Phase Change Material

The transient reflectivity response of phase-change Ge 2 Sb 2 Te 5 films to intense femtosecond laser pulses is studied by ultrafast coherent phonon spectroscopy. The three different phases ͑amorphous, fcc-, and hcp-crystalline͒, as well as laser-crystallized films, are investigated, featuring different photoexcited carrier and coherent optical phonon dynamics. At least two main phonon frequencies are identified for each phase/material and their evolution for increasing pump fluences is investigated for the fcc-crystalline phase and the laser-crystallized material, revealing strong differences. We find evidence that a considerable fraction of amorphous phase remains in the laser-crystallized material, which features a different phonon frequency, not related to other phases. These results are important for emerging strategies aimed at driving ultrafast phase transitions via coherent phonon excitation for applications in data storage.

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Kinetics of solidification of phase change materials (PCM) was analyzed for combined heat conduction in the PCM and container wall, and convection in the cold fluid. Stable and convergent numerical methods were derived after transformation to normalized size scale, and corresponding immobilization of the moving boundary. The accuracy was confirmed by comparing numerical solutions and corresponding analytical solutions for control by heat conduction in solidifying layers. The proposed method was used to assess when solidification is controlled solely by conduction in solidified layer, and to analyze relative roles of conduction through the container and convection in the cold fluid.

Although water is a cheap and effective medium for thermal energy storage, other options are currently being studied, to increase the storage density or to reduce the cost of the storage. The authors have been developing a system which combines the advantages of stratified sensible heat storage and latent, phase change heat storage; i.e. a hot water storage tank with stratification where a phase change material (PCM) is included into a spiral tube installed in the top of the tank. The PCM used was a granular PCM–graphite compound of about 90 vol.% of sodium acetate trihydrate and 10 vol.% graphite. This paper presents the results of an experimental investigation of the performance of the new storage concept, and of a conventional hot water storage tank for comparison. The data are further analysed with respect to the energetic and exergetic performance of the two systems. Copyright © 2007 John Wiley & Sons, Ltd.

Cooling and air-conditioning systems are responsible for the highest energy consumption in buildings located in hot areas. This high share does not only increase the building energy demand cost but also increases the environmental impact, the topmost awareness of the modern era. The development of traditional systems and reliance on renewable technologies have increased drastically in the last century but still lacks economic concerns. Passive cooling strategies have been introduced as a successful option to mitigate the energy demand and improve energy conservation in buildings. This paper shed light on some passive strategies that could be applied to minimise building cooling loads to encourage the movement towards healthier and more energy-efficient buildings. For this purpose, seven popular passive technologies have been discussed shortly: multi-panned windows, shading devices, insulations, green roofing, phase change materials, reflective coatings, and natural ventilation using...

The power of the handheld electronics devices continues to increase because packaging advances reduce their size even as more features are added and enhanced. The computational fluid dynamics (CFD) numerical simulation is performed on thermal management of the mobile phone using a heat storage unit (HSU) filled with the phase change material (PCM). The chips and HSU are embedded in an epoxy polymer casing. The PCM absorbs the heat dissipation from the chips and stores as latent heat to maintain the chip temperature below the allowable service temperature. Eight cases of simulations are carried out with or without PCM at several power dissipations. The use of PCM in mobile phone has shown to be effective in keeping the chip temperature down to acceptable level for certain duration.

a b s t r a c t An innovative refrigeration system incorporating phase change material (PCM) is proposed to maintain refrigerated trucks at the desired thermal conditions. The advantage of using PCM to maintain low temperatures is that a conventional refrigeration system does not have to be located on-board the vehicle. In addition, the system consumes less energy and produces much lower local greenhouse gas (GHG) emissions. The phase change thermal storage unit (PCTSU) is charged by a refrigeration unit located off the vehicle when stationary. The PCM is discharged and provides cooling when in service. A new PCM with a lower cost than currently available PCMs was developed, suitable for maintaining the refrigerated truck at a temperature of À18°C. The PCM has a melting temperature of À26.7°C and a latent heat of 154.4 kJ kg À1 . A prototype system was constructed and test results proved that the proposed refrigeration system is feasible for mobile transport. An analysis shows that delivery of refrigerated products can be made with a PCM system having a weight comparable to that of an on board conventional refrigeration system with less than half of the energy cost.

