Molten Salt Receiver Research Articles

Towards an optimal aiming for molten salt power towers

Finding a suitable aiming strategy for receivers of power towers can be challenging, especially for receivers using molten salt as heat transfer fluid as the allowable flux density decreases dramatically with increasing salt temperature. In this paper a very fast, steady-state model for the molten salt receiver is presented. This model is combined with a ray-tracing software and a metaheuristic optimization procedure. The thermal model is used to calculate the actual temperature and mass flow in the receiver which are then used to calculate the operational limits for the flux density. It is demonstrated that such an optimized aiming strategy can outperform a parameter based aiming strategies by more than 2%.

Integrated numerical study on the coupled photon-thermal conversion process in the central solar molten salt cavity receiver

In a solar power tower system, the concentrating and collecting subsystem, including the heliostat field and the receiver, contains some of the most costly and technically challenging components. Many types of solar receivers make up the solar power tower system. Among them, the cavity molten salt receiver has been widely applied due to its lower heat loss. The accurate simulation of the coupled heat transfer process in the cavity receiver and the prediction of receiver performance are of great importance for the receiver design and safe operation. The solar flux distribution on cavity interior surfaces is extremely non-uniform, which has great effect on the receiver thermal performance. Taking the non-uniform solar flux distribution into consideration, the photon-thermal conversion process is simulated integrally in this paper. A hybrid simulation approach, which couples Monte Carlo ray tracing and Gebhart methods, is applied to investigate the complete solar radiation transfer process in the solar power tower system with a cavity receiver and obtain the non-uniform solar flux distribution. On this basis, the coupled photon-thermal conversion process in the molten salt cavity receiver is simulated. The effects of the fluid flow layout on the receiver performance under the non-uniform solar flux distribution are particularly revealed. Also, the variation of the receiver performance with time is analyzed in detail. The results show that the solar flux distribution on the interior surfaces of the cavity receiver is extremely non-uniform, under which the molten salt flow layout has great effects on the receiver performance. In the case of flow layout that the cold molten salt flows into the receiver in the high-flux area, the temperatures of both the absorber wall and the molten salt increase rapidly along the flow path. The temperature of the whole absorber wall is relatively high, which leads to greater heat loss and less amount of hot molten salt, and even seriously, “heat transfer deterioration” near the outlet of the receiver. When the cold molten salt flows into the receiver in the low-flux area, the temperature of both the absorber wall and the molten salt increase slowly along the flow path. The temperatures of the whole absorber wall is relatively low, which results in less heat loss and larger amount of hot molten salt. Although the shape of the solar flux distribution on the interior surfaces varied greatly with time, the evolutions of the molten salt (absorber wall) temperature at different time display similar variation tendency along the flow path. Moreover, the reflective loss varies with time greatly, while the heat loss varies with time slightly.

Aiming strategy model based on allowable flux densities for molten salt central receivers

Abstract This study presents an aiming model to properly point heliostats at cylindrical molten salt receivers in Solar Power Tower. By means of two iterative algorithms (search and fit), the proposed strategy attempts to maximize the receiver thermal power output while preserving the receiver operational limits. Corrosion and thermal stress constraints are translated into allowable flux densities ( AFD ) that are handled by the model. The computer code accommodates the flux images produced by each heliostat in a field to accurately fit the AFD limit. In this paper, a Gemasolar-like field–receiver system serves to illustrate the aiming model. Compared to the equatorial aiming, receiver interception is slightly lower using the proposed strategy, but the receiver integrity is ensured; peak flux is significantly reduced up to 23%. It has been found that a favorable flux density profile generally has its peak displaced to the salt entrance at each receiver panel. Since external cylindrical receivers consist of a combination of up-flow and down-flow panels, the optimal flux profile is challenging for contiguous panels with contrary demands. In spite of that, remarkable matching is achieved by the fit algorithm. Because of its fast computation and automatic operation, the resulting tool can be applied to real-time control of existing heliostat fields and the integrated design of the coupled systems field and receiver.

Analysis of thermal stress and fatigue fracture for the solar tower molten salt receiver

In this paper, the thermal stress and fatigue fracture of a single tube for the solar tower molten salt receiver are presented. First, the temperature distribution of the tube is simulated. Second, the thermal stress is calculated by solving stress equations. Then the minimum heat flux formula is derived when damage occurs on wall based on the yield criterion and heat conduction equation. Finally, the critical crack length on outer wall is investigated by using the Crack Tip Opening Displacement (CTOD) method. The results show that the distribution of stress is similar to that of inner/outer wall temperature difference. The maximum stress, which causes plastic deformation, occurs on outer wall. According to the minimum heat flux formula q≥[σs]/(KA2+B2+C2−AB−AC−BC) derived in this paper, the heat flux is only 0.19 MW⋅m−2 when damage appears on the receiver tube if the inlet temperature and safety factor are 560 K and 1.5 respectively. Furthermore, the critical crack length is 17 mm when the heat flux is 0.7 MW⋅m−2.

