Two-tank System Research Articles

Small-scale CSP plant coupled with an ORC system for providing dispatchable power: the Ottana Solar Facility

This paper is focused on the ongoing studies at the Ottana Solar Facility, a new experimental power plant located in Sardinia (Italy). The facility consists of a 630 kW CSP plant and a 400 kW CPV plant. The CSP section includes a solar field based on linear Fresnel collectors using thermal oil as heat transfer fluid and a two-tank direct thermal storage system with a storage capacity of about 15 MWht. The electricity generation is carried out in the Turboden 6HR Special ORC unit. This paper presents a description of the CSP plant characteristics and an analysis of its expected performance. In particular, the results of the simulation activities aimed at assessing the influence of different operational strategies on the CSP plant behavior and, particularly, on the ORC performance are explained and discussed. The simulation models of the main sections of the CSP plant were developed in accordance with literature data and specific information given by the manufacturers. The results provide a useful basis for the future development of the management schemes during the experimental campaigns on the CSP plant.

A data-driven fault-tolerant control design of linear multivariable systems with performance optimization

In this paper, an integrated data-driven fault-tolerant control (FTC) design scheme is proposed under the configuration of the Youla parameterization for multiple-input multiple-output (MIMO) systems. With unknown system model parameters, the canonical form identification technique is first applied to design the residual observer in fault-free case. In faulty case, with online tuning of the Youla parameters based on the system data via the gradient-based algorithm, the fault influence is attenuated with system performance optimization. In addition, to improve the robustness of the residual generator to a class of system deviations, a novel adaptive scheme is proposed for the residual generator to prevent its over-activation. Simulation results of a two-tank flow system demonstrate the optimized performance and effect of the proposed FTC scheme.

Transient response simulation of a passive sensible heat storage system and the comparison to a conventional active indirect two-tank unit

This work presents a 1-D numerical model of a passive sensible thermal energy storage (TES) system using high-temperature concrete as storage medium. It is successfully validated against experimental data obtained from a pilot-scale concrete storage module. A specific Nusselt number correlation is developed providing better accuracy for the specific heat transfer fluid (Therminol VP1) and observed Reynolds numbers, than conventional general purpose heat transfer correlations for forced convection inside tubes. Then, the model is up-scaled to a commercial-sized thermal capacity of 504 MWh, providing grid-independent solutions and cyclic-steady-state initialization values for further use in a general purpose CSP model library. The proposed implementation in Modelica provides a flexible and intuitive simulation tool, which is not limited to a single simulation platform.Next, the presented model of the concrete TES system is used to compare its transient response with that of a conventional active indirect two-tank heat storage system. Whilst the concrete TES system shows lower maximum rates of change in HTF outlet temperature right after the switching of operating modes, the conventional two-tank TES system has the important advantage of steady-state operation, providing constant HTF outlet temperature, which is very favorable considering steam cycle efficiency and thermal fatigue management.

Probabilistic robust parity relation for fault detection using polynomial chaos

In this paper, a robust parity relation approach is proposed for fault detection of linear systems with the probabilistic time-invariant parameter uncertainties. To deal with polynomial dependence on uncertain parameters, a set of parity relations with polynomial parameterization is derived to decouple the unknown initial condition. Due to the polynomial structure having a finite degree, the generated residual vector admits an exact polynomial chaos expansion with the same degree, which enables an efficient online computation for quantifying the moments of the residual vector. The obtained moments are used to calculate a confidence set for checking the consistency between the observed system behavior and the parity relations. Compared to deterministic robust parity relation methods, the employment of probabilistic information allows a less conservative way to ensure a low false alarm rate while maintaining a high fault detection rate. In contrast to the existing polynomial chaos-based fault diagnosis literature, our proposed approach avoids truncation errors of polynomial chaos expansions, and is less computationally demanding. The proposed approach is illustrated by using a two-tank system example.

Dynamic simulation of two-tank indirect thermal energy storage system with molten salt

Thermal energy storage system, which can effectively store solar energy and make a solar power plant generate electricity in cloudy or rainy weather and nighttime, is a key part of a concentrating solar power plant, which makes solar power technology have unique advantages compared with other renewable energy power technology. Two-tank indirect thermal energy storage system with molten salt is most widely used and has been successfully commercialized in the field of solar power. In this passage, a universal dynamic simulation model of two-tank indirect thermal energy storage system with molten salt used for trough solar power plants based on the lumped parameter method is built, and the dynamic processes of thermal energy storage system charge and discharge, and the changes of heat transfer oil outlet temperature in the oil/salt heat exchanger, molten salt temperature and height in the molten salt tank when heat transfer oil mass flow rate has step disturbances are simulated on the STAR-90 simulation platform. The simulation results show that the model can simulate the charge and discharge dynamic characteristics of two-tank indirect thermal energy storage system with molten salt well, which can be good references for the system design, system control and system debugging.

