Flux distribution of solar furnace using non-imaging focusing heliostat

This paper discusses on the flux distribution of a quasi-single stage solar furnace which consists of a non-imaging focusing heliostat as the primary stage and a much smaller spherical concentrator as a secondary. As the optics of the primary stage heliostat is of non-imaging nature, the analytical method for studying the flux distribution of the hot spot of this type of solar furnace would be complicated. Therefore, a digital simulation approach has been employed. Flux distributions of the hot spot for several different incident angles, which have covered all the extreme cases of operating conditions have been simulated. Simulation result shows that a solar furnace using an 8 × 8 m non-imaging focusing heliostat with 289 mirrors coupled with a spherical concentrator with 0.7 m aperture and 27 cm focal length is theoretically capable of achieving flux concentration of 25,000 suns. Concentration contours of flux distribution for several interesting cases are presented, the different working areas of high flux footage from 5000 to 15,000 suns have been compiled.     

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.     

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.     

A novel procedure for the optical characterization of solar concentrators

A novel procedure for the optical characterization of solar concentrators is presented. The method is based on recording at night the light of a star reflected by the mirror. Images of the mirror taken from its focal region allow the reconstruction of the slope map. The application of this technique for the in situ characterization of heliostats is particularly simple and at very low cost. Results on first tests carried out with a heliostat of the CESA-I field at the Plataforma Solar de Almeria have shown the feasibility of this technique. Uncertainties in the reconstructed slopes of about 1.0 mrad have been estimated.     

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.     

A solar furnace using a horizontal heliostat array

A two-component solar furnace, condenser-heliostat combination, is described in which the condenser faces downward at 30° towards a heliostat comprised of numerous rows of plane mirrors mounted on a horizontal turntable. It is shown that for a south-facing condenser, with the angle of the final flux beam limited to 30° below the horizontal, the rows of heliostat mirrors may be mounted so they overlap, resulting in a reduction of the edge losses occurring when the heliostat mirrors are all held in a single plane. The over-all size of the heliostat turntable is calculated for a 6-hour workday throughout the year, and a suggestion is made for using the heliostat control mechanism to provide shutter action. The saving in flux possible by the elimination of an independent shutter is estimated at about eight per cent.     

Experimenting with concentrated sunlight using the DLR solar furnace

The high flux solar furnace that is operated by the Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) at Cologne was inaugurated in June 1994 and we are now able to look back onto one year of successful operation. The solar furnace project was founded by the government of the State Northrhine Westfalia within the Study Group AG Solar. The optical design is a two-stage off-axis configuration which uses a flat 52 m² heliostat and a concentrator composed of 147 spherical mirror facets. The heliostat redirects the solar light onto the concentrator which focuses the beam out of the optical axis of the system into the laboratory building. At high insolation levels (>800 W/m²) it is possible to collect a total power of 20 kW with peak flux densities of 4 MW/m². Sixteen different experiment campaigns were carried out during this first year of operation. The main research fields for these experiments were material science, component development and solar chemistry. The furnace also has its own research program leading to develop sofisticated measurement techniques like remote infrared temperature sensing and flux mapping. Another future goal to be realized within the next five years is the improvement of the performance of the furnace itself.     

Design of an optical thermal sensor for proton exchange membrane fuel cell temperature measurement using phosphor thermometry

Internal temperatures in a proton exchange membrane (PEM) fuel cell govern the ionic conductivities of the polymer electrolyte, influence the reaction rate at the electrodes, and control the water vapor pressure inside the cell. It is vital to fully understand thermal behavior in a PEM fuel cell if performance and durability are to be optimized. The objective of this research was to design, construct, and implement thermal sensors based on the principles of the lifetime-decay method of phosphor thermometry to measure temperatures inside a PEM fuel cell. Five sensors were designed and calibrated with a maximum uncertainty of ±0.6 °C. Using these sensors, surface temperatures were measured on the cathode gas diffusion layer of a 25 cm² PEM fuel cell. The test results demonstrate the utility of the optical temperature sensor design and provide insight into the thermal behavior found in a PEM fuel cell.     

