Review of thermal energy storage technologies based on PCM application in buildings

Energy and Buildings 67 (2013) 56–69

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Review of thermal energy storage technologies based on PCM application in buildings Michal Pomianowski a,∗ , Per Heiselberg a , Yinping Zhang b a b

Department of Civil Engineering, Aalborg University, Sohngaardsholmsvej 57, Aalborg, Denmark Department of Building Science, Tsinghua University, Haidian District, Beijing 100084, China

a r t i c l e

i n f o

Article history: Received 8 April 2013 Received in revised form 18 July 2013 Accepted 1 August 2013 Keywords: PCM Latent heat Thermal energy storage Heat transfer Building envelope

a b s t r a c t Thermal energy storage systems (TES), using phase change material (PCM) in buildings, are widely investigated technologies and a fast developing research area. Therefore, there is a need for regular and consistent reviews of the published studies. This review is focused on PCM technologies developed to serve the building industry. Various PCM technologies tailored for building applications are studied with respect to technological potential to improve indoor environment, increase thermal inertia and decrease energy use for building operation. What is more, in this review special attention is paid to discussion and identification of proper methods to correctly determine the thermal properties of PCM materials and their composites and as well procedures to determine their energy storage and saving potential. The purpose of the paper is to highlight promising technologies for PCM application in buildings with focus on room application and to indicate in which applications the potential is less significant. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In general, heat storage is a very interesting technique to decrease energy use in the buildings and to reduce the cost of operation of buildings. Therefore, during recent decades a variety of heat storage solutions for the building market have been developed. Some of the advantages of heat storage in the buildings are as follows: reduction of peak power for heating and cooling, possibility to shift peak heating and cooling loads to the low tariff hours, shifting temperature peaks to non-working hours, improvement of indoor environment, and efficient utilization of passive heating and cooling loads. Thermal energy storage (TES) can be divided into sensible heat storage and latent heat storage systems. It is worth mentioning that each latent heat storage system also always represents sensible heat storage, but this one is usually very small compared to the latent heat capacity, and therefore the latent heat storage is more interesting and has drawn much attention during the last decades. For example, ordinary building materials such as concrete and gypsum only represent the sensible heat storage capacity, which varies approximately between 0.75 and 1 kJ/(kg K) whereas, for example, some average paraffin materials, which undergo phase change have latent heat storage capacity of approximately 110 kJ/kg. This means that due to very high heat storage

∗ Corresponding author. Tel.: +45 9940 7234; fax: +45 9940 8552. E-mail addresses: [email protected] (M. Pomianowski), [email protected] (P. Heiselberg), [email protected] (Y. Zhang). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.08.006

potential, a much smaller volume of the material is needed to store the same amount of energy. The other advantage of phase change materials (PCM) is that within the phase change their temperature remains almost constant. Thanks to this characteristic, the temperature stratification in the spaces has been reported to be minimized, and too high temperatures of the surfaces of constructions prevented. However, despite of all the mentioned advantages and very large number of research projects on PCMs, the most of PCM products do not find commercial implementation and the ones that do are still in the niche. Even though some of the products are implemented in the buildings, there are no real case studies on the performance of PCM in buildings, and the only information available is limited to the small prototype laboratory tests. Moreover, although the thermal properties of many PCMs are well analyzed and documented, the whole picture of the conditions required to activate thermal mass are often omitted or unrealistic experimental scenarios are studied. As a result of that, either incomplete conclusion are drawn or overestimated performance is indicated. Furthermore, an economic analysis of the application of PCMs is never documented, and even simple information about cost of such materials is not easy to obtain by someone potentially interested. The purpose of this paper is to illustrate and sum up the research activities focused on PCM application in the buildings. The center of the focus is the room and PCM applications in the room. Additionally, the paper elaborates on the heat transfer within the room enclosures as this aspect is very often the key parameter that decides to which extent and how efficiently thermal mass of the building is activated.

