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Abstract: This paper presents an advanced methodology for the detailed modeling of the heat transfer and fluid dynamics phenomena in solar tower receivers. It has been carried out in the framework of a more ambitious enterprise which aims at modeling all the complex heat transfer and fluid dynamics phenomena present in central solar receivers. The global model is composed of 4 sub- models (heat conduction, two-phase flow, thermal radiation and natural convection) which are described. Results of the numerical model obtained so far are also presented and discussed [Copyright &y& Elsevier]

Abstract: The Australian National University has been undertaking a review of state-of-the-art in heliostat design, as part of the Australian Solar Thermal Research Initiative (ASTRI). Deep cost reduction is required to ensure solar tower technology becomes competitive, and to achieve the aggressive LCOE targets of ASTRI and other programs like U.S. Sunshot. This paper is a case study for a new heliostat design, and aims to provide directions and identify opportunities for cost reduction. The review examines trends at both a system level and at an individual collector level, to make sense of where the greatest potential for cost reduction lies. At a heliostat system level, we focus in particular on three factors critical to cost reduction: manufacturing and assembly, heliostat size and wind load analysis. A technological review of heliostat componentry highlights those areas we believe have most performance improvement and cost reduction potential, including mirrors, tracking systems, communication systems and structure. The review also identifies cost reducing design features of some recent unconventional heliostat designs, and provides a summary of those features we believe to hold the most promise. [Copyright &y& Elsevier]

Abstract: This paper describes the development of an inexpensive, highly portable photogrammetry technique for measuring the position and form of large mirror segments for solar collectors. The accuracy of the technique has been validated using a large Coordinate Measuring Machine (CMM) with results showing a measurement capability of better than 100μm. The surface form and resulting slope errors of the parabolic troughs have been measured and the impact of the mirror support system determined. This paper contains the results of photogrammetry and CMM comparison with details of the measurements, their analysis and further related experimental results obtained using both measurement techniques. [Copyright &y& Elsevier]

Abstract: The SpaceTube® is a new cost-optimized parabolic trough collector with an aperture width greater than 8m. This paper summarizes the development and testing of first-round SpaceTube® SCE prototypes and their related componentry. These tests showed good agreement with the modeled performance, confirming the commercial potential of the design. The optical accuracy of the SpaceTube® is higher than previous designs, due to improvements in the mirror quality and frame stiffness. To take advantage of these improvements, the concentrator geometry was analytically optimized for maximum performance under two different heat transfer fluid scenarios; current (oil-based) fluid in the range of 400˚C and next-generation (molten salt or direct steam) at temperatures above 500˚C. This led to an increase in the nominal heat collection element (HCE) diameter. Compared to the benchmark EuroTrough II, the SpaceTube® uses a 65% larger mirror module (SCE) and a more efficient frame structure, reducing the cost per unit aperture of the installed collector by approximately 40% [1]. A full-scale solar collector array (SCA) is now being constructed for on-sun thermal qualification with both oil and molten salt HTF mediums. Startup of the test loop is scheduled to occur in late 2013. [Copyright &y& Elsevier]

Abstract: Concentrating solar power (CSP) holds great promise for providing clean, economical and dispatchable thermal energy for use in electricity production via a turbine generator or directly in industrial processes. In particular, parabolic trough solar power plants have been operating in the Mojave Desert since the 1980s, with power purchase agreements now in the $0.06/kWh range, giving ample evidence that they will carry on delivering electricity for decades, long after their capital investment is paid off. Since 2005, nearly 2 GW of parabolic trough solar power plants have been installed around the world under various incentive and grant programs that make them financially viable. For this technology to reach its full market potential, it is necessary to drive down the cost of energy produced. This will be achieved by increasing efficiency, increasing volume of component production, decreasing cost of installation, and decreasing cost of finance. SkyFuel has addressed each of these areas for cost reduction in our optimized SkyTrough® design. In this paper we describe our approach and show the validated benefits. [Copyright &y& Elsevier]

Abstract: For non-standard structures, such as parabolic trough collectors, the application of codes of practice to compute load coefficients usually results in oversized wind loads. Therefore, it is common practice to support the analysis experimentally with wind tunnel test results. This approach, however, is time-consuming, expensive, flow intrusive and requires a new set of tests for any modification in geometry, configuration, topography or load conditions, entailing additional time and expense. The results obtained are restricted to a finite set of points and variables, and most importantly, the size and wind velocities in available boundary layer wind tunnels impose Reynolds numbers well below those occurring in open air full-scale structures of this type. This similitude limitation prevents a direct and fully accurate extension of the results to the characterization of full scale collector structures. These wind tunnel shortcomings have positioned CFD as an appealing alternative for determining wind load distributions over solar collectors, solving most of these problems. Although extensively used in other applications, however, the application of CFD methods requires additional development, further testing and verification, proper modeling conditions and additional validation to gain acceptance as an alternative to physical wind tunnel tests. In this paper, CFD has been used as a “virtual” wind tunnel to compute the three-dimensional flow around a single model-scale module for a range of yaw and pitch angles, and the resultant load coefficients have been compared with those obtained experimentally in a physical wind tunnel. After validation against experimental data, the computational methodology has been applied to compute the wind loads on a full-scale module (including the complete supporting structure) and an array configuration of such modules. It will be shown that the relative mean errors of the numerical results with respect to the reference experimental data are within 10%, thus of the same order as experimental uncertainty. [Copyright &y& Elsevier]

