Solar Receiver Research Articles

High-temperature corrosion of a nickel-based alloy in a molten chloride environment – The effect of thermal and chemical purifications

There is currently an ever-increasing demand for higher process efficiencies in next generation (Gen3) concentrating solar power (CSP). Higher process efficiencies may be procured by increasing the operating temperature, and simultaneously, minimizing the degradation of materials used for construction of CSP plants (e.g., piping, thermal storage tanks, solar receivers and heat exchangers). Thus, understanding materials corrosion in the presence of molten salt mixtures used as thermal energy storage media and heat transfer fluids is indispensable for CSP development. The present paper provides insights into the effects of salt purification on the corrosion of a nickel-based alloy (Haynes 230) isothermally exposed to a stagnant chloride-based salt mixture at 800 °C. The MgCl 2 -based salt mixture was thermally dehydrated and chemically treated with (0.1 and 0.5 wt %) elemental magnesium. Results reveal the electrochemical nature of the corrosion process, and the formation of corrosion products such as oxides (MgO, MgCr 2 O 4 , and MgAl 2 O 4 ) and nitrides (CrN) on the alloy surface and in sub-surface regions. Magnesium additions enhanced the ability to resist corrosion by reducing the concentration of impurities (H 2 O, MgOH + , OH − species) and polarizing the alloy surface. The formation of nitrides in all cases studied indicates the impact of using nitrogen as a protective gas in the system. Results also reveal that a single step treatment of the salt using metallic Mg could be considered as a measure to control the salt's impurity level, e.g., if required for system control. • Haynes 230 was exposed to several unpurified and purified MgCl 2 -based salt at 800 °C. • The corrosion behavior was systematically investigated using several analytical tools. • Abundant evidence for Cr- and Al-rich spinel and nitride formation was documented. • Possible formation mechanisms for the principal corrosion products were discussed. • Single-step purification of the salt substantially reduced the corrosion rate.

A New Multichannel Parallel Real-time FFT Algorithm for a Solar Radio Observation System Based on FPGA

The real-time fast Fourier transform (FFT) is the essential algorithm for signal processing in a solar radio receiver. However, field-programmable gate array (FPGA) computation resources have become the limitation of real-time processing of signals with increasing time and spectral resolutions. It is necessary to design a real-time parallel FFT algorithm with reduced resource occupation in the development of future receiving systems. In this paper, we developed a multichannel parallel FFT algorithm named the multichannel parallel real-time fast Fourier transform (MPR-FFT), which can greatly reduce FPGA resource occupation while increasing the real-time processing speed. In this algorithm, the 4L simultaneous N-point FFTs are first converted into L simultaneous 4N-point FFTs. Fusion processing is then performed to obtain the 4 ∗ L ∗ N-point spectrum. This method has been used in developing a solar radio spectrometer, which works in the frequency range of 0.5–15 GHz in the Chashan Observatory. In this spectrometer, 16 channel MPR-FFT with 8k-point data is realized in a Xilinx UltraScale KU115 FPGA. The MPR-FFT algorithm reduced the computational resources to a large extent compared to the Cooley-Tukey-based parallel FFT method; for instance, the Look-Up-Table, Look-Up-Table RAM, Flip-Flop, and Digital Signal Process slices were reduced by 37%, 50%, 17%, and 2.48%, respectively. Although the MPR-FFT consumes 14 block RAM resources more than the Cooley-Tukey-based parallel FFT, the MPR-FFT algorithm presents an overall reduction in resource usage.

