Etendue-matched two-stage concentrators with multiple receivers

A possible way to concentrate sun light is by using a Fresnel reflector: a large number of small mirrors (called heliostats) that mimic the behavior of a large concentrator, replacing it. These heliostats can move to track the sun, keeping its light concentrated onto the receiver. Fresnel concentrators, however, may have important losses. If the heliostats are spaced from each other, some light will miss them and be lost. If the heliostats are close to each other, they will block part of each other’s reflected light, also producing losses. One possible way to minimize these losses is to intersect two focusing Fresnel concentrators forming a Compact Linear Fresnel Reflector - CLFR. Although improving on a simple focusing Fresnel concentrator, these optics are still not optimal. Here new geometries for Fresnel reflectors are explored, minimizing their losses and increasing their concentration. This is achieved by changing the overall shape of the primary, making it a wave-shaped trough surface and/or by allowing for a variable size and shape of the heliostats as a function of the position in the heliostat field. These new Fresnel concentrators may also be combined with secondaries significantly improving their total concentration, which now approaches the theoretical maximum.     

EXTENSION OF THE HERMITE EXPANSION METHOD FOR CASSEGRAINIAN SOLAR CENTRAL RECEIVER SYSTEMS

An extension of the Hermite Expansion Method for performance simulation of central receiver plants is presented. This extension allows simulation and parametric study for a solar central receiver plant based on Solar Concentration Off-Tower (SCOT, or Reflective Tower) design. The extension includes mapping the physical receiver aperture into a Virtual Receiver located near the field's aim point, and performing the Hermite Expansion calculation on the Virtual Receiver. The calculation of aperture intercept/spillage and additional losses due to the Tower Reflector's finite size are discussed. Validation of the extension by comparison to ray-tracing simulation is presented for single heliostats, a group of heliostats and a complete surround field. The results match closely, showing the validity of the method and of its implementation.     

A cellwise method for the optimization of large central receiver systems

The total number of heliostats in the collecter field determines the approach to the optical simulation problem. For large central receiver systems, it is desirable to introduce a cell model which establishes an array of representative heliostats (see Ref.[1] for central receiver systems). We now have an arsenal of computer programs which allows us to optimize the arrangement of heliostats in the collector field subject to the approximations of the cell model. Each cell contains an arbitrary regular two dimensional array of heliostats. For practical reasons we have limited our current study of the 100 MWe commercial model to four categories of heliostats arrangement; (1) radial cornfields, (2) radial staggers, (3) N.-S. cornfields, and (4) N.-S. staggers.

The most important results from the 100 MWe commercial model optimization study are:

1. (1) Staggers are better than cornfields.

2. (2) The increased cost of the tower and receiver subsystems has moved the solution to a larger cell size and a shorter tower.

3. (3) No panels should be deleted from the south side of the cykindrical receiver, and

4. (4) The collector field trims to a 360° configuration.

The center of the collector field is north of the tower and some compromise may be made to prevent excessive panel power asymmetry. Currently, this problem is solved by using preheat panels in the southern part of the receiver.     

CRS4-2: A numerical code for the calculation of the solar power collected in a central receiver system

The analysis of the solar power collected at the receiver in solar tower systems requires the use of efficient and accurate numerical codes. This paper presents a new Fortran computer program, CRS4-2 (an acronym for Crs4 Research Software for Central Receiver Solar System SimulationS), for the simulation of the optical performance of a central receiver solar plant. The implemented mathematical algorithm allows for the calculation of cosine, shading and blocking effects for heliostats arbitrarily arranged in the solar field. Special attention has been given to ensure the maximum flexibility concerning the number, dimension, shape, and position of the heliostats. In the present implementation, the solar field can be composed of both square and circular heliostats possibly mixed together, each one of them characterized by the size and height from the ground. The modular design of CRS4-2 allows the extension to heliostats of arbitrary shape with only minor modifications of the code. Shading and blocking effects are computed by a tessellation of the heliostats: therefore, the numerical accuracy depends only on the refinement of the tessellation. The application to actual systems has shown that the approach is stable and general.     

