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The thermal response of a multilayered refractory-lined solar receiver has been compared with and without covering the aperture during the shutdown period. This includes an assessment of the influence of aperture cover on the overnight temperature drop, start-up time and thermal stresses. [ABSTRACT FROM AUTHOR]

In the Solar Central Tower, the receivers are used to convert the concentrated solar radiations into useful energy via appropriate thermic fluid. A suitable optical field design for the positioning of the heliostats on the ground and the design of receiver system at a calculated height are two of the challenging tasks in the field of concentrated solar thermal technologies (CSTT). To estimate the optical power incident on the receiver for a designed solar field ray tracing techniques have efficiently been used. The incidence of high concentration of solar radiation at the receiver has a potential of damaging it during the operational hours due to varying flux in the incoming solar radiation. To protect the receiver from thermal damages, it proposed to monitor the receiver remotely using an imaging optics kept at the ground. The Planckian radiation emanating from the receiver during the operational hour is envisaged for this purpose. The estimated power in the Planckian radiation for a designated aperture of the imaging optics at various intervals during the day. The detectors having responsivity between two different range of wavelength can be used to monitor the change in the temperature of the receiver. [ABSTRACT FROM AUTHOR]

This paper summarizes the objectives and experimental activity of the H2020 research project CAPTure, which officially lasted from May 2015 to July 2020. The main objective of the CAPTure project was the development and testing of several key components that allow the implementation of a solar powered combined cycle plant (topping Brayton cycle and bottoming Rankine steam cycle). The main innovation was related to the coupling of the open volumetric air receiver (OVAR) with the pressurized air stream of the Brayton cycle, i.e. the external heating of the Brayton cycle with hot air at atmospheric pressure. One key component was therefore a gas-gas heat exchanger, which was implemented as regenerator (atmospheric heating – pressurized cooling). A 300 kWth prototype, consisting of the solar receiver, the regenerative system and a small-scale Brayton cycle, was designed and implemented at CIEMAT-PSA. The paper describes the CAPTure prototype and the experimental activity performed until September 2021. [ABSTRACT FROM AUTHOR]

In the last years, it has been experimentally proven that the precursors of synthetic fuels can be produced exploiting the thermochemical water and CO2 splitting process, driven by concentrated solar radiation. This process requires large amount of thermal energy at very high temperature levels (1300-1400°C) far from being obtainable with conventional commercial receivers. For this reason, Synhelion SA is developing an innovative receiver design capable of operating at very high temperatures (in the order of 1500°C). To assist the design phase of the receiver, a CFD model was developed and exploited to run some CFD simulations campaigns. At first, an analysis on the effect of modeling the receiver behavior with different turbulence models (realizable k-ε and k-ω SST) was conducted with the aim of determining the most suitable for the cases under investigation. The CFD model was then applied to study the receiver behavior under different operating conditions. In particular, in terms of receiver operating pressure, two different cases were considered: ambient pressure and 10 bar. For each of them, a specific receiver design was described, and a relative simulations campaign, assuming different values of heat transfer fluid (HTF) mass flow rates, was performed not only to evaluate its thermo-fluid dynamics behavior but also to determine the most promising HTF inlet configuration between the two proposed of single and double inlets. Based upon the results obtained, the absorbing gas receiver resulted to have promising performance for all the working conditions evaluated. In the case of receiver operating at ambient pressure, the best resulting thermal efficiency values were in the range of 0.78 and 0.43, with relative HTF outflow temperature range of 1200°C and 1800°C. While, in the case of pressurized receiver, the best resulting efficiency values were in the range of 0.88 and 0.54, with relative HTF outflow temperature range of 1000°C and 1670°C. [ABSTRACT FROM AUTHOR]

Beam Down Receivers (BDR) are a promising alterative for small-to-medium tower-based Concentrating Solar Thermal (CST) plants. A novel particle receiver Solar Expanding Vortex Receiver (SEVR), that has been proposed to be configurated into a BDR for solarization of high temperature industrial applications, such as calcination process. This type of receiver might benefit from beam down optics via a ground-based receiver, a simplified tower, and key operational advantages (e.g., reduced pumping requirements, co-located receiver-storage, reduced particle egress). This study lays the groundwork for such a design via an optimization tool which uses the Monte Carlo Ray Tracing (MCRT) method. It was found that for a 50 MWth, system the optimal tower height is 75m and that this system can provide a LCOH of ∼34 USD/MWh. The radiation flux distribution on receiver aperture and the optical efficiency throughout the year were also studied. [ABSTRACT FROM AUTHOR]

Emerging directly irradiated particle receiver technologies have the capability of generating temperatures exceeding 800oC. This is important for industrial processes involving high temperatures, such as electricity generation and thermochemical processes, such as the Bayer alumina process. The utilization of particles in these devices is receiving growing attention due to their potential to withstand high temperatures and their effectiveness on solar radiation absorption. Among all particle receiver technologies being developed globally, the suspension flow receiver, better known as a vortex flow receiver, is receiving growing attention as a method to heat particles in suspension, and hence to heat both phases. In this study, a set of novel scale-up strategies was investigated for a 50-MW windowless directly irradiated vortex-based solar particle receiver to identify the strengths and limitations of each strategy. A receiver design was determined and chosen as the candidate for the current computational study. A systematic analysis was performed to assess the influence of particle mass loading and inlet air mass flowrate on the global performance of the receiver at scale (50-MW) under steady-state conditions with simulated solar flux, along with CARBO-CP used as the particle material. A fixed ratio of overventilation was applied to the induced draft fan to reduce particle egress. The numerical study was conducted using computational fluid dynamics (CFD) software (ANSYS/CFX 2020 R2). The result predicts that up to 60% thermal efficiency can be achieved under high air mass flowrates and particle loadings at scale for the operating conditions analyzed here. [ABSTRACT FROM AUTHOR]