Heat generation depth and temperature distribution in solar receiver tubes subjected to induction

Induction heating is commonly used in laboratory-scale facilities to replicate the heating conditions of the receiver tubes of concentrated solar power plants. This work aims at shedding light at the induction heating characteristics for such applications through the development of a multiphysics numerical model capable of replicating the experimental conditions of a molten salt loop locally heated by an induction heater. In the experiments, a stainless steel pipe is heated on its external surface by the induction heater, which is switched on and off during the experimental data acquisition while molten salts are continuously circulating in its interior. These conditions are replicated, for the first time, in a two-dimensional numerical domain fully coupling the electromagnetic and thermal physics, including thermally dependent material properties of the heated pipe. Once validated against the experiments, the numerical results revealed that the volumetric nature of the induction heating shall be considered for an accurate representation of the temperature profile inside the tube. As a novelty, different equivalent surface boundary conditions are presented and, despite the Gaussian-like behavior of the induction heating on the surface of the tube, the results indicate that there exists no equivalent wall boundary condition to fully replicate the temperature profile obtained with the induction heater. The effect of independently varying experimental parameters such as the geometry of the pipe (i.e., diameter and thickness) and its distance to the induction heating system is also evaluated. Using large diameters of the tube reduces the difference between the angular temperature profile obtained using induction heating and a simplified wall boundary condition. For small wall thicknesses, the induction heating is capable of penetrating along the whole thickness of the tube, the total heat generated in the volume of the tube being exposed to the counteracting effects of the volumetric generation and the enhancement of the heat dissipation by the molten salt, as both of them increase for small thicknesses. The distance of the inductor to the pipe wall appears to maintain the volumetric characteristics of the heating and only affects the induction heating magnitude and efficiency. • Fully coupled model of induction heating of a molten salt pipe is presented. • Induction promotes a volumetric heat generation in the pipe. • No equivalent surface heating can replicate induction heating. • Effect of different pipe geometries on the induction heating was characterized. • Induction reaches the whole pipe thickness for sufficiently thin tubes.

Numerical analyses of the concentrated solar receiver pipes with superheated steam

In this paper, numerical analyses on a concentrated solar receiver pipes have been performed. The fluid is considered to be superheated steam phase. The pressure drops and temperature distribution in the absorber tubes of the parabolic trough solar collector (PTSC) have been explored in detail. In the numerical studies, the experimental data received from the solar test facility located at the Plataforma Solar de Almeria, Spain has been used for the real world application in order to shed a light on the superheated phase of the plant. The RNG k-ε turbulence model is considered for the Computational Fluid Dynamics analyses. In the explorations, especially the fluid behavior at different mass flows has been taken into account. The process has been evaluated for different temperate T and pressure P cases such as Case 1: p = 3.21 MPa, T = 240.4°C and Case 2: p = 10 MPa, T = 500°C. According to the results, the numerical results have been in good agreement within the error rate of 0.79% for the experimental temperature measurements. Therefore, the proposed numerical technique can be generalized for different steam parameters of PTSC in order to estimate the steam characterization for the experimental facility.

Numerical Analysis of Central Solar Receivers with Various Geometries

Concentrated solar power (CSP) is a cutting-edge method of conserving renewable energy. The concentrated solar power is utilized as a heating source to increase the temperature of heat transfer fluid circulating in the piping of the central solar receiver. The solar central receiver is the most crucial part in solar tower power plants. In this study, a Computational Fluid Dynamics (CFD) framework was developed for analyzing four designs of the central tower receiver, namely, a conventional uniform tube diameter solar receiver (UTD), vertical variable tube diameter solar receiver (VTD), a circular solar variable tube diameter (CVTD) receiver and a leaf type circular solar receiver (LTSR). This analysis studied the solar radiation heat transfer efficiency, temperature distribution, and fluid outlet temperature; pressure and velocity distributions for the designs using CFD. It was found that the CVTD design helped achieve a higher rise in temperature of the heat transfer fluid (HTF) when the mass flow rate was in the range of 0.1 to 0.2 liter per minute. The CVTD and LSTR models of receiver were more efficient heat transfer receiver designs compared with other designs for same surface area and strength of beam radiations.

