Tower Power Plant Research Articles

Porous Monolithic Perovskite Structures for High-Temperature Thermochemical Heat Storage in Concentrated Solar Power (CSP) Plants and Renewable Electrification of Industrial Processes

A novel approach towards thermal energy storage of surplus renewable energy (RE) is introduced via a hybrid thermochemical/sensible heat storage concept implemented with the aid of porous structures made of redox metal oxides, capable of reversible reduction/oxidation upon heating/cooling in direct contact with air, accompanied, respectively, by endothermic/ exothermic heat effects and demonstrating fully reversible dimensional changes under cyclic operation. The proposed modular storage units can be heated during the day to a level exceeding the metal oxide’s reduction onset temperature either by hot air streams from air-operated Concentrated Solar Power (CSP) tower plants or via surplus/cheap RE-electricity from photovoltaics, wind, or other renewable sources (“charging”/energy storage step). When this RE sources become non-available or upon demand, the fully charged system can transfer its energy to a controlled airflow that passes through the porous oxide block and initiates the exothermic oxidation of the reduced metal oxide. Thus, a hot air stream is produced which can be used to provide electricity or exploitable heat for industrial processes. The present work elaborates on the operating principles and the potential application of this concept and reports progress in the preparation and shaping of reticulated porous ceramics (RPCs also known as “ceramic foams”) from CaMnO3-based perovskite compositions and their preliminary testing with respect to cyclic reduction-oxidation.

Comparison of Aiming Point Strategy Algorithms and Their Assessment for Volumetric Solar Receivers

The present work summarizes the work done within the scope of CATION, CHLOE, and HECTOR projects related to the aiming point strategies optimization algorithms applied to volumetric solar receivers. Several optimization methods have been applied in the literature for optimizing the aiming strategy of solar power tower plants. However, the use of these algorithms for the requirements of volumetric solar receivers and solar fuel applications has not been accomplished. Herein, the problem is formulated as a constrained optimization problem whose objective function is a combination of the total power on the receiver surface and the flux homogeneity. A comparison of the results is presented in this work. It will be seen that TABU search and SA algorithms are the most efficient in terms of computation time and, in addition, the first one shows very good results in power and spillage. The rest of the mentioned algorithms are much slower and, in general, the results are slightly worse.

Resources, Training, and Education Under the Heliostat Consortium: Industry Gap Analysis and Building a Resource Database

Abstract Concentrating solar power is not a widely deployed or known technology area, and the heliostat workforce community in the United States is currently small, with knowledge and expertise not widely available. The resource, training, and education (RTE) topic within the Heliostat Consortium (HelioCon) was established to address this. RTE encompasses resources, practices, and programs to ensure that (1) newcomers to the heliostat development community have an adequate knowledge base and training to conduct R&D efforts, (2) outsiders to the field are provided with resources and opportunities to join the workforce, and (3) the workforce community is a productive, healthy, and fulfilling environment for all workers. In the first year of the project, a roadmap study was conducted, in which the major gaps in RTE were identified by consulting experts in the industry, with the top gap being the lack of public accessibility to concentrating solar-thermal power (CSP) knowledge. To address this, the HelioCon team has been developing a centralized web-based resource database, containing a reference library, educational videos, lists of components suppliers and software/metrology tools, a power tower plant database, and information on existing standards/guidelines.

Heliostat field aiming strategy based on deterministic optimization: An experimental validation

In Solar Power Tower plants, the aiming strategy plays a key role in the performance and interaction between heliostat field and receiver. This work presents an aiming strategy based on a deterministic optimization that maximizes the flux uniformity and minimizes the spillage losses. The algorithm can be fed either by a flux mapping model or by the synthesis – superposition – of experimental images. Both approaches are experimentally tested at PROTEAS research facility, aiming 16 heliostats at a flat Lambertian target. In the comparison between experimental and computed flux maps, the cross-correlation coefficient exceeds 97%. The spillage loss factors are underestimated by the computations: 9.2 percentage points by the model-based and 6.7 points by the synthesis-based approach.

