•Stratified packed beds are preferred for low-temperature solar energy storage.•New researches in packed beds try to improve the stratification in the bed.•Fluidized beds can rapidly distribute ...concentrated solar energy in the whole bed.•Fluidized beds are preferred for thermochemical energy storage.•More studies of particles fluidized during long periods of time are necessary.
This review summarizes different solar thermal energy storage techniques from a particle technology perspective, including sensible, latent and thermochemical techniques for low- and high-temperature applications that use particles as the storage medium in the thermal energy storage system. The focus is on applications, experimental results, modeling and future trends. This review describes two different particle technologies used to store thermal energy: packed and fluidized beds. The advantages and disadvantages of both technologies are reviewed throughout different studies found in the literature for various thermal energy storage systems. Packed beds have the main advantage of thermal stratification, which increases the efficiency of solar collectors in low-temperature sensible energy storage systems and augments the exergy content in the bed. Moreover, they have been proven to be suitable as dual-media thermocline storage systems for CSP plants. In contrast, the high mixing rates of fluidized beds makes them suitable for the rapid distribution of concentrated solar energy in particle receiver CSP systems. In addition, their high heat and mass transfer rates, compared with those of packed beds, make them the preferred particle technology for thermochemical energy storage applications. This review also notes that it is important to find new materials with an appropriate size and density that can be properly used in a fluidized bed. Additionally, more specific research efforts are necessary to improve the understanding of the behavior of these materials during the fluidization process and over a high number of charging/discharging cycles.
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•Experiments of a tube of a receiver of a solar tower have been conducted under start-up conditions.•Surface temperature and deflection measurements are presented.•The inverse method ...is able to predict the incident heat flux and tube bending.•Higher thermal stresses are experienced under start-up than under normal operation.
During the startup operation of molten salt solar tower plants, the tubes of the receiver are subjected to a high nonhomogeneous heat flux along with a low heat transfer coefficient to the air inside the tube, which results in a highly uneven angular temperature distribution. This uneven temperature distribution causes thermal stresses and tube deflection in the receiver. The most important constraint for the design and operation of central receivers is to keep the intercepting solar flux within the tube mechanical safety limits. In this work, an experimental facility consisting of a molten salt loop that simulates a solar tower receiver tube is used to measure the outer surface temperature and the deflection of the tube under the startup operating conditions of a solar tower power plant. An inverse heat transfer problem is applied to obtain the heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient three-dimensional numerical model of an empty circular pipe subjected to a nonhomogeneous heat flux is developed. A good agreement between the experimental and calculated tube temperatures and deflection is observed, with differences of 7% and 10%, respectively. Moreover, the thermal stresses are calculated. It has been found that higher thermal stresses are obtained when the tube is preheated compared to the stress when the molten salt is flowing under similar heat flux conditions.
Induction heating is commonly used in laboratory-scale facilities to replicate the heating conditions of the receiver tubes of concentrated solar power plants. This work aims at shedding light at the ...induction heating characteristics for such applications through the development of a multiphysics numerical model capable of replicating the experimental conditions of a molten salt loop locally heated by an induction heater. In the experiments, a stainless steel pipe is heated on its external surface by the induction heater, which is switched on and off during the experimental data acquisition while molten salts are continuously circulating in its interior. These conditions are replicated, for the first time, in a two-dimensional numerical domain fully coupling the electromagnetic and thermal physics, including thermally dependent material properties of the heated pipe. Once validated against the experiments, the numerical results revealed that the volumetric nature of the induction heating shall be considered for an accurate representation of the temperature profile inside the tube. As a novelty, different equivalent surface boundary conditions are presented and, despite the Gaussian-like behavior of the induction heating on the surface of the tube, the results indicate that there exists no equivalent wall boundary condition to fully replicate the temperature profile obtained with the induction heater. The effect of independently varying experimental parameters such as the geometry of the pipe (i.e., diameter and thickness) and its distance to the induction heating system is also evaluated. Using large diameters of the tube reduces the difference between the angular temperature profile obtained using induction heating and a simplified wall boundary condition. For small wall thicknesses, the induction heating is capable of penetrating along the whole thickness of the tube, the total heat generated in the volume of the tube being exposed to the counteracting effects of the volumetric generation and the enhancement of the heat dissipation by the molten salt, as both of them increase for small thicknesses. The distance of the inductor to the pipe wall appears to maintain the volumetric characteristics of the heating and only affects the induction heating magnitude and efficiency.
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•Fully coupled model of induction heating of a molten salt pipe is presented.•Induction promotes a volumetric heat generation in the pipe.•No equivalent surface heating can replicate induction heating.•Effect of different pipe geometries on the induction heating was characterized.•Induction reaches the whole pipe thickness for sufficiently thin tubes.
•A dynamic model of a moving packed-bed particle-to-sCO2 heat exchanger is presented.•The aim of the heat exchanger is to raise the sCO2 temperature to 700 °C at a pressure of 20 MPa.•A control ...system based on adjusting both the particle and sCO2 mass flow rates is proposed.•The comparison between feed-forward and feedback control strategies is presented.
