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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.

Methanol synthesis offers a relatively fast transient response compared to other fuel synthesis technologies—a promising downstream alternative for solar-derived syngas. However, many non-intuitive design choices must be analysed to achieve an optimal techno-economic performance of this technology in the presence of variable solar energy. Using a combination of steady-state Aspen flowsheeting, dynamic simulation and techno-economic optimisation, this work presents the techno-economic performance of methanol production through solar-thermal supercritical water gasification (SCWG) of algae with three alternative methane reforming options. The syngas is conditioned for downstream use through either CO2 dumping or H2 addition. In a multi-objective optimisation, solar multiple and storage capacity are varied to minimise the levelised cost of fuel (LCOF) and maximise the capacity factor of the system. Results indicate that steam methane reforming (SMR) with CO2 dumping gives a minimum LCOF of ∼1.6 AUD per L. The lowest-cost reforming option changes if the cost of renewable H2 drops. Achieving high capacity factor (>98%) configurations requires large storage and causes LCOF to increase 4–5×. Scaling up the solar-thermal collector to 700 MWth yields LCOF reductions of ∼20% relative to the baseline 50 MWth scale, beyond which optical losses outweigh scaling benefits. Overall, the choice of methanol for downstream fuel synthesis appears to be beneficial due to its faster ramping and overall increased output compared to Fischer–Tropsch synthesis.

Algal biomass is an attractive feedstock for carbon-neutral fuel production due to high growth rates and its potential to be farmed in artificial ponds on non-arable land. Supercritical water gasification (SCWG) of algae is appealing because it eliminates a drying step and offers fast kinetics, low char/tar rates, high carbon efficiency and good scale-up potential. Limited understanding of the algae breakdown mechanisms in SCWG, including kinetics and char formation, together with high pressures and temperatures, challenging for containment materials, make reactor design difficult. Dewatering and pumping of algae at high concentration in a cost-effective manner is also challenging. Integration of concentrated solar-thermal (CST) heat into SCWG of algae offers the potential for lower-cost fuel production, since CST is a cheaper energy source than unprocessed algae. However, CST heat integration is challenging due to the additional impact of sunlight variability. Integration of intermittent solar synthesis gas flows with downstream liquid-fuel processing such as the Fischer–Tropsch or methanol synthesis raises other challenges. Off-sun SCWG reactors downstream of a high-temperature salt storage offer future potential. The falling cost of green hydrogen input offers new opportunities to achieve lower-cost solar-SCWG system configurations. This review provides a general overview of algae-fed SCWG technology and challenges, then discusses CST-integrated SCWG of algae and its prospects for lower-cost liquid fuel production.

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.

Background As practitioners and clinicians who work closely with children, young people and their families, we always assume we know what the families need from hospices. Aim(s) To review the current service offering of the hospice, to identify gaps in provision, to highlight any barriers there are to accessing services and to shape our delivery to meet the articulated future needs of our families and our commissioners. Methods We commissioned an independent research company to undertake the research project which included a desktop literature review, quantitative and qualitative interviews with 100 families and interviews with stakeholders across the North West including FOI requests to CCGs. Neonatal and bereaved families were not included in this study. Results Our families identified respite as the most important service offered by the hospice with wellbeing services identified as the second most important. Our stakeholders and CCGs acknowledged funding for palliative care is patchy across the North West and were concerned whether Derian House can continue to offer the services we currently provide. Additionally, the research also highlighted a big gap in transition services between children’s and adults’ hospices.. The results of the research have informed the strategy for the hospice for the next three years and CCGs will be using the research to revise and develop their own services for children and young people. Conclusions The results supported our continued provision of planned respite, it also highlighted areas which may act as barriers to accessing services which include transport and also the information provided to external services explaining the facilities offered to families. These barriers identified by this research project now form part of our strategy over the next three years. We will continue to review services and engagement with families, and will be further exploring needs for neonatal families and the need of families for end-of-life and bereavement.

