Stratified Water Storage Tank Research Articles

Numerical and experimental investigation of stratified water storage tanks: An enhanced adaptive-grid model

Stratified water storage tanks are key in thermal energy systems, effectively balancing energy supply with heat demand, thus facilitating operational flexibility. Accurately modeling both energy balance and thermocline evolution with minimal computational effort is essential for models implementation in real-time control applications. One-dimensional, static-grid models are favored for their easy implementation; however, they require dense grids in order to minimize numerical diffusion and to represent mixing effect correctly, resulting in rather expensive computational effort. This study introduces an adaptive-grid model that comprehensively represents the water storage tank’s geometrical features, including inlet and outlet duct heights and dead volume sizes. It also details the spatially discretized heat diffusion and characterizes the inflow region’s mixing using an empirical equation. Implemented in both TRaNsient SYstem Simulation (TRNSYS) and Modelica environments, the model is validated through laboratory experiments with a hot water storage tank operating under various mass flow and temperature conditions, representative of heat pump-driven heating systems. The results indicate improvements in computational efficiency compared to static-grid models, specifically a 55% reduction in simulation duration with the same level of uncertainty.

Data-driven turbulence modelling of inherently unsteady flow in stratified water storage tanks

The flow and thermal characteristics of thermal stratification appears in a wide range of industrial applications related to energy storage. In a previous study (Xu et al. submitted to Flow, Turbulence and Combustion), it has been shown that linear closure models in unsteady Reynolds averaged Navier–Stokes (RANS) calculations are unable to accurately capture the transient flow and thermal characteristics occurring in stratified water storage tank configurations. In the current study, we extend a data-driven framework to develop turbulence and heat flux closures for a three-dimensional (3D) geometry of a stratified water storage tank, using data from a highly resolved large eddy simulation. The transient high-fidelity data set is partitioned into several time-intervals used for closure development and testing. In detail, we first present the model formulations for 3D buoyant flow. We then use the reference data to extract the most suitable input features, i.e. variables used to regress the data driven model. In order to reduce the complexity of the resulting models and accelerate the training process, a recursive basis function elimination is performed, using alignment of the different basis functions with the target turbulence stress and heat flux as metric. Using the identified features and the subsets of basis functions, novel closures are then developed using a symbolic regression framework, based on Gene Expression Programming (GEP). Models are developed either for each time interval or for all intervals at once. The different machine-learnt turbulence and heat-flux closure models are then tested for two different tank configurations where the thermal stratification is affected by positively/negatively buoyant jets within simple or complex three-dimensional geometries. It is demonstrated that the extended GEP-based data-driven turbulence modelling approach is able to create models that can accurately predict flow with inherent unsteadiness in three dimensional complex-geometry cases.

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%.

Stratification efficiency of thermal energy storage systems – A new KPI based on dynamic hardware in the loop testing - Part II: Test results

Stratified water storage tanks are used for storing solar heat for space heating and domestic hot water in one device. When this kind of storage is used in combination with a heat pump, the temperature stratification of the storage is a decisive factor for the overall efficiency and thus for the consumed end energy of the system.Six storage devices from different manufacturers were tested in the laboratory. The results show that the new KPI for stratification efficiency of the storage system correlates well with the simulated and emulated electricity demand of a heat pump that is charging the storage. While the stratification efficiency was the decisive factor for the energetic performance of the system, the determined heat losses of the storage system showed no correlation with the overall energetic performance of the system.

Stratification Efficiency of Thermal Energy Storage Systems – A new KPI based on Dynamic Hardware in the Loop Testing - Part I: Test Procedure

Stratified water storage tanks are used for storing solar heat for space heating and domestic hot water in one device. When this kind of storage is used in combination with a heat pump, the temperature stratification of the storage is a decisive factor for the overall efficiency and thus for the consumed end energy of the system.A test method is introduced and a key performance indicator for stratification efficiency is defined based on the second law of thermodynamics, i.e. on the entropy balance of the storage system. The laboratory test is based on a 24 hour cycle where realistic and dynamic charging and discharging is applied according to boundary conditions of real weather data of a day in the year. This particular day of the year was chosen because of its representativeness for the effect of stratification on the system performance, such that the annual performance can be derived directly from the performance shown in the 24 h test cycle.

Experimental investigations of heat transfer characteristics of MPCM during charging

Experimental investigations of the heat transfer characteristics during charging were performed for a 25 wt% microencapsulated phase change material (MPCM), which was synthesized by polymerization. The shell and core were urea-formaldehyde and hexadecane (C16H34), respectively. The shell–core ratio of the MPCM was 3:7 by weight. A direct mixing stratified water storage tank was installed in the experimental system to decrease the chilled supply water temperature fluctuation. The influences of different stirring rates on the heat transfer performance of the MPCM slurry was also compared. The experimental results indicated that the volumetric thermal energy storage capacity of the MPCM slurry was 1.74 times that of pure water at 200 rpm. The charging rate and heat transfer coefficient increased rapidly in the phase transition region, however, there is no significant improvement compared with pure water during the whole charging process.

