Ray tracing and simulation for the beam-down solar concentrator

The ray tracing equations for the beam-down solar concentrator have been derived in this paper. Based on the equations, a new module for the simulation of the beam-down solar concentrating system has been developed and incorporated into the code HFLD. To validate the ray tracing equations, a simple beam-down solar concentrating system consisting of 3 heliostats and a hyperboloid reflector is simulated. The concentrated spots at the lower focal point of the hyperboloid reflector for the beam-down system are calculated by the modified code HFLD and then compared with that calculated by the commercial software Zemax. It is found that the calculated results coincide with each other basically. Furthermore, a beam-down solar concentrator consisting of 31 heliostats, a tower reflector and a CPC is designed and simulated by using the modified code HFLD. The concentrated spots of the beam-down solar concentrator are calculated.     

A new method for the design of the heliostat field layout for solar tower power plant

A new method for the design of the heliostat field layout for solar tower power plant is proposed. In the new method, the heliostat boundary is constrained by the receiver geometrical aperture and the efficiency factor which is the product of the annual cosine efficiency and the annual atmospheric transmission efficiency of heliostat. With the new method, the annual interception efficiency does not need to be calculated when places the heliostats, therefore the total time of design and optimization is saved significantly. Based on the new method, a new code for heliostat field layout design (HFLD) has been developed and a new heliostat field layout for the PS10 plant at the PS10 location has been designed by using the new code. Compared with current PS10 layout, the new designed heliostats have the same optical efficiency but with a faster response speed. In addition, to evaluate the feasibility of crops growth on the field land under heliostats, a new calculation method for the annual sunshine duration on the land surface is proposed as well.     

A new code for the design and analysis of the heliostat field layout for power tower system

A new code for the design and analysis of the heliostat field layout for power tower system is developed. In the new code, a new method for the heliostat field layout is proposed based on the edge ray principle of nonimaging optics. The heliostat field boundary is constrained by the tower height, the receiver tilt angle and size and the heliostat efficiency factor which is the product of the annual cosine efficiency and the annual atmospheric transmission efficiency. With the new method, the heliostat can be placed with a higher efficiency and a faster response speed of the design and optimization can be obtained. A new module for the analysis of the aspherical heliostat is created in the new code. A new toroidal heliostat field is designed and analyzed by using the new code. Compared with the spherical heliostat, the solar image radius of the field is reduced by about 30% by using the toroidal heliostat if the mirror shape and the tracking are ideal. In addition, to maximize the utilization of land, suitable crops can be considered to be planted under heliostats. To evaluate the feasibility of the crop growth, a method for calculating the annual distribution of sunshine duration on the land surface is developed as well.     

Modeling and simulation of the pioneer 1 MW solar thermal central receiver system in China

DAHAN, the pioneer 1 MWe CRS (central receiver system) funded by Ministry of Sciences and Technology (MOST), which can be regarded as the milestone in solar thermal power development in China, is now under construction at the foot of The Great Wall nearby Beijing. The major objective of the design and construction of DAHAN is to demonstrate the operation of CRS in China. A software tool HFLD is developed for heliostat field layout design and performance calculation. The simulation results from HFLD approximately agree very well with the published heliostat field efficiency data from Spain PS10. Based on that, the heliostat field layout of DAHAN is designed using HFLD and the whole CRS performance is simulated in the TRNSYS plant model. The modeling and simulation of this plant is presented in this paper.     

Accurate altitude-azimuth tracking angle formulas for a heliostat with mirror-pivot offset and other fixed geometrical errors

