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Heat collection performance simulations using the system advisor model (SAM) with typical meteorological year weather data from four geographic locations are used to investigate (a) the optimum overall tilt of piecewise-focusing (PWF) collectors, and (b) PWF collector performance in comparison to the SAM default central receiver system. Results show that the overall tilt angle is not critical, but values up to 50 deg are best at non-tropical latitudes, even when output in summer is more valuable than in winter. For tropical latitudes, 40 deg of tilt is sufficient. Increasing PWF collector width relative to height is advantageous. Depending on location, PWF collector performance is 66-90% better than for the SAM default 100 MWe central receiver system, per square meter of reflector or heliostat. Using SAM's detailed control over system parameters, it is shown that the PWF collector's superior performance is derived mainly from better geometry (smaller cosine losses), but the near-absence of atmospheric attenuation and the smaller receiver heat losses are also significant. [ABSTRACT FROM AUTHOR]

A computational fluid dynamics study about the aerodynamic loads over a heliostat due to an atmospheric boundary layer flow using a modified k-ɛ turbulence model is presented. A new formulation is used, in which the model quantities vary with the velocity field. Modified wall functions for roughness were used at the bottom of the computational domain to achieve horizontal homogeneity of the airflow. Good horizontal homogeneity for the streamwise and spanwise velocity and turbulence intensity profiles were found. The incident profiles were compared with the inlet ones. The average percentage differences were 0.22% for velocity and 0.43% for turbulent intensity. Good agreement was found between the numerical data and theoretical values of the streamwise and spanwise shear stress at the bottom of the domain. The aerodynamic coefficients of the heliostat at different elevation angles were obtained, and a good agreement was found between the numerical data concerning the wind tunnel experimental values. An average percentage difference of 3.1% was found for drag, 6.5% for lift, and 6.0% for overturning. A significant improvement was obtained by using this new formulation with respect to a non-modified k-ɛ turbulence model. The average differences of the aerodynamic coefficients were 6.6% for drag, 12.4% for lift, and 10.1% for overturning. The velocity, turbulent kinetic energy, and pressure fields at different elevation angles were analyzed. It was found that at an elevation of 60 deg, the stagnation point of the flow occurs at the superior edge of the heliostat, causing the maximum lift force over the structure. [ABSTRACT FROM AUTHOR]

Heliostat tracking is a critical component of the solar field of concentrating solar power tower (SPT) systems and can be the source of significant losses in power and profit when it lacks the necessary accuracy. This paper presents an advanced heliostat drive system for the SPT generation plant. An integrated model for the heliostat drive system based on dual axes tracking is proposed using an inexpensive angle sensor. The mathematical model of the integrated drive system is developed, including the solar, tower, and heliostat models. The matlab simulation model for the proposed integrated drive system is developed and evaluated. An experimental prototype for a dual-axis heliostat is built using Class-E direct current choppers and an inexpensive Gyro angle sensor. The prototype is tested and considered in the Dhahran region in Saudi Arabia under different operating conditions. A comparative study between simulation and experimental results is conducted to assess the efficacy and accuracy of the proposed controller drive system and validate the developed integrated model. Both simulation and experimental results demonstrate the effectiveness of the proposed dual-axis trackers to follow the sunbeams throughout the year. [ABSTRACT FROM AUTHOR]

SOLARPILOT is a software package that generates solar field layouts and characterizes the optical performance of concentrating solar power (CSP) tower systems. SOLARPILOT was developed by the National Renewable Energy Laboratory (NREL) as a stand-alone desktop application but has also been incorporated into NREL's System Advisor Model (SAM) in a simplified format. Prior means for user interaction with SOLARPILOT have included the application's graphical interface, the SAM routines with limited configurability, and through a built-in scripting language called "LK." This article presents a new, full-featured, PYTHON-based application programmable interface (API) for SOLARPILOT, which we hereafter refer to as CoPylot. CoPylot enables PYTHON users to perform detailed CSP tower analysis utilizing either the Hermite expansion technique (analytical) or the SolTrace ray-tracing engine. CoPylot's enables CSP researchers to perform analysis that was previously not possible through SOLARPILOT'S existing interfaces. This article discusses the capabilities of CoPylot and presents a use case wherein we populate a model that obtains optimal solar field aiming strategies. [ABSTRACT FROM AUTHOR]

Spinning-elevation (SE) tracking system produces a decent image on the receiver surface; however, it is subjected to large variations in tracking speed. In this research, a graphical ray tracing (GRT) model for center-oriented spinning-elevation (COSE) tracking method is developed to evaluate tracking angles. Instead of a target, a heliostat is pointed toward the on-field center point of the tower. Therefore, a spinning-axis of rotation is a line joining a heliostat, and a center of the tower and elevation-axis is perpendicular to it. This aiming strategy has shown a substantial reduction in rotations of spinning-motor. In contrast, the elevation-motor runs at slightly higher rotations than the target-oriented SE method for the same application. Also, COSE tracking method obtains better shape of the reflected image with less aberration on the receiver surface as compared with SE and the traditional Azimuth-elevation (AE) method. [ABSTRACT FROM AUTHOR]

