Multiple Phase Change Materials Research Articles

Mathematical Model of Packed Bed Solar Thermal Energy Storage Simulation

Abstract Mathematical model has been developed to assess the effects of using phase change materials (PCM) in a fully mixed water accumulation tank. Packed bed system of spheres with a diameter of 40 mm have been considered as an option to increase energy storage density. A continuous phase model has been applied to analyse the influence of phase change during a charging and discharging processes. The methodology for calculating energy mass flow, density of water, heat transfer between layers, temperature for each time step and energy transferred to PCM is given. Single type of PCM with melting temperature of 55 °C and multiple type PCM system (65, 55, 45 and 35 °C) are compared. It is found that multiple type PCM system compared to only water and single type PCM systems, provide lower return temperatures and higher energy density.

Experimental evaluation at pilot plant scale of multiple PCMs (cascaded) vs. single PCM configuration for thermal energy storage

The present paper provides on one hand, a literature review of the different studies available in the scientific literature where the concept of multiple phase change materials (PCM) configuration, also named cascaded or multi-stage, has been presented and on the other hand, an experimental evaluation of the advantages of using the multiple PCM configuration instead of the single PCM configuration in thermal energy storage (TES) systems at pilot plant to fill the gap of experimental and high scales studies on this concept in the literature. Two PCM with melting temperatures in a temperature range of 150–200 °C were selected due to their high value of heat of fusion and compared: d-mannitol and hydroquinone. Three configurations were evaluated: (1) single PCM with hydroquinone, (2) single PCM with d-mannitol and (3) multiple PCM with hydroquinone and d-mannitol. A discussion regarding the results on the specific energy stored and effectiveness as well as the evolution of the PCM and heat transfer fluid (HTF) through the time and at different and representative locations of the facility is presented. Results showed that the multiple PCMs configuration introduced an effectiveness enhancement of 19.36% if compared with single PCM configuration as well as a higher uniformity on the HTF temperature difference between the inlet and outlet.

PCM Thermal Energy Storage in Buildings: Experimental Study and Applications

Abstract The study aims at analyzing the performance of Phase Change Materials (PCMs) in residential housing for different climates. This paper presents the results of an experiment performed in the Concordia University Solar Simulator and Environmental Chamber research facility (SSEC, Montreal, Canada). PCM boards were embedded on the back wall of a test hut placedin the climatic chamber. Several experiments were performed to explore the potential for verification of the proposed analysis and to produce enough data to perform model calibrations. Results show a strong increase in the apparent thermal inertia of the room allowing for a reduction in daily temperature fluctuations in the test hut. Simulations were carried out by means of a calibrated EnergyPlus model to broaden the analysis in both a cold Canadian climate and in a Mediterranean temperate climate. Other configurations for the setup environment were studied to simulate the issues of very hot summers in Mediterranean climate. The different configurations ended up with mixed results for the two locations analyzed: the maximization of the solar gains and its storage in a high density multiple layer PCM wall is the better configuration for the cold climate analyzed, while its performance is non optimal for hot climates where the “emulation” of high thermal mass walls by PCM in lightweight structures requires a different positioning of the PCM panels.

Multiple phase change structure for the scalable phase change random access memory array

A multiple phase change structure with Sb79Te21 for switching and Ge1Cu2Te3 for memory was fabricated and evaluated for cell operation. It was confirmed that Sb79Te21 with smaller contact diameter is successfully operated or used as a switching device with selective current pulses. In addition, it was estimated that this structure provides acceptable on–off ratio and leakage current by structural consideration. From these results, we consider that the structure with multiple phase change materials is suitable as a new device with both switching and memory functions, which gives higher scalability in three-dimensional array architecture by adopting no additional selective devices, such as transistors and diodes. Therefore, we expect that this structure can be one of the candidates for the scalable phase change random access memory (PCRAM).

Energy and exergy evaluation of a multiple-PCM thermal storage unit for free cooling applications

Latent heat thermal storage (LHTS) is a technology that can help to reduce energy consumption for cooling applications, where the cold is stored in phase change materials (PCMs). Free cooling is a concept developed for air conditioning applications, in which coolness is collected from ambient air during night and released into the room during the hottest hours of the day. In this work, energy and exergy analyses are performed for a free cooling system using a LHTS unit employing multiple PCMs. The effects of inlet air temperature and air flow rate on the performance of the system are investigated. It is observed that the increase in exergy efficiency due to reducing inlet air temperature is more significant than effect from increasing the air flow rate during the charging process.