Supercritical Water Gasification of Biomass on CSP Plants: Comparison of On-Sun and Off-Sun Designs

A comparative study of On-Sun and Off-Sun designs for supercritical water gasification (SCWG) of biomass on concentrated solar power (CSP) plants was here presented. In the On-Sun design, the tubular reactor was placed on a cavity receiver of a solar power tower (SPT) plant and the heat employed to produce the gasification came directly from the sun (viz. the solar radiation was concentrated, thanks to the heliostat field, onto the tube's surface). In the Off-Sun designs, the gasification of the biomass was carried out inside a heat exchanger and it was produced thanks to the heat absorbed from the hottest fluid, which can be obtained from CSP plants or electrical heating. This work numerically investigated the thermomechanical performance of both designs. The results stood out the relation between temperature-gas yield-thermal stress. In On-Sun configurations the gas yield was 5 times higher than those obtained in the Off-Sun configuration. However, the highest values of both temperature and stress may produce the failure of the reactor. This fact could be avoided in an Off-Sun reactor, since its uniform heating conditions mitigate the maximum temperature and stresses.

Allothermally Heated Reactors for Solar-Powered Implementation of Sulphur-Based Thermochemical Cycles

Catalytic sulphur trioxide splitting is the highest-temperature (650-950oC), endothermic step of several sulphur-based thermochemical cycles targeted to production of hydrogen or solid sulphur. Concentrated solar power tower plants are an attractive renewable energy source to provide the necessary heat. Furthermore, the development of solar receivers capable of delivering solid or gaseous heat transfer fluids at these temperature ranges enable the implementation of such endothermic reactions in allothermally-heated reactors/heat exchangers placed away from the solar receiver. In this context, a 2-kW laboratory-scale shelland-tube reactor/heat exchanger to perform thermal sulphuric acid decomposition and catalytic sulphur trioxide splitting was in-house designed, built and tested with electrically heated bauxite particles, in the perspective of eventually coupling such a reactor with a centrifugal particle solar receiver. Thermal test runs demonstrated the in-principle feasibility of the concept. The temperatures reached were sufficient to ensure complete sulphuric acid evaporation. However, the ones in the SO3 splitting zone were of the order of 750°C, high enough to demonstrate SO3 splitting but not reaching the levels required for close-toequilibrium conversion of the Fe2O3 catalyst system used (~ 850oC). An improved version of the reactor is under construction incorporating design modifications based on lessons learned from the test campaigns, in the perspective of scaling up the process.

Performance analysis and optimization of a zero-emission solar-driven hydrogen production system based on solar power tower plant and protonic ceramic electrolysis cells

The solar-driven high-temperature steam electrolysis is promising for efficient large-scale H2 production. In this study, a comprehensive component-to-system model and optimization framework is developed to investigate the performance of a zero-emission H2 production system based on solar power plant and protonic ceramic electrolysis cell. Compared to previous system studies, the detailed description of cell internal operating characteristics is realized by integrating multi-physics simulation and artificial neural network. After parametric analyses, it is found that the system energy/exergy efficiency and co-generation performance are complicated by each subsystem. And the optimal system performance (ηth = 50.63 %, Z = 179.63 $·h−1 and ηex = 33.03 %, Z = 178.94 $·h−1, with LCOE = 0.172 $·kWh−1 and ZH2 = 6.497 $·kg−1) is obtained considering cell operating features and system energy-exergy-economic factors through multi-objective optimizations. Besides, the tradeoff between system maximum H2 production capacity and cell internal thermal conditions is revealed. This study can facilitate the development of zero-emission green H2 production driven by renewable energy.

Development, Construction, Operation and Maintenance of Shouhang Dunhuang 100 MW Molten Salt Solar Power Tower Plant

The Shouhang Dunhuang 100 MW molten salt solar power tower plant is the first 100 MW-scale commercial demonstration project in China. The plant started to break ground in October 2016, was completed and connected to the grid for power generation in December 2018, and achieved full-load operation in June 2019. This paper comprehensively introduces the development, design, and construction process of the project as well as its operation and maintenance over the past five years, and shares experiences and lessons learned from the project.

