Thermal Energy Storage Unit Research Articles

HEAT TRANSFER IN A FORM-STABLE DIRECT-CONTACT LATENT THERMAL ENERGY STORAGE UNIT

This study investigated various aspects of thermal storage concept, including material characterization and analysis of form-stable, unencapsulated phase-change materials (PCM) that underwent solid-solid phase transition and was in direct contact with the working fluid. The study focused on temperature range between 100-140°C (212-284°F). Mathematical heat transfer models were developed to examine the operating characteristics of the thermal energy storage unit, identify key parameters influencing storage, and conducted parametric studies. Both single-phase and phase-change working fluids were considered in the models. Experiments were conducted using a packed bed of PCM pellets and a single-phase working fluid (tri-ethylene glycol) to evaluate and demonstrate the heat storage concept during charging and discharging. The experimental results aligned well with the heat transfer models, validating their accuracy. Parametric studies explored a wide range of parameters not feasible in laboratory experiments, shedding light on charging, discharging, and thermal storage characteristics. These models facilitated the development and implementation of optimization algorithms for packed bed latent heat storage units. The findings indicated that form-stable latent heat units utilizing commercially available polymers undergoing solid-solid phase transition can serve as long-term stable thermal storage candidates for use with several single-phase working fluids as well as two-phase steam.

Case Study of Optimized Cascaded Phase Change Thermal Energy Storage Unit

Phase change materials are paid increasing attention by the scholars in the past decades. For maintaining a relatively constant renewable energy output as a competent alternative for fossil fuel replacement, phase change storage unit are widely studied. However, for better pursuit of thermal energy performance and energy efficiency, only single staged phase change storage unit is not enough for increasing thermal requirements. Therefore, a triple staged phase change thermal energy storage unit have been proposed in this study. Meanwhile, correspondent comparison with single staged counterpart have been conducted along with related optimization. It has been concluded that the proposed thermal storage unit achieved 334.95 J/s & 41.54% / 186.37 J/s & 53.22 % in thermal energy exchange rate & exergy efficiency during endothermic/exothermic respectively. 3/21.4 L/min are the optimal circulating heat transfer fluid flowrates for endothermic/exothermic (better discharging rate)/ exothermic (better energy efficiency). Moreover, 100/9/15 are the best temperatures of circulating water for endothermic/ exothermic (better discharging rate)/ exothermic (better energy efficiency).

Optimum design of a horizontal shell-and-tube latent heat thermal energy storage system via non-uniform upper-and-lower cascade PCMs

This paper concerns the optimum design of horizontal shell-and-tube latent heat thermal energy storage (LHTES) units that use symmetric splitter plates to structure non-uniform upper-and-lower (UAL) cascade PCMs with different combination ratios (2:1, 3:1, 5:1) further to improve melting/temperature uniformity and thermodynamic performance. The results illustrate that the three kinds of UAL cascade PCMs design (Case 2-T, Case 2-N, Case 2-S) can significantly improve the thermodynamic performance and melting/temperature uniformity compared with traditional horizontal shell-and-tube LHTES units (Case 1). For the UAL cascade PCMs design, Case 2-N can melt completely at nearly the same time for the two PCMs in the sector region and achieve the optimum melting performance. However, it is found that Case 2-S has advantages in minimizing the entropy generation number and improving the exergy efficiency. Besides, lower HTF temperature and mass flow rate can bring about higher exergy efficiency, but it also weakens the heat transfer rate. The paper can provide a certain reference value for the structural optimization of the horizontal shell-and-tube LHTES units.

A comprehensive and comparative study of an innovative constant-pressure compressed air energy storage (CP-CAES) system

Electricity and potable water are two vital resources for the world's population. A pioneering green energy storage system for power and potable water production has been introduced and investigated in this context. The innovative system integrates compressed air, pumped hydro, and thermal energy storage, along with multi-effect desalination. This combination enables the generation of power and potable water without exerting any adverse impact on the environment. This technology manages the start-up time of the desalination unit by using a low thermal energy storage unit and allows potable water to be generated throughout the day. By conducting a comprehensive analysis including energy, exergy, economic, and exergoeconomic, the performance of the system has been carefully examined and the effect of various parameters has also been considered. Finally, the advantages of the proposed system are compared to the conventional system. According to the study, round trip efficiency and total cost rate have been calculated as 62.18% and 618.6 USD/hr, respectively. Based on a case study conducted in the coastal areas of the state of California, the proposed system has the potential to generate 138 m3 of potable water and 88.11 MWh of electricity per day. This could lead to a total profit of 27.65 million dollars with a payback period of 3.82 years.

