Solar Receiver Research Articles

Analytical Determination of Nusselt Numbers for Convective Heat Transfer Coefficients in Channel Macroporous Absorbers.

This article introduces a novel analytical equation for computing the Nusselt number within the macroporous structures of channel absorbers utilized in high-temperature solar receivers. The equation incorporates heat and mass transfer processes occurring within boundary layers as fluid flows through complex-shaped macroporous absorber channels. The importance of accounting for the length of the thermodynamic boundary layer within channel-type macroporous media when calculating heat transfer coefficients using the Nusselt equation is demonstrated. By incorporating proposed indicators of porosity and flow characteristics, this method significantly enhances the accuracy of heat transfer coefficient calculations for such media. Discrepancies observed in existing calculation relationships and experiments are attributed to the omission of certain proposed values in the Nusselt number for macroporous media. To address this, empirical coefficients for the Nusselt number are derived using statistical methods. The resulting semi-empirical equation is applied to macroporous absorbers in solar receivers. The findings enable more accurate predictions of future absorber characteristics, enhancing their efficiency. The derived equation is successfully validated against numerical data across various geometric structures of absorbers in concentrated solar power plants.

Single-Source VLCP System Based on Solar Cell Array Receiver and Right-Angled Tetrahedron Trilateration VLP (RATT-VLP) Algorithm

A significant deployment limitation for visible light communication and positioning (VLCP) systems in energy- and light-source-restricted scenarios is the reliance of photodetectors (PDs) on external power supplies, compromising sustainability and complicating receiver charging. Solar cells (SCs), capable of harvesting and converting environmental light into electrical energy, offer a promising alternative. Consequently, we first propose an indoor VLCP system that utilizes an SC array as the receiver, alongside a right-angled tetrahedron trilateration visible light positioning (RATT-VLP) algorithm based on a single light source and multiple receivers. The proposed system uses an SC array in place of PDs, utilizing binary phase shift keying (BPSK) signals for simultaneous communication and positioning. In experiments, we verified the system’s error-free communication rate of 1.21 kbps and average positioning error of 3.40 cm in a 30 cm × 30 cm area, indicating that the system can simultaneously satisfy low-speed communication and accurate positioning applications. This provides a viable foundation for further research on SC-based VLCP systems, facilitating potential applications in environments like underwater wireless communication, positioning, and storage tank inspection.

A study of electro-magneto Darcy-Forchheimer Casson nanofluid flow under the effects of Cattaneo-Christov double diffusive model over linear vs nonlinear stretching sheets

The present study reveals that the two-dimensional incompressible Casson nanofluid saturated the porous media in the presence of Darcy-Forchheimer medium over a non-linear sheet stretching. It has immense practical applications in engineering, like paper production, solar receivers, aluminous plate cooling, plastic sheet extrusions, petroleum resources, soil physics, etc. Here, non-linear stretching is supposed to be the driving force. The electro-magnetized Cattaneo-Christov double diffusion model was imposed to enhance the heat and mass transfer along with the influences of ist order chemical reactions, Brownian diffusion, thermophoresis motion, internal heat source viscous-ohmic dissipation, and thermal radiation. The electro-magnetic impacts are uniformly normal to the direction of flow. The governing flow model, a system of highly coupled and non-linear partial differential equations, is converted into ordinary differential equations imposing appropriate similarity transformations, and solved analytically (differential transformation method via MAPLE) and numerically (Bvp4c via MATLAB). The final results are tabulated and demonstrated graphically for different control parameters. In addition, the contour graphs have been prepared. For engineering and industrial importance, a significant difference can be observed in the enhancement of heat and mass transfer through temperature and concentration profiles and tables of Nusselt as well as Sherwood numbers. Through histograms, it is concluded that the nonlinear stretching sheet provides a better heat and mass transfer rate as compared to the linear stretching sheet. The rate of heat transfer in the linear vs nonlinear stretching sheet is 25–75%, but the rate of mass transfer is 30–70%.

