Numerical study of heat transfer enhancement by unilateral longitudinal vortex generators inside parabolic trough solar receivers

This study presents numerical computation results on turbulent flow and coupled heat transfer enhancement in a novel parabolic trough solar absorber tube, the unilateral milt-longitudinal vortexes enhanced parabolic trough solar receiver (UMLVE-PTR), where longitudinal vortex generators (LVGs) are only located on the side of the absorber tube with concentrated solar radiation (CSR). The novel absorber tube and the corresponding parabolic trough receiver with smooth absorber tube (SAT-PTR) are numerical studied by combining the finite volume method (FVM) and the Monte Carlo ray-trace (MCRT) method for comparison and verification from the viewpoint of field synergy principle (FSP). Then the effects of Reynolds number, heat transfer fluid (HTF) inlet temperature, incident solar radiation and LVG geometric parameters were further examined. It was found that the mechanism of heat transfer enhancement of this novel absorber tube can be explained very well by the field synergy principle, and that the proposed novel UMLVE-PTR has good comprehensive heat transfer performance than that of the SAT-PTR within a wide range of major influence factors of diverse working conditions and geometric parameters.     

A technical note on application of internally finned tubes in solar parabolic trough absorber pipes

The heterogeneous incoming heat flux in solar parabolic trough absorber tubes generates huge temperature difference in each pipe section. Helical internal fins can reduce this effect, homogenising the temperature profile and reducing thermal stress with the drawback of increasing pressure drop. Another effect is the decreasing of the outer surface temperature and thermal losses, improving the thermal efficiency of the collector. The application of internal finned tubes for the design of parabolic trough collectors is analysed with computational fluid dynamics tools. Our numerical approach has been qualified with the computational estimation of reported experimental data regarding phenomena involved in finned tube applications and solar irradiation of parabolic trough collector. The application of finned tubes to the design of parabolic trough collectors must take into account issues as the pressure losses, thermal losses and thermo-mechanical stress, and thermal fatigue. Our analysis shows an improvement potential in parabolic trough solar plants efficiency by the application of internal finned tubes.     

Performance Characteristics of a Volumetric Solar Receiver: Presence of an Absorber Plate with a Selective Surface

Performance characteristics of a concentrated solar volumetric absorber are examined numerically. The thermal system considered consists of parabolic trough, glass tube, absorbing plate, and slurry containing 7% lauric acid as a phase change material and water as a carrier fluid. To assess the effect of the absorbing plate on performance characteristics, two locations of the absorbing plate on the glass tube surface are incorporated in the analysis. A selective surface is considered at the absorber plate surface for improved absorption of solar radiation and reduced thermal emission due to temperature increase at the surface. Temperature ratio, gain parameter, and pump power loss parameters are introduced to quantify the performance characteristics of the volumetric receiver. The study is extended to include the effect of Reynolds number on the receiver performance characteristics. A heating model incorporating radiation, convection, and conduction is adopted to simulate the thermal process. It is found that the gain parameter of the concentrated solar volumetric receiver improves by 15% when the absorber plate is located at the left face of the glass tube opposing the trough surface. The effect of Reynolds number on gain parameter is found to be inconsiderable.     

Analytic modeling of a solar power plant with parabolic linear collectors

An analytic model for a solar thermal electric generating system with parabolic trough collectors was developed. The energy conversion of solar radiation into thermal power along the absorber tube of the parabolic collector is studied, taking into consideration the non-linearity of heat losses and its dependence on the local temperature. The coupling between the collector and the thermodynamic cycle is made up of three heat exchangers, yielding the characteristic temperatures of the cycle. The conventional Rankine cycle is treated as an endo-reversible Carnot cycle, whereby the mechanical and electric power is calculated. For comparison, we refer to the Solar Electric Generating System VI (SEGS VI), installed in the Mojave desert-CA, whose solar field is composed by LS2 parabolic trough collectors. We simulated the efficiency curves of collectors LS2 with evacuated and non-evacuated absorbers and compared with experimental results. A second simulation was carried out to estimate the optimum quantity of non-evacuated LS2 collectors in series in a collectors’ row, when friction losses along the absorber tubes are considered. Also, the performance of a 30 (MWe) power plant, composed of 50 rows with 16 LS2 collectors in series (total 800 collectors) was simulated. Three fields of different collectors were considered, the first field with evacuated absorbers, the second with non-evacuated absorbers and the third with bare absorbers. Finally, the output power of the plant is analyzed as a function of the evaporation temperature of the water-vapor fluid. A large maximum of the overall cycle efficiency is found for evaporation temperatures around 320 °C. Good agreement is obtained when comparing the results of this model with experimental data belonging to the Solar Electric Generating Systems (SEGS) installed in the Mojave desert. The analytic model developed combines simplicity, precision and flexibility, making it an attractive tool for simulation and design of solar power stations.     

