Concentrated Solar Flux Research Articles

HEAT PIPES APPLICATION IN ELECTRONICS THERMAL CONTROL SYSTEMS

This paper introduces separate-type heat pipe (STHP) based solar receiver systems that enable more efficient operation of concentrated solar power plants without relying on a heat transfer fluid. The solar receiver system may consist of a number of STHP modules that receive concentrated solar flux from a solar collector system, spread the high concentrated solar flux to a low heat flux level, and effectively transfer the received heat to the working fluid of a heat engine to enable a higher working temperature and higher plant efficiency. In general, the introduced STHP solar receiver has characteristics of high heat transfer capacity, high heat transfer coefficient in the evaporator to handle a high concentrated solar flux, non-condensable gas release mechanism, and lower costs. The STHP receiver in a solar plant may also integrate the hot/cold tank based thermal energy storage system without using a heat transfer fluid.

Performance Evaluation of a Paraboloidal Concentrator

This research aims are to design, build and analyze the performance of concentrated solar energy onto a simple calorimeter, made of copper and stainless steel. A scrapped yard antenna dish with a diameter of 1.68 meters was lined with aluminum foil reflector sheet. The average dish aperture area of 2.21 m2, the system uses water as a heat transfer fluid distributed from overhead tank. Absorbed energy was investigated at a water flow rate of 0.5 L/min were performed in the experiment and the maximum useful energy was determined. Parabolic dish concentrator displacement angle was adjusted in every 15 degrees per hour, with the used of electric controlled sun tracking system. The experimental tests carried out in Faculty of Industrial Technology, Vallaya Alongkorn Rajabhat University Pathum Thani, under Thailand climatic conditions (14.134°N, 100.611°E) during 2 selected days of the months April 2013. The performance of a paraboloidal concentrator was assessed using open-air experimental measurements including the incoming heat, the energy absorbed by the water, and the concentrator efficiency. The experimental results shown the maximum thermal efficiency was 89.73%. It was also found that copper calorimeter can simply attain during operation relatively high water temperature levels oncoming 97 °C. The concentrated solar flux at the focal point was 442,073.11W/m2 for stainless steel calorimeter and 619,448.66 W/m2 for copper calorimeter, respectively.

Some physico-chemical aspects of the photocatalytic decomposition of water with a concentrated solar flux

The possibilities of increasing the hydrogen yields directly from water by the photocatalytic method by heating with a concentrated solar flux in two stages using two types of catalysts were studied. At the first stage, the possibility of the proton synthesis from water using solid water-insoluble acid catalysts was studied. At the second stage, acid catalysts, particularly, ones developed by the authors were used. In the use of H-kaolinite, Al2O3–B2O3, and Ti–SO3 acid catalysts for water treatment in the flow regime, 6 to 8.5% of protons were found in water. The formed protons are able to reduce at the second stage in a photoreactor until the hydrogen formation due to the interaction with electrons.

Numerical Study and Optimization of Parabolic Trough Solar Collector Receiver Tube

In this study, the parabolic trough collector's (PTC) performance is analyzed. In order to achieve this goal, the adopted procedure comprises two main steps. In the first step, the concentrated solar heat flux densities in the solar concentrator focal zone are calculated by soltrace software. In the second step, computational fluid dynamics (CFD) simulations are carried out to analyze and to optimize the thermal performance of the tube receiver. The calculated heat flux densities by soltrace software are used as wall heat flux boundary conditions for the receiver tube. The effect of the receiver tube diameter variation on the PTC thermal performance is studied. A new type of receiver tube is tested. This latter is covered with a metallic thickness. The performance of tube receiver covered with a metallic layer for different diameters is compared to those of the same diameters without the addition of metallic thickness. It has been found that increasing tube metallic thickness enhances the performance of PTC system comparing to the tubes of the same diameter and crossed by the same flow rates.

Diverging polygon-based modeling (DPBM) of concentrated solar flux distributions

This paper presents an efficient and robust methodology for modeling concentrated solar flux distributions. Compared to ray tracing methods, which provide high accuracy but can be computationally intensive, this approach makes a number of simplifying assumptions in order to reduce complexity by modeling incident and reflected flux as a series of simple geometric diverging polygons, then applying shading and blocking effects. A reduction in processing time (as compared to ray tracing) allows for evaluating and visualizing numerous combinations of engineering and operational variables (easily exceeding 106 unique iterations) to ascertain instantaneous, transient, and annual system performance. The method is demonstrated on a linear Fresnel reflector array and a number of variable iteration examples presented. While some precision is sacrificed for computational speed, flux distributions were compared to ray tracing (SolTrace) and average concentration ratio generally found to agree within ∼3%. This method presents a quick and very flexible coarse adjust method for concentrated solar power (CSP) field design, and can be used to both rapidly gain an understanding of system performance as well as to narrow variable constraint windows for follow-on high accuracy system optimization.