Pseudo-transient flow path optimization of a solar tower receiver operating with supercritical carbon dioxide

Aiming to further promote the decarbonization of energy production processes, this article simulates and optimizes the pseudo-transient behavior of a cylindrical solar receiver subjected to a spatially varying concentrated heat flux. The model employed, which accounts for its thermal–hydraulic behavior, uses supercritical carbon dioxide (s-CO2) as the heat transfer fluid and is based on a literature-validated hybrid formulation that axially discretizes the tube of the receiver and simultaneously solves the 2-D temperature distribution within the cross-section of each node. The operational conditions imposed on the receiver were determined such that it can produce 10 MW in a recompression Brayton cycle. Also, the receiver’s surface was exposed to a spatially variable concentrated solar boundary condition based on the direct normal irradiance (DNI) of a representative day for each month of the year, which was selected from the typical meteorological year (TMY) in Seville (Spain). The results showed that the combined thermal–hydraulic efficiency of the receiver increases with the number of panels and that the mass flow flowing through each symmetrical side of the receiver should not necessarily be identical for achieving maximal efficiency. More importantly, the results showed that by simply changing the location of the s-CO2 inlet and outlet ports, one can increase the efficiency of the receiver by about 3 % while considering the exact same concentrated solar irradiation.

Analysis of Heat Transfer of Molten Salts Startup Flow in Cold Pipes Avoiding Freezing in Solar and Nuclear Energy Systems

Abstract Molten salts are employed as the heat transfer fluid to carry the thermal energy from a solar receiver or a nuclear reactor for delivering to thermal storage systems or thermal power plants for power generation. For the startup operation, molten salts need to be pumped to flow into the pipes which may have lower temperature than the freezing point of molten salt due to the cold ambient temperature overnight or over the suspension of operation. Preventing the freezing of molten salt in cold pipes becomes a critical issue to the safe operation of a concentrating solar thermal power plant or a molten salt nuclear power plant. This study conducted a basic heat transfer analysis of the transient heat transfer from flowing molten salt to a cold pipe to determine the length from entrance to the onset of freezing of the fluid. From our modeling and analytical solution using method of characteristics, the correlation of the location of onset of freezing of molten salt with respect to the flow velocity, heat capacities of molten salt and pipes, dimension of the pipes, and the initial temperatures of salts and pipes, have been understood clearly. The modeling and computational tool can fundamentally help engineers to design a system to avoid freezing and clogging at cold startup when molten salt is applied as a heat transfer fluid.

Pathway to Multi-Response Characterization of the Improved Elliptical Vessel Solar Receiver for Efficiencies Maximization

Studies on the pathway to multi-response characterization of the improved elliptical vessel solar receiver for environmental sustainability has been studied. The materials were sourced based on categories of components element: support mechanisms made of mild steel plates, bolts, nuts, clamps, and water as heat transfer fluid. The reflector was made of aluminum foil tape while the vessel has a glass cover fitted with bolts and nuts, the receiver is made of copper pipe, aluminum pipe, galvanized iron pipes, and stainless steel pipes. They are fitted into the vessel with chlorinated polyvinyl chloride 3⁄4 pipes, and journal-bearing mechanisms. Furthermore, glass cover attachment reduces radiative heat loss coefficient by eliminating wind influence and increases heat flux inside the vessel thereby improving heat transfer, hence improving the overall system's efficiency. The pathway to multi-response characterization showed that the average experimental thermal efficiency rose from 9.83% to 12.55% and from 4.42% to 7.03% for Polyurethane coated Copper and Aluminum respectively. It reduced from 9.83% to 8.53% and from 8.10% to 6.50% respectively for Polyurethane coated Galvanized Iron and Aluminum. This depicts the gleam appearance of Polyurethane coating on Galvanized Iron and stainless steel thus reducing their heat absorption coefficient and in turn reducing their efficiency.

Enhanced thermal efficiency of solar liquid tin receiver with carbon black-reinforced carbon nanotube absorber

