Two-tank Systems Research Articles

A Comprehensive analysis of energy, exergy, economic and environment on integrated solarcombined cycle with various HTFs and thermal storage

Energy and exergy analysis are considered as important tools for the thermodynamic assessment of any power plant. The utilization of solar energy in electricity generation is progressively increasing day by day due to various issues associated with conventional power plants. In recent times, solar systems in integration with conventional power plants have emerged as popular and optimal solution. The current integrated solar combined cycle (ISCC) involves integrating a 15 MW linear Fresnel reflector with the existing 328.10 MW combined cycle power plant situated in North India. The proposed study examined the thermodynamic performance (energy and exergy) of ISCC and cascade thermal storage. The exergy loss in solar system, combined cycle, and thermal storage is quantified, followed by a comparative analysis of the impact of integrating the solar system and thermal storage on the exergy loss within the combined cycle plant. The results indicate that the collector-receiver experience the highest exergy loss at 69.41%, which is further reduced by 1.82% and 3.39% by employing therminol oil and molten salt, respectively, in thermal storage within the solar system. Additionally, integration of solar system & thermal storage with conventional system showed improvement in second law efficiency. The overall exegetic efficiency of the power system is improved by 1.38% and 3.12% utilizing molten salt as HTF with two-tank system and cascade TES respectively. CO2 emissions are reduced by 97,020 tons per annum with molten salt, which serves as a superior HTF compared to water. Cascade TES systems reduce a greater amount of CO2 emissions compared to two-tank TES systems, amounting to 172,260 tons per annum.

A transient heat losses model for two-tank storage systems with molten salts

Concentrating Solar Power (CSP) is a proven and mature technology for energy supply. In recent years, electricity generation based on this technology has increased worldwide, to a large degree due to its high dispatchability when coupled with an energy storage system. Commercially, most plants store the energy in molten salts in a two-tank configuration. This study focuses on this storage configuration and proposes a mathematical model for the thermal losses in these tanks, both at nominal conditions and during transients. With this model at its core, a computational tool for thermal performance analysis in OpenModelica is developed. This dynamic thermal model includes the estimation of local heat loss due to assembly defects, which are heat flows that cannot be determined by theoretical modelling. The development of a semi-empirical correlation for estimating local heat loss is also presented. Simulation results showed that this local heat loss may represent a share about 40% of the total heat loss in a small-scale tank. A comparison of the characteristics of the model proposed in a previous work is also presented to establish the innovation of the model. Two thermal storage systems with different tank designs and sizes were simulated to compare the results using the present model with data available in the literature. Results show good agreement in transient thermal behaviour of heat flows, temperatures, and cooling rates when compared with data from other authors for the same tank.

A metaheuristic approach for interval type-2 fuzzy fractional order fault-tolerant controller for a class of uncertain nonlinear system*

A new optimum interval type-2 fuzzy fractional-order controller for a class of nonlinear systems with incipient actuator and system component faults is introduced in this study. The faults of the actuator and system component (leak) are taken into account using an additive model. The Interval Type-2 Fuzzy Sets (IT2FS) is used to design an optimal fuzzy fractional order controller, and two different nature inspired metaheuristic algorithms, Follower Pollination Algorithm (FPA) and Genetic Algorithm (GA), are used to optimize the parameters of the fuzzy PID controller and Interval Type-2 Fuzzy Tilt-Integral-Derivative Controller (IT2FTID) for nonlinear system. The suggested control approach consists of two parts: an Interval Type-2 Fuzzy Logic Controller (IT2FLC) controller and a fractional order TID controller. Additionally, the two inputs of the IT2FLC are also calibrated using two fine tuning parameters and , respectively. The stability of the proposed controller is presented with some conditions. In addition to unknown dynamics, some unknown process disturbances, such as rapid changes in the control variable, are taken into account to check the efficacy of the proposed control scheme. Two nonlinear conical two-tank level systems are used in the simulation as a case study. The performance of the suggested approach is also compared to that of a widely recognized Interval Type-2 Fuzzy Proportional-Integral-Derivative (IT2FPID) Controller. Finally, the proposed control scheme's fault-tolerant behaviour is demonstrated using fault-recovery time results and statistical Z-tests for both controllers, and the proposed IT2FTID controller's effectiveness is demonstrated when compared to IT2FPID and existing passive fault tolerant controllers in recent literature.