Abstract: Nowadays, parabolic trough collectors constitute the first concentrating solar thermal technology regarding global installed power. Central receivers and Fresnel receivers appear with smaller value and commercial solar dishes power are practically negligible. Several features have driven parabolic troughs to this first position, but some disadvantages are appearing, i.e. the cost reduction is not as quick as expected, the flexible or ball joints constitute leak problems with high pressure and temperature, and simplification of the mechanical structures is quite difficult. At this point, Fresnel receivers appear as a main competitor with troughs, offering lower cost but also lower efficiency; if the reduction in cost is higher than in efficiency, or if Fresnel efficiency is improved, there are potential facts which can make this simpler technology more competitive than trough collectors. In order to improve the Fresnel capabilities, it could be useful to analyze its optical properties. One important feature of Fresnel solar fields is that receiver and concentrator are mechanically separated, providing the system of a new freedom degree in comparison with trough collectors. This degree of freedom can be used in multi-tube receivers, using the high intensity thermal flux impinging in the receiver to illuminate the tubes where the fluid has higher film coefficient (i.e. where the phase change occurs) to obtain a high thermal efficiency, and using the lower thermal flux for preheating and reheating. The idea presented in this paper is to use a multi-tube Fresnel receiver with an adequate optical system that permits tailoring the thermal flux mapping in the receiver in coherence with the convection process of the thermal fluid. In order to obtain this objective, the mass flow will undergo preheating through the peripheral tubes (where the radiation flux will be lower due to the tracking, mirrors shape and sun shape errors) and will be driven afterwards to the high intensity thermal flux, where boiling (that implies high film coefficient values) takes place. A four tube receiver configuration, according to these ideas, will have two main possibilities. The first one a one-way mass flow through the peripheral tubes (inlets to the receiver) and a second pass inside the internal tubes (outlets). The second possibility assumes the mass-flow inlet receiver through one peripheral tube and the exit at the other one, the mass flow passing consecutively through the other two tubes. [Copyright &y& Elsevier]

Abstract: eSolar has been designing small heliostats since the company's founding in 2007. One of the cornerstones of our approach to heliostat design is to deploy many factory-built small heliostats in high density fields. The fields are replicated and thus aggregated into solar plants supporting steam and molten salt electric power, integrated solar combined cycle (ISCC), and process heat applications, such as enhanced oil recovery and desalination. The small heliostat approach has been used in several pilot projects and was previously evolved into a commercially available product presented in 2011 at SolarPACES. In this paper we discuss the ground up redesign of our small heliostat hardware, with a focus of leveraging the proven approach into a higher reliability, globally applicable, more industrial, simple to source and most importantly total installed cost reduced solar collector product. During the redesign we discovered methods to simplify the design and confirmed that small, factory-built, affordable heliostats provide a compelling concentrated solar thermal technology. We discuss details on system optimization, concept selection and prototype development, including some results from design validation testing. Finally, we discuss how this new solar collector system (SCS5) performs and meets economic requirements. eSolar's SCS5 product will be commercially released early in 2014, and is currently available to quote for CSP projects. [Copyright &y& Elsevier]

Abstract: This paper describes work to compare the optical properties and surface texture of glass and polymer film collectors. We also present the results of experiments designed to simulate collector cleaning processes (both contact and non-contact), and the degradation of glass and polymer reflecting surfaces owing to sand and dust abrasion. Finally we present initial results on the applicability of anti-soiling and self-cleaning coatings on glass and polymer film collector surfaces. Measurements, which include specular and hemispherical reflectance, surface roughness, and electron microscopy, indicate the excellent performance of currently available polymer film in terms of its optical performance and robustness in comparison with traditional glass collectors in CSP applications. [Copyright &y& Elsevier]

Abstract: Heliostats constitute a major portion of the direct costs of a concentrating solar power tower plant. As a result, a significant amount of effort is being focused on designing and developing cheaper heliostats. The optical and structural performance of these new and existing heliostats under dynamic wind loads must be characterized and understood in order to meet both cost and performance objectives. This paper presents the second phase of a U.S. DOE-sponsored program at Sandia National Laboratories’ National Solar Thermal Test Facility (NSTTF) that includes dynamic testing and analysis of multiple full-scale heliostats. The objectives of these tests and analyses are to characterize and understand some differences in the impacts of dynamic wind loads on heliostat strain and cyclic fatigue between perimeter and inner-field heliostats. A weather tower with three tri-axial ultrasonic anemometers has been erected just outside the field to measure the approaching boundary winds, while a portable tower was set up to characterize wind velocities and turbulence between subsequent rows within the field. Anemometers have also been mounted to some heliostats to gather close range measurement of turbulence and wind frequencies and to provide a point of comparison for computational fluid dynamics models of wind flow over the field. This paper presents mean wind speeds and wind loads on heliostats as a function of field position. The calculated mean wind loads were used to assess the mean wind-load reduction correlation of Peterka [1]. [Copyright &y& Elsevier]

Abstract: In this article, an innovative solid particle receiver was developed which suitable to be used as fluidized bed solar receiver. The receiver is composed of five quartz tubes with the flowing silicon carbide particles as absorber. The thermal performance of the receiver was tested using 10kWth solar furnace system in Badaling solar thermal power experimental platform of China. Three days experimental results are listed in this paper. The maximum outlet air temperature is 624.1°C. The minimum temperature difference between particles and outlet air is below 10°C which shows good heat ransfer performance inside the receiver. The experimental results well verified the technical feasibility of such receiver design. [Copyright &y& Elsevier]

Abstract: This paper presents experimental and numerical studies of the turbulent heat transfer in solar thermal absorber tubes. The absorber tube is a significant component in a solar thermal power system. However, the tube heat transfer performance is different than with other tubes, only half of the circumference surface of the absorber tube is heated with the non-uniform heat flux and the other half is insulated. The non-uniform heat flux on an absorber tube can be up to 1.5MW/2 and generates significant temperature difference and thermal stress. The present study uses the experimental and computational fluid dynamics to reveal the turbulent convective heat transfer performance in the solar absorber tube within the range of Reynolds number from1.0×104 to 3.5×104. The results show that the temperature distribution of fluid and tube wall is very uneven in axial, radial and circumferential direction. The temperature of inner tube wall is an important parameter for preventing the decomposition of heat transfer fluid. The Dittus-Boelter equation is still applicable to calculate the heat transfer in a circular tube with non-uniform heat flux, but it is not suitable to calculate the wall temperature distribution in this condition. The wall temperature distribution of a circular tube with non-uniform heat flux varies with the circular angle of cross section, and this study presents the correlation. [Copyright &y& Elsevier]