High precision tracking formulas were developed for a receiver-oriented toroidal heliostat with the standard spinning-elevation tracking geometry in a previous paper. The spinning-elevation tracking geometry included some mirror-pivot offset, orthogonal intersecting rotational axes and the elevation axis being parallel to the mirror surface plane. This paper analyzes the tracking accuracy of these standard spinning elevation tracking formulas to show that they are accurate with negligible tracking error. Hence, the mirror-surface-center normal obtained from these formulas is accurate for any dual-axis tracking heliostat. Then, the accurate mirror-surface-center normal information is used to determine general altitude-azimuth tracking angles for a heliostat with a mirror-pivot offset and other geometrical errors. The main geometrical errors in a typical altitude-azimuth tracking geometry are the azimuth axis tilt from the vertical, the non-orthogonality between the two heliostat rotational axes, the non-parallel degree between the mirror surface plane and the altitude axis, and the encoder reference errors. An actual heliostat in a solar field is used as an example to demonstrate use of the general altitude-azimuth tracking formulas, with the tracking angles for this heliostat on typical days graphically illustrated. The altitude-azimuth tracking angle formulas are further verified by an indoor laser-beam tracking test on a specially designed heliostat model.     

Tracking formulas and strategies for a receiver oriented dual-axis tracking toroidal heliostat

A 4 m × 4 m toroidal heliostat with receiver oriented dual-axis tracking, also called spinning-elevation tracking, was developed as an auxiliary heat source for a hydrogen production system. A series of spinning-elevation tracking formulas have been derived for this heliostat. This included basic tracking formulas, a formula for the elevation angle for heliostat with a mirror-pivot offset, and a more general formula for the biased elevation angle. This paper presents the new tracking formulas in detail and analyzes the accuracy of applying a simplifying approximation. The numerical results show these receiver oriented dual-axis tracking formula approximations are accurate to within 2.5 × 10⁻⁶ m in image plane. Some practical tracking strategies are discussed briefly. Solar images from the toroidal heliostat at selected times are also presented.     

Efficiency assessment for a small heliostat solar concentration plant

In this paper, a small non‐imaging focusing heliostat is presented, and an analytical model for assessing its performance is described. The main novelty of the system lies in the tracking mechanism and the mirror mount, which are based on off‐the‐shelf components and allow a good trade‐off between accuracy and costs. The concentrator mirrors are moved by this two‐axis tracking machinery to reflect the sun's rays onto a fixed target, the dimensions of which can be varied to suit the user's needs. A prototype plant to be located in central Italy was designed and simulated with a ray‐tracing algorithm, and it comprises 90 heliostats for a total reflective area of 7.5 m². The reflected solar rays are tracked taking the mechanical positioning errors of the tracking system into account. The total flux of radiation energy hitting the target was determined, and intensity distribution maps were drawn. Simulations showed that the system's optical efficiency can exceed 90% in summer, despite the tracking errors, mainly because of the smaller distance between the heliostats and the receiver. The solar concentration ratio over a receiver of 250 mm in diameter reached 80 suns with a very good uniformity. Over a 400‐mm receiver, the concentrated radiation was less uniform, and the solar concentration ratio reached 50 suns, with a higher optical efficiency and collected solar radiation. The present concentration ratio is still suitable for many applications ranging from the electric power production, industrial process heat, and solar cooling. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

一种新型极轴跟踪式定日镜的研究

提出一种新型采用轮胎面聚光镜极轴跟踪式定日镜。分析了极轴跟踪式定日镜的跟踪原理,设计了用于该定日镜的轮胎面并分析其聚光性能。结果表明:该定日镜聚光性能较高,能够满足生活热水、太阳炉等应用领域的要求。     

Design optimization of a solar tower power plant heliostat field by considering different heliostat shapes

The present study focuses on the optimization of solar tower power plant heliostat field by considering different heliostat shapes including rectangular, square, pentagon, hexagon, heptagon, octagon, and circular heliostat shapes. The optimization is carried out using an in-house developed code-based MATLAB program. The developed in-house code is validated first on a well-known PS10 Solar Thermal Power plant having rectangular heliostats shape and the resulting yearly unweighted heliostat field efficiency of about 64.43% could be obtained. The optimized PS10 heliostat field using different heliostat shapes showed that the circular and octagon heliostat shapes provide better efficiency with minimum land area. The yearly efficiency is increased from 69.65% for the rectangular heliostat shape to 70.96% and 71% for the octagon and circular shapes, respectively. In addition, the calculated field area (land area) is reduced for the case of circular and octagon heliostat shapes with a gain of about 11.10% and 10.93% (about 42.0436 × 10³ and 41.4036 × 10³ m²), respectively, in comparison with the PS10 field area.     