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.     

An analytic evaluation of the flux density due to sunlight reflected from a flat mirror having a polygonal boundary

Computer algorithms for the flux density of reflected sunlight from a heliostat become an essential part of the optical simulation problem for the central receiver system. An exact analytic result is available for heliostats having polygonal boundaries. An analytical method for round heliostats is given in Appendix A, which is extremely complex and requires quartic roots. A useful numerical method is given in Appendix B for heliostats of arbitrary shape. A comparison is made between the analytic method and the Hermite function method, which is much faster but less accurate. The analytic method provides a basis for evaluating all other flux density calculations.     

Solar flux distribution on central receivers: A projection method from analytic function

This paper presents a methodology to project the flux distribution from the image plane into the panels of any central receiver in Solar Power Tower plants. Since analytic functions derived from the convolution approach are conveniently defined on the image plane, its oblique projection solves the distorted spot found in actual receivers. Because of its accuracy describing the flux distribution due to rectangular focusing heliostats, we make use of the analytic function on the image plane by Collado et al. (1986). Based on the projection method, we have developed a computer code successfully confronted against PSA measurements and SolTrace software, either for flat plate or multi-panel cylindrical receivers. The validated model overcomes the computation time limitation associated to Monte Carlo technique, with a similar accuracy and even higher level of resolution. For each heliostat in a field, the spillage is computed besides the rest of optical losses; parallel projection is used for shading and blocking. The resulting optical performance tool generates the flux map caused by a whole field of heliostats. A multi-aiming strategy is investigated on the basis of the radius of the reflected beams, estimated from error cone angles.     

An analysis of the terminal concentrator concept for solar central receiver systems

One of the most interesting approaches to the large scale development of solar energy for electric power production is the Central Receiver System Concept. The Central Receiver System contains a large number of individually guided heliostats that reflect sunlight towards a central receiver high above the field of heliostats. The system resembles a giant Fresnel mirror and provides a substantial concentration of the solar beam. If high concentration is desired, a terminal concentrator may be included.The terminal concentrator is a device designed to increase the concentration of solar flux reflected from the collector field. Our study depends on two assumptions: (1) the beam width formula for the reflected beams and (2) the uniformly bright collector field which is a gross simplification allowing us to deal with the terminal concentrator. We obtained the necessary design relations, including a lower bound for the rim angle φ_m, the average fraction reflected $\text{?}\text{(φ}{}_{\mathrm{m}}\text{)}$, a radiative stagnation temperature for the aperature, and the concentration ratios. The temperature and concentration ratio curves determine the optimum rim angle φ_m for each of several designs. When designed to provide maximum concentration, the terminal concentrator becomes excessively large. Consequently, we consider a design which produces 90 per cent of the maximum concentration and reduces the size of the conical reflector by 5–6 times. The effectiveness of this compromise design permits us to conclude that a practical terminal concentrator of the conical variety can almost double the concentration without any appreciable loss of total power. There will be losses due to reflectivity but not due to beam spillage because of the reduced aperture. The terminal concentrator will be economically desirable for small central receiver systems if it is cheaper than the incremental cost of the heliostat field due to the additional focusing required to produce the same concentration.     

Site selection for hillside central receiver solar thermal plants

In this article, a new tool is introduced for the purpose of locating sites in hillside terrain for central receiver solar thermal plants. Provided elevation data at a sufficient resolution, the tool is capable of evaluating the efficiency of a heliostat field at any site location. The tool also locates suitable sites based on efficiency and average annual normal insolation. The field efficiency, or ratio of radiation incident to the receiver to direct normal solar radiation, is maximized as a result of factors including projection losses and interference between heliostats, known respectively as cosine efficiency, shading, and blocking. By iteratively defining the receiver location and evaluating the corresponding site efficiency by sampling elevation data points from within the defined heliostat field boundary, efficiency can be mapped as a function of the receiver location. The case studies presented illustrate the use of the tool for two field configurations, both with ground-level receivers and hillside heliostat layouts. While both configurations provide acceptable efficiencies, results from case studies show that optimal sites for ground-level receivers are ones in which the receiver is at a higher elevation than the heliostat field. This result is intuitive from the perspective of minimizing cosine losses but is nevertheless a novel configuration.     

