Stratified Thermal Storage Tank Research Articles

Enhanced temperature regulation in compound heating systems: Leveraging guided policy search and model predictive control

With the widespread application of solar-energy-air-source heat pump composite heating, optimizing the regulation of air temperature in heating zones, enhancing system thermal comfort, and reducing energy consumption have become crucial. To ensure residential comfort while minimizing HVAC system energy costs, a control method combining guided policy search (GPS) with model predictive control (MPC) is proposed for composite heating systems. This method replaces state estimation in MPC with policy search and substitutes the offline trajectory optimization used in guided policy search with MPC, thus integrating the strengths of both approaches. This strategy not only improves control precision but also reduces energy consumption and enhances system robustness. The GPS-MPC control algorithm was validated against reinforcement learning and MPC. A simulation model was developed and validated on a real physical platform. Simulations compared the control effects of on-off control and MPC on pump frequency, flow rate, stratified thermal storage tank temperature, component, and system energy consumption. The data results indicate that the GPS-MPC algorithm offers superior predictive accuracy, efficiency, and robustness in composite heating control systems compared to conventional methods. Under the GPS-MPC strategy, indoor temperature fluctuations, pump frequency response speed, and energy consumption were significantly improved, with temperature fluctuation limited to only 0.8 °C, and the system achieving energy savings of over 12 %.

Transient modeling of stratified thermal storage tanks: Comparison of 1D models and the Advanced Flowrate Distribution method

Thermal energy storage (TES) is one of the key technologies for enabling a higher deployment of renewable energy. In this context, the present study analyzes the modeling strategies of one of the most common TES systems: stratified thermal storage tanks. These systems are essential to many solar thermal installations and heat pumps, among other clean energy technologies. Three different one-dimensional tank models are compared by their computing speed and resilience to long time steps. Two of the models analyzed are numerical, one being explicit and the other one implicit, and the other is analytical. The models are validated against data from experiments carried out considering small-scale stratified tanks, showing that their performance can be improved by using the Advanced Flowrate Distribution (AFD) method. The results show that the analytical model maintains its accuracy with longer time steps and is robust against divergence. Conversely, the numerical models show equivalent performance for short time steps, while the computation time is reduced. Although the AFD method shows promising results by achieving an improvement of 43% in terms of Dynamic Time Warping, its parameter optimization must be generalized for different tank designs, flow rates, and temperatures.

Compatibility assessment of thermal energy storage integration into industrial heat supply and recovery systems

Thermal energy storage (TES) systems can be used for recovering industrial waste heat and increasing energy efficiency, especially when coupled to batch thermal processes. Stratified water thermal storage tanks are the preferred technology for low-temperature applications, while molten salts are commonly used in medium and high-temperature applications with large storage capacities. No clear consensus exists on the appropriate TES technology for different industrial demands characteristics and their respective heat supply systems for medium and high-temperature applications. The present study analyzes several industrial sectors and their thermal processes, analyzing their temperature ranges, heat demands, and available TES technologies, which are classified by their operational conditions. The study presents two novel indicators for a preliminar compatibility assessment between TES and industrial sectors: a temperature compatibility indicator and exergy efficiency for TES and thermal processes. The results show that low and medium-temperature applications such as food, chemical, or textile industries exhibit high compatibilities with water (over 64%), high-temperature PCM (over 61%), and solid-state TES (100%), whereas molten salts and chemical looping demonstrate lower compatibility (below 24%). The exergy analysis for industrial cases shows that a lower temperature operating range for a TES induces low exergy efficiency. Regarding this scenario, high-temperature cPCM reaches efficiencies of over 44% for mid and high-temperature processes. Conversely, solid-state TES emerges as the most viable option for integration in high-temperature industries, exhibiting an efficiency of 62% with minimal exergy losses. The indicators defined in this study can be used for an early evaluation of TES integration in industrial applications, thus promoting emerging technologies selection through a quantitative comparison of the compatibility metrics.

A water heater that adjusts its temperature based on solar energy

The calculation method for a water distributor of a constant cross section uniformly perforated along the height for a multilayer stratified thermal storage tank used in solar heating and hot water supply systems was developed. The advantages of using self-regulating stratification water heat accumulators in solar heat supply systems are considered. It is shown that the use of stratification for short-term and long-term thermal accumulation leads to an increase in the use of solar heat by 15-20% compared to batteries with completely mixing water. In hot water systems, these batteries can provide earlier preparation of hot water with the required temperature for consumers. Based on the analysis of world experience in the development of water heat accumulator designs using temperature stratified water and the stability of the stratified flow of coolant in the accumulator volume, a new, self-regulating, water accumulator design has been proposed.