Two performance indices of TES apparatus: Comparison of MPCM slurry vs. stratified water storage tank

Microencapsulated phase change material (MPCM) slurry, which has a higher thermal storage capacity than water, was introduced as a storage medium in a coil-in-tank, but the key question is whether a competitive charging/discharging rate could be achieved. In this paper, two key indices, the volumetric thermal storage capacity at a given temperature difference and the charging/discharging rate variation over time, are newly defined. From the experimental results, it is estimated that the volumetric thermal storage capacity of the MPCM slurry is nearly twice that of water in the temperature range of 8–18°C. Finally, the overall heat transfer coefficient of the slurry storage device as a shell-tube heat exchanger was measured, and the external average convective heat transfer coefficient was also calculated. The two coefficients exhibited visible peaks during the phase change process with high-speed stirring and were much higher than that of water. However, the overall charging/discharging rates of the MPCM storage tank were observed to be much lower than the idealized stratified water storage tank (SWST), indicating that the design of an MPCM slurry thermal storage device needs to be further optimized.

Management of hybrid energy supply systems in buildings using mixed-integer model predictive control

In this paper a mixed-integer model predictive controller for hybrid energy supply systems in buildings is presented. This approach is based on a hierarchical building control concept where the energy supply level is coupled to the energy consumption level only by the heat load. The supply level is characterized by non-linear dynamics due to a stratified water storage tank and a switched heat pump with minimum on/off times. The mixed-integer model predictive controller optimizes the unit commitment problem at minimum costs while satisfying the consumption level’s predicted heat load. The hybrid system is formulated as a piecewise affine model comprising continuous and discrete system inputs. Moreover, the proposed controller is able to manage the stratified storage tank including switching sequences of the heat pump with respect to energy price forecasts. The effectiveness of this approach is shown by a comparison to a model predictive controller with an a priori fixed operation mode profile, where the heat pump is only operating at night, and discussing the effect of the variation of the stratified storage tank size. The proposed concept is able to flexibly manage all sizes of stratified storage tanks with better performance than the reference control strategy, which is only effective for larger tanks. Additionally, a robustness analyses demonstrates that the mixed-integer model predictive controller can handle errors in the heat load prediction from the consumption level. Both analyses show promising results for the practical use of the proposed controller within the hierarchical control concept or as a control module in a similar but more general application.

PERFORMANCE OF A DAY/NIGHT WATER HEAT STORAGE SYSTEM FOR HEATING AND COOLING OF SEMI-CLOSED GREENHOUSES IN MILD WINTER CLIMATE

A novel system for heating/cooling greenhouses based on air/water heat exchangers connected to a thermally stratified water storage tank was tested in a small greenhouse compartment at the Experimental Station of the Cajamar Foundation in Almeria, Spain. The system maintained a closed greenhouse (no natural ventilation) throughout the winter and spring during which a truss tomato crop was grown. Atmospheric CO2 concentration could be maintained during the daytime allowing for greater net photosynthesis in relation to a reference greenhouse in which traditional natural ventilation was used to control temperature and humidity. Three fine wire heat exchangers (Fiwihex®) were installed 1 m above the crop when at full development. The heat exchangers allowed for a very efficient transfer of the sensible and latent heat accumulated in the compartment. During the winter, the daytime temperatures were maintained below the ventilation set point (30°C). During the night, the system was able to maintain temperatures above the heating set point (12°C), with stored warm water temperatures between 15 and 17°C after clear days. After cloudy days, with water temperatures between 11 and 13°C, the system could always keep a temperature gradient with the exterior of 4-6°C, enough to maintain greenhouse air temperatures above 8°C the entire night. When seasonal night air temperatures exceeded the heating set point, the hot water at the top of the tank was cooled to the wet bulb temperature by means of an open cooling tower. The cooled water was used for cooling the greenhouse, which allowed for closing the greenhouse for a longer period. The water condensate during the cooling mode, allowed for a high water saving and maintained the relative humidity very constant during the day (around 80%). During the night, humidity was kept at 90% without ever reaching saturation.

Simulation of a solar absorption air conditioning system

This paper presents the simulation of a solar-powered absorption air conditioning system with the absorption pair of lithium bromide and water. An attempt is made to increase the COP of the system by partitioning a single storage tank into two parts. In the morning when sunshine is low, the upper part is activated, and in the afternoon, the whole (upper and lower) tank is connected to the collector. The analysis indicates that it is preferable to utilize a partitioned water tank rather than the normal stratified water tank because the cooling effect can be realized much earlier compared to the normal stratified water storage tank. Also, the overall cooling efficiency (cooling load to total solar energy ratio) was found to be higher for the partitioned storage air conditioning system.