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.     

Energetic and exergetic investigation on lauric and stearic acid phase‐change material–based thermal energy storage system integrated with engine exhaust

Energy and exergy analysis comparison of lauric and stearic acid phase‐change material (PCM)–based energy storage system integrated with engine exhaust have been investigated in the present study, which provides more realistic assessment than the conventional energy analysis. On the basis of thermodynamic laws, energy, exergy, charging efficiencies, and availability of PCM thermal storage with various mass fractions have been investigated at engine full load. The exergy saved for PCMs in the overall system is quantified and were compared. The results revealed a considerable enhancement in energy and exergy efficiency for thermal energy storage with lauric acid PCM due to its enhanced thermophysical properties. Energy and exergy of the storage medium for lauric acid PCM with 0.4 kg mass fraction, increased by 68% and 57.5% compared with stearic acid PCM thermal storage integrated with a diesel engine. Also, energy and exergy efficiency of charging and integrating the system with stearic acid PCM decrease with increase in mass fractions. Thus, lauric acid PCM can be used as thermal storage medium at high temperatures for exhaust heat recovery from engines and also an option for green technology.     

Analysis and optimization of a latent thermal energy storage system with embedded heat pipes

Latent thermal energy storage system (LTES) is an integral part of concentrating solar power (CSP) plants for storing sun’s energy during its intermittent diurnal availability in the form of latent heat of a phase change material (PCM). The advantages of an LTES include its isothermal operation and high energy storage density, while the low thermal conductivity of the PCM used in LTES poses a significant disadvantage due to the reduction in the rate at which the PCM can be melted (charging) or solidified (discharging). The present study considers an approach to reducing the thermal resistance of LTES through embedding heat pipes to augment the energy transfer from the heat transfer fluid (HTF) to the PCM. Using a thermal resistance network model of a shell and tube LTES with embedded heat pipes, detailed parametric studies are carried out to assess the influence of the heat pipe and the LTES geometric and operational parameters on the performance of the system during charging and discharging. The physical model is coupled with a numerical optimization method to identify the design and operating parameters of the heat pipe embedded LTES system that maximizes energy transferred, energy transfer rate and effectiveness.     

Microencapsulated phase change slurries for thermal energy storage in a residential solar energy system

Phase change materials (PCMs) are attractive for use in thermal energy storage applications and thermal regulation/control due to their high-energy storage density over a small temperature range. The direct use of phase change materials for energy storage and/or heat transfer applications has been limited due to the low thermal conductivity of the PCM particularly when solidifying on the heat transfer surface. A Phase change slurry (PCS) consists of small micro-encapsulated PCM particles suspended in a carrier fluid which enhances the heat transfer to the PCM. The PCS can serve not only as the thermal storage media but also as the heat transfer fluid, and hence may have many potentially important applications including in the field of heating, ventilation and air-conditioning (HVAC), refrigeration, solar energy and heat exchangers. A test system to examine PCS performance in residential thermal energy storage applications has been developed to both observe and characterise the thermal processes that occur in a thermal store with a helical coil heat exchanger. These test results will be used to improve the system design and identify limitations when used for intermittent application.     

Alternative storage options for solar thermal systems

The efficient implementation of solar systems in buildings depends on the storage of energy yielded, as it can both increase the solar system's autonomy and make it a feasible solution for zero energy buildings, and make storage vessels more compact, reducing precious space requirements. This is of particular important in places with reduced time of sunshine, where solar systems are less effective, because of the deviation between solar radiation and the demand. The traditional storage options use water, which is practical, safe and low‐cost, especially when the storage requirements are small. However, when larger storage is needed, limits concerning the use of water exist, mainly due to the need for larger installation space and the increased thermal losses. The use of phase change materials (PCM) for thermal energy storage seems an upcoming technology. The main idea is the substitution of water with PCM, which feature larger specific energy storage capacity compared to other conventional materials. In the context of the specific paper, a combined solar thermal system used for the preparation of domestic hot water (DHW) and space heating (Solar Combi System) with two different types of storage is studied, for two Greek cities. The aim is to find out which is the most efficient way of storing energy with respect to the autonomy of the system, for a solar combi system. This is being achieved by determining the comparative autonomy of PCM and water storage system for various climates. It was proven that using PCM is advantageous, as it can extend the autonomy duration of the solar system for 2 to 8 hours, depending on the season and the climatic conditions. However, it was also seen that in solar combi systems used throughout the whole year, PCM are inefficient during summer period.     