Abstract: While current commercial Concentrated Solar Power (CSP) plants utilize Rankine steam cycles in the power block, there is a goal to develop higher-efficiency plants based on Brayton cycles or on combined cycles. In this paper, we present an assessment of a gas turbine-driven CSP plant using small particle solar receivers. In particular, a recuperated, single-shaft gas turbine engine and a Small Particle Heat Exchange Receiver –a high temperature receiver for solar tower power plants developed in the framework of the U.S. DOE's SunShot Program– are the technologies employed for the gas turbine and the receiver, respectively. The curves of the solar receiver and the gas turbine engine were first obtained using in-house codes, and then coupled together. A backup combustor fueled with natural is used to compensate the variable nature of the solar resource. For a more flexible and optimum operation, the guide vane angle of the compressor is allowed to vary, and so is the position of the valves of the receiver and combustor bypasses. Two different operational strategies were analyzed: maximizing the overall efficiency of the plant and maximizing the net output power. Hence, the overall efficiency of a gas turbine-driven CSP plant based on a Small Particle Heat Exchange Receiver is estimated, and the potential to generate electricity is assessed. This analysis reveals the strengths of small particle receivers with respect to molten salt tubular receivers due to the much higher temperatures that can be achieved while maintaining (or even increasing) the receiver efficiency. [Copyright &y& Elsevier]

Abstract: Concentrating solar power currently relies on high temperature central receivers that utilize liquid cooling and operate in power steam cycles. However, highly efficient central receivers are being designed to operate at higher temperatures in a gas turbine power cycle. To address this, San Diego State University's (SDSU) Combustion and Solar Energy Laboratory is experimenting with a lab-scale Small Particle Heat Exchange Receiver (SPHER) in order to understand performance and develop experience for designing and operating a full-scale 5 MW design. The full-scale design will be tested at the National Solar Thermal Test Facility at Sandia National Laboratories as part of the Department of Energy SunShot Initiative grant. The SPHER relies on carbon nanoparticles as an absorption medium and air as a working fluid. The carbon particles are generated onsite by the Carbon Particle Generator (CPG) and are mixed with dilution air prior to entering the SPHER. Lab scale on-sun testing is carried out with a 15kWe solar simulator. The lab scale experimental goal is to achieve an outlet flow of 650°C at 5bar absolute operating pressure. To model the performance of the SPHER, CFD analysis is being used for comparison to lab scale testing. The lab scale SPHER is being modeled in ANSYS Fluent with coupled codes for oxidation and radiation input. In this paper, we present results of testing the lab-scale receiver and compare the measured outlet temperatures to predictions from the computer model. Finally, correlations are drawn for future experimenation and feasibility. [Copyright &y& Elsevier]

Abstract: For design and component specification of a Linear Fresnel Collector (LFC) cavity receiver, the prediction of temperature distribution and heat loss is of great importance. In this paper we present a sensitivity analysis for a range of geometry and material parameters. For the LFC receiver analysis we use two models developed at Fraunhofer ISE. One is a detailed model, combining the spatial distribution of reflected radiation via ray tracing with detailed convective simulations through computational fluid dynamics. The second one is a fast algorithm based on a thermal resistance model. It is applying a similar methodology as the well-known model for vacuum absorber, enhancing an absorber tube model by parameters describing the influence of the secondary mirror and cover glass. The thermal resistance model is described in detail. Obtained results indicate a significant effect of the secondary mirror temperature on heat loss for specific geometries. [Copyright &y& Elsevier]

Abstract: The falling particle receiver is a technology that can increase the operating temperature of concentrating solar power (CSP) systems, improving efficiency and lowering the costs of energy storage. Unlike conventional receivers that employ fluid flowing through tubular receivers, falling particle receivers use solid particles that are heated directly as they fall through a beam of concentrated sunlight for direct heat absorption and storage. Because the solar energy is directly absorbed by the particles, the flux limitations associated with tubular central receivers are mitigated. Once heated, the particles may be stored in an insulated tank and/or used to heat a secondary working fluid (e.g., steam, CO2, air) for the power cycle. Thermal energy storage costs can be significantly reduced by directly storing heat at higher temperatures in a relatively inexpensive, stable medium. This paper presents an overview of recent advancements being pursued in key areas of falling particle receiver technology, including (1) advances in receiver design with consideration of particle recirculation, air recirculation, and interconnected porous structures; (2) advances in particle materials to increase the solar absorptance and durability; and (3) advances in the balance of plant for falling particle receiver systems including thermal storage, heat exchange, and particle conveyance. [Copyright &y& Elsevier]

Abstract: The solar receiver tube is a key component to convert the solar energy into thermal energy in parabolic trough solar power system. The residual stresses which are generated during the cooling process of glass-to-metal sealing significantly influence the reliability of receiver tube. In order to lower the seal failure probability, the effects of geometry and material properties of glass- to-metal seals on the residual stress are analyzed using finite element method in this paper. The effects of the material properties of stainless steel 304, stainless steel 430, Kovar on the sealing residual stress can be presented. Although each of these different sealing material combinations can have enough sealing strength, the glass-to-metal seals with larger tensile stresses can cause the failure of the receiver tube. Moreover, effects of geometry of the metal tube and the glass tube on the residual stress are also analyzed. The stress distributions in the glass side analyzed by the finite element method (FEM) were in agreement with the ones calculated by analytic solution (AS) approach. The X-ray measurement results also proved the validation of the FEM. The dangerous tensile stresses mainly occur at the glass-metal interface. The results of this study have important implications on the optimization of seal configuration in the solar receiver tubes. [Copyright &y& Elsevier]

Abstract: Optical and thermal simulation of a new up-down arranged dual-receiver for solar tower plant is presented in this paper. The top receiver is an external absorber type to serve as the boiling section, the bottom receiver is a cavity type to serve as the superheating section. The heliostat field is divided into two parts respectively for boiling and superheating section, it is quick and simple to control the heat flux distribution on both section. Then multi-aiming strategy is used for avoiding appearance of heat spot. For cavity receiver, a optimized layout for tubes is to increase convective heat transfer coefficient in the high heat flux religions. The concept of this new receiver is illustrated by a 10 MWe solar power plant that produces main steam at 513.5°C and pressure of 10.12MPa. Finally, this dual-receiver has a thermal efficiency of 91%. [Copyright &y& Elsevier]

