Plataforma Solar De Research Articles

Developing Deformable Facets for Heliostats: An Increased Solar Concentration for Thermochemical Applications with Central Tower Receivers

In the present article a new design of a deformable facet and a heliostat with several deformable facets is presented. Previous work has been made around designing heliostats with concentrating facets in the facilities of the Plataforma Solar de Hermosillo. The evaluation and design process of facets and their evolution is also shown and reviewed. The facet design was initially conceived as a parabolic trough surface, however, through a modification of the facet, the surface was transformed into a concave surface. The resulting spot produced by the facet assumes nearly a spherical surface. A heliostat constructed with these facets shows similar optical performance with a previous one made with spherical facets; this spherical heliostat has a higher peak and mean flux. Nevertheless, the deformable facet heliostat presents a simpler construction and structure. A comparison between both heliostats will be presented.

Numerical analysis of the heat transfer performance of the absorber tube of a parabolic trough solar collector using the swirling flow technique

In the present work, the absorber tube of the DISS (Direct solar steam) solar plant in Plataforma Solar de Almeria (Spain) is powered by fluid flows in a swirl motion. Numerical simulations are used to predict the performance of a parabolic trough solar collector with inlets that allow the working fluid to flow in a swirl motion inside the absorber tube. There are numerous sites for tangential inlets distributed along the absorber tube. There is one inlet placed up, one located down, two located up, and two located down. High Reynolds numbers between 2.2 × 105 and 6.2 × 105 are used for the simulation. When the collector with the tangential inlet is compared to a conventional collector, the results reveal that using one inlet down causes a 9.1K decrease in the average temperature of the lower surface of the absorber tube. Compared to axial flow, one inlet down increases useful heat and heat transfer by 12.7 % and 41.7 %, respectively. The average friction factor of axial flow is 16.4 % higher than that of one inlet down. The collector with a tangential inlet reduces surface area by 10.9 % over Reynolds ranges when compared to an axial flow collector. Therefore, the thermal strain on the absorber tube and collector surface area is reduced by the swirling flow in the parabolic trough solar collector. As a helpful design tool, thermal performance correlations of absorber tubes with tangential inlets are constructed.

Enhancing solar furnace thermal stress testing using an adaptive model and nonlinear predictive control

Solar furnaces rely on renewable energy for thermal tests using concentrated solar direct irradiance. To address the complex dynamics and constraints of these systems, a novel control approach is introduced based on an adaptive model strategy. The proposed adaptive model representation with an Adaptive Practical Nonlinear Predictive Controller (A-PNMPC) strategy is employed in the solar furnace SF60 at Plataforma Solar de Almería (Spain), demonstrating enhanced model prediction that culminates in a finer control performance. A Recursive Least Squares solution for model adaptation adjusts the model dynamics to the actual behavior of the solar furnace. Then, the predictive control strategy uses the updated model to calculate optimal control actions that lead the sample temperature to the desired reference. Simulation studies demonstrated the superior performance of the A-PNMPC in temperature control and prediction compared to the constant parameter model PNMPC strategy. The proposed A-PNMPC was compared to the conventional PNMPC in simulations, showing up to a 5% improvement in control performance and an 80% in large model predictions. Additionally, the A-PNMPC was tested on the actual system for various temperature profiles, achieving a mean error of less than 10 °C in sample temperature ramp tracking

Typical solar extinction year at Plataforma Solar de Almería (Spain). Application to thermoelectric solar tower plants

The atmospheric extinction of solar radiation reflected by heliostats are recognized as an important factor of radiative losses in Concentrating Solar Power technologies in general and especially in thermoelectric solar tower plants. These types of plants are getting larger (≥100 MWe), and consequently the distances between the heliostats and the receiver very often exceed 1 km and radiative losses due to solar extinction on this path can represent a high percentage. The aerosols and water vapor along this route scatter and absorb solar radiation, preventing a percentage of it from reaching the solar receiver. For this reason, in the process of choosing a location for the design and construction of these plants, the radiative losses due to extinction in that place should be known in advance. Until now, Typical Meteorological Years have been available for the location chosen in the plant design stage, mainly considering Direct Normal Irradiance but not solar extinction. Unfortunately, ground-based measurement of solar extinction has never been properly considered because it was not known how to measure or estimate it adequately. The Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT, Spain) has developed a reliable solar extinction measurement system which has been recording accurate horizontal extinction values at Plataforma Solar de Almería (PSA) since 2017. Based on this unique in the world solar extinction database of more than five years, a Typical Solar Extinction Year has been obtained for the first time to rigorously validate extinction models that then allow knowing the extinction in any area of interest in the world for solar tower power plants and to choose the most convenient one. It has been found that the annual average extinction at PSA for the measurement distance (742 m) is 6 %, with a standard deviation of 2 % and a median of 6 %. This average annual extinction value corresponds to a horizontal extinction coefficient of (0.083 ± 0.029) km−1 and a Visual Range of 47 km (-12 km, +23 km). Frequent haze events have been observed at PSA mainly caused by Saharan dust, which can be considered one more symptom of the current increased desertification and climate change.