Robust fault detection and isolation scheme using fuzzy wavelet network with a hybrid design algorithm

This paper presents a new robust fault detection and isolation scheme using fuzzy wavelet network based on the bounded error approach. An ecient hybrid design algorithm, which consists of the orthogonal least square and the arti cial bee colony algorithms, is proposed to design fuzzy wavelet network for modeling normal and faulty behaviors of the system. The proposed model provides an alternative description of the behavior of the system with high accuracy, but it su ers from model uncertainty because of model-reality mismatch in practical applications. To overcome this diculty, the bounded error approach inspired from robust identi cation theory is applied to estimate the model uncertainty which de nes a con dence interval of the model output and derives adaptive threshold for residual evaluation. Also, online fault isolation process is performed using fuzzy wavelet network models of the faulty system and analyzing the relation between a bank of residuals. Performance and eciency of the proposed scheme is evaluated by simulating the nonlinear two-tank liquid level control system. Finally, some performance indexes are de ned, and then the Monte-Carlo analysis is carried out to evaluate the reliability and robustness of the proposed scheme.

Annual comparative performance and cost analysis of high temperature, sensible thermal energy storage systems integrated with a concentrated solar power plant

The present study conducts a comprehensive comparative techno-economic analysis of some near-term sensible thermal energy storage (TES) alternatives to the ‘standard’ two-tank molten salt system for concentrated solar power (CSP) plants. As such, we conducted detailed, relative annual transient simulations for single-medium thermocline (SMT), dual-media thermocline (DMT), and shell-and-tube (ST) systems. To be consistent with recent literature, the DMT and ST systems use concrete with a porosity of 0.2 (e.g. where concrete occupies 80% of the system) as their low cost filler material. The systems were integrated into a validated 19.9MWe Gemasolar CSP model, which has a solar multiple of 2.5. For a relative analysis, the storage capacity of each TES alternative was fixed at 722MWhth (15 h storage) for all TES alternatives. Based on this capacity, a geometric optimization was performed on DMT and ST systems to maximize the discharged power and minimize the pressure drop. Using the optimum designs, it was found that a CSP plant with a two-tank molten salt system enables the highest amount of electricity generation in a year followed by the SMT and DMT systems, which resulted in 7% and 9% less electricity generation, respectively. As the worst performer, a CSP plant integrated with a ST system generates 20% less electricity over a year. This implies that despite having the same theoretical capacity, the real performance is not same for the alternatives. While these losses may seem egregious at first, large TES cost reductions are made possible in these alternatives due to the fact that a single tank or concrete can be used (noting that concrete is <1/20th the price per kg of molten salt). We propose herein that the true techno-economic advantage (or lack thereof) of choosing alternative TES systems should be judged by a ‘normalized cost of thermal energy storage (NCOTES)’ which normalizes the cost of storage systems with regards to their annual electricity generation capacity. According to our analysis, optimized DMT and ST systems without embedded pipes (e.g. bored or formed concrete) can achieve 55% and 46% reductions relative to the NCOTES of the two-tank system, respectively (with 650K as cut-off discharge temperature), while the SMT system has an 13% lower NCOTES than the two-tank system. With embedded pipes, however, the ST design has a higher NCOTES (+1% to +100%, depending on the discharge cut-off temperature), indicating that embedded piping is the biggest cost driver and that the elimination of piping should be a priority in ST systems. Overall, this study provides a methodology for the relative comparison of various sensible TES alternatives, and it gives insight into the most promising alternatives for moving beyond two-tank molten salt systems.

Diagnostic and prognostic of hybrid dynamic systems: Modeling and RUL evaluation for two maintenance policies

In the industrial sector, maintenance plays a very important role in carrying out production by increasing system reliability and availability. Thee maintenance decision is based primarily on diagnostic modules, prognostics and decision support. Diagnostic consists of detection and isolation of faults, while prognostic consists of prediction of the remaining useful life of systems. Moreover, recent industrial systems are naturally hybrid: their dynamic behavior is both continuous and discrete. This paper presents an integrating architecture of diagnostic and prognostic in a hybrid dynamic system. Indeed, the diagnostic system is based on controlling task execution times during system operation. This method is based on a general modeling approach using hybrid automata. The model proposed is detailed by studying a two-tank system. To validate the model, a Stateflow controller is used. These failures are anticipated by a prognostics process based on a prediction of the remaining life for each component by taking maintenance policy into account. Two new methods are compared: ABAO (As Bad As Old) and AGAN (As Good As New), based on the type of repair strategy.

