Investigation of the effect of metal foam characteristics on the PCM melting performance in a latent heat thermal energy storage unit by pore-scale lattice Boltzmann modeling

Latent heat thermal energy storage (LHTES) has many advantages such as high energy density and phase change at a nearly constant temperature compared with sensible thermal energy storage or chemical energy storage techniques. However, one of its major drawbacks is the low thermal conductivity of phase change materials (PCMs) which impedes the heat transfer efficiency. High thermal conductivity metal foams could be added into the LHTES to enhance the heat transfer speed. Under this case, the investigation of the effects of metal foam porosity and pore size on the melting process is essential for improving the heat storage capability of LHTES. In this article, a pore-scale modeling of melting process in a LHTES unit filled with metal foams is carried out by enthalpy-based multiple-relaxation-time lattice Boltzmann method. The quartet structure generation set is used to generate the morphology of metal foams. In addition, a Compute Unified Device Architecture (CUDA) Fortran code is developed in this work for executing highly parallel computation through graphics processing units. The melting process in the PCMs is investigated in terms of porosity, pore size, nonuniform metal foam, hot wall temperature, and initial subcooled temperature to optimize the design of LHTES filled with metal foams.     

Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage

Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the most drawback which limits the utility area of them in thermal energy storage. The use of fatty acids as form-stable PCM will increase their feasibilities in practical LHTES applications due to reduced cost of the energy storage system. In this regard, a series of fatty acid/poly(methyl methacrylate) (PMMA) blends, SA/PMMA, PA/PMMA, MA/PMMA, and LA/PMMA were prepared as new kinds of form-stable PCMs by encapsulation of fatty acids into PMMA which acts as supporting material. The blends were prepared at different mass fractions of fatty acids (50, 60, 70, 80, and 90% w/w) to reach maximum encapsulation ratio. All blends were subjected to leakage test by heating the blends over the melting temperature of the PCM. The blends that do not allow leakage of melted PCM were identified as form-stable PCMs. The form-stable fatty acid/PMMA (80/20 wt.%) blends were characterized using optic microscopy (OM), viscosimetry, and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the PMMA was compatible with the fatty acids. In addition, thermal characteristics such as melting and freezing temperatures and latent heats of the form-stable PCMs were measured by using differential scanning calorimetry (DSC) technique and indicated that they had good thermal properties. On the basis of all results, it was concluded that form-stable fatty acid/PMMA blends had important potential for some practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floor impregnated with a form-stable PCM due to their satisfying thermal properties, easily preparing in desired dimensions, direct usability without needing an add encapsulation and eliminating the thermal resistance caused by shell and thus reducing cost of LHTES system.     

Economic evaluation of latent heat thermal energy storage using embedded thermosyphons for concentrating solar power applications

An economic evaluation of a latent heat thermal energy storage (LHTES) system for large scale concentrating solar power (CSP) applications is conducted. The concept of embedding gravity-assisted wickless heat pipes (thermosyphons) within a commercial-scale LHTES system is explored through use of a thermal network model. A new design is proposed for charging and discharging a large-scale LHTES system. The size and cost of the LHTES system is estimated and compared with a two-tank sensible heat energy storage (SHTES) system. The results suggest that LHTES with embedded thermosyphons is economically competitive with current SHTES technology, with the potential to reduce capital costs by at least 15%. Further investigation of different phase change materials (PCMs), thermosyphon working fluids, and system configurations has the potential to lead to designs that can further reduce capital costs beyond those reported in this study.     

基于改进粒子群算法的火储联合调频优化储能控制策略研究

为了解决火储联合调频协同控制效果与储能系统成本回收的问题,提出一种改进粒子群算法的储能优化控制策略,通过引入自适应权重调整全局搜索方向,并解决储能电荷状态与出力状态的耦合性问题,建立火储联合调频综合性能指标进行储能控制策略研究。案例仿真结果表明：本文所提算法比传统算法在收敛效果方面具有明显优越性,避免了局部最优解问题;当储能系统电荷状态设置在57.5%和42.5%时,提高了机组调频响应效果,延长了储能设备的使用寿命。     

