Thermocline Thickness Research Articles

Numerical analysis of thermocline evolution during charging phase in a stratified thermal energy storage tank

Thermocline thickness (TLT) is the best parameter to quantify the thermal performance of stratified thermal energy storage (TES) tanks as it defines the inactive part of a storage medium. A detailed literature review reveals that there is no consensus in the community on the temperature band where the TLT is quantified. In this study, a new criterion for quantifying TLT is proposed that addresses its dynamic nature and is based on 1K below Tmax and 1K above Tmin inside a TES tank. Additionally, an ideal charging diffuser is defined that can attribute to the minimum theoretical TLT at a certain TES tank height to diameter ratio and charging mass flow rate. Therefore, the effectiveness of any enhancement strategies for improving TES thermal performance in a real working condition can be compared against the ideal case. The proposed criterion is applied during the charging process of a cylindrical TES tank through computational fluid dynamic (CFD) simulation by solving the coupled hydrodynamic (continuity and momentum) and energy equations in a 2D axisymmetric computational domain for a real TES tank. The accuracy of the obtained results is validated against available experimental data and a good agreement is observed. The main observation is that the ideal minimum TLT evolves proportional to the square root of dimensionless charging time divided by Peclet number, however, with the actual diffusers when heat loss to the ambient air and mixing effect are present, TLT undergoes three formation stages, namely, mixing dominated, developing, and fully developed regions. The evolution of TLT in the dynamics of the TES charging process is reported at different working conditions.

Thermal stratification in a solar hot water storage tank with mantle heat exchanger

Solar hot water storage tanks with mantle heat exchangers are widely used in balcony wall-mounted solar heating system in China. The thermal behaviour of a novelty mantle solar water tank with vertical inlet and outlet water tubes has been investigated both experimentally and numerically. Dimensionless parameter can well characterize the stratification of water and thermocline dynamics in the water tank. In the discharging mode, as the dimensionless time increases, the thermocline thickness gradually increases, and the mixing in the water tank intensifies. The dimensionless exergy gradually decreases with the increase of the dimensionless time.

Thermal performance of naturally stratified water storage tank in a power plant: a comparative study

Thermal storage is one of the key technology in the transformation of thermal power flexibility. A model of naturally stratified water storage tank is established, and CFD analysis of heat release process is made. As can be seen from experimental results in a power plant, in the thermocline thickness in the tank increased from 0.43 m to 0.66 m, with an average hourly thickness of 0.06m, which has well agreement with the CFD analysis results. The effective height of the tank is 29.836m, from which we can calculate that its thermal storage efficiency is as high as 96.7%.

Effect of Diffuser Height on Thermocline in Stratified Chilled Water Storage Tank

Chilled water energy storage using thermal stratification technique currently used in the vast area because it contributes to reducing energy consumption and refrigeration capacity as well as its maintenance, operating and capital costs are low. In this paper, experimental tests were carried out on a small-scale vertical cylindrical storage tank equipped with an elbow-type conventional diffuser at inlet heights of 20, 170, 320 and 470mm for flow charging rates from 1.5-7.5l/min. in order to obtain a good thermal separation. The degree of stratification was estimated by means of temperature distributions and performance metrics, which involve thermocline thickness, the half-cycle figure of merit and equivalent lost tank height. The results show that the decrease in diffuser height above the tank floor tends to the steep thermocline or satisfactory thermal separation, the stratification and thermal performance were obtained at diffuser height of 20 mm within the limiting volume flow rates 1.5-4.5 l/min. better than those at volume flow rates ranging from 5.5-7.5l/min. and much better than at diffuser heights of 170, 320 and 470mm for various flow rates.