Abstract: In this paper a new design of a novel CSP cavity receiver for parabolic trough collector is analyzed by means of an analytical Matlab model. The receiver, designed by the Swiss company Airlight Energy Manufacturing SA, is 212 m long consisting essentially of a feed pipe, a run-back pipe and 4608 helically coiled heat exchangers designed to capture the incident solar energy concentrated by a parabolic trough. The heat transfer fluid is air heated to temperatures above 600°C. The analytical Matlab model based on a pneumatic - electric circuit analogy was developed to assess the receiver performance in terms of mass flow rate distribution, pressure drop, air outlet temperature and thermal efficiency. A solution was proposed to approximately ensure the same mass flow rate for each cavity. Different skew angles for the incoming solar radiation were considered and the receiver geometry was optimized minimizing the pressure drop and the thermal losses through the runback pipe. The main requirement was to achieve, at the outlet section of the receiver, an air temperature of 650°C; therefore, the total inlet mass flow rate was tuned accordingly. The helically coiled heat exchanger and the receiver insulation sub-models were validated against accurate computational fluid dynamics simulations. [Copyright &y& Elsevier]

Abstract: Introducing solar energy into the gas turbine of Combined Cycle systems (CC) offers significant advantages over other solar power plant concepts. A promising way to introduce solar power is solar preheating of the compressor discharge air before it enters the combustor of the gas turbine using a receiver built consisting of several metallic tubes. The main challenge during the design of such a receiver is the low solar flux capability caused by the limited convective heat transfer. To ensure a sufficient efficiency, the receiver is usually located in a cavity. The heat losses by conduction through the (insulated) cavity walls can be reduced by increased insulation thickness. Heat losses by radiation and convection to ambient depend strongly on the aperture area, receiver inclination and the receiver temperature. The receiver temperatures can be reduced by increasing the cavity si ze, as the flux distribution gets more homogenous and the overall flux density is reduced. Following heat losses by radiation and convection to ambient are reduced. On the other hand additional costs for the cavity walls have to be considered. The paper deals with the comparison of two different strategies to increase the receiver efficiency of a cavity receiver used to heat the compressed air of a 4.7 MWel turbine from 330°C up to 800°C, with a mass flow of 15.9kg/s at 10 barabs. The influence on the levelized cost of energy (LCOE) of different receiver sizes and one design option (transparent covering of aperture) for reducing the convections losses were analyzed and compared. The thermal and hydraulic layout was done for a thermal heat of 8.4MWth and 250 mbar pressure drop using thermal finite element (FE) models considering the local heat flux distribution, heat transfer to working fluid, radiation exchange between components and ambient as well as conduction and convection losses of the cavity. As the convective losses play a significant role, CFD models were used to evaluate the convective heat losses. [Copyright &y& Elsevier]

Abstract: SkyFuel has investigated the stresses and deformations occurring in parabolic trough receivers operating at temperatures above 425°C. Operating at these temperatures allows for direct molten salt storage and higher efficiency conversion from thermal to electric energy. However, at these temperatures, the typical stainless steels used in receiver construction are susceptible to chromium carbide precipitation. After the precipitation has occurred, the steel is vulnerable to intergranular corrosion, and the fatigue strength of the steel is reduced. Corrosion increases the stresses in the receiver walls, and the reduced fatigue strength lowers the stress limit where failure will occur. This paper presents the results of an analysis of these stresses and an evaluation of the receiver material at these operating temperatures. It is shown that parabolic trough receivers can be designed to mitigate the negative effects of chromium carbide precipitation and operate above 425°C without risk of premature failure. [Copyright &y& Elsevier]

Abstract: The effect of the choice of Reynolds-Averaged Navier-Stokes (RANS) type turbulence closure on the Computational Fluid Dynamics (CFD) prediction of convective heat losses from the Solar Two central receiver is considered in this paper for a simplified receiver geometry approximated by flat panels. Computed convective losses at steady state are ∼ 2-3% (1%) of the total power absorbed by the receiver, at high (low) wind speed, depending on the turbulence model chosen. The simulation results are consistent with those of available correlations for rough cylinders, if the macroscopic roughness due to the panel edges is accounted for, as well as with the low speed experimental results, within the respective error bars. [Copyright &y& Elsevier]

Abstract: The aim of this work was to study, by means of accurate 3D steady-state CFD simulations, the thermo-fluid dynamics behavior of a helically coiled heat exchanger (HCHE) constituting the receiving cavity of the novel CSP receiver based on Airlight Energy technology. In this innovative receiver design, air is used as heat transfer fluid (HTF), which, besides being inexpensive and environmentally friendly, is optimally suited for high temperature operation well beyond the limit of conventional HTFs. According to preliminary information related to the collectors orientation of the first 3.9 MWth Airlight Energy pilot plant, under construction in Ait Baha (Morocco), two reference skew angles of the incoming solar radiation were considered and the receiving cavity performance were evaluated in terms of thermal efficiency and pressure drop. Among all, one of the main requirements was to achieve, at the outlet section of the HCHE, an air temperature of 650°C; hence the mass flow rate was tuned accordingly. In order to minimize the pumping power requirements, the HCHE was designed to guarantee a laminar flow regime under all the operating conditions. Navier-Stokes, energy and radiation transport equations, the latter accounted for by the Discrete Ordinates (DO) model, were numerically solved, using the finite-volume method approach, with Fluent code from ANSYS. A meticulous experimental proof of concept was then carried out in Biasca (Switzerland) by the Swiss company Airlight Energy Manufacturing SA. The analysis of the experimental results, detailed in this paper, allowed to assess the reliability and effectiveness of this novel CSP receiver design in the solar energy harvesting. [Copyright &y& Elsevier]

Abstract: A transient analysis of a molten salt cavity receiver is presented. A two-node model, which is the theoretical basis for transient analysis, is developed for the receiver. During the transient process, the energy variation of different part of the model is researched experimentally. Especially, a transient variation process of both the receiver surface temperature and fluid temperature is analyzed detailedly in the paper. The transient process due to the sudden change of input power is divided into three stages. The transient characteristic of the receiver is analyzed based on the three stages. Also the time constant which reflects the influence of the thermal inertia of the receiver is got from the experiment. [Copyright &y& Elsevier]