A numerical study is conducted to evaluate the thermal characteristics of a PCM-based heat sink which can be potentially used for cooling of mobile electronic devices such as personal digital assistants (PDAs) and notebooks. The heat sink consists of a conventional, extruded aluminum sink embedded with appropriate PCMs. Some important parameters, such as PCM volume fraction, temperature difference, aspect ratio, and PCM properties, were studied to investigate their effects on the thermal performance of the hybrid cooling system.

An exergy analysis has been performed to determine the potential for useful work in a latent heat storage system with phase change material (PCM) for a flat-plate solar collector. Commercial paraffin wax is used as PCM to store and release energy in the solid-liquid transformation; this material is located in metal containers under the absorber plate on the bottom insulation of the collector. The exergy analysis is performed in outdoor conditions for days of low, medium and high radiation taken from October 2016 to March 2017 at Barranquilla city (latitude: 10º 59' 16" N, longitude: 74º 47' 20" O, Colombia). The system is evaluated throughout charge and discharge periods. The energy and exergy balance equations based on the first and second law of thermodynamics is formulated and solved for each element of the collector system as well as for the PCM. Results obtained show the energy distribution and energetic destruction for each system component and its variation as a time function. It was observed that the average energy and energetic efficiency are 28.7 %, 13.2 % for of low radiation days. 26.9%, 20.56% for of medium radiation days, and 23.2%, 18.6% for of high radiation days, respectively. Results of the analysis are shown in detail in the present paper.

A latent heat thermal storage prototype was tested under real working conditions with steam produced by a parabolic-trough collector test facility at the Plataforma Solar de Almería. The prototype contained KNO 3 /NaNO 3 eutectic mixture as phase change material (PCM) and expanded graphite fins arranged in a "sandwich configuration" for improving thermal conductivity. In this paper, experimental data such as steam quality, PCM temperature distribution, stored/delivered energy and thermal power have been analyzed for a selected day. A mismatch between steam quality results and the corresponding PCM temperature/time curves has been observed. Furthermore, it has been noted that stored/delivered energy and the resulting thermal power are 40 kW th h and 50 kW th , respectively, and hence, lower than the expected from design parameters. The reasons for these deviations seem to be deficient thermal insulation at the top of the prototype, use of working conditions other than design, and also thermal inertia introduced by excess PCM mass. In this paper, we also demonstrate the applicability of the quasi static model for describing the general performance of a latent thermal energy storage module with a sandwich configuration. In our particular case, the model fits the experimental data quite well when 8 W/mK is taken as the storage medium thermal conductivity. However, for a more accurate description, a sensible heat exchange term should be introduced in the model.

A key issue in nonvolatile storage is long-term data retention. This aspect is even more important in innovative storage technologies such as phase-change memory (PCM), which promises better performance and easier scalability with respect to traditional Flash memory and potential for multilevel storage. In this respect, we experimentally investigated the stability of intermediate states obtained by means of partial-SET programming. To this end, we analyzed the effects of the width and the amplitude of the programming pulses on the degradation of intermediate programmed resistance levels over time in PCM cells. Our study was carried out by considering the average behavior of an array of PCM cells, showing that data-retention properties degrade as the programming thermal stress increases.

Solar walls have been studied for decades as a way of heating building from a renewable energy source. A key ingredient of these wall is their storage capacity. However, this increases their weight and volume, which limits theirs integration into existing building. To aleviate this problem, storage mass is replaced by a phase change materials. These allow to store a large amount of energy in a small volume, which brings the possibility of retrofit trough use of light prefabricated module.