塔式太阳能热发电系统镜场调度方法的研究

提出一种塔式太阳能热发电系统中定日镜调度的方法。根据太阳、定日镜和接收面的光学成像关系,考虑太阳位置、镜面反射率和能见度等因素给出了镜场光能转换效率的计算方法,同时结合定日镜场状态及热力系统所需光功率建立了镜场调度模型。该文将定日镜的调度转化为一个0-1背包问题,设计了一种混合遗传算法来对其求解。采用该调度方法可得到各时刻转换效率最高时所需调用的定日镜数量及其分布,并可调整定日镜瞄准接收靶上分布的目标点,使吸热器上能流分布均匀,降低峰值能流密度,避免过热故障。仿真算例结果表明了该方法的有效性。     

An experimental and numerical study of the gap effect on wind load on heliostat

The main handicap of the concentrating solar power technology is still the higher cost compared with the conventional coal power plant. Heliostat arrays cause about 40% of the costs of central receiver power plants. The cost reduction of heliostats is of crucial importance to central receiver power plants. The reduction of wind load on heliostats will decrease the structural requirement for heliostats, and then cut the cost of heliostats. In this paper, different gap sizes (0–40 mm) between the facets of the heliostats were studied experimentally and numerically. Both of the results showed that the wind load increases slightly with the increase of gap size (0–40 mm). The result of the numerical simulation shows the flow pattern through the gap resembles a jet flow which does not affect the static pressure on the windward surface but does decrease the static pressure on the leeward surface of the facets. Consequently it increases the total drag force on the heliostat. However, the absolute increment of the wind load is very small compared with the overall wind load on the heliostat structure. It is not necessary to take account of the gap size effects on the wind load during the design process of heliostat.     

Multi-tower solar array

The multi-tower solar array (MTSA) is a new concept of a point focussing two-axis tracking concentrating solar power plant. The MTSA consists of several tower-mounted receivers which stand so close to each other that the heliostat fields of the towers partly overlap. Therefore, in some sectors of the heliostat field neighbouring heliostats are alternately directed to the receivers on different towers. This allows the MTSA to use radiation which would usually remain unused by a conventional solar tower system due to mutual blocking of the heliostats and permits an MTSA to obtain a high annual ground area efficiency (efficiency of usage of ground area). In the sectors close to the towers, where the shading effect predominates, all heliostats are directed to the nearest tower. In sectors further away from the towers, the heliostats are alternately directed to the receivers on two, three, or four different towers. To reduce dilution of the radiation from the field, the number of towers the heliostats in a specific region can be directed to may be limited to two, which causes almost no losses in the annual ground area efficiency.     

