Solar Furnace Research Articles

High-Temperature Enthalpy of La2Hf2O7 in the Temperature Range 490–2120 K

Lanthanum hafnate La2Hf2O7 was produced chemically by inverse precipitation from ammonia solution and a mixture of La and Hf nitrates, followed by hydroxide decomposition at 1250°C in air and melting of the oxide mixture in a solar furnace. The formation of La2Hf2O7 was ascertained by X-ray diffraction. The La2Hf2O7 enthalpy increment was measured in the range 490–2120 K (for the first time in the temperature ranges 490–988 K and 1740–2120 K) by drop calorimetry using a Setaram HT-1500 high-temperature differential calorimeter and a high-temperature calorimetric device. A fitted equation for the enthalpy increment was used to calculate the main thermodynamic functions (heat capacity, entropy, and Gibbs energy) in the temperature range 298–2120 K. The experimental results are compared with the published data and those assessed using the Neumann–Kopp rule.

Characteristic Features of the Energy Modes of a Large Solar Furnace with a Capacity of 1000 kW

The possible energy characteristics of the LSF (large solar furnace with a capacity 1000 kW) based on numerical calculations are analyzed. The technical characteristics of the LSF are presented. The energy characteristics of the total system with different inaccuracies of the reflecting surfaces, energy contributions of certain shelves and groups of heliostats, and the contributions of certain heliostats and shapes of their focal spot are determined. Empirical formulas are proposed to describe the obtained numerical results. The problem of implementing the possible energy modes of the LSF with and/or without the inclusion of certain shelves and groups of heliostats is analyzed. The problem of a change in the energy density distribution in the focal spot of the LSF during the day is considered.

Experimental study and thermal analysis of a tubular pressurized air receiver

Abstract Pressurized air receivers potentially used in concentrated solar power plants are being developed and studied to achieve higher systemic efficiency from solar to electricity. In this article, a novel receiver with double spiral tubes coiled into conical type was proposed and fabricated. The design shows a bending limitation of a diameter of 60 mm in the bottom of the receiver. Some experimental tests were conducted under a solar furnace in the real weather condition. The results reveal that this kind of receiver could obtain high-temperature outlet air at 908 °C with relatively low direct normal irradiation (DNI) of around 785 W/m2. Based on the comparisons of the outcomes of 9 test cases, it could be concluded that the outlet air temperature is mainly dominated by the value of DNI, while the pressure drop is determined by both DNI and mass flow rate of the air. Besides, the thermal inertia of the receiver demonstrates benefits for stable output and quick recovery induced by small adjustments in the real operation process. All of these results and analysis provide a helpful reference for validation, optimization and operational strategies for this kind of pressurized air receiver.

Modified xonotlite–type calcium silicate hydrate slabs for fire doors

Xonotlite-type calcium silicate hydrate slabs were examined under the thermal shock conditions in a solar furnace at the Plataforma Solar de Almeria which can reach a peak of 300 W/cm2. We have studied the original slabs as well as those modified with a mixture of liquid sodium silicate including montmorillonite as thermal insulation materials for fire doors applications. The slabs were kept at 950°C for 1 h. We performed X-ray diffraction, thermogravimetry and differential scanning calorimetry analysis, scanning electron microscopy, Fourier transform infrared spectroscopy, and thermal conductivity measurements and determined N2 adsorption/desorption isotherms. X-ray diffraction shows that during the thermal shock at 950°C xonotlite is converted to wollastonite. Specific surface areas of xonotlite slabs decrease due to release of crystalline water molecules. It is possible to maintain temperatures of the back door not exceeding 70°C while the front door is subjected to 950°C for 1-h time periods. The standard requires no more than 140°C at the back door.