Performance of a finned, latent-heat storage system for high temperature applications

A cost effective thermal storage system is required for the widespread application of the parabolic-trough solar thermal power generation technology. Such systems currently employ the sensible two-tank thermal storage system. The use of phase-change storage systems offers higher storage density and efficiency. However, due to the low thermal conductivity of suitable commercially-available phase-change materials (PCMs), heat transfer enhancement is required. Amongst heat transfer enhancement methods in latent-heat storage systems, the use of fins is the most practical for high-temperature applications. The heat transfer performance during the charging and discharging of a finned, latent-heat storage system was studied using CFD simulations for three commercially-available PCMs. During charging, natural convection plays a dominant role in the heat transfer. The quasi-steady effective heat transfer coefficient was found to depend on the heat transfer fluid (HTF) mass flow rate and the thermo-physical properties of the PCMs, but independent on the driving temperature difference. Although convection plays a role at the beginning of discharging, the main mode of heat transfer during the process is pure conduction. In this study, correlations that can be used for estimating the heat transfer rates during charging and discharging of a finned, latent heat storage system, using synthetic oil as the HTF, were developed.

Experimental comparison of new adaptive PI controllers based on the ultra-local model parameter identification

This paper is devoted to an experimental comparison between two different methods of ultra-local model control. The concept of the first proposed technique is based on the linear system resolution technique to estimate the ultra-local model parameters. The second proposed method is based on the linear adaptive observer which allows the joint estimation of state and unknown system parameters. The closed-loop control is implemented via an adaptive PID controller. In order to show the efficiency of these two control strategies, experimental validations are carried out on a two-tank system. The experimental results show the effectiveness and robustness of the proposed controllers.

Optimal design of a hybrid CSP-PV plant for achieving the full dispatchability of solar energy power plants

With the aim of providing fully dispatchable power using only solar energy, this paper focuses on a hybrid power generation system based on Concentrating Solar Power (CSP) and Photovoltaic (PV) plants. In particular, the CSP section is based on linear Fresnel collectors using thermal oil as heat transfer fluid, a two-tank direct TES system and an Organic Rankine Cycle (ORC) power plant. In the PV section, the PV array is coupled with a battery bank for electrochemical storage.The study evaluates the optimal design parameters (solar field area, TES capacity and ORC nominal power for the CSP section, nominal power of the PV array and battery capacity for the PV section) that minimize the energy production cost of the hybrid CSP-PV plant while the plant is constrained to follow a power output curve characterized by a constant power level. Moreover, to assess the influence of meteorological conditions, the study considers two different locations: Ottana (Italy) and Ouarzazate (Morocco).The results demonstrate that hybridization of PV and CSP technologies becomes highly cost-effective if a constant power output is required for daily time periods longer than about 16h, when the distinguishing feature of CSP plants of decoupling power generation from sunlight is effectively exploited, independently of the location under study. On the contrary, for production periods shorter than 8h, the use of a PV system coupled with a proper battery bank is the most cost-effective solution for both locations evaluated. Solar energy availability and unmet load fraction remarkably influence the hybrid plant design, especially for load duration periods in the range between 8 and 16h/day. As expected, due to the lower amount of available solar energy, the hybrid solar plant located in Ottana needs larger sizes of both CSP solar field and PV arrays than the same plant located in Ouarzazate resulting in higher energy production costs.

Embodied energy and cost of high temperature thermal energy storage systems for use with concentrated solar power plants

The intermittency of renewable energy systems remains one of the major hurdles preventing a large scale uptake of these technologies and concentrated solar power (CSP) systems are no different. However, CSP has the benefit of being able to store excess heat using thermal energy storage (TES). For the uptake of CSP with TES it must be demonstrated that the technology is both economically as well as environmentally feasible. This paper aims to investigate the economic and environmental impact of several TES options that are available for CSP systems. The investigated systems include an encapsulated phase change material (PCM) system, a coil-in-tank PCM system and a liquid sodium TES system. The economic impact in the current study refers to the capital cost (CAPEX) of each system including the tank, storage material, encapsulation cost (if applicable) and allowances for construction and engineering. The environmental impact of each system is accounted by calculating the embodied energy of each of the system components. Each storage system will be required to store a comparable amount of energy so that reliable conclusions can be drawn. The results from this analysis conclude that the encapsulated PCM (EPCM) and coil-in-tank system represent an embodied energy of roughly one third of the corresponding state-of-the-art two-tank molten salt system.Furthermore, the EPCM and coil-in-tank systems result in CAPEX reductions of 50% and 25% over the current state-of-the-art two-tank molten salt system. The liquid sodium system was found to result in higher embodied energy and CAPEX than any previously studied TES system. Finally, the advantages and disadvantages of each system was discussed and compared to previous literature.

