Tower Power Plant Research Articles

Thermo-economic assessment and systematic comparison of combined supercritical CO2 and organic Rankine cycle (SCO2-ORC) systems for solar power tower plants

Solar power towers (SPTs) integrated with thermal energy storage are promising solutions for solar energy utilisation. Supercritical CO2 power cycles are acknowledged as an attractive option for dry-cooling SPT plants. However, abundant low-grade waste heat exists in the SCO2 gas cooler, which is directly dissipated to the ambient. In order to further improve the performance of SCO2-based SPT plants, organic Rankine cycle (ORC) systems can be introduced as a bottoming cycle subsystem. In this paper, an ORC subsystem is added to four different SCO2 cycle layouts, i.e., recuperated (RE), recompression (RC), intercooling (IC), and partial cooling (PC) cycles, to form combined cycle systems for SPT applications. A parametric study reveals that the thermal efficiency of the power block (i.e., the whole power cycle system), and the salt temperature difference across the receiver are the determining factors of the thermo-economic performance of SPT-SCO2-ORC plants. Design optimisation and annual performance evaluations are implemented using actual weather data in Delingha, China. Compared to the SPT plant with a standalone recuperated SCO2 cycle system, the annual electricity generation is increased by 19 % by integrating a bottoming ORC subsystem. The SPT-RC-ORC system produces the maximum electricity generation of 123 GW·h/year, while the SPT-RE-ORC system achieves the lowest levelised cost of electricity (LCOE) of 0.12 $/kW·h and the minimum payback time of 8.8 years. A wide range of solar irradiance conditions is further considered to generalise the performance evaluation of such combined cycle systems, with results showing that with the highest solar irradiance investigated (3700 sunshine hours, 1000 W/m2·h), the LCOE of the SPT-RE-ORC plant can be as low as 0.07 $/kW·h and the payback time is as short as 6 years. This study investigates optimal SCO2-ORC configurations for SPT applications based on annual performance evaluations, and presents preliminary assessments of the thermo-economic potential of ushc SPT-SCO2-ORC plants across a variety of solar resource conditions.

Solar power tower plants with Bimetallic receiver tubes: A thermomechanical study of two- and three-layer composite tubes configurations

The present study numerically investigated the use of bimetallic tubes for concentrating solar energy applications. Specifically, a billboard receiver employing supercritical carbon dioxide (sCO2) as the heat transfer fluid is considered, with tubes made of stainless steel 316 and GRCop-84. Two- and three-layer tube configurations are compared, exploring the impact of more thermally conductive layer thickness and placement on temperature and stress fields. The findings demonstrate that the use of bimetallic tubes can effectively reduce temperature and stress in the receiver tubes. In light of the results, it can be concluded that the higher thermal conductivity of GRCop-84 leads to a more uniform temperature distribution, resulting in lower temperature peaks on the outer tube surface, and reduced maximum stresses. Furthermore, it has been found that the incident heat flux necessary to achieve the same temperature increment of sCO2 inside the panel is 1.7% lower in a two-layer tube configuration where GRCop-84 is placed in the outer layer. Besides, the stress in the 316 layer can be reduced up to 53.2% with the cited configuration. Nevertheless, it is observed that it is more beneficial to tube performance to place the more conductive layer inside, since it reduces stress in the GRCop-84 layer, and its compressive stress and corrosion-resistant properties help to avoid the risk of stress-corrosion-cracking. In a three-layer composite tube configuration, placing the more thermally conductive GRCop-84 layer close to the outer tube wall decreases the maximum temperature while increasing stress. The opposite effect is achieved by placing the more conductive layer closer to the inner tube wall. Overall, the results demonstrate the potential benefits of using bimetallic tubes for solar energy applications, when the layers have similar thicknesses, since their use can enhance the thermomechanical performance of conventional tubes made with one layer of 316 stainless steel. This fact has important implications for the design of efficient and reliable solar thermal systems.

