A MCRT and FVM coupled simulation method for energy conversion process in parabolic trough solar collector

A coupled simulation method based on Monte Carlo Ray Trace (MCRT) and Finite Volume Method (FVM) is established to solve the complex coupled heat transfer problem of radiation, heat conduction and convection in parabolic trough solar collector system. A coupled grid checking method is established to guarantee the consistency between the two methods and the validations to the coupled simulation model were performed. Firstly, the heat flux distribution on the collector tube surface was investigated to validate the MCRT method. The heat flux distribution curve could be divided into 4 parts: shadow effect area, heat flux increasing area, heat flux reducing area and direct radiation area. The heat flux distribution on the outer surface of absorber tube was heterogeneous in circle direction but uniform in axial direction. Then, the heat transfer and fluid flow performance in the LS-2 Solar Collector tube was investigated to validate the coupled simulation model. The outlet temperatures of the absorber tube predicted by the coupled simulation model were compared with the experimental data. The absolute errors are in the range of 1.5–3.7 °C, and the average relative error is less than 2%, which demonstrates the reliability of the coupled method established in this paper. At last, the concentrating characteristics of the parabolic trough collectors (PTCs) were analyzed by the coupled method, the effects of different geometric concentration ratios (GCs) and different rim angles were examined. The results show the two variables affect the heat flux distribution. With GC increasing, the heat flux distributions become gentler, the angle span of reducing area become larger and the shadow effect of absorber tube become weaker. And with the rim angle rising, the maximum value of heat flux become lower, and the curve moves towards the direction φ = 90°. But the temperature rising only augments with GC increasing and the effect of rim angle on heat transfer process could be neglected, when it is larger than 15°. If the rim angle is small, such as θ_rim = 15°, lots of rays are reflected by glass cover, and the temperature rising is much lower.     

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.     

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.     

Heat transfer analysis of receiver for large aperture parabolic trough solar collector

Parabolic trough solar collector (PTSC) is one of the most proven technologies for large‐scale solar thermal power generation. Currently, the cost of power generation from PTSC is expensive as compared with conventional power generation. The capital/power generation cost can be reduced by increasing aperture sizes of the collector. However, increase in aperture of the collector leads to higher heat flux on the absorber surface and results in higher thermal gradient. Hence, the analysis of heat distribution from the absorber to heat transfer fluid (HTF) and within the absorber is essential to identify the possibilities of failure of the receiver. In this article, extensive heat transfer analysis (HTA) of the receiver is performed for various aperture diameter of a PTSC using commercially available computational fluid dynamics (CFD) software ANSYS Fluent 19.0. The numerical simulations of the receiver are performed to analyze the temperature distribution around the circumference of the absorber tube as well as along the length of tube, the rate of heat transfer from the absorber tube to the HTF, and heat losses from the receiver for various geometric and operating conditions such as collector aperture diameter, mass flow rate, heat loss coefficient (HLC), HTF, and its inlet temperature. It is observed that temperature gradient around the circumference of the absorber and heat losses from the receiver increases with collector aperture. The temperature gradient around the circumference of the absorber tube wall at 2 m length from the inlet are observed as 11, 37, 48, 74, and 129 K, respectively, for 2.5‐, 5‐, 5.77‐, 7.5‐, and 10‐m aperture diameter of PTSC at mass flow rate of 1.25 kg/s and inlet temperature of 300 K for therminol oil as HTF. To minimize the thermal gradient around the absorber circumference, HTFs with better heat transfer characteristics are explored such as molten salt, liquid sodium, and NaK78. Liquid sodium offers a significant reduction in temperature gradient as compared of other HTFs for all the aperture sizes of the collector. It is found that the temperature gradient around the circumference of the absorber tube wall at a length of 2 m is reduced to 4, 8, 10, 13, and 18 K, respectively, for the above‐mentioned mass flow rate with liquid sodium as HTF. The analyses are also performed for different HTF inlet temperature in order to study the behavior of the receiver. Based on the HTA, it is desired to have larger aperture parabolic trough collector to generate higher temperature from the solar field and reduce the capital cost. To achieve higher temperature and better performance of the receiver, HTF with good thermophysical properties may be preferable to minimize the heat losses and thermal gradient around the circumference of the absorber tube.     