Experimental investigation on the energy and exergy performance of a coiled tube solar receiver

In this article, an experimental investigation is carried out to examine the heat transfer characteristics of a coil type solar dish receiver under actual concentrate solar radiation conditions. During the test, the concentrated solar flux is approximately 1000kW/m2 at aperture. The solar irradiance is almost unchanged (650W/m2) for continuous two hours in the afternoon, which is used to analyze the energy and exergy performance of the solar receiver. Experimental results show that, the efficiency of the solar receiver is normally above 70% with the highest efficiency of 82%, whereas at steady state, the efficiency is maintained at around 80%. A very low value of the heat loss factor (0.02kW/K) could be achieved during the current steady state operating conditions. The highest value of the exergy rate is around 8.8kW, whereas the maximum energy rate can reach 21.3kW. In addition, the highest exergy efficiency is approximately 28%, and the highest energy efficiency is around 82%.

Design of a Protection Thermal Energy Storage Using Phase Change Material Coupled to a Solar Receiver

AbstractThermal Energy Storage (TES) is the key for a stable electricity production in future Concentrated Solar Power (CSP) plants. This work presents a study on the thermal protection of the central receiver of CSP plant using a tower which is subject to considerable thermal stresses in case of cloudy events. The very high temperatures, 800 °C at design point, impose the use of special materials which are able to resist at high temperature and high mechanical constraints and high level of concentrated solar flux. In this paper we investigate a TES coupling a metallic matrix drilled with tubes of Phase Change Material (PCM) in order to store a large amount of thermal energy and release it in a short time. A numerical model is developed to optimize the arrangement of tubes into the TES. Then a methodology is given, based from the need in terms of thermal capacity, in order to help the choice of the geometry.

Thermal Presentation of Two-Phase Congested Thermosyphon in Submission of Determined Thermoelectric Dominance Producer by Means of Phase Change Material Thermal Storage

With the growing energy command due to growing world people and industrialization, utilizing strenuous solar energy for thermal and electrical power production will be the future renewable power source to decrease the confidence on fossil fuel and reduce carbon dioxide discharge. Besides by means of determined Photovoltaic (CPV) system, Concentrated Thermoelectric generator (CTEG) will be another possible option for sustainable power generation. The CTEG system utilizes concentrated solar flux as a heat source to the thermoelectric generating (TEG) module in generating direct current thermoelectricity which can be easily converted to alternating power using an inverter. By maintaining a temperature difference between the hot and cold sides of the thermoelectric cells (Seebeck effect), thermoelectricity is generated where its magnitude is a function of temperature difference. The main challenge is the effectiveness of excess heat removal which accumulated at the cold side of the thermoelectric cell to achieve greater power generation. Using acting cooling mechanisms are not energy efficient proposal as it requires power in operating them and significantly reduces the total power output generated. They are planned in the paper for working CTEG system through passive cooling and achieving invariable cooling. Two-phase closed thermosyphon is implement as an helpful heat transporting device for transfer excess heat from heated TEG part to the frozen PCM storeroom reservoir for heat luggage compartment. This investigation is to evaluate the thermal presentation of the proposed system.. The working fluid used in the thermosyphon was Acetone and its filling ratio was designated at 40% of the evaporator volume. Paraffin wax was selected as PCM with melting point of 47oC and acceptably high latent heat storage was selected in the thermal storage. The passive cooling mechanisms consist of PCM storage tank, heat pipe-based heat transfer system for transporting heat from TEG modules to the PCM thermal storage during the daytime and a similar heat pipe-based system for discharging heat from PCM storage tank to the cooler ambient during the night time.

