Single-tank Thermal Energy Storage Research Articles

Study on off-design performance of supercritical CO2 cycles coupled with single-tank thermal energy storage under variable heat source temperatures and partial loads

Study on off-design performance of supercritical CO2 cycles coupled with single-tank thermal energy storage under variable heat source temperatures and partial loads

Design and experimental analysis on a single tank energy storage system integrated with a cooking unit using funnel system

The paper presents a new innovation in small scale thermal energy storage system suitable for cooking application. A single tank thermal energy storage system integrated with a cooking unit has been developed and the performance analysed. The system consists of a heat storage tank, a heating chamber referred to as a funnel, and the cooking unit. The heating chamber is a pipe made of cast iron, and on top of the pipe is a cylindrical container made of Aluminium. A heating element of 1.5 kW 220 V was inserted inside the funnel and was used for heating the oil. The top of the funnel allows a cooking pot to be inserted inside it. The funnel can be raised or lowered using screws connected to the top of the tank; this regulates the temperature at which oil begins to overflow into the heat storage tank. The heat storage tank is made of stainless-steel, and contains sunflower oil as a heat storage medium. During charging, a small volume of oil about 2 L was heated inside the funnel, and the temperature rose rapidly to about 150 °C in less than 15 min. The system allows for cooking to begin immediately after charging has been started. As heating continues, the hot oil overflows into the storage tank, and cold oil at the bottom of the tank flows into the funnel where it is heated. The storage tank had 18 L of sunflower oil. During discharge, the direction of oil flow reverses; hot oil flows from the top of the tank into the funnel. The hot and the cold oil do not mix during discharge, and therefore, the heat front is preserved. Cooking tests were demonstrated during charging and discharging cycles. About 453.6 kJ of energy was used in boiling 1.5 L of water in 10 min, and 1.04 MJ was used in boiling 1 kg of dry beans in about 2 h.

Thermodynamic analysis of molten salt-based single-tank thermal energy storage system with heat transfer enhanced by gas injection

In this study, the feasibility of gas injection for enhancing heat transfer in a molten salt thermal storage system was experimentally investigated. The convective heat transfer coefficient induced by the gas injection was quantitatively measured. A predictive model was developed to estimate the heat transfer coefficient with respect to the gas injection rate. A comparative analysis between gas-injection and non-injection cases was also performed from a thermodynamic perspective to verify the feasibility of the method. The heat transfer coefficient was within the range of 359–1283 W/m2-K, depending on the gas injection rate. The developed model estimated the measured values within an average absolute error of 2%. From the comparative analysis, the gas injection method improved thermal efficiency from 86.3% to 95.1%. The energy loss induced by the gas injection was within the range of 0.2%–1.1% of the total stored energy. The gas injection also shortened the energy discharge time by 32%–41% owing to the heat transfer enhancement. The proposed method also improved the system performance by enhancing the exergy transfer rate from the salt to the water. These results indicate that a gas injection method is a potential candidate for improving molten salt thermal storage systems.

Research on dimensionless structure size of coil heat exchanger in heat discharging process of molten salt single tank heat storage system

• Optimum dimensionless size of coil heat exchanger was studied by simulation. • The optimal size of heat exchanger appears under the combined action of vortex and molten salt flow rate. • When the dimensionless pitch L/d =4, the heat exchanger has the best heat discharging performance. • When the dimensionless pitch is the best, the optimal dimensionless diameter is 0.25. There has been an increasing attention on single-tank thermal energy storage, attracted by the low cost, high efficiency and compactness in the operation. However, the most researches on single-tank thermal energy storage focused on the way of heat discharging and the arrangement of heat exchangers. Few studies on the dimensionless size of heat exchanger, which is important for the design of single-tank thermal energy storage, were carried out. In order to obtain higher heat exchange efficiency and design criteria of coil heat exchanger, the optimal dimensionless size of heat exchanger is studied by numerical simulation. The results show that both the dimensionless pitch and the dimensionless pipe diameter of the coil heat exchanger have a great influence on the heat discharging performance when the air is used as the heating medium and the molten salt is used as the heat storage medium. With the change of dimensionless pitch, the heat discharging performance and the flow field of molten salt are affected. When the dimensionless pitch L / d = 4, the heat discharging power and heat release are the highest. At this time, the temperature and flow field of molten salt are evenly distributed, and no temperature stratification and vortex are formed. When the dimensionless pipe diameter is changed on the premise of the optimal dimensionless pitch, the heat discharging power increases first and then decreases with the increase of dimensionless pipe diameter. When the dimensionless pipe diameter d / a =0.25, the molten salt flow rate in the single tank is relatively large and no vortex hindering heat transfer, and the heat discharging performance is the best. Therefore, under the joint action of molten salt velocity and vortex, the optimal dimensionless pitch and pipe diameter appear. The research results provide a theoretical basis and an example for the design of single tank energy storage system.