Abstract: This paper demonstrates a new concept of heat transfer fluid (HTF) for CSP applications, developed in the frame of both a National and a European project (CSP2 FP7 project). It involves a dense suspension of small solid particles. This innovation is currently. The dense suspension of particles receiver (DSPR) consists in creating the upward circulation of a dense suspension of particles (solid fraction in the range 30%-40%) in vertical absorbing tubes submitted to concentrated solar energy. So the suspension acts as a heat transfer fluid with a heat capacity similar to a liquid HTF but only limited in temperature by the working temperature limit of the receiver tubes. Suspension temperatures up to 750°C are expected for metallic tubes, thus opening new opportunities for high efficiency thermodynamic cycles such as supercritical steam and carbon dioxide. First experimental results were obtained during on-sun testing with CNRS solar facility of a single tube DSPR for an outlet temperature lower than 300°C. In this lab-scale experimental setup, the solar absorber is a single opaque metallic tube, containing upward solid circulation, located inside a cylindrical cavity dug in a receiver made of refractory, and submitted to the concentrated solar radiation through a 0.10m x 0.50m slot. The absorber is a 42.4 mm o.d. stainless steel tube. SiC was used because of its thermal properties, availability and rather low cost. The 63.9μm particle mean diameter permits a good fluidization with almost no bubbles, for very low air velocities. Solar flux densities in the range 200-250 kW/m2 were tested resulting in solid temperature increase ranging between 50 and 150°C. The mean wall-to-suspension heat transfer coefficient (h) was calculated from experimental data. It is very sensitive to the solid fraction of the solid suspension, which was varied from 27% to 36%. These latter values are one order of magnitude larger than the solid fraction in circulating fluidized beds operating at much higher air velocity. Heat transfer coefficients ranging from 140 to 500 W/m2.K have been obtained; i.e. 400 W/m2.K mean value for standard operating conditions at low temperature. [Copyright &y& Elsevier]

Abstract: Dish-Stirling systems have been demonstrated to provide high-efficiency solar-only electrical generation, holding the world record at 31.25%. This high efficiency results in a system with a high possibility of meeting the DOE SunShot goal of $0.06/kWh. Current dish-Stirling systems do not incorporate thermal storage. For the next generation of non-intermittent and cost-competitive solar power plants, we propose a thermal energy storage system that combines latent (phase-change) energy transport and latent energy storage in order to match the isothermal input requirements of Stirling engines while also maximizing the exergetic efficiency of the entire system. This paper reports on the technical advantages and challenges of dish Stirling with storage, to make a preliminary estimate as to the technical feasibility of such a system. The proposed system with storage incorporates high temperature latent transport and latent storage, providing an exergetic match to the isothermal input of the Stirling cycle. The transport from the receiver to the storage, and from storage to the engine, is accomplished with advanced sodium heat pipes. The storage is in a solid-liquid phase change material (PCM), likely a metallic eutectic to reduce exergy losses in thermal conduction. We model a dish Stirling system at a block level, using a combination of real data from several dish systems with and without heat pipe transport, and determine annual energy production and revenue streams based on Barstow California weather data and Southern California Edison Time of Day pricing. We optimize the system on solar multiple, capacity of storage, and several operational strategies. We find that a storage system using metallic eutectic phase change storage results in a feasible physical embodiment, with mass, volume, and complexity suitable for 25kWe dish Stirling systems. The results indicate a system with 6hours of storage and a solar multiple of 1.25 provides the optimum impact to LCOE and profit for the range of cases studied. A storage system applied to dish Stirling will leverage the current high performance systems, increasing the value to the utilities and transmission entities. A feasible embodiment has been proposed, which with sufficient development will re-establish dish Stirling as a leading energy option. [Copyright &y& Elsevier]

Abstract: This paper is focused on experimental and numerical investigation of a dual-media rock bed thermal energy storage. An experimental test loop whose main component is a 1 m diameter and 3 m high storage tank has been built to carefully study the thermal behavior of rock bed thermocline thermal storage. Experimental results show constant outlet temperature during discharge process and confirm that thermocline is suitable for integration in a CSP power plant. The influences of mass flow rate, charge and discharge temperature difference and partial load have been tested without significant effect on dimensionless axial temperature profile. This indicates the robust and controllable nature of dual-media thermocline energy storage. Numerical 3D simulations have been performed using the commercial code Fluent. Results clearly illustrate that the piston-like behavior of thermocline can be strongly degraded by flow non-uniformities. First comparisons with experimental data show global good agreement even if some improvements of the model are needed. [Copyright &y& Elsevier]

Abstract: Halotechnics, Inc. is developing an energy storage system utilizing a low melting point molten glass as the heat transfer and thermal storage material. This work is supported under a grant from the Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E). Advanced oxide glasses promise a potential breakthrough as a low cost, earth abundant, and stable thermal storage material. The system and new glass material will enable grid scale electricity storage at a fraction of the cost of batteries by integrating the thermal storage with a large heat pump device. Halotechnics is combining its proven expertise in combinatorial chemistry with advanced techniques for handling molten glass to design and build a two-tank thermal energy storage system. This system, operating at a high temperature of 1200°C and a low temperature of 400°C, will demonstrate sensible heat thermal energy storage using a uniquely formulated oxide glass. Our molten glass thermal storage material has the potential to significantly reduce thermal storage costs once developed and deployed at commercial scale. Thermal storage at the target temperature can be integrated with existing high temperature gas turbines that significantly increase efficiencies over today's steam turbine technology. This paper describes the development and selection of Halotechnics’ molten glass heat transfer fluids with some additional systems considerations. [Copyright &y& Elsevier]

Abstract: Thermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in CSP plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-PCM storage concept which aims at circumventing this issue. The concept proposed is built of different solid filler materials and encapsulated PCMs combined into a multi-layer storage tank with molten salt as heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until equilibrium is achieved, i.e. the same amount of energy stored in the charging phase is delivered in the discharge. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy stored/released, as well as the efficiencies of the storing process have been compared for the different thermocline, PCM-only and multi-layered thermocline-PCM configurations. Based on this analysis the selection of the best option for a given case/plant is proposed. [Copyright &y& Elsevier]

Abstract: This paper demonstrates the potential of the quartzite as an economic and efficient filler material in thermal oil direct thermocline storage system. This natural rock appears as a good candidate for thermal energy storage by sensible heat up to 350°C. In the present study, many experiences were conducted in order to choose the best rock among five varieties (quartzite, basalt, granite, hornfels and marble) which are found abundant in Morocco. The optimal rock was selected according to various criteria such as surface characteristics, porosity, density, calorific capacity, hardness, mechanical resistance. The chosen rock should also provide a thermal stability during energy exchange with the heat transfer fluid (HTF). It should be noted that choice of the rocks as a filler material may reduce the quantity of the HTF used for charging and discharging the thermal energy. Hence, the shrinkage of tank volume by thermocline sensible heat storage system using rocks as filler materials allows reducing the cost with an increase in the efficiency of the system. [Copyright &y& Elsevier]