Data-driven approaches to achieve energy transition: Statistical analysis of demand-side management and smart decision making in urban energy hub networks

Energy hubs (EHs) have emerged as a crucial solution to address resource scarcity and environmental concerns by integrating various technologies for energy production, conversion, and storage. These hubs encompass diverse elements such as integrated heat, electricity, and cooling systems, renewable energy sources, batteries, and thermal energy storage units (TES). This study focuses on a residential EH model that receives inputs in the form of electricity, gas, and solar energy to satisfy the household's electricity, heat, and cooling requirements. To enhance the operational flexibility of this energy hub, the study employs a Demand Response Program (DRP), which facilitates load shifting, load curtailment, and the modeling of flexible heating loads. The primary objective is to develop an efficient heat and power management system that optimally schedules household devices, generation equipment, and storage units. In pursuit of this goal, Long Short-Term Memory (LSTM) techniques have been selected for renewable energy, market price, and load forecasting, enhancing the system's realism and predictive accuracy. The Wild Goats algorithm is employed to solve an optimization problem for three distinct scenarios, aiming to minimize overall power costs while maintaining user comfort levels with regard to temperature and hot water availability. Furthermore, this study incorporates an environmentally conscious approach by considering the reduction of CO2, SO2, and NO pollution originating from consumers in a multi-objective optimization framework. The findings reveal that the implementation of DRP, intelligent Plug-in Hybrid Electric Vehicle (PHEV) management, TES technologies, and LSTM-based forecasting significantly contribute to reducing power costs within the proposed EH scheme. This research underscores the potential of data-driven strategies in advancing energy transition initiatives and fostering sustainable urban energy hubs while acknowledging the role of advanced forecasting techniques in enhancing system realism and efficiency.

Heat transfer performance enhancement and mechanism analysis of thermal energy storage unit designed by using a modified transient topology optimization model

The spatial layout of the highly conductive fin and phase change materials (PCM) and the thermophysical properties of PCM are important factors restricting the heat transfer rate of the latent heat thermal energy storage (LHTES). The current fin design concentrates on the limited design space, and rarely adopts the optimizing form of unconstrained structure evolution in the process. So this study proposes a modified transient topology optimization (TO) model to seek the optimal distribution of the fins embedded into PCM subject to the constraints of fin area and specified time. This topology optimization problem is formulated using the density-based material design variable in governing equations, physical constraints, etc. The design freedom and procedural iterative solving of TO model allow for the free evolution of innovative fin structures and elucidate the interaction mechanism between the fin design and physical occurring processes. The double interpolation function, variable regularization, adjoint-based sensitivity analysis and GCMMA algorithm are established to further improve TO model to obtain high-precision solutions. Using above methods, the regular evolution mechanism of fins and physical performance parameters and the trade-off between performance and structure are analyzed. Then, the effects and reasons of pure paraffin (PW) and the adding carbon nanotubes (nPW) on changes in topology structure and performance parameters are also comparatively studied. Finally, by comparing TO-fins with conventional rectangular fins, the superiority of the established method is verified, and the performance improvement mechanism and extent of TO-fins are revealed. Results show that TO-fin LHTES with nPW can achieve better performance than TO-fin LHTES with PW under fewer branches and shorter branch length. The uniform distribution and multi-branch characteristics of TO-fins greatly shorten the heat conduction path, and its local segmentation of whole PCM enhances the local convection effect.

Performance comparison and enhancement of the thermal energy storage units under two expansion methods

To improve the performance of the basic thermal energy storage unit, two expansion methods, modular combination and linear structural expansion, are proposed and compared through numerical simulations. The impacts of the two expansion methods on the performance of the storage units are compared by investigating the thermal storage and release processes. Following the numerical study, both expansion methods can increase the heat storage capacity and airflow rates compared to a basic phase change material (PCM) storage unit linearly. However, the linear structural expansion method will increase the PCM melting time from 147 min to 367 min when the heat storage capacity of the basic unit is increased two times, which is about 2.5 times longer duration than using the modular expansion method. As for the heat release process, the results indicate that thermal release performance using linear structural expansion is lower than that of the modular combination along with a decrease in heat release efficiency of about 32.53 %. In conclusion the modular combination method is proved to be more efficient compared to the linear structural expansion method for improving the performance of the PCM storage units.

