout combinations of aspect ratio, offset, ground-cover-ratio, and rotation. Collector position is optimised to reduce annual shading for a range of collector densities for the site of Barstow, California. Optimal layouts are rectangular in shape for ground cover ratios less than 0.23. Diamond layouts collect up to 1.4% more absolute annual solar energy for higher ground cover ratios. From these results, a correlation for determining optimal array layouts subject to the chosen position-dependent cost function is presented. This approach can be used for design of large arrays of two-axis sun-tracking collectors where collector position and costs proportional to land use affect levelised plant cost.

This paper presents a dynamic model of a once-through-to-superheat solar steam receiver for electricity generation. The receiver is a mono-tube cavity boiler mounted at the focal point of a 500 m2 paraboloidal dish concentrator at the Australian National University. The dynamic model is derived from physical principles of mass and energy conservation, and uses a moving boundary formulation, coupled with a switching approach, to represent outlet flow, ranging from sub-cooled liquid to superheated steam. A method to compute outlet mass flow rate for all three receiver outlet flow conditions is included. This modelling approach results in a compact state-space representation of the receiver which is useful in the development of model-based control strategies for the operation of the receiver in a concentrator plant. The model is implemented in TRNSYS 16 and validated with experimental data from the Australian National University 500 m2 dish system.

The Australian National University (ANU) has worked for many years on paraboloidal dish solar concentrators and demonstrated a 400 m 2 system in 1994. The commercialization of this technology has involved a re-design of the Big Dish concept for mass production. The new design is a 500 m 2 concentrator with 13.4 m focal length and altitude–azimuth tracking. It uses 380 identical spherical 1.17 m × 1.17 m mirror panels, which incorporate the Glass-on-Metal Laminate mirrors. Construction of a first prototype on the ANU campus began in the first quarter of 2008. The first on sun test was carried out on 29 June 2009.