This paper put forward a new kind of under-floor electric heating system with shape-stabilized phase change material (PCM) plates. Different from conventional PCM, shape-stabilized PCM can keep the shape unchanged during phase change process. Therefore, the PCM leakage danger can be avoided. This system can charge heat by using cheap nighttime electricity and discharge the heat stored at daytime. In the present work, the thermal physical properties of shape-stabilized PCM developed by us were measured. A prototype room with this system was set up in Beijing to testify its thermal performance and feasibility of this heating mode. The results show that temperature of the PCM plates upper surface can be kept near the phase transition temperature in whole day and a lot of off-peak period electricity can be used for space heating in stead of using peak period electricity, which obviously lowers the electricity tariff. #

Expanded graphite is a promising heat transfer promoter due to its high conductivity, which improves the thermal conductivity of organic phase change materials. Moreover, it can also serve as supporting materials to keep the shape of the blends stable during the phase transition. After various investigation, the results showed that the maximum weight percentage of polyethylene glycol was as high as 90% in this paper without any leakage during the melting period, with the latent heat of 161.2 J g À1 and the melting point of 61.46°C. It was found that the value of the latent heat was related to the polyethylene glycol portion, increased with the increase in polyethylene glycol content. Moreover, the measured enthalpy of the composite phase change materials was proportional to the mass ratio of the polyethylene glycol component. The melting temperatures were almost the same with different ratios of composites. The conductivity of blends was improved significantly with the high value of 1.324 W m À1 K À1 compared to the pure polyethylene glycol conductivity of 0.2985 W m À1 K À1 .

In a typical power electronics package, a grease layer forms the interface between the direct bond copper (DBC) layer or a baseplate and the heat sink. This grease layer has the highest thermal resistance of any layer in the package. Reducing the thermal resistance of this thermal interface material (TIM) can help achieve the FreedomCAR partnership goals of using a glycol water mixture at 105°C or even air cooling. It is desirable to keep the maximum temperature of the conventional silicon die below 125°C, trench insulated gate bipolar transistors (IGBTs) below 150°C, and silicon carbidebased devices below 200°C. Using improved thermal interface materials enables the realization of these goals and the dissipation of high heat fluxes. The ability to dissipate high heat fluxes in turn enables a reduction in die size, cost, weight, and volume. This paper describes our progress in characterizing the thermal performance of some conventional and novel thermal interface materials. We acquired, modified, and improved an apparatus based on the ASTM D5470 test method and measured the thermal resistance of various conventional greases. We also measured the performance of select phase-change materials and thermoplastics through the ASTM steady-state and the transient laser flash approaches, and compared the two methodologies. These experimental results for thermal resistance are cast in the context of automotive power electronics cooling. Results from numerical finite element modeling indicate that the thermal resistance of the TIM layer has a dramatic effect on the maximum temperature in the IGBT package.

In this paper, a finite element numerical algorithm for the simulation of two-dimensional problems of heat transfer with phase change is validated, comparing the numerical results with the experimental ones deriving from tests carried out on two different kinds of PCM containing sandwich panels. The very good approximation affecting the numerical results, as compared with the experimental ones, suggests that it can be a powerful design tool for every kind of PCM containing building elements.

In this paper, a PCM storage unit for a solar cooker was designed and developed to store solar energy during sunshine hours. The stored energy was utilised to cook food in the evening. Commercial grade acetamide was used as a latent heat storage material. Cooking experiments were conducted with dierent loads and loading times during the summer and winter seasons. The thermal performance of a solar cooker with the PCM storage unit was compared with a standard solar cooker. The experimental results showed that evening cooking is possible with a solar cooker having the PCM storage unit and is not possible in a standard solar cooker. 7

When the latent heat of a phase change material (PCM) with a lower melting point than ambient temperature was assessed according to the standard T-history method using a vertically oriented test tube, a temperature gradient occurred in the longitudinal direction of the tube due to natural convection. This led to a decrease in the accuracy of the latent heat of fusion measurement. In this study, the accuracy of the measurement with the original T-history method was improved without decreasing the test's simplicity and convenience by setting the test tube horizontally. The heat transfer to the vapor-layer of the tube under volume change during melting was assumed to be negligible and the results were calculated using the two inflection points of temperature as the start and end of latent heat period. Under these assumptions, the results agree closely with other reference data. And, the new method proposed in this study showed a remarkable reduction in data scattering.

