Stratified Hot Water Storage Tank Research Articles

Numerical approach and experimental investigation of a stratified hot water storage tank for domestic hot water production

Thermal stratification within storage tanks plays a critical role in the efficiency of thermal energy storage systems. In this paper, one-dimensional numerical model of a stratified storage tank is developed to simulate the thermal stratification process within a storage tank during the charging and the discharging processes, capturing the temperature distribution inside the storage tank over time. This work investigates the evolution and transient behavior of stratified flows during the heating and cooling process within sensible-heat energy storage systems under ReD=167 and ReD=616 for heating process. For that purpose, an experimental campaign was performed in an instrumented water storage tank to measure the stratification process when a hot water round jet was injected into a slender cylindrical tank filled with an initially uniform hydrostatic water field stabilized at a lower temperature. This information was used to develop a simple 1D model able to be applied during long-term charging/discharging transients of these kind of systems. The 1D model developed was validated against the experiments performed following prototypical charging/discharging tests according to the standard EN16147 and can be used to describe the energy performance of this kind of systems during long-term charging/discharging sequences. The present model provides a reduction of about 5% of the error of the time of the water cooling process with respect to other models in the literature.

Modeling and Simulation of Layered Water Tank Based on MATLAB/SIMULINK

In order to improve the accuracy of the stratified hot water storage tank model and simplify the modeling process of the stratified water tank, a modelling method of the stratified water tank based on MATLAB/SIMULINK is proposed. By comparing with TRNSYS platform simulation, the correctness and accuracy of this method are verified. At the same time, this method is used to establish a six-node stratified hot water storage tank. Through the simulation of 8 different working conditions, the influence of different inlet and outlet water flow rates and temperature of the inlet and outlet water on the stratification effect of the hot water storage tank is analyzed.

Photovoltaic thermal collectors: Experimental analysis and simulation model of an innovative low-cost water-based prototype

This paper presents an innovative water photovoltaic thermal collector prototype. One of the main novelties of such system is its economic affordability, obtained through low-cost materials. The collector, constructed and experimentally tested at the University of Patras (Greece), is composed of a polycrystalline photovoltaic module coupled to eleven plastic pipes for water heating, located under the PV panel in an aluminium box. The prototype, suitable for building architectonical integration, can provide domestic hot water and electricity to the building. In order to assess the energy, economic and environmental performance of the system under different weather conditions and for diverse building uses, a suitable dynamic simulation model was developed and validated vs. experimental data. To investigate the convenience of the presented prototype and the potentiality of the developed software, a suitable case study is presented. In particular, the photovoltaic thermal collector is coupled to a stratified hot water storage tank for supplying domestic hot water to a single-family house located in three different European weather zones: Freiburg, Naples and Almeria. The system layout optimization was also performed through an energy and economic sensitivity analysis to some design and operating parameters. Useful design criteria and interesting energy and economic results were obtained.

A validated model for mixing and buoyancy in stratified hot water storage tanks for use in building energy simulations

When evaluating the energy demand of buildings using stratified hot water storage tanks in an active demand response context, an accurate but low order storage tank model is required. Accurate modelling of buoyancy effects and mixing inside the storage tank as a result of direct inflows is key to obtain valid results. The use of one-dimensional storage tank models is common practice in building simulations, but as buoyancy and mixing are three-dimensional phenomena they cannot be incorporated directly in such a model and special formulations must be used. This paper presents a novel method for incorporating buoyancy and mixing in one-dimensional stratified storage tank models. The model parameters are derived from a series of computational fluid dynamics simulations and correlated with the appropriate non-dimensional parameters. The model is validated against an independent set of charging and discharging experiments. The model is found to represent buoyancy and mixing in the storage tank in a realistic way and to correspond well to experimental results. To allow researchers to assess the uncertainty on performance indicators in building energy simulations, an effective storage capacity ratio is defined and its variation within the model parameter uncertainty range is calculated and compared to other models. The results show a discrepancy between models commonly used in the literature and the presented validated model. When the influence of storage capacity on parameters of interest is known, the effective storage capacity ratio can also be used to estimate the uncertainty on these parameters caused by the storage tank model.