Direct measurements and prediction of the particle egress from a vortex-based solar cavity receiver with an open aperture

We report direct, in-situ measurements and numerical predictions of the normalised particle egress from a vortex-based particle solar receiver with an open aperture under iso-thermal conditions. These represent an important advancement toward meeting the challenge of developing both a suitable configuration and control strategy for operation of an open-to-atmosphere vortex reactor without significant particle egress. This control strategy induces a net inflow through the aperture by over-ventilating the outlet port as means to mitigate particle egress (here polymethylmethacrylate particles) for one configuration of the device. The influences of the level of over-ventilation, the Froude number and the Stokes number on the overall normalised particle number from the receiver were investigated using a planar laser-based Mie scattering method and computational fluid dynamics. It was found that, although this control strategy can be configured to mitigate particle egress from the vortex-based particle solar receiver with an open aperture, this will require significant over-ventilation of the cavity for the present configuration. The sensitivity analysis also found that slight over-ventilation is the most significant factor on controlling particle egress while significant over-ventilation is less effective considering the impact on thermal performance and the demand of fan power. This represents an important step toward the development of a windowless reactor, although further development of the configuration is needed to achieve efficient mitigation.

Optical and thermal integration analysis of supercritical CO2 Brayton cycles with a particle-based solar thermal plant based on annual performance

Central receiver concentrating solar power (CSP) plants based on particles as heat transfer fluid in solar circuits and supercritical CO2 (S–CO2) Brayton cycles can fulfil the requirements for next generation CSP to improve solar-to-electric efficiency and reduce energy storage costs. However, effective incorporation of these two concepts requires an in-depth understanding of their characteristics and an appropriate approach to match them. This paper addresses the importance of the design features and annualized performances of the optical subsystem (heliostat-receiver) and the thermal-to-electricity subsystem (solar receiver-energy storage-power block) on the global optimization of any integrated CSP plant. The analysis lies in a complete model of a particle-based CSP plant, which includes detailed modeling for the solar field, a cavity solar receiver with an up bubbling fluidized bed (UBFB) tubular panel, particles storage tanks and a recompression S–CO2 Brayton cycle. The design incident irradiance on the receiver aperture (IR) and the particles temperature at the receiver outlet (Tp) are identified as key parameters determining the solar-to-electric integration procedure and affecting the overall plant design and annual performance. Regarding subsystems located upstream and downstream of the receiver, the effects of heliostat and power block characteristics on the optimal IR and Tp are also evaluated, represented by the heliostat beam quality and main compressor inlet temperature. Results show that IR around 1,200–1,500 W/m2 provides the maximum system design efficiency and annual efficiency. Improvements on heliostat beam quality and power block efficiency help to increase the optimal IR and overall system efficiency. In the optimal range of IR, increasing Tp leads to higher system design efficiency, but lower system annual efficiency and annual electricity output. The optimal combination of IR and Tp contributes to a minimum heliostat design area, representing the integration trade-off between the system optical and thermal characteristics.

Pumped thermal energy storage systems integrated with a concentrating solar power section: Conceptual design and performance evaluation

A novel Pumped Thermal Energy Storage (PTES) system thermally integrated with a Concentrating Solar Power (CSP) plant is proposed and investigated. The two sections operate with the same working fluid, share several components and can operate simultaneously or independently of each other. A Thermal Energy Storage (TES) system composed of three thermocline packed-bed tanks is included. Specific mathematical models were developed to simulate the performance of the integrated PTES-CSP plant under nominal conditions and to evaluate the thermal profiles of the TES tanks. As a case study, an integrated PTES-CSP system characterized by a nominal power of 5 MW with a nominal storage capacity of 4 equivalent hours was considered. The influence of the main design parameters, namely the pressure ratio and the operating temperatures of the TES system, on the main performance indices was investigated. The results demonstrated that the exergetic roundtrip efficiency of the integrated plant reaches a maximum for a pressure ratio of about 5.2. A sensitivity analysis on the solar receiver temperature and turbomachinery polytropic efficiency and maximum allowable temperatures was also conducted. Finally, a feasible design for the PTES-CSP system characterized by an exergetic roundtrip efficiency of about 60% was proposed.