Validation of a closed-loop aim point management system at the Jülich solar tower

Due to modeling errors and disturbances caused by clouds, Solar Power Tower (SPT) plants consider high safety margins during operation leading to a significant loss of revenues. This conservatism contradicts the expansion of SPT plants despite their potential. In this work, an aim point management system is deployed at the open-volumetric receiver of the Jülich solar tower. It employs All Sky Imager (ASI)-based nowcasts as well as flux density and temperature measurements. While the temperature measurements provide feedback for an outer receiver control loop, the flux density measurements offer feedback for an inner heliostat field control loop of a cascade control. The receiver controller estimates flux density setpoints for the inner control loop. By comparing these flux density setpoints with measured flux densities, a static optimal control technique compensates for modeling errors and increases the intercept. This control requires a fast aim point optimization, the enhanced ant-colony optimization meta-heuristic. Furthermore, a feed forward controller compensates for disturbances due to clouds by scaling the heliostats’ reflected power used in the system model by DNI nowcasts. At the Jülich solar tower, the static optimal control compensates for changes in the allowable flux density in less than three control steps. The cascade control controls the subreceivers in a narrow temperature range of 15K, which has never been achieved before at this tower.

Thermomechanical behavior of mechanical attachments in solar power tower receivers under preheating conditions: A numerical study

Thermal deflection of receiver tubes in solar power tower plants is typically reduced by means of mechanical attachments, each one consisting of a clip and a guide through which a rod connected to a reradiating wall can freely slide. As a side effect, these mechanical attachments act as heat sinks, promoting a local temperature reduction in the tube during the receiver preheating, thus increasing the risk of crystallization of the molten salt during the subsequent filling of the receiver. Additionally, the mechanical attachments and the tube are subjected to thermal stresses during the preheating of the system, remarking the importance of considering both thermal and mechanical effects during the design of these elements. This study uses three-dimensional thermomechanical simulations to characterize the behavior of both the mechanical attachment and the tube during the preheating of a solar power tower receiver. The results revealed that the areas of maximum stress occur in the tube, clip, and guide sequentially as the preheating process progresses. The maximum stresses are located in the connections of the clip with both the tube and the guide. An extensive parametric sweep on the dimensions of the mechanical attachment revealed a trade-off between the minimum temperature and the maximum stress of the tube. Based on this analysis, an optimal solution for the dimensions of the clip and guide in terms of their height is proposed. The systematic study here presented can serve as a basis for the design of improved mechanical attachments able to ensure an adequate minimum temperature of the tube during the preheating while keeping the stress of the system within acceptable limits.

Performance analysis of a solar based novel trigeneration system using cascaded vapor absorption-compression refrigeration system

Solar power tower technique has a strong potential among several solar systems for large-scale power generation. It is crucial to make new, efficient trigeneration unit for solar power tower plant. This work makes a novel supercritical Brayton cycle based combined cycle in which helium is considered as the working fluid for power generation. The cascaded vapor absorption-compression refrigeration system is integrated with traditional Brayton cycle for recovering waste heat to produce additional heating and low temperature cooling effects for food storage. The energy, exergy efficiency and power output of the proposed plant were found as 28.82%, 39.53% and 14,865 kW respectively. The COPcooling and COPheating values were observed as 0.5391 and 1.539 respectively at 850 W/m2 of direct normal irradiation, 80 °C of generator temperature (Tc) and -20°C of evaporator temperature (Te). Solar sub system is responsible for high exergy destruction around 78.18% (22,763 kW) of total destruction of the overall plant. Moreover, parametric study reveals that performance of trigeneration system is highly affected by the heliostat and receiver efficiency, Tc, Te and inlet temperature of helium turbine. Also, a comparison with related earlier research has demonstrated that the performance of the current system is superior to that of systems based on the Rankine cycle and supercritical CO2 cycles.