Novel process for performance enhancement of a solar continuous adsorption cooling system

Adsorption cooling is regarded as one of promising clean technologies since it constitutes a rational way for the use of low-grade thermal energy (industrial waste, excess production, etc.) and renewable energy sources. The current paper provides fresh ideas on the relevant issues associated with the intermittence and low performance, which are considered among the main drawbacks of solar-driven adsorption cooling systems. Its primary aim is to design and analyze a solar continuous adsorption cooling system whose design consists mainly of combining a two-tank thermal energy storage system with two adsorbent beds. The operating process is based on storage of excess thermal energy, supplied by means of a solar parabolic trough concentrator, and on its subsequent recovery for producing an additional amount of cold.The simulation results indicate that, under the system conditions (design, working and weather), the solar coefficient of performance and daily cooling production are considerably improved when a latent heat storage unit is integrated with the system. The study reveals also that, thanks to this novel process, the number of refrigeration cycles achieved per day could be significantly increased, which means that the intermittence disadvantage could widely be overcome.

Thermo-mechanical parametric analysis of packed-bed thermocline energy storage tanks

A packed-bed thermocline tank represents a proved cheaper thermal energy storage for concentrated solar power plants compared with the commonly-built two-tank system. However, its implementation has been stopped mainly due to the vessel’s thermal ratcheting concern, which would compromise its structural integrity. In order to have a better understanding of the commercial viability of thermocline approach, regarding energetic effectiveness and structural reliability, a new numerical simulation platform has been developed. The model dynamically solves and couples all the significant components of the subsystem, being able to evaluate its thermal and mechanical response over plant normal operation. The filler material is considered as a cohesionless bulk solid with thermal expansion. For the stresses on the tank wall the general thermoelastic theory is used. First, the numerical model is validated with the Solar One thermocline case, and then a parametric analysis is carried out by settling this storage technology in two real plants with a temperature rise of 100°C and 275°C. The numerical results show a better storage performance together with the lowest temperature difference, but both options achieve suitable structural factors of safety with a proper design.

Temperature distribution and heat losses in molten salts tanks for CSP plants

Solar power plants have been deployed in the last 20years, so the interest in evaluating their performance is growing more and more. In these facilities, thermal energy storage is used to increase dispatchability of power. The two-tank molten salts storage system with “solar salt” (60wt.% NaNO3 and 40wt.% KNO3) is the one commercially used today. To be able to achieve a deep understanding of the two-tank solar storage systems with molten salts, in 2008 a pilot plant was built at the University of Lleida (Spain) and the experimental evaluation of the temperature distribution inside the tanks and their heat losses are presented in this paper. Therefore, this pilot plant is equipped with several temperature sensors inside the tank as well as in the different layers of external insulation. As expected, temperature is lower at the external part of the tank (near the cover, at the bottom and near the walls) and no stratification is seen. It is found that the influencing parameters in the temperature distribution of the salts inside the tank are: insulation, and the existence of different electrical resistances and the orientation and surroundings of the tank. Heat losses were measured and compared both with a simulated 1-D steady state model and previous literature. Measured heat losses were from 61W/m2 through the bottom to 80W/m2 through the walls (with 73W/m2 through the cover).

Attitude Dynamics and Control of Spacecraft with Multiple Liquid Propellant Tanks

AbstractThis paper focuses on the dynamics and control of spacecraft with multiple liquid propellant tanks. The liquid slosh dynamics are included by using an improved moving pulsating ball model. The moving pulsating ball model is an equivalent mechanical model that is capable of imitating the liquid reorientation process, especially for large-amplitude slosh. This model is improved by incorporating a static capillary force and an effective mass factor. The improvements are validated with previously published experiment results. The spacecraft attitude maneuver is implemented by the momentum transfer technique. The designed feedback control strategy is demonstrated to be efficient and robust for the coupled liquid-filled spacecraft system. For the two-tank system, two eccentric cases of concern are an asymmetric tank arrangement and an unbalanced propellant mass distribution in the two tanks. The effects of the two eccentric cases on sloshing torque and control torque are investigated. Some conclusions o...