Generation 3 particle-base concentrating solar power systems utilize solid media in the form of absorptive, sand-like particles as the heat-transfer fluid and thermal-energy-storage medium to increase the maximum operating temperature of concentrating solar power technology to over 800°C. The particles fall as a curtain through an open aperture receiver cavity to directly absorb thermal energy from the concentrated solar flux. Abrasion due to particle-to-particle and particle-to-surface interaction within particle based concentrating solar power systems creates dust that escapes through the receiver aperture. The dust present in the receiver cavity may result in beam attenuation and mass loss decreasing the efficiency of the receiver. A design for a dust removal loop is presented with preliminary testing demonstrating its ability to reduce the dust present in a 1 MWth falling particle receiver with an 8 kg/s mass flow rate. The particle dust removal loop reduced the mass of particle dust present in the receiver by 46% compared to steady operation with ambient temperature particles. [ABSTRACT FROM AUTHOR]

The centrifugal particle solar receiver (CentRec) is a promising technology to reduce the cost of heat and electricity generated by solar towers. In this study, a computationally inexpensive radiation model accounting for all radiative heat transfer mechanisms in the CentRec is addressed. A short-range radiation model to calculate the radiative heat transfer between close particle pairs and a radiation enclosure model to calculate diffuse radiation heat exchange between large surfaces of the receiver are developed. Moreover, a radiation penetration model to distribute the solar energy to particles, which was developed by authors previously, is integrated into the main model. The models are applied to a small-scale receiver and thermal steady state results are obtained. The model is compared with an in-house Monte Carlo ray tracing code for the case of a uniform heat flux of 3 MW/m^2 at the aperture. The results show that although the developed model matches particle outlet temperature by a narrow margin (10 K), the temperature distribution in the receiver is overestimated slightly by the current model. Moreover, while the current model calculates around 3.5% of reflection and emission loss, MCRT-based model gives around 5.5% of total radiative loss for the given solar flux distribution. Thus, there is still room for improvement. [ABSTRACT FROM AUTHOR]

Despite the large number of research studies about Solar Tower plants questions, remain on a wide range of topics including methods for the lifetime assessment of the receiver tubes under creep-fatigue damage. Most of the published works dealing with this task do not consider important aspects such as the impact of clouds passages, plastic deformations, or stress relaxation due to creep. In this study a methodology is proposed to assess the creep-fatigue lifetime of solar receivers based on real weather data. The presented approach requires as input the daily DNI profiles for the selected plant location which are used to assess the heat flux maps on the receiver. Then, the heat transfer phenomena occurring within the receiver are dynamically simulated in order to obtain the history of the temperature distributions on the receiver tubes. Lastly, the creep and fatigue damages accumulated during the operation are evaluated in each receiver point considering the effects of plastic deformations and stress relaxation. The proposed methodology is applied to a simple case study to estimate the creep and fatigue damages accumulated in a receiver flow path during a typical cloudy day. The results show that the fourth panel accumulates the maximum damage and the fatigue damage is negligible compared to the creep. [ABSTRACT FROM AUTHOR]

The usage of solid particles as solar thermal energy absorption/storage medium requires particle heating receiver to be developed. King Saud University and Georgia Institute of Technology have been collaborating in designing and testing obstructed particle heating receiver. Size of the receiver is 1.2 m x 1.2 m; the base of it is made of duraboard on which chevron-shaped meshes, made of Inconel 601, are installed as the obstructions in a staggered arrangement. The receiver is employed in the particle-based solar power tower experimental facility at King Saud University. To evaluate performance of the receiver, on-sun tests were conducted on two types of particle material, i.e., red sand and CARBOBEAD. Results show that CARBOBEAD absorbed more solar thermal energy and achieved higher temperature rise across the receiver compared to red sand, which is, most likely, due to a higher radiation absorptivity. It was also found that red sand tended to agglomerate at the tip of the chevron mesh during the test, blocking the concentrated sunlight. Moreover, as these agglomerates crumbled, they created many smaller agglomerates that can block the filter downstream, which will, consequently, block the particle flow path. Mesh material also needs to be analyzed further to avoid particle agglomeration, because it is expected that it has a significant role in this matter. [ABSTRACT FROM AUTHOR]

An experimental tube receiver was constructed, built and brought to operation. This receiver served as a model to try out the so-called scan-method and the measurement of the flux density by reflection off the receiver in real time during irradiation. A comparison with a flux map obtained by a radiometer-based method shows qualitatively similar results. It is concluded that the camera-method is applicable to tube receivers. Experiments at large-scale industrial receivers are planned. [ABSTRACT FROM AUTHOR]

The vacuum failure of the receivers will lead to a significant increase in its heat loss, the decline of its vacuum performance and the considerable loss of the whole solar thermal power generation system. This paper presents a method for measuring the vacuum performance of parabolic trough solar receivers based on spark discharge and emission spectroscopy. During discharge, the emission spectrum is detected when combined with the most common residual gases in the annulus of receivers: hydrogen and nitrogen. The emission spectrum calculates the electron temperature and electron density during discharge. The results show that the emission spectrum during discharge can be well detected by this method. In addition, the electron temperature and electron density during hydrogen and nitrogen discharge can be calculated through the emission spectrum obtained from the experiment, which can better understand the actual situation inside the annulus of receivers and provide reliable experimental results for improving the research of residual gas in the annulus of receivers. [ABSTRACT FROM AUTHOR]

In this paper a model predictive control (MPC) for operating a solar tower receiver is evaluated and compared with the performance of a PID-based control, that is considered as conventional control approach. The receiver outlet temperature is controlled by the mass flow, while the solar irradiance is considered as measurable disturbance. The control algorithms are tested via simulation on a virtual power plant based on a high-fidelity model of the receiver. For carrying out the evaluations, 24 validation cases are evaluated in terms of control error and compliance with limits for a high durability of receiver. As a result, the MPC has the potential to operate a solar tower molten salt receiver in a secure and optimal way as well as to increase the degree of automation. [ABSTRACT FROM AUTHOR]