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During recent years, many studies of PCMs and latent heat storage techniques have been reported. Moreover, research topics that can be found in the literature cover various areas of phase change application and features, and as a result it is sometimes difficult to have a clear overview of up to date information, since some of the newer results can be contradicted with regards to some earlier findings. For example, some of the studies are focused on developing new PCMs [1–3], whereas other categorize and sort out existing candidates with regards to, for example, their thermal and physical properties and melting ranges [4–6]. Other papers only deal with specific application and potential of technologies using PCMs, and this is a very large group of publications. There are also publications focusing on measuring procedures and techniques to determine the thermal properties of PCMs. These methodologies are not as straight forward as for example for ordinary materials representing only sensible heat [8,9]. Finally, some of the studies try to improve existing PCMs or latent heat storage solutions by modifying the thermal properties, geometry, or whole system configurations [10–12]. On top of that, some of the works combine different issues listed above in the text together in one holistic approach to solve the specific research problem. In this huge variety of subjects grouped around the latent heat storage issue, it is often a challenge to find a clear understanding of available studies focused on PCM applications tailored for the building market and – what is also very important – which of them might illustrate solutions that are more promising than others. Another very important issue is to be able to recognize proper testing procedures, boundaries and heating loads that should be taken under consideration, in order to obtain the correct performance potential of the investigated technology. The attempt of this paper is to gather publications treating PCM application developed for the building market. Different major disciplines are recognized, and relevant publications are grouped within each discipline. Moreover, results from different studies are not only summarized, but in many cases comments on possible improvement or missing information are indicated. In addition, scientific documentation on different technologies utilizing PCMs has been reviewed chronologically in some cases, since it was observed that in many consecutive studies, the outcome of the previous publications was the motivation to subsequent research. Furthermore, special care has been taken to not only list the available literature on the specific topics but also to shortly explain the content of research and the key conclusions. What is also worth highlighting is that the aim of this review is not to elaborate on the properties and the potential of different PCM as such, but always on the specific application in the buildings where the focus is on the PCM application in a room. Although no thermal properties of specific PCMs are listed in the paper, special attention is paid to elaborate on accurate methods available to determine the thermal properties of PCMs or theirs composites. The determination of the thermal properties of PCMs and theirs composites can be challenging. However, it is often an initial and unavoidable step to investigate specific PCM applicability and potential. Also, the focus on the realistic heat transfer within the building envelope enclosures is elaborated, as this is the key parameter influencing to what extent the thermal mass of the building is thermally activated. In the review, a special effort has been made to identify the success criteria of PCM application in buildings but also to identify the challenges and reasons why PCM has such a small share in the real building projects. Based on the scientific evidences, it is concluded when and where PCM has potential for successful application and which criteria has to be known and fulfilled. The article finishes with a recommendation for further research and indications of where further efforts should be made, in order to determine if PCM could be a beneficial and competitive technology.

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The paper starts with a summary of recent reviews on PCM materials and as well the technologies implementing PCM in building application. The outcome of the summary of reviews is to sort out the most important groups of PCM applications related to building solutions and also to indicate which research areas are of most interest. Consequently, different applications of PCM grouped into disciplines are discussed, and the most relevant works are presented. The following disciplines related to building application have been recognized: PCM in construction materials (passive), PCM in thermally activated constructions, PCM in glazing and shading devices, and PCM combined with ventilation and air-conditioning. What is more, the following issues closely related to the building design process and PCM thermal properties have been included in the paper: methods to determine thermal properties of PCMs and their composites, heat transfer enhancement in PCMs, and heat transfer in the room. The paper closes with discussion and conclusions.