A Review on the Performance Indicators and Influencing Factors for the Thermocline Thermal Energy Storage Systems

Thermal energy storage (TES) system plays an essential role in the utilization and exploitation of renewable energy sources. Over the last two decades, single-tank thermocline technology has received much attention due to its high cost-effectiveness compared to the conventional two-tank storage systems. The present paper focuses on clarifying the performance indicators and the effects of different influencing factors for the thermocline TES systems. We collect the various performance indicators used in the existing literature, and classify them into three categories: (1) ones directly reflecting the quantity or quality of the stored thermal energy; (2) ones describing the thermal stratification level of the hot and cold regions; (3) ones characterizing the thermo-hydrodynamic features within the thermocline tanks. The detailed analyses on these three categories of indicators are conducted. Moreover, the relevant influencing factors, including injecting flow rate of heat transfer fluid, working temperature, flow distributor, and inlet/outlet location, are discussed systematically. The comprehensive summary, detailed analyses and comparison provided by this work will be an important reference for the future study of thermocline TES systems.

Fuzzy-Optimized model reference adaptive control of interacting and noninteracting processes based on MIT and Lyapunov rules

Various system parameter variations occur during operations in several existing process industries. These parameter variations result in process shifts, thus, requiring adequate control strategies to compensate for these alterations, which consequently maintain desired system response. A paradigm is the coupled tank systems; in such systems, the level and flow of liquid must be adequately controlled to maintain the reaction equilibrium as well as to avoid spillage or equipment damage. The model reference adaptive control (MRAC) is an adaptive control strategy that creates a control law, subject to an adaptation gain, which causes the system’s plant to continuously track a reference model until a zero tracking error is achieved. The Massachusetts Institute of Technology (MIT) and Lyapunov approaches were used to develop the adaptation mechanism, which is used to adjust the parameters in the control law. Conventionally, a fixed value is adopted as the adaptation gain; however, the adaption gain can also be determined heuristically. The fuzzy logic control was used to optimally determine the value of the adaptation gain, which thus results in the fuzzy-optimized MRAC (FOMRAC) system. Consequently, these schemes were comparatively analysed for the control of the flow and level of liquid in coupled two-tank systems, arranged in noninteracting and interacting fashions. Using MATLAB/Simulink, results depicted that the FOMRAC systems had faster settling times in comparison with the fixed adaptation gain MRAC systems. Overall, the FOMRAC system based on Lyapunov rule yielded the lowest performance indices values. In addition, the scheme completely eliminated the overshoot that resulted from the implementation of the other schemes for the control of the interacting process.

Assessment of exergy delivery of thermal energy storage systems for CSP plants: Cascade PCMs, graphite-PCMs and two-tank sensible heat storage systems

Alternative cascade systems comprising of three, four, and five PCMs, PCM-graphite-PCM and a graphite system were compared with two-tank sensible heat storage systems. Numerical methods including an in-house code and Fluent were used to predict the transient heat transfer during the charging and discharging processes up to 6 h and 10 h, respectively. Energy and exergy analysis assisted in finding the configuration and size of cascade systems considering the material, temperatures and time constraints. Furthermore, the results of the energy and exergy analysis were used to compare the overall exergy efficiency for a useful work delivery of 680 MWhth to the power block. The exergy destructions due to heat transfer, the heat losses from the storage system during the charging process, and pumping power consumption were taken into account. These results show the high potential of cascade indirect systems of PCMs or graphite-PCMs to supply the required thermal energy to the power block with comparable exergy efficiency to the two-tank system. Moreover, this study presents a means for a first level design and selection of cascade systems to achieve the highest exergy efficiency for a specific exergy delivery bounded by a CSP cycle.