Experimental and numerical investigation on the cold harvest of composite phase change materials for building energy conservation

This paper investigates the cold energy storage of a horizontal shell-tube latent heat thermal energy storage (LHTES) unit for the cold harvest during night time. An experimental apparatus containing a horizontal circular tube with circumferential heating/cooling was designed and developed. Paraffin based phase change material (PCM) with a melting temperature of 27 °C was adopted as the cold harvest media. To improve the heat transfer rate, multi-walled carbon nanotubes (MWCNTs) were mixed with the paraffin to form a composite PCM with high thermal conductivity. A corresponding model of the composite PCM was developed and verified against the experimental data. Numerical simulations were performed for different boundary temperatures and concentrations of MWCNTs. The results show that when the heat transfer temperature difference is 20 °C, 0.05 wt% nanoparticles increase the melting rate of PCMs. When the heat transfer temperature difference was 40 °C, 0.10 wt% nanoparticles significantly affect the PCM melting rate. For low or high temperature (heat transfer temperature difference of 10 °C and 80 °C), the addition of nanoparticles is not conducive to improving the melting rate.

Effective one‐dimensional dynamic modelling of latent heat thermal energy storage units for heating applications

AbstractEffective use of the energy stored within thermal energy storage systems requires mathematical models that faithfully represent the dynamics of interest. Although three‐dimensional or two‐dimensional models may provide an accurate representation of the thermal store, these are computationally intensive and may not be suitable for control system design or to simulate complex networks. Following this line, a low‐order one‐dimensional model of a latent heat thermal store is presented. The model is based on energy balance, the specific heat–temperature curve of the storage medium, and the dynamic calculation of the heat transfer coefficient. The simplicity afforded by the model facilitates its implementation in any programming language, guaranteeing its compatibility with commercial software to simulate complex systems. The model was implemented in MATLAB/Simulink and verified against experimental data of the real unit and simulation results obtained with a two‐dimensional model. Simulation results for charging and discharging operations obtained with the one‐dimensional model exhibit a root mean square error of ≤0.53 °C and a mean square error of ≤0.32 °C when compared with experimental results of the output temperature of the heat transfer fluid. These outcomes are deemed acceptable considering the low order of the one‐dimensional model.

Accelerating melting time in a triplex tube heat exchanger thermal energy storage unit with micro-channels

This article studies optimal conditions to maximize heat transfer efficiency within a triplex tube heat exchanger (TTHX) filled with phase-change material. In this context, micro-channels (MCs) incorporated into TTHX for the first time for acceleration melting time. A two-dimensional numerical model has been developed, and pure conduction and natural convection are both simulated. The Reynolds numbers range from 1 to 100. The types of temporal velocity profiles into micro-channels were uniform, step, elliptic, sinusoidal, and power. The effect of gravity states, namely no gravity, with gravity, and (MCHE), has been studied with different numbers of microchannels (4, 6, 8, 10, 12, and 14). The results showed that there was no variation in melting duration observed when increasing the Re number from 50 to 100. Thus, the maximum reduction in melting period occurred at Re = 50. Moreover, sinusoidal velocity profiles were more effective and could help the process finish sooner. In addition, micro channel tube exchanger (MCTE) fitted or equipped with 10 micro-channels has an optimum condition considering pressure drop and complete melting time. Using the sinusoidal profile reduced the total fusion time by 325 s compared to uniform flow (8.47 % reduction). This indicates that the sinusoidal profile is an effective way to reduce the melting period.