Modelling of parabolic trough direct steam generation solar collectors

Solar electric generation systems (SEGS) currently in operation are based on parabolic trough solar collectors using synthetic oil heat transfer fluid in the collector loop to transfer thermal energy to a Rankine cycle turbine via a heat exchanger. To improve performance and reduce costs direct steam generation in the collector has been proposed. In this paper the efficiency of parabolic trough collectors is determined for operation with synthetic oil (current SEGS plants) and water (future proposal) as the working fluids. The thermal performance of a trough collector using Syltherm 800 oil as the working fluid has been measured at Sandia National Laboratory and is used in this study to develop a model of the thermal losses from the collector. The model is based on absorber wall temperature rather than fluid bulk temperature so it can be used to predict the performance of the collector with any working fluid. The effects of absorber emissivity and internal working fluid convection effects are evaluated. An efficiency equation for trough collectors is developed and used in a simulation model to evaluate the performance of direct steam generation collectors for different radiation conditions and different absorber tube sizes. Phase change in the direct steam generation collector is accounted for by separate analysis of the liquid, boiling and dry steam zones.     

The calculation and analysis of glass-to-metal sealing stress in solar absorber tube

The failure or degradation of solar absorber tubes is the single largest cost factor for current parabolic trough solar power plant. The main failure reason is that there are residual stresses in the glass-to-metal joint which are generated during the cooling process of sealing. According to the thin shell theory and thermal stress theory, this paper presents the analytic solution for the glass-to-metal sealing residual stress. It also analyses how the thickness of glass tube, thickness of metal ring, and thermal expansion coefficient affect the residual stress distribution. In order to verify the calculation results, the photoelastic technique is used to measure the residual stress and the tensile test is used to obtain the point of the most dangerous stress and the tensile strength for the sealed specimens. It can be concluded that the maximum tensile stress happens at some distance near the sealing interface on the outer surface of glass tube. The seal strength increases when the thickness of the glass tube is increased. The analytic solution is proved feasible to analyze the residual stress of glass-to-metal seals in solar absorber tubes.     

Influences of installation and tracking errors on the optical performance of a solar parabolic trough collector

The parabolic trough collector is an important component of parabolic trough solar thermal power generation systems. Coordinate transformations and the Monte Carlo Ray Trace (MCRT) method were combined to simulate the circumferential flux distribution on absorber tubes. The simulation model includes the optics cone with non-parallel rays, geometric concentration ratios (GCs), the glass tube transmissivity, the absorber tube absorptance and the collector surface reflectivity. The mode is used to analyze the effects of absorber tube installation errors and reflector tracking errors. The results are compared with reference data to verify the model accuracy. Influences of installation and tracking errors on the flux distribution are analyzed for different errors, incident angles and GCs. For a GC of 20 and 90° rim angle, X direction installation errors are −0.2%∼0.2%, Y direction installation errors are −1.0%–0.5%, and the tracking error should be less than 4 mrad. As the incident angle increases, the errors become larger, but the errors become smaller as concentration ratios are increased. The results provide foundations for heat transfer analysis of the absorber tube, for parabolic trough plant to ensure the safe intensity, and for economic analysis of the installation process and control system.     