Subtropical modulation of the equatorial undercurrent: a mechanism of Pacific variability

Combining data from Argo and the TAO buoy array we present new observations of variability in the Pacific Ocean. Argo profiles reveal the development of a thickness anomaly in the lower levels of the ventilated thermocline of the South Pacific in 2010. Data through 2017 show this anomaly propagating as a baroclinic wave westward and towards the equator. Theory suggests that this wave will reduce the velocity of the equatorial undercurrent (EUC) when it reaches the equator, transitioning the equatorial Pacific to a warm state. This is supported by TAO array observations that show a past decadal shift in EUC strength around 2000, as well as radiocarbon coral measurements which suggest a similar change in the 1970s, both of which align with phase changes in Pacific decadal variability. Using model simulations with enhanced vertical resolution in the thermocline, we affirm this link between the subtropical south Pacific thermocline and the EUC, which also manifests in eastern Pacific sea surface temperatures. We combine these results to hypothesize a mechanism that may explain some of the decadal variability observed in the Pacific. This mechanism relies on the propagation of anomalies in the structure of the ventilated thermocline from the southeastern Pacific to the equatorial Pacific, modulating the strength of the EUC. The cycle is enhanced by atmospheric teleconnections between the equatorial Pacific and the southeast Pacific that periodically reverse the anomaly in thermocline thickness. If correct, our hypothesis predicts a return to a warm state of the equatorial Pacific when the Kelvin wave reaches the equator and the thermocline adjustment slows the EUC.

Исследование временной изменчивости параметров вертикальной структуры вод в прибрежной зоне южного Крыма

The issues of the formation and temporal variability of the vertical structure of the Black Sea waters off the coast of the southern Crimea are considered. A large array of instrumental measurement data obtained in the second half of 2013 (June–December) during operational monitoring of the vertical temperature profile in the area of Stationary oceanographic platform of the Marine Hydrophysical Institute in the coastal zone near Cape Kikineiz is used. The measurements were carried out using a thermoline, which is a system of temperature sensors distributed in depth from the surface to the bottom. To study the vertical structure of the temperature field, a model of two-layer stratification is considered, when two quasi-homogeneous layers (the upper one is warmer and the lower one cold) are separated by a layer of large vertical temperature gradients—the thermocline. A number of parameters of the vertical structure of waters were calculated: the vertical temperature gradient, its maximum values and depth of occurrence, the temperature of the thermocline core (if it was recorded) and quasi-uniform layers, the temperature difference between these layers, and the thermocline thickness. The median estimates of the daily values of the listed parameters are analyzed. Specific cases of transformation of the vertical profile of water temperature during the upwelling and downwelling processes are considered. It is shown that the temporal variability of the parameters of the vertical structure is significant, especially in the summer hydrological season. The main contribution to the variability is caused by the seasonal course of the heat flow from the atmosphere through the sea surface and surge processes (upwelling and downwelling) in the summer period caused by abrupt changes in the alongshore wind component. In the autumn hydrological season, the thermocline is recorded sporadically with a slight gradient in the form of the lower boundary of the subsurface layer of daytime heating, or a thin bottom layer.

Comparative Evaluation of Circular Truncated-Cone and Paraboloid Shapes for Thermal Energy Storage Tank based on Thermal Stratification Performance

In solar water heating systems, geometrical configurations of thermal energy storage (TES) play a crucial role in the enhancement of system performances. Upon conducting a thorough study on the influence of different shapes and aspect ratios of TES tanks, the present study focuses on the choice of suitable TES tank geometry based on the key performance parameters, viz. Richardson number, stratification number, storage efficiency and the initial thermocline thickness. Thermal stratification characteristics are investigated and analyzed in detail for three chosen shapes of TES, namely truncated-cone, paraboloid, and cylinder, using a two-dimensional unsteady numerical model. Results of the numerical model on cylindrical TES is found to have good agreement with the findings of the in-house experiments. With increasing aspect ratio (AR) of the paraboloid and truncated cone TES tanks, initial thermocline thickness and the energy losses increase, whereas, Richardson number and average stratification number decrease, indicating better stratification performance in storage tanks with lower AR. However, there is a limit on reducing the AR. At every AR investigated for the two shapes of TES, Richardson number for the paraboloid TES is greater than that of the truncated-cone TES, qualitatively suggesting a better temperature stratification in the paraboloid TES. The effect of flow rate on the thermal stratification characteristics is also studied, which suggests that increasing the flow rate enhances mixing and deteriorates stratification. The findings can be used as guidelines for designing TES with improved thermal performance.