Abstract: In this paper, a mathematical model is developed to study the behavior of thermal energy storage (TES) under operation in the particular case of Adiabatic Compressed Air Energy Storage (A CAES). The A CAES consists of storing the available extra electrical energy of the electricity network in a form of compressed air (in a cavern) to discharge it during peak periods. The TES sub-system is used to charge and discharge the corresponding heat of compression, leading to a quasi adiabatic mode and an increase in the overall electricity storage efficiency (roughly from 50 to 70%) compared to diabatic CAES. The mathematical model has been converted into a computer simulation program with all the effective parameters of heat transfer in the storage reservoir. This model used to define a geometry reservoir able of storing a given power and restore it while maintaining a required temperature level at the output of unit. The influence of the input and output parameters on the storage efficiency is studied. The results illustrate the behavior of the storage reservoir under dynamic mode. [Copyright &y& Elsevier]

Abstract: This paper describes the modeling of a high-temperature storage system for an existing solar tower power plant with open volumetric receiver technology, which uses air as heat transfer medium (HTF). The storage system model has been developed in the simulation environment Matlab/Simulink®. The storage type under investigation is a packed bed thermal energy storage system which has the characteristics of a regenerator. Thermal energy can be stored and discharged as required via the HTF air. The air mass flow distribution is controlled by valves, and the mass flow by two blowers. The thermal storage operation strategy has a direct and significant impact on the energetic and economic efficiency of the solar tower power plants. [Copyright &y& Elsevier]

Abstract: The National Renewable Energy Laboratory is developing a thermal energy storage (TES) system that uses solid particles as the storage medium for a concentrating solar power plant. This paper focuses on the particle-TES performance in terms of three efficiency metrics: first-law efficiency, second-law efficiency, and storage effectiveness. The paper presents the derivation of the efficiency expression and their application in assessing the particle-TES performance and design. The particle-TES system uses low-cost stable materials that withstand high temperature at a fraction of the cost of the salt and metal containment vessels for high-temperature TES. Cost analysis indicates that particle TES costs less than $10/kWhth, which is less than half the cost of the current molten-salt-based TES and just a fraction of liquid heat transfer fluid storage at a similar high temperature of >700°C, due to its low cost of storage medium and containment. The fluidized-bed TES can hold hot particles of > 800°C with >95% exergetic efficiency, storage effectiveness, and thermal efficiency. [Copyright &y& Elsevier]

Abstract: Thermocline storage may be, today, the cheapest way to store solar thermal energy at high temperatures. Several estimations lead to a potential cost reduction of 35% compared to a two tank configuration while retaining a good efficiency. Thermocline tanks, in packed bed configurations, are mostly designed using filler materials to reduce the required amount of the expensive molten salt. The main problem of this technology is the thermal ratcheting which can lead to dramatic structural failure of the tank. One solution could be to use a structured bed configuration. With this goal, and as a part of the OPTS Project (OPtimization of a Thermal energy Storage system with integrated Steam Generator), after preliminary investigations driven by the necessity to have a low manufacturing cost, a brick design, made of industrial wastes, is proposed allowing both good thermal and mechanical properties. The implementation of the tank with the filler material of this geometry does not require a specific know- how other than building a classic wall. In order to study the behavior of the thermocline in this structured configuration, a numerical model is developed. A parametrical study was performed and the results are presented and discussed in this paper. The influences of the geometrical ratio of our ‘pattern’, of the physicals properties of the storage material and of the flow rate of the solar salt inside the tank are characterized. The aim is to study the theoretical feasibility of a direct high temperature structured thermocline tank for concentrated solar power, using cheap filler materials. [Copyright &y& Elsevier]

Abstract: The availability of cost-effective heat storage solutions is one of the key elements for the successful implementation of Direct Steam generation (DSG) solar plants. The most promising storage system for DSG solar plants consists of a 3-stage system whose key element is a Phase Change Material (PCM) latent heat module. In the framework of two large Concentrated Solar Power (CSP) projects, CEA-INES is involved in the development of heat storage systems for large scale DSG solar plants. Within these projects CEA-INES is setting up a unique testing platform consisting of all the required tools for the full qualification of the critical PCM storage modules. The PCM storage testing platform includes low temperature testing rigs to test the storage phenomenology and validate the performance simulation models for the selected PCM heat exchanger elements, testing equipment for the durability assessment of the key components up to a high pressure water steam testing rig able to test at lab-scale the PCM modules in real operating conditions. Most of these tools are currently in operation and the few remaining are about to be commissioned. The paper illustrates the CEA-INES design methodology for the development of the PCM storage modules and describes the most relevant tools of the CEA-INES test platform discussing how these tools are integral part of the PCM storage module design process. A few case studies and the results of the validation activity for some reference geometries are also presented. [Copyright &y& Elsevier]

Abstract: For each solar technology the dispatchable CSP plant scheme that maximizes the solar to electrical energy conversion and minimizes the costs associated to such transformation is searched. In recent years double tank TES systems are evolving towards more simple configurations of a single tank [1], [2], [3], also known as thermocline tank TES systems and, even going beyond, there are already thermocline tanks in process of development where the steam generator (SG) is integrated into the storage tank (European project OPTS). A prototype with integrated SG has been erected for testing purpose in the Casaccia Research Centre of ENEA (Italy) [4]. The model presented in this paper reproduces the experimental data of such prototype using the CFD commercial code STAR-CCM + [5]. Unlike very few published until now [6]. The simulated system account for molten salts behavior, not only for the bulk molten salts, but also for the circulation of the molten salts inside the SG (with three steam coils). The molten salts move by induced natural convection and the steam temperatures and pressures are up to ∼500° C/40bar. Temperature gauges situated in the bulk and the molten salts mass flow inside the SG are the main variables considered for validating the model. From the numerical point of view is a transient simulation and the model is 2D-axisymmetric. The required features of the system mesh and the physical models used are presented in this work. [Copyright &y& Elsevier]