Device for Measuring Forces and Torques in Flexible Connection Joints for Parabolic Trough Collector

Measurement of forces and torques in flexible connection joints for parabolic trough collectors (PTC) during their life time is important to prevent damage or breakage of these elements. Plataforma Solar de Almería (PSA) has developed and patented a device for measuring these forces and torques in flexible connection joints installed in solar power plants. The device consists of three dynamometers and one torque transducer coupled to a rotation assembly that can be externally coupled to a ball joint installed in interconnections of parabolic-troughs, with no interference in the hydraulic circuit. To calibrate the device and perform initial performance checks at lab scale, a test bench was prepared for applying forces and torques on a set-up composed of a multicomponent sensor connected in series to a standard rotary ball joint and measuring simultaneously the forces and torques by mean of the designed device. Calibration tests have been performed applying a range of forces and torques to the multicomponent sensor-ball joint at different rotation positions. Multicomponent sensor and developed device measurements have been compared to obtain a calibration matrix for the last one. Results show good performance of the device once calibrated, but that will be improved measuring angular deviations from reference axes position of each of the dynamometers that compose the device during the operation, to increase the accuracy of the measurements.

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.

Thermo-Mechanical Cycling of the ROTAJOINT Flexible Interconnection Installed in a Parabolic Trough Test Facility

Rotation and expansion performing assemblies (REPA) are one of the key components of parabolic-trough collectors (PTC) solar fields. This element is a flexible connector that connects the moving absorber tubes between adjacent PTCs or the absorber tubes to the fixed pipes of the solar field. A hybrid REPA model consisting of a flexible hose and a swivel joint, manufactured by the Spanish company ROTARM, has been installed in one of the PTC of a pilot plant at the Plataforma Solar de Almería (Spain) and tested to evaluate its life-time performance. Due to the lack of standardized procedure for testing this type of components for use in PTC solar fields, a test procedure has been defined consisting of a continuous thermo-mechanical cycling of the component to evaluate its performance under real conditions and to check any possible wear or failure. The total number of accumulated cycles has been 4000, which means that the tested REPA can withstand at least 10 years of trouble-free operation, if the periodic revisions and maintenance actions are carried out according to the manufacturer’s specifications.

Design and Characterization of the New FAHEX100 Concentrator of PSA's SF60 Solar Furnace

The facets of the old Mc Donnell Douglas concentrator of Plataforma Solar de Almería –PSA- SF60 solar furnace were degraded after thirty years of service, which had a great influence on its thermal response and efficiency, as the reflectivity decreased significantly due to the reflective surface being corroded and scratched. That is why, it was decided to replace them by new facets with higher reflectivity and better optical conditions.
 This paper describes the new facets, as well as the new support structure and the assembly and alignment of the concentrator facets. Finally, the technical characteristics of the new concentrator, and the power, size and flux distribution on the focus, are presented.
 For the new facets we selected a thin, hexagonal-shaped mirror bonded to an aluminum plate.
 Therefore, new facets with improved optical conditions were designed and manufactured and, for the assembly a new tubular structure was used to conform the new FAHEX100 concentrator
 To anticipate the behavior of the new FAHEX100 concentrator, a model was built in Tonatiuh, for which a geometric analysis of the facets was previously performed by means of photogrammetry.
 Once the facets were mounted on the concentrator, they were aligned by the Reverse Ilumination Method -RIM–, and the flux distribution on the focus was measured with a CCD camera.
 The performance of the SF60 solar furnace was improved significantly with the new FAHEX100 concentrator, including power, peak concentration, flux distribution on the focus, focus diameter and optical error σopt.