Currently, there are several simulation software based mainly on two different calculation methods that are used to design, analyze and optimize optical systems by simulating them with an ongoing effort to improve its velocity without losing accuracy. This paper presents two types of software. The first software uses the raytracing algorithm providing high accurate results but with very long execution time while the second one is an analytic software based on convolution methods where the results accuracy decreases and the execution time decreases dramatically too. The main goal of this work is to compare the results obtained from the simulations of central receiver plants using both types of software and demonstrate that the analytic software can achieve an acceptable accuracy (less than 1% comparing with ray-tracing) saving a great time of execution and consequently, make its use practical to the design of solar central tower plants. [ABSTRACT FROM AUTHOR]

An accurate and computationally fast ray-tracer is the key part for the simulation of the optical irradiation of a heliostat field in a solar central receiver system. Within this work we present how the ray-tracing for central receiver systems can be accelerated while obtaining highly accurate results. The runtime improvement is achieved by an enhanced blocking and shading routine and a parallelized evaluation of the ray-tracer using a graphical processing unit (GPU). Our new SunFlower GPU ray-tracer is implemented with the Monte-Carlo methodology such that direct run-time comparisons to other Monte-Carlo ray-tracers are possible. Within a case study we demonstrate that the improved and GPU-parallelized ray-tracer computes up to 99.95% accurate annual simulations of heliostat fields consisting of 8600 heliostats in less than 2.5 seconds. [ABSTRACT FROM AUTHOR]

A Haynes 230 micro-pin solar receiver has been designed and fabricated using a wide variety of manufacturing techniques. This receiver utilizes a modular design that allows for high temperature and pressure operation in a gas-phase CSP system. Several failure modes related to design and fabrication processes were identified in the first receiver prototypes. This paper reports the efforts to resolve the failure modes related to diffusion bonding, brazing and welding of the Haynes 230 micro-pin receiver. A systematic approach starting with separate effects and then integrated testing was used to qualify each process. At the end of this process, a 15 cm x 15 cm prototype solar receiver was fabricated and proof test to a pressure of 41.4 MPa, qualifying it for on-sun testing in future work. [ABSTRACT FROM AUTHOR]

Direct particulate heating receivers (DPHR) are the most important part of the particle-based concentering solar power (PBCSP) systems in which the particles are exposed to direct high-concentrated sunlight. One variation of DPHRs is the obstructed flow receiver, which King Saud University (KSU) and the Georgia Institute of Technology have been developing for years. In this type of PHR, the particles fall freely in the form of a curtain through some obstruction to decelerate flow and allow the falling particles to absorb a considerable amount of thermal energy. The discrete structure presented in this design uses chevron-shaped mesh made of metallic alloy (Inconel 601) that was fixed on a Duraboard HD (an alumina-silica ceramic fiberboard). The design has been successfully tested using red sand and CARBOBEAD as heat transfer media. However, the interaction between the particulate and chevron mesh screen needs to be considered carefully at high operating temperatures. Careful consideration is the softening of the tip of the metallic chevron facing the sun. The tip of the mesh becomes soft at high temperatures and deforms which causes the particles to agglomerate on the chevrons, clog the chevron opening, and prevent particles from moving down to the end. Also, the particles get exposed to high flux for a long time which leads to particle fusing. Finally, a high degradation in the thermal efficiency of the system occurs as a result of this agglomeration and stops the whole operation of the plant. So, it is very important to study the interaction between the particulates and obstacles at high temperatures before installing it in the PHR cavity. This paper addresses these issues by presenting a study of the interaction between several chevron candidate materials and high-temperature candidate particles. The test was conducted in two stages: On-sun test and the lab test. Lab test is done by immersing the mesh screens with particulate materials and then heating at a specific temperature for several hours. For the chevron material, three candidates were considered: (a) stainless steel 316, (b) Inconel 601, and (c) ceramic mesh (WHIPOX). Three candidate particulate materials were considered: (a) Riyadh red sand, (b) Riyadh white sand, and (c) CARBOBEAD CP. Three temperature levels were considered in the lab test: (a) 800℃, (b) 1000℃, and (c) 1200℃. [ABSTRACT FROM AUTHOR]

The present work aims to study the feasibility to deposit selective absorbing coatings by using low-power plasma spraying systems. Three powders systems, namely pure chromium carbides, WOKA 7104 that is a commercially available mixture of chromium carbide and Ni20Cr metallic particles, and Manganese/Cobalt/Oxygen spinel were investigated, and the effect of processing parameters on the coating morphology and absorptance were assessed. The design of experiments approach was applied for the selection of the processing parameters. The results indicate that the compact plasma spray was not able to process the pure chromium carbide without the application of a bond coat, achieving a poor or null deposition for almost all process configurations. Woka and MnCoO spinel attained an average value of absorptance between 0.85 and 0.91, strongly influenced by the current and the plasma gas rate, which in turn dictate the power absorbed by the plasma equipment. Satisfactory values for absorptance and coating thickness can be achieved after optimization of the processing parameters, however, the results indicated that morphological characteristics and optical performance can have a competitive behavior, and their maximation can not be reached simultaneously. Therefore, requirements in mechanical and optical performances need to be balanced when the optimization of the deposition process is wanted. [ABSTRACT FROM AUTHOR]

In solar central receivers, temperature gradients arisen by incident heat flux are a key factor to their lifetime, since they cause thermal stresses on receiver tubes. In this work, a numerical analysis of how functionally graded materials could help to reduce thermal stresses, is carried out. Using an elastic modulus and thermal expansion coefficient variation throughout the tube radius, the influence of different material gradations between inner and outer tube walls have been studied. Results show how thermal expansion coefficient gradation may have a key role on thermal stress reduction, being necessary to have a material with a lower value at the outer radius compared with the inner radius material in order to homogenize the deformation of the cross section. Although a material with a lower elastic modulus in the outer radius reduces thermal stress, its influence is lower compared with thermal expansion coefficient. Optimizing the volume fraction of each constituent material also helps to enhance the stress reduction on solar receiver tubes. [ABSTRACT FROM AUTHOR]