In this paper, a continuous tracking strategy for the heliostat in the James S. Markiewicz Concentrated Solar Energy Research Facility at Valparaiso University is developed. A model of the nonlinear dynamics of the heliostat motion is developed and the open-loop control strategy is presented. Asymptotic stability of the heliostat control using the Lyapunov and LaSalle's theorems was proven. Simulations using the nonlinear dynamic model are presented and interpreted to identify the feedback gain that maximizes the time response of the heliostat without introducing oscillations in its motion. Finally, the control strategy is put to the test during summertime operation. The data presented show that the tracking strategy has an root mean square (RMS) tracking error of 0.058 mrad, where the error is defined as the difference between the desired and actual heliostat positions. Images of the aperture of a high-temperature solar receiver over 8 h of testing are also presented to qualitatively demonstrate the success of the tracking strategy. [ABSTRACT FROM AUTHOR]

Maintaining receiver's thermal stresses and corrosion below the material limits are issues that need careful attention in solar thermal towers. Both depend on heliostats' aiming points over the central receiver and available direct solar radiation at any instant. Since this technology relies on an unavoidable time-changing resource, aiming points need to be properly manipulated to avoid excessive hot spots. This paper proposes a new aiming point strategy based on a multivariable model predictive control (MPC) approach. It shows an alternative approach by introducing an agent-based group behavior over heliostats' subsets, which makes possible either concentrating or dispersing solar radiation as required by the MPC algorithm. Simulated results indicate that it is feasible to develop a closed-loop control procedure that distributes solar irradiance over the central receiver according to the predefined heat flux limits. The performance of the proposed approach is also compared with the results found in the available literature that uses a different methodology. [ABSTRACT FROM AUTHOR]

Beam-down concentrating solar tower (BCST) is known for its merits in easy installation and maintenance as well as lower convection heat loss of the central receiver (CR) when comparing to a traditional concentrated solar tower system. A point-line-coupling-focus (PLCF) BCST system using linear Fresnel heliostat (LFH) as the first stage concentrator (heliostat) and hyperboloid/ellipsoid reflector as the tower reflector (TR) is proposed and theoretically analyzed and compared in this paper. Theoretical investigation on the ray concentrating mechanism with two commonly used reflector structures, namely, hyperboloid and ellipsoid, is conducted utilizing Monte Carlo ray-tracing (MCRT) method. The objective of this study is to reveal the achievable optical performance of these types of TRs in the PLCF system considering the effect of LFH tracking errors on TR astigmatism as well as the differences of optical efficiency factors and power transmission in a large-scale biomimetic layout. Results indicate that the ellipsoid system is superior in terms of interception efficiency over the hyperboloid system due to smaller astigmatism at the CR aperture, especially at larger facet tracking error. However, the ellipsoid reflector shows significantly lower TR shading efficiency resulting from the larger TR surface area compared to that of the hyperboloid reflector. The total optical efficiency of the hyperboloid system is always better than that of the ellipsoid system, and this efficiency gap decreases as the ratio e increases. The hyperboloid TR is proved to be more promising and practical for the PLCF system. [ABSTRACT FROM AUTHOR]

Reducing levelized electricity costs of concentrated solar power (CSP) plants can be of great potential in accelerating the market penetration of these sustainable technologies. Linear Fresnel reflectors (LFRs) are one of these CSP technologies that may potentially contribute to such cost reduction. However, due to very little previous research, LFRs are considered as a low efficiency technology. In this type of solar collectors, there is a variety of design approaches when it comes to optimizing such systems. The present paper aims to tackle a new research axis based on variability study of heliostat curvature as an approach for optimizing small and large-scale LFRs. Numerical investigations based on a ray tracing model have demonstrated that LFR constructors should adopt a uniform curvature for small-scale LFRs and a variable curvature per row for large-scale LFRs. Better optical performances were obtained for LFRs regarding these adopted curvature types. An optimization approach based on the use of uniform heliostat curvature for small-scale LFRs has led to a system cost reduction by means of reducing its receiver surface and height. [ABSTRACT FROM AUTHOR]

A high-temperature, high-pressure solar receiver was designed as part of the advanced thermal energy storage project carried out in collaboration with Abengoa Solar NT at CSIRO Energy Centre in Newcastle, Australia, with support through the Australian Renewable Energy Agency (ARENA). The cavity-type receiver with tubular absorbers was successfully installed and commissioned, using concentrated solar energy to raise the temperature of CO2 gas to 750 °C at 700 kPa in a pressurized, closed loop system. Stand-alone solar receiver tests were carried out to investigate the thermal characteristics of the 250 kWt solar receiver. The on-sun full-load test successfully achieved an outlet gas temperature of 750 °C while operating below the maximum allowable tube temperature limit (1050 °C) and with a maximum pressure drop of 22 kPa. The corresponding estimated receiver thermal efficiency values at full flow rate were 75% estimated based on measured receiver temperatures and heat losses calculations for both single aim-point and multiple aim-point heliostat control strategies. The use of a quartz glass window affixed to the receiver cavity aperture was tried as a means for improving the receiver efficiency by reducing convective heat losses from the receiver aperture. However, while it did appear to significantly reduce convective losses, a more effective metal support frame design is necessary to avoid damage to the window caused by stresses introduced as a result of distortion of the supports due to heating by the spillage of rays from the heliostat field. [ABSTRACT FROM AUTHOR]

