Solar Flux Concentration Research Articles

A Planar-Cavity Receiver Configuration for High-Temperature Solar Thermal Processes

Next-generation concentrating solar thermal power (CSP) technologies target a wide spectrum of applications including electricity generation, thermochemical processes, and industrial process heat for broad decarbonization potential. Many of these applications require higher temperatures than those of current commercial nitrate salt systems. Particulate materials are promising candidates for next-generation high-temperature heat transfer and low-cost storage media and can facilitate operation over a wide temperature range. However these materials necessitate novel receiver configurations to accept the high incident flux concentrations that enable high solar-to-thermal receiver efficiency. One option is a novel light-trapping planar cavity receiver configuration in which small cavity-like structures are formed from opaque planar surfaces such that a high incident flux concentration at the cavity aperture is reduced to a wall-absorbed solar flux concentration that is manageable within limited wall-to-particle heat transfer rates. The paper introduces the receiver configuration and provides calculated optical performance for a preliminary 50 MWt receiver design.

The effect of space charge neutralization on the parametric design of photon enhanced thermionic solar converters

Photon-enhanced thermionic emission (PETE) is a promising method for increasing current density at relatively low cathode temperatures, but is susceptible to the space charge effect. In this study, we propose a novel PETE device model in which the space-charge barrier is relaxed by photon-excited cesium ions. The decrease in maximum efficiency with interelectrode gap is more gradual compared to previous studies. We investigate the effects of cathode materials properties (band gap and electron affinity) and operating conditions (cathode temperature and interelectrode gap) on PETE performance, and identify optimal performance conditions. Our findings show that the optimal gap range exceeds 5 μm and near field radiation loss can be minimized, providing an important opportunity for designing PETE device with larger interelectrode gaps and higher solar flux concentrations.

Solar-driven biomass steam gasification by new concept of solar particles heat carrier with CPFD simulation

Solar-driven biomass gasification is a promising solar thermochemical technology to produce green H2-enriched syngas, and contribute to solar energy upgrade and efficient conversion. Whereas, the continuous and stable operation of solar-driven biomass gasification is challenging due to the non-uniform concentration of solar flux and its fluctuating nature. In this regard, a novel concept of solar-driven gasification is proposed, using solar-heated particles as the heat carrier, and biomass steam gasification in a fluidized bed reactor is numerically investigated based on the computational particle fluid dynamics (CPFD) model. The complex distributions of gas and particle phases, as well as the effects of operation parameters on syngas species are comprehensively analyzed. Results indicate that the biomass material and solar-heated particles can be well-mixed through continuous feeding, and the internal heat transfer and gasification kinetics are enhanced. The mole fraction of H2 and CO in the produced syngas reach to 50.86% and 18.97% under the benchmark operating condition. By using the high-temperature quartz sand as the solar particles heat carrier, the long-term and continuous operation of the endothermic gasification process can be achieved. The newly developed solar-driven biomass gasification method provides a promising approach to mitigate the impact of the fluctuating solar irradiation and enhance the operational flexibility of gasification process.

Association Between Magnetic Pressure Difference and the Movement of Solar Pores

Solar pores are closely related to the concentration, dissipation, and transportation of solar magnetic flux. Their observable characteristics can provide constraints on models and simulations of magnetic flux emergence and formation. The specific property investigated in this study is their horizontal movement. The aim is to investigate whether the movement is correlated with any observable quantities. Our statistical analysis of 61 compact pores identified from the Spaceweather HMI Active Region Patches (SHARP) from 2011 to 2018 indicates that the direction of movement is often either parallel or antiparallel to the direction of maximum magnetic pressure difference (dP mag) at the opposite sides of the edges of the pores. The correlation coefficients for both the parallel and antiparallel cases are higher than 0.74. Despite the high correlation, our analysis using the transfer entropy indicates no significant causal relationship between the direction of motion and the direction of maximum dP mag.

A solar tower fuel plant for the thermochemical production of kerosene from H2O and CO2.

