Design criteria in PCM wall thermal storage

Performance criteria for a wall with an enclosed phase-change material (PCM) are discussed. Qualitative conclusions about how best to choose the PCM are derived and illustrated by means of computer simulations. Material properties dominating the heat-transfer process are identified and relations between them are determined.     

An analysis of a packed bed latent heat thermal energy storage system using PCM capsules: Numerical investigation

This paper is aimed at analyzing the behavior of a packed bed latent heat thermal energy storage system. The packed bed is composed of spherical capsules filled with paraffin wax as PCM usable with a solar water heating system. The model developed in this study uses the fundamental equations similar to those of Schumann, except that the phase change phenomena of PCM inside the capsules are analyzed by using enthalpy method. The equations are numerically solved, and the results obtained are used for the thermal performance analysis of both charging and discharging processes. The effects of the inlet heat transfer fluid temperature (Stefan number), mass flow rate and phase change temperature range on the thermal performance of the capsules of various radii have been investigated. The results indicate that for the proper modeling of performance of the system the phase change temperature range of the PCM must be accurately known, and should be taken into account.     

An organic PCM storage system with adjustable melting temperature

A proper storage temperature is an important criterion for selecting a phase change material (PCM) for a passive solar heating application. Here we describe a novel procedure to produce a mixture of carboxylic acids with a melting temperature adjustable to the climate specific requirements. The approach is based on the ideal solution model and differential scanning calorimetry (DSC). The applicability of the method is demonstrated and it is also applied to a PCM wall design. The accuracy of the theoretical model is ±2°C in the temperature range of 20°–30°C and even a ±0.5°C accuracy can be obtained by the experimental procedure.     

Numerical analysis of melting with constant heat flux heating in a thermal energy storage system

Melting in a finite slab with a second kind boundary condition is studied numerically in order to simulate the charging process of a thermal energy storage system. A dimensionless model is given, from which it is concluded that the main factors that influence the melting process are the dimensionless heating flux, the modified Stefan number, the relative thermal diffusivity and the relative thermal conductivity. The influence of preheating or solid subcooling is studied. It is found that though preheating does not have very important effects on the melting time, it does influence the interface marching velocity significantly. The melt fraction and the melting time are calculated extensively for various dimensionless numbers. The numerical results show that the ratio of the thermal conductivity of the solid to that of the liquid has little effect on the melting time, and the time for finishing melting can be expressed as a function of the dimensionless heating flux, the modified Stefan number and the relative thermal diffusivity, and the possible function form is suggested.     

Numerical analysis of a cool-thermal storage system with a thermal conductivity enhancer operating under a freezing condition

Enhancing the cooling process of water in a cool-thermal storage system is investigated in this paper. The system is utilized to cool air during on-peak power consumption hours to save energy. The system consists of parallel plates filled with water and triangular corrugated fins. A finite element model for the system is used to predict the cooling time of water for different water's initial temperatures, and also to study the effect of fins design on the cooling process. The result indicates that the aspect ratio of the triangular fin has a significant effect the cooling process of water, and cooling rate increases for the aspect ratio greater or less than 0.75. Temperature contours, average water temperature, and Nusselt number are presented for the cooling process.     

Numerical analysis of natural convection for a porous rectangular enclosure with sinusoidally varying temperature profile on the bottom wall

Numerical investigations of steady natural convection flow through a fluid-saturated porous medium in a rectangular enclosure with a sinusoidal varying temperature profile on the bottom wall were conducted. All the walls of the enclosure are insulated except the bottom wall which is partially heated and cooled. The governing equations were written under the assumption of Darcy-law and then solved numerically using finite difference method. The problem is analyzed for different values of the Rayleigh number Ra in the range 10 ≤ Ra ≤ 1000, aspect ratio parameter AR in the range 0.25 ≤ AR ≤1.0 and amplitude λ of the sinusoidal temperature function in the range 0.25 ≤ λ ≤ 1.0. It was found that heat transfer increases with increasing of amplitude λ and decreases with increasing aspect ratio AR. Multiple cells were observed in the cavity for all values of the parameters considered.     

