CSP Applications Research Articles

Thermal Desalination Through Forward Osmosis Coupled With CO2-Mixture Power Cycles for CSP Applications

This work, performed in the framework of the H2020 EU project “DESOLINATION”, analyses the coupling between CSP plants using transcritical power cycles with CO2-mixtures and an innovative thermal desalination technique based on Forward Osmosis. Calculations are presented for a large scale CSP plant with central tower receiver and direct storage with solar salts in Dubai, adopting the mixtures CO2+SO2 and CO2+C6F6 in the power cycles. The heat rejected from the cycle condenser is recovered directly by the FO plant, where the draw solute is heated up from 40 °C to 76 °C, to allow for the regeneration of the draw solution used in the forward osmosis membrane. The thermo-responsive polymer adopted is PAGB2000, already considered in literature as a promising option. Results show a very effective synergy between the electricity and the freshwater production: high yearly solar to electric efficiencies are possible (around 19%), with a low freshwater specific thermal consumption (around 100 kWhth/m3). The proposed desalination method is more effective than a conventional MED system (with + 50% of yearly freshwater produced), while a larger solar field (+ 28% in surface area) is necessary for a PV+RO plant to produce annually both the energy and freshwater produced by the CSP+FO plants.

Transcritical Cycles With CO2-based Mixtures for CSP Applications: An Overview of the SCARABEUS Findings

This work summarizes the methodology developed within the H2020 EU project SCARABEUS for the analysis of innovative CO2-based mixtures used as working fluid in transcritical cycles for CSP applications. By adding a specific quantity of carefully selected dopant to CO2 it is possible to reach a high mixture critical temperature, suitable for air cooled cycles at very high minimum temperature in hot environments with high efficiencies. Along the methodology, some results related to the design of the turbine and the heat exchangers for the innovative mixtures are presented, including a focus on the system integration between the power block and the solar plant: as such, these results can be considered as interesting research outcomes to widen the knowledge on innovative solutions for efficient power cycles. The new power cycles are foreseen to drastically increase the profitability and cut the specific cost of CSP systems with respect to the state-of-the-art

Development of Self-Passivating, High Strength Ferritic Alloys for CSP and TES Application

The addition of aluminum to ferritic stainless steels can result in self-passivation by the formation of a compact Al2O3 top layer, which exhibits significantly higher corrosion resistance to solar salt compared to a Cr2O3 surface layer. The development and qualification of realistic experimental methods for fatigue testing under superimposed salt corrosion attack will enable safe component design. Salt corrosion experiments were carried out at 600 °C with and without mechanical fatigue loading at a novel, self-passivating trial steel, using “solar salt” (60 wt.% NaNO3 and 40 wt.% KNO3). Cyclic salt corrosion tests at 600 °C under flowing synthetic air (without mechanical loading) showed that self-passivation to molten salt attack and mechanical strengthening by precipitation of fine Laves phase particles is possible in novel ferritic HiperFerSCR (Salt Corrosion Resistant) steel. A compact, continuous Al2O3 layer was formed on the surface of the model alloys with Al contents of 5 wt.% and higher. A distribution of fine, strengthening Laves phase precipitates was achieved in the metal matrix.

New Coating Paradigm to Boost Performance and Durability of Solar Receivers

The development of renewable energy sources is nowadays of enormous importance, not only for the climate change fight but also for the security of the energy supply. The latest geopolitical unfortunate events in Europe have highlighted our energy dependency on foreign fossil fuels. In this context, solar technologies are already playing an essential role in shifting towards neutral carbon economies, ensuring a reliable energy supply. In this regard, dispatchability provided by CSP plants is key to pave the way towards energy transition. It is worth noting that the long-term durability and performance of solar components in arid regions are crucial to increase the reliability and performance of CSP plants while reducing O&M costs. In this work, an innovative approach based on nano-structuring the solar receiver tube glass is presented, which provides improved anti-reflective (AR) and anti-soiling (AS) properties, also showing good durability with respect to abrasion. Spectral transmittance improvement, soiling rate decrease, and durability measurements are presented for nano-structured glasses, comparing with current state of the art glass performance. The achieved experimental results suggest that the new structured glasses would be good candidates for CSP applications

Pathways to IEC Standards for Heliostat Design Qualification and Site Acceptance in Central Receiver CSP Applications