An experimental investigation of temperature distribution and heat loss in molten salt tanks in concentrating solar power plants

Two-tank molten salt storage systems, which increase the dispatchability of power, are widely utilized in commercial concentrating solar power plants. In the present work, a pilot solar thermal power plant was created to investigate temperature distribution and heat loss in the molten salt tank. The results showed that the molten salt had no temperature stratification at different liquid levels and operating temperature conditions except at the unprotected position near the electric heater. However, there was an obvious temperature difference in the upper atmosphere: the temperature dropped by 5.5 °C to 6.5 °C with every 400 mm increase in the height. During the cooling process, the temperature gradient in the tank foundation was basically stable. Experiments indicated that the liquid level of the salt affected the cooling rate of the molten salt significantly, while it had a little effect on the total heat loss in the tank. As the liquid level decreased from 2040 to 1039 mm, the temperature difference of the molten salt in the cooling process increased from 22.1 °C to 41.5 °C, but the total heat loss only changed by 4.3%. However, the molten salt operating temperature had a great influence on both the temperature change and the heat loss in the molten salt tank. When the operating temperature decreased from 557.7 °C to 376.9 °C, the temperature difference of molten salt in the cooling process decreased from 37.4 °C to 19.7 °C and the total heat loss in the tank decreased by 48.2%.

Experimental Study on Temperature Distribution and Heat Losses of a Molten Salt Heat Storage Tank

Two-tank molten salt heat storage systems are considered to be the most mature thermal storage technology in solar thermal power plants. As the key part of the system, the thermal performance of molten salt tanks is of great importance. An experimental thermal storage system with a new type of molten salt as a thermal energy storage medium has been built to investigate the temperature distribution of molten salt inside the tank during the cooling process from 550 °C to 180 °C. The temperature distribution of the salt was obtained, which reveals that temperature stratification appears at the bottom of the tank within the height of 200 mm. The position, with the maximum temperature difference of 16.1 °C, is at the lower edges of the molten salt storage tank. The temperature distribution was also measured to deepen our understanding of the insulation foundation, which shows that the maximum temperature appears at the middle upper part of the foundation and decreases radially. The heat losses of the molten salt tank were calculated by the classical equation, from which it was found that the heat loss decreases from 3.65 kWh to 1.82 kWh as the temperature of the molten salt drops from 550 °C to 310 °C. The effect of temperature stratification on the heat losses of the tank’s bottom was also analyzed.

The potential of metal hydrides paired with compressed hydrogen as thermal energy storage for concentrating solar power plants

Concentrating solar power (CSP) plants require thermal energy storage (TES) systems to produce electricity during the night and periods of cloud cover. The high energy density of high-temperature metal hydrides (HTMHs) compared to state-of-the-art two-tank molten salt systems has recently promoted their investigation as TES systems. A common challenge associated with high-temperature metal hydride thermal energy storage systems (HTMH TES systems) is storing the hydrogen gas until it is required by the HTMH to generate heat. Low-temperature metal hydrides can be used to store the hydrogen but can comprise a significant proportion of the overall system cost and they also require thermal management, which increases the engineering complexity. In this work, the potential of using a hydrogen compressor and large-scale underground hydrogen gas storage using either salt caverns or lined rock caverns has been assessed for a number of magnesium- and sodium-based hydrides: MgH2, Mg2FeH6, NaMgH3, NaMgH2F and NaH. Previous work has assumed that the sensible heat of the hydrogen released from the HTMH would be stored in a small, inexpensive regenerative material system. However, we show that storing the sensible heat of the hydrogen released would add between US$3.6 and US$7.5/kWhth to the total system cost for HTMHs operating at 565 °C. If the sensible heat of released hydrogen is instead exploited to perform work then there is a flow-on cost reduction for each component of the system. The HTMHs combined with underground hydrogen storage all have specific installed costs that range between US$13.7 and US$26.7/kWhth which is less than that for current state-of-the-art molten salt heat storage. Systems based on the HTMHs Mg2FeH6 or NaH have the most near term and long term potential to meet SunShot cost targets for CSP thermal energy storage. Increasing the operating temperature and hydrogen equilibrium pressure of the HTMH is the most effective means to reduce costs further.