Modeling and simulation of a molten salt cavity receiver with Dymola

Molten salt receivers play an important role in converting solar energy to thermal energy in concentrating solar power plants. This paper describes a dynamic mathematical model of the molten salt cavity receiver that couples the conduction, convection and radiation heat transfer processes in the receiver. The temperature dependence of the material properties is also considered. The radiosity method is used to calculate the radiation heat transfer inside the cavity. The outlet temperature of the receiver is calculated for 11 sets of transient working conditions. The simulation results compare well with experimental data, thus the model can be further used in system simulations of entire power plants.

Numerical Model and Validation for Simulating the Cold Filling of the Molten Salt Receiver Tube

Abstract Anti-protecting against freezing and preheating the components are the most critical problems of molten salt receiver. Cold filling is a potential solution to these problems. This paper presents a numerical model for simulating the cold filling of a receiver tube. The model uses the volume of fractionmethod to track the interface betweenthe molten salt and the air and the enthalpy method to describe the liquid-solid phase change of the molten salt. This model was used to simulate the cold filling tests of the Molten Salt Electric Experiment (MSEE) receiver. This model was then use to simulate the dynamic process of cold filling a vertical receiver tube. Three modes of cold filling, including successful filling, partial frozen filling and fully frozen filling, were demonstrated. The results show that high initial temperature of the receiver tubes is benefit for successful filling.

Thermal Analysis of a Multi Effect Distillation Plant Powered by a Solar Tower Plant

Abstract Decreasing water resources and steadily rising water demand drive research towards new approaches for safe and reliable water supply for the municipal, agricultural and industrial sectors. One solution for fresh water provision is sea water desalination using solar thermal energy. This paper describes the coupling of a Multi Effect Distillation (MED) plant with a Clausius-Rankine cycle powered by a solar central receiver system. A steady state model of an MED plant is developed and a correlation for the Gained Output Ratio (GOR) as function of heating steam temperature, specific seawater massflow and specific heat transfer surface of the desalination plant is deduced. This correlation is integrated into a model of a central receiver plant, comprising the heliostat field, a molten salt receiver system, two-tank storage and the steam cycle. A showcase simulation for the location Al-Kosseir, Egypt is performed to show the trade-off between electricity and water production for a cogeneration situation.

Molten Salt Receivers Operated on Parabolic Trough Demo Plant and in Laboratory Conditions

Concentrating solar power (CSP)technologies constitute achievable commercial options for large scale power plants as wellas for smaller electricity and heat generating device. Archimede Solar Energy (ASE), with the strong contribution of his shareholder Chiyoda and thanks to the important support given by industrial partners†, set recently in motion a Molten Salt Parabolic Trough (MSPT) Demo Plant, with the aim to deepen the knowledge about economy and reliability ofMolten Salt Parabolic-Trough receivers and Power Plants. We present the first characterization of ASE receiver tubes operated on the MSPT Demo Plant and on an indoor test benchas well as the Demo plant outcome regarding operational temperatures and heat loss.

Design Features and Control Concepts of ALSTOM Molten Salt Receiver

The tower type molten salt solar thermal power plant consists of a large heliostat field consisting of thousands of mirrors with capabilities to track the sun, positions itself in order to reflect the solar energy onto the molten salt central receiver (MSCR) atop a tower optimally located in the heliostat field. The receiver consists of tube banks arranged in panels with molten nitrate salt flowing within. The concentrated energy from the heliostat field heats the molten salt from 290oC to 565oC while it passes through the receiver. The hot molten salt is stored in a tank and utilized to produce steam for power generation as per the demand. The molten salt central receiver serves as the interface between the solar field and the molten salt heat transfer medium. The various components comprising the MSCR have been designed to maximize the received solar energy, the transfer of solar energy to salt, and the capacity of hot salt production. The state of the art for molten salt central receiver technology offer new opportunities to optimize the design and control of the same. This paper discusses the advanced design features and control concepts of the ALSTOM molten salt central receiver. It includes an optimized receiver surface to minimize pressure drop while maximizing the utilization of the heliostat field, modular design which is easy to construct and maintain, multiple system features for controlling salt temperatures and a health monitoring system for accurate, on-line life assessment, as prominent features.