A comparison of optical performance between evacuated collector tubes with flat and semicylindric absorbers

In this article, concepts of solar irradiance ratio and absorbed energy factor on the surface of the evacuated collector tube absorbers were presented respectively. For evacuated collector tubes with flat and semicylindric absorbers, we used a solar simulator as a light source, measured separately distribution of the solar irradiance ratio that varies with incident angles on various points on the absorber surface in a glass-covered tube, and gave their three-dimensional regressive equations correspondingly. Experimental measurement of solar irradiance ratio and solar absorptance of coatings on absorber surfaces was carried out. On this basis, rules of absorbed energy factors on absorbers in two shapes that vary with incident angles were analyzed and studied. According to clear-day model, the daily absorbed energy and its annual changes of single evacuated collector tubes with absorbers in two shapes placed under 40° northern latitude, 40° inclined angle and south orientation were calculated and compared. The results show that the annual absorbed energy of evacuated collector tube with a semicylindric absorber is 15.9% higher than that with a flat absorber. In addition, optimized incident angles for the absorber in two shapes of evacuated collector tubes operated in a whole year were tentatively investigated.     

管簇结构腔体式吸收器热效率所受边界条件的影响

分析了槽形势物镜聚光集热器中管族结构腔体式吸收器的热物理特征，依据其热阻网络、太阳能一般性控制方程和计算框图对其热效率在自动跟踪条件下所受边界条件影响作了数值分析。     

Behavior of the compound wall copper–steel receiver with stratified two-phase flow regimen in transient states when solar irradiance is arriving on one side of receiver

Direct steam generation (DSG) in parabolic trough concentrators can cause some instabilities in steel absorbers. These instabilities are caused mainly along the stratified two-phase flow related to the boiling process of water. These instabilities are manifested as a transient state and shown as a bending of the absorber, causing a rupture of the glass envelope of the receiver. This bending has been associated with the thermal gradient produced by the boiling of water, and the effects are related to the thermal conductivity of the absorber, the flow pattern and heating of the absorber from below around solar noon.When solar beam irradiance is arriving on one side of the absorber, the bending, which is always upwards, can be up to 50 mm. This is the worst condition because the temperature difference is at its maximum at a transient state. This paper presents the results of a bimetallic absorber (copper–steel), which should reduce such deflections due to the high thermal conductivity of the copper and the mechanical resistance of the steel. In order to compare the results of the theoretical model with experimental results, it was necessary to build a bimetallic absorber 3 m long with a 31.7 mm internal diameter and a thickness of 2.6 mm. The bending obtained experimentally was between 5 and 10 mm, depending on the mass flow and the temperature difference in the wall receiver.As a main conclusion, the bending of the receiver is acceptable, eliminating the rupture of the glass envelope, and it may be used in DSG of parabolic trough collectors at low flows. The objective of this work is to apply DSG to low power generation with parabolic trough plants.     

Investigation of evacuated tube heated by solar trough concentrating system

Two types of solar evacuated tube have been used to measure their heating efficiency and temperature with fluids of water and N₂ respectively with a parabolic trough concentrator. Experiments demonstrate that both evacuated tubes present a good heat transfer with the fluid of water, the heating efficiency is about 70–80%, and the water is easy to boil when liquid rate is less than 0.0046 kg/s. However, the efficiency of solar concentrating system with evacuated tube for heating N₂ gas is less than 40% when the temperature of N₂ gas reaches 320–460 °C. A model for evacuated tube heated by solar trough concentrating system has been built in order to further analyze the characteristics of fluid which flow evacuated tube. It is found that the model agrees with the experiments to within 5.2% accuracy. The characteristics of fluid via evacuated tube heated by solar concentrated system are analyzed under the varying conditions of solar radiation and trough aperture area. This study supports research work on using a solar trough concentrating system to perform ammonia thermo-chemical energy storage for 24 h power generation. The current research work also has application to solar refrigeration.     

Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube

The thermal performance of a glass evacuated tube solar collector is numerically and experimentally investigated. The solar collector considered in this paper consists of a two-layered glass tube and an absorber tube. Air is used as the working fluid. The length and diameter of this glass tube are 1200 and 37 mm, respectively. Four different shapes of absorber tubes are considered, and the performances of the solar collectors are studied to find the best shape of the absorber tube for the solar collector. Beam irradiation, diffuse irradiation, and shade due to adjacent tubes are taken into account for a collector model to obtain a realistic estimation. In addition, a single collector tube with only beam irradiation is studied as a simplified model, and the results of the simplified model are compared to those of the collector model to identify the difference between these two models. The performance of a solar collector is affected by the shape of the absorber, incidence angle of solar irradiation, and arrangement of collector tubes. The results obtained from the simplified model are very different from those from the collector model, which considered not only beam and diffuse irradiation but also shade due to adjacent tubes.     

On the deduction of single coal particle combustion temperature from three-color optical pyrometry

Temperature–time histories of burning single coal particles can be obtained with multi-color (multi-wavelength) optical pyrometry. With this method, a number of different temperatures can be deduced from the resulting number of two-color ratios. However, these two-color temperatures do not always agree, causing considerable uncertainty in the temperature measurement. This work used a three-color pyrometer and focused on identifying and minimizing the causes of disparity among the three deduced temperatures. Components of the pyrometer (such as dichroic filters, interference filters and photo-detectors) were modeled mathematically, taking into account their wavelength-dependent properties. The pyrometer was calibrated with both a high-temperature pre-calibrated tungsten lamp, and a moderately-high temperature blackbody cavity, to span the temperature range of interest in pulverized coal combustion. Temperatures were deduced based not only on a suitably-modified pyrometric signal ratio method but also, on a similarly modified pyrometric signal non-linear least-square method, to provide comparison. Results are exemplified by presenting radiation-signal-time and temperature–time profiles of single particles burning in air. The variation of the projected luminous area of burning particles was also computed using both methods, and area–time profiles are presented herein. The char particle emissivity was either treated as a quantity independent of the wavelength (i.e., assuming gray-body behavior), or as a quantity assumed to depend linearly on the wavelength and using pertinent published emissivity data. Finally, a sensitivity analysis was performed to investigate individual effects of parameters, such as the calibration method, the wavelength dependencies of filter transmissivities, and the photo-detector responsivities on the pyrometric signal ratio method temperature consistency.     

太阳能集热组合墙系统的耦合传热与流动分析

针对太阳能集热组合墙系统，分析了太阳辐射及环境温度变化时，组合墙内传热与流动变化。太阳能集热组合墙系统中，多孔介质起半透明隔热体和蓄热体的作用。多孔介质的空隙率、粒径大小对系统的采暖效果影响较大。     

Simulation model of a CPC collector with temperature-dependent heat loss coefficient

We describe a mathematical model for the optical and thermal performance of non-evacuated CPC solar collectors with a cylindrical absorber, when the heat loss coefficient is temperature-dependent. Detailed energy balance at the absorber, reflector and cover of the CPC cavity yields heat losses as a function of absorber temperature and solar radiation level. Using a polynomial approximation of those heat losses, we calculate the thermal efficiency of the CPC collector. Numerical results show that the performance of the solar collector (η vs. ΔT_f(0)/I_coll) is given by a set of curves, one for each radiation level. Based on the solution obtained to express the collector performance, we propose to plot efficiency against the relation of heat transfer coefficients at absorber input and under stagnation conditions. The set of characteristic curves merge, then, into a single curve that is not dependent on the solar radiation level. More conveniently, linearized single plots are obtained by expressing efficiency against the square of the difference between the inlet fluid temperature and the ambient temperature divided by the solar radiation level. The new way of plotting solar thermal collector efficiency, such that measurements for a broad range of solar radiation levels can be unified into a single curve, enables us to represent the performance of a large class of solar collectors, e.g. flat plate, CPC and parabolic troughs, whose heat loss functions are well represented by second degree polynomials.     