Exergetic and performance analyses of two-layered packed bed latent heat thermal energy storage system

In concentrating solar power (CSP) plant, a novel method involving the use of thermocline can be employed to augment the capability of the thermal energy storage system (TES). The rate of thermocline degradation can be reduced by packing encapsulated phase change material (PCM) in the TES. The thermal performance of the packed bed latent heat thermal energy storage system (PBTES) can be further enhanced by employing different diameters of PCM capsules arranged in multiple layers. In this paper, the thermal and exergetic performance of single-layered and two-layered PBTES is evaluated for varying mass flow rate, PCM capsule diameter and bed height of larger PCM capsules using a dynamic model based on simplified energy balance equations for PCM and heat transfer fluid (HTF). The single-layered PBTES has a lower TES latent charging rate than the two-layered PBTES. The charging efficiency and charging time of two-layered PBTES are increased by 15.85% and 16.85%, respectively for reducing the HTF mass flow rate by 14.29%. A higher stratification number can be achieved by using a two-layered PBTES instead of a single-layered PBTES filled with the corresponding larger diameter PCM capsules. The second law efficiency of the two-layered PBTES is found to be less than that of the single-layered PBTES. A decrease in the bed height of larger PCM capsules decreases the exergetic efficiency of the two-layered PBTES by 3.27%. The findings from this study can be used in further designing and optimising the multi-layered PBTES.     

High temperature latent heat thermal energy storage using heat pipes

A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.     

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.     

Experimental validation of a CFD model for tubes in a phase change thermal energy storage system

A computational fluid dynamic (CFD) model for tubes in a phase change thermal energy storage system has been developed and validated with experimental results. The heat transfer fluid (HTF) flows in tubes which are configured in a unique arrangement during the charging and discharging processes. Water was used as the phase change material (PCM) which was contained in a cylindrical tank with four tubes coiled inside it. Experiments were conducted for both freezing and melting processes. A three-dimensional CFD model using Ansys code was developed and validated with experimental results. This model endeavoured to describe both the freezing and melting processes of the PCM. The inlet and outlet HTF temperatures as well as nine temperature locations in the PCM were compared with the CFD results. The average effectiveness as well as the duration of the phase change process of each experimental point was also compared with results from the CFD. From this study, it was concluded that the CFD model developed can accurately predict the behaviour of the thermal storage system during charging and discharging. The paper gives details of the CFD model and compares results from the model and experiments.     

Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating

The continuous increase in the level of greenhouse gas emissions and the rise in fuel prices are the main driving forces behind the efforts for more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. In this study, the thermal performance of a phase change thermal storage unit is analyzed and discussed. The storage unit is a component of ten pieced solar air collectors heating system being developed for space heating of a greenhouse and charging of PCM. CaCl₂6H₂O was used as PCM in thermal energy storage with a melting temperature of 29 °C. Hot air delivered by ten pieced solar air collector is passed through the PCM to charge the storage unit. The stored heat is utilized to heat ambient air before being admitted to a greenhouse. This study is based on experimental results of the PCM employed to analyze the transient thermal behavior of the storage unit during the charge and discharge periods. The proposed size of collectors integrated PCM provided about 18–23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the conventional heating device.     

Review on storage materials and thermal performance enhancement techniques for high temperature phase change thermal storage systems

Designing a cost-effective phase change thermal storage system involves two challenging aspects: one is to select a suitable storage material and the other is to increase the heat transfer between the storage material and the heat transfer fluid as the performance of the system is limited by the poor thermal conductivity of the latent heat storage material. When used for storing energy in concentrated solar thermal power plants, the solar field operation temperature will determine the PCM melting temperature selection. This paper reviews concentrated solar thermal power plants that are currently operating and under construction. It also reviews phase change materials with melting temperatures above 300 °C, which potentially can be used as energy storage media in these plants. In addition, various techniques employed to enhance the thermal performance of high temperature phase change thermal storage systems have been reviewed and discussed. This review aims to provide the necessary information for further research in the development of cost-effective high temperature phase change thermal storage systems.     

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.     

Energy analysis of space solar dynamic heat receivers

Energy analysis of space solar dynamic heat receivers employing solid–liquid phase change storage is developed. The heat receiver is a critical component of a solar dynamic system. Phase change thermal energy storage is used in the heat receiver. The energy analysis presented here can be used to understand the energy transfer in the heat receiver and thermal energy storage in phase change materials (PCM). The heat receiver cavity radiation mathematical model and the working fluid tube heat model are established. Energy loss, energy absorbed by gas, the latent and sensible thermal energy storage in PCM, maximum tube temperature, gas outlet temperature and liquid PCM fraction were calculated. The results are analyzed and could be used in heat receiver design.     