Thermal performance of PCM thermal storage unit for a roof integrated solar heating system

The thermal performance of a phase change thermal storage unit is analysed and discussed. The storage unit is a component of a roof integrated solar heating system being developed for space heating of a home. The unit consists of several layers of phase change material (PCM) slabs with a melting temperature of 29 °C. Warm air delivered by a roof integrated collector is passed through the spaces between the PCM layers to charge the storage unit. The stored heat is utilised to heat ambient air before being admitted to a living space. The study is based on both experimental results and a theoretical two dimensional mathematical model of the PCM employed to analyse the transient thermal behaviour of the storage unit during the charge and discharge periods. The analysis takes into account the effects of sensible heat which exists when the initial temperature of the PCM is well below or above the melting point during melting or freezing. The significance of natural convection occurring inside the PCM on the heat transfer rate during melting which was previously suspected as the cause of faster melting process in one of the experiments is discussed. The results are compared with a previous analysis based on a one dimensional model which neglected the effect of sensible heat. A comparison with experimental results for a specific geometry is also made.     

Optimising PCM thermal storage systems for maximum energy storage effectiveness

A new performance parameter for PCM thermal storage systems, the energy storage effectiveness, is defined. This parameter can be used to optimise the design of any PCM thermal storage system to maximise the use of the thermal storage media. The paper presents results of a parametric study using an experimentally validated numerical model for PCM encapsulated in plates. The results are used to calculate the energy storage effectiveness which is ultimately used to optimise the useful energy that can be stored in the PCM thermal storage system. The energy storage effectiveness is also used to compare the useable storage capacity of the PCM relative to a sensible energy storage system.     

Numerical modeling of aquifer thermal energy storage system

The performance of the ATES (aquifer thermal energy storage) system primarily depends on the thermal interference between warm and cold thermal energy stored in an aquifer. Additionally the thermal interference is mainly affected by the borehole distance, the hydraulic conductivity, and the pumping/injection rate. Thermo-hydraulic modeling was performed to identify the thermal interference by three parameters and to estimate the system performance change by the thermal interference. Modeling results indicate that the thermal interference grows as the borehole distance decreases, as the hydraulic conductivity increases, and as the pumping/injection rate increases. The system performance analysis indicates that if η (the ratio of the length of the thermal front to the distance between two boreholes) is lower than unity, the system performance is not significantly affected, but if η is equal to unity, the system performance falls up to ∼22%. Long term modeling for a factory in Anseong was conducted to test the applicability of the ATES system. When the pumping/injection rate is 100 m³/day, system performances during the summer and winter after 3 years of operation are estimated to be ∼125 kW and ∼110 kW, respectively. Therefore, 100 m³/day of the pumping/injection rate satisfies the energy requirements (∼70 kW) for the factory.     

Numerical simulation of a multi-layer latent heat thermal energy storage system

A computational model for the prediction of the thermal behaviour of a compact multi-layer latent heat storage unit is presented. The model is based on the conservation equations of energy for the phase change material (PCM) and the heat transfer fluid (HTF). Electrical heat sources embedded inside the PCM are used for heat storage (melting) while the flow of an HTF is employed for heat recovery (solidification). Parametric studies are performed to assess the effect of various design parameters and operating conditions on the thermal behaviour of the unit. Results indicate that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, on the thermal diffusivity of the liquid phase, on the thickness of the PCM layer and on the HTF inlet mass flowrate and temperature. It is, on the other hand, nearly independent of the wall thermal diffusivity and thickness and of the maximum operating temperature. Correlations are proposed for the total energy stored and the output heat load as a function of the design parameters and the operating conditions. © 1998 John Wiley & Sons, Ltd.     

Numerical analysis and evaluation of an open-type thermal storage system using composite sorbents

A family of composite sorbents has been acknowledged as promising thermal storage materials for low grade thermal energy storage owing to its high specific storage capacity and low regenerating temperature. This paper reports a simplified numerical model aiming to determine the dynamic characteristics of the composite sorbents and evaluate the specific capacity and COP of the open-type thermal energy storage system. The computational results were validated with the experimental measurements carried out on an open-type thermal energy storage set-up filled with composite sorbents. By using the simplified numerical model, the dynamic characteristics of the composite sorbents in the thermal energy storage process were determined. The effects of the composite sorbents and the operating parameters on thermal energy storage system performance were also evaluated.     