A dynamic model of a moving packed-bed particle-to-sCO2 heat exchanger and control system for concentrating solar power (CSP) applications is presented. The shell-and-plate heat-exchanger model allows for numerically investigating the transient operation and control of the heat addition to the power cycle in a particle-based CSP plant. The aim of the particle-to-sCO2 heat exchanger is to raise the sCO2 temperature to 700 °C at a pressure of 20 MPa. The control system adjusts both the particle and sCO2 mass flow rates as well as an sCO2 bypass to obtain the desired sCO2 turbine inlet and particle outlet temperatures for a prescribed thermal duty. The control system is demonstrated for disturbances in particle and sCO2 inlet temperatures as well as changes in thermal duty for part-load operation. A feed-forward control strategy that adjusts the sCO2 and particle mass-flow rates as functions of measured inlet temperatures and a steady-state model solution was able to return the heat exchanger to the desired operating condition, but not without experiencing significant deviations in the sCO2 turbine inlet and particle outlet temperature (>40 °C) during the transient. To reduce both sCO2 and particle temperature deviations, a feedback control strategy was investigated, where sCO2 and particle mass-flow rates based on the steady-state model solution were corrected based on measured outlet temperature deviations. The feedback control strategy maintains sCO2 turbine inlet and particle outlet temperature to within 16 °C of the set points with a three-minute settling time for step changes in inlet conditions and thermal duty. This finding demonstrates the possibility of dynamically dispatching next-generation particle-based CSP plants driving sCO2 power cycles.
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•A molten salt loop that simulates a tube of a solar tower receiver is presented.•The tube conducting molten salt receives a high nonuniform heat flux provided by an induction ...heater.•The surface temperature and the deflection of the tube under solar power tower operating conditions are measured.•An inverse problem is applied to accurately obtain the heat flux reaching the tube.•The measurement of the deflection of the tube is used to verify the inverse problem.
Inverse heat transfer problems typically rely on temperature measurements for estimating unknown boundary heat flux, such as that in the water tubes of steam boilers or central receivers in solar tower power plants. In this work, an experimental facility consisting of a molten salt loop that simulates a tube of a solar tower receiver is presented to obtain the outer tube surface temperatures under solar tower power plant operating conditions. The external surface f the pipe in the test section is heated in a controlled manner with an induction heater, which provides a very high nonuniform heat flux. An inverse thermal method has been applied to obtain the incident heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient two-dimensional numerical model of a circular pipe flowing molten nitrate salt and subjected to a nonhomogeneous circumferential heat flux has been developed. The heat flux calculation with the inverse method is in accordance with the heat flux estimation based on the calibration of the induction heater. A good agreement between the experimental and calculated temperatures is observed. Furthermore, the deflection of the tube caused by the nonhomogeneous heat flux is measured and is compared to the deflection calculated from the radial temperature profile from the inverse problem solution, and a good agreement between both results is observed.
The importance of considering the water price in the analysis of the impact of dry versus hybrid condensing systems in the thermo economical performance of solar tower plants was demonstrated in this ...work. The dry condensing system consists of several induced-draft air-cooled condenser cells (ACCs) and the hybrid system consists of a parallel system where the condensing steam is split between the ACCs and a surface steam condenser where circulating water is cooled in a wet mechanical-draft cooling tower. The influence of the operating parameters of either the dry or wet cooling systems on the cooling load and fan power consumption were studied. Then, for a given condensing system (a system with a defined number of installed ACCs units and cooling tower units) and given the dry-air and wet-bulb air temperatures, the operating parameters were optimized to maximize the revenues of the power plant. This optimization depends on the water-to-electricity price ratio R, showing that at low ambient temperature when this ratio increases it is not profitable to turn on the cooling towers since the water cost is not counterbalanced by the higher cycle efficiency obtained with the lower condensation temperature. Finally, the annual operation and the LCOE and NPV of the CSP plant located in Dunhuang were analyzed for both dry and hybrid condensing systems with different number of ACCs and wet towers, showing that the most cost-effective configuration is the 16 ACCs with 3 wet cooling towers for water-to-electricity price ratio R=4 ($/m3)/($/kWhe) and R=5 ($/m3)/($/kWhe), but for R=10 ($/m3)/($/kWhe), the best option is with only 2 wet towers.
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•The thermo-economical behavior of dry and hybrid condensing systems is compared.•The annual operation analysis of a real solar tower plant is presented.•The condensing systems operation parameters were optimized to maximize the revenues.•The water cost effect in the thermo-economical performance of the solar plant is shown.
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•A simplified model of a thermocline tank including the composite tank wall is proposed.•Temperature profile and mechanical stress in the tank wall are obtained.•The effects of molten ...salt flow rate on the results are analyzed.•The effects of bed to wall heat transfer on the results are analyzed.
Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-media thermocline tanks in solar energy applications. To study this phenomenon, it is necessary to develop a comprehensive model of a thermocline tank that includes both the heterogeneous filler region and the composite tank wall. Because CFD models require a high computational cost to simulate a thermocline tank considering transient state operation, a simplified dual-phase model that includes the unsteady heat transfer through a multiple layer wall has been developed. The filler region consists of a rock bed with interstitial molten salt, and the tank wall is composed of a steel shell with two layers of insulation (firebrick and ceramic). In this simplified model, the fluid flow inside the tank is considered to be one-dimensional along the tank axis direction, whereas the heat conduction in the composite wall is considered to be two-dimensional. Therefore, a convective heat transfer coefficient from the bed to the wall is necessary to couple the molten salt flow with the heat transfer in the tank shell. In this work, the effects of both convective heat transfer from the bed to the wall and molten salt flow rate on the time-dependent thermal response of both the steel shell and molten salt have been analyzed. The simplified model is able to predict the temperatures of the molten salt, filler material and layer wall as well as the mechanical stress in the tank shell.
•A storage system to recover energy during startups of combined cycles is proposed.•Low payback period 3–3.4 years and energy recovery efficiency up to 80 %•The optimum solutions present a net ...present value between 2.53 and 4.45 M€.•The net present value grows up to 60% considering the benefits from carbon credits.•Between 2 175–3 640 tons of CO2 emissions per year can be avoided.
This work presents a novel steam accumulator and concrete-block storage system (SACSS) to recover part of the energy lost through the steam cycle side during startups of combined cycle power plants (CCPPs). The steam accumulators are integrated with sensible-heat concrete storage to provide superheated steam resulting then to a higher efficiency and safer steam turbine operation compared with systems based only on saturated steam. An economic analysis is performed considering two different scenarios: i) a CCPP able to execute fast startups using a Benson-type heat recovery steam generator (HRSG) and ii) a CCPP operated with conventional startups which employs a typical drum-type HRSG. It is worth mentioning that the second scenario is based on measured data. The economic optimization of the SACSS is carried out focusing in four design variables: number of steam accumulator units, storage pressure, concrete-block length and outer concrete diameter. The optimum solution presents a net present value of 4.45 M€ and a payback period of 3 years for the CCPP suitable for fast startups. For the CCPP operated with conventional startups, a net present value of 2.53 M€ and a payback period of 3.4 years are obtained. The net present value grows around 60 % in both cases if the benefits from carbon credits are considered. In addition to the efficiency improvement, the SACSS could be used to preheat critical sections of the heat recovery steam generators, reducing the thermal stress and the fatigue damage during fast startups. Finally, the emissions avoided thanks to SACSS are estimated to be around 3 640 and 2 175 tons of CO2 per year, for fast and conventional startup cases, respectively.
Climate projections under the high-emission representative concentration pathway “RCP-8.5” scenario are used to show the effects of global warming on the energy production of two similar concentrated ...solar tower power plants in Spain and Chile from 2025 to 2060. Results show a reduction of the annual energy production of almost 1.8% for Chile and an increase of almost 6% in the Spanish plant, but an increase of 10% water consumption due to both the increase in dry- and wet-bulb temperatures and changes in the direct normal irradiation caused by climate change. When the effect of water restrictions is included in the model caused by the foreseen reduction in water availability in Spain, the increase of the annual energy production is only 2%. Finally, the levelized cost of energy (LCOE) calculated during the whole power plant lifetime (35 years) with respect to the LCOE obtained at the beginning of the project is increased by 1.1% in the Chilean plant, whereas in the Spanish plant, it can be reduced more than 2.5% if there are no water restrictions, while a reduction of 0.4% is expected in a scenario with water restrictions. Results show that careful consideration of climate projections should be considered to properly estimate the economic viability of solar tower plants.
The use of recycled aggregate concrete (RAC) acquires particular interest in civil construction regarding sustainable development. Recycled aggregates usually present greater porosity and absorption, ...and lower density and strength than natural aggregates. Microstructural studies on RAC indicate differences in the characteristics of the interfacial transition zones between the cement paste and the aggregates. At the same time most experiences verify that reduction in concrete stiffness is higher than in strength. The failure mechanisms in RAC can be affected by the above stated factors. In this paper, three Series of concretes with different compressive strength levels are presented. Each Series includes a reference concrete prepared with natural crushed stone and two RAC prepared with two coarse aggregates obtained by crushing a normal strength and a high strength concrete. Flexural tests on notched beams and uniaxial compression tests on standard cylinders were performed. In addition, the characteristics of the fracture surfaces were analysed in order to determine the amount of broken aggregates. RAC present lightly lower strengths (1–15%), lower modulus of elasticity (13–18%) and significant reductions in the energy of fracture (27–45%) and, consequently on the fracture zone size, when it is compared with a concrete prepared with natural coarse aggregates.
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