This paper surveys the existing landscape of standards relevant to heliostats, identifies their gaps, and proposes a path forward to a comprehensive set of heliostat guidelines, technical specifications, and standards under the framework of International Electrotechnical Commission (IEC) TC 117. Gaps in existing guidelines and standards are surveyed using a three-tiered taxonomy: component-level, heliostat-level, and field-level. At each level, the gap analysis is followed by a proposal for a coordinated path forward on the development of standards. At the component level, advances in the understanding of wind loading should inform a technical specification for drives and structures. Reflectors require consolidation of measurement guidelines into existing standards documents. Communications & controls require technical standards to inform their selection and secure implementation. At the heliostat level, IEC 62817 (solar trackers) adequately characterizes drive systems, structures, and electronics, but requires adaptation to heliostats’ use patterns, operating modes, and expected life cycles. IEC 62817 does not address heliostat beam quality and pointing accuracy, but the process for determining both is elaborated in the SolarPACES Guideline for Heliostat Performance Testing. This SolarPACES document requires two main modifications: adaptation to IEC language and inclusion of testing after heliostats which have undergone accelerated weathering and mechanical cycling (to understand performance degradation). At the field level, IEC 62862-4-2 addresses the function and control of heliostat fields but does not cover the statistically rigorous testing of heliostat groups, or field performance factors like security and soiling. The addition of documents under IEC-62862-4 is proposed to address this gap.

Development of a PHIL Real-Time Simulation Testbed for Optimization of Hybrid Power Plant Generation

Future electrical grids will likely include a diverse group of power generation plants. With the growing list of new generation options comes many opportunities for more optimal energy production management and effective resource utilization. Microgrids are an ideal environment for research, development, and validation of those energy optimization strategies. This paper establishes a method for integrating a virtual thermal power plant model with a physical microgrid in a power hardware in the loop testbed. One advantage of these testbeds is their variety of configuration options. In this paper, the combined effects of photovoltaic and concentrating solar power generation are investigated. A conventional thermal power plant with components sized for CSP applications is modeled in the Simscape environment. Actual solar panels provide the photovoltaic element of this experiment. How the plant model and physical hardware interact is described in detail. A PV startup and shutdown event is simulated in real-time. The model responses are shown to be successfully and correctly coupled to electrical power flow in the testbed.

Thermal desalination from rejected heat of power cycles working with CO2-based working fluids in CSP application: A focus on the MED technology

This work analyses the integration of concentrated solar power plants based on innovative sCO2 cycles and transcritical CO2-based mixtures cycles with thermal desalination plants adopting the conventional MED technology. In these cogeneration plants, all heat rejected from the cycle is exploited by the desalination system, avoiding any parasitic electric consumption of the fans of the air-cooled heat rejection unit. The MED layout proposed exploits both latent effects, from 3 to 8, and sensible effects, to match at best the temperature level at which the heat is available from the power cycles, delivered to the desalination plant through an intermediate loop of demineralized water.The cogenerative solution is designed in all its components and proposed in this work as a 100 MWel solar tower CSP plant located in Sevilla, resulting in a yearly production of around 400 GWhel/year and between 3.5 and 4.2 Mm3 of freshwater produced, depending on the configuration analysed. Various power cycle layouts are investigated, working with both sCO2 and the innovative CO2 + C6F6 mixture as working fluids. Regarding the solar plant, detailed models for the solar field optical analysis and the receiver thermal analysis are adopted. The seawater desalination plant, when coupled with this category of CSP plants, presents a thermal consumption between 180 and 140 kWh/m3. Finally, the cogenerative plants performances are compared in terms of levelized cost of electricity, with a slight edge for the innovative mixture cycles, and levelized cost of water, in a range between 1 and 2 $/m3.

Off-design of a CO2-based mixture transcritical cycle for CSP applications: Analysis at part load and variable ambient temperature

This work focuses on the off-design analysis of a simple recuperative transcritical power cycle working with the CO2 + C6F6 mixture as working fluid. The cycle is air-cooled and proposed for a state-of-the-art concentrated solar plant with solar salts as heat transfer fluid in a hot region, with a cycle minimum and maximum temperature of 51 °C and 550 °C at design conditions. The design of each cycle heat exchanger (primary, recuperator and condenser) is carried out in MATLAB with referenced models and the turbine designed in CFD, providing performance maps adopted by the cycle operating in sliding pressure. The off-design of the cycle is developed with a routine simulating the thermodynamic conditions of the cycle at variable ambient temperature and thermal inputs down to 40 % of the nominal value. The results show that the cycle can efficiently run in a wide range of part load conditions and ambient temperatures, from around 0 °C to over 40 °C, with net electric cycle efficiencies from 45 % to 36 %: according to the control philosophy proposed, the condenser fans are fixed at design speed, while the cycle operates in sliding pressure, when is possible. The results evidence the flexibility and good performances of the proposed system in various operating conditions.