Real‐time thermal states monitoring of absorber tube for parabolic trough solar collector with non‐uniform solar flux

With the world energy shortage problem becoming increasingly prominent, more and more attentions have been paid to the development of renewable energies. Among these sources, solar energy has received extensive attention with its excellent characteristics. The thermal state affects the safety of the solar heat collection system. In this paper, real‐time monitoring of the input heat flux on the inside wall and the temperature field simultaneously of an absorber tube for parabolic trough solar collector were studied. Based on the measured temperatures on the outside wall, the fuzzy adaptive Kalman filter coupled with weighted recursive least squares algorithm (WRLSA) was employed to monitor the heat states of the absorber tube inversely, in which WRLSA was used to acquire the heat flux while fuzzy adaptive Kalman filter was adopted to monitor the temperature field. The method showed strong robustness to resist the ill‐posedness. Accurate monitoring results also can be acquired when there are random disturbances of the heat transfer condition on the inner wall.     

Numerical simulation of a parabolic trough solar collector with nonuniform solar flux conditions by coupling FVM and MCRT method

In this paper, a more detailed three-dimensional computational model of the whole parabolic trough solar collector (PTC) system and corresponding numerical simulations by combining the Finite Volume Method (FVM) and the Monte Carlo Ray-Trace (MCRT) method were presented. Corresponding codes and solving methods were also developed and applied to simulate and analyze the total involuted photo-thermal conversion process of an experimental LS2 PTC system. The numerical results were compared with experimental data and good agreement was obtained, proving that the model and method used in the present study is feasible and reliable. More details of the characteristics of solar concentrating, solar collecting, fluid dynamics, coupled heat transfer and the whole flow and temperature fields in the receiver were also revealed and discussed. Then some typical heat transfer fluid (HTF) types and residual gas conditions were further studied. It was revealed that the properties of these HTFs/conditions and their varying relations of the fluid temperature affected the characteristics of fluid dynamics, coupled heat transfer and the whole temperature distributions in the receiver, thus affected the thermal loss and the collector efficiency synthetically.     

具有二次聚光器的新型大开口槽式太阳集热器设计与光学性能分析

针对大开口和更高运行温度的槽式太阳能热发电系统,提出一种可实现高聚光比、低辐射热损及能流密度均匀的新型槽式太阳集热器,即在集热管内放置外壁具有太阳选择吸收膜层和内壁具有反射膜层二次聚光器的大开口槽式太阳集热器。建立圆弧为微元段的自适应设计新方法,提出3种典型的二次聚光器面型,利用蒙特卡洛光线追迹方法仿真新型集热器的能流密度分布特性,验证该光学仿真方法,分析影响集热器光学性能的各种因素。结果表明,该集热器可显著提升集热效率。     

Modeling of the Dish Receiver With the Effect of Inhomogeneous Radiation Flux Distribution

In this work, a new modeling coupling the inhomogeneous radiation flux distribution for the dish receiver is proposed and developed. The radiation transmission and absorbing process of the dish concentrating system is achieved by using the Monte Carlo ray tracing method (MCRT method), which reveals the high-order nonuniformity of the irradiance flux distribution on the inner wall of the dish receiver. The implementation of the three-dimensional numerical simulation coupling the heat loss of the dish receiver is by combining the microscopic MCRT method and the macroscopic SIMPLE method. In addition, a coupled photon statistic method is established to ensure the accuracy of heat flux distribution computation. The modeling result reveals that the temperature distributions of the inner receiver surface are significantly influenced by the inhomogeneous radiation flux. The temperature of the high local heat flux density area that lies in the middle part of the inner surface reaches 1374.8 K, which is even higher than the top area. In addition, the combined heat losses from natural convection and surface radiation are analyzed and compared respectively. It is found that the surface radiation heat loss is the predominant heat loss pattern of the combined heat transfer, and the natural convection loss is sensitive to solar intensity and the orientation of dish cavity receiver but changes little with the emissivity of the inner surface.     

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.     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.     

Optical evaluation and analysis of an internal low-concentrated evacuated tube heat pipe solar collector for powering solar air-conditioning systems

An optical evaluation and analysis of an internal low-concentrating evacuated tube heat pipe solar collector designed to enhance the collection of solar radiation for medium temperature applications is presented in this paper. The internal low-concentrating evacuated tube heat pipe solar collector was designed with an acceptance angle of 20° given a geometrical concentration ratio of 2.92. The truncation of the upper part of the reflector giving a geometrical concentration ratio of 1.95 was carried and enabled the internal low-concentrating evacuated tube heat pipe collector to be enclosed by a borosilicate glass tube with 100 mm and 93 mm outer and inner diameters, respectively. Ray trace analysis at different transverse angles determines optical efficiencies, related optical losses and flux distribution on the absorber of the internal low-concentrating evacuated tube heat pipe solar collector. A detailed two dimensional ray trace techniques considering only the direct insolation component predicated overall ray’s acceptance of 93.72% and optical efficiency of 79.13% from transverse angles of 0° to 20°.     