Concentrated solar power (CSP) plants have the potential to reduce the consumption of nonrenewable resources and greenhouse gas emissions in electricity production. In CSP systems, a field of heliostats focuses solar radiation on a central receiver, and energy is then transferred to a thermal power plant at high temperature. However, maximum receiver surface fluxes are low (30-100 W cm-2) with high thermal losses, which has contributed to the limited market penetration of CSP systems. Recently, small (~4 cm2), laminated micro pin-fin devices have shown potential to achieve concentrated surface fluxes over 100 W cm-2 using supercritical CO2 as the working fluid. The present study explores the feasibility of using these microscale unit cells as building blocks for a megawatt-scale (250 MW thermal) open solar receiver through a numbering-up approach, where multiple microscale unit cell devices are connected in parallel. A multiscale model of the full-scale central receiver is developed. The model consists of interconnected unit cell and module level (i.e., multiple unit cells in parallel) submodels which predict local performance of the central receiver. Each full-scale receiver consists of 3000 micro pin-fin unit cells divided into 250 modules. The performance of three different full-scale receivers is simulated under representative operating conditions. The results show that the microscale unit cells have the potential to be numbered up to megawatt applications while providing high heat flux and thermal efficiency. At the design incident flux and surface emissivity, a global receiver efficiency of approximately 90% when heating sCO2 from 550 °C to 650 °C at an average incident flux of 110 W cm-2 can be achieved. [ABSTRACT FROM AUTHOR]

The design of a solar field is one of the crucial aspects when a solar tower system is realized. In general, shading and blocking effects, which are the main causes of solar power losses, are minimized displacing the heliostats each other quite distant, with typical land coverage less than 20%, and thus, strongly limiting the construction of these plants to low value lands. A new method is proposed here to improve the collected energy for solar tower systems with high land coverage (greater than 30%), based on the chance for each heliostat to rotate about the normal passing through the center of its surface. Then, shading and blocking are minimized by optimization of the relative orientations. To this aim, a small solar field composed of 150 rectangular flat heliostats has been considered, and its performances with and without the proposed optimization have been computed and compared for a wide variety of cases. In particular, a systematic analysis is presented to study the effect of the shape of the heliostats on the solar field performance: in a series of simulations, maintaining constant the area of each heliostat, the ratio between its two sides has been varied in a range between 1 (squared heliostats) and 3 (very stretched heliostats), and optimized and nonoptimized systems have been compared. Also, the total energy collected by the solar field has been calculated for optimized and nonoptimized heliostats' orientations, considering towers of different heights. Finally, the real PS10 solar plant has been considered, demonstrating that also for an optimized, very low coverage plant (about 14%), heliostats rotation can still improve the energy collection efficiency by a non-negligible amount. [ABSTRACT FROM AUTHOR]

This paper investigates different strategies for the reduction of peak heat fluxes on the receiver of a solar tower plant through the variation of the heliostats aiming points. The analysis is performed for two different solar tower receivers and heliostat field layouts. The innovative aspect of the work is in the methodology proposed: the effect of different aiming points is evaluated at different sun positions, and the yearly optical efficiency is calculated to determine drawbacks in terms of energy production. The optical simulation of the solar plant is performed with delsol through a matlab suite to easily manage the input and output. Preliminary assessments showed that the most important displacement is the vertical one, and the variation of the aiming point is important for the rows that are closer to the tower. With the appropriate strategy, the peak heat flux can be reduced by about 40% with limited spillage increase compared to the reference case. This result is similar for the two investigated plants, and it is confirmed also at different sun positions. The yearly optical efficiency with the optimal aiming strategy is reduced by less than 0.5% points. Future analysis will assess potential cost reductions and thermal efficiency increase brought about by the proposed strategies. [ABSTRACT FROM AUTHOR]

An experimental and numerical modal analysis was performed on an 8 m2 T-sliaped heliostat structure at different elevation angles. The experimental results were used to validate a finite element (FE) model by comparing natural frequencies and mode shapes. The agreement between experiments and simulations is good in all operating points investigated. In addition, damping coefficients were determined experimentally for each mode, in order to provide all necessary information for the development of a dynamic model. Furthermore, potentially critical operating conditions caused by vortex shedding were identified using a simple approach. [ABSTRACT FROM AUTHOR]