Performance Analysis of Thermal Energy Storage System Integrated with a Cooking Unit

This paper presents an experimental study on a single tank thermal energy storage (TES) system integrated with a cooking unit. The tank had a capacity of 45 L of oil. The cooking chamber was embedded in the storage tank, thereby eliminating the use of pumps and connecting pipes between the cooking unit and the storage unit. The system was designed to make good physical contact, circumferential and basally, with the cooking pot, to improve the rate of heat transfer. Experimental tests were performed with oil only and oil–rock pebbles as sensible heat storage materials. The charging unit was connected to the TES unit in such a way that it allowed circulation of oil between them during charging, using the thermosiphon principle. An electric heater rated at 800 W 240 V was inserted into the charging unit to charge the system. The thermal performance of the TES systems was evaluated in terms of the charging temperature, heat retention capacity, energy stored and cooking efficiency, and the overall heat lost coefficient. The results showed that the oil–rock system performed best, with a cooking efficiency of 64.9%, followed by the oil-only TES system, with 60.3%. Further tests on cooking indicated that the system was able to cook beans in 2.25 h and 2.0 h using the oil only and oil–rock pebbles thermal energy storage systems, respectively.

Study on Thermal Performance of Single-Tank Thermal Energy Storage System with Thermocline in Solar Thermal Utilization

For the intermittence and instability of solar energy, energy storage can be a good solution in many civil and industrial thermal scenarios. With the advantages of low cost, simple structure, and high efficiency, a single-tank thermal energy storage system is a competitive way of thermal energy storage (TES). In this study, a two-dimensional flow and heat transfer model of a cylindrical storage tank with water as heat transfer fluid (HTF) is developed, in which the effects of time, flow velocity, and height-to-diameter ratio of the tank on the thermocline thickness have been highlighted. The results show that the thermocline thickness in the storage tank is increasing during the charging and discharging processes, and it increases with the increase of the inlet flow velocity and the height to diameter ratio. It is emphasized that in our cases when the time period of t is 14,400 s, the fluid inlet velocity of uin is 4.577 × 10−4 m/s, and the height-to-diameter ratio of H/D is 1.2, the performance evaluation index reaches the maximum 0.9575, and the efficiency of the system is the highest. It is expected that all of the findings herein can provide a fundamental understanding of the design and operation of the single-tank thermal energy storage system in water heating for civil and industrial applications.

Three-Dimensional Thermocline Dynamics in Thermal Storage Tanks

In this work, a series of three-dimensional unsteady numerical simulations are performed to study the stability and interface dynamics of a thermocline-based lab-scale single tank Thermal Energy Storage system (TES). The stability of thermocline is analysed by introducing relatively cold fluid for a short period at the inlet of the TES. Numerical simulations are performed for three inlet flow disturbances (weak, medium and strong) and three stratification levels (sharp, moderate and large). The fluid injected at the inlet rolls-up and interacts with the thermocline which causes spatio-temporal disruption of the stable stratification inside the TES. It was found that the three-dimensional simulations bear some resemblance to the two-dimensional case but also show crucial differences. The propagation of the injected cold fluid and the subsequent interaction with the thermocline are analysed. A wide gamut of flow structures is identified inside the TES depending on the degree of stratification and level of disturbance. Finally, the oscillatory nature of interface and associated mixing mechanism are addressed. The simulation indicates that the oscillations at the interface are through the successive generation of countersign vorticity which retards/suppresses the propagation of the vortex ring. In the case of large interface, internal waves are generated by the periodic array of vortices which generates a standing wave pattern near the Brunt-Va ̈isa ̈la ̈ frequency‎.