Solar receiver endurance assessment under non-conventional operation modes

Central receiver endurance in solar power tower plants is a critical aspect to ensure their viability. In this work, two receiver configurations, with and without flow path crossover, are compared in thermal and mechanical performance. Considering their thermomechanical limits during operation, results show that panels in the crossover configuration would endure from 1.22 to 2 times that in the no-crossover, just marginally penalising its thermal efficiency. Additionally, two plant operation strategies are compared: continuous operation and nighttime strategy. The former results in 1.89-times more damage but roughly a 1.9-times increase in energy output, obtaining a practically constant trade-off between damage and energy generation for these two strategies. Lastly, the effect of considering either a linear or bilinear (interaction point set at 0.05,0.05) creep/fatigue damage interaction is explored. While producing more conservative results, linear interaction error is below 3 % in the most critical receiver regions. Such a low discrepancy is caused by the creep-damage dominance over fatigue in the most critical regions. Other areas show heavier creep-fatigue interaction, resulting in larger error, but these areas obtain times to failure much longer than expected receiver lifespan.

Experimental isochoric apparatus for bubble points determination: Application to CO2 binary mixtures as advanced working fluids

Carbon dioxide binary mixtures are increasingly considered as working fluids in transcritical power cycles, due to the capability to perform liquid-phase compression even at high environmental temperatures. However, a robust thermodynamic model is essential for optimal and reliable design conditions. It is widely recognised that fine-tuning the equation of state with experimental vapour-liquid equilibrium data of the mixture significantly enhances its reliability.In this work, a new apparatus dedicated to vapour-liquid equilibrium measurements of mixtures is presented. The proposed method consists of a constant-volume system, where bubble points are identified from the divergence of slope of the isochoric lines between the two-phase and liquid regions, in the temperature-pressure plane. The temperature and pressure limits of the apparatus are 503 K and 25 MPa.Bubble points of CO2 binary mixtures with hexafluorobenzene (C6F6) and n-pentane (C5H12) have been measured and compared with previous literature data for validation purposes. Then, the CO2 mixture with octafluorocyclobutane (c-C4F8) is experimentally studied, addressing a literature gap in bubble point data. The data are used to calibrate the thermodynamic model, leading to affordable design conditions of the power cycle compared to the non-optimised thermodynamics scenario, in a concentrated solar power tower plant.

Commissioning of a Preheat Strategy of a Molten Salt Test Receiver

Solar power tower plants that are operated with molten salt offer the advantage that the molten salt can be used as a heat transfer medium as well as storage medium. Consequently, solar energy can be stored easily and efficiently in only one loop without transformation losses. Furthermore, by using large-scale storage facilities, almost a 24/7 base load operation is possible. Since the molten salt begin to crystallize at temperature of 238 °C, the absorber tube must be preheated before the plants’ operation can begin. During the preheating a subset of heliostats will be aimed on the receiver surface to ensure a solar flux density that leads to tube temperatures above the solar salt crystallization point. Since the absorber tubes are empty during the preheating, there is a risk of high temperature gradients and transients and thus high thermal stresses that may lead to fatigue damage. To prevent this, a preheating strategy for a molten salt test receiver was developed and tested, taking into account ambient conditions and time. The present work shows the results of the commissioning of the developed preheating strategy and discusses its potential for improvement.