Current heliostats cost ~$200l/m² of reflective area and are estimated to contribute up to 50% of the total solar power tower plant costs. A drastic overall cost reduction is required in order for concentrated solar thermal power to he economically viable. The Department of Energy has set forth the SunShot initiative targeting a levelized cost of energy (LCOE) of $0.06/kWh by the year 2020. The cost of each heliostat must be brought down to an estimated $75/m² to achieve this rigorous goal. One of the driving aspects of heliostat design and cost are the heliostat optical errors. At the moment, it is relatively unclear about the amount of error that can be present in the system while still maintaining low cost and high optical accuracy. The optical errors present on heliostat mirror surfaces directly influence the plant LCOE by causing beam spillage. This can result in an increase in the number of heliostats, an increased receiver size, and decreased thermal efficiency. Assuming a fixed heliostat cost of $75/m², the effects of optical errors on LCOE were evaluated within the software delsol. From a probabilistic analysis, beam quality errors (i.e., slope error, alignment errors, etc.) were shown to have more importance on the LCOE than tracking errors. This determination resulted in a realization that the tracking errors and beam quality errors could be combined into a "bundled" root-sum-square (RSS) error value and produce similar results in delsol. A "bundled" error value of 2 mrad resulted in an LCOE of $0.06lkWh. This "bundled" value was the basis for a new optical error budget and is decomposed into five individual errors. These five errors can be used as design specifications for new generation heliostats. [ABSTRACT FROM AUTHOR]

The present study focuses on transient energy and exergy analyses of an integrated heliostat field, gas-turbine cycle and organic Rankine cycle system capable of generating power and heat in a carbon-free manner. A parametric study is carried out to ascertain the effect of varying the exit temperature of salt and the pressure ratio (PR) on the network output, rate of heat lost from the receiver, and energy and exergy efficiencies for 365 days of the year and from 10:00 am to 2:00 pm. The results are obtained for the city of Toronto, Canada and indicate that the network output increases from 1481 to 3339 kW with a rise in the exit salt temperature from 1200 to 1600 K. The energy and exergy efficiencies of the integrated system vary from 0.72 to 0.78 and 0.36 to 0.46, respectively, with a rise in the exit salt temperature. The energy and exergy efficiencies vary from 0.68 to 0.73 and 0.35 to 0.39, respectively, with an increase in the PR from 10 to 20. [ABSTRACT FROM AUTHOR]

Flux density measurement applied to central receiver systems delivers the spatial distribution of the concentrated solar radiation on the receiver aperture, measures receiver input power, and monitors and might control heliostat aimpoints. Commercial solar tower plants have much larger aperture suifaces than the receiver prototypes tested in earlier research and development (R&D) projects. Existing methods to measure the solar flux density in the receiver aperture face new challenges regarding the receiver size. Also, the requirements regarding costs, accuracy, spatial resolution, and measuring speed are different. This paper summarizes existent concepts, presents recent research results for techniques that can be applied to large-scale receivers and assesses them against a catalog of requirements. Direct and indirect moving bar techniques offer high measurement accuracy, but also have the disadvantage of large moving parts on a solar tower. In the case of external receivers, measuring directly on receiver suifaces avoids moving parts and allows continuous measurement but may be not as precise. This promising technique requires proper scientific evaluation due to specific reflectance properties of current receiver materials. Measurement-supported simulation techniques can also he applied to cavity receivers without installing moving parts. They have reasonable uncertainties under ideal conditions and require comparatively low effort. [ABSTRACT FROM AUTHOR]

The optical performance of a novel solar concentrator consisting of a 400 spherical heliostat array and a linked two-axis tracking system is analyzed using the Monte Carlo ray-tracing technique. The optical efficiency and concentration ratio are compared for four different heliostat linkage configurations, including linkages of 1x1, 1x2, 2x2, 4x4, and 5x5 heliostats for 7-hour operation and the selected months of June and December. The optical performance of the concentrator decreases with the increasing number of heliostats in the individual groups due to increasing optical inaccuracies. In June, the best-performing linked configuration, in which 1 heliostat in the east-west direction and 2 heliostats in the north-south direction are linked, provides a monthly-averaged 7-hour optical efficiency and average concentration ratio of 79% and 511 suns, respectively. In December, the optical efficiency and the average concentration ratio decreases to 61% and 315 suns, respectively. [ABSTRACT FROM AUTHOR]

A modeling procedure was developed to simulate design and off-design operation of hybrid solar gas turbines in a combined cycle (CC) configuration. The system includes a heliostat field, a receiver, and a commercial gas turbine (GT) interfaced with a conventional steam Rankine cycle. Solar power input is integrated in the GT combustor by natural gas. Advanced commercial software tools were combined together to get design and off-design performance prediction: TRtySYS® was used to model the solar field and the receiver while the gas turbine and steam cycle simulations were performed by means of Thermoflex®. Three GT models were considered, in the 35-45 MW range: a single shaft engine (Siemens SGT-800) and two two-shaft engines (the heavy-duty GT Siemens SGT-750 and the aero derivative GE LM6000 PF). This was in order to assess the influence of different GT spool arrangements and control strategies on GT solarization. The simulation method provided an accurate modeling of the daily solar hybrid CC behavior to be compared against the standard CC. The effects of solarization were estimated in terms of electric power and efficiency reduction, fossil fuel savings, and solar energy-to-electricity conversion efficiency. [ABSTRACT FROM AUTHOR]