SummaryDeveloping solar technologies for producing carbon-neutral aviation fuels has become a global energy challenge, but their readiness level has largely been limited to laboratory-scale studies. Here, we report on the experimental demonstration of a fully integrated thermochemical production chain from H2O and CO2 to kerosene using concentrated solar energy in a solar tower configuration. The co-splitting of H2O and CO2 was performed via a ceria-based thermochemical redox cycle to produce a tailored mixture of H2 and CO (syngas) with full selectivity, which was further processed to kerosene. The 50-kW solar reactor consisted of a cavity-receiver containing a reticulated porous structure directly exposed to a mean solar flux concentration of 2,500 suns. A solar-to-syngas energy conversion efficiency of 4.1% was achieved without applying heat recovery. This solar tower fuel plant was operated with a setup relevant to industrial implementation, setting a technological milestone toward the production of sustainable aviation fuels.

Evaluation of external tubular configurations for a high-temperature chloride molten salt solar receiver operating above 700 °C

Next-generation concentrating solar power (CSP) tower technologies target operating temperatures exceeding 700 °C to increase the thermal-to-electric conversion efficiency. Molten chloride salts are one possible alternative to current commercial molten nitrate salts to enable the higher operating temperature. This paper analyzes the predicted optical, thermal-fluids, and structural performance of traditional external tubular solar receiver configurations applied with a chloride salt heat transfer fluid (HTF) and inlet/outlet temperatures of 500 °C/735 °C, and considers sensitivity analysis and optimization relative to receiver sizing, tube sizing, number of panels, flow circuit configurations, and solar flux concentration under constraints on internal velocity, pressure drop, wall thickness, and required creep-fatigue lifetime. The high temperature conditions increase the significance of inelastic deformation mechanisms such as creep relative to that expected in commercial 565 °C nitrate salt designs. High-temperature creep and creep-fatigue damage in the metal alloy tubes are the key factors that limit allowable solar flux concentration and achievable receiver thermal efficiency at the near-800 °C wall temperature conditions. For a traditional external cylindrical receiver configuration, the design parameters and conditions capable of satisfying all constraints produced, at best, a design point receiver efficiency of 78.2%, or 80.5% when excluding receiver intercept efficiency. Variation in the optimal receiver performance relative to uncertainty in the binding maximum velocity, minimum wall thickness, and minimum lifetime constraints is presented.

Numerical investigations on metal foam inserted solar parabolic trough DSG absorber tube for mitigating thermal gradients and enhancing heat transfer

Absorber tube in the superheating section of a direct steam generating solar parabolic trough collector field is subjected to large circumferential temperature gradients due to non-uniform concentration of solar flux and low thermal conductivity of steam. The thermal stresses developed due to the thermal gradients can lead to bending and may eventually result in the structural failure of the glass tube enclosing the absorber tube. Therefore, thermal analysis of the absorber tube for identifying the overheated sections and methods to mitigate the thermal gradients are very essential. In the present study, numerical investigations are carried to evaluate the performance of absorber tube with metal foam inserts subjected to non-uniform solar flux. Configurations of absorber tube with circumferential metal foam inserts in the lower half of the absorber tube are considered for the numerical analysis. Moreover, configurations of metal foam inserts with a novel shape based on the non-uniform flux around absorber tube are also analyzed. The metal foam inserted absorber tube configurations are found to be very effective in reducing the circumferential thermal gradients. In comparison to the clear tube, reduction of about 47% to 72% in maximum temperature difference is found for the range of operating conditions considered. The Nusselt number and performance evaluation criterion are found to be enhanced by a factor 10 and 5 respectively. Moreover, maximum gain of 3.71% in the net energy efficiency and 2.32% in exergy efficiency is found for the collector with metal foam inserted absorber tube configurations. The results of the numerical study conclude the insertion of metal foams as an effective method for mitigating thermal gradients around the tube and enhancing the heat transfer performance.

A spectral splitting solar concentrator for cascading solar energy utilization by integrating photovoltaics and solar thermal fuel

Full-spectrum solar energy utilization has drawn widespread attention for cascading solar energy utilization. The hybrid approach integrating photovoltaic generation and solar thermochemical reaction is attractive to convert full-spectrum solar energy into electricity and chemical energy. This paper proposes a concentrating solar photovoltaic/thermochemical hybrid system. Meanwhile, a spectral splitting concentrator is proposed and designed. The proposed concentrator consists of above-mirror and sub-mirror. The above-mirror enables the visible spectrum to be concentrated onto photovoltaics. The sub-mirror allows the infrared and ultraviolet spectra to be concentrated onto a solar thermochemical reactor. The design methodology based on ray tracing is described. The distribution of solar radiation on the photovoltaics and the thermochemical reactor is made analysis. The results show that solar radiation can be uniformly cast to the photovoltaic surface without using secondary optical element; the common solar flux concentration of thermochemical reactor is also mitigated. The proposed spectral splitting concentrator is introduced in the hybrid system. The total conversion efficiency of the solar energy can exceed 20%. Compared with individual photovoltaic electricity and solar thermal fuel, this hybrid system has the potential to increase the conversion efficiency of solar energy into both electricity and fuel, with greater than 5% absolute enhancement. The design of the spectral splitting concentrator provides the possibility of cascading utilization of full-spectrum solar energy.