Parameter analysis and rapid design of porosity gradient distribution for open-cell metal foam in the latent thermal energy storage unit

The positive gradient porosity design of metal foam is used to improve the thermal performance of the vertical shell-and-tube latent thermal energy storage (LTES) unit. To optimize the gradient design, quantitative analysis is of vital importance. Therefore, the relative offset number EX is proposed in this study, which indicates the porosity difference between upper/down porosity and the mean porosity. Based on the numerical model of a shell-and-tube LTES unit filled with composite phase change material (CPCM) on the shell side, the effects of parameters, including the inlet HTF temperature and velocity, LTES dimensions, porosity, and thermal conductivity of the metal foam on the optimal gradient design are discussed. The optimal EX exhibits little relation with the inlet temperature but increases with the LTES height and thermal conductivity, while decreases with inlet velocity, porosity, and LTES outer radius. With the correlation between the optimal EX and the relative dimensionless number, the effects of these parameters on the optimal EX are integrated and quantified as experience equations so that the optimal porosity gradient of metal foam and the relative reduction melting time can be rapidly obtained under a wide variety of practical applications.

Experimental study of thermal energy storage system for solid particles/ heat transfer oil in shell and tube heat exchangers with H-shaped fins

The solid-state sensible heat storage method is cost-effective, technically simple, and works well across wide temperatures. Using return fines (RFs) as the heat storage medium (HSM) can ease problems like RFs in excess and high energy consumption in the sintering process. This article first characterizes the thermal properties of RFs. Results show a specific heat capacity of 0.67–0.97 kJ/(kg·°C) within 20–380 °C, with stable thermal properties from 100 to 1000 °C. Then, the heat transfer performance of RFs and heat transfer oil (HTO) in a shell and tube heat exchanger is experimentally investigated. H-shaped fins are added to enhance the heat transfer. Stacking solid particles on the shell side can avoid direct contact between the HSM and HTO, the tiny size limitations of HSM particles, and cracking problems caused by thermal expansion. The thermal energy storage (TES) unit operates within 170–270 °C. Effects of different flow rates (30, 20, 10 L/min) and charging/discharging temperatures (300/160, 290/150, 280/140 °C) on system performance are explored. Increasing the HTO flow rate and charging temperature and lowering the discharging temperature can reduce charging/discharging time and enhance average power. Cycle efficiencies for flow rates of 30, 20, and 10 L/min are 86.9%, 85.2%, and 79.5% respectively. The 30 L/min working condition shows the best performance. Remarkably, system performance improvements are relatively limited compared to the 20 L/min setting. Regarding charging/discharging temperatures of 300/160, 290/150, and 280/140 °C, cycle efficiencies stand at 73.3%, 85.2%, and 70.6%, respectively, with the 290/150 °C setting displaying the most favorable characteristics, including the shortest total charge and discharge time of 59.3 min and the highest cycle efficiency. Efforts to enhance the system's insulation and maintain a substantial temperature differential between HTO and HSM are pivotal in accelerating the charge/discharge process and minimizing heat loss.

Design and Thermodynamic Investigation of a Waste Heat‐Assisted Compressed Air Energy Storage System Integrating Thermal Energy Storage and Organic Rankine Cycle

Compressed air energy storage (CAES) technology has attracted growing attention because of the demand for load shifting and electricity cost reduction in energy‐intensive industries. To increase the round‐trip efficiency and energy storage density and simplify the structure of advanced adiabatic CAES (AA‐CAES) systems, a waste heat‐assisted CAES (WH‐CAES) design integrating a tube‐in‐tube thermal energy storage unit and an organic Rankine cycle (ORC) is described herein. Thermodynamic models of the proposed WH‐CAES system are established, and the effects of waste heat temperature, ratio of compression ratios, ratio of expansion ratios, and air storage cavern pressure on the performance of different systems, including AA‐CAES and WH‐CAES with and without ORC, are investigated. The results show that when the waste heat temperature is 500 °C, the input work of the compressors reaches its minimum value at a ratio of compression ratios of 1.14, and the optimal ratio of expansion ratios is 0.39. Under these optimal operating conditions, compared with the AA‐CAES system, the round‐trip storage efficiency, energy storage density, and system energy efficiency of the WH‐CAES system can be increased significantly. The findings validate the potential of the proposed WH‐CAES system.