A new method is described to determine irradiance distributions on receivers and targets from heliostats or other collectors for concentrating solar power applications. The method uses a digital camera, and, unlike previous beam characterization systems, it does not require additional sensors, calorimeters, or flux gauges on the receiver or target. In addition, spillage can exist and can also be measured (the beam does not need to be contained within the target). The only additional information required besides the images recorded from the digital camera is the direct normal irradiance and the reflectivity of the receiver. Methods are described to calculate either an average reflectivity or a reflectivity distribution for the receiver using the digital camera. The novel feature of this new photographic flux (PHLUX) mapping method is the use of recorded images of the sun to scale both the magnitude of each pixel value and the subtended angle of each pixel. A test was performed to evaluate the PHLUX method using a heliostat beam on the central receiver tower at the National Solar Thermal Test Facility in Albu-querque, NM. Results showed that the PHLUX method was capable of producing an accurate flux map of the heliostat beam on a Lambertian suiface with a relative error in the peak flux of ~2% when the filter attenuation factors and effective receiver reflectivity were well characterized. Total relative errors associated with the measured irradiance using the PHLUX method can be up to 20%-40%, depending on various error sources identified in the paper, namely, uncertainty in receiver reflectivity and filter attenuation. [ABSTRACT FROM AUTHOR]

Rigorous computational fluid dynamics (CFD) codes can accurately simulate complex coupled processes within an arbitrary geometry. CFD can thus be a cost-effective and time-efficient method of guiding receiver design and testing for concentrating solar power technologies. However, it can be computationally prohibitive to include a large multifaceted dish concentrator or a field of hundreds or thousands of heliostats in the model domain. This paper presents a method to allow the CFD code to focus on a cavity receiver domain alone, by rigorously transforming radiance distributions calculated on the receiver aperture into radiance boundary conditions for the CFD simulations. This method allows the incoming radiation to interact with participating media such as falling solid particles in a high-temperature cavity receiver. The radiance boundary conditions of the CFD model can take into consideration complex beam features caused by sun shape, limb darkening, slope errors, heliostat facet shape, multiple heliostats, off-axis aberrations, atmospheric effects, blocking, shading, and multiple focal points. This paper also details implementation examples in ANSYS FLUENT for a heliostat field and a dish concentrator, which are validated by comparison to results from DELSOL and the ray-tracing code ASAP, respectively. [ABSTRACT FROM AUTHOR]

This paper presents a modular and scalable approach to concentrated solar power (CSP) harvesting by using low-profile, light-weight, sun-tracking, millimeter-to-centimeter-scale mirror arrays that can be wirelessly controlled to reflect the incident solar energy to a central receiver. Conventional, utility-scale CSP plants use large-area heliostats, parabolic troughs, or dish collectors that are not only heavy and bulky, but also require significant labor for installation and maintenance infrastructure. Furthermore, form-factors of current heliostats are not compatible with low-profile roof-mountable systems, as seen by the dominance of the conventional Photovoltaic systems for roof-top installations. Solar TILE (STILE) technology to be presented in this work enables concentrated solar power harvesting on a given surface with form factor and weight per unit area comparable to those of ceramic tiles used on walls/floors or that of Photovoltaic modules. Self-powered operation by integrated solar cells, elimination of wiring for power transfer, wireless control, and weather-proof enclosure of moving parts help STILE technology promise lower installation and maintenance costs than PV approaches, while enabling novel beam-redirection applications over large surfaces. As the STILEs are made of mostly plastic, which costs at least an order of magnitude less than solar grade silicon, associated material costs could potentially be much cheaper than silicon PV cells. After a description of the STILE technology and a discussion of mirror scaling, we present a prototype tile with dimensions 33 × 33 × 6.4 cm³ and detail its wireless operation. [ABSTRACT FROM AUTHOR]

In the framework of the French PEGASE project (Production of Electricity by GAs turbine and Solar Energy), CNRS/PROMES laboratory is developing a 4 MWth pressurized air solar receiver with a surface absorber based on a compact heat exchanger technology. The first step of this development consists in designing and testing a pilot scale (1/10 scale, e.g., 360 kWth) solar receiver based on a metallic surface absorber. This paper briefly presents the hydraulic and thermal performances of the innovative pressurized air solar absorber developed in a previous work. The goal is to be capable of preheating pressurized air from 350 °C at the inlet to 750 °C at the outlet, with a maximum pressure drop of 300 mbar. The receiver is a cavity of square aperture 120 cm × 120 cm and 1 m deepness with an average concentration in the aperture of more than 300. The square shaped aperture is chosen due to the small scale of the receiver; indeed, the performances are not enhanced that much with a round aperture, while the manufacturability is much more complicated. However in the perspective of PEGASE, a round aperture is likely to be used. The back of the cavity is covered by modules arranged in two series making the modular and multistage absorber. The thermal performances of one module are considered to simulate the thermal exchange within the receiver and to estimate the energy efficiency of this receiver. The results of the simulation show that the basic design yields an air outlet temperature of 739 °C under design operation conditions (1000 W/m² solar irradiation, 0.8 kg/s air flow rate). Using the cavity walls as air preheating elements allows increasing the air outlet temperature above 750 °C as well as the energy efficiency up to 81% but at the cost of a critical absorber wall temperature. However, this wall temperature can be controlled by applying an aiming point strategy with the heliostat field. [ABSTRACT FROM AUTHOR]