Enhanced heat transfer performances of molten salt receiver with spirally grooved pipe

The enhanced heat transfer performances of solar receiver with spirally grooved pipe were theoretically investigated. The physical model of heat absorption process was proposed using the general heat transfer correlation of molten salt in smooth and spirally grooved pipe. According to the calculation results, the convective heat transfer inside the receiver can remarkably enhance the heat absorption process, and the absorption efficiency increased with the flow velocity and groove height, while the wall temperature dropped. As the groove height increased, the heat losses of convection and radiation dropped with the decrease of wall temperature, and the average absorption efficiency of the heat receiver can be increased. Compared with the heat receiver with smooth pipe, the heat absorption efficiency of heat receiver with spirally grooved pipe e/d = 0.0475 can rise for 0.7%, and the maximum bulk fluid temperature can be increased for 31.1 °C. As a conclusion, spirally grooved pipe can be a very effective way for heat absorption enhancement of solar receiver, and it can also increase the operating temperature of molten salt.

Phase change of molten salt during the cold filling of a receiver tube

The most critical problems for the molten salt solar power plant are protecting against freezing and preheating of the flow pipes and equipments. A candidate way to solve these problems is cold filling. In this study, the dynamic process of molten salt cold filling into a receiver tube was simulated by combining the volume of fluid method and the enthalpy method. The detailed mechanism of melting-solidification and the evolution of the pressure drop along the tube were demonstrated. The influence of cold filling on the temperature across the tube wall was analysed as well. The results indicate that a lower pressure drop can be obtained by increasing the salt temperature and the initial tube temperature, and that an optimal velocity is required to get minimum pressure drop. The temperature difference between the external and internal surfaces of the tube wall increases with the filling velocity and the filling temperature whereas decreases with the initial tube temperature and the heat flux on the external surface. A large filling velocity and a high filling temperature that results in large temperature difference across the wall of the tube should be avoided. This study gives practical reference to the application of cold filling in molten salt receiver.

Design Space Exploration of a 5 MWth Small Particle Solar Receiver

We present a design space exploration of a 5 MWth Small Particle Solar Receiver for solar tower power plants. This new solar receiver, developed under the support of the U.S. Department of Energy's SunShot Program, aims to volumetrically absorb concentrated solar irradiation using an air-particle mixture to drive a gas turbine or a combined cycle at much higher temperature than the state-of-the-art molten salt receivers. Among other advantages, the thermodynamic efficiency of the power block and the overall efficiency of the plant would considerably increase with this technology. The design space consists of the wall angle of the receiver, the geometry of the window (necessary to allow the solar irradiation to enter into the receiver) and the radiative properties of the walls. The constraints are based on material limits, ensuring the mechanical integrity of the quartz window, and other technical issues; though some of them are imposed via a penalty method. The design space is explored through parametric studies and a multidisciplinary approach is adopted. The aluminum oxide walls, the 45° spherical-cap window and the 45° wall- angle receiver are preferred due to their best compromise between thermal efficiency and wall temperature.

ESolar's Modular, Scalable Molten Salt Power Tower Reference Plant Design

eSolar has completed design of a molten salt solar power tower with storage based on a 50-MWt module comprised of a tower- mounted molten salt receiver surrounded by a heliostat field utilizing eSolar's small heliostat technology. This basic thermal module can be replicated, without scaling or redesign, as many times as required to create plant sizes from 50 to 200 MW with capacity factors ranging from 20 to 75%. This paper describes the details of this molten salt reference plant design as developed over the past two years, as well as a potential scenario for staged initial commercial deployment that manages both risk and cost.

Allowable flux density on a solar central receiver

The allowable flux density on a solar central receiver is a significant receiver parameter and is related to the receiver life span and economics. The allowable flux density has gradually increased as receiver technologies have developed and is related to various factors, such as the material characteristics, tube sizes, and internal tube flow conditions. A mathematical model was developed to calculate the allowable flux density for the Solar Two receiver which agrees well with published data. The model was then used to show that a higher allowable flux density can be obtained by increasing the allowable strain of the tube material and the fluid velocity and decreasing the tube thermal resistance, the convective thermal resistance, and the tube diameter and wall thickness. A sensitive analysis shows that the most important influence is the wall thickness, followed by the tube diameter and fluid velocity. Finally, a molten salt receiver gives a much higher allowable flux density than water/steam receivers and is even better than a supercritical steam receiver.