Transient study of thermal stratification of full-scale chilled water storage tank during optimum discharge condition

Thermal stratification of full-scale Chilled Water Storage Tanks (5855 m3) with 18 m tank diameter, and 23 m water depth during discharge mode and optimum condition was studied. The experimental and numerical analyse of stratified thermal storage tank in full-scale dimension and discharge mode has been studied. A 3D and 2D numerical model was performed. The case simplified to divide into two parts, the first part was to simulate flow in pipes and radial diffuser, and the second part included simulation in the tank according to the part one results. The optimum operation condition which found from many tests of tank operation conditions according to high thermal stratification and low thermal mixing inside the tank. Validation test with experimental data obtained from the present work and literature. The results included to analyse the thermal stratification and the flow characteristics during the discharging mode. Also, thermal performance parameters such as discharge efficiency, Richardson number, stratification efficiency, thermocline thickness, the average temperature in the tank, and discharge temperature were presented. The important result of the above analysis was the optimum stratification phenomena in the tank with low thermal mixing. The contribution of this study was the numerical modelling and simulation setting to capture the experimental results of these phenomena in the full-scale thermal storage tank, moreover to specify the optimum condition of thermal stratification of this tank dimension, diffuser type, and flow rate.

Calculation and experimental study of water distributor of stratification heat accumulator of solar heating system

The calculation method for a water distributor of a constant cross section uniformly perforated along the height for a multilayer stratified thermal storage tank used in solar heating and hot water supply systems was developed.

Surrogate modeling and optimization for the unequal diameter radial diffuser of stratified thermal energy storage tanks

AbstractStratified thermal energy storage (TES) tanks are widely used in thermal power plants to enhance the electric power peak load shifting capability and integrate high renewable energy shares. In this study, a data‐driven surrogate modeling and optimization study of the unequal diameter radial diffuser previously proposed by the present authors is conducted. First, based on the orthogonal experimental design, numerical experiments are performed to generate the performance database. Then, the database is used to establish the data‐driven surrogate model via the support vector machine. Subsequently, the single‐objective optimization and multiobjective optimization of an unequal diameter radial diffuser are conducted using the genetic algorithm. For the single‐objective optimization, the optimal thermocline thickness is 0.829 m when the diameter ratio of the long baffle and the tank is 0.426, the diameter ratio of the short baffle and the long baffle is 0.823, and the distance between the two baffles is 228.51 mm. For multiobjective optimization, the obtained Pareto optimal solutions are obtained. Under the premise of maintaining excellent thermal stratification, the selected Point C can reduce the steel cost by 88.1%. The research results are helpful for designing efficient and economical unequal diameter radial diffusers for TES tanks.

Research of water distribution in stratification heat accumulator of a solar heating system

The calculation method for a water distributor of a constant cross section uniformly perforated along the height for a multilayer stratified thermal storage tank used in solar heating and hot water supply systems was developed.

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.

Numerical simulation of stratified thermal energy storage tanks

ABSTRACTStratified tanks are being widely used for thermal energy storage (TES), allowing a decoupling between the energy source (commonly a renewable energy source or heat rejected by some industrial process) and the demand. A high degree of stratification is required to make more efficient use of the stored energy, and several design configurations have been developed in order to create and maintain a good stratification. Numerical and experimental approaches are used to monitor and design the process, and also to predict the behavior of these tanks under different conditions. Detailed numerical simulation, however, is computationally expensive and not suitable for the long-time simulations (typically years) required to assess the performance and the economics of energy systems which include TES tanks.In this paper, a relatively simple, computationally inexpensive numerical approach, called Multi-node model, is used to simulate the energy transfer and the stratification inside the tank. The results obtained are compared to a software package for simulation of energy systems called TRNSYS and some experimental results to validate the model.

Performance assessment of a novel diffuser for stratified thermal energy storage tanks – The nonequal-diameter radial diffuser

Owing to their simple structure, easy installation, low cost, and excellent thermal stratification, radial diffusers have been widely used in large-scale stratified thermal energy storage (TES) tanks. The current work proposes a novel nonequal-diameter radial diffuser. The short baffles for the top diffuser and bottom diffuser are above and below, respectively. On the one hand, the new diffuser design is beneficial to thermal stratification. On the other hand, it can reduce the cost of the radial diffuser. The thermal stratification and cost analysis of the proposed nonequal-diameter radial diffuser is performed based on the computational fluid dynamics. Moreover, the influence of the structural parameters on the diffuser performance is analyzed. The research results demonstrate that the nonequal-diameter radial diffuser has nearly the same thermal stratification performance as the equal-diameter radial diffuser and can significantly reduce the cost (37.5% when the diameter of the short baffle is 2/3 that of the long baffle). The thermal stratification performance of the stratified TES tank is optimized when the diameter ratio of the long baffle and the tank is 1/3, the diameter ratio of the short baffle and the long baffle is 1/3, and the distance between the two baffles is as small as possible. The research results are of considerable significance to the design, operation, and optimization of stratified TES tanks.