Indonesia is a country that has abundant solar energy. Cooking with solar energy is one way to reduce dependence on fossil fuels. In this research, a Fresnel solar cooker (FSC) will be designed with a solar tracker. PMMA Fresnel lens is used to concentrate Direct Normal Irradiance (DNI) in a cooking pot with a conical cavity receiver cover. The thermal performance of the FSC was investigated by simulation and experiment. The simulation results show that the disorientation of the focal point causes a deceleration in the thermal distribution. The experiment results show that the focal point temperature of the Fresnel is influenced by Direct Normal Irradiance (DNI). The increase in solar radiation produces a focal temperature, exponentially. The test results show that DNI = 862.50 W/m2 can produce a focal temperature, Tf = 1064.00°C. [ABSTRACT FROM AUTHOR]

The cost of variable renewable power in certain applications is now lower than 'new build' fossil power (1). However, the value of variable renewable power has plummeted even further when demand is low. While renewable energy generation (wind and PV) is only 9.5% of global power (2), this value reaches over 50% in certain locations at certain times, contributing to significant problems with grid stability and demand-supply mismatch. These factors can, at times, trigger curtailment of renewable generation capacity, making their variable power worthless. This is impeding the expansion of renewable energy generation, since asset utilization and the return on investment is decreasing. If we are to reach zero emissions by at least 2050, clean electricity must contribute over 60% to global electricity supply (3). To achieve this, the value of solar power must be restored such that profitable business can drive the necessary 5-fold expansion of PV alone in the next 10 years (3). This requires both flexible stabilizing storage and market recognition of the value that this brings. A key enabler to restore the value of variable renewable power will be low cost, long duration power storage which is flexible and clean. The authors have devised a method of exploiting the unique synergies between central receiver PV, heat engines and simple water storage to enable an integrated, dispatchable power storage system. This system permits solar power delivery 'on demand', and can also take excess, low-cost power from the grid to stabilize (and thereby maintain the value of) other renewable energy sources, such as wind or PV. This paper describes the high-level features of such a system which uses proven off the shelf equipment to provide GWh scale, long duration storage at a fraction of the cost of batteries. A 4MWe central receiver PV plant with integrated 17-hour storage of 3MWe-50MWhe is being constructed in Australia to demonstrate our concept for 'dispatchable PV' power. [ABSTRACT FROM AUTHOR]

In this paper, elliptical hyperbolic solar concentrator (EHC) was designed with an angle of lateral truncation, using a simulation in the (Zemax) optical design program. The design consists of a conical basin in the form of a hyperbola that has a reflective inner surface, and an entrance aperture that allows solar rays to enter the (EHC). To be reflected repeatedly on the inner walls of it to fall onto the output aperture containing the receiver. The receiver collects the solar thermal rays and stores them through a warehouse that contains a liquid with high thermal conductivity. Heat is transferred from the warehouse to another thermal tank to exploit the thermal energy for multiple purposes. In this paper, there are many of optical parameters were used to demonstrate their effect on the optical efficiency of the (EHC). Such as concentrator length, concentration ratio, aspect ratio and truncation angle. The results showed the necessity to use a solar tracking system for this design, as it has a relatively small acceptance angle due to the nature of its geometric shape. [ABSTRACT FROM AUTHOR]

The article deals with the problem of synthesis of the control system of the power unit of a solar space gas turbine power unit. To ensure the uninterrupted operation of the turbine in the shadow region of the orbit, a thermal storage unit is used as an energy source, which prudently stores solar energy in the light region of the orbit. Operation of the turbine is provided by the system "receiver of solar radiation – thermal storage", which forms a single power unit. The problem of structural and parametric identification of its mathematical model is solved. Thermal storage efficiency is justified by maximum possible temperature of working fluid at turbine inlet. The criterion of optimality of choice of operating parameters of power plant is to ensure constancy of thermal storage characteristics at maximum thermodynamic efficiency of the system, which is equivalent to maximization of exergy efficiency of the system. The results of calculations have shown the possibility of using the proposed structural solutions to control the capacity of the power unit. [ABSTRACT FROM AUTHOR]

A novel integrated model is used to evaluate the technical feasibility of a large scale Concentrating Solar Power (CSP) plant with thermochemical energy storage based on the Calcium-Looping (CaCO3/CaO) process. Instead of using a solar particle receiver to carry out the calcination of limestone, as the usual solution considered in previous literature, this work proposes an indirect calcination system based on using a pressurized Heat Transfer Fluid (HTF) heated up inside a tubed volumetric cavity receiver, to heat indirectly the particles for calcination to take place in an Entrained Flow (EF) reactor. After the reactor, the HTF follows a typical closed combined cycle path for power production before entering again in the receiver. On the other hand, the CaO particles and CO2 are stored for power production under demand. Results show that full calcination of 10 kg/s can be achieved in a 51 m length and 2.5 m diameter downer reactor from an HTF releasing 17.2 MWth at temperatures higher than 1100°C. This opens a promising research line for the development of systems based on materials calcination from renewable energies. [ABSTRACT FROM AUTHOR]

Liquid metals have remarkable heat transport capabilities and are, thus, promising heat transfer fluids in thermal receivers in solar thermal electricity systems with high heat loads, such as central receiver systems. For thermal energy storage, a dual-media storage system with solid filler material is proposed. This configuration increases the storage capacity and decreases the storage material costs compared with a direct two-tank system. It also improves the storage performance, when comparing with a single-tank thermocline system without filler. Theoretical results show that the discharge efficiencies are highest for the largest storage heights and higher for heavy metals (lead and lead-bismuth eutectic) compared with sodium, both due to the decreased axial heat conduction and thus, minimized degradation of the thermocline. At the Karlsruhe Liquid Metal Laboratory (KALLA) at the Karlsruhe Institute of Technology (KIT) a lab-scale prototype of a dual-media storage system with filler material and lead-bismuth eutectic (LBE) as the heat transfer fluid is currently taken into operation. The first results without filler material show a good agreement between the theoretical model and the experimental results. In parallel, the compatibility of filler material candidates with LBE is investigated by storing the material in stagnant LBE for several weeks at 500°C and afterwards examining the filler material with a scanning electron microscope (SEM). The results indicate that ceramics are the most promising candidates. [ABSTRACT FROM AUTHOR]