2. Summary of reviews Over the last decades, several reviews on latent heat storage materials and systems have been published. In the following, some of the most significant ones are shortly summarized and put in chronologic order [5,6,13–17]. In [5], over 230 references related to various topics considering PCMs are given. Firstly, an extensive list of materials representing latent heat storage properties is presented. In the review, PCMs are classified into paraffin, fatty acids, salt hydrates and eutectics. What is more, [5] treats, among others, issues such as thermal properties, stability and heat transfer in the latent heat storage materials. In [5], one can find numerous references to various studies that investigate different application of PCMs, among them also application of PCMs in the buildings. The year after [6] was published, which summarizes investigations into PCM application in the buildings. The focus of the review is to gather works regarding PCM integration in the building construction materials such as wallboards, gypsum and concrete. In [6], there is also a discussion on the advantage of the development of microencapsulated PCMs and their superiority with regards to macro-encapsulated PCMs. The review finishes with a short summary of fire retardation of PCM-treated construction materials. In 2007 [7], was published which gathers previous investigations on PCM application in the building envelope. The review indicates that with suitable PCM incorporation in the buildings, it can be an interesting technology and an efficient solution to decrease energy use to heat and cool buildings. The paper, however, points out that several problems have to be tackled before this kind of TES can become reliable and applied in practice. The review is finished with a list of pending activities that should be carried out to illuminate the energy saving potential and reliability of latent heat storage technologies. In 2008 [13], was published which reviews thermal energy storage systems using PCM capsules. The paper presents developed latent heat thermal energy storage systems and aspects related to heat storage, such as, encapsulation, heat transfer, applications and materials properties. The consequent review [14] (on PCMs application in buildings) elaborates on researches dealing with passive performance of PCM in the buildings, load shifting and as well active building systems with PCM. In the paper, there is a summary and a comparison of the performance of various simulated technologies utilizing PCMs over the last years. In the more recent publication [15], the latest developments of PCM and various applications in the buildings are shown. The review also presents a very broad list of latent heat storage

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candidates that were listed in the other recent reviews and publications. The review pays special attention to the thermal properties and issues related to encapsulation of PCMs. This paper presents illustrations and pictures of the several latest developed products and concepts integrating microencapsulated PCMs. However, the performance and the energy saving potential of the products are not discussed in detail. Another recently published review, [16] carries out a compilation of researches on PCM slurries and PCM emulsions. The review presents information on various PCM emulsions and slurries and heat transfer phenomena, and indicates some of the applications. One of the key conclusions of the review is that, at present, it cannot be determined whether PCM emulsions and PCM slurries can improve the heat transfer phenomenon and installations with water based systems used as reference one. One of the reasons why it is presently difficult to indicate the potential of this type of material is due to very few installations and experimental examples conducted so far. In [17], the focus of the review is on cooling systems with PCMs related to buildings. A review is carried out focusing on the information concerning the following four types of cooling systems: free cooling applications, encapsulated PCM systems, air-condition and sorption cooling systems. It was concluded that PCMs have a positive impact on the energy use for cooling the buildings. On the other hand, the authors indicate that the problem with heat transfer between PCM and the fluid and the amount of the material needed for thermal storage constitute the major challenge to make PCM solutions economical and attractive from an energetic point of view. Based on the summary of reviews, the conclusion is that there are an extensive number of publications and reviews dealing with the subject of latent heat storage. What is more, it can be concluded that in the most cases the focus is on including and repeating a bit of information about everything: general information about heat transfer, thermal properties and a list of available PCMs. Each review focuses on several types of PCM applications in the building, and do not elaborate on other systems and solutions. As a result, the holistic overview of possibilities and the potential of PCMs applications in buildings are ambiguous. In addition, the extensive number of reviews and publications published over the years and some of the information found in the earlier sources is incompatible with the findings presented in the more recent publications. Furthermore, based on the scientific evidence the competitiveness of PCM applications to other available passive and energy saving solutions is still doubtful. Although some of the conceptual investigations have evolved into the first available commercialized products, most of the concepts are still in the prototype stage, and PCM applications are not “broadly” used in the buildings. An attempt to answer to that situation will be made in this paper review.