A self-driven solar air heater integrated with a thermal energy storage unit: Design and experiment study

The Self-driven solar air heater (SDSAH) can operate independently without reliance on an external power grid through the incorporation of photovoltaic (PV) panels, however it is still subject to intermittent operation and large temperature fluctuations. In this study, a novel SDSAH integrated with a thermal energy storage (TES) unit was proposed and the operational thermal and electrical performance of the SDSAH with TES unit were experimentally investigated. The results showed that the SDSAH could output heat continuously throughout the day (24 h) in a self-sustaining mode and automatically adjust the airflow rate according to solar irradiance. The overall thermal efficiency of this system averages at 52.73 % throughout the day. The introduction of the TES unit could lower the peak temperature of the outlet air by more than 9 K and improve the electrical efficiency of PV panels. However, due to heat loss at night, the whole-day thermal efficiency will be slightly lower than that of SDSAH without a TES unit. The energy payback time for this system was 42 months. This study offers new ideas and solutions for a distributed agricultural drying system.

Design and assessment on a bottom-cut shape for latent heat storage tank filled with metal foam

The utilization of phase change materials (PCMs) holds tremendous potential of heat storage domain. The PCM's refractory at the latent heat thermal energy storage (LHTES) unit bottom hinders the heat storage efficiency, despite the significant improvement in thermal conductivity achieved through the addition of metal foam. This study employs numerical simulation to examine the impact of applying bottom cross-cut on PCM's spatial distribution in a horizontal LHTES unit. The manuscript analyzes parameters including melting fraction, complete melting time, Rayleigh number, natural convection heat transfer gain, melting phase interface, velocity and temperature distributions, and heat storage. The findings indicate that the proximity to the heating tube results in a reduction of solid volume at the LHTES unit bottom. A 0.6 bottom cross-cut ratio leads to an 18.84 % faster heat storage rate compared to a concentric-circle unit. Furthermore, a bottom cross-cut ratio of 0.5 enhances natural convection heat transfer gain by 3.28 times.

Design and Research of Heat Storage Enhancement by Innovative Wave Fin in a Hot Water–Oil-Displacement System

Energy storage technology provides a new direction for the utilization of renewable and sustainability energy. The objective of this study is to introduce a novel, wavy, longitudinal fin design, which aims to improve heat transfer in the melting process of a Latent Heat Thermal Energy Storage (LHTES) unit. The main goal is to mitigate the negative effects caused by the refractory zone at the end of the melting phase. A two-dimensional numerical model of LHTES unit is established by using the enthalpy porosity method and verified by experimental data. Through the quantitative comparison between the traditional rectangular fin and the innovative wave fin, the influence of wave fin on the heat transfer mechanism in the heat storage process is revealed. The results show that the average heat storage rate of five and six wave fins is 3.70% and 12.98% higher than that of conventional rectangular fins, respectively, and the average temperature response of six wave fins is 17.78% higher than that of conventional rectangular fins. The addition of the wave fin weakens the negative effect of the refractory zone, but prolongs the heating time of the initial melting point.

Numerical analysis and optimization of the heat transfer enhancement from the heat transfer fluid side in a shell-and-tube latent heat thermal energy storage unit: Application to buildings thermal comfort improvement

This study investigates the impact of enhancing the convective heat transfer on the Heat Transfer Fluid (HTF) side by passive flow manipulation on the melting and solidifying kinetics of a Phase Change Material (PCM) in a shell-and-tube Latent Heat Thermal Energy Storage (LHTES) unit. The PCM used is PureTemp 23 (PT23), a commercial bio-sourced product. The targeted application is the development of a LHTES unit for improving the thermal comfort in buildings through a combined heating and cooling approach. The study is carried out using the commercial code STARCCM+ and the enthalpy-porosity method, with appropriate implementation of the buoyancy source term. The numerical approach used for phase change modeling has been validated by comparisons with reference data from the open literature. Delta Wing pairs (DWg) and Delta Winglet pairs (DWt) vortex generators (VGs), were first inserted into the tube in which the HTF flows. Parametric analysis on the aspect ratio, position, and VGs number were performed, and optimization was done on the attack and rolling angles. The Performance Evaluation Criteria (PEC) index, which takes into account the heat transfer enhancement achieved and the related pressure losses, was used as an assessment criterion to evaluate the heat transfer augmentation of the studied enhanced tube with VGs. The results show that maxima can be reached for the different parametric studies carried out. While PEC increases with the attack angle until a maximum beyond which it decreases, it decreases consistently with the rolling angle. Once the enhanced optimal tube equipped with a shell containing the PCM, the results of the detailed study made on the kinetics for charging and discharging period of the enhanced LHTES unit reveal important gain for the storage unit. The heat transfer enhancement achieved result into more than 110 % increase in the PCM's melting/solidifying kinetics in the shell, thanks to an increase in the heat duty exchanged between the HTF and the PCM, leading to a considerable reduction of complete melting/solidification time. It was observed that the enhanced and optimized LHTES unit developed in the present study has a huge potential for improving buildings seasonal thermal comfort.