A numerical approach to the flux density integral for reflected sunlight

A computer model of the central receiver system must evaluate the flux density on the receiver due to sunlight reflected by the heliostats in the collector field. Several approaches are available but each has its limitations. The Monte-Carlo approach represents all of the heliostat behavior but is relatively slow in terms of CPU time and is not suitable for optimization purposes. FLASH is an analytically exact approach for flat polygonal heliostats but is slow and not applicable to dished heliostats or aureole effects. Cone optics programs evaluate the flux density by a direct numerical integration of the double integral, but this method is very slow if accuracy is required. HCOEF is a two dimensional Hermite polynomial method which is relatively fast and can be extended to include canting, focusing, solar limb, and guidance error effects. However, the polynomial approximation breaks down for near heliostats, small guidance errors, and aureole effects. The new image generators based on KGEN overcome this limitation, but running times compare to FLASH and are 3 or 4 slower than HCOEF.The new approach proposed in this study assumes isotropic gaussian guidance errors. Hence, the flux density integral reduces to several iterated single integrals which can be precalculated and stored in a table for interpolation as needed. The LBL solar telescope data are fed into a convolution integral which represents the guidance errors. Aureole effects can be switched on or off at this point. A vector of convoluted solar data is input to another integration which gives the table of normalized flux contributions. The tabular values depend on the position of the flux point with respect to an edge of the heliostat as seen in the image plane. The image map of the heliostat is linear unless ripples or irregularities occur; hence, effects due to canting and dishing can be included by a ray trace of the heliostat vertices.The use of tabular interpolation is not as fast as expected because of the time required to calculate the distance between the flux point and the image of the vertices. The accuracy of this method is limited by interpolation errors, and better results can be obtained with the same CPU time if more core is used for a larger table. It is possible to eliminate the table by introducing a Romberg type of integrator which bisects the interval until sufficient accuracy is achieved; however, this approach is inefficient unless the images are relatively small compared to the receiver.The convolution process in KGEN is fast and can be used to calculate moments for HCOEF and coefficients for FLASH which utilize the LBL data.     

Concentrated solar power on demand

A concentrating solar power system is presented which uses hillside mounted heliostats to direct sunlight into a volumetric absorption molten salt receiver with integral storage. The concentrated sunlight penetrates and is absorbed by molten salt in the receiver through a depth of 4-5 m, making the system insensitive to the passage of clouds. The receiver volume also acts as the thermal storage volume eliminating the need for secondary hot and cold salt storage tanks. A small aperture and refractory-lined domed roof reduce losses to the environment and reflect thermal radiation back into the pond. Hot salt is pumped from the top of the tank through a steam generator and then returned to the bottom of the tank. An insulated barrier plate is positioned within the tank to provide a physical and thermal barrier between the thermally stratified layers, maintaining hot and cold salt volumes required for continuous operation. As a result, high temperature thermal energy can be provided 24/7 or at any desired time.The amount of storage required depends on local needs and economic conditions. About 2500 m³ of nitrate salt is needed to operate a 4 MW_e steam turbine 24/7 (7 h sunshine, 17 h storage), and with modest heliostat field oversizing to accumulate energy, the system could operate for an additional 24 h (1 cloudy day). Alternatively, this same storage volume can supply a 50 MW_e turbine for 3.25 h without additional solar input. Cosine effect losses associated with hillside heliostats beaming light downwards to the receiver are offset by the elimination of a tower and separate hot and cold storage tanks and their associated pumping systems. Reduced system complexity also reduces variable costs. Using the NREL Solar Advisor program, the system is estimated to realize cost-competitive levelized production costs of electricity.     

Compact Linear Fresnel Reflector solar thermal powerplants

This paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scale solar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linear receivers elevated on tower structures that are close enough for individual mirror rows to have the option of directing reflected solar radiation to two alternative linear receivers on separate towers. This additional variable in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirror inclination can be set up such that shading and blocking are almost eliminated while ground coverage is maximised. Preferred designs would also use secondary optics which will reduce tower height requirements. The avoidance of large mirror row spacings and receiver heights is an important cost issue in determining the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam line cost. The improved ability to use the Fresnel approach delivers the traditional benefits of such a system, namely small reflector size, low structural cost, fixed receiver position without moving joints, and non-cylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes as the receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorber orientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector field configurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was the development of specific raytrace and thermal models to simulate the new concepts.     