Properties of a general azimuth–elevation tracking angle formula for a heliostat with a mirror-pivot offset and other angular errors

The solar field of a central receiver system (CRS) is an array of dual-axis tracking heliostats on the ground beside or around a central tower, with each heliostat tracking the sun to continuously reflect the solar beam onto the fixed tower-top receiver. Azimuth–elevation tracking (also called altitude–azimuth tracking) is the most common and popular tracking methods used for heliostats. A general azimuth–elevation tracking angle formula was developed previously for a heliostat with a mirror-pivot offset and other typical geometric errors. The angular error parameters in this tracking angle formula are the tilt angle, ψ_t, and the tilt azimuth angle, ψ_a, for the azimuth axis from the vertical direction, the dual-axis non-orthogonal angle, τ₁ (bias angle of the elevation axis from the orthogonal to the azimuth axis), and the non-parallel degree, μ, between the mirror surface plane and the elevation axis (canting angle of the mirror surface plane relative to the elevation axis). This tracking angle formula is re-rewritten here as a series of easily solved expressions. A more numerically stable expression for the mirror-center normal is then presented that is more useful than the original mirror–normal expression in the tracking angle formula. This paper discusses some important angular parametric properties of this tracking angle formula. This paper also gives an approach to evaluate the tracking accuracy around each helistat rotational axis from experimental tracking data using this general tracking angle formula. This approach can also be used to determine the heliostat zero angle positioning errors of the two rotational axes. These supplementary notes make the general azimuth–elevation tracking angle formula more useful and effective in solar field tracking designs.     

Modernization of an automated controlling system for heliostat field of big solar furnace

This article presents the results of scientific–technical and engineering works on modernizing a big solar furnace (BSF) developed by the Institute of Materials Science, Academy of Sciences of the Republic of Uzbekistan, including works concerning the development of an automated control system for heliostats and the development of microprocessor local control systems for each individual heliostat that are managed by a mainframe computer with the relevant software, highly-precise sensors of the rotation angles of heliostat axes, and digital sun-tracking sensors. The system under consideration provides for a two-way command and data exchange between all local systems and the mainframe computer. This exchange will make it possible to perform the software control of the operation of each individual heliostat, including sun tracking based on a solar sensor, tracking based on mirror turning, and direct positioning, as well as to continuously monitor the heliostat field state, perform self-diagnostics, and maintain an automated heliostat field operation log. This article also describes works on modernizing mechanisms for heliostat rotation and motion control: the replacement of brush-type dc motors by more reliable asynchronous motors with inverters, installation of angular sensors, and reconstruction of rotation mechanisms.     

Sensor-controlled heliostat with an equatorial mount

A heliostat having a photo-sensor sun-tracking system was developed and evaluated. The sensor was composed of a set of two photo-cells placed side by side on the bottom of the small box. Sun-tracking can be achieved by rotating the heliostat equipped with the sensor, while maintaining the two photo-cells under illumination by the sun through a slit in the box. A preliminary tracking evaluation of the sensor was carried out with the aid of a mirror-telescope system, and the tracking error was estimated to be less than 0.6 mrad in clear weather. The developed heliostat employed an equatorial mount system that permits the rotating speed of the right-ascension axis to be nearly constant for the diurnal motion of the sun. The use of two additional sensors, a cloud sensor and a primary sensor, permitted stable tracking with high accuracy even in a cloudy sky. Field tests of the heliostat revealed that an angular error within 2 mrad was achieved in fine weather. In cloudy weather, the heliostat operated stably with the cloud sensor within an error of 10 mrad.     