In many concepts for the solar thermochemical production of fuels in a large scale, it is planned to place an array of receiver-reactors on a single solar tower. We investigate how each of these reactors needs to be operated and how the available power of the heliostat field should be distributed to the respective receiver-reactors to obtain maximum amount of solar fuel. Therefore, we developed a model for a single receiver-reactor, which is then used to analyze the single reactor operation. Based on that we present a heuristic which tries to maximize the hydrogen produced by an array of receiver-reactors for a given solar power. As our results are preliminary, we give an outlook on ongoing and future work in the end. [ABSTRACT FROM AUTHOR]

The heliostat collector field is the front-end of large solar power tower plants. Any negative performance impacts on the collector field will propagate down the stream of subsystems, which can negatively impact energy production and financial revenues. An underperforming collector field will provide insufficient solar flux to the receiver resulting in the receiver running at below capacity and not producing the thermal energy required for thermal storage and to run the power block at optimum efficiency. It is prudent to have an optimally operating collector field especially for future Gen3+ plants. The performance of a deployed collector field can be impacted by mirror quality (surface and shape), mirror canting errors, tracking errors, and soiling. Any of these error sources can exist during installation and further degrade over time and, if left unattended, can drastically reduce the overall performance of the plant. Concentrating solar power (CSP) plant operators require information about the collector field performance to quickly respond with corrections, if needed, and maintain optimum plant performance. This type of fast response is especially critical for future Gen3+ plants, which require high collector field performance consistently. However, power tower operators have struggled with finding or developing the right tools to assess and subsequently fix canting errors on in-field heliostats efficiently and accurately. Sandia National Laboratories National Solar Thermal Test Facility (NSTTF) is developing an aerial imaging system to evaluate facet canting quality on in-situ and offline heliostats. The imaging system is mounted on an unmanned aerial system (UAS) to collect images of targets structures in reflection. Image processing on the collected images is then performed to get estimates of the heliostat canting errors. The initial work is to develop the system definition that achieves the required measurement sensitivities, which is on the order of 0.25-0.5 mrad for canting errors. The goal of the system is to measure heliostat canting errors to <0.5 mrad accuracy and provide data on multiple heliostats within a day. In this paper, the development of the system, a sensitivity analysis, and initial measurement results on two NSTTF heliostats are provided. [ABSTRACT FROM AUTHOR]

This work presents the design and analysis of the optical performance of a 3D-CPC of a 10 kWe solar micro- tower in the Sahelian zone. The tower consists of a field of 60 heliostats of 2 m2 and a receiver with a CPC of height equal to 18 m and inclined at an angle of 45 °. The CPC is dimensioned from an opening radius corresponding to the radius of concentrated solar flux of the heliostats field. This work presents on the one hand a method to model the 3D-CPC from its characteristics and on the other hand an approach to incorporate a complex geometry like the 3D-CPC in Monte Carlo ray tracer Soltrace. The simulation results show that the optical performance of the tower improves with the presence of the CPC which increases the solar flux at the receiver entrance while reducing the opening area of said receiver. The average concentration ratio of 3D-CPC is equal to 4.7. [ABSTRACT FROM AUTHOR]

The receiver of the Solar Power Tower plants is one of the subsystems under the most highly demanding working conditions. In this study, the lifetime of various external tubular receiver configurations, also under different heat flux distributions, are obtained in order to test how they respond under both design and external variations, aiming to set certain guidelines that may aid the central receiver selection. To achieve it, an analytical approach is followed, which allows to obtain the estimated operation days in terms of the creep and fatigue damage. [ABSTRACT FROM AUTHOR]

The standard Concentrating Solar Thermal (CST) mono-tower technology, which uses one receiver placed on top of a tower to which all heliostats in the heliostat field aim to, is regarded as one of the best and most promising technologies for various CST-driven applications, namely CST power plants, solar metallurgical processes, thermochemical production of solar fuels and waste materials recycling. However, the technology has some technical challenges concerning optical performance/tolerances, system dimensions, operation and maintenance issues, etc. An alternative to this standard CST mono-tower technology is the so-called beam-down technology, where a special mirror is placed on the top of the tower, instead of a receiver, to redirect the incident radiation from the heliostat field onto a receiver/reactor placed closer to the ground and potentially delivering higher concentrations at the receiver than the standard CST mono-tower technology. This paper presents a new approach to improve the optics of beam-down systems, applies it to the optical design of a specific system, and shows the optical behavior of this design at two locations: Évora (Portugal) and Hurghada (Egypt). The approach uses etendue-matching between all the optical stages to minimize the optical losses between them. To analyze the optical behavior of the system designed, as an example, using the etendue-matching approach, raytracing simulations were carried out and are presented also in the paper. [ABSTRACT FROM AUTHOR]

Falling particle receivers have the potential to increase the maximum operating temperature of CSP systems by directly heating a solid particle heat transfer fluid. Particle abrasion in FPR systems can generate dust which can escape out of open receiver designs. The characterization and capture of this dust can help mitigate health risks and increase the optical and thermal efficiency of the receiver. Particle fines were generated and captured by fluidizing a bed of nominally sized particles and filtering out the entrained particulate from the air exiting the bed. Particle fine size distribution, composition, and rate of generation was found for a specific mass of fluidized particles using optical microscopy, SEM, XRD, and aerosol sampling equipment to better inform dust filtration in falling particle receiver systems. [ABSTRACT FROM AUTHOR]

