Stratified Thermal Storage Research Articles

Heat loss characteristics for a typical solar domestic hot water storage

It is common practice to predict the performance of solar domestic hot water (SDHW) systems by computer simulation. This process relies on the accurate specification of the system's physical and thermal characteristics, and is often based on a number of simplifying assumptions. An important aspect of system performance is storage heat loss characteristics; however, these are often represented by an average heat loss coefficient or U-value that does not account for the complex geometry of the thermal storage or the interaction of the various inlet and outlet ports that may act as thermal conduits. In addition, most solar storage models assume that the tank temperature profile is one-dimensional and that conduction within the tank wall is negligible. To investigate these effects, tests were conducted on a typical thermal storage used in SDHW applications and included a cool-down test and a heat diffusion test sequence. The values derived from these test sequences were then compared to computer predictions based on estimated thermal properties. In addition, the basic assumptions typically used in the computer modelling of solar storage heat losses (e.g., one-dimensional temperature profiles, minimal tank wall conduction, uniform wall heat loss) were investigated, particularly in the context of a thermally stratified thermal storage.

Simulation study on a Domestic Solar/Heat Pump Heating System Incorporating Latent and Stratified Thermal Storage

Both solar and heat pump heating systems are innovative technologies for sustaining ecological heat generation. They are gaining more and more importance due to the accelerating pace of climate change and the rising cost of limited fossil resources. Against this background, a heating system combining solar thermal collectors, heat pump, stratified thermal storage, and water/ice latent heat storage has been investigated. The major advantages of the proposed solar/heat pump heating system are considered to be its flexible application (suitable for new and existing buildings because of acceptable space demand), as well as the improvement of solar fraction (extended solar collector utilization time, enhanced collector efficiency), i.e., the reduction of electric energy demand for the heat pump by management of the source and sink temperatures. In order to investigate and optimize the heating system, a dynamic system simulation model was developed. On this basis, a fundamental control strategy was derived for the overall co-ordination of the heating system with particular regard to the performance of the two storage tanks. In a simulation study, a fundamental investigation of the heating system configuration was carried out and an optimization was derived for the system control, as well as the selection of components and their dimensioning. The influence of different parameters on the system performance was identified, where the collector area and the latent heat storage volume were found to be the predominant parameters for system dimensioning. For a modern one-family house of 120 m2 living area with a specific annual heat demand of 60 kWh/(m2 a) for both heating and domestic hot water, a solar collector area of 30 m2, and a latent heat store volume of 12.5 m3 are proposed for the location of Wuerzburg (Germany). In this configuration, the heating system reaches a seasonal performance factor of 4.6, meaning that 78% of the building’s and users’ heat demand are delivered by solar energy. The results show that the solar/heat pump heating system can give an acceptable performance using up-to-date components in a state-of-the-art building.

Thermodynamic optimization of a stratified thermal storage device

Thermodynamic optimization combines exergy analysis with dynamical modeling to determine optimal design parameters. In this work we apply exergy analysis to an analytically solvable model of a single-medium stratified thermal storage device. We consider a simple one-dimensional model that includes conductive losses to the environment. Competition between mixing and conductive losses gives rise to a thermodynamically optimum charging time for the system. The model can treat the full range of stratification from the fully-mixed limit to perfect stratification and we are therefore able to determine the effect of stratification on the thermodynamic optimum. We show that stratification increases the overall efficiency and shortens the thermodynamically optimum charging time.

The effect of diffuser configuration on thermal stratification in a rectangular storage tank

This study is to systematically analyze the effect of various kinds of design factors on the stratification performance of a rectangular storage tank. Special interest is focused on the diffuser configuration, which crucially impacts the performance of a storage tank. Herein, a new diffuser shape is proposed, which exemplifies improved performance. Three-dimensional unsteady numerical experiments are conducted for four design parameters of a stratified thermal storage tank: Three design parameters with three levels (i.e., the Reynolds number=400, 800, and 1200; the Froude number=0.5, 1.0, and 2.0; and the area ratio of the diffuser to tank cross-section=0.0327, 0.0582, and 0.131) and one design parameter having two levels (i.e., diffuser type=radial plate type and radial adjusted plate type). Orthogonal array L 18(2×3 7) is adopted for the analysis of variance. The result gives quantitative estimation of the various design parameters affecting the performance and helps to determine the main factors for the optimum design of a stratified thermal storage tank. In the range of parameters considered, the Reynolds number is found to be the most dominant parameter. Moreover, the diffuser shape plays a significant role on the performance of a stratified thermal storage tank.