Improving the optical efficiency of a concentrated solar power field using a concatenated micro-tower configuration

The concatenated micro-tower (CMT) is a new configuration for concentrated solar power plants that consists of multiple mini-fields of heliostats. In each mini-field, the heliostats direct and focus sunlight onto designated points along an insulated tube, where thermal receivers are located. The heat transfer fluid, flowing through a multitude of discrete receivers, is combined and directed towards a single power block. The key advantages of CMT are its dual-axis tracking system and dynamic receiver allocation, i.e., the ability of each heliostat to direct sunrays towards receivers from adjacent mini-fields throughout the day according to their optical efficiency. Here we compare between the annual optical efficiencies of a conventional trough, large tower, and CMT configuration, all located at latitude 36 N. For each configuration, we calculated the annual optical efficiency based on the cosine factor and atmospheric transmittance. CMT’s dynamic receiver allocation provides more uniform electricity production during the day and throughout the year and improves the annual optical efficiency by 12-19% compared to conventional trough and large tower configurations.     

Concentration of the solar radiation in a solar furnace

Energy concentration in a solar furnace is greatly influenced by the optical accuracy of the reflecting surface of the mirror. The highest density of a heat flux on the heating surface of a specimen is determined by its concentration and by the reflectivity of the mirror.

Since the apparent diameter of the moon is almost equal to that of the sun and the illumination of the moon may be adapted to the photographic determination of flux density, the rate of energy concentration in the authors' solar furnace was studied by the moon's image projected upon the heating zone in place of that of the sun.

As a result, about 300 watts per cm was obtained as the highest possible density of the heat flux in this solar furnace. This figure also agreed with the results of an optical analysis based on the practical finishing of the present mirror surface. With a heat flux of 300 watts per sq cm, in accordance with Stefan-Boltzmann's law of radiation, the highest temperature attainable is estimated to be approximately 2700°K. Besides the above, in actual experiment, approximately 2300°C was obtained in observing the melting points of binary mixtures, e.g., MgO---CoO and MgO---Cr₂O₃. Thus, the above-mentioned three approaches to the problem are in fairly good agreement.     

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.     

浅析一种用于塔式太阳能热发电站的定日镜全自动清洗车

在塔式太阳能热发电站中,定日镜的表面清洁度与其聚光集热效率直接相关,合理的清洗策略和高效的清洗设备有助于提升定日镜镜场的平均清洁度,提高其聚光集热效率,进而提升整个太阳能热发电站的发电效率。介绍了一种定日镜全自动清洗车,其基于导航系统和镜场整体控制来完成定日镜镜场的自动清洗工作。     

Nontracking solar concentrators with louvered heliostats: A calculation algorithm

An algorithm for calculating the solar radiation flux on the receiving surface of a nontracking parabolic trough concentrator with a louvered heliostat is presented. Cylindrical geometry of the concentrator and shape and arrangement of mirror lamellae of the heliostat make it possible to consider sunlight passage across the mirror system of the module as a whole band, which significantly increases the computational efficiency of the algorithm proposed.     

An analytical method for reflected flux density calculations

Conception, evaluation and real time control of solar “power tower” systems require the use of fast and accurate computer programs for calculating the flux density distributions on the receiver. Since the classical methods of “cone optics” and “hermite polynomial expansion” have some limitations of speed and accuracy, we have built an analytical model for calculating the convolution of the solar brightness distribution with the principal image of a heliostat (i.e. the fictive image for a “point sun”). We first characterize a principal image of a focusing heliostat by its shape and its geometrical concentration factor. Then this image is projected back onto the central plane (which passes through the center of the mirror), and considered as a flat reflecting surface. And the problem is reduced to density calculation for a flat heliostat. For each point of the receiver, the density of flux reflected by a heliostat is obtained by direct resolution of a convolution integral. The different formulations used to express the density function correspond to the various types of intersections between the image of the solar disk for the considered point and the principal image of the heliostat. Confrontation of this method with a program based on “cone optics” shows a good concordance of results and a strong decrease of computation time. We want to apply this method to the existing “THEMIS” solar plant built in France and to compare our results with real observations. Our density calculation programs will help conceiving fields of focusing heliostats for a new generation of power systems (gaz turbine systems).     

Optical analysis for simplified astigmatic correction of non-imaging focusing heliostat

In the previous work, non-imaging focusing heliostat that consists of m × n facet mirrors can carry out continuous astigmatic correction during sun-tracking with the use of only (m + n − 2) controllers. For this paper, a simplified astigmatic correction of non-imaging focusing heliostat is proposed for reducing the number of controllers from (m + n − 2) to only two. Furthermore, a detailed optical analysis of the new proposal has been carried out and the simulated result has shown that the two-controller system can perform comparably well in astigmatic correction with a much simpler and more cost effective design.     