Improved thermocline initialization through optimized inlet design for single-tank thermal energy storage systems

Single tank thermal energy storage systems based on the thermocline concept have attracted large interest in the last years at both, scientific and industrial levels, as cost-effective alternative to the commercially available and proven molten salt double tank storage system. Recently, many experimental and modeling results of these systems validating the technology and addressing guidelines to optimize its thermal performance have been reported. However, high cyclic efficiencies predicted in simulation have not been reported to be achieved experimentally. This paper addresses one of the reasons for this discrepancy, i.e., the fact that the additional temperature stratification volume generated by the fluid distribution at the tank's inlet is typically not considered in theoretical models. This paper demonstrates that assuming idealized flow distribution in the tank's cross-section will not represent reality adequately. Using this approach, the thermocline thickness at the beginning of the charge operation can be misestimated by more than 50%. Furthermore, a sensitivity analysis of the geometric characteristics of a specific radial fluid distributor type is included in this work. A theoretical model is developed for this purpose and validated with experimental data. The results of the analysis show that the fluid distributor should always be located in the upper-most part of the tank to minimize the initial thermocline thickness. Besides, the analysis of the cross-sectional area occupied by the distributor has been analyzed. It is shown that an optimal value exists for this parameter in order not to inhibit thermal stratification and to create further temperature gradients by subdividing the tank and thereby creating fluid-dead volumes.

Novel designs of hybrid thermal energy storage system and operation strategies for concentrated solar power plant

Packed-bed thermal energy storage (PBTES) has advantage of being relatively low cost, but suffers from low utility factor, compared with two-tank thermal energy storage (TTES). This paper proposes two new designs of hybrid thermal energy storage system (HTESS), consisting of PBTES and TTES, and corresponding operation strategies: HTESS-TS for thermocline storage and HTESS-OTC for outlet temperature control. Firstly, structures and operation strategies of HTESS-TS and HTESS-OTC are described in detail. Then, thermal and economic performances of HTESS and single-tank thermal energy storage system (STESS) only containing PBTES in stand-alone state are compared. Next, effects of cut-off temperature and thermal capacity of TTES are analyzed. Finally, under realistic solar radiation, annual performance of concentrated solar power plant (CSP) with different thermal energy storage systems are compared. Results show that compared with STESS, utility factors of HTESS-TS and HTESS-OTC are improved by 12.5% and 22.1% respectively. Meanwhile, unit cost of HTESS-OTC is 8.6% lower than that of STESS. In addition, for a broad range of outlet temperature limits, HTESS-OTC can maintain more stable outlet temperature, higher utility factor than STESS. Compared with STESS, annual generated electricity induced by HTESS-TS and HTESS-OTC increase by 9.8% and 14.1% respectively.

Single-tank thermal energy storage systems for concentrated solar power: Flow distribution optimization for thermocline evolution management

Concentrated Solar Power (CSP) technology captures solar radiation and converts it into heat for electricity production. It has received an increasing attention because integrated thermal energy storage (TES) systems can largely enhancing the reliability and the dispatchability. Over the last decade, low-cost single storage tank based on the thermocline technology becomes an alternative to commonly-used two-tank TES system. However, the improper inlet/outlet manifolds may cause the strong mixing of hot and cold fluids and disturb the temperature stratification, resulting in reduced thermal performances of the storage tank. This study aims at solving the flow maldistribution problem in the single-tank thermocline storage system by appropriately structuring the inlet/outlet manifolds. The technical solution is based on the insertion of optimized perforated baffles in the manifolds. 2D Computational fluid dynamics simulations were performed to calculate the transient flow and temperature profiles in the storage tank during the charging and discharging operations. The optimal size distribution of orifices on the upper baffle has been determined for homogenizing passage times of the thermal front, so as to enhance the temperature stratification. A novel intermediate evaluation indicator was introduced to characterize the real-time thermal behavior, which could reduce the computational cost of the optimization problem by a factor of 6 at least. Numerical results shown that the proposed optimization algorithm could significantly improve the thermal performances, indicated by the increased values of charging/discharging efficiency, the capacity ratio and the overall efficiency, ex., the fully charging efficiency be increased by 29% by comparing the unstructured manifold geometry and the one with optimized baffles. The parametric study on certain geometry and operating factors also demonstrated that the proposed method for flow distribution optimization was robust, effective and efficient.