Simulations in Development of Nonlinear Control for a Solar Thermal Power Plant

This paper focuses on the utilization of dynamic simulation models in the planning of experiments for control development. A set of models based on the first principles for system level simulation of the complete new TCP-100 research facility at Plataforma Solar de Almería (CIEMAT) was planned for the development of control solutions for this new research facility which replaced the 32-year-old ACUREX facility. Many advances in Automatic Control have been reached by using the ACUREX field. Simulation experiments with the parabolic trough (PTC) field would require more tuning and adaptive parts before getting the required experimental data for typical operating conditions. The analysis operates for all state variables, which are temperatures, and input variables, which include solar radiation, ambient temperature and several setpoints. The nonlinear scaling approach keeps the algorithms unchanged by focusing on the meanings of the measured variables. The scaling functions are variable specifIc. For the irradiation, the functions do not change which means that also the indicator of the cloudiness remains the same. The algorithms are not changed and the data analysis is for a limited set of measurements and subsystems. The simulation experiments need to be first focused on the loops and modules of the solar field and the full model need to be extended before going to the full simulation tests and the test campaigns with the new facility.

EU-SOLARIS: The European Research Infrastructure Consortium for CSP Technologies

After many years of fruitful collaboration in the field of research infrastructures (RIs) for concentrated solar power/solar thermal energy applications, several European countries decided to apply to the European Commission to go a step forward and create a European Research Infrastructure Consortium (ERIC), a new legal form of association which poses a number of advantages in the endeavour for a further development of this technology in Europe at all levels.
 This new consortium, EU-SOLARIS ERIC, is likely to be legally implemented in October 2022, after a long negotiation and evaluation process, where the support of the respective Governmental research agencies of the countries involved has been crucial.
 EU-SOLARIS ERIC will be a legal entity and its main goal will be the improvement of the interoperability, accessibility and quality of services provided by the existing European research infrastructures to the CSP/STE research communities and industrial stakeholders.
 Although the current Members of the EU-SOLARIS consortium are Germany, France, Cyprus and Spain (Statutory Seat at Plataforma Solar de Almería, Spain), plus Portugal as an Observer, it is open to any other country willing to participate as Member or Observer.

Development, verification and experimental validation of a 3D numerical model for tubular solar receivers equipped with Raschig Ring porous inserts

This study presents a numerical investigation of gaseous tubular absorbers used in a solar furnace and introduces a 3D model to simulate porous inserts with random packing of metallic Raschig Rings (RR) at pore-scale. A comprehensive verification and validation process was conducted based on the experimental data obtained at Plataforma Solar de Almeria (SF60). The evaluations were carried out on three samples, including a smooth pipe (SP) and two enhanced pipes (20RR and 40RR) with 20 mm and 40 mm RR inserts. The effects of several influential parameters, such as the fluid turbulence model, the effective thermal conductivity of the porous material, and the randomness of the RR in the porous structure, were studied to assess the accuracy and repeatability of the model in predicting thermal and fluidic characteristics of air inside the absorber pipes under different working conditions. A detailed thermo-hydraulic analysis revealed an irregular flow pattern inside the porous zone, while the air recirculation at the porous entrance and exit provides an uneven pressure drop distribution in the azimuthal direction. Moreover, fluid turbulence is enhanced downstream of the porous insert thanks to several stream jets formed by uneven flow discharge at the insert outlet face, which improves heat transfer in that area. The analyses demonstrated a significant improvement in heat transfer, with a maximum enhancement factor of 10–15 than the smooth tube. Additionally, in the evaluation of thermo-hydraulic performance, the enhanced absorbers exhibit Performance Evaluation Criteria up to 2, compared to the tube without RR inserts.

Influence of internal design on the performance of pilot vacuum-assisted air-gap membrane distillation modules for brine concentration with solar energy

Membrane distillation (MD) in batch operation is proposed for brine concentration with solar energy in zero liquid discharge (ZLD) desalination processes, but there are some research gaps yet. Energy efficiency is key for implementing the technology with solar energy and there is not enough experience and experimental results of operation of MD pilot modules at high salinity feeds. This research focuses on the experimental evaluation and comparison of two multi-envelope spiral wound commercial membrane distillation modules operating in vacuum-assisted air gap MD at the solar MD pilot facilities of Plataforma Solar de Almería in Spain. These modules have similar membrane area (24 and 25.9 m2) and residence time (large, to maximize thermal efficiency), but different internal design: 6 envelopes 5 m long and 12 envelopes 2.7 m long, respectively. This difference affects mostly the feed circulation velocity for equal feed flow rate. Performance parameters such as permeate production and specific thermal energy consumption (STEC) are compared and experimental models performed and validated for different feed salinities. Permeate quality and specific electrical energy (SEEC) consumption are also evaluated. The results show that the best module for batch operation is the one with a larger number of channels but of shorter length.