The thermochemical process of syngas production, exploiting concentrated solar power, requires thermal energy at very high temperature, in the order of 1000-1500 °C, well beyond the maximum operating temperature of actual commercial receivers. The absorbing gas solar receiver, proposed by Synhelion SA, represents a breakthrough in the point focus solar technology allowing to operate at temperature levels higher than 1500 °C. This innovative cavity-type receiver exploits thermal radiation, as major heat transfer mechanism, for directly heating the gaseous heat transfer fluid (water vapor or carbon dioxide). Given the complexity of the physical phenomena taking place into the receiver, a CFD-based approach was followed to accurately replicate its thermo-fluid dynamics behavior under different operating conditions. In detail, a total of three CFD simulations campaigns, assuming the receiver operating at ambient pressure or at 10 bars, were performed with the aim of evaluating the effect of important parameters, such as heat transfer fluid entrance angle, gravity and realistic concentrated solar flux distribution into the cavity, on the receiver performance. An incoming concentrated solar flux on the aperture of 1.2 MW/m2 (corresponding to 600 kW/m2 on the absorptive surfaces) was assumed as reference leading to a total input power of 120 MW and 240 kW in the case of unpressurized and pressurized receiver respectively. According to the results obtained, gravity resulted to be the parameter with major influence followed by realistic concentrated solar flux distribution and heat transfer fluid entrance angle. However, a minimum receiver thermal efficiency of 66%, at about 1600 °C outflow temperature, was observed under the worst operating conditions considered indicating the reliability and robustness of this innovative receiver deign. [ABSTRACT FROM AUTHOR]

A refractory-lined particle receiver has been analyzed during its start-up under transient operating conditions. The influence of solar resource variability on the start-up time and overall thermal losses associated with particle receivers has been investigated using real-time minutely variability of solar resource on a clear sky and cloudy day. New insight is provided on the role of the refractory lining thickness on receiver's start-up time and thermal losses. [ABSTRACT FROM AUTHOR]

Solar particle receivers promise higher operating temperatures compared to molten salt-based receivers. The Discrete Element Method (DEM) tool LIGGGHTS is used to simulate the particle motion in a centrifugal solar particle receiver. A separate thermal model is developed in MATLAB, which uses the particle positions calculated with the DEM simulation. Heat transfer at the particle scale is accounted for with models of particle-particle conduction, particle-fluid- particle conduction, and a newly developed short-range radiation model. Radiation between large internal surfaces of the receiver are modeled with a view factor based long-range radiation model, and concentrated incident radiation is modeled with the Monte Carlo method. A thermal simulation for a lab-scale receiver is run for a relevant set of operational parameters, with key results being the temperature distribution and thermal efficiency of the receiver. [ABSTRACT FROM AUTHOR]

Operation strategy optimization for concentrating solar power (CSP) plants has been a long-studied topic in solar energy. In our work, an effective and systematic approach has been developed to harvest optimal operation strategies for CSP plants via reinforcement learning (RL). Our goal is to find the optimal strategy, which instructs the operation of different key operating variables of CSP plants in order to maximize daily power generation under a given solar irradiation series. Key variables are extracted through data mining techniques, and Deep Q-Network (DQN) is applied to find the optimal solution to the problem. A case study based on a 10MW central tower receiver solar thermal plant's operating data is carried out, which shows great improvement in power generation through our proposed method. [ABSTRACT FROM AUTHOR]

Concentrated Solar Technologies can be applied in the heating and drying processes in industry. This work proposes a design for a particle receiver based on rotating vibratory motors, creating an easily adjustable vibratory conveyor. This way, knowing the total mass of the conveyor we can regulate the flow rate of particles by adjusting the vibration angle. Since the main advantage of the proposed technology is the mixing of the top and bottom layers of particles we also explore the effect of different parameters in the behavior of the particles inside the bed and their influence on the mixing rate. [ABSTRACT FROM AUTHOR]

In solar gas turbines, the conventional integration of the core compressor and turbine into a single "spool" can impede optimal flow through the blade passages in the components for a given thermal energy input and load. This affects the efficiencies of the compressor and turbine components, and the performance of other components in the flow circuit that are sensitive to the pressure and mass flow rate of the working fluid, such as the solar receiver. In principle, flow compatibility between the various components in a solar gas turbine can be improved if the compressor and turbine rotor speeds can be made to adapt to changing conditions in the flow circuit. Such deconstructed configurations are used in electrically assisted turbochargers and appear in various concepts for next-generation jet engines. In this work, different variable speed gas turbine configurations are simulated, and their performance compared with that of conventional single- and two-shaft engines to quantify the improvement, if any, that can be attained. The findings show that variable rotor speeds can enable the average quantity of thermal energy harnessed by the solar receiver to be increased by 8.46 %, and the average fuel consumption in solar hybrid operation to be reduced by 15.1 % compared to the reference. The effects of the component sizes on the observations are described, limitations of the analysis are identified, and possible implications of the findings for the costs and emissions of solar gas turbine systems are discussed. [ABSTRACT FROM AUTHOR]

Power tower or central receiver systems use hundreds to ten thousands of two-axis tracking mirrors, so called heliostats, which reflect and focus the sunlight onto a receiver on top of a tower during the day. As basis for a well performing heliostat field, a single heliostat has to perform "correctly". In order to describe and measure the performance of a single heliostat, the SolarPACES Guideline for Heliostat Performance Testing has been developed by a group of R&D and industry experts during the last years. However, at the end, the performance of the whole field, which means the superposition and interaction of all heliostats determines the energy collected in the aperture of a solar central receiver. For that reason, a second guideline is currently outlined, the SolarPACES Guideline for Heliostat Field Performance Testing. Both guidelines aim to be commonly agreed protocols between R&D centers and industry in the field of heliostat performance testing. Recently, a third guideline activity has been started to improve and unify heliostat wind load design methods. The once finalized guidelines (or their concepts) should be included in international standards (e.g. IEC) and used by national organizations like DKE, AENOR, ASME, ASTM. This paper gives an update of the state of the three guidelines being developed in the SolarPACES task III-heliostat working group. [ABSTRACT FROM AUTHOR]