Heat discharging and natural convection heat transfer performance of coil heat exchanger in single molten salt tank

There has been an increasing attention on single-tank thermal energy storage, attracted by the low cost, high efficiency and compactness in the operation. The most researches on single-tank thermal energy storage focused on temperature stratification to improve its heat discharging efficiency using water as heat storage media. The present research aims at investigating heat discharging performance by improving natural convection heat transfer of molten sat around coil heat exchanger in single heat storage tank. The effect of different parameters of the coil heat exchanger on the heat discharging performance, flow field and temperature distribution are analyzed and discussed in the single molten salt tank. The results show that the heat discharging performance of the coil heat exchanger can be enhanced by increasing the buoyancy-driven flow around the coil heat exchanger. The heat discharging efficiency can be improved by the optimal arrangement of coil heat exchanger for the highest velocity can be induced around it, which results in a fully mixed of molten salts and its temperature turns to uniform, instead of temperature stratification. Based on the results, a correlation among the natural convection Nu number with Ra number and other heat exchanger parameters in a single molten salt tank is obtained by numerical results. The research results provide a theoretical basis and paradigm for the design of single thermal storage tank energy storage system.

Comparative experimental study on parabolic trough collector integrated with thermal energy storage system by using different reflective materials

The present study focuses on the performance analysis of parabolic trough collector (PTC) incorporated with single-tank thermal energy storage (TES) system with the use of two different reflective materials in PTC which includes glass and high-reflective stainless steel sheet. It consists of TES tank which contains 110 L of Therminol-66 oil (heat transfer fluid). The experiments are conducted for the use of glass reflective material PTC and stainless steel reflective material PTC when both are integrated with TES system. The various performance parameters like useful energy gained by the PTC, energy collected in the PTC, PTC efficiency, heat loss coefficient and the stored amount of energy in TES tank are calculated to know the overall efficiency of the system during daytime. It has been concluded that the use of glass as the reflective material in PTC has better useful heat gain and high average PTC efficiency in comparison with stainless steel reflective material. However, the overall system performance is almost equal to both reflective materials. Silver-coated stainless steel reflective material may be preferred in PTC when it is coupled with single TES tank system, due to cost effectiveness and ease availability with a high-reflective power about 98.9%.

CSP plants with thermocline thermal energy storage and integrated steam generator – Techno-economic modeling and design optimization

Although CSP has reached technological maturity, high capital investment and specific electricity cost remain the major development barriers. To reduce them, highly efficient, integrated, and cheaper CSP components are urgently needed. In this paper, we investigate a novel CSP plant configuration with a single-tank Thermal Energy Storage (TES) fully integrated with the steam generator.The objective of this research is twofold: i) provide a reliable model of single-tank thermal storages with integrated steam generator; ii) identify two optimized CSP plant designs to achieve best energetic and economic performances. To achieve these aims we developed a numerical model of the main system components and validated it against experimental data. This model was then integrated in a full simulation and heuristic design optimization of the plant.The results revealed that the system proposed can generate electricity in middle-Italy (Rome) at a cost of 230.25 $/MWh with a 15% reduction compared to the double tank option. Furthermore, if cogeneration is used to recover the waste heat, this system is an interesting option for users such as small districts, university campuses and hospitals. In the latter case, the optimized system pays off in 6 years and covers 80% of the heating and cooling requirements.

Short and long-term sensitivity of lab-scale thermocline based thermal storage to flow disturbances

Molten-salt thermocline-based systems are a low-cost option for single-tank thermal energy storage in concentrated solar power plants. Due to the high variability in solar energy availability, these energy storage devices are subject to transient heat loads during charging that can affect the storage efficiency. Numerical simulations were conducted to analyze the stability characteristics of a lab-scale thermocline tank subject to a flow disturbance during charging under different operating temperatures. The charging process was first simulated at a constant Reynolds number for three different Atwood numbers; a stably stratified fluid layer develops inside the storage tank in all cases. A flow disturbance was then introduced at the inlet of the stratified thermocline tank by inserting colder fluid for a short period of time. The disturbance interacts with the thermocline and causes oscillations and mixing. The thermocline oscillations are under-damped and lead to an increase in thermocline region thickness. The transient behavior of the thermocline and the decay rate in its oscillations were analyzed; the damping time depends on the Atwood number. The persistence of flow disturbance effects during long-term cyclical operation was also investigated. Several charge/discharge cycles were simulated at constant Reynolds number to obtain a time-periodic thermal response for each Atwood number. The characteristic flow disturbance was introduced at the inlet during a single charging process, and the thermocline region was observed during several subsequent charge/discharge cycles to assess the long-term temporal attenuation of the disturbance. The thermocline almost fully recovers to the time-periodic behavior after a single cycle.