Latent heat augmentation of thermocline energy storage for concentrating solar power – A system-level assessment

Molten-salt thermocline tanks are a low-cost energy storage option for concentrating solar power plants. Despite the potential economic advantage, the capacity of thermocline tanks to store sufficient amounts of high-temperature heat is limited by the low energy density of the constituent sensible-heat storage media. A promising design modification replaces conventional rock filler inside the tank with an encapsulated phase-change material (PCM), contributing a latent heat storage mechanism to increase the overall energy density. The current study presents a new finite-volume approach to simulate mass and energy transport inside a latent heat thermocline tank at low computational cost. This storage model is then integrated into a system-level model of a molten-salt power tower plant to inform tank operation with respect to realistic solar collection and power production. With this system model, PCMs with different melting temperatures and heats of fusion are evaluated for their viability in latent heat storage for solar plants.Thermocline tanks filled with a single PCM do not yield a substantial increase in annual storage or plant output over a conventional rock-filled tank of equal size. As the melting temperature and heat of fusion are increased, the ability of the PCM to support steam generation improves but the corresponding ability of the thermocline tank to utilize this available latent heat decreases. This trend results from an inherent deconstruction of the heat-exchange region inside the tank between sensible and latent heat transfer, preventing effective use of the added phase change for daily plant operations. This problem can be circumvented with a cascaded filler structure composed of multiple PCMs with their melting temperatures tuned along the tank height. However, storage benefits with these cascaded tank structures are shown to be highly sensitive to the proper selection of the PCM melting points relative to the thermocline tank operating temperatures.

Thermal performance optimization of free cooling systems using enhanced latent heat thermal storage unit

Free cooling systems use phase change materials (PCMs) to store outdoor cold during the night and to supply it to the indoor environment during the day when the need for cooling increases. This work presents a numerical investigation of the performance enhancement of a free cooling system using a latent heat thermal storage (LHTS) unit employing multiple PCMs. The PCM storage system consists of several flat PCM slabs arranged in layers with a passage in between for the heat transfer fluid (HTF). Using the effective heat capacity method, the process of melting and solidification of the PCM is predicted. The energy storage effectiveness (as a new performance parameter for LHTS units) and coefficient of performance (COP) of the system are calculated. These two performance indicators are used to optimize the system. The suitability of both methods for optimization purpose is discussed indicating the superiority of the energy based (COP) method.

Investigation of a selective switching device using a phase-change material for a 3-dimensional PCRAM array

A selective switching device utilizing a phase-change material was investigated. In this work, we present a new concept to realize serially a selective switching and memory operation in a multiple phase-change memory with only phase-change materials without any semiconductor switching device. A phase-change material for selective switching can be expected to have a higher resistance amorphous phase and to show lower melting and crystallization temperatures than a phase-change material for a memory. Here, we present a structural method to obtain the above requirements. In addition, we confirm the switching operation by a selective current pulse for multiple phase-change materials from the experiment. From these results, we expected that one of the multiple phasechange materials can be replaced in a switching device without the need for an additional selective device, and that such a device would be feasible for 3-dimensional PCM architecture.

Multistage latent heat cold thermal energy storage design analysis

Thermal energy storage in cooling applications contributes to improvements in overall system efficiency as well as to better energy quality management. Latent heat thermal energy storage (LHTES) is used to provide load shifted thermal energy at small temperature swing with high storage density, hence an overall more compact energy system. However, the low thermal conductivity of the majority of the phase change materials (PCMs) necessitates delicate design of the active storage unit to meet power demand (high enough energy extraction/storage per amount of time).A performance analysis of two LHTES configurations is carried out in this work. Thermal charge and discharge rate of single PCM is compared with multistage LHTES using a cascade design of multiple PCMs at various phase change temperatures in a submerged finned pipe heat exchanger design. The work is conducted with a validated finite element based numerical simulation for evaluation of both full charge/discharge cycle and continuous half charge/discharge cycles.The results show that in full charge/discharge mode, the thermal performance of a multi-PCM LHTES may be improved by 10–40% as compared to that of a homogeneous single-PCM storage unit in terms of thermal charge/discharge rate. This is due to the capability of the multistage LHTES to maintain a higher driving temperature difference for the heat transfer process in the charging and discharging processes. In half charge/discharge cycling mode, however, the thermal power rating performance of multi-PCM storage converges towards that of the single-PCM storage in melting process, reducing thus the multi-PCM enhancement. This work provides preliminary insights to multistage latent heat cold thermal energy storage design with finned pipe heat exchanger.