Application of Cold Sintering Process for Stabilizing Heavy Metals in Municipal Solid Waste Incineration Fly Ash

Municipal solid waste incineration fly ash (MSWI FA) consists predominantly of compounds comprising elements such as calcium, aluminum, silicon, sodium, and others. Additionally, it encompasses a complex mixture of heavy metals, chlorides, sulfates, organic pollutants, and other constituents. The effective and economically viable treatment of MSWI FA poses a formidable challenge for resource cycling at the current stage. In this research report, we adopt a novel low-temperature sintering method called the “Cold Sintering Process” (CSP) as a means to immobilize heavy metals within the fly ash. By utilizing a Taguchi orthogonal array method, we will adjust five control factors in the CSP, including sintering temperature, uniaxial pressure, sintering time, initial water addition, and sodium carbonate dosage. The leaching of cadmium from the fly ash, as measured by the Toxicity Characteristic Leaching Procedure (TCLP), will serve as the quality indicator of products. Through the application of CSP, MSWI FA was transformed into structurally stable ceramic blocks, and the heavy metals within the blocks were effectively immobilized. The results of the experiments showed that MSWI FA under the conditions of a temperature of 300 °C, uniaxial pressure of 312 MPa, sintering time in 60 min, 25 wt% water addition, and 9 wt% Na2CO3 addition could effectively reduce the leaching of cadmium by 77.71%, lead by 21.14%, zinc by 42.37%, and chromium by 99.99%, as compared to the original MSWI FA TCLP results. The X-ray Diffraction (XRD) results indicate that during the CSP, fly ash forms phases such as calcium silicate, rankinite, hydrogrossular, anorthite, and marilite. These phase transformations are considered beneficial for preventing the leaching of internal heavy metals. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy (SEM-EDS) results reveal that CSP is advantageous for compacting the overall structure, and EDS results further demonstrate that some of the Pb and Zn are carried out from the interior of the blocks, with uneven distribution on the surface of fly ash particles. The aforementioned experimental results serve as preliminary indications of CSP’s capability to stabilize detrimental components within high-purity fly ash. Future research endeavors may entail the refinement of material proportions, modification of experimental parameters, and other methodologies, thus facilitating potential scalability to industrial applications. Such developments align with the overarching goal of resource utilization.

Characterization and experimental study of volcanic rocks in fluidized beds with a beam-down solar reflector for CSP applications

This work experimentally studied the thermal behavior of volcanic rocks in a lab-scale bubbling fluidized bed with concentrated irradiation from the top. Following an exhaustive review of natural particles that can be radiated and fluidized, volcanic particles are considered a suitable material thanks to their origin (from crystallization upon solidification from temperatures close to 1200 °C), and their ease of volatilization. In the search for natural materials, this work tested two volcanic sands (red and black pozzolana volcanic sand), comparing their results with other solid particles tested previously under similar experimental conditions. This comparison yielded ratios of thermal energy stored versus pumping power of the fluidizing air between 34% and 58% higher than other typical solid particles used for this purpose, such as sand or Silicon carbide (SiC). These two volcanic rocks were also experimentally tested over 10 cycles of charging/discharging without apparent deterioration of their thermal properties, as the thermal response in each cycle was practically the same with a maximum variation of 7% in the mean temperature.

Numerical Investigation of the (Mono-Hybrid) Nanofluid Thermophysical Properties for Concentrated Solar Power Plant

Nanofluids became an essential solution for the improvement of efficient heat transfer fluids. Thus, it’s necessary to optimize their propreties. This paper investigates the effect of the temperature and the volume fraction on the thermo-physical properties of different nanofluids (Mono and hybrid) such as: Density, thermal conductivity, dynamic viscosity, kinematic viscosity, heat capacity and enthalpy in various nanoparticule concentrations and operating temperature. Two nanoparticles Al2O3, CuO were added to three different conventional base fluids namely: Therminol VP-1; Sylthrem 800; Dowtherm A, with several volume fractions, and various temperatures (200–400 °C). A numerical model was developed using MATLAB software, to evaluate the behavior of each thermo-physical property of the nanofluid that can be used as a working fluid in CSP applications and compared with their conventional fluids. The results show an improvement in thermo-physical properties compared to pure fluids for an optimal value of 4% for Al2O3. Also, the increase in temperature plays an important role in the decrease in viscosity, and their influence on other properties has also been noticed while the addition of nanoparticles to the pure fluid allow to increase the thermal conductivity by 13%. Finally, the (Al2O3 + CuO/Dowtherm A) hybrid nanofluid sems to be attractive to use in CSP applications.