This work reports the laboratory test results of a Li-ion battery designed for electric scooter applications. Four different modes of heat dissipation were investigated in this experimental study: (1) natural convection cooling; (2) presence of aluminum foam heat transfer matrix; ...

Greenhouses provide a controlled environment that is suitable for plants growth and cultivation. In this paper the maximum temperature change inside the greenhouse is to be reduced by the use of energy storage in a phase change material PCM. A mathematical model is developed for the storage material and for the greenhouse. The coupled models are solved using numerical methods and Java code program. The effect of different parameters on the inside greenhouse temperature is investigated. The temperature swing between maximum and minimum values during 24 h can be reduced by 3-5°C using the PCM storage. This can be improved further by enhancing the heat transfer between the PCM storage and the air inside the greenhouse.

Within a development project for the characterization of sputtered thin films for phase change optical disks, amorphous and crystalline phases of Ge2Sb2Te5 (GST) and AgInSbTe (AIST) phase change materials have been investigated. Sputtered thin films (200nm) on polycarbonate were characterized before and after laser initialization using X-ray diffraction methods. The crystalline phase of GST was identified as fcc rock-salt like

The present paper is confined to the discussion of technical grade paraffin waxes as phase change material (PCM) for cool storage and the cool storage systems capital cost investment. The objective of this study is to determine the potential for using cool storage systems. The thermal properties of technical grade paraffin waxes as PCM are investigated, and static and dynamic cool storage processes are discussed. The capital cost of the cooling storage system is estimated, which indicates the cooling storage system not only saves energy and other operation and maintenance costs, but also saves a significant fraction of the initial capital costs. Installing cool storage, at less cost than conventional nonstorage equipment, can often increase the cooling capacity of the existing system. Ó

Elevated operating temperatures reduce the efficiency of photovoltaic devices. The use of a phase change material to moderate building integrated photovoltaic temperature rise has been investigated by experiments and numerical simulations. Experimental data are used to validate the previously developed two-dimensional finite volume heat transfer model conjugated hydrodynamically to solve the Navier-Stokes and energy equations. A parametric study of a design application is also reported. Temperatures, velocity fields and vortex formation within the system were predicted for a variety of configurations using the experimentally validated numerical model. Temperature distributions predicted for different insolation and ambient temperatures at the photovoltaic surface show that the moderation of temperature achieved can lead to significant improvements in the operational efficiency of photovoltaic facades.

a b s t r a c t Transient mathematical models are presented for a single slope-single basin solar still with and without phase change material (PCM) under the basin liner of the still. Analytical expressions for temperatures of the still elements and the PCM have been obtained. The still performance has been investigated by computer simulation. Numerical calculations have been carried out, using stearic acid as a PCM, on typical summer and winter days in Jeddah (lat. 21°42 0 N, long. 39°11 0 E), Saudi Arabia. Effect of mass of the PCM (m pcm ) on the daylight P dl , overnight P on and daily productivity P d and efficiency g d of the still for different masses of basin water m w has been investigated. It is found that P dl decreases as m pcm increases; but P on and P d increase significantly with an increase of m pcm due to the increased amount of the heat stored within the PCM. During discharging of the PCM, the convective heat transfer coefficient from the basin liner to basin water is doubled; thus, the evaporative heat transfer coefficient is increased by 27% on using 3.3 cm of stearic acid beneath the basin liner. Therefore, on a summer day, a value of P d of 9.005 (kg/m 2 day) with a daily efficiency of 85.3% has been obtained compared to 4.998 (kg/m 2 day) when the still is used without the PCM. The PCM is more effective for lower masses of basin water on winter season.