Structural Performance-Based Design Optimisation of a Secondary Mirror for a Concentrated Solar Power (CSP) Plant

Concentrated Solar Power (CSP) plants use mirrors to reflect and concentrate sunlight onto a receiver, to heat a fluid and store thermal energy, at high temperature and energy density, to produce dispatchable heat and/or electricity. The secondary mirror is a critical component in the optical system of certain Solar Power Tower plants (SPT), as it redirects the concentrated sunlight from the primary mirror onto the receiver, which can be arranged at ground level. In this study, we propose a design optimisation for the secondary mirror of a CSP plant. The design optimisation method consists of two steps. The first step involves the use of the finite element simulation software Abaqus 2022 to analyse the structural performance of the secondary mirror under thermal loads and wind. The second step consists of the use of simulation results to identify the combination of design parameters and best performances, with respect to both design constraints and structural safety. This is carried out by developing an algorithm that selects those configurations which satisfy the constraints by using safety coefficients. The proposed optimisation method is applied to the design of a potential configuration of a secondary mirror for the beam-down of the CSP Magaldi STEM® technology, although the methodology can be extended to other components of CSP plants, such as primary mirrors and receivers, to further enhance the structural performance of these systems.

Waste heat recovery potential in the thermochemical copper–chlorine cycle for hydrogen production: Development of an efficient and cost-effective heat exchanger network

With the finite nature of fossil fuel resources and related environmental issues, it is necessary to replace fossil-fuel-based hydrogen production methods with more sustainable ones. For the clean production of hydrogen, the copper-chlorine (Cu–Cl) thermochemical cycle is a favorable choice. However, internal waste heat recovery of the cycle is essential to improve the efficacy of the system. In this paper, the waste heat recovery potential of the four-step Cu–Cl cycle is investigated, to support the design of a new cost-effective and high-performance heat exchanger network. Pinch analysis is executed to find the optimum rates of cold and hot utilities for this cycle. Three heat exchanger network cases are investigated which include case 1, without heat recovery; case 2, with 16 heat exchangers; and case 3, with 11 heat exchangers. The effect of varying design parameters on the energy system's efficiency and cost-related parameters are also explored here. Based on the pinch analysis results, the minimum heating and cooling and the maximum recoverable heat, on a normalized basis, are 333.3 kJ/mol H2, 76.9 kJ/mol H2, and 134.4 kJ/mol H2, respectively. Moreover, for cases 1, 2, and 3 the thermal efficiencies are calculated as 33.3%, 41.1%, and 40.5%, respectively. Additionally, it is observed that, when the thermal energy of the cycle is supplied from a solar power tower plant, the levelized cost of hydrogen for case 1 is 9.6 $/kg H2, for case 2 is 7.9 $/kg H2, and for case 3 is 7.8 $/kg H2.

Co-optimisation of the heliostat field and receiver for concentrated solar power plants

In a concentrated solar power (CSP) tower plant, it is essential to understand the performance of the subsystem formed by the heliostat field and the receiver, operated with an optimal aiming strategy that guarantees the safety and lifetime of the receiver while maximising performance. State-of-the-art studies optimise the heliostat field, aiming strategy and the receiver independently. However, the field and the receiver are interdependent and co-optimisation of the field-receiver subsystem is necessary to obtain the optimal configuration. Fast and accurate annual performance assessments of the subsystem are needed to calculate the annual energy output and the Levelised Cost of Energy (LCOE) of the full CSP system. In this study, a co-optimisation method is proposed based on coupled instantaneous optical, thermal and mechanical models integrated into a system-level model for annual simulation of full system and economics. The resulting system-model is then used for design optimisation based on a genetic algorithm. Several techniques are implemented to make this complex and computationally expensive problem tractable. The proposed method is used to optimise the design of two systems composed of a surround field and a liquid sodium-cooled cylindrical external receiver for first the annual performance and then the LCOE. The good behaviour of the method is confirmed by a sensitivity study. The LCOE-based optimisation leads to a less efficient system than the efficiency-based optimisation but a higher capacity factor. The methods presented are applicable to other CSP plant configurations, including state-of-art molten salt power tower plants.