Laminar film condensation driven latent thermal energy storage in rectangular containers

This paper focuses on numerically analyzing the thermal transport phenomena in the transient conjugate problem of melting and laminar film condensation. The key focus is to identify an optimum container aspect ratio/shape and conditions for which the heat storage time and the storage capacity are minimum and maximum respectively. Since most solid–liquid phase change materials (PCMs) suffer from poor thermal conductivities, the major resistance to heat transfer comes from PCM. Hence, high thermal conductivity, low-cost metal foam is suggested for use along with PCM to minimize this resistance. The conjugate transient problem of film condensation driven solid–liquid phase change of PCM impregnated inside porous metal foam is numerically analyzed. An effective heat capacity formulation is employed for modeling the transient PCM phase change in porous foam and is solved using finite element method. It is coupled with laminar film condensation on the outside of the storage container. The model is then used for selecting the best aspect ratio for thermal energy storage (TES) containers that enables to store comparatively the maximum heat. The results of the developed model showed that the major resistance to heat transfer and hence efficient thermal energy storage depends strongly on the aspect ratio of the PCM storage containers.     

Numerical study on a low energy architecture based upon distributed heat storage system

Basic performance of a hybrid heating system was investigated numerically through several case studies including examinations of effects of PCM as a heat storing materials. A simple test room assuming passive utilization of solar energy was used with a thermal storage wall (Trombe wall) inside it. Unsteady simulation was performed with a CFD code developed by authors. As the outdoor conditions, standardized weather data of Sapporo city, a cold climate district in Japan, were used. In the simulation, the room air was controlled with the heater operations setting the target air temperature at 18 degree Celsius. Simulated results indicate the effectiveness of PCM and suggest the possibility of developing low energy houses with hybrid system introduced in this study.     

Experimental and numerical investigation of thermal energy storage with a finned tube

A latent heat thermal energy storage system using a phase change material (PCM) is an efficient way of storing or releasing a large amount of heat during melting or solidification. It has been determined that the shell‐and‐tube type heat exchanger is the most promising device as a latent heat system that requires high efficiency for a minimum volume. In this type of heat exchanger, the PCM fills the annular shell space around the finned tube while the heat transfer fluid flows within the tube. One of the methods used for increasing the rate of energy storage is to increase the heat transfer surface area by employing finned surfaces. In this study, energy storage by phase change around a radially finned tube is investigated numerically and experimentally. The solution of the system consists of the solving governing equations for the heat transfer fluid (HTF), pipe wall and phase change material. Numerical simulations are performed to investigate the effect of several fin parameters (fin spacing and fin diameter) and flow parameter (Re number and inlet temperature of HTF) and compare with experimental results. The effect of each variable on energy storage and amount of solidification are presented graphically. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.     

Numerical and experimental investigation of heat transfer in a shell and tube thermal energy storage system

A numerical and experimental investigation of phase change process dominated by heat conduction in a thermal storage unit is presented in this paper. The thermal energy storage involves a shell and tube arrangement where paraffin wax as phase change material (PCM) is filled in the shell. Water as heat transfer fluid (HTF) is passed inside the tube for both charging and discharging cycles. According to the conservation of energy, a simple numerical method called alternative iteration between thermal resistance and temperature has been developed for the analysis of heat transfer between the PCM and HTF during charging and discharging cycles. Experimental arrangement has been designed and built to examine the physical validity of the numerical results. Comparison between the numerical predictions and the experimental data shows a good agreement. A detailed parametric study is also carried out for various flow parameters and system dimensions such as different mass flow rates, inlet temperatures of HTF, tube thicknesses and radii. Numerical study reveals that the contribution of the inlet temperature of HTF has much influence than mass flow rate in terms of storage operating time and HTF outlet temperature. Tube radius is a more important parameter than thickness for better heat transfer between HTF and PCM.     

Capric-myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage

Phase change materials (PCMs) can be incorporated with building materials to obtain novel form-stable composite PCM which has effective energy storage performance in latent heat thermal energy storage (LHTES) systems. In this study, capric acid (CA)-myristic acid (MA) eutectic mixture/vermiculite (VMT) composite was prepared as a novel form-stable PCM using vacuum impregnation method. The composite PCM was characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. Thermal properties and thermal reliability of the composite PCM were determined by differential scanning calorimetry (DSC) analysis. The CA-MA eutectic mixture could be retained by 20 wt% into pores of the VMT without melted PCM seepage from the composite and therefore, this mixture was described as form-stable composite PCM. Thermal cycling test showed that the form-stable composite PCM has good thermal reliability and chemical stability although it was subjected to 3000 melting/freezing cycling. Thermal conductivity of the form-stable CA-MA/VMT composite PCM was increased by about 85% by introducing 2 wt% expanded graphite (EG) into the composite. The increase in thermal conductivity was confirmed by comparison of the melting and freezing times of the CA-MA/VMT composite with that of CA-MA/VMT/EG composite. The form-stable PCM including EG can be used as energy absorbing building material such as lightweight aggregate for plaster, concrete compounds, fire stop mortar, and component of interior fill for wallboards or hollow bricks because of its good thermal properties, thermal and chemical reliability and thermal conductivity.