This paper addresses a numerical and experimental investigation of a cold thermal energy storage system involving phase-change process dominated by heat conduction. The problem involves a fluid flowing inside a horizontal finned tube surrounded by a phase-change material (PCM). The objective of this paper is to predict the temperature distribution, the phase front distribution along the tube and to analyze the effect of fin density and size on the dynamic performance of the system. The problem is modeled as axisymmetric and two-dimensional, and a control volume computer code has been developed for the solution of the corresponding mathematical model. In the experimental arrangement of the tube configuration, two different fin diameters; D fn = 2.7 and 3.2 are considered and the fin density at each fin diameter is varied in the range of N fn = 14-31 fins·m −1 . For a particular geometry then the heat transfer fluid inlet temperature assumes values between −10 • C and −20 • C, and the flow Peclet number is varied in the range from 14 350 to 200 900 accordingly. Comparison between the numerical predictions and the experimental data shows good agreement, even though some effects that are produced by heat transfer to the environment especially at high flow rates neglected in the model but unavoidable in the experiments. Finally, time-wise variation of energy stored by the system is evaluated through instant images of solidification fronts and the combined effect of fin parameters and the flow rate on energy storage is discussed.

Thermal energy storage in general, and phase change materials (PCMs) in particular, have been a main topic in research for the last 20 years, but although the information is quantitatively enormous, it is also spread widely in the literature, and difficult to find. In this work, a review has been carried out of the history of thermal energy storage with solid-liquid phase change. Three aspects have been the focus of this review: materials, heat transfer and applications. The paper contains listed over 150 materials used in research as PCMs, and about 45 commercially available PCMs. The paper lists over 230 references.

Paraffin waxes have been used in many latent thermal energy storage applications because of their advantageous thermal performances. In this paper, the liquid-solid phase diagram of the binary system of tetradecane and hexadecane has been used to obtain information of the phase transition processes for cool storage applications. The analysis of the phase diagram indicates that, except for the minimum-melting point mixture, all mixtures melt and freeze in a temperature range and not at a constant temperature. The latent heat of fusion evolves throughout this temperature range. Differential scanning calorimetry (DSC) was used to determine the thermophysical properties of the binary system. Depending on the DSC settings throughout the measurements, varying results were obtained. For example, when the DSC runs at a high heating/cooling rate, it will lead to erroneous information. Also, the correct phase transition temperature range cannot be obtained simply from DSC measurement. By combining phase equilibrium considerations with DSC measurements, a reliable design method to incorporate both the heat of phase change and the temperature range is presented. #

A lithium-ion battery employing a novel phase change material (PCM) thermal management system was designed for an electric scooter. Passive thermal management systems using PCM can control the temperature excursions and maintain temperature uniformity in Li-ion batteries without the use of active cooling components such as a fan, a blower or a pump found in air/liquid-cooling systems. Hence, the advantages of a compact, lightweight, and energy efficient system can be achieved with this novel form of thermal management system. Simulation results are shown for a Li-ion battery sub-module consisting of nine 18650 Li-ion cells surrounded by PCM with a melting point between 41 and 44 • C. The use of aluminum foam within the PCM and fins attached to the battery module were studied to overcome the low thermal conductivity of the PCM and the low natural convection heat transfer coefficient. The comparative results of the PCM performance in the presence of Al-foam and Al-fins are shown. The battery module is also simulated for summer and winter conditions. The effect of air-cooling on the Li-ion battery was also studied. These simulation results demonstrate the successful use of the PCM as a potential candidate for thermal management solution in electric scooter applications and therefore for other electric vehicle applications.

A passive thermal management system is evaluated for high-power Li-ion packs under stressful or abusive conditions, and compared with a purely air-cooling mode under normal and abuse conditions. A compact and properly designed passive thermal management system utilizing phase change material (PCM) provides faster heat dissipation than active cooling during high pulse power discharges while preserving sufficiently uniform cell temperature to ensure the desirable cycle life for the pack. This study investigates how passive cooling with PCM contributes to preventing the propagation of thermal runaway in a single cell or adjacent cells due to a cell catastrophic failure. Its effectiveness is compared with that of active cooling by forced air flow or natural convection using the same compact module and pack configuration corresponding to the PCM matrix technology. The effects of nickel tabs and spacing between the cells were also studied.

This paper proposes a multilevel Non-Return-to-Zero (NRZ) coding technique for the transmission of digital signals. The Multilevel Technique presented here helps in removing certain problems associated with Bipolar and Manchester coding techniques. This multilevel technique utilizes different D.C. levels for representing a ‘0’ and ‘1’ with a NRZ method. The PSD (power spectral density) of the encoded signal is analyzed and possible generation method is also shown.