Study of the distribution of the absorbed solar radiation on the performance of a CPC-type ICS water heater

An Integrated Collector Storage (ICS) solar water heater was designed, constructed and studied with an emphasis on its optical and thermal performance. The ICS system consists of one cylindrical horizontal tank properly mounted in a stationary symmetrical Compound Parabolic Concentrating (CPC) reflector trough. The main objective was the design and the construction of a low cost solar system with improved thermal performance based on the exploitation of the non-uniform distribution of the absorbed solar radiation on the cylindrical storage tank surface. A ray-tracing model was developed to gauge the distribution of the incoming solar radiation on the absorber surface and the results were compared with those from a theoretical optical model based on the average number of reflections. The variation of the optical efficiency as function of the incident angle of the incoming solar radiation along with its dependence on the month during annual operation of ICS system is presented. The ICS device was experimentally tested outdoors during a whole year in order to correlate the observed temperature rise and stratification of the stored water with the non-uniform distribution of the absorbed solar radiation. The results show that the upper part of the tank surface collects the larger fraction of the total absorbed solar radiation for all incident angles throughout the year. This is found to have a significant effect on the overall thermal performance of the ICS unit. In addition, the presented results can be considered important for the design and the operation of ICS systems consisting of cylindrical tank and CPC reflectors.     

多孔介质太阳能集热组合墙的耦合传热与流动分析

针对接触型和分隔型多孔介质太阳能集热组合墙系统,分析了太阳辐射及环境温度变化时,组合墙内传热与流动变化.多孔介质太阳能集热组合墙中,多孔介质起半透明隔热体和蓄热体的作用.多孔介质集热层的孔隙率、粒径、材料热导率和多孔介质集热层在组合墙中的位置对系统的采暖效果影响较大.     

Flow distribution in a solar collector panel with horizontally inclined absorber strips

The objective of this work is to theoretically and experimentally investigate the flow and temperature distribution in a solar collector panel with an absorber consisting of horizontally inclined strips. Fluid flow and heat transfer in the collector panel are studied by means of computational fluid dynamics (CFD) calculations. Further, experimental investigations of a 12.5 m² solar collector panel with 16 parallel connected horizontal fins are carried out. The flow distribution through the absorber is evaluated by means of temperature measurements on the backside of the absorber tubes. The measured temperatures are compared to the temperatures determined by the CFD model and there is a good similarity between the measured and calculated results.

Calculations with the CFD model elucidate the flow and temperature distribution in the collector. The influences of different operating conditions such as flow rate, properties of solar collector fluid, solar collector fluid inlet temperature and collector tilt angle are shown. The flow distribution through the absorber fins is uniform if high flow rates are used. By decreased flow rate and decreased content of glycol in the glycol/water mixture used as solar collector fluid, and by increased collector tilt and inlet temperature, the flow distribution gets worse resulting in an increased risk of boiling in the upper part of the collector panel.     

槽式太阳能集热器热性能实验测试

针对天津市槽式太阳能集热系统性能测试平台,对不同太阳辐射强度、入口流体温度以及不同工质流量状况下集热效率和集热管压降变化规律进行实验测试,通过测试数据对槽式太阳能集热器热性能进行分析。试验结果表明:在天津地区槽式太阳能集热器集热效率可以达到66.1%;太阳辐射强度的增强,会提高集热效率,并且集热器进出口的压降会随之降低;在太阳辐射强度和导热油流量一定时入口流体温度存在最佳值;集热效率和集热器进出口的压降随着流量的增加而增大。     

Heat extraction efficiency of a concentric glass tubular evacuated collector

The results of detailed measurements and calculations of the properties of Sydney University/Nitto Kohki evacuated collector tubes have been used to develop a formula for the instantaneous heat extraction efficiency η of a collector panel incorporating the evacuated tubes. The instantaneous efficiency depends on ambient temperature, mean fluid temperature in the collector, solar flux and the design of the manifold used to extract heat from the glass absorber tubes. Manifold design determines the mean temperature difference between absorber tube surface and mean fluid temperature for given operating conditions, and strongly affects the efficiency η of a collector panel. Neither changes in the number of evacuated tubes per unit area of collector, nor variations in solar flux, significantly alter the efficiency decrement Δ η₀ associated with a particular manifold design. Calculated efficiencies agree well with experimental results for collector panels incorporating manifolds of various designs. The formula for efficiency η allows detailed analysis of the relative importance of various energy loss mechanisms in a collector.