Numerical Study of a Shell-and-Tube Latent Thermal Energy Storage Unit Heated by Laminar Pulsed Fluid Flow

The present study aims to investigate the effect of the pulsed fluid flow on the thermal performance of a latent heat storage unit (LHSU). The storage unit consists of a shell-and-tube in which phase change material (PCM) occupied the shell space and the heat transfer fluid (HTF) flows in the inner tube. The present study is motivated by the need to intensify heat transfer and accelerate melting process in LHSU. A mathematical model based on the conservation equations of energy in both HTF and PCM has been developed. The finite volume approach was used for the discretization of equations. The developed model has been validated by comparing the obtained numerical results with experimental, analytical, and numerical data found in literature. The effects of the pulsation frequency and amplitude, the Reynolds and Stefan numbers on the thermal performance and behavior of the LHSU were investigated. The parametric study showed that the pulsating parameters (frequency and amplitude) affect the thermal performance of the LHSU. The results reveal reduction in the melting time for low pulsating frequency (less than 0.052) and high pulsating amplitude. For pulsating amplitude of 6 and pulsating frequency of 0.01, a reduction up to 13% (at Reynolds number of 500 and Stefan number of 0.16) was obtained. The results also showed that the Reynolds and Stefan numbers strongly affect the heat transfer rate, and the low melting time is obtained for high Reynolds and Stefan numbers.     

Analysis of solidification in a finite PCM storage with internal fins by employing heat balance integral method

Here, a simplified analytical model has been proposed to predict solid fraction, solid–liquid interface, solidification time, and temperature distribution during solidification of phase change material (PCM) in a two‐dimensional latent heat thermal energy storage system (LHTES) with horizontal internal plate fins. Host of boundary conditions such as imposed constant heat flux, end‐wall temperature, and convective air environment on the vertical walls are considered for the analysis. Heat balance integral method was used to obtain the solution. Present model yields closed‐form solution for temperature variation and solid fraction as a function of various modeling parameters. Also, solidification time of PCM, which is useful in optimum design of PCM‐based thermal energy storages, has been evaluated during the analysis. The solidification time was found to be reduced by 93% by reducing the aspect ratio from 8 to 0.125 for constant heat flux boundary condition. While, for constant wall temperature boundary condition, the solidification time reduces by 99% by changing the aspect ratio from 5 to 0.05. In case of convective air boundary surrounding, the solidification time is found to reduce by 88% by reducing the aspect ratio from 8 to 0.125. Based on the analytical solution, correlations have been proposed to predict solidification time in terms of aspect ratio and end‐wall boundary condition.     

Research on melting and solidification processes for enhanced double tubes with constant wall temperature/wall heat flux

Latent heat thermal energy storage (LHTES) improves the energy utilization efficiency between energy supply and energy demand of heating storage in buildings and liquid desiccant air conditioning systems. The present work is focused on validated numerical investigation of the thermal performances of LHTES inside enhanced double tubes. The effects of the number of fins ranging from 2 to 10 and boundary conditions of the inner tube wall on the melting and solidification processes are examined. The results indicate that number of fins and wall boundary conditions play an important role in the thermal performances of LHTES. It is noted that recirculation flow in the liquid phase change material region is formed remarkably. The enhancement ratio for constant wall temperature is more significant than that of constant wall heat flux during the melting process. However, the discrepancy of the enhancement ratio for different inner wall temperatures is limited during the solidification process.     

Constructal enhancement of thermal transport in metal foam-PCM composite-assisted latent heat thermal energy storage system

The aim of this study is to augment thermal transport in latent heat thermal energy storage (LHTES) system by the optimum allocation of metal foam-phase change material (PCM) composite. This study emphasizes on the optimal volume and distribution of metal foam-PCM composite (MFPC) to enhance melting performance without delay in the total melting time. Therefore, a MFPC is designed according to constructal theory. The fundamental principle of the theory is to configure high thermal conductivity agents at optimal thermal energy flow path for effective heat exchange. A numerical code based on local thermal nonequilibrium approach equipped enthalpy porosity method is formulated, and evaluated. The results of the proposed configuration show that the provision of MFPC only at high local temperature gradient enhances the conductive transport with improvement in the overall thermal transport. It is derived that the elimination of metal foam volume at low temperature gradient incorporates the advantageous effect of natural convective transport, which is seen to be suppressed. Additionally, the proposed configuration may increase the volume of PCM, thus, the TES capacity. It also reduces the total weight and economy of energy storage system. The overall melting rate is improved by 11.11% in comparison with the LHTES with full volume of this high thermal conductivity agent.     