Impact of heliostat curvature on optical performance of Linear Fresnel solar concentrators

The present paper gives a numerical investigation of the effect of mirror curvature on optical performance of a Linear Fresnel Reflector solar field installed recently in Morocco. The objective is to highlight and discuss the effect of mirror curvature on the flux density distribution over the receiver and the system optical efficiency. For this purpose, a Monte Carlo-ray tracing simulation tool is developed and used to optimize the optical design taking into account the curvature degree of the heliostat field. In order to assess the accuracy of the numerical code developed and the validity of simulation results, a set of verification tests were developed and detailed within this article. Then, the optical performance of the system is evaluated as a function of mirror curvature and receiver height. The major challenge of this study is to find a trade-off between heliostat curvature and receiver height since lower and smaller receivers may reduce the system cost. It has been found that the flux distribution over the receiver and the optical efficiency of the system are relatively sensitive to the mirror curvature. We have demonstrated quantitatively how the use of curved mirrors can enhance the optical performance and reduce the required receiver size.     

Measurements of solar flux density distribution on a plane receiver due to a flat heliostat

An experimental facility is designed and manufactured to measure the solar flux density distribution on a central flat receiver due to a single flat heliostat. The tracking mechanism of the heliostat is controlled by two stepping motors, one for tilt angle control and the other for azimuth angle control. A x-y traversing mechanism is also designed and mounted on a vertical central receiver plane, where the solar flux density is to be measured. A miniature solar sensor is mounted on the platform of the traversing mechanism, where it is used to measure the solar flux density distribution on the receiver surface. The sensor is connected to a data acquisition card in a host computer. The two stepping motors of the heliostat tracking mechanism and the two stepping motors of the traversing mechanism are all connected to a controller card in the same host computer. A software “TOWER” is prepared to let the heliostat track the sun, move the platform of the traversing mechanism to the points of a preselected grid, and to measure the solar flux density distribution on the receiver plane. Measurements are carried out using rectangular flat mirrors of different dimensions at several distances from the central receiver. Two types of images were identified on the receiver plane—namely, apparent (or visible) and mirror-reflected radiation images. Comparison between measurements and a mathematical model validates the mathematical model.     

Performance of low cost solar reflectors for transferring sunlight to a distant collector

Efficiency of reflection and optical transmission to a distant collector is a critical parameter, along with cost per unit area, in the selection of a heliostat design for the Central Collector Solar Electric Plant. Efficient optical transmission is not easily accomplished because of the large distance to be spanned in a multi-MW facility. Depending on heliostat location, the transmission distance may vary from a few hundred to thousands of feet.Design conditions which influence optical transmission over these long distances are: heliostat pointing accuracy; spreading of the reflected solar beam due to the finite size of the Sun's image; beam spreading due to reflector misalignment or waviness; aberration present if curved heliostat reflectors are used and beam spreading due to microscopic irregularities (characteristic length less than 0.1 mm) in the reflective surface. These factors increase in importance as the transmission distance from heliostat to collector increases. Even the most preliminary heliostat design activity requires a detailed evaluation of beam spreading before the most cost effective heliostat concept, or family or concepts depending on transmission distance, can be defined.Data are presented here which will be of value in assessing one of the factors causing beam spreading. An experimental method has been utilized to determine beam spreading due to microscopic surface irregularities prevalent with “mill finished” materials. The test method provides a nearly independent measure of the effect of surface imperfections.Data are presented for five candidate materials and, as reference, an optical quality first surface mirror.     

Low-profile heliostat design for solar central receiver systems

Heliostat designs intended to reduce costs and the effect of adverse wind loads on the devices were developed. Included was the low-profile heliostat consisting of a stiff frame with sectional focusing reflectors coupled together to turn as a unit. The entire frame is arranged to turn angularly about a center point. The ability of the heliostat to rotate about both the vertical and horizontal axes permits a central computer control system to continuously aim the sun's reflection onto a selected target. A schematic of the heliostat design is shown in Fig. 1. An engineering model of the basic device was built and is being tested. Control and mirror parameters, such as roughness and need for fine aiming, are being studied. The fabrication of these prototypes is in process. The model was also designed to test mirror focusing techniques, heliostat geometry, mechanical functioning, and tracking control. The model can be easily relocated to test mirror imaging on a tower from various directions. In addition to steering and aiming studies, the tests include the effects of temperature changes, wind gusting and weathering. The results of economic studies on this heliostat are also presented.     