Abstract: The current paper proposes three thermal storage concepts for solar thermal power plants, which are all based on sand powder as storage medium. The solid powder is used in a manner similar to fluid storage media such as molten salt. Fluidization is needed in order to produce fluid-like behavior of the powder. Applying powders as storage media for thermal energy storages offers advantages such as low cost, no freezing danger and hence no melting effort, no corrosion, local availability and high allowable temperatures. The storage media demand for all three concepts is estimated. [Copyright &y& Elsevier]

Abstract: One possibility to increase the efficiency and thus economic viability of solar thermal power plants is to increase their operating temperature. This approach demands the substitution of the state-of-the-art heat transfer fluid (HTF) that limits the operating temperature to roughly 400°C. Promising heat transfer fluids for future applications are molten salts or water/steam. If water/steam is used as HTF, the feed-water from the power block is fed to the solar field (SF) and directly evaporated and superheated. This process is called direct steam generation (DSG). A recent study [1] has pointed out that the economic potential of the DSG process is utilized only, if the SF design is simplified and a competitive thermal storage is available. Thus, an R&D project was launched in Germany to develop a complete storage system covering the energy of the evaporation as well as of the pre- and superheating section. It consists of a phase change material (PCM) storage for evaporation and a molten salt storage for pre- and superheating. One specific feature of superheated steam is its changing specific heat capacity with temperature. Using molten salt as storage medium with a nearly constant specific heat and the application of an obvious simple heat exchange would lead to an inefficient process. A significantly reduced live steam temperature and thus power block efficiency during discharge would be the results. Furthermore, the specific storage density of the molten salt system would be reduced too. In this paper this effect will be discussed in more detail. The consequences for the storage system will be discussed and solutions of the developed processes for the integration of such storage into a DSG power plant will be presented that reduce or overcome the mentioned restrictions. [Copyright &y& Elsevier]

Abstract: A stratified single tank is an alternative to a two-tank storage system for solar thermal power plants since it is promising cost reductions and shows more potential for further technical development. Direct integration of the steam generator is one of the possibilities. In this paper a higher order finite volume modeling approach for the storage is presented which reduces numerical diffusion but applies a turbulence factor which determines stratification. The discharge of the storage and the circulation of molten salt through the shell side of the internal steam generator by a forced flow is compared to natural circulation. The buoyancy driven flow strongly depends on the system design. The performance is improved with higher positioning of the heat sink in the storage system since a lower minimum charging level of the storage is required to obtain the design flow rate. The difference between the operation modes is more distinctive in the morning and the evening, when the charging level is relatively low and during days with low irradiation. With the chosen configuration both operation modes lead to the same circulation rate as soon as the charging level exceeds 24%. An annual simulation showed a reduction of total produced electricity by 3% for natural circulation compared to forced circulation. In case of forced circulation 95% of the cumulated charging power can be discharged, pure natural circulation achieves a discharge rate of 91%. To achieve the same performance the buoyancy driven flow needs to be supported by an impeller in 9% of the operational time. [Copyright &y& Elsevier]

Abstract: In this paper, the application of two types of heat exchangers under molten salt service in thermal energy storage plants is investigated. At first, relevant TES processes (direct and indirect) are analyzed and boundary conditions for the heat exchangers are defined. As a more detailed example, application for indirect storage in a parabolic trough plant employing HTF-oil is investigated and a comparison of shell-and-tube and coil-wound type exchanger is presented. It is shown, that the coil-wound type exchanger can leverage its specific advantages as e.g. compactness, higher efficiency of heat transfer and inherent ability to withstand thermal shocks leading to a cost-effective and innovative solution which ultimately enhances operation of a thermal energy storage plant and reduces investment cost in various aspects. [Copyright &y& Elsevier]

Abstract: Regenerator heat storage is a cost-effective solution to provide solar tower power plant with operational flexibility and load- following capability – a key factor for marketability. The recently completed project HOTSPOT addresses open design questions of this storage technology and reduces technical risks with respect to thermal design, fluid-dynamic and thermo-mechanical aspects. For the first time, design solutions based on packed beds have been looked at, and their specific technical risks have been systematically dealt with. The present paper reports on progress made and summarises some design recommendations. [Copyright &y& Elsevier]

Abstract: Several dynamic models of concentrated solar power plants were developed. The component models used belong to the ThermoSysPro (Modelica-based) library developed by EDF and released under open source license. The main objective of ThermoSysPro is to provide a generic library for the modeling and simulation of power plants and other kinds of energy systems. The meaning of the word ‘generic’ is here to be understood as the possibility to use the same components to model different kinds of energy systems (thermal, nuclear, solar, etc.) for different types of studies (design, control system verification, etc.). To that end, the library contains over 200 0D/1D models of components such as heat exchangers, steam and gas turbines, compressors, pumps, drums, tanks, volumes, valves, pipes, furnaces, combustion chambers, linear parabolic trough solar collector, linear Fresnel reflector, solar field and heliostat, etc. In particular, one and two-phase water/steam flow, one phase oil flow, as well as flue gases flow are handled. The library has been validated against several test-cases belonging to all the main domains of power plant modeling, namely the nuclear, thermal, biomass and solar domains. The paper describes the structure of the library. Then the test-cases belonging to the solar domain (both solar and power cycle performance are modelled) are presented: a dynamic model of a Linear Parabolic Trough Solar power plant and a model of a Solar Hybrid Combined-Cycle Power Plant with a Linear Fresnel Field. Several transients are simulated, the objective being to reduce the uncertainty of the prediction on the yearly electricity production (simulation with yearly DNI). [Copyright &y& Elsevier]

Abstract: The interest for hybrid solar gas-turbine systems (HSGT) in solar tower plant technologies is growing This is due to the high conversion efficiency and to the low water consumption that are achieved when a combined cycle is implemented. This paper presents a simulation tool which is dedicated to the performance analysis of the top cycle featuring a thermal energy storage unit (TES). The influence of the TES operating conditions on the power plant production is highlighted. A major advantage of the storage is to stabilize the air temperature at the combustion chamber inlet in order to keep the operating conditions of the combustion chamber close to the design point. This work establishes that a higher and more stable electrical generation is achieved through this concept. According to the storage capacity, the TES unit increases the daily average solar share of the power plant. This study is conducted within the framework of the French PEGASE project (Production of Electricity from Gas and Solar Energy) which aims at setting up and testing at THEMIS site (France) a demonstration plant based on HSGT technology. [Copyright &y& Elsevier]