A systematic pinch approach to integrate stratified thermal energy storage in buildings

In this study, dynamic streams including space heating, domestic hot water, renewable solar thermal energy and waste heat such as gray water are extracted from a typical multi-family building under extreme winter conditions for one day or 24 h. The maximum heat recovery (MHR) is targeted through an adapted time slice model (TSM) of pinch analysis. Time slices are selected such that all changes in dynamic heat capacity flow rates can be included in the pinch analysis. Moreover, charging and discharging streams of the thermal energy storage (TES) is calculated based on the adapted TSM graphical approach. The integration of a heat pumping system, gray water and solar thermal collectors through mixed direct/indirect heat recovery (i.e. via TES) can reduce hot utility usage in the studied case by as much as 72 percent. An appropriate heat exchanger (HE) and TES network is proposed for the test building to benefit from this underutilized resource. Additionally, the dynamic thermal behavior of the proposed stratified TES is numerically investigated. The results reveals that combined heat loss and thermocline thickness can reduce the heat recovery from TES tank by 10 percent, which is 2 percent of MHR in the test building.

Numerical investigation on simultaneous charging and discharging process of molten-salt packed-bed thermocline storage tank employing in CSP plants

Based on the local thermal non-equilibrium theory, a modified transient two-dimensional numerical model is developed to investigate the simultaneous charging and discharging performance of a molten-salt packed-bed thermocline tank employed in concentrated solar power plants. Two different types of operation including that of stable and periodic charging with steady discharging are analyzed. Three different solid fillers are utilized, including quartzite rock, slag pebbles, and alumina ceramics. The effects of charging/discharging flow rate ratio and non-charging periods on thermocline thickness and energy storage power have been investigated. The results show significant influences of the charging/discharging flow rate ratio. When charging is steady, the discharge performance of a packed-bed tank is better than that of a pure molten-salt tank at lower flow rate ratios. The thermocline thickness of the packed-bed tank is higher than that of a pure molten-salt tank for different solid materials. Compared with quartzite rock and slag pebbles, alumina ceramics achieves a higher thermal gradient. In periodic charging mode, as the cycle’s number of charging periods increases, thermocline thickness increases linearly. Operating at higher charging rates and shorter non-charging periods, the discharge outlet temperature of molten-salt becomes more stable for the steam generation process and hence, the discharge performance is improved.

Dynamic characteristics of solid packed-bed thermocline tank using molten-salt as a heat transfer fluid

Thermocline characteristics and the discharge performance of a molten-salt packed-bed energy storage system are analyzed numerically. For this purpose, a transient two-dimensional model based on local non-equilibrium thermal theory in a porous medium is developed. Three different solid fillers are utilized, including that of quartzite rock, slag pebbles, and alumina ceramics. Molten-salt serves as a heat transfer fluid (HTF). The effects of fluid inlet flow rate and the particle size diameter on the thermocline thickness through temperature profiles of packed-bed systems have been investigated. The discharging power is discussed in a detailed description of various operating conditions through the discharge outlet temperature stability period. The results obtained are compared with that of the pure molten-salt tank. It is revealed that the thermocline thickness of the molten-salt packed-bed tank is higher than that of pure molten-salt tank, while the latter is better in thermal stratification. As the inlet flow rate increases, both the discharging power and thermocline thickness increase. The results also indicated that decreasing particle size diameter, results in a decrease in thermocline thickness, and the discharging performance becomes more stable. Slag pebbles as a filler material is more effective than quartzite rock in thermal energy storage (TES). The results can be beneficial for the design and optimization of the packed-bed thermocline tank.