Optimum Design and Control of Heat Pumps for Integration into Thermohydraulic Networks

Germany has become one of the leading players in the transformation of the electricity sector, now having up to 42% of electricity coming from renewable sources. However, the transformation of the heating sector is still in its infancy, and especially the provision of industrial process heating is highly dependent on unsustainable fuels. One of the most promising heating technologies for renewable energies is power-to-heat, especially heat pump technology, as it can use renewable electricity to generate heat efficiently. This research explores the economic and technical boundary conditions regarding the integration of heat pumps into existing industrial thermohydraulic heating and cooling networks. To calculate the optimum design and control of heat pumps, a mixed-integer linear programming model (MILP) is developed. The model seeks the most cost-efficient configuration of heat pumps and stratified thermal storage tanks. Additionally, it optimizes the operation of all energy converters and stratified thermal storage tanks to meet a specified heating and cooling demand over one year. The objective function is modeled after the net present value (NPV) method and considers capital expenditures (costs for heat pumps and stratified thermal storage tanks) and operational expenditures (electricity costs and costs for conventional heating and cooling). The comparison of the results via a simulation model reveals an accuracy of more than 90%.

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.

SIMULATION STUDY OF PARABOLIC TROUGH SOLAR POWER PLANTS IN BRAZIL

Various data reveals the potential of concentrated solar technologies for the electricity production. With global growing energy demand and green-house gas emission, concentrating solar power is considered as one of the promising options and has invited wide attention. In this work, a model for a 30 MW parabolic trough solar power plant system was developed for 31 different locations in Brazil, using TRNSYS simulation software, and TESS and STEC libraries. The power system consists of a parabolic trough solar collector loop connected to a power block by a series of heat exchangers. The solar collector loop consists of a field of parabolic trough collectors, stratified thermal storage tank, pump and heat exchangers to drive the power block and uses Therminol VP1 as heat transfer fluid. The results show that the cities of Recife (PE), Fortaleza (CE), Belterra (PA), Salvador (BA) and Petrolina (PE) stand out for their high monthly values of direct normal irradiation and, resulting the highest production of energy by the same configuration of Solar Central Power Plant.

A 1-dimensional continuous and smooth model for thermally stratified storage tanks including mixing and buoyancy

To mitigate the effects of the intermittent generation of renewable energy sources, reliable and efficient energy storage is critical. Since nearly 80% of households energy consumption is destined to water and space heating, thermal energy storage is particularly important. In this context, we propose and validate a new model for one of the most efficient heat storage systems: stratified thermal storage tanks. The novelty of the model is twofold: first, unlike the non-smooth models from the literature, it identifies the mixing and buoyancy dynamics using a smooth and continuous function. This smoothness property is critical to efficiently integrate thermal storage vessels in optimization and control problems. Second, unlike models from literature, it considers two types of buoyancy: slow, linked to naturally occurring buoyancy, and fast, associated with charging/discharging effects. As we show, this distinction is paramount to identify accurate models. To show the relevance of the model, we consider a real tank that can satisfy heat demands up to 100 kW. Using real data from this vessel, we validate the proposed model and show that the estimated parameters correctly identify the physical properties of the vessel. Then, we employ the model in a control problem where the vessel is operated to minimize the cost of providing a given heat demand and we compare the model performance against that of a non-smooth model from literature. We show that: (1) the smooth model obtains the best optimal solutions; (2) its computation costs are 100 times cheaper; (3) it is the best alternative for use in real-time model- based control strategies, e.g. model predictive control.

Modelling stratified thermal energy storage tanks using an advanced flowrate distribution of the received flow

Energy storage plays a central role in managing energy resources and demand. Among the numerous energy storage technologies, stratified storage tanks are a promising option, but their operation requires to be finely tuned in order to optimize their utilization. Accurate models are required to properly design and control such systems. In this paper, an advanced flowrate distribution of the flow entering the tank is developed for modelling stratified storage tanks based on a nodal approach. The model is calibrated and validated with the measurements of a 240-m3 water tank used in a solar community district heating system. The effects of the model parameters and the simulation time step on the model accuracy are also investigated and the selection of the optimal number of nodes must be carefully performed along with the simulation time step. Overall, the proposed model achieves better results than the traditional method and is suitable to be used in applications such as design optimization problems and model predictive control.