Durability of solar reflector materials for secondary concentrators used in CSP systems

Secondary concentrators are used in solar concentrating systems to redirect solar beams reflected by the primary concentrators to the focal point or line. These components allow to increase the concentrated solar flux density and hence to lower thermal radiation losses. Solar reflectors for secondary concentrators are permanently exposed to environmental conditions, high radiation fluxes and elevated temperatures that potentially cause stress and degradation throughout the time. Therefore, analyzing solar reflectors of secondary concentrators by simulating these conditions is crucial. No previous research works about the durability of solar reflector materials for secondary concentrators have been reported. The present work is focused on studying the degradation of the reflector materials by simulating accelerated aging, caused by several ambient parameters and the effect of concentrated radiation. Both cooled and uncooled systems for secondary concentrators are included in this study. According to results obtained, aluminum reflectors and thin silvered-glass reflectors glued to an aluminum structure showed minimum reflectance losses and structural degradation under the operation conditions of cooled 3D secondary concentrators (tower systems). Following critical aspects to avoid reflector degradation were identified: to select a suitable adhesive material to glue the thin silvered-glass reflector to the support aluminum structure, to properly protect reflectors edges, to design a suitable cooling system and to avoid the combination of high radiation fluxes with mechanical stress. In addition, laminated silvered-glass reflectors have shown to be suitable for uncooled 2D secondary concentrators (Fresnel collectors). Furthermore, a comparison with naturally aged secondary concentrators using silvered-glass reflectors glued to an aluminum structure revealed that the simulated degradation under accelerated conditions performed in this work did reproduce the most frequent degradation patterns suffered in real operating conditions.

Influence of circumferential solar heat flux distribution on the heat transfer coefficients of linear Fresnel collector absorber tubes

The absorber tubes of solar thermal collectors have enormous influence on the performance of the solar collector systems. In this numerical study, the influence of circumferential uniform and non-uniform solar heat flux distributions on the internal and overall heat transfer coefficients of the absorber tubes of a linear Fresnel solar collector was investigated. A 3D steady-state numerical simulation was implemented based on ANSYS Fluent code version 14. The non-uniform solar heat flux distribution was modelled as a sinusoidal function of the concentrated solar heat flux incident on the circumference of the absorber tube. The k–ε model was employed to simulate the turbulent flow of the heat transfer fluid through the absorber tube. The tube-wall heat conduction and the convective and irradiative heat losses to the surroundings were also considered in the model. The average internal and overall heat transfer coefficients were determined for the sinusoidal circumferential non-uniform heat flux distribution span of 160°, 180°, 200° and 240°, and the 360° span of circumferential uniform heat flux for 10m long absorber tubes of different inner diameters and wall thicknesses with thermal conductivity of 16.27W/mK between the Reynolds number range of 4000 and 210,000 based on the inlet temperature. The results showed that the average internal heat transfer coefficients for the 360° span of circumferential uniform heat flux with different concentration ratios on absorber tubes of the same inner diameters, wall thicknesses and thermal conductivity were approximately the same, but the average overall heat transfer coefficient increased with the increase in the concentration ratios of the uniform heat flux incident on the tubes. Also, the average internal heat transfer coefficient for the absorber tube with a 360° span of uniform heat flux was approximately the same as that of the absorber tubes with the sinusoidal circumferential non-uniform heat flux span of 160°, 180°, 200° and 240° for the heat flux of the same concentration ratio, but the average overall heat transfer coefficient for the uniform heat flux case was higher than that of the non-uniform flux distributions. The average axial local internal heat transfer coefficient for the 360° span of uniform heat flux distribution on a 10m long absorber tube was slightly higher than that of the 160°, 200° and 240° span of non-uniform flux distributions at the Reynolds number of 4000. The average internal and overall heat transfer coefficients for four absorber tubes of different inner diameters and wall thicknesses and thermal conductivity of 16.27W/mK with 200° span of circumferential non-uniform flux were found to increase with the decrease in the inner-wall diameter of the absorber tubes. The numerical results showed good agreement with the Nusselt number experimental correlations for fully developed turbulent flow available in the literature.

A design method and numerical study for a new type parabolic trough solar collector with uniform solar flux distribution

The non-uniform concentrated solar flux distribution on the outer surface of the absorber tube can lead to large circumferential temperature difference and high local temperature of the absorber tube wall, which is one of the primary causes of parabolic trough solar receiver (PTR) failures. In this paper, a secondary reflector used as a homogenizing reflector (HR) in a conventional parabolic trough solar collector (PTSC) was recommended to homogenize the solar flux distribution and thus increase the reliability of the PTR. The design method of this new type PTSC with a HR was also proposed. Meanwhile, the concentrated solar flux distribution was calculated by adopting the Monte Carlo ray-trace (MCRT) method. Then, the coupled heat transfer process within the PTR was simulated by treating the solar flux calculated by the MCRT method as the heat flux boundary condition for the finite volume method model. The solar flux distribution on the outer surface of the absorber tube, the temperature field of the absorber tube wall, and the collector efficiency were analyzed in detail. It was revealed that the absorber tube could almost be heated uniformly in the PTSC with a HR. As a result, the circumferential temperature difference and the maximum temperature could be reduced significantly, while the efficiency tended to decrease slightly due to the inevitably increased optical loss. Under the conditions studied in this paper, although the collector efficiency decreased by about 4%, the circumferential temperature difference was reduced from about 25 to 3 K and the maximum temperature was reduced from 667 to 661 K.