Thermal performance of a multiple PCM thermal storage unit for free cooling

As demand for refrigeration and air conditioning increased during the last decade, the opportunities have expanded for using thermal energy storage (TES) systems in an economically advantageous manner in place of conventional cooling plants. Many cool storage systems use phase change materials (PCMs) and achieve peak load shifting in buildings. This work presents numerical investigations of the performance enhancement of a free cooling system using a TES unit employing multiple PCMs. The TES unit is composed of a number of rectangular channels for the flowing heat transfer fluid, separated by PCM slabs. Using the effective heat capacity method, the melting and solidification of the PCM is solved. The forced convective heat transfer inside the channels is analyzed by solving the energy equation, which is coupled with the heat conduction equation in the container wall. The effect of design parameters such as PCM slab length, thickness and fluid passage gap on the storage performance is also investigated using an energy based optimization. The results show that a system which can guarantee comfort conditions for the climate of Tabriz, Iran has an optimum COP of 7.0. This could be achieved by a combination of CaCl2·6H2O with RT25 with the optimum air channel thickness of 3.2mm, length of 1.3m and PCM slab thickness of 10mm.

Improved design for heat transfer performance of a novel phase change material (PCM) thermal energy storage (TES)

This study evaluates numerically various configurations of PCM thermal energy storage devices, e.g., U-tube, U-tube with in-line fins, U-tube with staggered fins and a novel festoon design. The conjugate heat transfer between the heat transfer fluid (HTF) and PCM, which undergoes a cyclic melting and freezing process, is solved numerically using the computational fluid dynamic approach utilizing enthalpy-porosity formulation. The results indicate that our novel festoon channel design yields improved heat transfer rate for both charging and discharging stages. To further enhance heat transfer performance, we introduce use of multiple PCMs with various configurations of different PCM arrangement. Advantages and limitations of each design are evaluated with respect to their heat transfer performance vis-à-vis heat storage capacity. Further R&D and experimentation is needed to arrive at commercially viable designs of PCM heat storage units.

Diffuse Solar System Design and Utilization in Agriculture and Domestic Applications

Diffuse solar radiation is a component of total solar radiation that is good for low temperature grade heating. Since the portion of the scattered radiation from the sun, which consists of short and long waves, that reaches the earth is diffused, its utilization in Agriculture as this paper suggested, has multiple phase change material, which as well could serve as heat exchange for domestic (residential) applications. The Diffuse Solar System designed, consists of three functional units: the solar collector, thermal storage (heat sink) and a dryer cabinet. The thermal storage ranges from 140 – 1500F (60 – 500C) for heating system and 60 – 700F (15 – 200C) for residential cooling. Likewise, a natural convection solar dryer with collector efficiency of about 42% and drying efficiency of about 53% would dry tomatoes with initial moisture contents of 94% wet basis, to safe moisture content of about 4% wet basis and 35.2 – 86.8% ascorbic acid (vitamin C) retention with acceptable colour and texture. Keywords: Solar radiation, Heat sink, Ascorbic acid, Collectors, Reflectors, Yeast/mould count and Efficiency.

A carbon nanotube-based composite for the thermal control of heat loads

Design optimization of an encapsulated carbon composite thermal control (TC) system is presented. The composite TC system consists of multiple phase change materials (PCM) doped with carbon nanotubes and enclosed in a casing of carbon/carbon composite sheets. Using the concept of global thermal resistance an analytical model was formulated to predict the transient temperature distribution through the composite system. The temperature data was then used to estimate maximum energy storage and heat dissipation rates. A substantial reduction in weight and size of the TC composite was observed corresponding to the optimized design. The use of carbon nanotubes both as additives with optimal loading and as a thermal interface material significantly reduced the maximum junction temperatures for different constant power loads for the multiple PCM composite as compared to its original size used for the experimental work. The optimized composite minimized the total thermal resistance through the composite sample and thereby increased its thermal response as indicated by approximately 4 times increase in the heat dissipation rate.

Prediction of Melting of Organic Phase Change Materials (PCMs) for Energy Storage with Simple Low Eutectic Systems

The paper predicts the melting of solid-liquid organic PCMs for energy storage by the use of activity coefficient. Single parameter Margules Equation was built for alcohol - carboxylic acid series of binary systems by improving single parameter Margules Equation and introducing new variable temperatures, from which T-X phase diagram was obtained. The results are helpful with prediction of multiple phase change materials with the suitable phase transition temperature.