Analysis of a Thermal Energy Storage Tank in a Large District Cooling System: A Case Study

This study’s primary goal is to evaluate the performance of a large thermal energy storage tank installed in a Gas District Cooling (GDC) plant. The performance parameters considered in this study include thermocline thickness (WTc), Cumulated Charge (Qcum), and Half Figure of Merit (½ FOM). The operation sensor data of a large Thermal Energy Storage (TES) tank was acquired for this analysis. The recorded temperature sensor from the 1st to 7th January and from 12th to 17th October 2019 was considered in this research. GraphPad prism computer software was deployed for analyses, and the temperature distribution data were analyzed to determine the four temperature parameters (hot water temperature (Th), cool water temperature (Tc), cool water depth (C), and slope gradient (S)) using a non-linear regression curve fitting technique and sigmoid Dose Responses function as integrated with the software. At the end of this research, the relationship between the growth of the determined performance parameter with charging hours was analyzed and presented. The research results proved the ability of GraphPad Prism software to assess the temperature distribution in the TES tank and also the corresponding effects on the overall Tank performance. The software offers better advantages in evaluating the performance parameter of the TES tank accurately.

Heat transfer performance of a storage system of molten-salt thermocline-packed bed

A packed bed is a good heat-storage method because of its simple structure, low cost, and good heat-transfer performance. Packed beds generally have a thermocline temperature distribution. This study analyzes a molten-salt thermocline storage system packed with a sensible heat material based on simulation and experimental research. The relationships between heat transfer and the characteristic parameters of the filling material, charging time, and initial conditions are obtained. Investigation results show that the thickness of the thermocline layer reduces with the increase in volume heat capacity but increases with time. The equivalent diameter of particles almost has zero effect on the thermocline thickness, and porosity has an indirect impact on the thickness of the thermocline layer but influences the movement speed of the thermocline. The maximum temperature difference between the fluid and solid decreases, but the range of temperature differences increases with time.

Investigation on Simultaneous Charging and Discharging Process of Water Thermocline Storage Tank Employed in Combined Heat and Power Units

Abstract Thermal energy storage (TES) has become a key component in combined heat and power (CHP) generation, which enhances the load regulation capability and overall thermal performance. In line with that concept, the present work addresses a numerical study that aims at investigating and predicting the transient thermal behavior of a water thermocline storage tank that integrates into a CHP plant. Investigations are carried out on two distinct two-dimensional models. The first model studies thermocline characteristics of the heat storage system through a single-charge/discharge operation. The influences of various operating parameters such as the inlet flowrate and the inlet temperature of HTF are investigated. Results indicate that the thermocline thickness increases as the inlet flowrate increases, and consequently, the heat storage/release period decreases. The second modified model discusses the technique of simultaneous charging and discharging operation applied in the CHP unit. Two types of operations are analyzing with (a) stable charging with steady discharging and (b) periodic charging with steady discharging. The results ravel out that the mixing ratio of charging and discharging flowrate (Qc/Qd) has a strong influence on the operation performance. When the ratio of Qc/Qd >1, the thermocline forms, and its thickness increases with time. The thermocline thickness remains constant as this ratio declines to 0.625. In the case of periodic charging, both larger charging flowrate and shorter non-charging periods result in maintaining the discharge outlet temperature of HTF stable for the heat-supply net, and discharge performance improved.

Performance assessment of stratified chilled water thermal energy storage tank at district cooling plant

Thermal energy storage (TES) is the key component of the district cooling (DC) plants. Its performance is important to be analysed. Various works have been carried out to analyse the TES tank performance. Among the methods, the most common are thermocline thickness and figure of merit. In this study nonlinear regression is used to analyse the temperature profiles of a stratified TES tank at DC plant at Universiti Teknologi PETRONAS. The TES performance was also evaluated using thermocline thickness and 1/2FOM. Results indicate curve fitting approach is enabled to plot temperature profiles during charging. The plots assist in understanding the development of thermocline within the TES tank during charging process. Results on the thermocline thickness and 1/2FOM during the understudy charging period indicate increased of thermocline thickness and decreased of 1/2FOM values respectively during the study period. Hence it can be concluded that deterioration occurred to the TES tank during the study period.