Aging of solar absorber materials under highly concentrated solar fluxes

This article introduces an experimental approach to study the aging of absorber materials used in the receivers of CSP power plants. The term ‘aging’ is discussed in the case of solar absorber materials and a definition is suggested. A two-layer material (metal+paint coating) that is commonly used in solar tower receivers is studied. Accelerated aging tests are performed on this material with the Solar Accelerated Aging Facility (SAAF) available at PROMES laboratory (Odeillo, France). To evaluate the aging state of the samples after being solar-treated, the thermoradiative and thermophysical properties that characterize the aging of the coating are estimated with two experimental devices. The results show the influence of the mean irradiance, amplitude, period, and exposure time of the radiative cycles on the estimated properties. The absorptance appears to be the most sensitive property causing the thermal performance to decrease. Particular attention should be paid to the application of the paint coating which needs to be vitrified carefully in order to obtain optimal and stable absorption properties.

Optical Analysis and Thermal Modeling of a Window for a Small Particle Solar Receiver

Concentrated solar power (CSP) systems use heliostats to concentrate solar radiation in order to produce high temperature heat, which drives a turbine to generate electricity. A 5 MWth Small Particle Solar Receiver is being developed for power tower CSP plants based on volumetric absorption by a gas-particle suspension by the support from the U.S. Department of Energy under the SunShot Initiative. The radiation enters the pressurized receiver (0.5MPa) through a curved window, which must sustain the thermal loads from the concentrated solar flux and infrared reradiation from inside the receiver. The thermal load from the solar flux on the window is calculated by using the computer code MIRVAL from Sandia National Laboratory which uses the Monte Carlo Ray Trace (MCRT) method, along with two other codes developed by the authors. Thermal loading was calculated from energy absorbed at various points throughout the window from both the heliostat field and inside the receiver. Transmission and reflective losses were also calculated for different window materials in order to find out how much radiation will enter the receiver or will be lost. The three dimensional temperature distribution of the window is analyzed by an energy balance taking into account spectral volumetric absorption, spectral surface emission, conduction within the window, and convection from both surfaces. A maximum window temperature of 800°C must be enforced to prevent cracking and/or devitrification due to overheating. Several grades of quartz are considered for this study with detailed spectral calculations. For a chosen material, the window temperature was found to be held under 800°C. The results showed that most of the heat load on the window comes from the inside of receiver due to spectral variation.

Thermal Stresses Analysis of a Circular Tube in a Central Receiver

The tubes of central receiver power plants are designed to work under very demanding conditions, with heat fluxes varying between 0.2 to 1 MW/m2, depending on both the weather conditions and the operating regime of the plant. Also, the heat flux is localized along the circumferential coordinate of the tube, on the outward facing side of the tube. This results in a non-uniform temperature distribution with strong gradients both in the heat transfer fluid and the pipe walls. Predicting this distribution is important since it can induce high thermal stresses on the pipe walls and it is a key factor for the fluid decomposition.In this work we have studied different configurations to analyze the effects of tube diameter and wall thickness in the temperature distributions in the fluid and solid, for a typical concentrated solar heat flux distribution. HITEC salt and 316L stainless steel have been used as the heat transfer fluid and the material of the tubes in the receiver, respectively. A numerical method has been used to calculate the temperature distribution of the fluid and the convection coefficient along the circumference. Finally the Von-Mises thermal stresses of the tube receiver have been calculated.

Comparative study of different means of concentrated solar flux measurement of solar parabolic dish

The purpose of this work is to evaluate the energy coming to the focus of a solar parabolic concentrator SPC using four types of absorbers: flat plate, disk, water calorimeter (WC) and solar heat exchanger (SHE), developed in the Research and Technologies Centre of Energy in Borj Cedria in Tunisia (CRTEn). The temperature of the absorber, the solar energy absorbed by the receiver, the mean concentration ratio and both energy and exergy efficiency of the system were obtained experimentally. In order to validate the experimental results of the SHE and to determine the SPC efficiency using the disk, an analytical study based on thermodynamics analysis is carried out. We note that a good agreement between the four experimental methods has been obtained with the four receivers used in this work. The thermal energy efficiency varies from 40% to 77%, the concentrating system reaches an average exergy efficiency of 50% and a concentration factor around 178.