Design and evaluation of esolar’s heliostat fields

Central receiver concentrating solar power plants offer significant performance advantages over line-focus systems. However, the high cost of the heliostat field remains a barrier to the widespread adoption of such plants. eSolar has approached the problem of heliostat field cost by emphasizing small size, low cost, easy installation, and high-volume manufacturing of heliostat field components.During 2008 and 2009, eSolar designed, constructed, and began operation of its demonstration facility, which comprises two towers each with heliostat subfields to the north and the south. These heliostat fields are composed of large numbers of small heliostats, creating an arrangement unlike other central receiver plants. This paper describes the design, construction, startup, and testing of these heliostat fields, showing that they perform well and represent a viable alternative to more traditional fields of large heliostats.     

塔式太阳能电站聚光镜场的土地利用率研究

塔式太阳能热发电站的聚光镜场大多是由按一定规律排列的矩形定日镜组成,在相邻定日镜间无机械碰撞的情况下,聚光镜场的最大土地利用率仅为58%。文章提出了选用规则交错排列的聚光镜场布置方案,建立不同形状定日镜的土地利用模型,并计算出不同情况下的最大土地利用率。通过仿真得出,矩形定日镜和六边形定日镜在一定长宽比时可获得最大土地利用率,其中六边形定日镜的土地利用率最高,约为100%。     

Optical analysis of solar facility heliostats

Useful insights into significant operating parameters are gained by treating the mirror collector system of a central receiver solar power station as an optical system, no matter how large. We at Sandia Laboratories have developed a method for estimating the approximate size of the solar image cast by individual heliostats. As a consequence from the Coddington equations, we have developed a simple analysis of astigmatism, the major aberration which describes optical off-axis image behavior of a focusing collector system. These predictive equations agree well with experiments performed with a spherical mirror over a range of angles of incidence exceeding 60°. Other than flat mirros, several heliostat configurations were proposed and explored and were found amenable to the same analysis. All desings which attempt to superpose the reflected energy from several mirros mounted and tracked together on the same frame are subject to the same simple rules. Although this analysis is not rigorous, it does give useful insight into the cause of the large image spread found under some operating conditions and it does indicate methods for optimizing the system. Conclusions reached are explicitly related to the off-axis performance of the mirror collectors of a central receiver solar power station.     

Mathematical formulation of a graphical method for a no-blocking heliostat field layout

The graphical method for a no-blocking radial staggered layout was introduced within the joint work between the Center For Solar Energy Studies (CSES), Tripoli, and Atlantis Energy Ltd, Bern. It locates the heliostats in the field of a solar central receiver plant so that they provide no blocking losses over the year. In this method the field is divided into certain groups to increase the efficient use of land. The method is a simple one when compared to cell-wise procedures, making it more suitable for preliminary design of heliostat fields. At the same time, the method can be represented by a set of mathematical equations, consequently facilitating its computer implementation. In this paper a mathematical formulation of the method will be introduced, as well as its algorithm. Also, a criterion for the transfer to a new heliostat group is proposed based on mirror density.     

An artificial vision-based control system for automatic heliostat positioning offset correction in a central receiver solar power plant

This paper presents the development of a simplified and automatic heliostat positioning offset correction control system using artificial vision techniques and common CCD devices. The heliostats of a solar power plant reflect solar radiation onto a receiver (in this case, a volumetric receiver) placed at the top of a tower in order to provide a desired energy flux distribution correlated with the coolant flow (in this case air mass flow) through the receiver, usually in an open loop control configuration. There exist error sources that increase the complexity of the control system, some of which are systematic ones, mainly due to tolerances, wrong mirror facets alignment (optical errors), errors due to the approximations made when calculating the solar position, etc., that produce errors (offsets) in the heliostat orientation (aiming point). The approximation adopted in this paper is based on the use of a B/W CCD camera to correct these deviations in an automatic way imitating the same procedure followed by the operators. The obtained images are used to estimate the distance between the sunbeam centroid projected by the heliostats and a target placed on the tower, this distance thus is used for low accuracy offset correction purposes. Basic threshold-based image processing techniques are used for automatic correction.