A numerical study of geopolymer concrete thermal energy storage: Benchmarking TES module design and optimizing thermal performance

Geopolymer (GEO) concrete emerges as a potential high-temperature thermal energy storage (TES) material, offering a remarkable thermal storage capacity, approximately 3.5 times higher than regular Portland cement (OPC) concrete, without compromising its environmentally benign nature.This research dissects the application of GEO concrete as a high-temperature TES material, primarily focusing on its optimization and scalability. The introductory part of the study involves the development and validation of a three-dimensional numerical model using computational fluid dynamics (CFD). The model demonstrated an average accuracy rate of 5 %, as justified by empirical data. Later, a two-tiered investigation to determine the optimal design for GEO concrete TES systems was investigated. Three different geometries plus the impact of crucial parameters such as air velocity, tube diameter, and module size on the thermal storage capacity (Q) studied. It further extends into a parametric examination, exploring a variety of tube sizes, arrangements, and configurations. It is found that air velocity primarily influences Q.A subsequent phase provides an analysis of the thermodynamic effects brought by the inclusion of tubes within TES modules through an equivalent parametric study. It exposes the thermal resistance resulting from tube insertion. The study reinforces the superior thermal performance of tubeless GEO concrete TES configurations, as signified by overall heat transfer rate (Q̇). The study also signals the significant roles of key parameters in determining the temperature (T) and Q within TES unit using Pearson's correlation coefficient equation.As a final observation, this work emphasizes the sustained significance of on-site evaluations to consistently monitor the interplay between TES materials and high-temperature fluids (HTFs) over extended periods for viability analysis purposes.

A proposal for advanced supplementary technologies and a hybrid system with gas-cooled fast reactor concept ALLEGRO

A coupling of advanced reactors with high-capacity energy storage and other innovative non–nuclear energy technologies is a way to increase the flexibility and utilisation factor of the future nuclear systems and to diversify its energy outputs. In this paper, the basic layout of a hybrid system for a gas-cooled fast reactor is proposed. The system is composed of a high-temperature thermal energy storage unit coupled with a supercritical CO2 conversion cycle and both low-temperature and high-temperature hydrogen production and utilization. As a reference GFR reactor, the ALLEGRO concept is considered. The paper describes state-of-the-art and overviews potentially applicable technologies. Selection and justification of concepts of the individual parts of the hybrid system including operational conditions is also given. The R&D needs necessary for the potential implementation of the system are identified and the activities ongoing in the Czech Republic and especially in Centrum Výzkumu Řež are also described.

A New Approach of Solidification Analysis in Modular Latent Thermal Energy Storage Unit Based on Image Processing

The solidification process of RT18HC in a cylindrical shell and tube storage unit has been studied using a new methodology based on image processing. The main idea of the algorithm is to label the region of solidification and use statistical functions to calculate the dimensions of the solidification front over time. Said analysis includes two methods. The first method is to measure the solid fraction changes during solidification. The novelty of this method, as compared to other literature findings, is that pre-processing and calculation process occurs automatically via a calculation algorithm. This method is used to calculate the solid fraction of RT18HC which is reported to be a bit fast at the beginning that 40 % of its volume solidified in 1000 s while the rest of the process is completed in almost 6500 s. The second method is used to measure and calculate the thickness of the solid front by using image processing. This method’s error is calculated to be less than 7% throughout the entire process. The second method also acts as an experimental database of front thickness to use in a novel, simplified, semi-theoretical model proposed to calculate the solid front thickness as a function of time in this paper. It is also worth presenting solution extended by a general definition of thermal resistance for a cylindrical partition. The above study will enable the development of an enhanced and optimized model for complex geometries based on image processing techniques in the future. It will also allow the investigation of both processes i.e. solidification and melting alongside other influencing parameters such as the geometry of the storage unit in future.