Spatio-temporal disruption of thermocline by successive laminar vortex pairs in a single tank thermal energy storage

The stratification efficiency of single tank thermal energy storage is affected by the internal mixing processes, especially in the thermocline region due to disturbances of different kinds. To study the mixing dynamics at the interface, we have conducted detailed numerical and supporting experimental studies for different Atwood numbers (stratification levels). Numerical experiments were conducted with two successive vortex pairs with three different time-lags (short, medium and long). For the short time-lag case, the preliminary vortex pair merges with the ensuing vortex pair. The merged single vortex pair peels back the thermocline layer causing mixing of the hot and cold fluids. The thermocline thickness increases as a result of the entrainment of the cold fluid into the hot fluid. The peeling process continues until buoyant forces leads to plume like structures that penetrate into the lighter fluid. For the medium and large time-lag cases, such merging of vortices was not observed. The vortex pair interacts separately with the thermocline region. The plume structure created by the first vortex pair interacts with the ensuing vortex pair. The altered interface (thermocline) thickness strongly depends on the nature of the vortex–thermocline interaction mechanisms. The thermocline effectiveness decreases consequent to such interactions and have been quantified in details in the current work.

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.

CFD-based reduced model for the simulation of thermocline thermal energy storage systems

Thermocline thermal storages are widely used in energy systems. Computational Fluid Dynamic (CFD) can be used for an accurate simulation of the physical phenomenon but its implementation in system-level annual simulations is hardly possible because of the huge computational time required. The present paper proposes a novel approach for the utilization of CFD simulation results in system-level annual simulations and optimizations. An analytical function able to represent the dimensionless vertical temperature profile inside the tank is parameterized statistically using the results of multiple simulations of a CFD model, which have been previously validated with experimental data. The reduced model obtained is then compared to other CFD simulations under highly variable conditions, showing a satisfactory degree of agreement (the mean absolute error and the error standard deviation are calculated to be 1.52 K and 1.93 K respectively). Furthermore, it is demonstrated that this approach can be conveniently adopted for the modeling of a wide range of systems with a single tank thermal energy storage, from Concentrated Solar Power to District Heating.

Spatial and temporal modeling of sub- and supercritical thermal energy storage

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system that can operate from the two-phase (liquid–vapor) to supercritical regimes for storage fluid temperatures typical of concentrating solar power plants. State-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high capital cost of the salts (International Renewable Energy Agency, 2012). The alternate approach explored here opens up the use of low-cost fluids by considering operation at higher pressures associated with the two-phase and supercritical regimes.The main challenge to such a system is its high pressures and temperatures which necessitate a relatively high-cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers.A thermodynamic model is used to evaluate system performance; in particular it predicts the volume of tank wall material needed to encapsulate the storage fluid. The transient temperature of the tank is observed to remain hottest at the storage tank exit, which is beneficial to system operation. It is also shown that there is an optimum storage fluid loading that generates a given turbine energy output while minimizing the required tank wall material.Overall, this study explores opportunities to further improve current solar thermal technologies. The proposed single-tank system shows promise for decreasing the cost of thermal energy storage.

Optimal Design and Operation of a Solar Energy Receiver and Storage

Optimization of design and operation is presented for a solar energy receiver combined with a thermal energy storage. The concentrated solar power on-demand (CSPonD) concept, which can be described, in brief, as a volumetric solar energy receiver system combined with a modified raft thermocline concept, is considered. The CSPonD concept is assumed to be providing heat for a general cogeneration scheme where power production is the main product of the cogeneration. With a constant power production, a secondary process is assumed to consume the process heat from the CSPonD and power cycle. Models are developed for thermal analysis of the energy storage, taking into account hourly and seasonal variations in the solar energy as well as the heliostat field efficiency. Nonlinear programming (NLP) is used for optimization of the design and operation. The sequential method of optimization and a heuristic approach (parallel computing) are implemented using an equation-oriented modeling environment and gradient-based local solvers. A strategy is presented to design and operate the plant, considering the significant seasonal variations in the solar energy. Three case studies are presented. The first one optimizes the design based on a design day and a desired thermal duty. The other two address optimal yearly operation of the plant. The results of the optimization case studies show that (a) the CSPonD concept aids in handling variations (hourly, daily, and seasonal) in solar energy, (b) CSPonD is a promising concept for cogeneration, (c) the mass of salt required in the CSPonD concept is not significantly lower than the salt required in a single-tank thermal energy storage system.