An operation strategy and off-design performance for supercritical brayton cycle using CO2-propane mixture in a direct-heated solar power tower plant

Adding propane to CO2 can improve the thermal efficiency of the supercritical Brayton cycle under high condensation temperatures. Integrating the thermal energy system in a direct-heated solar power tower plant can maximize the thermal utilization of solar energy. In this paper, the thermodynamic model of the plant is constructed, and an operation strategy is proposed to realize round-the-clock operation by flexible scheduling of solar, molten salt, and fossil fuel energy supply. Notably, the potential of adding propane to CO2 in the plant was analyzed by comparing the off-design performance and the operation behavior under typical days. The results show that the thermal efficiency of the plant using CO2-propane is not only 1.63% higher than the original, but also the adequate utilization time of solar energy will be prolonged by 0.3 h. In addition, the hot molten salt filling rate of the CO2-propane system is more sensitive to changes with direct normal irradiance. However, the performance of the system will deviate from the optimal design value when the heating mode is switched. The achievements from this paper have testified the CO2-propane cycle is better than the initial cycle in the direct-heated solar power tower plant under off-design conditions. The system scheme and operation strategy can be regarded as valuable references for CO2-based mixtures application.

Techno-economic assessment and optimization of the performance of solar power tower plant in Egypt's climate conditions

Egypt is a sunbelt nation with plenty of solar resources and significant wasteland areas, particularly in its southern and western regions. Concentrating solar power technologies are tried-and-true renewable energy systems that use solar radiation to produce electricity in nearby nations. These technologies have the potential to reduce costs in the future. Comprehensive techno-economic research has not been conducted to assess the potential for the solar power tower technique in Egypt; instead, solar photovoltaic and parabolic trough systems are given the majority of the attention. The major goal of this article is to persuade the Egyptian government to pursue solar power tower technology and meet the accompanying difficulties. Using system advisor model software, six distinct locations were used to simulate a 100 MW plant for solar power tower technology. Based on a thorough technical solar power tower site evaluation research, these zones were examined. To achieve the lowest levelized cost of electricity (LCOE), thermal energy storage and solar field size optimization are carried out. The Benban location, with outputs of 521,228.8 MWh and 0.1130 $/kWh, respectively, has the greatest produced energy and the lowest levelized cost of electricity, according to the findings. The ideal design for the solar power tower plant was shown by the results to be a solar multiple of 2.8 with a thermal energy storage of 8 h. The solar power tower plant's lowest levelized cost of electricity might then be reduced, in accordance with the optimum configurations, to 0.1057 $/kWh.

Optimization and performance analysis of a near-zero emission SOFC hybrid system based on a supercritical CO2 cycle using solar energy

Renewable energy represented by solar energy has the characteristics of intermittency and fluctuation. To ensure that the grid can operate safely and stably after the large-scale grid connection of renewable energy power, a new hybrid system consisting of a solid oxide fuel cell (SOFC), a solar power tower plant, and a supercritical CO2 (S-CO2) Brayton cycle is proposed in this study from the perspective of energy, exergy, and economy. By utilizing the cold source loss of the S-CO2 cycle to provide part of the energy required for the alkaline electrolyzer, it can efficiently produce and store hydrogen, achieving zero greenhouse gas emissions while increasing efficiency. Meanwhile, the high-temperature exhaust gas from SOFC is recovered by the S-CO2 cycle. The system considers two operation modes, switching two types of S-CO2 cycle configurations to efficiently utilize solar energy according to the variation of solar radiation energy. The thermal efficiency of the proposed system is improved by 16.61% and 6.88% compared with the other two typical S-CO2 cycle power generation systems, and the net power output reaches 8.0 MW. A parameter study shows that increasing the split ratio is beneficial to improve the thermodynamic performance of the system. However, at a split ratio less than 0.76, it is beneficial to the total cost rate of the system. The results of multi-objective optimization show that the exergy efficiency of the combined system and the total system cost rate reach 56.86%, 513.10$/h and 56.45%, 481.59$/h for the system with an additional simple S-CO2 cycle and the system with an additional recompression S-CO2 cycle, respectively, and the optimum point is selected by the TOPSIS and LINMAP methods.