Experimentation of surface areal irradiance method for solar flux measurement in compound parabolic collector

Flux measurement is one of the major considerations in the optical analysis of solar collector to improve its performance. An uneven distribution of concentrated solar rays at the receiver creates the need of finding a region of high concentration flux and its measurement. This paper describes the experimentation of the author’s recent study of surface areal irradiance (SAI) method performed on compound parabolic collector (CPC) with low acceptance angle. This method uses a 2D graphical ray tracing techniques to determine the region of high concentration of solar flux and its intensity in that region. Also the present study refines the concept of SAI method with geometric cosine factors for incident and reflected rays. The result of flux curves by SAI method and experimentation show the identical characteristics and it is plotted along with solar direct irradiation and variable collector area. SAI method uses the reflector area instead of aperture area to determine concentrated flux which is helpful in designing solar collectors as per the requirements.

Experimental analysis and evaluation of a vacuum enclosed concentrated solar thermoelectric generator coupled with a spectrally selective absorber coating

ABSTRACTSignificant research in the past decade has been focused on quantitatively and qualitatively validating potential of solar thermoelectric modules to harness electricity. In the present study, we have experimentally analysed steady-state temperature variation of a spectrally selective solar absorber coating (α = 0.954, ε = 0.13) with variation in solar irradiation flux (concentration ratios = 39, 50 and 65) using Fresnel lens and vacuum enclosure pressure (200 mbar to 900 mbar in steps of 100 mbar). It is observed that the experimental results so obtained go hand in hand with a COMSOL simulation model of the set-up. Further, we have carried out performance analysis of a solar thermoelectric generator (STEG) set-up enclosed in vacuum conditions equipped with Fresnel lens and absorber set-up coupled to Bi2Te3 thermoelectric module array electrically connected in series. The results depict a maximum power output of 0.91 W and a peak efficiency of 2.21% at a hot-side temperature of 642 K.

Flux concentration on tubular receiver of compound parabolic collector by surface areal irradiance method of ray tracing

Ray tracing for solar collectors finds wide application for designing the components. In this research, a solar flux concentration on the tubular receiver of compound parabolic collector (CPC) is determined by a novel approach of surface areal irradiance (SAI). The objective of this method is to determine the solar reflected irradiance at the receiver by incorporating effective areas of reflector and receiver. These areas are determined by 2D graphical ray tracing technique. Multiple ray tracing iterations are performed on various geometric possibilities of receiver–collector combinations by varying receiver diameter and height. The collector dimensions are fixed as per authors previously designed CPC to reduce the number of possibilities of receiver – collector combinations. The results of these iterations are presented in terms of utilization ratio (U), projection ratio (P) etc. based on which concentrated solar flux and heat equations are determined. It is observed that there is saturation point for all receiver diameters at which a receiver – collector combination gives highest utilization ratio and the corresponding height gives the best location for that receiver. SAI method gives a strong methodology for designing solar collectors on the basis of utilization ratios and heat equations.

Properties of silicon solar cells with a holographic concentrator

The processes of photovoltaic conversion in the silicon solar cells (SE) with р+–n–n+ structure have been studied while lighting with the holographic concentrator. The dependences of the open-circuit voltages, short circuit current of the solar cell on the lighting level in the range of the solar flux concentration 1–10 units without special cooling.

Optimal Positioning of Reflectors to Increase Heat Interception of a Compound Parabolic Solar Collector using Taguchi Method

This work represents the optimization of reflector positioning of compound parabolic collectors for enhanced heat interception. The heat intercepted by a compound parabolic collector can be improved either by increasing the quantity of incident solar radiation or by absorbing a greater percentage of the available radiation. In the redesigned system, the utilization of incident heat has been increased by increasing the utilization of incident heat, instead of increasing the quantity of incident heat. We have increased the overall efficiency of the compound parabolic collector by applying Taguchi’s method, increasing the maximum concentration ratio to a value almost equal to the geometric concentration ratio and therefore achieved maximum utilizations of the available solar radiation. We have increased the heat intercepted by 23% from 3.7 KW to 3.75 KW and by applying electronic and mechanical tracking, an increase in average solar flux concentration by 10 times is observed.