Carbon nanotube (CNT) heat absorbers were developed to enhance the direct solar heating of liquid tin as a heat-transfer fluid in ground solar receivers. To improve the thermal transfer of absorbed solar energy to tin, absorbers reinforced with carbon black (CB) were fabricated by compression molding. As the compressive strength increased, the CNT absorber formed denser nanotube arrays, which increased the thermal conductivity by 1.68 times. Increasing the CB content further enhanced the conductivity by 1.19 times through grape cluster-like structures. The thermal properties of CNT absorbers predominantly depend on the compressive strength and CB content, and a correlation is proposed for predicting the conductivity. The most effective absorber was prepared at 50 kPa with a CB/CNT ratio of 1.0. The heat absorption characteristics of the solar receivers (50 mm i. d., 60 mm high) with the prepared CNT absorbers were determined. Tin temperatures with CNT absorbers rose more steeply than in bare tin systems, exhibiting a higher temperature increase per irradiance. The efficiency of the bare tin system improved from 24.8 % to 45.4 % using absorbers at 50 kPa and to 46.4 % using a CB/CNT ratio of 0.75 at 20 kPa. Under wind-free conditions, the receiver with the highest thermal conductivity achieved 67.7 % efficiency, a 1.69-fold increase over a tin-only receiver. This study confirmed that the improved thermal conductivity of the CNT absorbers significantly enhanced the receiver efficiency by facilitating rapid solar-energy transfer to liquid tin.

Numerical and experimental analysis of a cross-finned solar receiver for parabolic dish collectors

A parabolic dish solar collector (PDC) is used to concentrate solar radiation on a receiver to produce hot water or steam for various thermal applications. The solar receiver of the PDC is one of the significant components to improve the overall efficiency of the PDC. The present study is a numerical and experimental investigation of the thermal performance of a solar receiver with vertical fins with and without cross fins. The fin orientation was investigated using 0.056 kg/s, 0.083 kg/s, and 0.111 kg/s water flow rates. COMSOL Multiphysics® was used to simulate the receiver to determine the receiver’s temperature behavior. The thermal performance parameters such as heat gain, heat loss, convective heat transfer coefficient, and thermal, and exergy efficiency were analyzed under various flow conditions. The cross fins effectively prolong the water residence, increasing turbulence, and heat transfer and reducing heat loss to the surroundings. The receiver with cross fins at higher flow rate conditions shows higher thermal performance than lower flow rate and without fins conditions. The solar radiation during the outdoor testing is in a range of 300–950 W/m2 and the ambient temperature varies from 30-37 °C. The peak average thermal and exergy efficiency obtained for the cross-finned receiver at 0.111 kg/s is 64.4 %, and 6.3 %, respectively. Cross-finned receiver operating at 0.111 kg/s reduced CO2 mitigation rate, estimated at approximately 0.29 tons of CO2 per year, along with a lower carbon credit gain at $0.47. Thus, the proposed cross-fin solar receiver demonstrates improved thermoeconomic performance than conventional receiver designs, offering a more cost-effective design with minimal investment.

Two-Fluid and Discrete Element Modeling of a Parallel Plate Fluidized Bed Heat Exchanger for Concentrating Solar Power

Abstract A novel high-temperature particle solar receiver is developed using a light trapping planar cavity configuration. As particles fall through the cavity, the concentrated solar radiation warms the boundaries of the receiver and in turn heats the particles. Particles flow through the system, forming a fluidized bed at the lower section, leaving the system from the bottom at a constant flowrate. Air is introduced to the system as the fluidizing medium to improve particle heat transfer and mixing. A laboratory scale cavity receiver is built by collaborators at the Colorado School of Mines and their data are used for model validation. In this experimental setup, near IR quartz lamp is used to provide flux to the vertical wall of the heat exchanger. The system is modeled using the discrete element method and a continuum two-fluid method. The computational model matches the experimental system size and the particle size distribution is assumed monodisperse. A new continuum conduction model that accounts for the effects of solid concentration is implemented, and the heat flux boundary condition matches the experimental setup. Radiative heat transfer is estimated using a widely used correlation during the post-processing step to determine an overall heat transfer coefficient. The model is validated against testing data and achieves less than 30% discrepancy and a heat transfer coefficient greater than 1000 W/m2 K.