Predicting heating demand and sizing a stratified thermal storage tank using deep learning algorithms

This paper evaluates the performance of deep recurrent neural networks in predicting heating demand for a commercial building over a medium-to-long term time horizon (⩾1 week), and proposes a modeling framework to demonstrate how these longer-term predictions can be used to aid design of a stratified thermal storage tank. The building sector contributes significantly to primary energy consumption in the US, and as such, there is a need to predict heating demand in buildings over longer time horizons, and to develop methods that can facilitate installation, planning and management of distributed generation and thermal storage to meet these heating demands. Key objectives of this paper are: (a) Investigate how a deep recurrent neural network model performs in predicting heating demand in campus buildings at University of Utah over multiple weeks, and (b) develop an optimization framework that which can provide definitive guidelines on sizing a stratified thermal storage tank without requiring high performance computing resources. The results showed that the predictions by the deep RNN are comparatively more accurate than those by a 3-layer MLP, and that these deep RNN predictions can adequately serve as proxy for future demand while considering sizing in the design of a complementary stratified thermal storage tank.

Investigation of Stratified Thermal Storage Tank Performance for Heating and Cooling Applications

A large amount of energy is consumed by heating and cooling systems to provide comfort conditions for commercial building occupants, which generally contribute to peak electricity demands. Thermal storage tanks in HVAC systems, which store heating/cooling energy in the off-peak period for use in the peak period, can be used to offset peak time energy demand. In this study, a theoretical investigation on stratified thermal storage systems is performed to determine the factors that significantly influence the thermal performance of these systems for both heating and cooling applications. Five fully-insulated storage tank geometries, using water as the storage medium, were simulated to determine the effects of water inlet velocity, tank aspect ratio and temperature difference between charging (inlet) and the tank water on mixing and thermocline formation. Results indicate that thermal stratification enhances with increased temperature difference, lower inlet velocities and higher aspect ratios. It was also found that mixing increased by 303% when the temperature difference between the tank and inlet water was reduced from 80 °C to 10 °C, while decreasing the aspect ratio from 3.8 to 1.0 increased mixing by 143%. On the other hand, increasing the inlet water velocity significantly increased the storage mixing. A new theoretical relationship between the inlet water velocity and thermocline formation has been developed. It was also found that inlet flow rates can be increased, without increasing the mixing, after the formation of the thermocline.

One-dimensional model of a stratified thermal storage tank with supercritical coiled heat exchanger

This paper presents a semi-transient model of the thermal stratification in a hot water storage tank used as a gas cooler in a transcritical CO2 heat pump system. It includes two different kinds of coiled heat exchangers. The first heat exchanger model is based on the analytical solution of the 1D flow in a pipe with constant thermodynamic properties and negligible pressure loss. The second coil model is discretized along the flow direction, the properties are updated along the coil and the velocity, enthalpy and pressure field are calculated. Both heat exchanger models are steady state and the axial diffusion heat transfer in the storage tank is transient. The novelty here is that the second exchanger model accounts for supercritical and two-phase CO2 flow in the pipe immersed in the reservoir. This can thus be considered as a generalisation of TRNSYS Type 534. A numerical comparison is carried out between the actual model and the TRNSYS Type 534 from the TESS library to assess its validity. A validation of the CO2 heat exchanger is also carried out using experimental and numerical results of a tube-in-tube heat exchanger available in the literature. Finally, numerical results where the tank is used as a transcritical heat pump gas cooler were produced.

Modelling Techniques Used in The Analysis of Stratified Thermal Energy Storage: A Review

Thermal energy storage plays an important role in the energy management and has got great attention for many decades; stratification is a key parameter to be responsible for the performance of the stratified thermal energy storage tank. In this paper detailed study of modelling techniques used to analyse thermal energy storage has been conducted. The division of literature has been made as numerical, analytical, and artificial neural network-based. Numerical modelling being very physical based and required more specific software’s tools remain costly and computationally very complex at the same time it provides the detailed insights into the system, analytical model provide the exact solutions but need some assumptions which make the system unrealistic in some cases but is easy and flexible in terms of computational requirements, ANN though recently used modelling technique is a black box model which merely needs the data rather than any physical based complex calculations is attracting the scientific community.