Improving Temperature Tracking Control for Solar Collector Fields Based on Reference Feedforward

Improving temperature reference tracking in solar collector fields is essential for enhancing the performance of solar thermal plants. Conventional control strategies are usually employed as static reference feedforwards (FFs) to reduce rise time when reference changes occur. Nevertheless, strict performance requirements may demand precise control behavior that classical reference FFs cannot deliver. Therefore, from theoretical analysis, this work proposes two different control strategies based on reference FF structure to accomplish a low rise time and avoid producing overshoot for temperature reference tracking, namely, a lead–lag and a nonlinear reference FF. The structures are studied for suitable implementation, assuming an adaptive control framework to keep the same control performance under different operating conditions. Simulation experiments are developed to investigate and compare the proposed strategies with the conventional static FF with and without a reference filter. Furthermore, the proposed strategies are successfully tested in an actual solar plant AQUASOL-II, located in the Plataforma Solar de Almería (PSA), Almería, Spain. The promising results have concluded that the lead–lag and nonlinear reference FF associated with the adaptive structure have attended the proposed control requirements of low rise time and no overshoot. Moreover, the results have demonstrated the compromise between the lead–lag and the nonlinear controller, where the final choice should be made as a tradeoff between performance and control effort.

Data-driven online feedback optimization of solar membrane distillation systems operating in batch mode

Membrane distillation is a separation technology that can be powered with low-temperature solar energy and perform brine concentration treatments. However, the operations under changing radiation and salinity conditions make it necessary to use specific control and optimization strategies to maximize the MD system’s performance. In this work, an online feedback optimization technique is proposed to address this issue in real-time. This strategy allows us to intrinsically manage constraints on the input and output of the process while minimizing its thermal energy consumption, which makes progress concerning previous approaches in the literature for MD systems. In addition, the online feedback optimization controller is combined with the least squares technique, which turns the approach into a model-free controller that helps to overcome the difficulty in modeling MD systems. The proposed strategy was tested in a trustworthy simulation scenario, using a validated model and real data from a facility located at Plataforma Solar de Almería (Spain). The results show its superior performance over a previous controller in the literature and non-optimal approaches, allowing us to reduce the thermal energy consumption by 0.5% and up to 6.7%, respectively, which has a direct impact on the cost of water treatment. On the whole, the proposed controller results in an attractive plug-and-play strategy with energy savings achievements for general use in commercial MD modules.

Experimental Validation of Double Paraboloid Reflection for Obtaining Quasi-Homogeneous Distribution of Concentrated Solar Flux

This work demonstrates that the quasi-homogeneous distribution of concentrated solar flux is achievable by using double paraboloid reflection, with a primary reflector to concentrate the sunlight, and a secondary reflector to homogenise the radiation flux. For that, three slightly different secondary reflectors were designed and manufactured, matching the specifications of the paraboloid concentrator of the SF60 solar furnace located in PSA—Plataforma Solar de Almería, which was used as primary reflector. Starting from preliminary simulations of the optical apparatus, the secondary geometries were selected and then the reflectors were manufactured from 7075-T6 aluminium alloy, using conventional and CNC machining technologies, with further processing to achieve a mirror-like finish. The results obtained from solar irradiation tests corroborate that the “double paraboloid reflection” methodology proposed in previous theoretical works seems to be technically feasible and can be a solution for obtaining homogeneously distributed fluxes of highly concentrated solar radiation.

Experimental investigation on an air tubular absorber enhanced with Raschig Rings porous medium in a solar furnace

An experimental study was carried out to assess the performance of a tubular absorber enhanced with Raschig Rings (RR) porous medium for CSP applications. Two alternative designs with different porous lengths of 20 and 40 mm were fabricated and compared with two conventional tube designs with and without surface coating. Several tests were conducted at the solar furnace SF60 of the Plataforma Solar de Almería (PSA) within the international access program of the SFERA III project, financed by the EU. The main scope of the study was to provide comprehensive detail on the hydraulic and thermal characteristics of the modified tube for further optimization and deployment in point-focusing solar systems. Therefore, evaluations were directed to determine the effects of each design on the pressure losses and the tube wall temperature, as well as on the useful heat gain. Results indicated that although the porous inserts rise the pressure losses through the fluid flow, the higher wetted area in the porous zone for heat transfer between the air and the heated plate reduces the wall temperature significantly. Moreover, applying the PYROMARK 2500 as the surface coating has a high influence on increasing solar absorption and reducing thermal losses. Further investigations revealed that the integration of the porous medium changes the temperature profile formed all over the tube, transforming a Gaussian shape in the plain pipes to a spline shape with two peaks in the modified tubes. Increasing the energy and exergy efficiencies of the solar absorber up to 30–50% and 60–70%, respectively, demonstrated the improving effects of the proposed porous material for future applications in the solar industry.