Negative space charge effects in photon-enhanced thermionic emission solar converters

In thermionic energy converters, electrons in the gap between electrodes form a negative space charge and inhibit the emission of additional electrons, causing a significant reduction in conversion efficiency. However, in Photon Enhanced Thermionic Emission (PETE) solar energy converters, electrons that are reflected by the electric field in the gap return to the cathode with energy above the conduction band minimum. These electrons first occupy the conduction band from which they can be reemitted. This form of electron recycling makes PETE converters less susceptible to negative space charge loss. While the negative space charge effect was studied extensively in thermionic converters, modeling its effect in PETE converters does not account for important issues such as this form of electron recycling, nor the cathode thermal energy balance. Here, we investigate the space charge effect in PETE solar converters accounting for electron recycling, with full coupling of the cathode and gap models, and addressing conservation of both electric and thermal energy. The analysis shows that the negative space charge loss is lower than previously reported, allowing somewhat larger gaps compared to previous predictions. For a converter with a specific gap, there is an optimal solar flux concentration. The optimal solar flux concentration, the cathode temperature, and the efficiency all increase with smaller gaps. For example, for a gap of 3 μm the maximum efficiency is 38% and the optimal flux concentration is 628, while for a gap of 5 μm the maximum efficiency is 31% and optimal flux concentration is 163.

Thermal Stress Analysis of Absorber Tube for a Parabolic Collector under Quasi-Steady State Condition

The design of parabolic trough collectors is based on the concentration of solar heat flux on its absorber tube. This may be subjected to high thermal stresses which could cause deflection of the absorber tube and failure of its cover glass tube. Investigation of earlier studies on thermal gradient and thermal stresses demonstrates that so far, the issue is considered for steady state condition based on the assumption of constant solar heat flux distribution during the day hours. However, in practice, solar heat flux distribution on the receiver tube has continues changes with respect to time and deformation is basically transient. Due the low speed of sun during a day and for specific time intervals, the average solar radiation could be considered constant for thermal stress and deflection analysis. Hence, in this study, thermal gradients as well as thermal stresses in the absorber tube are numerically investigated in quasi steady condition. Due to the wide changes of solar radiation during a year, computations are performed for four specific days of Spring Equinox, Summer Solstice, Autumnal Equinox and Winter Solstice. To compute solar heat flux distribution, SolTrace software is used and three dimensional heat flux distribution is applied as an external boundary condition to calculate temperature distribution in the absorber tube. By using Von-Misses theory, maximum equivalent of total stress is computed. Maximum deflection is evaluated for various different inlet temperatures and hot oil mass flow rates in the common range of solar thermal power plants for atypical parabolic trough collector which is under construction in Shiraz, Iran.

Demonstration of the Entire Production Chain to Renewable Kerosene via Solar Thermochemical Splitting of H2O and CO2

The European consortium SOLARJET has experimentally demonstrated the first ever production of jet fuel via a thermochemical H2O/CO2-splitting cycle using simulated concentrated solar radiation. The key component of the production process of sustainable “solar kerosene” is a high-temperature solar reactor containing a reticulated porous ceramic (RPC) foam structure made of pure CeO2 undergoing a 2-step redox cyclic process. During the first endothermic reduction step at 1450–1600 °C, the RPC was directly exposed to concentrated thermal radiation with power inputs ranging from 2.8 to 3.8 kW and mean solar flux concentration ratios of up to 3000 suns. In the subsequent exothermic oxidation step at 700–1200 °C, the reduced ceria was stoichiometrically reoxidized with CO2 and/or H2O to generate CO and/or H2. The RPC featured dual-scale porosity: millimeter-size pores for volumetric radiation absorption during reduction and micrometer-size pores within its struts for enhanced oxidation rates. For a cycle durati...