Experimental study of erosion in heat exchangers immersed in fluidized beds

Fluidized beds can be used as solar energy receivers in beam-down concentrated solar power plants. The fact that solid particles can withstand temperatures over 1,000°C is promising because it would mean higher thermal efficiencies than conventional concentrated solar power plants (CSP, that use molten salts, whose maximum operating temperature is limited to about 560°C). Nonetheless, immersed heat exchangers are exposed to erosion because fluidized particles impinge on their surfaces. The multiple variables of a multiphase flow and the uncertainties of the material detachment process make erosion a complex problem that stills not fully solved.The aim of this study is to conceive an experimental procedure to measure erosion on cylindrical probes immersed in a lab-scale high temperature fluidized bed (able to operate up to 1,000°C and atmospheric pressure), simulating a tube of a heat exchanger in a fluidized bed working at expected temperatures in CSP applications.Preliminary results at ambient temperature are presented in this study. Silica sand particles were fluidized with air and erosion was measured on cylindrical probes made of aluminium 5027. Silica sand was characterized with a microscope before and after each test to determine its size distribution and shape. Several parameters (diameter, mass, and circumferential profile) of each probe were also studied before and after each test. Different fluidization velocities were implemented to assess the influence of the flow rate on erosion.

Annual performance evaluation of a hybrid concentrated solar–micro gas turbine based on off-design simulation

As the adoption of solar hybrid systems continues to rise due to their potential to compensate for the fluctuation of solar irradiation, it becomes imperative to accurately evaluate their performance, considering the variation of off-design conditions. This paper introduces a detailed analysis method for evaluating the annual performance of a solar-MGT system under transient boundary conditions for a whole-year operation range. A hybrid system of a micro gas turbine, recuperator, and solar dish is considered, and an off-design simulation model is developed and verified with available experimental results. Two different configurations for a recuperated cycle are considered, and simulations are conducted for a test case in Pretoria, SA. The results for Jun.21 and Dec.21 as low and high solar energy days are reported with more details to compare the configurations and demonstrate the effect of ambient temperature on the heat loss of the solar receiver and the overall performance of the system. The alternative configuration reduces heat loss with a lower temperature receiver but has higher fuel consumption compared to the conventional configuration. Operating strategies for different hours of operation from 1 h to 24 h per day are simulated for 365 days, based on real meteorological data, and compared with the operating in solar available hours. It is shown that the whole-year simulation of the system considering the variation of boundary conditions can change the estimation of fuel consumption by 25 %.

Design modification of elliptical vessel solar receiver by response surface methodology

Design modification of elliptical vessel solar receiver system by response surface methodology has been carried out. The materials used in this study were locally sourced from Kenyeta Market Enugu, Onitsha Bridge Head Market, and Idumota Market Lagos. These materials were sourced based on categories of components element: support mechanisms made of mild steel plates, bolts, nuts, clamps, and water as heat transfer fluid. The reflector is made of aluminum foil tape while the vessel has a glass cover fitted with bolts and nuts, the receiver is made of copper pipe, aluminum pipe, galvanized iron pipes, and stainless steel pipes. The pipes were fitted into the vessel with chlorinated polyvinyl chloride 3⁄4 joint pipes, and journal-bearing mechanisms. Other features include the tracking system made of light dependent resistance sensors, a direct current motor, a pulley, a belt, an Arduino controller, and a thermal energy storage tank. The lagging material was an expanded Polyethylene sheet. Experimental data were measured with thermocouples, a digital panel, a Uni-T digital anemometer (UT363), and a digital solar power meter (SM206-SOLAR). Matrix Experimental design was used to develop an experimental model for the system. The developed system was tested to investigate the effect of various heat collectors with and without coating on its performance. It was established from the response optimization that the intercept factor was improved by 32.2%. Similarly, the theoretical efficiency was improved by 8.19% while the experimental thermal efficiency was improved by 6.99%.

Multi-objective optimization of the hydrogen fuel production of a solar-based cobalt-chlorine (Co–Cl) thermochemical cycle with grey relational analysis

In this paper, multi-objective optimization of the hydrogen production (m˙H2) of a solar-based cobalt-chlorine (Co–Cl) thermochemical cycle with grey relational analysis (GRA) has been performed. Monthly total irradiance averages (A), wind velocity (B), receiver height (C), receiver radius (D) and number of heliostats (E) are considered to achieve the highest the useful energy absorbed by the solar receiver (Q˙abs), efficiency of the receiver (ηrec), the overall efficiency of the system (ηoverall) and hydrogen production (m˙H2). Orthogonal experimental design is planned according to L18 orthogonal array created by Taguchi method. The percent contribution rates (PCR) on the Q˙abs, ηrec, ηoverall and m˙H2 are determined with the help of ANOVA analysis. The optimal parameters are identified as monthly total irradiance average of 143.8 kW h (A3), wind velocity of 5 m/s (B2), receiver height of 8 m (C1), receiver radius of 3 m (D1) and number of heliostats of 850 (E3), respectively. The highest Q˙abs, ηrec, ηoverall and m˙H2 values corresponding to A3B2C1D1E3 are obtained as 50.2 GW; 64.1%; 22.2% and 122.59 (tons/month), respectively. The PCR of the A, B, C, D and E parameters influencing the f-factor are determined to be 12.6 %, 1.92 %, 0.48 %, 15.03 %, and 53.20 %, respectively. Also, empirical equations are developed with regression analysis (RA) to estimate the Q˙abs, ηrec, ηoverall, m˙H2 values. The statistical coefficients (R2-value) for training and testing data's are determined in the range of 0.771167–0.99608 for Q˙abs, ηrec, ηoverall and m˙H2, respectively.