Atmospheric air freshwater using TPMS compact heat exchangers

The technology of extracting freshwater from atmospheric air is currently accessible in tropical regions and is experiencing substantial growth in areas where freshwater scarcity is prevalent. Moreover, the water-cooling towers in power plants and district cooling chillers produce saturated air, which serves as a significant supply of humid air and consequently freshwater. Dehumidification is the necessary procedure for extracting water vapor from moist and saturated air in order to generate water. An efficient heat exchanger with a high ratio of surface area to volume is necessary to enhance the production of freshwater by condensing the water vapor present in the air. Triply periodic minimum surface (TPMS) structures possess intricate geometries, resulting in a high ratio of surface area to volume and intense turbulence. Consequently, these structures have exceptional heat transmission capabilities. The internal network structure of the TPMS enables the formation of a consistent layer of freshwater by direct interaction with humid airflow, resulting in a continuous flow. The objective of this study was to examine the structural architectures of TMPS that maximize the generation of freshwater from moist air under various humidity and flow conditions. This was achieved by employing accurate 3D computational fluid dynamics (CFD) models. The purpose of developing these models is to analyze the performance of the Gyroid-Solid TPMS structure and compare it to a vertical flat plate with the same surface area. Furthermore, the study examines the freshwater production efficiency of various TPMS designs (Gyroid-Solid, Gyroid-Sheet, and Diamond-Solid) with same porosity but varying levels of humidity in the air. The findings indicate that the Gyroid-Solid structure enhances the condensation flow rate by a threefold factor in comparison to the flat plate across different levels of humidity in the air. Furthermore, the findings indicated that Diamond-Solid exhibited superior performance when subjected to low airflow Reynolds numbers, whereas at high Reynolds numbers The Gyroid-Sheet exhibits the highest level of condensation among the three solid structures, surpassing the Diamond-Solid by 25% and the Gyroid-Solid by 65 %. The findings demonstrated a notable increase in the production of freshwater through the utilization of TPMS structures, thereby deepening our understanding of the crucial influence of geometry on the condensation process. Empirical formulae have been formulated to calculate the Nusselt number of atmospheric air for TPMS heat exchangers, taking into account various design and operational factors.

First Self-Aligned Heliostat Prototype at the Plataforma Solar de Almeria

It is a generally admitted fact that the optical aligning process of faceted heliostats in a Solar Power Tower Plant (SPTP), also called heliostat canting, introduces an assembly time penalization as well as a potential error in heliostat optics conformation [1, 2]. This double penalization influences both the solar field setup time and the optical quality of the SPTP. Aware of this fact, in 2014 the Plataforma Solar de Almeria (PSA) developed and patented a method to self-align facets and thus avoid the critical canting phase of a heliostat [3]. To experimentally demonstrate the feasibility of the self-aligning facet process, a small reflector composed of six facets and 9m2 of reflecting surface was designed and fabricated. The success of that experience has led to the development of a first 18.5m2 prototype of a self-aligned heliostat, which has been installed at the CESA-1 Heliostat Field (PSA) in 2022.

Estimation of solar receiver corrosion conditions during operation to aid in the design of receiver-corrosion lab tests

Solar power tower plants widely rely on molten nitrite salt as heat transfer fluid and storage media. However, its corrosiveness endangers several metallic components, with central receivers being one of the most compromised. Great efforts are being made to establish molten salt corrosion in different metals. Recently, the importance of the amount of salt used in these experiments has been highlighted. Yet, most tests disregard the hot-receiver-surface/salt ratio that would represent the receiver operation conditions. Commonly, larger metal surfaces are employed, accelerating molten salt thermal decomposition kinetics. Hence, this study seeks to develop basic design conditions for experimental corrosion tests to realistically mimic receiver-tube corrosion. Consequently, the high-temperature operation of two receiver designs, resembling those of Crescent Dunes and Gemasolar plants, with their tubes manufactured in HA230, is analysed. The yearly salt degradation in these plants is found to be nearly negligible due to the large salt volume stored and the relatively limited receiver surface above concerning temperatures, with the whole salt volume being exposed to it for around 20 min during a 25-year operation for large receivers. Moreover, the hot-metal-surface/solar-salt ratio ranges from 5 to 7 mm2/kg, depending on the receiver design, which should be preserved in a corrosion experiment. Metal loss is also estimated: less than 7 % of the receiver would lose at least 50 % of the initial tube thickness. Still, lower metal loss rates are expected since coefficients from static-isothermal tests were employed for the calculation, which also disregarded the metal surface/salt proportion, leading to an overestimation.