Shell-and-tube or packed bed thermal energy storage systems integrated with a concentrated solar power: A techno-economic comparison of sensible and latent heat systems

A detailed techno-economic comparison—using annual, transient integrated system modelling—was conducted for sensible and latent heat thermal energy storage (TES) systems. As the most viable near-term competitors, thermocline/packed-bed and shell-and-tube configurations were compared to the conventional two-tank molten salt system. Concrete and a range of phase change materials (PCMs) were considered as the storage mediums in this study. All analyses were conducted for 15 h of storage capacity for the 19.9 MWe Gemasolar concentrated solar power (CSP) plant as a real-world case study.It was found that when the CSP plant is constrained to the operational within the tight bounds of the standard two-tank system, the dual-media thermocline (DMT) system with concrete or encapsulated PCM shows the best performance. Imposing rigid operational boundaries significantly disadvantages shell-and-tube (ST) systems (e.g. resulting in up to ∼50% less annual electricity production than a CSP plant with two-tank system). However, the results of this study reveal that extending the TES charge and discharge cut-off temperatures closer to the plant’s maximum and minimum operating range (i.e. 800 K and 650 K) can maximize the potential of dual-media TES alternatives. Under these loose operational conditions, the CSP plant will be required to occasionally operate at conditions which are far from its nominal design point (i.e. up to 75% variation). This flexibility allows all the TES alternatives considered in this study to achieve similar CSP annual electricity output as the two-tank system. In this case (e.g. all TES systems achieve the same annual output), the TES alternatives can be compared based on their specific costs rather than their levelized cost of electricity. Compared to the specific cost of two-tank molten salt systems, ∼24.5 US$ kWhth−1, a 62% reduction of specific storage cost was found to be achievable with concrete storage a dual-media thermocline (DMT) system, representing the best techno-economic option. This was followed by 49% cost reduction for a pipeless shell-and-tube (ST) system incorporating concrete or a PCM with 1 mm pipe thickness. Without minimizing the capsule thickness to 0.1 mm, the packed bed system with PCM would never have economic justification. Overall, this study reveals that if CSP systems can be designed to have more flexibility in their operational temperature range, a significant cost savings is available in moving to alternative TES systems.

Analysis of the effects of climatic conditions, loading level and operating temperature on the heat losses of two-tank thermal storage systems in CSP

This work presents a mathematical model to evaluate heat losses of a sensible heat storage tank for a concentrated solar power plant with solar salt as the storage material. The transient mathematical model takes into account the heat losses to the environment by evaluating the levels of solar irradiation, ambient temperature, and wind speed. Besides that, the model is simple and flexible and may not require considerable computational power. The model was validated using published experimental data for the solar power plants Solar Two and Andasol I. The transient analysis was performed by solving the diffusive terms of the differential equations resulting from energy balance and heat exchanges using the Finite Difference Method (FDM) in the implicit formulation for system discretization and the TriDiagonal Matrix Algorithm (TDMA) for solving the equations. This model allows evaluating individually the effects of several parameters inherent to the thermal storage system by two tanks. In this paper, thermal losses and storage efficiency were evaluated regarding the effects of climatic conditions, loading level, and operating temperature. Heat loss rates calculated by the proposed model differ from the experimental results by 4.55% for the hot tank, and 4.82% for the cold tank. The tank had no significant sensitivity to climatic conditions exhibiting a difference of less than 1% for storage efficiency. Heat losses relatively sensitive to the tank loading level, presenting a difference of 37.35% between the loaded and unloaded tank. The operating temperatures were also quite influential on the thermal storage efficiency, being 10.78% for the tank at 300 °C and 89.42% for the tank at 600 °C. In general, the tank showed little sensitivity to changes in climatic conditions, however, it showed considerable sensitivity to loading level and operating temperature. The heat loss of a heat storage system is a function of many variables and understanding the influence of each parameter in the system is essential for technological advances.

Determining the optimal cultivation strategy for microalgae for biodiesel production using flow cytometric monitoring and mathematical modeling

Trade-off between growth and lipid accumulation is one of the main issues in cultivation of microalgae for biodiesel production. Advantages of two-phase cultivation systems, with growth and lipid accumulation occurring in separate tanks, have been proposed. However, because the two-phase cultivation system requires more cultivation space and complex operation procedures compared to a single tank system, its advantage for lipid production is still uncertain. Furthermore, previous studies have not explicitly determined an optimal cultivation schedule, defining timing of the transfer of cells to the second tank and harvesting in the second tank. We therefore developed a model for lipid accumulation in the microalgae Chlorella sorokiniana to compare theoretical maxima of lipid productivity in single- and two-tank cultivation systems. We first established a monitoring method of growth and lipid accumulation using a flow cytometer, and then modeled the data by a logistic equation to derive a model for lipid accumulation. This model was used to theoretically optimize cultivation methods and harvest times to maximize lipid productivity in four different cultivation systems: single- or two-tank setups, with batch or fed-batch cultures, respectively. Results theoretically demonstrated that a two-phase cultivation system is slightly more productive than a single-tank one. The mathematical model analysis indicates strategies for further improving the productivity of each cultivation system without any additional investments in cultivation systems (i.e., nutrient, light, CO2). Our model analysis approach provides a theoretical basis for determining an optimal cultivation strategy of microalgae and identifies experimental data helpful for further improvement of cultivation systems.