Plate Diffuser Performance in Spherical Tank Thermocline Storage System

Thermal energy storage (TES) systems that store sensible heat in liquid media require the use of storage tanks. Spherical tanks require less building material and insulation, which might reduce the overall cost of a TES system while providing structural rigidity. The current study investigates an optimized plate diffuser in a thermocline spherical tank storage system to possibly increase the discharge flow rate without disrupting the thermocline region and without reducing the tank thermal efficiency. For low temperature (10–90 °C heat storage applications), such as heating, ventilation, and air conditioning (HVAC) and thermal water desalination, storing hot water in a thermocline system can increase the system thermal efficiency by up to 40% when compared to a fully mixed water tank and reduce the installation cost by 30% compared to a two-tank system. This study examines using a spherical tank in a thermocline system for such applications. A computational fluid dynamic (CFD) study simulated the discharge process from a spherical storage tank thermocline water system. Thermocline thickness and temperature profile in the tank were numerically determined for Reynolds number, Re = 600 and Froude number, Fr = 1.2; results were then experimentally validated. A CFD parametric study with (500 &lt; Re &lt; 7500) and (0.5 &lt; Fr &lt; 3.3): (i) determined the influence of tank flow dimensionless numbers (Reynolds, Froude, Richardson, and Archimedes) on thermal efficiency and thermocline thickness, (ii) produced an equation to predict the tank thermal efficiency using flow dimensionless numbers, and (iii) estimated the thermocline region volume occupation as a percentage of the total volume. The study of an optimized plate diffuser produced an equation for thermal efficiency based on Re and Fr numbers and estimated a thermocline volume equal to 15% of total tank volume. Flow rate ramp up by a factor of 3 was possible after the thermocline region was formed without losing tank thermal efficiency.

Thermodynamic analysis and optimization of an integrated Rankine power cycle and nano-fluid based parabolic trough solar collector

In this paper, the performance of an integrated Rankine power cycle with parabolic trough solar system and a thermal storage system is simulated based on four different nano-fluids in the solar collector system, namely CuO, SiO2, TiO2 and Al2O3. The effects of solar intensity, dead state temperature, and volume fraction of different nano-particles on the performance of the integrated cycle are studied using second law of thermodynamics. Also, the genetic algorithm is applied to optimize the net output power of the solar Rankine cycle. The solar thermal energy is stored in a two-tank system to improve the overall performance of the system when sunlight is not available. The concept of Finite Time Thermodynamics is applied for analyzing the performance of the solar collector and thermal energy storage system. This study reveals that by increasing the volume fraction of nano-particles, the exergy efficiency of the system increases. At higher dead state temperatures, the overall exergy efficiency is increased, and higher solar irradiation leads to considerable increase of the output power of the system. It is shown that among the selected nano-fluids, CuO/oil has the best performance from exergy perspective.

Online identification for batch processes in closed loop incorporating priori controller knowledge

It is of great importance to develop an online modeling method for chemical processes operated in closed loop for better understanding, monitoring the process or other purposes without endangering the system. This paper intends to devise an online system identification method, particularly for the batch process, by fully exploiting its intrinsic repetitiveness. It properly uses the information from the time direction and the batch direction, thus leading to a gradual performance enhancement. In addition, the identification method formulates the priori controller knowledge such as closed-loop stability as optimization constraints to refine the parameter estimates. A trust region method is employed to overcome the significant computation burden of directly handling these constraints such as solving Lyapunov inequalities. An adaptive filter is introduced to further smooth the parameter estimates. Finally, the effectiveness of the approach is verified by three numerical examples including a two-tank system.

Molten salt facilities, lessons learnt at pilot plant scale to guarantee commercial plants; heat losses evaluation and correction

This paper presents the importance of the thermal losses in the performance evaluation of thermal storage systems. In order to reinforce this statement, an evaluation of a pilot plant whose size is sufficiently representative for the extrapolation of results at larger scales has been carried out. The evaluation of the heat losses of a molten salt pilot plant with 8.1 MWhth built in Spain by Abengoa is presented. While the storage materials development has attracted a lot of attention from the researchers, the performance of a two-tank storage system has not been evaluated in detail. The relevance of the design of conventional systems such as insulation, mechanical assembly or foundation, are found to be the key for the feasibility of a TES system. Different performance scenarios were performed and based on experimental results, decisions for reengineering of the pilot plant could be taken to improve commercial storage plants.