The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermalstorage systems for heat pumps, solar engineering, and spacecraft thermal control applications. The uses of PCMs for heating and cooling applications for buildings have been investigated within the past decade. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This paper also summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for use in different applications. #

This study aimed determination of proper amount of paraffin (n-docosane) absorbed into expanded graphite (EG) to obtain formstable composite as phase change material (PCM), examination of the influence of EG addition on the thermal conductivity using transient hot-wire method and investigation of latent heat thermal energy storage (LHTES) characteristics of paraffin such as melting time, melting temperature and latent heat capacity using differential scanning calorimetry (DSC) technique. The paraffin/EG composites with the mass fraction of 2%, 4%, 7%, and 10% EG were prepared by absorbing liquid paraffin into the EG. The composite PCM with mass fraction of 10% EG was considered as form-stable allowing no leakage of melted paraffin during the solid-liquid phase change due to capillary and surface tension forces of EG. Thermal conductivity of the pure paraffin and the composite PCMs including 2, 4, 7 and 10 wt% EG were measured as 0.22, 0.40, 0.52, 0.68 and 0.82 W/m K, respectively. Melting time test showed that the increasing thermal conductivity of paraffin noticeably decreased its melting time. Furthermore, DSC analysis indicated that changes in the melting temperatures of the composite PCMs were not considerable, and their latent heat capacities were approximately equivalent to the values calculated based on the mass ratios of the paraffin in the composites. It was concluded that the composite PCM with the mass fraction of 10% EG was the most promising one for LHTES applications due to its form-stable property, direct usability without a need of extra storage container, high thermal conductivity, good melting temperature and satisfying latent heat storage capacity.

Thermal energy storage is one of the most efficient ways to store solar energy for heating air by energy collected from sun. The relative studies are involved to the type of collection with the type of storage, i.e. separated to each other or integrated. This review summarizes the previous works on solar air heaters with storage materials include greenhouse, encapsulation, and the latest development in the solar thermal energy storage with air as a heat transfer fluid. The recent researches focused on the phase change materials (PCMs), as latent heat storage is more efficient than sensible heat storage. It has been appeared that PCM with high latent heat and suitable geometry are required for optimum thermal performance of solar air heater. The recent designs of solar air heaters with thermal storage units reduced the cost and the volume when integrated in one product.

A box-type solar cooker with latent heat storage has been designed and fabricated for the composite climatic conditions of India. The experimental results demonstrate the feasibility of using a phase change material as the storage medium in solar cookers, i.e. it is possible to cook the food even in the evening with a solar cooker having latent heat storage. It also provides a nearly constant plate temperature in the late evening. The experimental results have also been compared with those of a conventional solar cooker. 0 1997 Elsevier Science Ltd. All rights reserved

Solar water heaters (SWHs) are a well-established renewable energy technology that have been widely adopted around the world. The downfall of this technology is rooted in the inconsistency of solar intensity from day to night. In the recent studies, the application of energy storage materials such as phase change materials (PCMs) has attracted many attentions; however, PCM by itself may not be effective due to the poor heat transfer rate, low thermal diffusivity and thermal conductivity. This paper aims to explore the thermal performance of energy storage-transfer materials to be applied in conjunction with PCMs. The selected types of PCMs are paraffin waxes with melting point temperatures of 28–72°C. In the first analysis, silicone oil is selected as the heat transfer medium with high thermal stability. The melting point and specific heat capacity were measured by a modulated differential scanning calorimeter (MDSC). The obtained results show that silicone oil will lead to melting ...

The continuous increase in the level of greenhouse gas emissions and the climb in fuel prices are the main driving forces behind efforts to more effectively utilise 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. However, the large-scale utilisation of this form of energy is possible only if the effective technology for its storage can be developed with acceptable capital and running costs. One of prospective techniques of storing solar energy is the application of phase change materials (PCMs). Unfortunately, prior to the large-scale practical application of this technology, it is necessary to resolve numerous problems at the research and development stage. This paper looks at the current state of research in this particular field, with the main focus being on the assessment of the thermal properties of various PCMs, methods of heat transfer enhancement and design configurations of heat storage facilities to be used as a part of solar passive and active space heating systems, greenhouses and solar cooking.