The heat transfer coefficient associated with a moving packed bed of silica particles flowing through parallel plates

Concentrating Solar Power (CSP) with thermal energy storage has the potential to be a renewable energy technology with long duration, inexpensive energy storage. Higher temperature operation increases the conversion efficiency and reduces the cost of energy storage. Several emerging CSP designs utilize particles as the solar receiver due to their high temperature stability and low cost. In some designs, the particles are also used as the thermal energy storage (TES) media. In either configuration, an energy transfer is required between the hot particles and the working fluid in the power cycle in a Particle-to-Fluid Heat Exchanger (PtFHX). Understanding the heat transfer between a moving packed bed and a stationary surface is critical to the successful design of a PtFHX.In this paper, a test facility is described in which a moving packed bed of silica sand with particle size 100–600 μm is introduced into the channel formed by two parallel plates, one of which is heated. The effective static thermal conductivity of the particles used for the test are separately measured over the entire range of test temperatures. The inlet and outlet bulk temperatures of the particle flow are measured as are the surface temperatures at several axial locations along the centerline of the plate. The result is the measurement of heat transfer coefficient as a function of temperature for several velocities. The uncertainty of the measurements is presented and the results are compared to model results found in the literature.

Comparison of different safety concepts for evaluation of molten salt receivers

Solar power tower plants are a promising technology for renewable energy supply, because of their base load capability. Nevertheless, this type of power plant is in competition with various other technologies. In order to ensure the competitiveness of power tower plants, there are several approaches to reduce investment and operating costs.Huge amounts of these costs are defined in the lifetime calculation of the solar receiver. For this purpose, based on ASME BPVC Section III Division 5 (Nuclear Code), a number of safety concepts have been developed in the last decades, which are adapted to solar applications using reduced conservatism. However, a generally established approach has not yet been developed. Therefore, this study introduces a novel approach that makes it possible to directly compare the different safety concepts on the level of permissible strains and stresses. The comparison with these so-called interaction maps illustrates the range of variation of the allowable strains (factor of 1.5) and stresses (factor of 2.2), depending on the applied safety concept. Ultimately, a preferred method is identified that is both easy to apply, since it relies on generally available material data, and economically advantageous compared to the approach defined in the Nuclear Code.Additionally, it is demonstrated that the interaction maps can also be used for preliminary design of the receiver components. A change of the absorber material from the currently used nickel-based alloys to the significantly more cost-effective stainless steels offers options for an economically advantageous design, especially in the regions of low receiver temperatures.

A particle receiver-driven thermochemical cycle employing elemental sulphur for solar thermochemical energy storage: Investigation of particles as concentrated sunlight harvesting media and sulphur trioxide splitting catalysts

The present work is part of an approach targeted on demonstrating a concentrated solar energy-driven thermochemical cycle employing solid elemental sulphur as a chemical energy vector and a seasonal solar energy storage medium. In exploring the concept of coupling this cycle to a centrifugal particle solar receiver, bauxite-based particles, commercial and composition-modified, were tested and validated with respect to their suitability for simultaneous operation as solar energy harvesting media and sulphur trioxide splitting catalysts.Long-term catalytic activity tests exceeding 1000 h of operation at 850 °C, demonstrated that properly composition-modified bauxite particles were capable of achieving stable SO3 conversion exceeding 70% (about 80% of equilibrium value under conditions studied) and exhibiting no or negligible loss of catalytic performance after this extended operation. In addition, such composition modification enhanced the particles’ solar irradiation absorptance, which resulted in substantially higher than that of their commercial, non-modified bauxite counterparts. Commercial bauxite particles on the other hand, demonstrated much better mechanical strength, flowability, and attrition- and thermal shock resistance together with measurable, yet lower catalytic activity, which deteriorated with prolonged on-stream exposure. The combined results obtained advocate for the eventual selection of an “allothermal” cascaded sulphur trioxide splitting/sulphuric acid decomposition reactor, containing a non-moving catalytic bed that is heated indirectly by a moving bed of high-temperature (≥900 °C) non-modified commercial bauxite particles irradiated in the centrifugal solar particle receiver.