Medium- and High-Entropy Rare Earth Hexaborides with Enhanced Solar Energy Absorption and Infrared Emissivity.

The development of a new generation of solid particle solar receivers (SPSRs) with high solar absorptivity (0.28-2.5 μm) and high infrared emissivity (1-22 μm) is crucial and has attracted much attention for the attainment of the goals of "peak carbon" and "carbon neutrality". To achieve the modulation of infrared emission and solar absorptivity, two types of medium- and high-entropy rare-earth hexaboride (ME/HEREB6) ceramics, (La0.25Sm0.25Ce0.25Eu0.25)B6 (MEREB6) and (La0.2Sm0.2Ce0.2Eu0.2Ba0.2)B6 (HEREB6), with severe lattice distortions were synthesized using a high-temperature solid-phase method. Compared to single-phase lanthanum hexaboride (LaB6), HEREB6 ceramics show an increase in solar absorptivity from 54.06% to 87.75% in the range of 0.28-2.5 μm and an increase in infrared emissivity from 76.19% to 89.96% in the 1-22 μm wavelength range. On the one hand, decreasing the free electron concentration and the plasma frequency reduces the reflection and ultimately increases the solar absorptivity. On the other hand, the lattice distortion induces changes in the B-B bond length, leading to significant changes in the Raman scattering spectrum, which affects the damping constant and ultimately increases the infrared emissivity. In conclusion, the multicomponent design can effectively improve the solar energy absorption and heat transfer capacity of ME/HEREB6, thus providing a new avenue for the development of solid particles.

Dynamic thermal response characteristics of a metal foam enhanced horizontal loop thermosyphon

A metal foam horizontal loop thermosyphon (MF-HLTS) for a solar receiver was proposed and investigated under the variable conditions. Meanwhile, a dynamic response model for predicting the thermal response characteristics of MF-HLTS was established using Nusselt theory and lumped parameter method. Results of the dynamic model show good agreement with the experimental data, with a maximum deviation of 3.5 % for temperature response. Under sudden power conditions, the temperature variation curve of MF-HLTS shows a progressive monotonic change with two stages: both rising and decreasing rates of temperature show linear and rapidly changes at first stage, and them begins to decrease and gradually tends to 0 at second stage. The time required to reach steady-state increases with the increase of power step. For start-stop of power and cooling, the recovery time gradually increases with increasing stop time, but start-stop ratio firstly decreases rapidly and then tends to be flat. Dynamic response of MF-HLTS is relatively fast with a temperature time lag of 2 s to 4 s, which can meet the requirements of dynamic regulation of system temperature under actual conditions.

Typical solar extinction year at Plataforma Solar de Almería (Spain). Application to thermoelectric solar tower plants