The current benchmark for CSP is the molten salt tower. The next generation of CSP plant should keep the general architecture of the current benchmark – namely the molten salt tower with its direct storage – but should operate at higher temperatures in order to downsize its solar field through higher efficiency. Solid particles are the best candidate today to replace the molten salt as storage medium. They are used as both storage medium and heat transfer fluid that works as an in-tube upward bubbling fluidized bed in the solar receiver. Next-CSP is a project funded by the European Union's H2020 program that aims at developing a new concept of particle solar receiver and validating it with a 1.2 MWe demonstration plant. The project also includes the study of a future utility-scale plant based on the concept in order to assess its technical and economic feasibility. This paper outlines the preliminary design of such a 150 MWe Next-CSP plant. Cost estimates were made with reasonable uncertainties to make design choices but are not dealt with in this paper. The plant is designed as a peaker that generates power during the evening. Due to inherent limitations of the concept, the solar receiver is a cavity one with a limited thermal output, which makes a multi-tower configuration mandatory. Our plant has six towers, each one with one receiver. Bucket elevators were chosen to lift the particles from ground level to the receivers. The layout of the whole solar island was optimized to minimize the cumulated length of the network that horizontally conveys the particles between the storage system and the six towers. The chosen layout, named "Vertical Star", allows for a cumulated length of 4.0 km, which is still very challenging in terms of Capex and thermal losses. Continuous-flow conveyors and proper design limit the thermal losses to 5%. The power cycle is an externally-heated gas turbine operated in combined cycle. Whilst the bottoming steam cycle is standard, the gas turbine features a double reheat in order to achieve a combined cycle efficiency approaching 50%. The heat exchangers that provide the heat from the particles to the gas turbine are numerous (ten) and bulky. To conclude, the deployment of a utility-scale Next-CSP plant can realistically be envisioned; however, some technical challenges must be dealt with carefully, especially the thermal losses of the solar receiver and the particle conveying network. [ABSTRACT FROM AUTHOR]

The control of the profile on the surface of heliostats is crucial because the mirror can be imperfectly manufactured, and the distribution of solar irradiation on the receiver is sensitive to the curvature of the surface of the heliostat. In practice, irregularities in the surface of the heliostat introduce aberrations that contribute to enlarge the spot on the receiver and the resulting energetic losses depend on the collection characteristics of the solar plant. Since the solar field can be formed by thousands of mirrors, it is essential to have a quick and economical control method to be used during manufacturing operations and successively also in the field during installation. The proposed method of optical control is based on techniques of image analysis. It assesses the heliostat surface quality by means of the examination of the acquired image in comparison with a reference image. This on-site optical control could be applied on various mirror types. The application of this profile control requires only a photographic camera and a screen with a calibrated pattern. The main advantages are ease of application, low-cost equipment, and simplicity of processing. [ABSTRACT FROM AUTHOR]

We present the design and development of a novel compact single-source high-flux solar simulator. The simulator will be used to study properties of fluids subjected to radiation-induced natural convection and volumetric heating, and for design optimization of volumetric solar thermal receivers. The simulator incorporates a pre-assembled light source unit with a custom frame made from common off-the-shelf components. The light source unit comprises a 6.5kWe Xenon-Mercury arc lamp coupled with a truncated ellipsoid rear reflector, while the frame is constructed from aluminum extrusions for simple integration of additional optical elements. The novel simulator design features a moveable light source suspended directly above the target plane. The simulator configuration allows for experimental set-ups not possible with traditional configurations, including sensitive or high-temperature experiments during which the sample cannot be manipulated, and experiments which require a variable beam diameter. The footprint of the simulator is 5' by 5', allowing the system to integrate into small lab spaces. Finally, a light-attenuating curtain was designed and characterized using a ray tracing model. Key curtain parameters were established, and the influence of the curtain on the flux distribution was characterized. [ABSTRACT FROM AUTHOR]

A fixed-bed regenerative heat exchanger has been developed as a system for enabling the thermohydraulic connection to be made between a volumetric solar receiver (non-pressurized air circuit) and a Brayton turbine (a pressurized air circuit) in the context of the CAPTure project. The design of the ceramic beds is addressed in terms of selecting ceramic block geometries and matrix sizing. The control strategy for the regenerative system is also developed, thus establishing the proper sequence for connection changes to enable the turbine to operate continuously. Both designs are approached and verified via simulation, for which detailed, dynamic bed and complete system models are developed. [ABSTRACT FROM AUTHOR]

An accurate and computationally fast ray-tracer is the key part for the simulation and optimization of the optical irradiation of a heliostat field layout in a solar central receiver system. Within this work we present an analytical ray-tracer which is fast in runtime while obtaining highly accurate results. The runtime improvement is achieved by a faster integration method that does not require a discretization of the receiver. This allows for discretizing the heliostat surface into smaller cells each having a representative flux function to better account for a variety of optical errors. Our new ray-tracer is implemented on the same C++ platform as other existing ray-tracers, such that reasonable cross validation with direct run-time comparisons are possible. Within a case study we demonstrate that the new convolution method decreases the run-time by a factor of 20 compared to HFLCal, and a factor of three compared a bidirectional Monte-Carlo ray-tracer, while achieving a stable accuracy of 99.98 %. [ABSTRACT FROM AUTHOR]

This paper describes a software tool for predicting the service life of a tubular panel solar receiver operating at elevated temperatures. The current version of the tool, available at https://github.com/Argonne-National-Laboratory/srlife as open-source software, provides full 1D, 2D, or 3D thermal and structural analysis and creep-fatigue service life prediction for a single material, Alloy 740H. Future version will include an expanded material library as well as heuristics designed to reduce the cost of the receiver analysis. The goal for this package is to become a part of a software stack, including solar field and thermohydraulic simulations, for optimizing receiver designs to meet service life and economic targets. These design and optimization frameworks could be crucial in producing viable high temperature receiver designs, operating at outlet temperatures targeting high efficiency energy conversion and storage systems. [ABSTRACT FROM AUTHOR]