3. PCM in construction materials (passive) During recent decades, several bulk encapsulated PCMs were developed for the building applications. However, as stated in [6], the surface area of the most encapsulated commercial products was inadequate to deliver heat to the building after PCM was melted. Therefore, the majority of studies have been focused on PCM integration into the construction elements of the building, such as walls, ceilings and floors. These construction elements offer large areas for heat transfer within building enclosures. In Sections 3.1–3.4, various construction materials are presented (gypsum, concrete, bricks) which were blended or combined with PCMs. The activation of latent heat storage in all presented cases is due to passive activation, and it means that thermal mass represented by the constructions is heated up or cooled down only due to indoor

temperature fluctuations that were not caused by any mechanical additional cooling and heating means. In sections 3.1–3.5, focus is on testing procedures of the new materials and their energy performance with regards to indoor temperature stabilization and potential for energy savings due to temperature peak reduction.

3.1. PCM in gypsum and wallboards This section presents studies that were conducted on composites of either PCM gypsum boards or PCM wallboards. During the last couple of years, numerous researchers have studied and developed a vast variety of this type of materials. The main purpose of integrating PCM into lightweight construction materials is to increase their thermal mass. As a result, such products could be used to decrease temperature fluctuations in existing and renovated buildings as well new lightweight buildings. In Ref. [18] authors have developed the gypsum board with integrated microencapsulated PCM, mineral aggregates and have added some admixtures to improve working properties of the board. The incorporated PCM had a melting range between 25 ◦ C and 28 ◦ C. The sensible and latent heat of the material was measured with differential scanning calorimetry (DSC) with a constant heat and cooling rate of 2 K/min. The thermal conductivity of PCM-modified gypsum was determined with use of a laser flash instrument. The developed PCM boards were tested in the special light weight chambers. The two identical test chambers were built next to each other, and in the first one, walls were covered with PCM plaster boards and in the second one with ordinary plaster boards. The thickness of the gypsum board was varied between 1 cm and 3 cm. The test series were carried out under controlled variable conditions. It was discovered that during warm days a reduction of the peak temperature of about 3 K in comparison to the room without PCM could be achieved. On the other hand, temperature in the test chamber was allowed to fluctuate from very low to very high temperatures (approximately 14–35 ◦ C). In real building conditions, such high temperature amplitude would not be acceptable and therefore also utilization of the latent heat of PCM in the gypsum boards would be decreased. Additionally, authors do not elaborate on obtained PCM to gypsum ratio in the developed gypsum boards. In [19], measurements of a full-size room equipped with microencapsulated-PCM plaster boards are presented. Prior to the full scale measurements, some small scale experiments with specially designed plate apparatus to test wall samples have been conducted. A small sample of 50 cm × 50 cm area was pressed between two copper plates, which can be heated and cooled independently. The thermal performance of the wall samples with PCM was tested for the constant heat flux on both sides of the sample and temperature in the middle of the sample was registered. It was discovered that for the samples with PCM temperature instead of rising linearly begins to deflect within PCM melting temperature range. Consecutively, the full scale measurements have been conducted in the specially built light weight test rooms. One room was equipped with the ordinary reference plaster and the other with the PCM plaster. Both rooms were facing south. In the article, it was not written how much of internal area was covered with gypsum and where the plaster boards was located. Within the project, two different PCM products were tested: dispersion based plaster with 40% weight PCM and 6 mm thickness and gypsum plaster with 20% weight PCM and 15 mm thickness. The experimental study indicated that PCM gypsum helped to decrease high and low temperature peaks. Over period of 3 weeks, the reference room was warmer than 28 ◦ C for about 50 h while the PCM room was only 5 h above 28 ◦ C. Authors pointed out that microencapsulated PCM has the advantage of easy application and there is no danger of leakage like with macroencapsulated PCM.