Determination of the angular parameters in the general altitude–azimuth tracking angle formulas for a heliostat with a mirror-pivot offset based on experimental tracking data

A set of general altitude–azimuth tracking angle formulas for a heliostat with a mirror-pivot offset and other geometrical errors were developed previously. The angular parameters with respect to the geometrical errors are the tilt angle, ψ_t, and the tilt azimuth angle, ψ_a, of the azimuth axis, the bias angle, τ₁, of the altitude axis from the orthogonal to the azimuth axis, and the canting angle, μ, of the mirror surface plane relative to the altitude axis. In view of the importance the zero angle position errors of the two rotational axes (α₀ is for the zero angle position error of the altitude axis and γ₀ for the zero angle position error of the azimuth axis), the original general tracking angle formulae have been slightly modified by replacing the tracking angles in the original tracking formulas with the difference between the nominal tracking angles and the zero angle position errors. The six angular parameters (ψ_a, ψ_t, γ₀, τ₁, α₀, μ) for a specific altitude–azimuth tracking heliostat could be determined from experimental tracking data using a least squares fit and the classical Hartley-Meyer solution algorithm. The least squares model is used on data for a specially designed heliostat model with two sets of laser beam tracking test data to show the effectiveness of the least squares model and the Hartley-Meyer algorithm.     

基于三角腔体吸收器的透射式菲涅尔定焦线系统聚光特性分析

为解决线性菲涅尔太阳能集热系统单轴跟踪过程中出现的聚光焦线偏移以及降低系统跟踪能耗等问题,提出一种透射式菲涅尔定焦线太阳能聚光器.该聚光器采用极轴跟踪方式与线性菲涅尔透镜定期滑移调节方式相结合,可实现固定焦线聚光.将该聚光器与三角腔体吸收器所组成的太阳能集热系统,利用基于蒙特卡罗光线追迹法的TracePro光学软件分析...     

Mirror rearrangement optimization for uniform flux distribution on the cavity receiver of solar parabolic dish concentrator system

The nonuniform and high‐gradient solar radiation flux on the absorber surface of solar dish concentrator/cavity receiver (SDCR) system will affect its operational reliability and service lifetime. Therefore, homogenization of the flux distribution is critical and important. In this paper, 2 mirror rearrangement strategies and its optimization method by combining a novel ray tracing method and the genetic algorithm are proposed to optimize the parabolic dish concentrator (PDC) so as to realize the uniform flux distribution on the absorber surface inside the cavity receiver of SDCR system. The mirror rearrangement strategy includes a mirror rotation strategy and mirror translation strategy, which rotate and translate (along the focal axis) each mirror unit of the PDC to achieve multipoint aiming, respectively. Firstly, a correlation model between the focus spot radius and mirror rearrangement parameters is derived as constraint model to optimize the PDC. Secondly, a novel method named motion accumulation ray‐tracing method is proposed to reduce the optical simulation time. The optical model by motion accumulation ray‐tracing method and optimization model of SDCR system are established in detailed, and then, an optimization program by combining a ray‐tracing code and genetic algorithm code in C++ is developed and verified. Finally, 3 typical cavity receivers, namely, cylindrical, conical, and spherical, are taken as examples to fully verify the effectiveness of these proposed methods. The results show that the optimized PDC by mirror rearrangement strategies can not only greatly improve the flux uniformity (ie, reduce the nonuniformity factor) and reduce the peak local concentration ratio of the absorber surface but also obtain excellent optical efficiency and direct useful energy ratio. A better optimization results when the PDC is optimized by mirror rotation strategy at aperture radius of 7.0 m, focal length of 6.00 m, and ring number of 6; the nonuniform factor of the cylindrical, conical, and spherical cavity receivers is greatly reduced from 0.63, 0.67, and 0.45 to 0.18, 0.17, and 0.26, respectively; the peak local concentration ratio is reduced from 1140.00, 1399.00, and 633.30 to 709.10, 794.00, and 505.90, respectively; and the optical efficiency of SDCR system is as high as 92.01%, 92.13%, and 92.71%, respectively. These results also show that the dish concentrator with same focal length can match different cavity receivers by mirror rearrangement and it can obtain excellent flux uniformity.