This paper evaluates the annual dynamic performance and techno-economic feasibility of a solar fuels process driven by concentrated solar power (CSP). The process integrates solar supercritical water gasification (SCWG) of microalgae, syngas storage, and downstream Fischer–Tropsch (FT) synthesis, to produce renewable drop-in liquid transport fuels. Performance curves for key components, derived from design-point and off-design simulations in steady-state flowsheet models established in the previous work of the authors, are incorporated into a dynamic energy-based model of the overall plant in OpenModelica. Control units are implemented to model the high-level state transitions of the plant relating to ramp-up and ramp-down of solar-SCWG and FT units as well as syngas storage and dispatch. There is significant uncertainty about the achievable ramping time for FT plant, owing to the dynamics of the synthesis of long-chain hydrocarbons in the FT reactor, so this is modelled as a variable-duration ramp-up with no useful product output. A perfect one-day-ahead forecast scheduler is developed to regulate the supply of solar syngas to the FT unit to minimise these ramp-up events. The levelised cost of fuel (LCOF) for the plant is minimised using a genetic algorithm to adjust the solar multiple and syngas storage capacity of the plant, with weather data and location of Geraldton in Western Australia. The optimal configuration has a solar multiple of ∼4 and syngas storage of ∼20 h, and achieves an LCOF of ∼3.5 AUD/L (∼2.5 USD/L) and capacity factor of ∼72%. Sensitivity analysis shows that the price of the farmed microalgae feedstock has the strongest influence on the LCOF. The one-day-ahead syngas dispatch scheduler contributes an LCOF reduction of 19% compared to an immediate dispatch strategy. The importance of the FT ramping time in these simulations suggests that fast-ramping FT reactors (e.g. microchannel reactors) will be essential for the techno-economic success of solar-gasified FT fuels, and motivates further research and development on this topic.

Central receiver in a solar tower power system absorbs the solar energy and transfers heat to the working fluid. It is of great importance to reduce the heat losses of the receiver. In the present work, a plane air nozzle was installed on top of a solar cavity receiver in order to create an air curtain. The effects of air curtain on the natural convection of the cavity receiver were numerically analyzed. Four parameters of the air nozzle, including the inclination (θnoz), the width (Lnoz), the outlet temperature (To, noz) and the outlet velocity (uo, noz), were considered as the factors. The results indicate that the air curtain could effectively prevent the hot air from flowing out of the cavity. It can enlarge the stagnation zone inside the cavity and suppress the natural convection on the internal surfaces. The convective heat loss monotonically decreases with increasing θnoz. The downward facing (θnoz = 90°) air nozzle produced the highest drop in the convective heat loss. A reduction of 28.6% can be achieved when a downward facing nozzle with 600 mm in width is used. The optimal uo, noz decreases with increasing Lnoz.

This study reviews basic relations, classification, and characteristics of concentrating collector systems for high-temperature solar thermal applications.

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.

This paper focuses on how the low-cost renewable H2 can change a process design configuration in solar fuel plant. As a case study, an industrial process is considered in detail at ANU to convert the algal biomass into Fischer–Tropsch liquid fuels via solar-powered supercritical water gasification (SCWG). The yield gases from the gasifier mainly contain methane, which is then converted into the suitable composition of syngas in the steam methane reforming (SMR) process. Two scenarios are evaluated here to balance the H2:CO ratio for the downstream process: (i) dumping a fraction of carbon in the form of CO2, (ii) supplying make-up H2 from PV-driven electrolysis unit. A detailed steady-state model of the SCWG-SMR and FT plants is developed in Aspen Plus software. The performance curves of gasification/FT units at design and off-design points together with a set of control logics is used to form an energy-based system-level dynamic model in OpenModelica. The levelised cost of fuel (LCOF) as a key parameter for system optimisation is calculated for the considered scenarios. It is revealed that the preferred process design choice of the whole plant is highly affected by the H2 price from a techno-economic standpoint. If the H2 cost falls by 42% (i.e. 5.6 AUD/kg), the SMR-H2 configuration is more economically feasible as compared to the SMR-dumping scenario.

In this study, Monte Carlo ray tracing (MCRT) is coupled with heat transfer models to analyse the efficiencies and energy losses of a cylindrical cavity receiver in a dish system. The progressive Monte Carlo evaluation (PMCE) method is used to determine good geometric designs with high receiver efficiencies. The spillage skirt, which consists of several coiled pipes outside the cavity inlet, is added and optimised to increase the receiver efficiency. Three different types of secondary reflectors, including conical, trumpet and compound parabolic concentrator (CPC) reflectors are also investigated to decrease the spillage 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 can increase the efficiency by reflecting rays outside the cavity inlet into the receiver. CPC secondary reflectors experience more internal reflections which cause high secondary absorption loss and consequently cannot achieve the equivalent efficiency compared to other designs. With a combination of spillage skirt and conical or trumpet reflectors, the receivers obtain the highest efficiency, with a 1.0% increment in receiver efficiency and 12.9% relative reduction of losses compared to the best cylindrical receiver. Limitations of the current study are that the effects of skirt and reflectors on convective losses are not considered and that the primary concentrator design is fixed.