Real-Time Optimization of Heliostat Field Aiming Strategy via an Improved Swarm Intelligence Algorithm

Optimizing the heliostat field aiming strategy is crucial for maximizing thermal power production in solar power tower (SPT) plants while adhering to operational constraints. Although existing approaches can yield highly optimal solutions, their considerable computational cost makes them unsuitable for real-time optimization in large-scale scenes. This study introduces an efficient, intelligent, real-time optimization method based on a meta-heuristic algorithm to effectively and reliably manage SPT plant operations under varying solar conditions, such as cloud shadowing variations. To minimize redundant calculations, the real-time optimization problem is framed in a way that captures the operational continuity of the heliostat, which can be utilized to streamline the solution process. The proposed method is tested in a simulation environment that includes a heliostat field, cylindrical receiver, and cloud movement model. The results demonstrate that the algorithm presented in this paper offers higher intercept efficiency, improved robustness, and reduced optimization time in more complex scenes.

Evaluation of the Levelized Cost of Energy With New Costs for Concentrating Solar Power Tower Plants in Northern Chile and Impact of Green Taxes

The world is undergoing an energy transformation, from a system based on fossil fuels to a system based on renewable energy, in order to reach the Paris Agreement target. Chile is no exception, and through the Asociación de Concentración Solar de Potencia (ACSP) has generated a new cost structure for tower-based Concentrated Solar Power (CSP) technology in the framework of the Long-Term Energy Plan, which updates the costs reported in previous studies. The changes experienced in this cost structure make it necessary to study the impact of this cost structure on the levelized cost of energy (LCOE). This work develops simulations of CSP tower plants analyzing their generation and the LCOE based on the new cost structure mentioned, for the north of Chile between the Arica and Parinacota region and the Coquimbo region. The most relevant results show a significant reduction in the LCOE, compared to studies from 2020, reaching a minimum LCOE value of 52,6 USD/MWh. On the other hand, the impact on the net social benefit of including the green tax to the merit order of each of the generating plants that make up the electric park until 2021 is studied, which yields a negative impact under current legislation.

Sample-Efficient Hyperparameter Optimization of an Aim Point Controller for Solar Tower Power Plants by Bayesian Optimization

This work introduces a sample-efficient algorithm to optimize the control parameters of an aim point controller for solar power tower plants. Optimizing the control parameters increases the performance of the aim point controller, and thus the efficiency of the plant. However, optimizing the parameters in simulation will not yield the true optimal parameters at the real plant due to mismatches between simulation and reality. Thus, optimization must be done at the real tower to find a true optimum. As this can be time consuming and costly, the optimizer should require a minimum number of steps. Hence, a sample-efficient optimization strategy is needed. This work introduces a new algorithm based on Bayesian Optimization (BO), which leverages multiple sets of simulation data to accelerate the optimization. The algorithm is tested on a six-dimensional test function representing an arbitrary aim point controller. The proposed algorithm outperformed standard Bayesian Optimization by reaching near optimal parameter configurations of 95% accuracy within 33% less optimization steps. In a second test, the proposed algorithm is used to optimize a simulated Vant-Hull aim point controller with two hyperparameters. Here, the algorithm also needs 33% less optimization iterations than the standard BO.

Thermo-economic design of an electric heater to store renewable curtailment in solar power tower plants

Thermo-economic design of an electric heater to store renewable curtailment in solar power tower plants

Research on Modeling Simulation and Optimal Layout of Heliostat Field Optical Efficiency for Solar Power Tower Plant

Research on Modeling Simulation and Optimal Layout of Heliostat Field Optical Efficiency for Solar Power Tower Plant

Analytical investigation of wind loads on heliostats

Applications of Heliostats for solar power tower plants increased in the recent years. Most of the solar power tower plants installed are on flat terrain and they may be subjected to wind loads. In the present study, a refined method for calculating the wind loads on heliostats is presented, here called the Sandia 92-method. The first step in the Sandia-92 method is to adjust the required wind speed value to the wind speed at the center of the mirror array at a height H above ground. Hence, the performance of heliostats with different pedestal heights can be compared. The design of heliostat must be calculated in-field and the calculation of isolated heliostat wind loads should not be taken as a reference. Also, the presence of well studied fences reduces considerably wind loads on heliostats placed in the front rows.