Next-generation central receiver concentrating solar power systems (Gen3 CSP) are targeting operating temperatures above 700°C and use of a closed Brayton power cycle with supercritical CO2 (sCO2) as the working fluid. These systems intend to deliver greater value through improved operating efficiency, dramatic cost reductions, and improved ability to provide grid benefits. EPRI conducted production cost modeling to quantify potential system benefits for a particle-based Gen3 CSP technology developed by Sandia National Laboratories. The model explores dispatch strategies for a Particle CSP Plant and determines changes in annual system operating cost, plant revenue, and locational marginal price (LMP). Sensitivity analysis is used to identify scenarios and market conditions in which the technology is competitive, such as high penetrations of variable solar photovoltaics (PV) and wind. The model is intended to quantify the unique value proposition for the Particle CSP Plant. [ABSTRACT FROM AUTHOR]

Designing next generation concentrated solar power plant solar receivers with current metallic alloys is extremely challenging due to high creep damage accumulation under high temperature operating conditions. New structural materials with high creep resistance must be sought to achieve a desired design life of 30 years to recover the plant capital cost. MAX Phase materials are known to exhibit high creep resistance as well as high fracture toughness and therefore could be a viable option for high temperature receiver designs. This work assesses Ti3SiC2 MAX Phase as a potential candidate for high temperature receivers through evaluating the creep-rupture life of a reference molten salt receiver. Assessments are made for two different grain sizes – a fine-grained (3-5 µm) and a coarse-grained (∼ 30 µm) Ti3SiC2 MAX Phase. Results indicate a design life of several tens of years can be achieved for the reference receiver using Ti3SiC2 MAX Phase compared to less than a year design life using a high temperature nickel based alloy A740H. [ABSTRACT FROM AUTHOR]

This paper describes a framework for the design of non-metallic concentrating solar power components. The next generation of solar receivers may be required to withstand material temperatures in excess of 800(in order to increase energy conversion and storage efficiency. At these temperatures, even high temperature Ni-based alloys may not provide practical, economic designs. Ceramics and other non-metallic materials have the required strength at these temperatures, but component design approaches for these materials must consider a different set of failure modes and mechanisms, when compared to metals. This paper outlines existing, applicable design methods and provides a draft set of discriminatory categorization tests aimed at placing non-metallic materials in categories with well-established existing design methods. However, the paper notes the existence of a category of high temperature materials – brittle or quasi-brittle materials with time-dependent strength – where there are no well-established design rules. A statistical design process for these types of materials is outlined here, though developing this outline into a practical design method will require substantial additional work. [ABSTRACT FROM AUTHOR]

In order to solve the basic problem of the supercritical carbon dioxide (S-CO2) Brayton cycle integrated with solar power tower (SPT) station which used solid particle solar receiver (SPSR), a new extremum-seeking control method was applied by manipulating the particle and S-CO2 mass inventory in the integrated system. By designing different system operation modes and corresponding control logic, then using this control method to carry on the dynamic simulation. The results show that power generation in summer and winter solstice by extremum-seeking control is improved by 2.1% and 1.6%, respectively. The results could be references for control strategies as well as the safe operation of the system. [ABSTRACT FROM AUTHOR]

The low cost fabrication of thermally stable solar coatings having high photo-thermal performance through high deposition rate processes represents one of the most demanding challenges in linear focusing Concentrated Solar Power (CSP) technology. Since 2005, ENEA has been developing and patenting solar coatings suitable for medium and high temperature applications based on the technology of double nitride cermet. The absorber layer of these coatings is characterized by a graded multilayer cermet of tungsten nitride and aluminium nitride (WN-AlN), while the ceramics of the antireflection filter are constituted by aluminium nitride (AlN) and silica (SiO2). The technology employed to deposit the ceramic component of all cermet materials, as well as of the antireflection filter, is reactive magnetron sputtering in poisoned mode, which does not allow obtaining the best trade-off between deposition rate and energy consumption. In order to improve the economic feasibility of linear focusing CSP solar plant, a cost-effective process was developed in the present work to produce spectral selective coatings for solar receiver tubes. To achieve this objective, the reactive magnetron sputtering technology in transition mode was applied to deposit all ceramic constituents of solar coatings. In detail, the antireflection ceramics were deposited through "dual magnetron" reactive sputtering in transition mode with Medium Frequency (MF) supply and using a Plasma Emission Monitoring (PEM) control system to maintain unchanged the target conditions during the process. It was found that, when using this technology in place of that in poisoned mode, the deposition rate improves by 1.6 and 2.3 times for AlN and SiO2, respectively. Regarding the graded multilayer cermet, while the ceramic component was deposited by "dual magnetron" reactive sputtering in transition mode with MF supply, the metallic component was deposited by "standard magnetron" sputtering with Direct Current (DC) supply. The process control through PEM system was simplified in this case by introducing N2 only from the gas-ring around the Al targets. Since in this configuration the reactive gas flow was not sufficient to promote the formation of WN as metallic component inside the cermet, the double nitride cermet WN-AlN was replaced by W-AlN. A fast procedure was developed to grow the W-AlN multilayer cermet by employing only one hysteresis curve to control the deposition process and grow the cermet layers in sequence. Specifically, this method presents the advantage that it is not necessary to stop and re-start PEM control, power supply and gas injection between the deposition of consecutive cermet layers. The fast procedure was applied to fabricate the absorber layer of the solar coating allowing to obtain a 44% reduction in the energy consumption by replacing WN with W as a metallic component of the multilayer cermet. The coating was subjected to a stability study at high temperature including the estimation of thermal degradation of the photo-thermal parameters after heat treatments under vacuum (2·10−2 Pa) for a total duration of 33 days at the temperature of 620 °C. The results revealed that no appreciable modification occurred in terms of solar absorptance (αs), while a very small increment of thermal emittance (εth) was evaluated at 400°C, thus demonstrating the excellent thermal stability of the produced coating. [ABSTRACT FROM AUTHOR]

As part of a DOE Small Business Innovative Research (SBIR) project, CTP explored options to enhance the solar optical properties of its solar receiver tube. Sandia National Laboratories (Sandia) working with CTP, performed high-flux solar simulator tests on several receiver-tube specimens. The results demonstrated the excellent durability and optical performance of SiOC composite receiver tubes at 800°C surface temperatures. [ABSTRACT FROM AUTHOR]