Implications of the modelling of stratified hot water storage tanks in the simulation of CHP plants

This paper considers the effect that different hot water storage tank modelling approaches have on the global simulation of residential CHP plants as well as their impact on their economic feasibility. While a simplified assessment of the heat storage is usually considered in the feasibility studies of CHP plants in buildings, this paper deals with three different levels of modelling of the hot water tank: actual stratified model, ideal stratified model and fully mixed model. These three approaches are presented and comparatively evaluated under the same case of study, a cogeneration plant with thermal storage meeting the loads of an urbanisation located in the Bilbao metropolitan area (Spain). The case of study is simulated by TRNSYS for each one of the three modelling cases and the so obtained annual results are analysed from both a First and Second-Law-based viewpoint. While the global energy and exergy efficiencies of the plant for the three modelling cases agree quite well, important differences are found between the economic results of the feasibility study. These results can be predicted by means of an advanced exergy analysis of the storage tank considering the endogenous and exogenous exergy destruction terms caused by the hot water storage tank.

A novel experimental investigation of a solar cooling system in Madrid

This paper reports novel experimental results derived through field testing of a part load solar energized cooling system for typical Spanish houses in Madrid during the summer period of 2003. Solar hot water was delivered by means of a 49.9 m 2 array of flat-plate collectors to drive a single-effect (LiBr/H 2O) absorption chiller of 35 kW nominal cooling capacity. Thermal energy was stored in a 2 m 3 stratified hot water storage tank during hours of bright sunshine. Chilled water produced at the evaporator was supplied to a row of fan coil units and the heat of condensation and absorption was rejected by means of a forced draft cooling tower. Instantaneous, daily and period energy flows and energy balance in the installation is presented. System and absorption machine temperature profiles are given for a clear, hot and dry day's operation. Daily and period system efficiencies are given. Peak insolation of 969 W m −2 (at 12:30 solar time on 08/08/03) produced 5.13 kW of cooling at a solar to cooling conversion efficiency of 11%. Maximum cooling capacity was 7.5 kW. Cooling was provided for 8.67 h and the chiller required a threshold insolation of 711 W m −2 for start-up and 373 W m −2 for shut-down. A minimum hot water inlet temperature to the generator of 65 °C was required to commence cold generation, whereas at 81 °C, 6.4 kW of cooling (18.3% of nominal capacity) was produced. The absorption refrigeration machine operated within the generation and absorption temperature ranges of 57–67 and 32–36 °C, respectively. The measured maximum instantaneous, daily average and period average COP were 0.60 (at maximum capacity), 0.42 and 0.34, respectively. Energy flows in the system are represented on a novel area diagram. The results clearly demonstrate that the technology works best in dry and hot climatic conditions where large daily variations in relative humidity and dry bulb temperature prevail. This case study provides benchmark data for the assessment of other similar prototypes and for the validation of mathematical models.

Experimental and Theoretical Investigation of Mixed and Stratified Hot Water Storage Tanks

This paper investigates the performance of a 1.1 m3 stratified hot water storage tank coupled to a vapour compression heat pump system. A comprehensive data acquisition system has been used to obtain the experimental data from a series of static and dynamic tests. In the static experiments a well-defined thermocline has been achieved and the effects of insulation and tank wall thickness on the preservation of the thermocline have been determined. The results indicate that thermal losses in stratified tanks are about 22 per cent higher than the losses in fully mixed tanks. The dynamic experiments have been conducted with an upward-moving thermocline and the major factors influencing its stability have been correlated in terms of the Archimedes number (Gr/Re2). It has been found that good stratification performance can be maintained with Archimedes numbers in the range between 35000 and 55000. A simplified one-dimensional model of the storage tank has been developed and validated against experimental results. The model will be linked to dynamic models of the heat pump and the building to simulate the performance of a heat store/heat pump energy management system.

Experimental study of thermally stratified hot water storage tanks

Temperature stratification in hot water storage systems was studied experimentally. In particular, high extraction rates from plastic cylindrical vessels were emphasized. Data were taken at various length to diameter ratios, inlet-outlet temperature differences and mass flow rates. The effect of inlet and exit port configuration on thermal stratification was also studied. The data were empirically correlated to yield useful relations for the design of effective hot water storage systems. Finally, a novel inlet and exit configuration scheme was designed for an 80 gal. (300 l.) and a 500 gal. (1900 l.) storage tank.