Experimental evaluation of vitrified waste as solid fillers used in thermocline thermal energy storage with parametric analysis

This work presents the first use of Cofalit® (vitrified asbestos-containing waste) as a solid filler in pilot-scale thermocline thermal energy storage (TES). The thermocline size is 4 m³ connected to the MicroSol-R installation at the PROMES research facility in Odeillo, France. The study compares the thermal performance of the thermocline filled with Cofalit® to the reference case of alumina spheres for typical charge and discharge operations. It evaluates the actual thermal behavior of thermocline considering three leading performance indicators: process duration, thermocline thickness, and process efficiency. The investigation shows a 22% shorter charge duration in Cofalit® compared to alumina and 16% shorter discharge duration. Cofalit® exhibits about 16% lower thermocline thickness during both charge and discharge compared to alumina. The charge efficiency is slightly better in Cofalit® than alumina with an efficiency of 82% and 78%, respectively. Also, Cofalit® has a better discharge efficiency, 90% with respect to 84% for alumina. These results confim a good thermal performance of Cofalit® as filler material in thermocline TES. Considering the cost-saving and positive environmental impact of using Cofalit® as well as the good thermal performance of the thermocline filled with it, Cofalit® appears a very good filler material in TES. The obtained temperatures from radial positions indicate no significant variation during charge and discharge, and this confirms the one-dimensional thermal behavior of this setup. A parametric analysis is performed using a 1D continuous solid (C-S) model to investigate the influence of particle diameter, porosity, thermal conductivity, and volumetric heat capacity on the thermal performance of the thermocline. The analysis confirms the experimental findings, and it indicates that about a 10.9% longer process duration is associated with a 10% larger volumetric heat capacity and less affected with other parameters. Thermocline thickness is mainly affected by the diameter as well as the volumetric heat capacity of the solid filler; it grows 2.2% for each 10% diameter increase and around 3.23% for doubling the volumetric heat capacity. Charge efficiency demonstrates independency from evaluated properties. While discharge efficiency increases sharply at a tiny particle diameter before an optimum diameter value is reached, then it starts to decrease by 1.4% to each 10% bigger diameter.

A comprehensive parametric study on integrated thermal and mechanical performances of molten-salt-based thermocline tank

The numerical model of a molten-salt-based thermocline tank with multi-layer wall structure is established. A comprehensive parametric study on integrated thermal and mechanical performances of the tank is conducted by using the CFD approach. The effects of eight factors are investigated. For all factors, complete operation cycle simulations of the tank as well as mechanical calculations are launched. The effect laws are revealed and briefly summarized. The results indicate that the inlet molten salt velocity, cold molten salt temperature, porosity of porous bed, diameter, thermal conductivity and specific heat of solid filler particle all have relatively obvious impacts on the discharging performance of the tank. The thermocline thickness during the discharging process can be decreased by increasing the inlet molten salt velocity, cold molten salt temperature or specific heat of filler particle, or by reducing the porosity of porous bed, diameter or thermal conductivity of solid filler particle. The maximum mechanical stress of the steel wall can be decreased by increasing the inlet molten salt velocity, cold molten salt temperature or firebrick thickness, or by decreasing the steel wall thickness, porosity of porous bed, diameter, thermal conductivity or specific heat of solid filler particle.

Performance Analysis of Target Depth Classification Algorithm Based on Sea Experiment Data