Dynamic Thermal Transport Characteristics of a Real-Time Simulation Model for a 50 MW Solar Power Tower Plant

A dynamic simulation model of a heliostat field and molten salt receiver system are developed on the STAR-90 simulation platform. In addition, a real-time simulation model coupling the above two models is built to study the photothermal conversion process of Delingha’s 50 MW solar power tower plant. The nonuniform and time-varying characteristics of the energy flux density on the receiver surface and the dynamic characteristics under different operating conditions are studied. The operational process of the receiver of a typical day is simulated. It was found that there was a strong positive correlation between the energy flux and DNI, and the maximum energy flux density on the surface of the heat absorbing tube panel moved from the first tube panel to the fourth in sequence from 12:00 to 18:00. At the same time, the energy flux density of the last four panels decreased gradually along the arrangement order of the panels. DNI, molten salt mass flow rate and inlet temperature step disturbance simulations are carried out, and the response curves of the molten salt outlet temperature and tube wall temperature are obtained. The conclusion of this paper has important guiding significance for the establishment of an operational strategy for photothermal coupling in a molten salt solar power tower plant.

A feedforward-feedback control strategy based on artificial neural network for solar receivers

In a solar power tower plant, the stability of the receiver’s outlet temperature is required for high-efficiency and safe operation. However, the dramatic variation of solar energy caused by clouds is a severe challenge for keeping the temperature steady at the outlet. To solve this problem, a feedforward-feedback control strategy based on an artificial neural network is proposed herein to cope with the fluctuation of solar energy by regulating the receiver’s mass flow rate. The feedforward controller based on artificial neural network can quickly respond to the change of solar energy, while the feedback controller can make the receiver achieve an expected ultimate temperature. The performance of the proposed control strategy is comprehensively evaluated and compared with PID controller under different conditions. The results show that the proposed control strategy can significantly reduce the fluctuation of receiver’s outlet temperature. For the step variation of direct normal irradiance ranging from −15 % to 15 %, the proposed control strategy can confine the temperature deviation within ± 1 °C. For the real dramatical and continuous direct normal irradiance variation, the temperature deviation is limited to 5 °C under the proposed control strategy, while it exceeds 30 °C under the PID controller only. The results provide an alternative efficient strategy for the receiver’s mass flow control to keep a steady outlet temperature under the fluctuation of the solar resource.

Influence of eccentricity on the thermomechanical performance of a bayonet tube of a central solar receiver

This work numerically evaluates the thermal and mechanical behaviors of eccentric bayonet tubes to be used in external central receivers of solar power tower plants. A bayonet tube is composed of two tubes, one inside the other, creating circular and annular sections, through which the molten salt of the receiver sequentially flows. Eccentricity in the annular section is achieved by displacing the axis of the interior tube with regard to the exterior one. For comparative purposes, two examples of conventional tubes (single tubes with circular cross-sections with diameters of 25mm and 50mm) are also investigated in this work to compare their performances with those of bayonet tubes. The results obtained with the eccentric configurations show an enhancement of the heat transfer to the molten salt and a reduction of the tube wall overheating compared with the concentric bayonet tubes and the largest simple tube. For conditions representative of the normal operation of a solar power tower, eccentric bayonet tubes could reduce the pressure drop by 30.8% and increase the convective heat transfer achieved in a concentric configuration of the bayonet tube by 26.1%. Nevertheless, this pressure drop was considerably higher than those obtained in the smallest and largest simple tubes, which were 1.28 bar and 0.13 bar, respectively. To investigate whether the enhancement of the convection heat transfer experienced by bayonet tubes compensates for their higher pressure drop or not, a Performance Evaluation Criterion (PEC) was proposed and used to compare the global performance of bayonet tubes with that of conventional tubes. The bayonet tubes with eccentricity 0.45 obtained the largest PEC, which was up to 13% higher than reference conventional tubes. Enhancement of the tube wall refrigeration produced when increasing the eccentricity is reflected in the maximum tube temperature and thermal stresses, which are found to diminish by approximately 8.8% with the highest eccentricity. In addition, the largest eccentric bayonet tube layout obtains the smallest peak temperatures compared to conventional tubes. The lower inertial moment of the smallest conventional tube indicates that its thermal stress is 2.1% lower than the stress obtained in the most eccentric layout analyzed in this work. Nevertheless, the time to rupture associated with creep damage of the eccentric bayonet tube is 1.04 times higher than that obtained in the smallest simple tube, demonstrating that bayonet tubes could be a potential alternative to the current tubes of external tubular receivers.