A system-level evaluation of a concentrating solar power (CSP) configuration, with high-temperature sodium boiler receiver, direct-contact NaCl phase change material (PCM) storage and a Stirling engine array at 1 MWe scale was performed using the modelling framework of SolarTherm to provide an estimate of system costs and comparison to a reference 100 MWe two-tank molten salt system. Assuming an allowable receiver peak flux limit of 1.75 MWth/m2 and a Stirling engine efficiency of 34.6%, a levelised cost of energy (LCOE) of 137 USD/MWhe is obtained via genetic optimisation of parameters including solar multiple, receiver size, and storage component geometry. For an optimistic power cycle efficiency at 75% of the Carnot limit, the calculated LCOE is 109 USD/MWhe. Assuming safety risks can be mitigated without major additional cost, this novel concept holds promise when compared to the reference case that has an LCOE of 123 USD/MWhe, especially for small-scale opportunities either off-grid or at the fringe-of-grid. As this concept operates at very high temperature, the thermal efficiency of the receiver is lower than for a conventional CSP plant, and the storage system is more expensive on a unit-cost basis due to the use of high strength alloys in the tank. However, these drawbacks are offset by the use of a low-cost steel lattice tower design which is feasible at small scale, and by high efficiency power conversion at ∼800°C.

In recent years, electricity costs in the Australian National Electric Market (NEM) have increased primarily due to the network costs associated with grid infrastructure upgrade and operation. Distributed power generation through small-scale concentrated solar power (CSP) can mitigate these network costs if deployed at critical points in the network where low capacity transmission lines most constrain transmission. This study identified cost-effective configurations of small-scale CSP plants, from 10 to 50 MWe, via levelised cost of electricity (LCOE) by adjusting the solar field layout and size, tower height, receiver dimensions and storage capacity. The model was implemented in SolarTherm, and parametric sweeps were carried out by varying solar multiple, storage capacity, and receiver size and tower height to determine the effect of these parameters in the LCOE. Also, two material options were considered for tower construction: reinforced concrete and steel truss. Results showed that the highest capacity factor is found at a 3.4 solar multiple and 15–16 hours of storage capacity at all CSP sizes, achieving an LCOE between 13.7–16.7 ¢USD/kWh depending on CSP size and assuming a 1.5 MW/m2 peak heat flux. Besides, if peak flux is reduced to 0.85 MW/m2, then LCOE ranges between 20.5–26.2 ¢USD/kWh. The use of steel towers is suitable for plant scales below 20 MWe, reducing the LCOE by 2.5–4.2% compared to concrete towers. Meanwhile, concrete construction is the best option for 30–50 MWe.

A bladed receiver concept, intended for use with conventional NaNO3:KNO3 solar salt, was developed and tested using a novel technique where water and air were used as working fluids, to estimate the losses that would be experienced with salt. The (hot) air and (cold) water tests allowed the optical and thermal losses to be separately observed. As well as the bladed receiver, tests were also conducted with an optimised flat receiver. Results showed the total efficiency (reflection, emission, convection losses) was 0.65% higher for the bladed vs. flat receiver, lower than the >2% improvement expected. The lower efficiency is tentatively attributed to lower flux, lower temperatures and windy conditions, compared to the design-point modelling.

As concentrating solar power systems push towards higher temperatures and lower costs, it is critical that losses of overall system performance can be attributed correctly to the appropriate source. Up to now, this has been poorly done for the case of optical errors, since applicable methods do not exist to quantify how much different imperfections contribute to reducing the upper-bound efficiency of the overall system. Here, the exergy impact of varied optical design parameters—slope error, rim angle, mirror reflectance and sun-shape—is calculated for the first time. Slope error is shown to have the strongest impact. Also, dishes with rim angles significantly wider than the conventional 45° are shown to yield the best overall energy conversion. The resulting analysis method, broadly applicable in concentrating solar power, enables a new approach to quantitative optical system design.

Mixed convection around rectangular structures has great scientific value with various industrial applications. In this study, heat transfer in the mixed convection regime around a tilted rectangular cuboid is investigated experimentally and numerically. The cuboid has adiabatic wall boundaries, except for its frontal wall which is set isothermal and facing downwards when tilted. The effect of an upcoming horizontal flow on mixed convection around the cuboid was investigated focusing on flow characteristics for Reynolds numbers in the range of 10 4 Re 1.8 × 10 5 with low turbulence intensities of TI 5.5 % and Richardson numbers of 5 × 10 - 3 Ri 2 , well within the mixed convection regime. A numerical simulation based on the three-dimensional SST k - ω turbulence model, validated against data from wind tunnel experiments, was used to accurately estimate the total heat transfer rate and the average Nusselt number as a function of cuboid inclination and wind direction at different Ri and TI. The results show that convective heat transfer is enhanced at a characteristic inclination of ∼ 80 ° by ∼ 30 % in comparison to the case where the freestream flow is perpendicular to the isothermal wall. Flow behaviour in the vicinity of the cuboid demonstrated that laminar separation and turbulent reattachment were responsible for this enhancement, which is a behaviour not observed for isothermal flat plates but reported for flows around cylinders and spheres.

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.