Heat transfer and exergy analysis of cascaded latent heat storage with gravity-assisted heat pipes for concentrating solar power applications

A thermal network model is developed to predict the performance of latent heat thermal energy storage (LHTES) systems including cascaded phase change materials (PCMs) and embedded heat pipes/thermosyphons. Because the design of LHTES systems involves a compromise between the amount of energy stored, the heat transfer rate, and the quality of the released thermal energy, an exergy analysis is also carried out to identify the preferred LHTES design. It is found that the LHTES with the lowest melting temperature PCM yields the highest exergy efficiency. However, a cascaded LHTES recovers the largest amount of exergy during a 24 h charging–discharging cycle. Quantitatively, the cascaded LHTES recovers about 10% more exergy during a 24 h charging–discharging cycle compared to the best non-cascaded LHTES considered in this work.     

基于火积理论的多级套管式相变蓄热系统优化

本文基于最小火积耗散热阻原理,在考虑相变材料导热热阻以及非稳态传热过程的基础上,对多级套管式相变蓄热系统的融化温度进行了数值优化,获得了最优融化温度分布。在此基础上,研究了相变材料导热系数和传热管长度对最优融化温度、火积耗散热阻和平均蓄热速率的影响。研究结果表明,与现有理论优化方法相比,本文提出的数值优化方法具有更好的适用性;优化后多级套管式相变蓄热系统可有效提高相变蓄热系统的平均蓄热速率,降低火积耗散热阻;随着相变材料导热系数增大和传热管长度增加,多级套管式相变蓄热系统最优融化温度的温差愈加明显,其强化传热性能呈上升趋势。     

Free cooling of a building using PCM heat storage integrated into the ventilation system

This article presents a study of the free cooling of a low-energy building using a latent-heat thermal energy storage (LHTES) device integrated into a mechanical ventilation system. The cylindrical LHTES device was filled with spheres of encapsulated RT20 paraffin, a phase-change material (PCM). A numerical model of the LHTES was developed to identify the parameters that have an influence on the LHTES’s thermal response, to determine the optimum phase-change temperature and to form the LHTES’s temperature-response function. The last of these defines the LHTES’s outlet-air temperature for a periodic variation of the inlet ambient-air temperature and the defined operating conditions. The temperature-response function was then integrated into the TRNSYS building thermal response model. Numerical simulations showed that a PCM with a melting temperature between 20 and 22 °C is the most suitable for free cooling in the case of a continental climate. The analyses of the temperatures in a low-energy building showed that free cooling with an LHTES is an effective cooling technique. Suitable thermal comfort conditions in the presented case-study building could be achieved using an LHTES with 6.4 kg of PCM per square metre of floor area.     

Chance-constrained energy-reserve co-optimization scheduling of wind-photovoltaic-hydrogen integrated energy systems

An optimal energy-reserve scheduling model of wind-photovoltaic-hydrogen integrated energy systems (WPH-IES) with multi-type energy storage devices including electric, thermal and hydrogen is presented in this paper. The chance-constrained programming (CCP) theory is utilized to model the impact of renewable and load uncertainties on reserve constraints. Considering the non-convex of proposed CCP model, an improved discretized step transformation (DST) method is proposed to transform the CCP problem into a solvable mixed integer linear programming (MILP) formulation. In addition, a critical threshold value selection approach is developed to reduce the number of constraints and improve solution efficiency. Case studies demonstrate that the proposed energy-reserve model can reduce the total operating cost on the premise of ensuring the safety of system operation. The combined improved DST and critical threshold value selection method can reduce computational burden and improve the accuracy of scheduling results.     