Experimental Study on Performance Enhancement of Evacuated Tube by Constant Heat Flux Mode

The evacuated tube collector with U shape copper absorber tube is considered for the analysis. The experimental investigation is conducted on parabolic trough collector with U shape tube as absorber tube. The effect of the sudden fluctuations in the solar radiation on the performance of the collector is reduced by means of evacuated tube collector filled with thermic fluids. The analysis is performed with different thermic fluids such as dowtherm, therminol66, glycol water and ethylene glycol, are filled in the annular space between inner glass tube and U shape copper absorber tube. The experimentation is carried out at various mass flow rates from 20 to 100 LPH with the step-up flow rate of 20 LPH. A comparative study is carried out on various parameters such as effect of mass flow rate over instantaneous efficiency, useful heat gain and work input, etc. The characteristic curve of cylindrical parabolic trough collector (PTC) is also discussed. Experimental results show that, ethylene glycol gives better efficiency over mass flow rate and therminol66 gives best power heat ratio. Heat transfer mediums and its properties [specific heat capacity, thermal conductivity and dynamic viscosity] for all specified heat transfer fluids are also discussed. The results obtained with various specified heat transfer fluids filled in the annulus space of evacuated tube are compared with plain evacuated tube. It is observed that there is significant enhancement of overall instantaneous collection efficiency of the parabolic trough collector.     

Experimental and numerical investigation on solar parabolic trough collector integrated with thermal energy storage unit

Solar parabolic trough collector (PTC) is the best recognized and commercial‐industrial‐scale, high temperature generation technology available today, and studies to assess its performance will add further impetus in improving these systems. The present work deals with numerical and experimental investigations to study the performance of a small‐scale solar PTC integrated with thermal energy storage system. Aperture area of PTC is 7.5 m², and capacity of thermal energy storage is 60 L. Paraffin has been used as phase change material and water as heat transfer fluid, which also acts as sensible heat storage medium. Experiments have been carried out to investigate the effect of mass flow rate on useful heat gain, thermal efficiency and energy collected/stored. A numerical model has been developed for the receiver/heat collecting element (HCE) based on one dimensional heat transfer equations to study temperature distribution, heat fluxes and thermal losses. Partial differential equations (PDE) obtained from mass and energy balance across HCE are discretized for transient conditions and solved for real time solar flux density values and other physical conditions of the present system. Convective and radiative heat transfers occurring in the HCE are also accounted in this study. Performance parameters obtained from this model are compared with experimental results, and it is found that agreement is good within 10% deviations. These deviations could be due to variations in incident solar radiation fed as input to the numerical model. System thermal efficiency is mainly influenced by heat gain and solar flux density whereas thermal loss is significantly influenced by concentrated solar radiation, receiver tube temperature and heat gained by heat transfer fluid. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermal stress analysis of eccentric tube receiver using concentrated solar radiation

In the parabolic trough concentrator with tube receiver system, the heat transfer fluid flowing through the tube receiver can induce high thermal stress and deflection. In this study, the eccentric tube receiver is introduced with the aim to reduce the thermal stresses of tube receiver. The ray-thermal-structural sequential coupled numerical analyses are adopted to obtain the concentrated heat flux distributions, temperature distributions and thermal stress fields of both the eccentric and concentric tube receivers. During the sequential coupled numerical analyses, the concentrated heat flux distribution on the bottom half periphery of tube receiver is obtained by Monte-Carlo ray tracing method, and the fitting function method is introduced for the calculated heat flux distribution transformation from the Monte-Carlo ray tracing model to the CFD analysis model. The temperature distributions and thermal stress fields are obtained by the CFD and FEA analyses, respectively. The effects of eccentricity and oriented angle variation on the thermal stresses of eccentric tube receiver are also investigated. It is recommended to adopt the eccentric tube receiver with optimum eccentricity and 90° oriented angle as tube receiver for the parabolic trough concentrator system to reduce the thermal stresses.     

Thermal enhancement of solar parabolic trough collectors by using nanofluids and converging-diverging absorber tube

Parabolic trough collectors are the most mature technology for utilizing the solar energy in high temperature applications. The objective of this study is the thermal efficiency enhancement of the commercial parabolic collector IST-PTC by increasing the convective heat transfer coefficient between the working fluid and the absorber. There are two main factors which influence on this parameter, the working fluid type and the absorber geometry. For this reason three working fluids are investigated, thermal oil, thermal oil with nanoparticles and pressurized water. Moreover, a dimpled absorber tube with sine geometry is tested because this shape increases the heat transfer surface and increases the turbulence in the flow. The final results show that these two techniques improve the heat transfer coefficient and the thermal efficiency of the collector. More specifically, the use of nanofluids increases the collector efficiency by 4.25% while the geometry improvement increases the efficiency by 4.55%. Furthermore, collector parameters such as the heat loss coefficient, the exergetic efficiency, the pressure losses and the absorber temperature are presented for all the examined cases. The model is designed with Solidworks and is simulated by its flow simulation studio.     