THERMAL EVALUATION OF A SOLAR POWER TOWER EXTERNAL RECEIVER WITH LIQUID METAL AS HEAT TRANSFER FLUID IN NORTHERN CHILE

Molten salts have been the main heat transfer fluids used in solar power tower plants in recent times, but their limited operating temperatures limit the efficiency of current solar plants and their competitiveness. One option to improve the efficiency is to use liquid metals instead of molten salts, which in addition to withstanding temperatures even above 1000°C, have excellent thermophysical properties. This paper shows the comparative thermodynamics evaluation of a solar power tower external receiver using different liquid metals and molten salts as heat transfer fluid (HTF), located in northern Chile, a location with a high solar potential and where an important plant of this type recently started operation and others are projected. Tonatiuh and Engineering Equation Solver software were used to develop the work and implement the model. The results show that the operation with liquid metals produces lower energy efficiency than molten salts due to the higher operating temperatures, which significantly increase thermal losses, mainly radiative ones. However, the better thermophysical properties of some liquid metals, especially sodium, make the efficiency reduction not so relevant (only 2 percentage points in this analysis). It would be expected that this reduction in efficiency could be compensated by the increase in the thermal efficiency of the thermomechanical cycle in the case of electricity production due to the fact of operating at higher temperatures, but this work shows that the exergy efficiency of the process with liquid sodium is only 2 percentage points higher than with solar salt. Therefore, the reduction of losses, especially radiative losses, which can be as much as 400% greater than that ocurring with solar salt, becomes a necessity if we want to take advantage of the potential represented by being able to operate solar systems at temperatures higher than the current ones, with liquid metals or any other material.

Thermodynamic assessment of the dry-cooling supercritical Brayton cycle in a direct-heated solar power tower plant enabled by CO2-propane mixture

This paper proposes an improved recompression combined cycle configuration to explore the potential of using CO2-propane in a direct-heating solar power tower (SPT) plant. The effects of critical parameters on overall performance were detailly analyzed. The exergy distribution of CO2 and CO2-propane cycles was compared respectively after being optimized by a genetic algorithm. The results show that the exergy loss of regenerators was reduced, and the entire exergy efficiency of the SPT plant was improved by configuring two high-temperature regenerators. The cycle performance of propane, serving as the second additive, becomes more significant when the cooling temperature is increased. The overall thermal efficiency of the CO2-propane-configured SPT plant is 2.08% higher than using CO2 alone at a condensation temperature of 50 °C. Meanwhile, the exergy efficiency of the SPT plant with CO2-propane cycle decreases less (1.67%) than CO2 (2.02%) when the condensation temperature rises from 42 °C to 50 °C. However, the temperature difference of the receiver would decrease when using CO2-propane. This study provides a valuable reference for the application of CO2-propane mixture in the dry-cooling Brayton cycle of the direct-heated SPT plant.

Solar simulator based on induction heating to characterize experimentally tubular solar central receivers

Solar receiver tubes in solar power tower plants are subjected to non-homogeneous heat flux and temperature that produce early failure derived from the thermal stress. To reduce these problems, the thermomechanical characterization of the receiver is required. However, there are limitations in the data acquisition during operation due to the extreme working conditions, the different scale lengths, and the disturbance of the normal operation, being not possible to determine experimentally the thermomechanical behaviour of this device or to validate specific models developed.In this work an experimental facility able to reproduce the operation conditions of solar receivers has been designed and tested. To obtain the non-homogenous heat flux an induction heating system has been used. The facility has also been equipping of sensors and cameras that allow to measure the temperature distribution, the displacement and the strain field in a tube stretch. It is obtained a circumferential variation of the temperature of 160 °C, and a tube displacement due to the bending produced by the temperature gradient in combination to the mechanical boundary conditions of 2.251 mm. The data obtained with the cameras have been checked with the displacement and temperature sensors, obtaining maximum differences of 3 %, it means 5 °C and 0.05 mm. Thus, it is possible to derivate the tube displacement to obtain the hoop and axial strain field. These results are the first step for the thermomechanical characterization of tubes with non-homogeneous temperature profiles in the three directions. The proposed solar simulator will allow the validation and development of thermomechanical models able to reproduce the operation conditions of the receiver with the final goal of improving the receiver design and operation.