An experimental study: Thermal performance of molten salt cavity receivers

In solar power plants, a molten salt receiver always works in unsteady state conditions. Therefore, it is necessary to research the thermal performance of a receiver in an unsteady state condition. For this purpose, an indoor testing system with a molten salt cavity receiver was developed. Experimental research was conducted to determine the thermal performance of a 100 kWt molten salt receiver. The effect of the input power and flow rate on the thermal performance of the receiver was investigated. In addition, a simple unsteady model was established to research the characteristics of the variation of the internal energy of the receiver and the characteristics of the heat loss. The results indicated that the efficiency of the receiver was in direct proportion to the flow rate. However, the influence was small. In the initial stage of the transient process, the increments of the internal energy of the receiver and the fluid were large (approximately 20% in the energy which is not removed by the mass flow of the fluid). Over time, the thermal inertia of the receiver decreased with the transient process. As a result, any energy not taken away by the fluid was transformed into heat loss.

Experimental and theoretical analysis of a dynamic test method for molten salt cavity receiver

Test methods for estimating the thermal performance of the molten salt receiver are a matter of ongoing concern. To date, test methods in the literature require receiver to be operated in steady state or quasi-steady state. However, the receiver is always operating in the unsteady state with ongoing changes in power absorption and flow rate. Therefore, research into dynamic test method for the molten salt cavity receiver is required. The Transfer Function Method (TFM) is a successful dynamic test method for solar collectors. In this paper, a theoretical analysis of the TFM was applied to the molten salt cavity receiver and then verified by indoor transient experiments. The TFM predicted outlet temperature of the receiver was compared with experimental data. The results showed that the TFM accurately predicted the outlet temperature trends despite some errors between predicted and measured outlet temperature. The errors may have originated from the changing flow rate. The TFM is a good candidate as a dynamic test method for the concentrated solar receiver.

Concentrated solar power on demand

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

Heat transfer enhancement and performance of the molten salt receiver of a solar power tower

This paper investigates the interaction between the heat transfer performance and the thermal efficiency of a molten salt receiver used in the solar power tower plant. A test-bed is built, and a series of experiments of heat transfer enhancement for two types of molten salt receiver tubes, including smooth and spiral tubes, have been carried out under the high temperature and the high heat flux conditions. The experimental results show that the Nusselt numbers of spiral tube with heat transfer enhancement are in the range of 400–1200, which is about 3 times than that of the smooth one on average. The wall temperature of the spiral tube is decreased by about 30 °C comparing with that of the smooth tube under the identical heat transfer conditions. The results of the experiment show that, by using the spiral tube as the heat transfer tube, the heat transfer performance of the molten salt receiver is obviously improved, and the radiation and convection losses are significantly reduced. The results will be helpful for the design of the molten salt receiver.

High Flux Central Receivers of Molten Salts for the New Generation of Commercial Stand-Alone Solar Power Plants

Molten salt technology represents nowadays the most cost-effective technology for electricity generation for stand-alone solar power plants. Although this technology can be applied to both concentrating technologies, parabolic through and central receiver systems (CRSs), CRS technology can take advantages from its higher concentration, allowing to work at higher temperatures and therefore with a reduction in the size and cost of the storage system. The receiver system is the “door” for which the energy passes from the field collector to the thermal-electric cycle; it represents, therefore, the core of the CRS and its performance directly affects plant production. Starting from the published lessons from SOLAR TWO receiver technology, the validation of an improved receiver for molten salt technology was assumed as part of the SOLAR TRES solar thermal power commercial plant development. Main challenges for the new receiver were to increase its allowable peak flux up to 1MW∕m2 in order to maximize the thermal efficiency of the CRS solar power plant, and to improve its safe life without limiting the incident fluxes that the field of heliostats is able to deliver with an optimized pointing strategy. Several advanced features in geometric and thermodynamic aspects and in its material selection have been implemented on the receiver. With the results of a sensitivity analysis carried out with an own code developed by SENER (SENREC), a prototype receiver panel was designed, fabricated, and installed in a proper test bed at the PSA. Test validation on this panel was carried out in 2007. The initial test results show a very good behavior of the prototype receiver, which allows to anticipate that the objectives of its design can be fulfilled. SENER and CIEMAT have joined forces to face up the challenge of sizing and designing a new molten salt receiver of high thermal efficiency, able to operate at high fluxes without compromising its durability (at least 25years). Main challenges for the new receiver design were to optimize the receiver dimensions and receiver tube sizes and material selection to surpass the operating conditions in the new plants with respect to SOLAR TWO.

Quick evaluation of the annual heliostat field efficiency

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