Optimal energy use of the collector tube in solar power tower plant

As one of the most important parts of the solar power tower plant, the receiver plays an important role in the high-efficiency operation of the solar power tower system. Obtaining the maximum available energy in the receiver is highly desired in real-world operations. In this paper heat transfer and exergy transfer methods are used to model the energy transfer process in a collector tube. Different from common analysis methods, in order to ensure the molten salt to obtain the maximum available energy, an objective function is proposed to convert the task into a constrained optimization problem. The gravitational search (GS) algorithm is employed to search for the optimal solution of the proposed objective function. Numerical results indicate that the proposed optimization method can find the optimal operation parameters under different conditions. The heat transfer and exergy transfer characteristics along the collector tube under the optimal working condition are revealed, which quantifies the available energy along the collector tube, as well as reveals the location of energy degradation in the tube. The research findings will provide a beneficial reference for the effective use of the solar energy.     

Effect of using nanofluids on efficiency of parabolic trough collectors in solar thermal electric power plants

In this paper, forced convection heat transfer nanofluid flow inside the receiver tube of solar parabolic trough collector is numerically simulated. Computational Fluid Dynamics (CFD) simulations are carried out to study the influence of using nanofluid as heat transfer fluid on thermal efficiency of the solar system. The three-dimensional steady, turbulent flow and heat transfer governing equations are solved using Finite Volume Method (FVM) with the SIMPLEC algorithm. The results show that the numerical simulation are in good agreement with the experimental data. Also, the effect of various nanoparticle volume fraction on thermal and hydrodynamic characteristics of the solar parabolic collector is discussed in details. The results indicate that, using of nanofluid instead of base fluid as a working fluid leads to enhanced heat transfer performance. Furthermore, the results reveal that by increasing of the nanoparticle volume fraction, the average Nusselt number increases.     

Heat transfer analysis of solar-driven high-temperature thermochemical reactor using NiFe-Aluminate RPCs

Converting solar energy efficiently into hydrogen is a promising way for renewable fuels technology. However, high-temperature heat transfer enhancement of solar thermochemical process is still a pertinent challenge for solar energy conversion into fuels. In this paper, high-temperature heat transfer enhancement accounting for radiation, conduction, and convection heat transfer in porous-medium reactor filled with application in hydrogen generation has been investigated. NiFe-Aluminate porous media is synthesized and used as solar radiant absorber and redox material. Experiments combined with numerical models are performed for analyzing thermal characteristics and chemical changes in solar receiver. The reacting medium is most heated by radiation heat transfer and higher temperature distribution is observed in the region exposed to high radiation heat flux. Heat distribution, O₂ and H₂ yield in the reacting medium are facilitated by convective reactive gas moving through the medium's pores. The temperature gradient caused by thermal transition at fluid-solid interface could be more decreased as much as the reaction chamber can store the transferred high-temperature heat flux. However, thermal losses due to radiation flux lost at the quartz glass are obviously inevitable.     

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.     

Analysis of wind flow around a parabolic collector (2) heat transfer from receiver tube

Parabolic collectors of commercial solar thermal power plants are subject to variable convection heat transfer from the receiver tube. In the present study heat transfer from a receiver tube of the parabolic trough collector of the 250 kW solar power plants in Shiraz, Iran, is studied taking into account the effects of variation of collector angel of attack, wind velocity and its distribution with respect to height from the ground.The governing equations for the two-dimensional steady state wind flow include continuity, momentum and energy equations and RNG-based k–ε model for turbulence scheme. Finite volume discretization method is used to solve the governing equations with wall function boundary condition and the SIMPLE approach is employed to iterate for the pressure correction and convergence of the velocity field. The momentum equation contains buoyancy force when the buoyancy effect is high and force convection effect is low.Computation is carried out for various wind velocities and different collector orientations with respect to wind direction. For solution of the energy equation, temperature of the receiver tube is taken as 350 K and ambient temperature is assumed to be 300 K. Various recirculation and temperature fields were observed around the receiver tube for different flow conditions. Effect of collector orientation on the average Nu number for the receiver tube was found negligible when the wind speed is low (Re⩽4.5×10⁵ based on the collector aperture). But when the wind velocity is high (Re>4.5×10⁵), the collector effect on the variation of Nu around the glass cover of the absorber tube is considerable.