On Simplified Models for the Rate- and Time-Dependent Performance of Stratified Thermal Storage

In direct sensible thermal storage systems, both the energy discharging and charging processes are inherently time-dependent as well as rate-dependent. Simplified models which depict the characteristics of this transient process are therefore crucial to the sizing and rating of the storage devices. In this paper, existing models which represent three distinct classes of models for thermal storage behavior are recast into a common formulation and used to predict the variations of discharge volume fraction, thermal mixing factor, and entropy generation. For each of the models considered, the parametric dependence of key performance measures is shown to be expressible in terms of a Peclet number and a Froude number or temperature difference ratio. The thermal mixing factor for each of the models is reasonably well described by a power law fit with Fr2Pe for the convection-dominated portion of the operating range. For the uniform and nonuniform diffusivity models examined, there is shown to be a Peclet number which maximizes the discharge volume fraction. In addition, the cumulative entropy generation from the simplified models is compared with the ideally-stratified and the fully-mixed limits. Of the models considered, only the nonuniform diffusivity model exhibits an optimal Peclet number at which the cumulative entropy generation is minimized. For each of the other models examined, the cumulative entropy generation varies monotonically with Peclet number.

Transition from gravity- to inertia-dominated behavior computed for the turbulent stably-stratified filling of an open enclosure

This paper treats the unsteady mixed convection processes occurring during the stably-stratified bottom filling of an open enclosure with application to stratified thermal storage devices. Central to the performance of these devices is the extent of thermal mixing which occurs at the interface between the hot and cold fluid volumes as one or the other is displaced from the vessel. Useful volume fraction, a parameter reflecting the temperature limited usefulness of the stored fluid, and internal entropy generation are used to characterize the extent of thermodynamic losses in time and their variation with flow rates ranging from well-stratified to highly-mixed conditions. The transition to highly-mixed behavior is critically important since it limits the achievable energy discharge rates. Distinct trends in the variation of cumulative performance with Reynolds number indicate this transition occurs at Froude numbers of approximately one and lead to identification of a gravity-dominated and an inertia-dominated regime. In the gravity-dominated regime, the useful volume fraction increases linearly with time after an initial adjustment period and the turbulent diffusivity affects the variation of entropy generation only at the edges of the relatively distinct thermocline. Within the thermocline, the turbulent kinetic energy is completely damped by gravity. However, in the inertia-dominated or highly-mixed regime, the useful volume fraction starts to drop in the later stages of the filling process and appreciable entropy generation levels are distributed across a relatively wide thermocline with turbulent diffusivity playing a significant role throughout.

Application of temperature-distribution models to evaluate the energy and exergy of stratified thermal storages

Vertical stratification occurs in many thermal energy storages, and is often promoted to increase efficiency. Six temperature-distribution models for vertically stratified Thermal Energy Storage (TES) systems are described and applied for determining their energy and exergy efficiencies. The models are Linear, Stepped, Continuous-linear, Basic Three-zone, General Three-zone and General-linear. The models are intended to allow accurate yet relatively simple determinations of TES energy and exergy while providing physical meaning and understanding. An application of the temperature distribution models to a TES is presented. The results are expected to assist in the design and improvement of TES systems.

Effect of stratification on energy and exergy capacities in thermal storage systems

The increase in exergy storage capacity that is attained in thermal storages through stratification is assessed. A design-oriented temperature-distribution model for vertically stratified thermal storages that facilitates the evaluation of storage energy and exergy contents is utilized. The paper is directed towards demonstrating the thermodynamic benefits achieved through stratification, and increasing the utilization of exergy-based performance measures for stratified thermal storages. A wide range of realistic storage-fluid temperature profiles is considered, and for each the relative increase in exergy content of the stratified storage compared to the same storage when it is fully mixed is evaluated. The results indicate that, for all temperature profiles considered, the exergy storage capacity of a thermal storage increases when it is stratified, and increases as the degree of stratification, as represented through greater and sharper spatial temperature variations, increases. Furthermore, the percentage increase in exergy capacity is greatest for storages at temperatures near to the environment temperature, and decreases as the mean storage temperature diverges from the environment temperature (to either higher or lower temperatures). It is concluded that (i) the use of stratification in thermal storage designs should be considered as it increases the exergy storage capacity of a thermal storage and (ii) exergy analysis should be applied in the analysis and comparison of stratified thermal storage systems.