Cyclic performance of cascaded and multi-layered solid-PCM shell-and-tube thermal energy storage systems: A case study of the 19.9 MWe Gemasolar CSP plant

A shell-and-tube heat exchanger which incorporates a sensible or phase change material (PCM) as the storage medium offers a potentially commercially viable alternative to the two-tank molten salt system. In particular, cascaded PCMs and multi-layered solid-PCMs (MLSPCMs) were investigated as proposed systems which can reduce the amount of storage material used and ensure optimal storage utilization. In this work, the performance of various thermal energy storage (TES) alternatives integrated into the 19.9 MWe Gemasolar concentrated solar power (CSP) plant (located in Seville, Spain) were compared with the conventional two-tank system. These alternative storage configurations were characterized by a single tank filled with a single, cascaded, or multi-layered storage media. Importantly, as a system-level study, this paper compared the performance of the design alternatives integrated with other CSP components in order to capture the effect of dynamic interactions between the storage system and other CSP components. Through a validated numerical investigation of the annual performance of the integrated systems, all the design alternatives were compared in the context of annual electricity generation, which represents the ultimate criterion to judge the true potential of each alternative. To conduct an apples-to-apples comparison, the storage capacity and geometric parameters were fixed. The design alternatives were categorized based on the storage materials involved and their percentages of occupancy in the TES tank (i.e. 12 storage groups and a total number of 45 design alternatives). It was found that the well-designed TES designs with cascaded PCMs performed similarly in charging and discharging (i.e. with a similar amount of total stored or delivered energy per cycle). This contrasts with a single PCM system, where there exists a significant difference between charging and discharging performance. The results of annual cyclic performance, under real-time operational conditions, indicated that a MLSPCM design configuration that was filled with a high melting point PCM in the top 25% of the tank, sensible concrete in the middle 50%, and a low melting point PCM in the bottom 25% of the tank had the best performance among all design alternatives studied. Moreover, it was found that changing the filler portions any one cascaded PCM group cannot significantly change the annual performance of the system. Contrary to much of the available literature – literature which does not consider system integration – it was shown that the shell-and-tube alternatives can only approach the annual performance of two-tank systems under ‘extended’ operational conditions (i.e. allowing temperature set points to float relatively far away from their fixed design points).

Thermomechanical testing under operating conditions of A516Gr70 used for CSP storage tanks

Thermal energy storage (TES) in molten salts is the storage dominating technology in solar power applications today. In two-tank molten salt storage systems energy density ranges from 30 to 70kWh/m3 are achievable. The salt material used is a binary system, composed of 60% NaNO3 and 40% KNO3. In the 8 MWhth pilot plant built and tested by Abengoa, the storage tanks were made of steel A516Gr.70 using the Appendix M of code API 650 for their design. A specific testing device was developed to evaluate thermo-mechanical properties, and a study was conducted in order to evaluate tensile properties of A516Gr.70 specimens under operation conditions for the hot tank at the pilot plant that is in contact with molten salts at 380°C. Results confirmed the outcomes of the work: the reduction of the yield limit, elongation before fracture, and Young modulus at 380°C after having been 5min immersed in molten salts. Moreover, after a creep-test simulating operating 7 days conditions during, an additional reduction of the yield limit was measured.

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.