Coalitional model predictive control of parabolic-trough solar collector fields with population-dynamics assistance

Parabolic-trough solar collector fields are large-scale systems, so the application of centralized optimization-based control methods to these systems is often not suitable for real-time control. As such, this paper formulates a novel coalitional control approach as an appropriate alternative to the centralized scheme. The key idea is to split the overall solar collector field into smaller subsystems, each of them governed by a local controller. Then, controllers are clustered into coalitions to solve a local optimization-based problem related to the corresponding subset of subsystems, so that an approximate solution of the original centralized problem can be obtained in a decentralized fashion. However, the operational constraints of the solar collector field couple the optimization problems of the multiple coalitions, thus limiting the ability to solve them in a fully decentralized manner. To overcome this issue, a novel population-dynamics-assisted resource allocation strategy is proposed as a mechanism to decouple the local optimization problems of the multiple coalitions. The proposed coalitional methodology allows to solve the multiple local subproblems in parallel, hence reducing the overall computational burden, while guaranteeing the satisfaction of the operational constraints and without significantly compromising the overall performance. The effectiveness of proposed approach is shown through numerical simulations of a 10- and 100-loop version of the ACUREX solar collector field of Plataforma Solar de Almería, Spain.

Nonlinear Predictive Control for Temperature Regulation of Solar Furnaces

In the context of material synthesis and thermal stress tests, solar furnaces play a fundamental role. These experimental systems are usually operated manually by trained operators. However, the use of automatic controllers can be essential to take into account the different dynamics of the treated materials, follow the different temperature profiles required in the tests, meet system security requirements, and face the intermittency of the main energy source, the solar energy. On these bases, the present work proposes a control system for temperature regulation in solar furnaces. The controller is based on a nonlinear model predictive control strategy that deals, in a single control layer, with the nonlinear behaviour of the system, the effect of disturbances, and the system's operating constraints. The controller was experimentally tested in the solar furnace SF60 of Plataforma Solar de Almería (southeast Spain), showing adequate performance against setpoint temperature changes, reaching a settling time of the order of 54 % faster than the open-loop system's time constant, and effectively rejecting irradiance disturbances. In addition, the operational constraints were ensured in almost all operating circumstances, observing only slight deviations of less than 2°C. These results allow us to position the proposed controller as a relevant tool for daily operation.

Improvements in the measurement of high solar irradiance on a 300 kWth volumetric receiver

The current solar tower plants demand the measurement of high solar irradiance on the receiver. This measurement would facilitate the operation of the plant, the estimation of the receiver efficiency and would contribute to the safety of the receiver. Nowadays, these plants don't have a system to measure the flux because there are some limitations, as the difficulty to add new elements to the design and the structure of the tower. It is necessary to include a flux measurement system in the planning and construction of the commercial plants. This work presents a validation of the new indirect method to measure the flux which only use the surface of the receiver, a radiometer and a digital camera. In the context of an international project, the new method to measure the flux has been tested with a different receiver and with different working conditions. The receiver tested in this project is a volumetric receiver made of silicon carbide. The flux measurements have been done in the solar tower facility of the Plataforma Solar de Almería, the SSPS-CRS. The results obtained with this system have been compared to the results obtained with the reference flux measurement system based on a white lambertian target, obtaining a good level of agreement. This reference method can't be extrapolated to the commercial plants because it has moving parts that can't be added to the design of the towers. The new method avoids these moving elements and simplify the measurement of the flux distribution on the receiver.

Canting heliostats with computer vision and theoretical imaging

Solar Power Tower technology requires accurate techniques to ensure the optical performance of the heliostats both in commissioning and operation phases. This paper presents a technique based on target reflection to detect and correct canting errors in heliostat facets. A camera mounted on the back of a target heliostat sees an object heliostat and the target facets in reflection. The pixels difference between detected and theoretical borders determines the canting errors. Experiments in a lab scale testbed show that canting errors can be corrected up to an average value of around as low as 0.15 mrad. Experiments were also performed on a real heliostat at Plataforma Solar de Almería. As a result, canting errors (up to 5 mrad) have been reduced below 0.75 mrad. Mirror slope errors, which can be noticeable in large facets, becomes the largest source of inaccuracy in the presented method.