The Influence of Technological Regimes of Synthesizing a Solar Furnace on the Phase Composition of TiO2-CuO Cermets and the Optical Properties of Coatings on Their Basis

We present the results of studying the effect of technological synthesis regimes of a solar furnace using the method of a partial metal reduction of one of the oxides on the phase formation of cermet composite materials of the TiO2-CuO system. It has been established that the phase composition of the synthesized cermet composite materials depends on the carbon concentration, melting temperature and cooling rate. The dependence of the spectral-optical properties of selectively absorbing coatings on the production technology and properties of synthesized composite materials has been presented. It has been found that the coatings fabricated by melting in air with overheating at a melt cooling rate of about 105–106°C/s have the highest values of the integral absorption coefficient, α s = 91.0–94.5%.

Sun light transmission through silica optical fibers for lighting: an experimental study

The process of sunlight transmission through optical fibers has received considerable interest from the scientific community in recent times. The interdisciplinary emerging field has found many applications in the field of solar lighting, solar furnace, three dimensional solar cells etc. Most of the optical fiber based solar lighting systems have employed plastic fibers as the light transmission media and the use of silica fibers in such systems has not been given much attention. Further, the actual measured data/values of the light emanating from the output ends of fibers in such systems are rarely reported. In the present study, merits and demerits of employing silica and plastic fibers in transmitting sun light are discussed. A silica optical fiber based solar lighting system has been developed and after installing it on an indigenously fabricated sun tracker, its performance has been measured and reported. Based on experimental and simulated results, multimode silica fibers could turn out to be better light transmission media than the plastic fibers, particularly for the case when light has to be transported over longer distances. The findings reported herein stress the need for tailor made customized solar lighting design for different buildings.

Увеличение эффективности органических солнечных элементов с помощью просветляющих покрытий на основе фторидных композиций

AbstractResults are presented obtained in measurements of current–voltage characteristics of organic solar cells with and without antiref lection coatings produced by melting in a solar furnace of 95 : 5 and 55 : 45 wt % MgF_2 and CaF_2 fluoride mixtures. The relative increase in the efficiency of the organic solar cells was 3.2% in the first case and 3.3% in the second.

Aperture size adjustment using model based adaptive control strategy to regulate temperature in a solar receiver

One of the main challenges of solar thermal technology is the intermittency of solar radiation which adversely affect temperature stability of the solar receiver. A promising technique to tackle this problem is the use of a variable aperture mechanism to regulate the light entry into solar receiver. Efficiency analysis confirms the advantage of this control technique over shutter adjustment method, which is also based on regulation of solar radiation entry. In order to regulate the temperature in a closed loop circuit based on aperture size adjustment, a model based control strategy was developed. To show the robustness and comprehensiveness of this control strategy, it was applied to a cavity receiver heated by two different radiative heat sources demonstrating the applicability of this control strategy consistently in most commonly practiced solar thermal systems. The first heat source studied is a solar furnace housing a parabolic dish, whereas the second one is a high flux solar simulator. For each radiative heat source, flux entering the receiver was determined using Monte Carlo ray tracing (MCRT) method. MCRT model was then coupled with energy balance equations to derive numerical model describing dynamic temperature variation in solar receiver. Comparison of simulated and experimentally measured temperatures showed appreciable accuracy of the dynamic model. Simulation results of the numerical model were then used to identify a nonlinear adaptive model for use in designing a model predictive controller (MPC). Parameters of the adaptive model were updated continuously to make the controller more robust against model mismatches and external disturbances. Simulation results for both radiative heat sources showed that the proposed controller yields faster response with less overshoot compare to proportional integral derivative (PID) controller. Results showed that this controller exhibits robust performance during sunrise and sunset times as well as passing clouds conditions where significant fluctuations in solar radiation is experienced.