Techno-economic assessment of thermal energy storage solutions for a 1 MWe CSP-ORC power plant

Organic Rankine Cycle (ORC) have principally been used in the past couple of decades to recover medium grade heat from sources such as geothermal, biomass and the exhaust of a realm of different industrial processes (waste heat recovery). In the past few years, this power generation technology has also been proposed in concentrating solar thermal power applications, aiming to exploit its features in intermediate temperature systems (low water consumption, water-free operation, scalability). These inherent features are best exploited if coupled with thermal energy storage systems to enable good performance in spite of the intermittent energy supply and also to increase the average load factor. To achieve these objectives though, a proper storage system tailored to the heat profile captured by the solar collector and to the characteristics of the power cycle must be identified.This paper discusses the cited design criteria and presents an analysis aimed at identifying the potential storage solutions to be implemented into a Concentrated Solar Power (CSP) plant for electricity generation operating at temperatures between (170°C) and (300°C). The system so developed will be integrated in the 1MWe CSP-ORC facility based on Fresnel technology that is currently under construction at Iresen’s premises in Benguerir, Morocco. Detailed transient models of performance of two-tank and thermocline storage systems are presented. The annual simulations carried out reveal that thermocline solutions are globally more attractive, since they exhibit similar thermal performance but at a much lower (30% lower) cost.

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).

Exergy analysis of discharging multi-tank thermal energy storage systems with constant heat extraction

Abstract In this work, an exergy analysis of discharging multi-tank thermal energy storage systems with constant heat extraction is studied. Theoretical models are developed to analyze the effect of the induced stratification through storage division into several tanks for different system configurations: multi-tank systems with even mass distribution, and two-tank systems with uneven and variable mass distribution. The instantaneous and the process efficiencies are defined to assess the performance of the system as function of operation parameters. For two- and four-tank systems with mass evenly distributed, efficiency increments around 4% and 7%, respectively, are possible. For two-tank systems, higher efficiencies are accomplished for short times when the mass is unevenly distributed and larger amount of the mass is in the hotter tank. For longer times, the even distributed mass situation provides the highest performance. To achieve superior efficiency of two-tank systems during the entire timespan, a time varying mass distribution χ = 0.5 ( 1 + e - 2 t ∗ ) is proposed. Finally, the energy losses as well as the pumping effects on the performance of one- and two-tank systems are analyzed. It is found that, both, the pumping effects and the energy losses are generally negligible and the efficiency can be improved dividing the storage system, rather than increasing the mass flow rate.

Single-tank TES System – Transient Evaluation of Thermal Stratification According to the Second-law of Thermodynamics

Abstract Single-tank thermal energy storage (TES) systems represent a valuable alternative, to the most common two-tank systems with molten slat, to effectively store thermal energy in concentrating solar power (CSP) applications.From an economic standpoint, the gap between the two TES solutions is relevant. A remarkable cost reduction can be achieved if a single-tank TES system, with a low-cost filler material, is exploited.In this kind of TES system,the buoyancydriven effects of the heat transfer fluid are exploited to establish and maintain a thermocline zone which separates the hot region on top and the cold region at the bottom of the tank.The thinner the thermocline thickness, the higher the thermodynamic quality of the stored energy. As soon as the TES is charged for the first time, i.e. startup of the system, the extent of thermal stratification may vary sharply during the first cycles before achieving a stable condition. For this reason, this study aims at evaluating, by means of accurate time-dependent 3D CFD simulations, the transient evolution of thermal stratification of a single-tank TES system exploited to fulfill the round-the-clock energy requirement of a reference 80 MWeCSP plant which uses air as heat transfer fluid.A total of 30 consecutive cycles, composed by charge/discharge phases, were simulated.Since the thermal energy stored is exploited to produce electrical energy, the performances of the TES system, operating under cyclic conditions, were qualitatively characterized by means of a stratification efficiency indexbased upon the second-law of thermodynamics.

Performance comparison of two-tank direct and thermocline thermal energy storage systems for 1 MWe class concentrating solar power plants

This paper compares the performance of medium size CSP (Concentrating Solar Power) plants based on an ORC (Organic Rankine Cycle) power generation unit and using linear Fresnel collectors, thermal oil as heat transfer fluid and two-tank direct and thermocline energy storage systems. The comparative performance analysis was carried out by means of specifically developed simulation models and considering different values of solar multiple and thermal energy storage capacity. The results of the performance assessment demonstrate that if the CSP plant has to be optimized for the highest specific energy production, that is, for the highest solar energy conversion efficiency, two-tank energy storage systems show slightly higher performance than thermocline storage systems. However, the study also demonstrates that thermocline storage systems can be an interesting option to reduce the energy production cost of CSP plants.