Optical performance evaluation of a solar furnace by measuring the highly concentrated solar flux

We evaluated optical performance of a solar furnace in the KIER (Korea Institute of Energy Research) by measuring the highly concentrated solar flux with the flux mapping method. We presented and analyzed optical performance in terms of concentrated solar flux distribution and power distribution. We investigated concentration ratio, stagnation temperature, total power, and concentration accuracy with help of a modeling code based on the ray tracing method and thereby compared with other solar furnaces. We also discussed flux changes by shutter opening angles and by position adjustment of reflector facets. In the course of flux analysis, we provided a better understanding of reference flux measurement for calibration, reflectivity measurement with a portable reflectometer, shadowing area consideration for effective irradiation, as well as accuracy and repeatability of flux measurements. The results in the present study will help proper utilization of a solar furnace by facilitating comparison between flux measurements at different conditions and flux estimation during operation.

Multiphysics well-stirred reactor modeling of coal gasification under intense thermal radiation

Gasification of carbonaceous materials using concentrated solar radiation can be an efficient and cost-effective way of supporting the endothermic reactions. To understand the effect of concentrated solar energy on the gasification process, we applied a multiphysics constant-volume well-stirred reactor model. The model considers detailed gas-phase chemistry, devolatilization kinetics, particle-phase reactions, film diffusion, pore evolution, as well as full coupling between the two phases at multiple scales for mass, species, and energy exchange. Numerical simulations were conducted using an in-house code to understand the effects of concentrated solar energy flux on conversion time, syngas yield, solar-to-fuel conversion efficiency, and solar-to-chemical enthalpy conversion percentage.

Control of the flux distribution on a solar tower receiver using an optimized aiming point strategy: Application to THEMIS solar tower

Life time of components is one of the technological bottle-necks in the development of solar tower power plant technology. The receiver, which is subjected to high and variable concentrated solar flux density is particularly affected: High, variable and non-homogeneous solar flux on the solar receiver walls results in strong stresses because of high temperatures, thermal shocks and temperature gradient that contribute to the reduction of the life time of this key component. This work aims to present an open loop approach to control the flux density distribution delivered on a flat plate receiver for a solar power tower. Various distributions of aiming points on the aperture of the receiver are considered. The flux density distribution on the aperture is simulated by a computer code. A specific neighborhood is defined for the TABU optimization meta-heuristic according to the size of the image of each individual heliostat. This modified algorithm is implemented to select the best aiming point for each heliostat. This approach has been validated using the example of THEMIS solar power tower in Targasonne, France.

Experimental and numerical investigation on solar concentrating characteristics of a sixteen-dish concentrator

The concentrated solar flux distributions of a sixteen-dish concentrator (SDC) were measured applying a thermal infrared imager in combination with water-cooled Lambert target, and predicted using a Monte Carlo ray tracing method (MCRT). A slope error of 2.2 mrad is detected by comparing the experimental and numerical results. Then, a two-stage concentrator system, formed by the SDC in tandem with a three-dimensional compound parabolic concentrator (3D CPC–SDC), is constructed based on the geometrical optics approach. The interception performances and the energy concentration ratio images (ECR) are presented for both the SDC and the 3D CPC–SDC. The results show that the ECR profiles of the SDC depend on the receiver sizes, whereas that of the 3D CPC–SDC is rather steady because most sunlight enters the receiver via several reflections with the 3D CPC mirror. The 3D CPC–SDC is capable of increasing the geometric concentration ratio (GCR) at the expense of a little interception efficiency.

Solar induced chemical vapor deposition of carbon from ethanol

Solar induced chemical vapor deposition (S-CVD) process using the concentrated solar radiation flux provided at the focus of a Fresnel lens (diameter 40 cm, focal length 40 cm) has been investigated. Carbon deposits on silicon and on silica have been produced using ethanol as the carbon precursor, highly diluted in hydrogen. A simple optical modeling results in a spot of 0.36 cm diameter with maximum radiation flux of 0.388 kW cm−2. The deposited spots morphology observed by Scanning Electron Microscopy (SEM) are round shaped with dimensions smaller than 500 × 500 μm2. Several flakes and holes of diameters around 2 μm are easily seen on both substrates. Raman results suggest that the deposits may be composed of complex structured carbon.