Review of thermal energy storage for air conditioning systems

Thermal energy storage is very important to eradicate the discrepancy between energy supply and energy demand and to improve the energy efficiency of solar energy systems. Latent heat thermal energy storage (LHTES) is more useful than sensible energy storage due to the high storage capacity per unit volume/mass at nearly constant temperatures. This review presents the previous works on thermal energy storage used for air conditioning systems and the application of phase change materials (PCMs) in different parts of the air conditioning networks, air distribution network, chilled water network, microencapsulated slurries, thermal power and heat rejection of the absorption cooling. Recently, researchers studied the heat transfer enhancement of the thermal energy storage with PCMs because most phase change materials have low thermal conductivity, which causes a long time for charging and discharging process. It is expected that the design of latent heat thermal energy storage will reduce the cost and the volume of air conditioning systems and networks.     

Optimal exponent heat balance and refined integral methods applied to Stefan problems

When using a polynomial approximating function the most contentious aspect of the Heat Balance Integral Method is the choice of power of the highest order term. In this paper we employ a method recently developed for thermal problems, where the exponent is determined during the solution process, to analyse Stefan problems. This is achieved by minimising an error function. The solution requires no knowledge of an exact solution and generally produces significantly better results than all previous HBI models. The method is illustrated by first applying it to standard thermal problems. A Stefan problem with an analytical solution is then discussed and results compared to the approximate solution. An ablation problem is also analysed and results compared against a numerical solution. In both examples the agreement is excellent. A Stefan problem where the boundary temperature increases exponentially is analysed. This highlights the difficulties that can be encountered with a time dependent boundary condition. Finally, melting with a time-dependent flux is briefly analysed without applying analytical or numerical results to assess the accuracy.     

Enhancement of latent heat energy storage using embedded heat pipes

Latent heat thermal energy storage (LHTES) utilizing heat pipes or fins is investigated experimentally. Photographic observations, melting and solidification rates, and PCM energy storage quantities are reported. Heat pipe effectiveness is defined and used to quantify the relative performance of heat pipe-assisted and fin-assisted configurations to situations involving neither heat pipes nor fins. For the experimental conditions of this study, inclusion of heat pipes increases PCM melting rates by approximately 60%, while the fins are not as effective. During solidification, the heat pipe-assisted configuration transfers approximately twice the energy between a heat transfer fluid and the PCM, relative to both the fin-assisted LHTES and the non-heat pipe, non-fin configurations.     

Transient calculation of charge and discharge cycles in thermally stratified energy storages

A stable thermal stratification in solarthermal storage tanks increases the energy efficiency of these systems. Especially in charging and discharging cycles, mixing occurs due to jet flows. The reliable prediction of the influence of the storage and of the charging device geometry on the loading behaviour is essential for the layout and improvement of stratified storage systems. A model approach for the computational calculation of the time-dependent temperature distribution in stratified storage tanks based on the one-dimensional heat transport equation is described in the present study. The numerical solution was obtained by application of the first order Upwind-discretization scheme. This basic approach was further refined by the consideration of charging jet flows and local turbulences in the area of stratification according to the strategies of Jirka, 2004, Mott and Woods, 2009 and implemented in MATLAB. Two simulation examples of different complexity have shown that the enhanced model could increase the calculation accuracy in comparison to similar CFD and experimental studies. The results of the MATLAB program were reached with much less calculation effort than the results of the CFD simulation.     

An effective conduction length model in the enthalpy formulation for the stefan problem

In the fixed-grid finite-volume formulation, so called the enthalpy formulation, for the Stefan problem, the temperature and the front movement show step-like history, which is a well-known characteristic of the enthalpy method. This paper presents an effective conduction length model to mitigate such an oscillatory behavior as well as to support the physical reasoning. The proposed model is based on the simple fact that the heat flux across the boundary of phase-change cells should be estimated with the distance between the phase front and the center of neighboring cell. The model is applied to one-dimensional Stefan problems with various Stefan numbers. The numerical results show that the proposed model can smooth the spurious oscillation of the history of temperature and the evolution of front movement.     