A review study on solar tower using different heat transfer fluid

The object of research is distinguishing the different heat transfer fluids (HTF) in concentrating solar power (CSP). CSP technologies are gaining more attention these years due to the fact that the world is facing significant problems, especially concerning environmental issues and the increasing electricity demand. The world countries are currently committed to mitigating climate change and limiting greenhouse gas emissions to keep the global temperature rising below 2 °C. As a result, renewable energy sources are required for power generation. One of the most widely used technologies is the solar tower, where mirrors reflect solar radiation into a central receiver on top of a tower that contains a working fluid known as heat transfer fluid. The HTF is one of the most important components in solar power tower plants used to transfer and store thermal energy to generate electricity. This study focuses on the HTF used in solar power towers and how it can affect the efficiency of the plant. The HTF discussed in this study are air, water/steam, molten salts, liquid sodium, and supercritical CO2. Among the review of HTFs in the solar tower system, the result of the research shows that the Air can reach the highest temperature while liquid sodium achieves the highest overall plant efficiency.

World map of low-layer atmospheric extinction values for solar power tower plants projects

Atmospheric extinction is defined as the ability of the atmosphere to attenuate the solar radiation beam. In the case of concentrating solar power tower plants, low-layer atmospheric extinction plays an important role in the assessment of the solar resource by attenuating the radiation on its way from the heliostats to the receivers. The lack of values of the low-layer atmospheric extinction could lead to incorrect decisions in solar projects and investments. In this paper, we present a world map of annual low-layer atmospheric extinction values. The world map has been elaborated by processing the 30-year database provided by MERRA-2 and the use of atmospheric radiative transfer codes. The results show that the atmospheric extinction values are not negligible and reach high values in many areas of the sun belt. Global daily averages of atmospheric extinction are between approximately 3% and 30%, implying the similar values of annual energy losses in solar power tower plants. The southern hemisphere is characterised by low extinction values, while hight attenuation values are found in the northern hemisphere, especially in West and Central Africa, the Arabian Peninsula, Turkmenistan, India, and China, where annual atmospheric extinction exceeds 20%, reaching values of 30% in some places. These results have a major impact on the economic evaluation of solar tower projects. For atmospheric extinction values above 20%, the impact on the Levelized Cost Of Energy implies upward corrections of more than 25%, approaching 50% for values such as those obtained in Chat or Niger. Geographical information of low-layer atmospheric extinction values will help to reduce uncertainty margins in studies related to tower technology solar plants.

Optical characterization of heliostat facets based on Computational Optimization

In solar power tower plants, knowing the optical quality of heliostats makes it possible to predict relevant information on the receiver surface, such as the irradiance concentration factor and spillage. However, there are no standardized routines for optical characterization in commercial facilities because the process is challenging, multidisciplinary, and time-demanding. This article revises the traditional optical characterization methodology followed at the Solar Platform of Almería (PSA). The process starts with the acquisition of the image of the studied optical system. After that, the picture must be fitted to an analytical model, which requires finding the variables that best reproduce the reality. The traditional method for accomplishing this task is iterative, semi-automatic, and contains trial-and-error components. This work studies how to replace this part with heuristic optimizers and considers using the state-of-the-art methods TLBO, UEGO, and Multi-Start Interior-Point (MSIP). Their effectiveness has been compared to the results manually achieved by an expert with three different heliostat facets. According to the results obtained, the parameter sets found by TLBO and UEGO outperform those obtained through the traditional method.