Experimental evidence of an observer effect in high-flux solar simulators

High-flux solar simulators consist of lamps that mimic concentrated sunlight from a field of heliostats or parabolic dish. These installations are used to test promising solar-thermal technologies for technical viability. Ideally, the conditions in a given high-flux solar simulator closely approximate those in solar furnaces, experimental facilities that are driven by actual sunlight. During the characterization of a new high-flux solar simulator at the University of Colorado an artifact predicted in high-flux solar simulators, but not solar furnaces, was observed experimentally. Specifically, power measurements from the 18 lamp, 45kWelectric, device increased by 11.1% depending on the wider optical environment, which was either specularly reflective or diffusely absorbing. This calorimetry result was confirmed by a flux gauge, which showed that optical conditions could inflate incident flux by more than 20% at the radiation target. An “observer effect” in high-flux solar simulators was first suggested by Monte Carlo ray traces, and refined ray traces recapitulated the experimental results. Thus, solar-thermal designs may behave differently when evaluated in solar furnaces, versus high-flux solar simulators. Artifacts can be minimized by insuring that solar targets, including radiation measurement instrumentation, present diffusely absorbing surfaces to high-flux solar simulators. Under these conditions the new high-flux solar simulator featured a peak flux of 12.50MW/m2 and delivered 9.076±0.190kW onto a ∅10cm target for a mean flux of 1.155MW/m2.

Solar furnace temperature control with active cooling

The article describes a control architecture for solar furnaces where active cooling is employed to improve the tracking of a time-varying temperature reference. This capability is important during the decreasing phase of the temperature reference where heat loss must be increased. The results of two different control methodologies, exact linearization and model predictive control with integral action, are shown with active cooling that is done in coordination with the command of the shutter which adjusts the solar incident power.The controller parameters are computed from the temperature dynamics which is identified off-line from collected process data. This approach is used to avoid online adaptation mechanisms of the controller parameters, that may cause stability problems during the controller startup, and may melt the testing material sample.The novelty of the present work is to present a control architecture that coordinates the operations of the shutter together with the application of active cooling. This methodology improves temperature reference tracking and increases the usability and the operation of solar furnaces.

Titanium Alloys Thin Sheet Welding with the Use of Concentrated Solar Energy

The present study deals with the welding of titanium alloys thin sheets 1.3 mm thick, with the use of concentrated solar energy. The experimental part of the work took place at a medium size solar furnace at the installation of the Centre National de la Recherche Scientifique, at Odeillo, in Southern France, where similar and dissimilar defect-free welds of titanium Grades 4 and 6 were achieved, in the butt joint configuration. After the determination of the appropriate welding conditions, the optimum welded structures were examined and characterized microstructurally, by means of light optical microscopy, scanning electron microscopy, and microhardness testing. In addition, test pieces extracted from the weldments were tested under uniaxial tensile loading aiming to the estimation of the strength and the ductility of the joint. The analysis of the experimental results and the recorded data led to the basic concluding remarks which demonstrate increased hardness distribution inside the fusion area and severe loss of ductility, but adequate yield and tensile strength of the welds.

Concentrated solar energy used for sintering magnesium titanates for electronic applications

Solar energy is an important renewable source of energy with many advantages: it is unlimited, clean and free. The main objective of this work was to sinter magnesium titanate ceramics in a solar furnace using concentrated solar energy, which is a novel and original process. The direct conversion of solar power into high temperature makes this process simple, feasible and ecologically viable/environmentally sustainable. We performed the solar sintering experiments at Plataforma Solar de Almeria-CIEMAT, Spain. This process takes place in a vertical axis solar furnace (SF5–5kW) hosting a mobile flat mirror heliostat, a fixed parabolic mirror concentrator, an attenuator and a test table the concentrator focus. We sintered (MgO)0.63(TiO2)0.37, (MgO)0.49(TiO2)0.51, (MgO)0.50(TiO2)0.50 ceramics samples in air at about 1100°C for a duration of 16min, 1h, 2h and 3h in the solar furnace. The MgO/TiO2 ratio and the dwell time was varied in order to obtain phase pure MgTiO3 ceramic. We obtained a pure MgTiO3 geikielite phase by solar sintering of (MgO)0.63(TiO2)0.37 samples at 1100°C (16min–3h). Samples of (MgO)0.63(TiO2)0.37, solar sintered at 1100°C for 3h, resulted in well-sintered, non-porous samples with good density (3.46g/cm3). The sintered samples were analyzed by XRD for phase determination. The grain and surface morphology was observed using SEM. Electrical measurements were carried out on solar sintered samples. The effect of processing parameters on microstructure and dielectric properties were investigated and is presented.