Measured and Modeled Charging of a Stratified Chilled Water Thermal Storage Tank with Slotted Pipe Diffusers

As part of ASHRAE Research Project 1185, field data from the constant flow rate charging of a stratified chilled water storage tank with double-ring octagonal slotted-pipe diffusers serving a university chilled water system were compared with results of a transient axisymmetric computational fluid dynamics (CFD) model. Charge processes at flows near design flow rate and 50% of design were modeled. Laminar and turbulent simulations were performed for a range of slot dimensions. Performance of the model was assessed by direct comparison of temperature profiles and by comparison of common thermal performance metrics, including thermocline thickness, lost capacity, and equivalent lost height. The inlet Richardson number based on slot hydraulic radius was found to have the strongest effect on model results. The model Richardson number giving best agreement with the lost height of the field data for near design flow rate was roughly an order of magnitude greater than the Richardson number of the actual diffuser. It is conjectured that this is due to inherent differences between the flow produced by multiple three-dimensional jets in the actual diffuser and the continuous slot of the axisymmetric model. The effect of turbulence varied from negligible to moderate as the Richardson number decreased.

Numerical and experimental study on the design of a stratified thermal storage system

The purpose of this study is to figure out the thermal stratification mechanism of a storage tank and thereby to determine optimum design and operating conditions. To this end, a computer program is developed to investigate the fluid flow in a tank, using Patankars SIMPLE algorithm. The validation of program is made successfully by the comparison against experimental data measured with a bench scale facility. Further a systematic investigation has been made in terms of important design and operational parameters such as storage tank size (commercial-scale and bench-scale), loading time, shape of diffuser, turbulence model and inlet velocity or Fr No. Considering the thermal efficiency of storage tank is critically impaired by the effect of flow recirculation and mixing by turbulence, a novel model with minimum mixing between hot and cold water is proposed for the evaluation of the performance of storage system by the assumption of the uniform plug-type flow. This is made by solving only the following governing equation of temperature which has no convective mixing with constant axial velocity, that is, ∂(ρT) ∂t +u ∂(ρT) ∂x =∇· k c p ∇T Based on a series of parametric investigation, a number of useful results can be drawn. In general the large scale system shows better storage performance than the small system. As the increase of loading time, the degree of stratification lowered due to the increased effect of heat transfer by convection and diffusion via thermal stratification region. And the curved type diffuser shows better performance of thermal stratification compared to the diffuser of flat type. As for the case of large-scale tank, the effect of Fr No., or inlet velocity does not show any significant effect on the thermal stratification compared to the small size one. The calculated results show that the model of ideal type-plug flow can be used as a possible tool for the evaluation of performance of storage system by giving the reference condition of best thermal stratification or least mixing condition. But in the case of full scale system, the difference between ideal and actual cases not significant compared to the case of bench scale. It is considered possibly due to the relatively decreased heat transfer effect by the increased effect of water flow in the full scale system. Two turbulence models, say standard k– ε and RNG k– ε models, show no visible difference in this study.

The exergy of stratified thermal energy storages

The performance of energy and exergy analyses of TES systems incorporating thermal stratification are described, along with the resulting insights and benefits. Six temperature-distribution models for stratified TESs are considered (linear, stepped, continuous-linear, general-linear, basic three-zone and general three-zone) which facilitate the evaluation of energy and exergy contents. The selection of a particular distribution is seen to involve a trade-off between result accuracy and calculational effort. It is shown using the temperature-distribution models how improving stratification can increase TES efficiency, and how the use of stratification can increase the exergy storage capacity of a thermal storage. Consequently, exergy analysis is shown to provide illuminating and meaningful assessments and comparisons of such TES systems that can assist in improving and optimizing designs. It is hoped that the results will facilitate the development of standardized exergy-based methodologies for the evaluation and comparison of stratified TES systems.

The exergy of stratified thermal energy storages

The performance of energy and exergy analyses of TES systems incorporating thermal stratification are described, along with the resulting insights and benefits. Six temperature-distribution models for stratified TESs are considered (linear, stepped, continuous-linear, general-linear, basic three-zone and general three-zone) which facilitate the evaluation of energy and exergy contents. The selection of a particular distribution is seen to involve a trade-off between result accuracy and calculational effort. It is shown using the temperature-distribution models how improving stratification can increase TES efficiency, and how the use of stratification can increase the exergy storage capacity of a thermal storage. Consequently, exergy analysis is shown to provide illuminating and meaningful assessments and comparisons of such TES systems that can assist in improving and optimizing designs. It is hoped that the results will facilitate the development of standardized exergy-based methodologies for the evaluation and comparison of stratified TES systems.

Approximate analytical solutions for stratified thermal storage under variable inlet temperature

This paper deals with approximate analytical solutions to the two-region one-dimensional model describing the charging process of stratified thermal storage tanks with variable inlet temperature as well as momentum-induced mixing. The inlet temperature, which is allowed to increase arbitrarily except that it always remains higher than (or equal to) the temperature at the tank top, is broken down into step changes and intervals of continuous change. Assuming that each continuous interval can be approximated as a finite number of piecewise linear functions, there is admitted an analytical solution for the perfectly mixed region. Based on the principle of superposition, the temperature profiles in the plug flow region (assuming both semi-infinite and adiabatic ends) are successfully derived, in terms of well-defined functions. The effect of the end condition proves to be negligible under the practical operating conditions. For a quadratic variation of inlet temperature, the approximate solution modeled on a moderate number of pieces agrees well with the corresponding closed-form solution.