Heliostat is an important component in the solar power tower station because its cost, optical performance, and mechanical performance have great influence on the overall property evaluation of the station. The T-shape heliostat tracking the sun in the azimuth-elevation mode has been commonly developed and successfully applied in many stations. In recent years, some researchers have developed spinning-elevation heliostats and have analyzed their optical performance. In this paper, the structure of a 16 m² toroidal heliostat tracking the sun in the spinning-elevation mode is designed, and the mechanical performance of the structure under wind load is analyzed by means of the finite element method. The trends of force, moments, and vibration modes with the change in elevation have been presented, and the classic status is analyzed in detail when their maximum values occur. Furthermore, the advantages and disadvantages of a toroidal heliostat in reducing the cost are discussed compared with those of a traditional T-shape heliostat. [ABSTRACT FROM AUTHOR]

Tracking motions in current heliostats are usually performed in discrete steps, even though the motion of the sun is continuous. Aiming errors due to the discrete steps are often about 1 mrad or more. A significant reduction of tracking error by smooth continuous tracking is presented. The implementation uses an electronic speed control unit to modify the rotational speed of the two AC motors on an existing heliostat. The continuous tracking system was implemented and successfully operated at the Weizmann Institute heliostat field. Measurements of heliostat motion show that aiming error due to tracking intervals was practically eliminated. A comparison of heliostat motions and flux on the target in step-tracking and continuous tracking modes is reported. [ABSTRACT FROM AUTHOR]

The basic mathematics and structure of heliostat have remained unchanged for many decades. Following the challenge first made by Ries et al., the non-imaging focusing heliostat recently proposed by Chen et al. provides an alternative in the field of concentrated solar energy. This paper investigates the performance of a heliostat field composed of the newly proposed heliostats. In contrast to the dynamic curvature adjustment proposed in our previous work for a solar furnace, a fixed asymmetric curvature is used here with the spinning-elevation tracking method. This restriction is intended to equalize the manufacture cost of the new heliostat with that of traditional heliostats with azimuthelevation tracking and spherical curvature. Fixing the curvature results in only partial aberration correction, compared to full correction using the dynamic adjustment of curvature. Nevertheless, the case studies presented in this paper show that the new heliostat design can reduce the receiver spillage loss by 10-30%, and provide a much more uniform performance without large variations with time of day. [ABSTRACT FROM AUTHOR]

Producing hydrogen in a cost-effective manner while minimizing environmental impacts is a big challenge. Hydrogen can be generated with carbon as a by-product from thermal decomposition of natural gas. A system using a solar reactor to produce hydrogen on-site for fueling stations was examined for its technical and economic feasibility. Integrated energy and material balance calculations were made to determine the amount of hydrogen that could be produced from a given reactor sire and heliostat field area. Hourly solar data were applied to the model to properly estimate real storage requirements. This paper gives the results of the study including the greenhouse gas emissions and energy balance. [ABSTRACT FROM AUTHOR]

Investigates thee role of flux density in the design and operation of molten-salt solar central receivers. Identification of the insolation status of heliostat; Impact of illuminated heliostats on flux density; Function of heliostat field.

Examines the use of large heliostat fields and solar receivers in central receiver systems. Implementation of hybrid system in electricity production; Characteristics of solar thermal power central receiver systems; Absorption and heat transfer of tubular and volumetric receivers.

Describes the optical design characteristics, main engineering features, and operating performance of a high-flux solar furnace. Arrangement of a sun-tracking flat heliostat on axis with a stationary primary paraboloidal concentration; Dimensions of the primary parabolic concentrator; Chemical storage and transport of solar energy.

Discusses an application of geometric and thermodynamic principles to derive upper limits on the performance of a solar energy collection system. Key collection system components including an axisymmetric heliostat field, a solar power, secondary optics and a black receiver; Performance limits on collected power, concentration and work output; Tower systems with secondary optics options.