Quick evaluation of the annual heliostat field efficiency

The recent world wide interest in solar power tower justifies the presentation of a simplified model that allows quick evaluations of the annual overall energy collected by a surrounding heliostat field, which is sent to the electric power generating system (EPGS). The model is the combination of an analytical model of the flux density produced by a heliostat from Zaragoza University, an optimized mirror density distribution developed by University of Houston for the Solar One project and molten salt receiver efficiencies measured during the Solar Two project. The abilities of the model are successfully compared against the scarce open data about the next Solar Tres demonstration plant-a 15 MWe solar tower with molten salts storage. This simplified model could be valid for rather preliminary optimizations, although it should not substitute much more accurate discrete evaluations that manage thousands of individual heliostats with their actual shadowing and blockings, performed every few minutes using actual meteorological data.     

Energy and exergy analyses and optimization study of an integrated solar heliostat field system for hydrogen production

In this paper, we propose an integrated system, consisting of a heliostat field, a steam cycle, an organic Rankine cycle (ORC) and an electrolyzer for hydrogen production. Some parameters, such as the heliostat field area and the solar flux are varied to investigate their effect on the power output, the rate of hydrogen produced, and energy and exergy efficiencies of the individual systems and the overall system. An optimization study using direct search method is also carried out to obtain the highest energy and exergy efficiencies and rate of hydrogen produced by choosing several independent variables. The results show that the power and rate of hydrogen produced increase with increase in the heliostat field area and the solar flux. The rate of hydrogen produced increases from 0.006 kg/s to 0.063 kg/s with increase in the heliostat field area from 8000 m² to 50,000 m². Moreover, when the solar flux is increased from 400 W/m² to 1200 W/m², the rate of hydrogen produced increases from 0.005 kg/s to 0.018 kg/s. The optimization study yields maximum energy and exergy efficiencies and the rate of hydrogen produced of 18.74%, 39.55% and 1571 L/s, respectively.     

An analytic function for the flux density due to sunlight reflected from a heliostat

This paper presents an analytical model for the flux density due to a focused heliostat over the receiver plane of a tower solar plant. The main assumptions are: spherical and continuous surface of the mirror, linear conformal transformation in the complex plane equivalent to the reflection mapping between an on-axis aligned heliostat and the objective located on the receiver at the slant range necessary to produce the minimum circle of confusion, circular Gaussian distribution of the effective sunshape and the concentration function constant on the receiver or the image plane. Under the hypotheses presented earlier an exact convolution is obtained. The result, an analytic flux density function, relatively simple and very flexible, is confronted with experimental measurements taken from four heliostat prototypes of second-generation placed at the Central Receiver Test Facility (CRTF), Albuquerque, New Mexico, and compared indirectly with the predictions of the Helios model for the same heliostats. The model is an essential tool in the problem of the determination of collector field parameters by optimization methods.     

Off-axis aberration correction surface in solar energy application

A new fixed aberration correction surface for applications in solar energy is proposed. The surface equation was derived based on the theory of focusing heliostat and the technique of canting the mirror array at a particular incident angle. The performance of the optimized new surface is simulated and compared with that of a spherical surface for two different ranges of incident angles, i.e., 0°–33° and 33°–57°. The comparison result shows that there are considerable improvements: not only is the size and concentration of the sun’s image improved, but it is also possible to shift the hot spot from noon time to early morning and late afternoon.     

A high pressure ignition delay time study of 2-methylfuran and tetrahydrofuran in shock tubes

Ignition delay time studies for tetrahydrofuran (THF) and 2-methylfuran (2MF) as well as optical investigations of combustion for 2MF have been carried out using two shock tubes. The experiments with undiluted THF/air mixtures were performed at 20 and 40 bar in a high pressure shock tube (HPST) at an equivalence ratio of Ф = 1 covering an overall temperature range of 780–1100 K and 691–1006 K, respectively. Undiluted 2MF/air mixtures (Ф = 1) were also investigated in the HPST at 40 bar in the temperature range of 820–1215 K. The experimental data of 2MF obtained at 40 bar were supported with kinetic simulations of existing models from literature. Additionally, sensitivity analyses of 2MF at several temperatures were performed for finding out the most sensitive reactions. Schlieren imaging was employed in a rectangular shock tube (RST) utilizing a high speed video camera through which the ignition process was captured for a stoichiometric 2MF/O₂/Ar mixture at pressures of about 10 bar and in the temperature range of 871–1098 K.