Experiment study of a quartz tube falling particle receiver

This paper presents an experimental evaluation of a specially designed falling particle receiver. A quartz tube was used in the design, with which the particles would not be blown away by wind. Concentrated solar radiation was absorbed and converted into thermal energy by the solid particles flowed inside the quartz tube. Several experiments were conducted to test the dynamic thermal performance of the receiver on solar furnace system. During the experiments, the maximum particle temperature rise is 212°C, with an efficiency of 61.2%, which shows a good thermal performance with a falling distance of 0.2 m in a small scale particle receiver. The average outlet particle temperature is affected by direct normal irradiance (DNI) and other factors such as wind speed. The solid particles obtain a larger viscosity with a higher temperature while smaller solid particles are easier to get stuck in the helix quartz tube. The heat capacity of the silicon carbide gets larger with the rise of particle temperature, because as the temperature of solid particles increases, the temperature rise of the silicon carbide decreases.

Preparation of Finely Crystalline Yttrium-Aluminum Garnet in Solar Furnaces

The possibility is considered of preparing finely crystalline yttrium-aluminum garnet using high-temperature processes in solar furnaces. The effect of concentrated radiation heat flow density during treatment of a mixture of yttrium and aluminum oxides on phase composition of the melt obtained is revealed. The optimum radiation flux density for preparing specimens of finely crystalline single-phase yttrium-aluminum garnet is 8.4 MW/m2.

Transient behavior of a multi-tubular cavity solar thermochemical reactor

A three-dimensional transient heat transfer model is presented to predict the start-up operation of a multi-tubular cavity reactor under concentrated irradiation in a solar furnace. The reactor consists of a cavity containing nine absorber tubes, through which a suspension of CeO2 in Argon flows. An iterative splitting scheme coupling a Continuous Random Walk, a Finite Volume, and a Ray-Tracing Monte Carlo methods, is implemented to estimate the temperature gradients in the tubes and gas-particle media. The radiation heat transfer among the tubes and cavity walls is considered, as well as conduction and convection in the tubes and the particle suspension. During the initial heating stage, gradients are mainly angular, while in steady-state they are primarily axial. The former may cause tube bending or cracking, and strategies to reduce them are examined. In particular, different heating ramps were simulated, which was found to reduce these initial thermal gradients.

Contact sensors for measuring high surface temperature in concentrated solar radiation environments

Surface temperature is a highly desired but difficult measurement especially in concentrated solar context. In this work a method for surface temperature measurement based on contact sensors is presented. In the case of materials with high thermal conductivity, contact sensors positioned in the back of the material sample and very close to the surface is the most accurate way to measure surface temperature. Computational Fluid Dynamics simulations have shown the truth of this statement. The higher thermal conductivity of the material, the lower the uncertainty in the measurement of surface temperature using this methodology. This measurement procedure has been applied to AISI 310S steel samples in the Plataforma Solar de Almería vertical axis solar furnace SF5 confirming the validity of the simulations.