Thermodynamic and economic analysis of a novel combination of the heliostat solar field with compressed air energy storage (CAES); a case study at San Francisco, USA

In spite of the various important features of the compressed air energy storage (CAES), this technology suffer from some environmental effects because of the burn of fossil fuels in the combustor that reduces its widespread use. To address this problem, a novel green and effective concept based on the combination of the CAES with the concentrated solar power and absorption chiller is introduced. This hybridization not only helps the further development of both the CAES and solar sites as a type of renewable energy, but it also simultaneously generates power, cooling capacity, and hot water that are the main components for reducing peak energy demand, especially in hot climates. Thermodynamic analyses revealed that this combination results in a round trip efficiency and an exergy efficiency of 67.5 and 45.6%, respectively. In addition, solar receiver tower, heliostat field, and pressure regulator are the equipment with the highest exergy destruction. To approve the economic feasibility of the proposed system, a precise economic analysis was done for the case study of San Francisco, USA, concluding that the system has a payback time of 1.7 years and profitability of $231 M during the 30 years of its service time.

Thermal performance and thermal stress analysis of a supercritical CO2 solar conical receiver under different flow directions

To ensure the safe and efficient operation of the receiver in the concentrated solar power system, it is necessary to study its performance characteristics. Different from considering the thermal performance alone, this study comprehensively analyzed the thermal and mechanical performance of the solar receiver by using a coupled thermal-mechanical model. The critical factors including geometric parameters (aspect ratio H/D and angle ratio φ/φmax), operation parameters (mass flow rate, inlet temperature and direct normal irradiance (DNI)) and flow directions (up-flow and down-flow) were investigated. The results demonstrated that the optimum thermal efficiency achieved 85.67% when H/D = 1.5 and φ/φmax = 0.25, and the maximal thermal stress was 87.61 MPa. Increasing the flow rate from 0.01 kg/s to 0.06 kg/s improved the thermal performance, but increased the thermal stress. An opposite trend was observed when the inlet temperature increased from 573 K to 773 K. As the DNI increased from 200 W/m2 to 1000 W/m2, the maximal thermal stress increased by approximately 3 times, when the DNI >800 W/m2, the thermal performance would be deteriorated. Although the thermal performance under up-flow was better, the thermal stress was greater, the receiver should adopt an up-flow flow direction.

Comparative Analysis of Rankine Cycle Linear Fresnel Reflector and Solar Tower Plant Technologies: Techno-Economic Analysis for Ethiopia

The need to meet the world’s growing demand for energy in an environmentally sustainable manner has led to the exploration of various renewable energy (RE) resources for power generation. The objective of this study is to examine the techno-economic potential of concentrated solar power plants (i.e., linear Fresnel reflector (LFR) and central receiver system (CRS) for electricity generation in Eastern African countries with a case study on Ethiopia. The study was conducted using the System Advisor Model (SAM). In order to estimate the economics of the two power plants, the Levelized cost of energy (LCOE) and the net present value (NPV) metrics were used. According to results obtained from the simulations, the LFR produced annual energy of 528 TWh at a capacity factor (CF) of 60.3%. The CRS also produced a total of 540 TWh at a CF of 61.9%. The LCOE (real) for the CRS is found to be 9.44 cent/kWh against 10.35 cent/kWh for the LFR. The NPV for both technologies is found to be positive for inflation rates of 2% and below. An inflation rate above 2% renders the two power plants financially impracticable. A real discount rate above 9% also renders both projects economically unviable. Based on the obtained results, the CRS system is identified as the best technology for electricity generation under the Jijiga climatic condition in Ethiopia.

Obtaining Titanium–Nickel Cermets on a Solar Furnace and Creation of Selectively Absorbing Coatings Based on Them for Solar Heat Receivers

Obtaining Titanium–Nickel Cermets on a Solar Furnace and Creation of Selectively Absorbing Coatings Based on Them for Solar Heat Receivers

Image Analysis of Particle Flow in Centrifugal Solar Particle Receiver

Abstract Particle solar receivers promise economical and operational advantages compared to the molten salt-based solar receivers. In this study, an experiment is designed to observe the particle flow characteristics in the Centrifugal Particle Solar Receiver. A set of experiments for various receiver rotational speeds and particle mass flowrates is conducted, and experimentally obtained raw results are post-processed by means of an Image Processing Routine based on 4BestEstimate algorithm. The axial advance of the particles in one receiver rotation, the particle film thickness, and the ratio of the length of the stationary zone to the receiver circumference is measured to be later used in the validation study of the discrete element method-based numerical model.