Performance enhancement of a solar dynamic LHTS module having both fins and multiple PCMs

Latent heat thermal storage units span a wide and varied range of applications in the domestic, industrial and space based activities. Numerical investigations on the performance enhancement of a solar dynamic latent heat thermal storage (LHTS) unit employing multiple phase change materials (PCM) and fins are made. The LHTS unit has been studied for the charging mode alone. Enthalpy based formulation of the energy equations governing the behaviour of the LHTS system has been made and compared with the response of a single PCM unit. The governing conjugate equations have been solved employing finite difference techniques. The results show an appreciable enhancement in the rate of melting of PCM and nearly uniform exit temperature of heat transfer fluid (HTF) in the multiple PCM LHTS unit.

Second law analysis of latent thermal storage for solar system

A theoretical model has been developed for analysis and optimization of the solar system using phase change material (PCM). The later consists of a solar air heating collector coupled with a cylindrical storage tank which contains spherical capsules filled with a PCM. Energy and exergy analyses are carried out to understand the behavior of the system using single PCM or multiple PCMs. Numerical results show that the performance of the latent thermal storage system can be ameliored by the judicious choice of the melting temperature of the PCM.

Cascaded latent heat storage for parabolic trough solar power plants

The current revival of solar thermal electricity generating systems (SEGS) unveils the still existing need of economic thermal energy storages (TES) for the temperature range from 250 °C to 500 °C. The TES-benchmark for parabolic trough power plants is the direct two tank storage, as it was used at the SEGS I plant near Barstow (USA). With the introduction of expensive synthetic heat transfer oil, capable to increase the operating temperature from former 300 °C up to 400 °C, the direct storage technology became uneconomical. Cascaded latent heat storages (CLHS) are one possible TES alternative, which are marked by a minimum of necessary storage material. The use of a cascade of multiple phase change materials (PCM) shall ensure the optimal utilization of the storage material. This paper reports experimental and numerical results from the investigation of cascaded latent heat storages with alkali nitrate salts like NaNO 3, KNO 3 and others more. The experiments were conducted with vertical shell and tube type heat exchanger devices under realistic operation parameters. The experimental results were used for a numerical model to simulate different CLHS configurations. Dymola/Modelica was used to conduct the simulation. The outcome of this work shows on the one hand, that the design of CLHS for this temperature range is more complex than for the temperature range up to 100 °C. And on the other hand, the low heat conductivity of available PCM is an obstacle which must be overcome to make full use of this promising storage technology.

Effects of different multiple PCMs on the performance of a latent thermal energy storage system

The present study presents a theoretical model for the performance of a shell and tube latent thermal energy storage (LTES) unit using multiple phase change materials (PCMs). The model is based on the enthalpy method. Numerical simulations are carried out to investigate the effects of different multiple PCMs on the melted fraction, stored thermal energy and fluid outlet temperature of the LTES unit. Numerical results indicate that PCMs’ fractions and melting temperatures play an important role in the performance of the LTES unit. As a result, appropriate choosing of multiple PCMs is very significant for the performance improvement of the LTES unit.

Thermal performance analysis for a heat receiver using multiple phase change materials

To solve the problems associated with employing the single melt point phase change material in a heat receiver for the NASA 2 kW solar dynamic power system, this paper presents a practically easy to carry-out PCM receiver model composed of three different phase change temperature materials together with the corresponding physical model. A numerical solution is also given by which the maximal temperature for heat transfer, working fluid exit temperature, and liquid PCM fraction of the total heat transfer tube in whole are calculated. Furthermore, the results are compared with those obtained from the single PCM heat receiver. The results show that it is possible to improve the receiver performance and to reduce both the fluctuation of working fluid temperature and the weight of the heat receiver. All results of the calculation can be used to guide the heat receiver design.

Optimization of a Class of Latent Thermal Energy Storage Systems With Multiple Phase-Change Materials

This paper presents an analysis of a class of latent thermal energy storage (LTES) system. The analysis is based on a simplified model that allows the system performance to be evaluated in terms of a small set of parameters, while still retaining the main thermodynamic aspects associated with their operation. This analysis therefore permits the broad-based application potential of these systems to be viewed. The paper also discusses the applicability of the model to practical systems. This paper analyzes LTES with multiple energy storage cells and multiple phase-change materials (PCMs). The most general case of infinite energy storage cells and PCMs is solved, for the charge process only, as well as for the overall charge-discharge process. The results yield the optimum phase change temperature, expressed as a continuous function of position along the LTES. The method is equally applicable to the case of a finite number of storage cells. An example of the application of the method to this case is also included. The results show the optimum phase change temperatures for each of the problems being considered, along with the corresponding optimum exergetic efficiencies. The solutions to the optimization problems are surprisingly simple to express, considering the difficulty of the problems, and indicate the potential advantages of using LTES with multiple PCMs.