... Ahmet Koca a , Hakan F. Oztop b , Tansel Koyun c and Yasin Varol a , Corresponding Author Contact Information , E-mail The Corresponding Author. ... from 1.22 to 2.63 kW, whereas the rate of heat stored in the LHS unit were 111.2 and 79.9 W. Sari and Kaygusuz [11] conducted ...

a b s t r a c t A significant portion of the total primary energy is consumed by today's buildings in developed countries. In many of these buildings, the energy consumption can be significantly reduced by adopting energy efficiency strategies. Due to environmental concerns and the high cost of energy in recent years there has been a renewed interest in building energy efficiency. This article strives to make an exhaustive technical review of the building envelope components and respective improvements from an energy efficiency perspective. Different types of energy efficient walls such as Trombe walls, ventilated walls, and glazed walls are discussed. Performance of different fenestration technologies including aerogel, vacuum glazing and frames are presented. Advances in energy efficient roofs including the contemporary green roofs, photovoltaic roofs, radiant-transmittive barrier and evaporative roof cooling systems are discussed. Various types of thermal insulation materials are enumerated along with selection criteria of these materials. The effects of thermal mass and phase change material on building cooling/heating loads and peak loads are discussed. Application of thermal mass as an energy saving method is more effective in places where the outside ambient air temperature differences between the days and nights are high. Air tightness and infiltration of building envelopes are discussed as they play a crucial role in the energy consumption of a building. Energy efficiency approaches sometimes might not require additional capital investment. For example, a holistic energy efficient building design approach can reduce the size of mechanical systems compensating the additional cost of energy efficiency features.

Recently, thermal energy storage (TES) has received increasing attention for its high potential to meet cities' need for effective and sustainable energy use. Traditionally, energy was stored in the form of sensible heat which requires large volume of storage material. The storage volume can be significantly reduced if energy is stored in the form of latent heat and thus can benefit enormously practical applications. The existing approaches in the design, integration and application of phase change materials (PCMs) in domestic hot water tanks (HWT) and transpired solar collector (TSC) using water/air as the heat transfer media are reviewed. Crucial influencing factors are considered, including thermo-physical properties of different PCMs, different configurations of PCMs in HWT and TSC, and the limitations of each technique. This paper also discusses the existing simulation, design tools and experimental studies related to PCMs usage in HWT and central thermal storage.

Thermal mass combined with other passive strategies can play an important role in buildings energy efficiency, minimizing the need of space-conditioning mechanical systems. However, the use of lightweight materials with low thermal mass is becoming increasingly common. Phase Change Materials (PCMs) can add thermal energy storage benefits to lightweight constructions. There are many studies about the use of PCM in buildings, but there are still some difficulties for effective use and practical application of these materials. The study of applications used in real buildings could help us find solutions to these difficulties. This paper analyzed different PCM applications presented in highly efficient lightweight construction houses that have participated in the American Solar Decathlon, an international competition organized by U.S. Department of Energy. These houses have been tested and monitored in Washington DC, place with suitable climate conditions for short term thermal storage systems. The study started with a classification of the PCM applications and included an analysis of the systems, materials, switching temperatures, containments and design strategies used to improve the houses energy performance. Also, it included results of numerical simulations and experimentation that the participant team had done, complemented with information available in the literature about similar materials or applications.

The use of a latent heat storage system using phase change materials (PCMs) is an effective way of storing thermal energy and has the advantages of high-energy storage density and the isothermal nature of the storage process. PCMs have been widely used in latent heat thermalstorage systems for heat pumps, solar engineering, and spacecraft thermal control applications. The uses of PCMs for heating and cooling applications for buildings have been investigated within the past decade. There are large numbers of PCMs that melt and solidify at a wide range of temperatures, making them attractive in a number of applications. This paper also summarizes the investigation and analysis of the available thermal energy storage systems incorporating PCMs for use in different applications. #