A survey of hitherto concepts for cost reduction of heliostats is given. The survey might serve as a base for the development of low cost heliostats that are needed to meet the current challenging cost objectives. The concepts are related to the main heliostat subfunctions and to basic approaches for cost reduction found so far. Based on the main advantages and drawbacks of every concept, the most promising ones are indicated. [ABSTRACT FROM AUTHOR]

The optimization of the selection of heliostat aim points in a solar power tower plant with the objective of an increased overall efficiency represents a NP-hard optimization problem of high dimension. This paper presents a universal procedure for the purpose of aim point optimization based on the ant colony optimization metaheuristic that uses the principles of swarm intelligence. The applicability of the developed aim point optimization procedure to central receiver systems is demonstrated on a test case, for which the electrical power of a concentrated photovoltaic (CPV) receiver is maximized for a selected operating point. The example of a CPV receiver was chosen due to its nonlinear and nonmonotonous dependency of efficiency and flux density. It is shown that the optimization result is very close to the theoretical maximum. [ABSTRACT FROM AUTHOR]

Sandia Optical Fringe Analysis Slope Tool (SOFAST) is a mirror facet characterization system based on fringe reflection technology that has been applied to dish and heliostat mirror facet development at Sandia National Laboratories and development partner sites. The tool provides a detailed map of mirror facet surface normals as compared to design and fitted surfaces. In addition, the surface fitting process provides insights into systematic facet slope characterization, such as focal lengths, tilts, and twist of the facet. In this paper, an analysis of the sensitivities of the facet characterization outputs to variations of SOFAST input parameters is presented. The results of the sensitivity analysis provided the basis for a linear uncertainty analysis, which is also included here. Input parameters included hardware parameters and SOFAST setup variables. Output parameters included the fitted shape parameters (focal lengths and twist) and the residuals (typically called slope error). The study utilized empirical propagation of input parameter errors through facet characterization calculations to the output parameters, based on the measurement of an Advanced Dish Development System (ADDS) structural gore point-focus facet. Thus, this study is limited to the characterization of sensitivities of the SOFAST embodiment intended for dish facet characterization, using an LCD screen as a target panel. With reasonably careful setup, SOFAST is demonstrated to provide facet focal length characterization within 0.5% of actual. Facet twist is accurate within ±0.03 mrad/m. The local slope deviation measurement is accurate within ±0.05 mrad, while the global slope residual is accurate within ±0.005 mrad. All uncertainties are quoted with 95% confidence. [ABSTRACT FROM AUTHOR]

This work has been realized in the framework of the Elioslab project, financed by the Italian Ministry of Education, University and Research (MIUR), which aims to create a research platform in order to develop components and systems for the production and utilization of medium and high temperature heat using concentrated solar energy. As regards high temperature heat production, a 30?kW solar furnace that consists of a heliostat with flat mirrors and a parabolic concentrator with off-axis alignment is being realized in order to achieve a solar radiation concentration peak of about 2000 suns. The energy flux relative to the concentrated radiation will be converted to high temperature heat by a cavity receiver cooled with CO2, and finally transferred to a device operating at high temperature consisting in a thermochemical reactor for hydrogen production. Due to the intermittency of solar radiation, a high temperature (>800°C) packed bed sensible heat storage system, with alumina balls as heat storage material, has been developed in order to provide continuity to the user operation. This paper focuses on the parametric analysis that has been carried out by means of numerical simulations to evaluate the set of variable parameters that maximize the efficiency of the heat storage system of the solar furnace. The charging and discharging phases of the heat storage tank have been numerically simulated by means of an analytical model that takes into account the conductive, convective, and radiative heat transfer as well as turbulent diffusion due to the solid-fluid interaction. The results of the numerical parametric analysis are presented together with the experimental validation of the adopted analytical model accomplished by using a reduced-scale high temperature storage system. [ABSTRACT FROM AUTHOR]

The cost of solar tower power plants is dominated by the heliostat field making up roughly 50% of investment costs. Classical heliostat design is dominated by mirrors brought into position by steel structures and drives that guarantee high accuracies under wind loads and thermal stress situations. A large fraction of costs is caused by the stiffness requirements of the steel structure, typically resulting in ∼20 kg/m² steel per mirror area. The typical cost figure of heliostats (figure mentioned by Solucar at Solar Paces Conference, Seville, 2006) is currently in the area of 150 €/mm caused by the increasing price of the necessary raw materials. An interesting option to reduce costs lies in a heliostat design where all moving parts are protected from wind loads. In this way, drives and mechanical layout may be kept less robust, thereby reducing material input and costs. In order to keep the heliostat at an appropriate size, small mirrors (around 10x10 cm²) have to be used, which are placed in a box with a transparent cover. Innovative drive systems are developed in order to obtain a cost-effective design. A 0.5x0.5 m² demonstration unit will be constructed. Tests of the unit are carried out with a high-precision artificial sun unit that imitates the sun's path with an accuracy of less than 0.5 mrad and creates a beam of parallel light with a divergence of less than 4 mrad. [ABSTRACT FROM AUTHOR]

This paper discusses the tracking speed for different types of sun tracking algorithms, particularly the latitude-oriented spinning-elevation tracking mode in the solar energy application. It is found that the tracking speed of the spinning axis of a latitude-oriented type of heliostat varies drastically during solar noon around days of equinox. We analyze the time intervals during which the heliostat requires faster tracking speed and discuss the causes and possible solutions. [ABSTRACT FROM AUTHOR]