Abstract: Experience curve approach has been used before to predict cost reductions in renewable power technologies. For wind and PV, this prediction came true but not for Concentrating Solar Thermal (CST). The CST has not been able to enjoy sufficiently high deployment rates and therefore has not been able to achieve significant cost reductions. This paper examines the technical reasons for the relatively slow CST uptake over the last two decades and how this can be remedied in the future using different technology. [Copyright &y& Elsevier]

Abstract: The increase of the process temperature of concentrating solar power plants above the degradation temperature of thermal oil (400°C) opens the way for increased power block efficiency and thus reduced cost of electricity production. Direct solar steam generation is one technical option to follow this path. The paper presents different power block designs for direct steam generation parabolic trough and linear Fresnel power plants. Based on a systematic modelling approach, results for efficiency gains are derived and compared against a reference case of an oil-based plant. The results show that different reheat configurations are feasible and that efficiency gains in the range from 4 to 6% can be expected based on todays or near future solar collector technology. [Copyright &y& Elsevier]

Abstract: Evaporation cooling systems are currently deployed in the majority of operating Concentrating Solar Power (CSP) plants. However, the fundamental drawback of this approach is that large quantities of water are used in the cooling tower, so that this solution will be not applicable for a large-scale CSP development in arid regions. In fact, in most of the sites suitable for CSP applications, ambient temperatures are typically high and water is scarcely available, or the cost of transporting water to these sites is prohibitive. For this reason, at these sites dry cooling will be the only viable option. This paper analyses the impact of dry cooling systems on technical and economic plant performances, considering several condenser layouts, different operation strategies and economic boundary conditions. In particular, the capacity and the operation of the thermal energy storage can be optimized in order to maximize power production, e.g. by preferential plant commitment at night hours. The analysis is carried out for three selected locations with real meteorological data by means of annual simulations with hourly time steps. [Copyright &y& Elsevier]

Abstract: Recent research suggests that an emerging power cycle technology using supercritical carbon dioxide (s-CO2) operated in a closed-loop Brayton cycle offers the potential of equivalent or higher cycle efficiency versus supercritical or superheated steam cycles at temperatures relevant for CSP applications. Preliminary design-point modeling suggests that s-CO2 cycle configurations can be devised that have similar overall efficiency but different temperature and/or pressure characteristics. This paper employs a more detailed heat exchanger model than previous work to compare the recompression and partial cooling cycles, two cycles with high design-point efficiencies, and illustrates the potential advantages of the latter. Integration of the cycles into CSP systems is studied, with a focus on sensible heat thermal storage and direct s-CO2 receivers. Results show the partial cooling cycle may offer a larger temperature difference across the primary heat exchanger, thereby potentially reducing heat exchanger cost and improving CSP receiver efficiency. [Copyright &y& Elsevier]

Abstract: Concentrating solar power (CSP)-biomass hybrids plants are becoming increasingly interesting as a low cost option to provide dispatchable renewable energy since the first reference plant commenced operation late 2012, 22.5MWe Termosolar Borges in Spain. The development of such project is a complex task with not only one but two energy sources required to make the project successful. The availability of several studies but only one reference plant worldwide is proof of that. This paper investigates the hybridisation of a biomass power plant with a molten salt solar tower system. The benefit of this combination is a high cycle efficiency as both the steam generators can provide steam at 525°C and 120bar to the steam turbine. A case study approach is used to provide technical, economic and environmental benefits of a 30MWe CSP-biomass plant with 3h thermal storage in Griffith, New South Wales. At this site such a plant could provide annually 160,300MWh of electricity with an annual average electricity price of AU$155/MWh. Compared to a standalone CSP plant with 15h of thermal storage the hybrid plant investment is 43% lower, providing a possibility to fast-track CSP implementation in countries where CSP is struggling to enter the market due to low wholesale electricity prices, such as Australia. [Copyright &y& Elsevier]

Abstract: In this paper the results of the preliminary performance assessment of an emerging hybrid CHP-technology using Organic- Rankine-Cycle technology comprising biomass combustion and concentrating solar thermal power in areas with low DNI is presented. The study was conducted in course of the research project BIOconSOLAR, which was funded by the climate fund of the Austrian government. The assessment is based on the technical design and economic conditions of an existing CHP-plant with nominal electric power output of 1.5 MW, which is located in the city Salzburg in Austria. The solar thermal energy provided by a parabolic trough collector field is primary used for electric power production in order to reduce biomass consumption and operation costs accordingly, but also to boost the thermal energy supply for the district heating of the city Salzburg. For electric power production the solar thermal energy is fed into the ORC-power cycle at a temperature level of at least 270°C. A transient simulation model of both the biomass CHP-plant and the parabolic trough plant was developed in IPSEpro. Based on the results of the process simulation the economic performance was assessed by conducting a dynamic investment calculation. Despite the technical and economic uncertainties of this preliminary assessment the retrofitting of biomass-solar CHP-plants with CSP in areas with low DNI is a promising option to improve the economic performance of about 100 CHP-plants in operation in Central Europe. A feed-in-tariff for solar thermal electricity in the same order of magnitude as for photovoltaic could trigger the retrofit of biomass CHP-plants. [Copyright &y& Elsevier]

Abstract: In this paper, a supercritical cycle coupled with an earth-air heat-exchanger (EAHE) has been studied for power generation from low to medium temperature heat sources. A number of organic refrigerants (R32, R125, R134a, R143a, R170 and R218) were studied as working fluids. The temperature range of 125-200°C was considered for the heat source while ambient air, cooled using EAHE, was used as the sink. The effect of various parameters (operating pressure, outlet temperature of the heat source and geometry of the EAHE) on the efficiency of the thermodynamic cycle was studied. An optimum operating pressure was obtained for all the fluids studied, except R32. Practical limitations, such as vapor quality in the turbine and pinch point in the heat exchangers, were considered in the analysis. Sinusoidal yearly variation in daily average air temperature was also taken into account in modelling the system. The soil temperature increased only for a short distance around the pipe while the bulk temperature remained unaffected. [Copyright &y& Elsevier]