Heat Efficiency of Trough Solar Vacuum Receiver

The heat loss and thermal performance of solar parabolic trough vacuum receiver were experimentally measured and analyzed by heat transfer model. According to the present experiments, the heat loss of solar parabolic trough vacuum receiver has good agreement with the heat loss of vacuum receiver from Solel company. As the wall temperature increase from 108°C to 158°C, the heat loss of solar parabolic trough vacuum receiver remarkably increases from 35 Wm-2to 57 Wm-2. The heat transfer model of parabolic trough solar receiver is then theoretically investigated due to the energy balances between the heat transfer fluid, absorber tube, glass envelope and surroundings. When solar radiation flux is constant, the heat efficiency of solar parabolic trough system decreases with the wall temperature and oil temperature. When solar radiation flux or solar concentration ratio increases, the heat efficiency of solar parabolic trough system increases.

Análisis de los datos horarios de radiación solar y abundancia de ozono del Distrito Metropolitano de Quito del 2007 al 2012

En este trabajo se presenta un análisis de los datos horarios de radiación solar y de concentraciones de ozono a nivel de la superficie en el norte y sur de Quito del 2007 al 2012. Los datos fueron obtenidos de los archivos públicos de la red de monitoreo del aire de Quito.En este estudio se elaboró perfiles diurnos mensuales de la radiación solar y el ozono en ambas regiones. Se encontró una dispersión muy marcada, sobre todo en el juego de datos de radiación solar. La región norte es, en general, más soleada que la sur. Esta última, sin embargo, presenta concentraciones de ozono más elevadas, revelando diferencias importantes en la calidad y abundancia de sus precursores. Los meses del verano presentan las mayores concentraciones de ozono, correlacionándose directamente con los niveles de radiación solar. Las concentraciones de ozono en horas de luz del día en el sur de la ciudad pueden llegar a ser hasta cuatro veces mayores que en el norte. En horas de la noche, en la región sur, se identificó concentraciones de ozono tan altas como valores diurnos en los meses del verano, debido a un posible atrapamiento del ozono de la capa residual en la capa límite nocturna. Los resultados encontrados sugieren posibles experimentos atmosféricos futuros que podrían diseñarse para probar las hipótesis propuestas.

Satellite‐derived estimates of ultrafine particle concentrations over eastern North America

High concentrations of ultrafine particles (UFP, i.e., particles with diameter < 100 nm) impact both human health and Earth's climate. Recent innovations in remote sensing technologies and data retrievals offer the potential for predicting UFP concentrations based on data from satellite‐borne instrumentation. Herein we present a physically based statistical algorithm to estimate UFP concentrations across eastern North America using remotely sensed aerosol optical depth, Ångstrom exponent, ultraviolet solar radiation flux, and ammonia and sulfur dioxide concentrations. The proposed algorithm is built and independently evaluated using an array of in situ observations. The algorithm is able to capture up to 60% of the variability in daily measured UFP number concentrations at a regionally representative reference site and is thus applied to generate seasonal UFP concentration estimates across eastern North America. The resulting UFP concentrations are cross‐evaluated with simulations from a global aerosol microphysics model. There is a negative bias in the model output relative to the satellite‐driven proxy, which is largest (up to 76%) in summer and may be due to overestimation of UFP from the satellite‐based algorithm derived herein, due to the higher availability of remote sensing data in clear‐sky conditions or uncertainty in the model simulation of new particle formation. Nevertheless, the model and algorithm indicate similar spatial and seasonal variability (spatial correlation coefficients of 0.10 to 0.56), indicating the value of the satellite‐based UFP proxy in global and regional model evaluation exercises and in efforts to identify regions where future in situ data collection should be prioritized.

Solar Thermochemical CO2 Splitting Utilizing a Reticulated Porous Ceria Redox System

A solar cavity-receiver containing a reticulated porous ceramic (RPC) foam made of pure CeO2 has been experimentally investigated for CO2 splitting via thermochemical redox reactions. The RPC was directly exposed to concentrated thermal radiation at mean solar flux concentration ratios of up to 3015 suns. During the endothermic reduction step, solar radiative power inputs in the range 2.8–3.8 kW and nominal reactor temperatures from 1400 to 1600 °C yielded CeO2−δ with oxygen deficiency δ ranging between 0.016 and 0.042. In the subsequent exothermic oxidation step at below about 1000 °C, CeO2−δ was stoichiometrically reoxidized with CO2 to generate CO. The solar-to-fuel energy conversion efficiency, defined as the ratio of the calorific value of CO (fuel) produced to the solar radiative energy input through the reactor’s aperture and the energy penalty for using inert gas, was 1.73% average and 3.53% peak. This is roughly four times greater than the next highest reported values to date for a solar-driven d...