CFD modelling of an indirect thermocline energy storage prototype for CSP applications

Thermocline thermal energy storage is a (potentially) cost-effective alternative to the more widespread two-tank solution, as both the hot and the cold medium are stored in a single tank.An innovative single-medium indirect thermocline technology was recently developed by ENEA and a prototype was experimentally tested at the Casaccia laboratories. The storage tank is equipped with two flat-coil heat exchangers (HXs) located at the bottom and at the top of the tank, for the charge and discharge phases, respectively. The storage medium is a mixture of molten salt (Hitec XL) and the heat transfer fluid is a mineral oil (Delcoterm Solar E15). An internal vertical channel, which assists the motion of the storage medium during the charge and discharge transients, connects the two HXs.In this paper, a detailed, transient 2D axisymmetric Computational Fluid Dynamics (CFD) model of the prototype has been developed. The model simulates the charge and discharge transients, determining the heat losses and the temperature distribution of the molten salt in the tank. The computational domain includes the storage medium, the tank insulated walls and the main components immersed in the molten salt, i.e., the heat exchangers and the vertical channel.The model has been first calibrated by best-fitting the data measured during a test conducted without any thermal load, then validated against another independent set of experimental data.A first comparison against a charge transient showed some discrepancies between the computed and the measured temperatures of the salt, which could be explained by assuming the presence of a bypass at the location of the HXs. This assumption is discussed and justified in the paper. The CFD domain has then been modified to include the bypass, and this allowed to successfully validate the model against experimental data in both charge and discharge phases.The validated model is finally exploited to assess how a variation in the diameter of the internal vertical channel could affect the thermal performance of the storage system in a charge transient. The results indicate that decreasing the channel diameter leads to an increase of the salt temperature at the very top of the tank at the cost of a longer time to fully charge the storage.

Temperature dependence of high-temperature corrosion on nickel-based alloy in molten carbonates for concentrated solar power applications

Concentrated solar power plants present several issues due to salt-induced corrosion, and the modelling of these mechanisms is needed for estimating power plants lifetime predictions. Moreover, a small variation of temperature can lead to microstructure changes due to allotropic transformations and thus free energy of formation of phases within the materials. Therefore, a corrosion study of molten carbonates, being high potential heat transfer fluids for CSP applications, on H230 alloy was performed at 700ºC, 750ºC and 800ºC up to 1000 h, in air atmosphere. The diffusion processes inducing the corrosion mechanism were analyzed and modelled with microstructural characterizations and thermogravimetric studies.

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.

Zirconia-based coatings for the protection of ferritic-martensitic steels in a molten salt environment for CSP technology

The ferritic-martensitic 9Cr-1Mo steel was coated with a sol-gel ZrO2-3%molY2O3 solution by dip-coating. The protective properties of the coating were evaluated in contact with Solar Salt for CSP applications. The coated and uncoated steels were tested at 500 °C in contact with the Solar Salt for 2000 h. Gravimetric, microstructural and compositional studies were performed to compare their behaviour, against corrosion. Results revealed the beneficial influence of the coating on the corrosion resistance of P91 in contact with molten nitrate salt. Weight gain of the coated samples remained practically unaltered throughout the test, while the uncoated samples showed a weight increase with the formation of multi-layered, non-adherent corrosion layers that were spalled during the test. The microstructural and compositional study performed on the samples shows the maintenance of the coating layer and the presence of a 5.18 μm-thick diffusion layer primarily composed of Fe-Zr species, which reduce the ionic diffusion and improve the protective properties. Results reveal the possible suitability of the proposed system as a potential solution to the severe corrosion problems industrial tanks and pipes in contact with Solar Salt.Graphical The coating of 9Cr-1Mo ferritic-martensitic steel with a sol-gel solution of ZrO2 3% mol Y2O3 achieved a substantial improvement avoiding corrosion problems and postulating the proposed system as a potential solution to the severe corrosion problems of industrial tanks and pipes in contact with solar salt.