High-efficiency solar power towers using particle suspensions as heat carrier in the receiver and in the thermal energy storage

Solar thermal electricity generated by concentrated solar power (CSP) plants is increasingly implemented. CSP plants can supply electricity on a fully matched supply-demand basis if equipped with a thermal energy storage. To increase the efficiency and reduce both capital and operating costs, a next generation of CSP concepts is required. Particle suspensions can be applied to meet these targets and can be used throughout the CSP conversion process, as high temperature heat transfer medium in the receiver, for heat storage, and in the power block of the plant. This work presents the novel concept of using particle suspensions as heat carriers, mostly further to initial testing at the CNRS 1 MW solar furnace of Odeillo Font-Romeu (F). Values of the heat transfer coefficient up to 1100 W/m2K (bare tubes) and 2200 W/m2K (finned tubes) were obtained for operation at low superficial gas velocities of 0.04–0.19 m/s, thus limiting heat losses by the exhaust air. Despite additional costs for particle handling and for an appropriate boiler, the required overall investment and operating costs are expected to be significantly lower than for common equivalent molten salt CSPs, leading to a reduction in Levelized Cost of Electricity (LCOE) from approximately 125 €/MWh to below 100 €/MWh.

4-4-5 45kW_ KIER太陽炉によるCeO_2/MPSZ反応デバイスを用いた水分解サイクルの太陽試験(4-4 太陽熱・水素製造,Session 4 新エネルギー)

4-4-5 45kW_ KIER太陽炉によるCeO_2/MPSZ反応デバイスを用いた水分解サイクルの太陽試験(4-4 太陽熱・水素製造,Session 4 新エネルギー)

Hazards Caused by UV Rays of Xenon Light Based High Performance Solar Simulators.

BackgroundSolar furnaces are used worldwide to conduct experiments to demonstrate the feasibility of solar–chemical processes with the aid of concentrated sunlight, or to qualify high temperature-resistant components. In recent years, high-flux solar simulators (HFSSs) based on short-arc xenon lamps are more frequently used. The emitted spectrum is very similar to natural sunlight but with dangerous portions of ultraviolet light as well. Due to special benefits of solar simulators the increase of construction activity for HFSS can be observed worldwide. Hence, it is quite important to protect employees against serious injuries caused by ultraviolet radiation (UVR) in a range of 100 nm to 400 nm.MethodsThe UV measurements were made at the German Aerospace Center (DLR), Cologne and Paul-Scherrer-Institute (PSI), Switzerland, during normal operations of the HFSS, with a high-precision UV-A/B radiometer using different experiment setups at different power levels. Thus, the measurement results represent UV emissions which are typical when operating a HFSS. Therefore, the biological effects on people exposed to UVR was investigated systematically to identify the existing hazard potential.ResultsIt should be noted that the permissible workplace exposure limits for UV emissions significantly exceeded after a few seconds. One critical value was strongly exceeded by a factor of 770.ConclusionThe prevention of emissions must first and foremost be carried out by structural measures. Furthermore, unambiguous protocols have to be defined and compliance must be monitored. For short-term activities in the hazard area, measures for the protection of eyes and skin must be taken.

Non-destructive testing method to quantify aging of materials by its apparent emissivity: Case of glass-based reflectors

In this paper, the measurement capabilities of apparent emissivity are tested by means of a generic LWIR system to quantify the aging of materials. A theoretical frame permitted to elaborate abacuses and to correlate a scaling law to identify the relevant parameters, and to foretell performance in terms of absolute aging time interval resolution as well. The predictions are done for both indoor and outdoor situations. They are compared with measurements performed on glass-based mirrors of a heliostat of the solar furnace of Odeillo after having undergone a Damp Heat accelerated aging test. Apparent emissivity measurements implement an indoor home-made device which had already been characterized. The performances are in accordance with the predictions and allow to discriminate initial natural aging of glass mirror combined with accelerated aging with an interval resolution from 4 to 10years keeping in mind that we are in quest of differentiating two mirror glasses or of quantifying the absolute aging of each mirror. Limitations of indoor measurements come mainly from the accuracy of Non-Uniformity Correction and from the internal drift compensation inherent to any IR system. Outdoor performances are foreseen to be as efficient by optimizing the contrast tapp.-tenv.. If we consider an uncertainty of emitting temperature of 0.25°C and the one of apparent temperature of 0.1°C, a standard outdoor thermography would enable to discriminate absolute aging produced by Damp Heat test with a 10 standard year resolution.