Optical performance analysis of an innovative linear focus secondary trough solar concentrating system

The parabolic trough solar concentrating system has been well developed and widely used in commercial solar thermal power plants. However, the conventional system has its drawbacks when connecting receiver tube parts and enhancing the concentration ratio. To overcome those inherent disadvantages, in this paper, an innovative concept of linear focus secondary trough concentrating system was proposed, which consists of a fixed parabolic trough concentrator, one or more heliostats, and a fixed tube receiver. The proposed system not only avoids the end loss and connection problem on the receiver during the tracking process but also opens up the possibility to increase the concentration ratio by enlarging aperture. The design scheme of the proposed system was elaborated in detail in this paper. Besides, the optical performance of the semi and the whole secondary solar trough concentrator was evaluated by using the ray tracing method. This innovative solar concentrating system shows a high application value as a solar energy experimental device.     

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。     

Numerical Investigation of Energy-Efficient Receiver for Solar Parabolic Trough Concentrator

In this paper, a thermal analysis of an energy-efficient receiver for solar parabolic trough concentrator is presented. Various porous receiver geometries are considered for the performance evaluation of a solar parabolic trough concentrator. Numerical models are proposed for a porous energy-efficient receiver for internal heat gain characteristics and heat loss due to natural convection. The internal flow and heat transfer analysis is carried out based on a RNG k-? turbulent model, whereas external heat losses are treated as a laminar natural convection model. The numerical models have been solved using the commercial engineering package, FLUENT. The thermal analysis of the receiver is carried out for various geometrical parameters, such as fin aspect ratio, thickness, and porosity, for different heat flux conditions. The inclusion of porous inserts in tubular receiver of solar trough concentrator enhanced the heat transfer about 17.5% with a pressure penalty of 2 kPa. The Nusselt number correlation is proposed based on the extensive numerical data for internal heat transfer inside the receiver. The proposed model is compared with more well-known natural convection models. A comparative study is carried out with different porous geometries to evolve an optimum configuration of energy-efficient receivers.     

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.     

Numerical investigation of an enhanced PTC absorber tube using cylindrical inserts

The main purpose of this study is to investigate numerically the thermal performance of a parabolic trough solar collector's absorber tube that contains a novel kind of inserts with the objective to improve the heat transfer between the heat transfer fluid and the absorber tube. In the first part of this paper, the diameter and the length of the cylindrical inserts are investigated based on finite volume method and Monte Carlo ray tracing method for Reynolds number ranges from 2.36 × 10⁴ to 7.09 × 10⁴. In the second part, the eccentricity of the cylindrical inserts is investigated under the same operating conditions. The Therminol®VP1 is the HTF that is used in this investigation's intermediate fluid. The numerical simulation indicates that the perturbators enhance the thermal behavior of the receiver and reduces the absorber tube's temperature difference.     

Performance analysis of Azimuth Tracking Fixed Mirror Solar Concentrator

The fixed mirror solar collector (FMSC) fixes reflector and mobiles receiver to collect solar energy. However, this type of concentrator has a low efficiency and short operating duration in practical applications. In this paper, we propose to install the FMSC on an azimuth tracking device (ATFMSC) and the reflectors are arranged by intermission to avoid the shading of neighbor reflector for incidence angle of less than 10° to improve its optical performance. Through the integration of the reflected solar radiation distribution function over any reflection point, and then the whole collector aperture, we develop the analytical expressions of various system efficiencies to numerically simulate the performance of ATFMSC with evacuated tube receiver in different design parameters. It is validated by the ray tracing results. The result shows that the mean annual net heat efficiency of the whole system would be up to 61% with the operating temperature of 400 °C, which is higher than parabolic trough collector and traditional FMSC. This is because the longitudinal incidence angle of ATFMSC always remains zero by tracking the sun azimuth, so the end loss of the concentrator can be avoided and enables it to operate with high efficiency continually.     

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.     