Cross-spectrum signals can be calculated by the pressure signals. The sign distribution of cross-spectrum active component can be effectively used for target depth classification algorithm. The algorithm is applicable for depth classification of targets where frequencies can only excite the first two normal modes. The corresponding research results are mainly based on the theoretical study. There are few researches on the algorithm performance based on experiment results. To overcome this research lack, based on the effective depth model, the effects on various receiving depth, source frequency, and received signal-to-noise ratio on the algorithm performance have been studied in this paper. The influence of sound velocity profile parameters (negative gradient, thermocline intensity, thermocline thickness, and up-boundary depth) on the algorithm performance has also been researched. According to the simulation results, proper adjustment of the receiving depths can effectively improve the algorithm performance. The source frequency primarily affects the position of the ideal receiving depth which can be appropriately adjusted according to the depth classification requirements of the real sea environment. The algorithm performance improves gradually with the increase of signal-to-noise ratio. Moreover, the algorithm can also be applied under the conditions of negative gradient and thermocline. The comprehensive sound velocity profile parameters have a large impact on the depth classification performance of the algorithm. Even in the case of strong negative gradient or strong thermocline, the robustness of the algorithm is still high. The feasibility of our presented method has been verified by sea experiment. The practical application value of the ideal receiving depth has been researched and validated. The factors affecting the algorithm performance including line spectrum continuity and received signal-to-noise ratio have also been analyzed in our simulation and real sea experiments.

Thermodynamic Analysis of Packed Bed Thermal Energy Storage System

A packed-bed thermal energy storage (PBTES) device, which is simultaneously restricted by thermal storage capacity and outlet temperatures of both cold and hot heat transfer fluids, is characterized by an unstable operation condition, and its calculation is complicated. To solve this problem, a steady thermodynamics model of PBTES with fixed temperatures on both ends was built. By using this model, the exergy destruction, thermocline thickness, thermal storage capacity, thermal storage time, and other key parameters can be calculated in a simple way. In addition, the model explained the internal reason for the change of thermocline thickness during thermal storage and release processes. Furthermore, the stable operation of the PBTES device was analyzed, and it was found that higher inlet temperature of hot air, and lower temperature difference between cold and hot air can produce less exergy destruction and achieve a larger cycle number of stable operation. The work can be employed as the basis of the design and engineering application of PBTES.

An Integrated Thermal and Mechanical Performance Analysis of Effect of Cold Molten Salt Temperature for Thermocline Tank

To study the effect law of cold molten salt temperature on both the thermal energy storage and mechanical performances of a thermocline molten salt tank, a numerical model of the tank which includes a multi-layer wall is established. The tank wall consists of three layers, which are firebrick, steel, and ceramic, respectively. Fluent code is employed to carry out the simulations of five different complete operation cycles of the thermocline tank. An integrated thermal and mechanical performance analysis of the effect of cold molten salt temperature for the thermocline tank is carried out. The analysis results reveal that for all the simulated cases, the thermocline is steady, and both the charging and discharging processes of the tank are stable. The thermocline thickness of the charging process can be smaller than that of the discharging one. The axial temperature of steel wall does not change synchronously with the central axial temperature of molten salt during both charging and discharging processes. As the cold molten salt temperature decreases, the thermocline thickness as well as the maximum mechanical stress of the steel wall increases. Hence, it can be drawn that increasing the cold molten salt temperature properly will be of benefit to both the energy storage performance and stability of the thermocline tank structure.

Seasonal variation of the upper-layer seawater properties in the Banda Sea: observed from an autonomous CTD Argo float

An autonomous CTD (conductivity-temperature-depth) Argo float was deployed in the deepest Weber Basin of Banda Sea to investigate seasonal changes of the upper-layer seawater properties from the first year of CTD time-series data between August 2017 and July 2018. It is found that the float trajectory formed distinct elliptical-shapes, exhibiting a new picture of deep clockwise circulation of the float parking depth at 1000 m. On seasonal scale, large fluctuation of properties, mixed-layer depth, and thermocline thickness, were associated with seasonal alternating Ekman upwelling and downwelling dynamics. Upwelling “colder” event (July-September) was characterized by outcrops of 27°C isotherm, 34 psu isohaline and 22 kg/m3 isopycnal at sea surface and large lateral variation of salinity in the upper 50 m. In contrast, these properties lines deepen down to about 100 m depth during downwelling “warmer” event (December-February). Amplitude of seasonal temperature change in the upper-layer was about 3°C. The very fresh water episode in the upper-layer was found during the monsoon break in April.