The atmospheric extinction of solar radiation reflected by heliostats are recognized as an important factor of radiative losses in Concentrating Solar Power technologies in general and especially in thermoelectric solar tower plants. These types of plants are getting larger (≥100 MWe), and consequently the distances between the heliostats and the receiver very often exceed 1 km and radiative losses due to solar extinction on this path can represent a high percentage. The aerosols and water vapor along this route scatter and absorb solar radiation, preventing a percentage of it from reaching the solar receiver. For this reason, in the process of choosing a location for the design and construction of these plants, the radiative losses due to extinction in that place should be known in advance. Until now, Typical Meteorological Years have been available for the location chosen in the plant design stage, mainly considering Direct Normal Irradiance but not solar extinction. Unfortunately, ground-based measurement of solar extinction has never been properly considered because it was not known how to measure or estimate it adequately. The Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT, Spain) has developed a reliable solar extinction measurement system which has been recording accurate horizontal extinction values at Plataforma Solar de Almería (PSA) since 2017. Based on this unique in the world solar extinction database of more than five years, a Typical Solar Extinction Year has been obtained for the first time to rigorously validate extinction models that then allow knowing the extinction in any area of interest in the world for solar tower power plants and to choose the most convenient one. It has been found that the annual average extinction at PSA for the measurement distance (742 m) is 6 %, with a standard deviation of 2 % and a median of 6 %. This average annual extinction value corresponds to a horizontal extinction coefficient of (0.083 ± 0.029) km−1 and a Visual Range of 47 km (-12 km, +23 km). Frequent haze events have been observed at PSA mainly caused by Saharan dust, which can be considered one more symptom of the current increased desertification and climate change.

Erosion behaviour of low-pressure cold-sprayed Ni coatings for concentrating solar power receivers

Nickel-based coatings provide outstanding wear and erosion resistance making them particularly suitable for several applications, from aerospace and naval to automotive, energy, and electrical. Cold spray (CS) is encountering a growing interest as a promising technology to deposit thick and dense coatings for components used in energy power generation systems, such as the concentrating solar power (CSP) plants. In this article, nickel coating was deposited onto steel substrates by using a low-pressure CS. The erosion behaviour of the coating was evaluated by a solid particle impact test at two different impact angles. The coating erosion rate was 10 × 10−4 at a 60° impact angle, showing a combination of ploughing and cutting mechanisms.

A Review of the Modeling of Parabolic Trough Solar Collectors Coupled to Solar Receivers with Photovoltaic/Thermal Generation

This paper is a summary of the last ten years of work on the study of parabolic trough collectors (PTCs) and compound parabolic collectors (CPCs) coupled to photovoltaic and thermal solar receiver collectors (SCR-PVTs). While reviewing the state of the art, numerous review papers were found that focused on conventional solar receiver collector (SRC) technology for solar thermal generation. However, there is a lack of review papers summarizing SRC-PVT hybrid technology for solar electric/thermal generation, which would be beneficial for researchers interested in this area of research. This paper provides a review of SRC-PVT hybrid technologies. The theoretical foundations for analyzing and modeling PTC and CPC concentrators coupled to SRC-PVT are described, with an emphasis on modeling through thermal resistances and energy balances. Additionally, this section provides a concise overview of previous studies that have addressed the modeling of PTC and CPC collectors coupled to SCR-PVT, as well as experimental information useful for the validation of new mathematical models of SRC-PVT.

Parametric analysis of evacuated annular parabolic solar receiver integrated solar stills: a relative optimization approach

Parametric analysis of evacuated annular parabolic solar receiver integrated solar stills: a relative optimization approach

Performance evaluation of a partially-filled porous foam cylindrical tubular receiver realizing Ni foam material reduction

The optimization of metal foam structure parameters is crucial for enhancing the heat transfer and pressure drop performance of cylindrical tube heat exchangers. In this study, thermal output performance of 16 different nickel foam-filled solar receivers with varying thicknesses and positions is numerically simulate. An optical-thermal conversion model is established using the Monte Carlo ray tracing (MCRT) method and user-defined functions. The influences of foam filling thickness and position on heat absorber, wall hot spot distribution, heat loss type, thermal efficiency, pressure drop, thermohydraulic performance, exergy loss, and outlet temperature were analyzed. Furthermore, a multi-objective optimization algorithm called non-dominated sorting genetic algorithm II (NSGA II) is used to generate the best trade-off solutions between improvement of thermal efficiency and pressure drop. The results show that enhancing the thickness of the filled foam within the CTR tube enhances thermal efficiency and elevates pressure drop. However, optimizing the length and filling position of the foam can effectively mitigate excessive pressure drop caused by longer foam, without compromising thermal efficiency. This approach results in a reduced pressure drop while maintaining optimal performance. The optimal solution of multi-objective optimization is that the 123 mm thickness foam is filled at 0 mm from the inlet of the tube, and the pressure drop and thermal efficiency of the solar receiver are 830.2Pa and 83.1 %, respectively, when the mass flow rate is 0.01 kg/s.