Numerical simulation of parabolic trough solar collector: Improvement using counter flow concentric circular heat exchangers

Detailed numerical simulations of thermal and fluid-dynamic behavior of a single-pass and double-pass solar parabolic trough collector are carried out. The governing equations inside the receiver tube, together with the energy equation in the tube walls and cover wall and the thermal analysis in the solar concentrator were solved iteratively in a segregated manner. The single-pass solar device numerical model has been carefully validated with experimental data obtained by Sandia National Laboratories. The effects of recycle at the ends on the heat transfer are studied numerically shown that the double-pass can enhance the thermal efficiency compared with the single-pass.     

Study of thermal characteristics of a heating module with parabolic trough concentrator and linear wedge-like photoelectric receiver

New solar modules intended for typical solar collectors containing semiparabolic trough concentrators and receivers that convert solar energy into thermal energy are considered. Mathematical modeling is carried out to develop an algorithm for estimating the structure of a heating module with the assigned energy parameters according to the laws of geometrical optics, as well as heat and mass transfer. When using such modules, which are based on a parabolic concentrator and a receiver with a system of coolant flow, cogeneration plants can be designed to produce electricity and heat. The mockups developed using this procedure are studied on the corresponding facilities and are tested under in-situ conditions. A solar module with an asymmetric parabolic trough concentrator and a linear wedge-like photoelectric receiver of concentrated radiation with a system of coolant flow provides the maximum power of 386 W at a temperature of 40°C and an efficiency of 60%, and 319 W at 60°C and 49%, respectively. Such modules are proposed for use to design solar collectors with the required performance.     

EXPERIMENTAL INVESTIGATION ON CONCENTRATOR-ASSISTED SOLAR-STILLS

The performance of a simple basin solar-still and a similar still coupled to an external trough type concentrator is investigated. The stills are basin-mounted inverted-V roof type, 1.67 m² area each. The assisting concentrator is a single trough-type finned tube 840 cm² receiver area with an adjustable trough half angle. The receiver tube and the basin form a thermosyphone closed loop generating a weak motion of the saline inside the stilt basin. Experiments have been conducted at Jeddah, Kingdom of Saudi Arabia ( Latitude 21^°45') for an average daily solar flux 300–500 w/ m² with water levels of 5–7 cm. A single correlation to predict the stills yield is obtained on basis of data for stills of different compass orientation. The productivity of the assisted still is 22% higher than that of the simple still under the same climatic conditions. For the present results, with fixed concentrator inclination, variation of the trough half angle has no appreciable effect upon the distillation process.     

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.     

Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.     

An optical analysis of a static 3-D solar concentrator

Concentrating technology is long established in the field of solar thermal applications. However, there is still scope for improvement due to innovation in design, materials and manufacturing methods. The optical efficiency of a solar concentrator depends largely on the geometry of the concentrator profile. This paper evaluates the optical performance of a static 3-D Elliptical Hyperboloid Concentrator (EHC) using ray tracing software. Ray tracing has been used extensively to calculate the optical efficiency of the static 3-D EHC. Performance parameters such as effective concentration ratio, optical efficiency and geometric concentration ratio are also evaluated for different aspect ratios of the elliptical profile. Optimization of the concentrator profile and geometry is also carried out to improve the overall performance; this parametric study includes the concentrator height, solar incidence angle and aspect ratio of the ellipse. The overall performance of the concentrator was assessed based on the acceptance angle, effective concentration ratio and optical efficiency. Finally, the flux distribution on the receiver area for different concentrator heights is also presented.     

Parabolic trough collector with rhombus tube absorber for higher concentration ratio

In the present study, the concentration ratio of the parabolic trough collector using rhombus tube absorber has been estimated. An analytical technique has been developed to determine the optimum size of the rhombus tube absorber for given trough dimensions. The optimum size of the rhombus tube absorber is 13.8% smaller than the circular tube absorber for the LS3 trough with no change in intercept factor. The maximum improvement in the concentration ratio is found to be 31.5% for the troughs with rim angle 90° in comparison to circular tube absorber. Results also indicate that rhombus tube absorber can be employed for a range of rim angle 75 to 90 degree.