A simple analytical thermal model of solar cavity receivers

This article presents a closed form analytical solution to estimate solar receiver surface and fluid temperatures. An approximation and its domain of validity (in term of the value of a small parameter) are also proposed. These simple models are then applied to a large and a small cylindrical cavity. Finally, the model is applied to an experimental hemispherical coiled cavity.

CFD Assessment of Circular and Spiral Recipients in CSP Plants

The primary goal of this research is to create a new billboard-style central spherical solar receiver with varying header and tube diameter, for which two spiral central receiver designs and a new circular central solar receiver with varying pipe radius and varying mass circulation are utilized. The exhaust temperature of the water was determined employing a computational fluid dynamics assessment employing ANSYS Fluent. Following the verification of the standards models, a novel central solar receiver project was made that accomplishes computational fluid dynamics assessment with almost the same boundary condition.The heat export fluid is water, and the spiral coil tube is copper. The mass circulation rate varies among 0.1 LPM, 0.2 LPM, and 0.3 LPM, with the inlet temperature of water being 299K at continuous solar irradiation. The findings demonstrate that at 0.1 LPM, the redesign of the central solar receiver produces a temperature of 383.9 K, which is 28.39 percent higher than the inlet temperature and 7.32 percent higher than the spiral central receiver. As a consequence, the circular central solar recipient to varying pipe radius is suggested and considered in upcoming usages

Heat Transfer Enhancement in Parabolic through Solar Receiver: A Three-Dimensional Numerical Investigation.

Parabolic trough collectors (PTC) are one of the most established solar concentrating systems which have been used in a wide variety of applications. Enhancing their performance is critical to establish them as a viable technology. Internal obstacles are an intriguing way for improving the collector’s performance. However, the usage of obstacles results in increasing pressure loss. The purpose of this research is to numerically explore the impact of introducing obstacles to the receiver tube of a parabolic trough collector on heat transmission in PTCs and its overall thermal performance. The first part analyzed the effects of geometrical parameters, orientation angle (α = 45°, 90° or 135°), and spacing of obstacles (P/D = 1, 2, or 3) on the fluid motion, heat transfer, and performance. Then, a non-uniform heat flow was applied to the absorber’s outer surface. The effects of nanoparticles type, temperature profile, and heat transfer performance of three different nanofluids (Cu/thermal oil, Al2O3/thermal oil, andTiO2/thermal oil) were studied in the second part. The simulation results show that, the friction factor increased when P/D decreases, and that the absorber tube with obstacles discs (α = 90°) and P/D = 2 achieved the best thermal performance. Additionally, increasing the concentration of solid nanoparticles in thermal oil improves heat transmission, and the Cu nanofluid has the greatest Nusselt number.

A comparison between lumped parameter method and computational fluid dynamics method for steady and transient optical-thermal characteristics of the molten salt receiver in solar power tower

Lumped parameter method (LPM), which neglects the change of the temperature within the cross-section of the molten salt, is widely used to predict solar receiver's steady and transient optical-thermal characteristics. To consider the influences of the temperature variation inside molten salts on the performance of solar receivers, the receiver's steady and transient thermal performance are comparatively studied by computational fluid dynamics method (CFD) and LPM. The results show that the rise time of the outlet temperature predicted by the CFD model under the start process, the change of mass flow rate, and the change of direct normal irradiance are more than two times those of the LPM model. Take the start process on spring equinox as an example, the rise time predicted by the CFD and LPM model are 52s and 22s, respectively. Therefore, the temperature distribution within the molten salt should be considered for predicting a more precise transient thermal performance of the receiver. The predicted outlet temperatures in the steady state by the two models are nearly the same, with an absolute difference less than 2.0 °C, which indicates that whether considering the temperature distribution within the molten salt does not influence receiver's steady thermal characteristics.