NaNO3-KNO3-KCl/K2CO3 with the elevated working temperature for CSP application: Phase diagram calculation and machine learning

Mixed nitrates with relative low melting point have widely been used as thermal storage medium in the concentrated solar power stations. However, their maximum working temperatures aren’t high enough to integrate with more efficient power generation systems such as the supercritical CO2 cycle. In the present work, mixed nitrates with elevated working temperature were obtained by adding KCl/K2CO3. With the help of FactSage software and back-propagation (BP) algorithm combined with genetic algorithm (GA), the eutectic compositions and melting points of NaNO3-KNO3-KCl and NaNO3-KNO3-K2CO3 were predicted, and the validity of BP algorithm combined with GA was verified. Based on DSC and TG curves, the melting points of the as-prepared NaNO3-KNO3-KCl and NaNO3-KNO3-K2CO3 are 206.12 °C and 207.24 °C, respectively, which are in agreement with predicted values. Moreover, their decomposition temperatures were increased to more than 640 °C. In addition, the specific heat capacity of both mixed molten salts was measured, and it also was calculated by using FactSage software. This work provides two new ternary molten salts based on nitrates with higher working temperature and develops the prediction method on phase diagram calculation and machine learning.

Flow and heat transfer analysis of a gas–particle fluidized dense suspension in a tube for CSP applications

This work presents a numerical study of the flow of particles in a gas–particle fluidized dense suspension for CSP applications using the Multi-Phase Particle in Cell (MP-PIC) method, implemented in CPFD-Barracuda software. The study covers two different numerical simulations. The first is a cold and isothermal model in which the fluctuations and control of the mass flow of particles ascending along the vertical tube was studied. In the second, a high-temperature boundary condition was imposed on the external surface of the tube and the energy equation was solved. In this second case, the heat transfer coefficient between the inner surface of the tube and the particles was numerically computed.The numerical results in the cold model are highly consistent with experimental data available in the literature (with values up to 150kg/h and differences of approximately ±10kg/h) and underline the significant impact of the pressure at the bottom of the bed and of the aeration flow rate on the mass flow of particles. The results of the non-isothermal case present heat transfer coefficients in the range of 300–400W/(m2K) with transient fluctuations during the fluidization process. These fluctuations may be an influence on the mechanical damage of the tube, which is exposed to high levels of concentrated irradiation.

Electrochemical Impedance Analysis for Corrosion Rate Monitoring of Sol-Gel Protective Coatings in Contact with Nitrate Molten Salts for CSP Applications.

The protective behaviour of ZrO2-3%molY2O3 sol-gel coatings, deposited with an immersion coating technique on 9Cr-1Mo P91 steel, was evaluated with corrosion monitoring sensors using the electrochemical impedance spectroscopy technique. The tests were carried out in contact with solar salt at 500 °C for a maximum of 2000 h. The results showed the highly protective behaviour of the coating, with the corrosion process in the coated system being controlled by the diffusion of charged particles through the protective layer. The coating acts by limiting the transport of ions and slowing down the corrosive process. The system allowed a reduction in the corrosion rate of uncoated P91 steel. The estimated corrosion rate of 22.62 μm·year-1 is lower than that accepted for in-service operations. The proposed ZrO2-3%molY2O3 sol-gel coatings are an option to mitigate the corrosion processes caused by the molten salts in concentrated solar power plants.

Influence of the Diffusing Elements on the Corrosion Mechanism of a Ni-Based Alloy in Molten Chlorides at High-Temperature for Csp Applications

Influence of the Diffusing Elements on the Corrosion Mechanism of a Ni-Based Alloy in Molten Chlorides at High-Temperature for Csp Applications

Experimental study of different coatings on silica sand in a directly irradiated fluidised bed: Thermal behaviour and cycling analysis

The present work shows the experimental results obtained with sand particles, which where coated to increase the thermal absorptivity for CSP applications. The particles were tested in a lab-scale fluidised bed with concentrated irradiation on the top. The experimental results indicates that two of the three coatings tested, based on graphite and carbon black, worked properly and absorbed between 30 and 40% more energy than raw sand due to the higher thermal absorptivity. Both coatings were also experimentally tested during 10 cycles of charging/discharging without apparent deterioration of their thermal properties. Sand coated with graphite exhibited colour change, from an initial dark black to a greyish tone that did not have an impact on the particles thermal response. The extrapolation of the results observed at lab-scale, to the maximum expected temperatures in new generation of CSP plants with solid particles, up to 1250 K, shows that coated particles may enhance the energy effectively stored in the bed by 60–80% compared to raw sand.