THERMAL STORAGE AND EXERGY ANALYSIS: THE IMPACT OF STRATIFICATION

The importance and usefulness of second-law analysis for evaluating and comparing thermal energy storage (TES) systems in meaningful ways is discussed. Some of the main thermodynamic factors to be considered in evaluating TES performance are identified and described, including determining important analysis quantities, obtaining appropriate measures of efficiency, pinpointing losses, assessing the effects of stratification, and accounting for time duration of storage. As an illustrative example, exergy analysis, a particular type of second-law analysis, is applied to stratified TES systems in order to evaluate the effects and benefits of thermal stratification. It is concluded based on the results of the example that (i) the use of stratification in thermal storage designs should be considered as it increases the exergy storage capacity of a thermal storage, and (ii) exergy analysis should be applied in the analysis and comparison of stratified thermal storage systems.

Transient analysis of energy storage in a thermally stratified water tank

A one-dimensional transient heat conduction model to describe the decay of the thermocline in a stratified water tank is presented. The problem is formulated as an initial boundary value problem and the resulting governing equations in the fluid and in the storage wall are solved numerically to obtain the temperature profiles in the wall and the fluid. The formulation considers the axial conduction of heat, both in the fluid and in the solid wall. The mixing parameters introduced in the boundary conditions at the top and bottom of the tank in the fluid region account for mixing due to inlet and outlet streams with the stored fluid. The model is applicable to the storage of both hot and chilled water. The model is validated with experimental data from the literature. The parameters that influence the operation of a stratified thermal energy storage for both heat and cool storage are examined. © 1998 John Wiley & Sons, Ltd.

Transient analysis of energy storage in a thermally stratified water tank

A one-dimensional transient heat conduction model to describe the decay of the thermocline in a stratified water tank is presented. The problem is formulated as an initial boundary value problem and the resulting governing equations in the fluid and in the storage wall are solved numerically to obtain the temperature profiles in the wall and the fluid. The formulation considers the axial conduction of heat, both in the fluid and in the solid wall. The mixing parameters introduced in the boundary conditions at the top and bottom of the tank in the fluid region account for mixing due to inlet and outlet streams with the stored fluid. The model is applicable to the storage of both hot and chilled water. The model is validated with experimental data from the literature. The parameters that influence the operation of a stratified thermal energy storage for both heat and cool storage are examined. © 1998 John Wiley & Sons, Ltd.

Chilled Water Thermal Energy Storage without Electric Rate Incentives or Rebates

Stratified chilled water thermal energy storage (TES) is an accepted load shifting technology for chilled water plants serving large cooling loads. Much of the success experienced by TES technology in the past can be attributed to electric demand charges and to capital cost incentives offered through utility rebate programs. Concern that TES is not a “green” technology coupled with changes in the power production industry may eliminate both demand charges and rebates. If TES technology is to survive in this environment, TES systems must be equal or lower in capital cost than conventional systems, or they must generate sufficient energy and maintenance cost savings to offset any added capital cost. Using an example facility, this paper investigates the use of TES in such an environment and draws conclusions as to the sustainability of the technology.

96/05345 Analytical solutions to a one-dimensional finite-domain model for stratified thermal storage tanks

96/05345 Analytical solutions to a one-dimensional finite-domain model for stratified thermal storage tanks

Analytical solutions to a one-dimensional finite-domain model for stratified thermal storage tanks

A set of analytical solutions to a one-dimensional finite-domain model is obtained to analyze the charging process in stratified thermal storage tanks. An approximate but practically acceptable short-time solution which is compactly expressed in terms of well-defined functions is preferred to the mathematically exact but long-time solution. From the order of magnitude analysis on the series solution, all the higher order terms other than the zeroth order one are truncated, and the resultant solutions prove to be almost exact for the practical charging time range. The earlier Cabelli's solutions based on the semi-infinite domain are found to be a subsect of our solutions for which the adiabatic end condition is imposed. In view of the generality and completeness of the model and analysis, it is expected that the present solution may serve as a rigorous theoretical reference for related works.

Study on Optimization of Thermal Storage Performance for a Temperature Stratified Thermal Storage Tank

Study on Optimization of Thermal Storage Performance for a Temperature Stratified Thermal Storage Tank