Parametric study and approximation of the exact analytical solution of the Stefan problem in a finite PCM layer in a steady periodic regime

In this paper a parametric study and an approximation of the exact analytical solution of the Stefan problem in steady periodic regime conditions (Mazzeo et al., 2015) is presented. The physical model describes the thermal behaviour of a PCM (phase change material) layer subject to phase transition, and the considered thermal regime ensures continuous cycles of phase changes under the action of the periodic boundary conditions.The exact analytical solution determines, through implicit transcendental equation with complex thermal parameters and unknowns, the bi-phase interface position, the thermal field and the sensible and latent stored energy in the layer. The dimensionless solution is a function of the Fourier and Stefan numbers calculated in the two phases, and of the dimensionless bi-phase interface position corresponding to the steady regime.For the parametric study, the thermophysical properties of the most commonly used PCMs and the variation range of attenuation and time lag between the temperature loadings operating on the internal and external surfaces were considered. This study has allowed for the identification of the thermal parameters that mainly influence the dynamic thermal behaviour of the PCM layer and the mathematical structure of the frequency response of the layer.Since an analytical expression in an explicit form of the position of the bi-phase interface in the layer is not available, an approximate analytical solution was obtained, which makes the bi-phase interface position depends on the product between the Fourier number and the Stefan number calculated in the two phases. The limits of validity of such a solution were determined evaluating the relative error, which is committed in the determination of the amplitude and of the argument of the oscillating component of the bi-phase interface position. Finally, the fields of variation of the thermal parameters that ensure a relative error value lower than 3% and the corresponding values of the maximum errors of the amplitude or the argument are determined. In PCM layer thermal analysis it is useful to have an expression in an explicit form of the oscillating component of the bi-phase interface position to obtain the mathematical expression of the temperature and heat flux field as a function of only dimensionless thermal parameters and boundary loadings.     

Micro-encapsulated paraffin/high-density polyethylene/wood flour composite as form-stable phase change material for thermal energy storage

Six novel polymer-based form-stable composite phase change materials (PCMs), which comprise micro-encapsulated paraffin (MEP) as latent heat storage medium and high-density polyethylene (HDPE)/wood flour compound as supporting material, were prepared by blending and compression molding method for potential latent heat thermal energy storage (LHTES) applications. Micro-mist graphite (MMG) was added to improve thermal conductivities. The scanning electron microscope (SEM) images revealed that the form-stable PCMs have homogeneous constitution and most of MEP particles in them were undamaged. Both the shell of MEP and the matrix prevent molten paraffin from leakage. Therefore, the composite PCMs are described as form-stable PCMs. The differential scanning calorimeter (DSC) results showed that the melting and freezing temperatures as well as latent heats of the prepared form-stable PCMs are suitable for potential LHTES applications. Thermal cycling test indicated the form-stable PCMs have good thermal stability although it was subjected to 100 melt–freeze cycles. The thermal conductivity of the form-stable PCM was increased by 17.7% by adding 8.8 wt% MMG. The results of mechanical property test indicated that the addition of MMG has no negative influence on the mechanical properties of form-stable composite PCMs. Taking one with another, these novel form-stable PCMs have the potential for LHTES applications in terms of their proper phase change temperatures, improved thermal conductivities, outstanding leak tightness of molten paraffin and good mechanical properties.     

Maximum efficiency point tracking for an ocean thermal energy harvesting system

Limited energy is the most critical factor that restricts the persistent presence of underwater vehicles in the oceans; thus, harvesting the ocean's thermal energy that is stored in the water column between the sea surface and deep water is a particularly promising solution for the current power shortage. This paper has designed a new ocean thermal energy conversion system which using phase change material as energy storage medium, and proposed a novel maximum efficiency point tracking (MEPT) method for energy conversion. This new method, which is integrated with a radial basis function neural network (RBFNN), particle swarm optimization (PSO) and the proportion integration differentiation (PID) control method, could effectively improve the efficiency of energy conversion. Compared with the power generation system that does not use the MEPT method, experimental results show that the proposed method can improve the efficiency of the power generation from less than 19.05% to more than 34.3% and has higher stability (using this method: the efficiency changes from 34.3%-34.7%; without using this method: the efficiency changes from 13.56% -19.05%) when the load changes. This novel method can be used in many conditions, especially when the mathematical model of the generation system is unknown or researchers want to use fewer sensors for maximum efficiency point tracking.