Abbreviations: CR: concentration ratio; CSP: concentrated solar power; MCRT: Monte Carlo ray tracing; PTC: parabolic trough collector     

Modeling and simulation of ray tracing for compound parabolic thermal solar collector

A ray tracing model for the compound parabolic collector (CPC) is presented in this work. The pertinent parameters for the compound parabolic thermal solar collector are analyzed and calculated, and the ray tracing model is further investigated. The ray tracing model is validated by comparing our ray tracing model results with a commercial optical software. Each ray is traced by the CPC model, so the incident angle is calculated when solar ray enters the absorption tube. The ray tracing model was applied to the thermal efficiency analysis of the CPC, and the thermal performance results obtained by the model and test results were compared.     

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.     

Prediction and optimization of the performance of parabolic solar dish concentrator with sphere receiver using analytical function

Parabolic solar dish concentrator with sphere receiver is less studied. We present an analytic function to calculate the intercept factor of the system with real sun brightness distribution and Gaussian distribution, the results indicate that the intercept factor is related to the rim angle of reflector and the ratio of receiver angle to the optical error when the optical error is larger than or equal to 5 mrad, but is related to the rim angle, receiver angle and optical error in less than 5 mrad optical error. Furthermore we propose a quick process to optimize the system to provide the maximum solar energy to net heat efficiency for different optical error under typical condition. The results indicate that the parabolic solar dish concentrator with sphere receiver has rather high solar energy to net heat efficiency which is 20% more than solar trough and tower system including higher cosine factor and lower heat loss of the receiver.     

Conical receiver for a paraboloidal concentrator with large rim angle

The design and optimization of novel type of receiver for a paraboloidal concentrator with 90° rim angle is carried out by means of detailed ray tracing simulations. Cylindrical, conical, and spherical geometries are compared and their dimensions optimized. The chosen design is based on a conical cavity, which differs from similar receivers developed for concentrators with smaller rim angles. In particular, the receiver is able to catch concentrated solar energy both on its outer side and on the inner walls. Water flows inside the receiver along the conical geometry, in a double layer configuration. The receiver was built and implemented in a 90° rim angle paraboloidal concentrator. Thermal efficiency of the system is evaluated for different flow rates and inlet temperatures, both in stationary and in transient regimes, and results for fluid temperatures are compared with the results predicted by a thermal model. The time constant is evaluated.     

太阳能热发电系统中腔式吸热器的光学性能

建立了球形、圆柱形、圆锥形和平顶圆锥形4种典型腔式吸热器与抛物面聚光器的三维模型,利用蒙特卡洛光线追踪法预测了4种典型腔式吸热器内部辐射能流的分布,其中球形吸热器内部的辐射能流分布均匀性最好,且辐射峰值最小,具有较好的光学性能。通过统计逸出腔口的反射光计算出这4种腔式吸热器的反射光损,其中球形吸热器的反射光损最小。在聚光器反射率为0.9,腔体内壁吸收率为0.9时,球形吸热器反射光损仅为0.66%,聚光器/球形吸热器的光学效率为88.9%。     

Optimum aperture size and operating temperature of a solar cavity-receiver

For solar cavity-receivers operating at high temperatures, the optimum aperture size results from a compromise between maximizing radiation capture and minimizing radiation losses. When the absorbed solar energy is utilized as high temperature process heat, the energy conversion efficiency can be represented as the product of the energy absorption efficiency and the Carnot efficiency. We describe a simple, semiempirical method to determine the optimum aperture size and optimum operating temperature of a solar cavity-receiver for which its energy conversion efficiency is maximum. Such optimization strongly depends on the incident solar flux distribution at the aperture plane of the receiver. We analytically examine the case of a Gaussian distribution of the incident power flux, and we compare theoretical results with the results obtained when using an optically measured flux distribution. Using Monte-Carlo ray tracing, we further investigate the influence of sunshape on the optimal parameters of a cavity-receiver in a paraboloidal concentrator.