A completely new ray tracing software has been developed at the German Aerospace Center. The main purpose of this software is the flux density simulation of heliostat fields with a very high accuracy in a small amount of computation time. The software is primarily designed to process real sun shape distributions and real highly resolved heliostat geometry data, which means a data set of normal vectors of the entire reflecting surface of each heliostat in the field. Specific receiver and secondary concentrator models, as well as models of objects that are shadowing the heliostat field, can be implemented by the user and be linked to the simulation software subsequently. The specific architecture of the software enables the provision of other powerful simulation environments with precise flux density simulation data for the purpose of entire plant simulations. The software was validated through a severe comparison with measured flux density distributions. The simulation results show very good accordance with the measured results. [ABSTRACT FROM AUTHOR]

Heliostat canting (alignment of mirror facets) is known to have a major influence on the optical efficiency of heliostat fields and therefore on the power output of solar tower plants. In recent years several canting concepts were used, mainly on- and off-axis canting. Several new canting concepts, such as stretched-parabolic or target-aligned canting, were proposed in order to improve the performance of heliostats. As solar power plants become economically more attractive, knowledge about the influence of canting becomes more important. In this context, the influence of several factors on the canting method is discussed and optimal canting strategies are described. The considered factors comprise plant power level, heliostat position in the field, heliostat area, receiver dimension, and site latitude. It is concluded that the target-aligned tracking method is superior to all other variants in the majority of cases. As for the standard azimuth-elevation tracking methods, not one of these exhibits a clear advantage. It is only the on-axis method that performs worst in all cases. [ABSTRACT FROM AUTHOR]

The "beam down" optics or "solar tower reflector" has been successfully used recently for testing in different projects at the Weizmann Institute of Science. There are currently sufficient data on this technology to evaluate its upscaling for commercial uses. The sizing of a tower reflector (TR) is directly linked to the layout of the heliostat field and the geometry of the ground secondary concentrator (compound parabolic concentrator (CPC)). It depends on its position relative to the aim point of the field, amount of spillage around it, and the allowable solar flux striking the TR. Its position influences the size of the image at the entrance plane of the ground CPC and the spillage around the CPC aperture. The spillage around the CPC is also directly related to the exit diameter of the CPC (equal to the entrance opening of the solar reactor, matching the CPC exit) and therefore linked to the input energy concentration, thermal losses, and working temperature in the reactor. Restrictions on the size of the exit of the CPC can influence the entire design of the optical system. This paper provides the correlations between the main design parameters and their sensitivity analysis. These correlations are based on edge-ray methodology, which provides a quick and sufficiently accurate means for preliminary evaluating large-scale beam down solar plants without the need for detailed design of the heliostat field and considering their errors. The size of the TR and the geometry of the CPC are correlated to the size of the reflective area of the heliostats field (and the power output). Thermal modeling of the TR has been performed, showing the maximum energy flux allowed on the reflector to avoid overheating, using natural cooling to the surrounding air. The current mirrors of the TR are limited to working temperatures of 120-130°C to achieve reasonable lifetime. This parameter must be considered when determining the TR position. A key issue discussed in this paper is the amount of spillage around the CPC entrance. To reduce the spillage losses, one needs to increase the size of the exit aperture (although there are practical limitations to this, e.g., due to the size of the reactor's window). This, however, reduces the concentration and increases the thermal losses from the reactor and requires optimization work. [ABSTRACT FROM AUTHOR]

In standard heliostat design, the usual strategy of cost reduction is to increase the mirror area of the heliostats. This not only leads to a reduction of specific drive cost but also to an increase of the torques caused by the wind loads, resulting in higher specific weight and higher specific drive power. These drawbacks are avoided when small heliostats are coupled. At the torque tube heliostat (TTH), the mirrors are mounted on torque tubes to simplify the coupling mechanism for the elevation angle. Ray tracing calculations were done to obtain optimized heliostat configurations and to compare the energy yield of a heliostat field built of 288 m² TTHs with a conventional heliostat field. To get an idea of the weight reduction potential of the TTH, the dimensions of the torque tubes, the secondary axis tubes, and the mirror support structure were weight optimized via finite element method. The layout criterion was a mean mirror error of 2.5 mrads under wind load and gravity. [ABSTRACT FROM AUTHOR]

The most general form of solar tracking formulae of an arbitrarily oriented heliostat toward an arbitrarily located target on the Earth is presented. With this complete solution, the used azimuth-elevation, spinning-elevation tracking formula, etc., are the special cases of it. Therefore, more application may be sought out for many individual cases in solar engineering. The new form of tracking solution could bring changes in the geometry and structure of the design of heliostat to meet various requirements. [ABSTRACT FROM AUTHOR]

Describes a proof-of-concept subscale prototype of a light-weight, low-cost inflatable heliostat with a spherical inflatable structure and a reflective Mylar membrane mirror on its equatorial plane for solar thermal powerplants. Production application; Use of polar bearings and base rollers; Features of the prototype transparent surface.