Characterising PCM thermal storage systems using the effectiveness-NTU approach

Thermal performance characterisation of thermal storage systems employing phase change materials (PCMs) has been carried out predominantly through numerical modelling. These models do not provide a direct representation of the storage system, and cannot readily be used to design a thermal storage unit (TSU) for a particular application to meet the performance specification. Furthermore, limited consideration is given to exergy efficiency of the PCM system being studied. The phase change profiles identified in detailed simulation of PCM encapsulated in flat containers, have been employed to analytically formulate the TSU effectiveness using the familiar effective-NTU (ε-NTU) approach. A one and two dimension formulation were developed with respect to the phase change fraction. This single parameter function can be used for sizing a TSU, by determining the redundant amount of PCM defined by the minimum effectiveness during discharging. Furthermore by optimising design parameters, exergy losses can be minimised by maximising the effectiveness over the full charging – discharging cycle. The study demonstrated that for air based systems phase change can be represented in one-dimension, however for liquid based systems two-dimensional phase change needs to be considered.     

Thermal performance of a non-air-conditioned building with PCCM thermal storage wall

This paper presents a time-dependent periodic heat transfer analysis of a non-air-conditioned building having a south-facing wall of phase-changing component material (PCCM). A rectangular room (6 × 5 × 4 m) based on the ground is considered. The effects of heat transfer through walls and roof, heat conduction to the basement ground and furnishings, heat gain through window and heat loss due to air ventilation have been incorporated in the periodic time-dependent heat transfer analysis. The time-dependent heat flux through the PCCM south-facing wall has been obtained by defining the effective thermal properties of the PCCM for a conduction process with no phase change. Numerical calculations are made for a typical mild winter day (7 March 1979) at New Delhi for heat flux entering through the wall and inside air temperature. Further, a PCCM wall of smaller thickness is more desirable, in comparison to an ordinary masonry concrete wall, for providing efficient thermal energy storage as well as excellent thermal comfort in buildings.     

PCM thermal storage in buildings: A state of art

A comprehensive review of various possible methods for heating and cooling in buildings are discussed in this paper. The thermal performance of various types of systems like PCM trombe wall, PCM wallboards, PCM shutters, PCM building blocks, air-based heating systems, floor heating, ceiling boards, etc., is presented in this paper. All systems have good potential for heating and cooling in building through phase change materials and also very beneficial to reduce the energy demand of the buildings.     

Numerical heat transfer analysis of encapsulated ice thermal energy storage system with variable heat transfer coefficient in downstream

During charging and discharging processes, the heat transfer behavior of the encapsulated ice thermal energy storage (TES) system changes during downstream case and this should be taken into account since the temperature of heat transfer fluid (HTF) and especially the heat transfer coefficient varies considerably around each capsule. This requires a careful study of the problem with variable heat transfer coefficient to contribute to the state-of-the-art. This has been the primary motivation behind the present study. Here, we first develop a new heat transfer coefficient correlation by simulating a series of 120 numerical experiments for different capsule diameters, mass flow rates and temperatures of HTF and second undertake a comprehensive numerical analysis using the temperature based fixed grid solution with control volume approach for studying the heat transfer behavior of an encapsulated ice TES system. Thirdly, we validate the present numerical model and the new correlation with some experimental data obtained from the literature, and hence a good agreement is obtained between the model results and experimental data. The results indicate that the heat transfer coefficient varies greatly during downstream and highly affects the heat transfer taking place during the process. So, the solutions with constant heat transfer coefficient appear to be unreliable for analysis and system optimization. The results also show that the solidification process is chiefly governed by the magnitude of Stefan number, capsule diameter and capsule row number.     

Numerical analysis of convective flow and thermal stratification in a cryogenic storage tank

In this study, the liquid–vapor mixture model was used for a numerical study of natural convective flow in a cryogenic tank with a capacity of 4.9?m³ under various conditions of heat flux and filling level to understand the early stages of convective flow phenomena and the consequent thermal stratification of cryogenic liquid. Two cryogens—liquefied natural gas (LNG) and liquefied nitrogen (LN₂)—were compared to observe their effects. LN₂ exhibited faster vaporization owing to its lower heat of vaporization. It was observed that higher heat flux and lower filling level led to faster vaporization and relatively vigorous heat transfer, showing early thermal stratification.     

内翅式套管相变蓄热器性能研究

相变蓄热能有效地解决太阳能利用过程中供需不匹配的问题。文章对内翅式太阳能套管式相变蓄热器的蓄热性能进行了研究,结果表明:供热流体入口速度越大,熔化过程所需要的时间越短;速度为2.5 m/s时,熔化过程需要225 min左右;与未加翅片的蓄热器相比,在相变材料侧增加环形翅片时蓄热时间缩短10%左右。