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In [20], experimental tests on composite PCM product ENERGAIN® from Dupont de Nemours Society that contained 60% of microencapsulated paraffin and rest was copolymer can be found. The thermal conductivity has been measured using a guarded hotplate apparatus and in the liquid state it was at 0.22 W/(m K) and in the solid at 0.18 W/(m K). The enthalpy of the composite PCM has been measured using DSC method at heating cooling rate of 0.05 K/min. Melting peak temperature was obtained at 22.2 ◦ C and freezing peak at 17.8 ◦ C. Composite boards were tested in the specially designed full scale test room MINIBAT. The test room was equipped with thermal guard surrounding room where the temperature was stabilized at certain temperature of 20.5 ◦ C. The test cell was also equipped with a solar simulator located in the climatic chamber that was attached to the test chamber to simulate external thermal condition. The climatic chamber was separated from the test room with a glass fac¸ade. The temperature inside the climatic chamber could vary between −10 ◦ C and 40 ◦ C and could be dynamically controlled so that any temperature evolution can be generated. In the investigation, three types of test were conducted: • A summer day: temperature in the climatic chamber varied between 15 ◦ C and 30 ◦ C, there was night cooling to improve PCM storage/release effect. • A mid-season day: temperature in the climatic chamber varied between 10 ◦ C and 18 ◦ C. • A winter day: temperature in the climatic chamber varied between 5 ◦ C and 15 ◦ C, heating system in the test room was turned on when temperature in the room dropped below 20 ◦ C. For all tested cases, the solar flux is preserved as the same. The experiment was conducted in a comparative manner for the test room with PCM and without PCM boards on the walls. Based on the results obtained, authors concluded that PCM composite is an interesting solution for the building application to enhance the human thermal comfort due to three reasons: • The PCM included in the walls reduced the overheating effect and energy stored was released to the room when temperature was minimum. • The wall surface temperature peaks were flattened. • The stratification of air temperature in the room with PCM was not observed as it was for room without PCM. Still the allowed temperature fluctuations for the tests were very high. For example, for a summer day the air temperature was allowed to fluctuate between approximately 19 ◦ C and 32 ◦ C for the room without PCM and from 19 ◦ C to 29 ◦ C for the room with PCM. Based on that, it can be concluded that if the test rooms were equipped with some additional measures to reduce temperature fluctuations, for example solar shadings, the utilization of PCM in the room would be smaller and also the improvement with regards to the room without PCM would drop. During the recent years, many more experimental studies on PCM gypsum and PCM wallboards have been documented, for example, [21–25]. It was documented that in gypsum materials can be combined up to 45% by weight of PCM when reinforcing the structure with some additives and up to 60% by weight in wall board composites. It can be concluded, that the passive (thermal activation of thermal mass is driven only due to indoor temperature fluctuation) performance of PCM gypsum and boards is usually investigated in the comparative manner. Two chambers are exposed to the same loads and one is furnished with PCM boards whereas the reference one accommodates ordinary gypsum boards. The performance of the PCM boards is assessed based on the decrease of temperature amplitude with regards to room with ordinary finish. The method

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is simple, easy to control and time efficient, however, permitted temperature fluctuations in the test chambers are often too high and do not represent realistic conditions that would be the case in real buildings. As a result of that, overestimation of the heat storage performance of PCM gypsum products can be expected. On the other hand, it has to be kept in mind that also too small internal heat loads and insignificant diurnal temperature variations would result in underestimation of PCM potential. Therefore, heat loads and allowed diurnal internal temperature fluctuations have to be always chosen within reasonable range. Moreover, authors often do not mention essential information such as, ratio of PCM plaster surface area to total internal surface area of the room or the percentage content of PCM in the plaster. Lack of only one of the mentioned information is enough not to be able to reproduce experiment or to conclude on potential of specific type of PCM product.

3.2. PCM in concrete The general purpose of combining PCM in concrete materials is to further increase heat storage of heavy construction materials. A combination of the high latent heat capacity of PCM and the high density of heavy weight concrete is an interesting concept for the new technology for storing heat in building constructions and thus energy savings for heating and cooling of buildings. Over the last years, several studies on PCM concrete have been documented and some of them will be presented here. An experimental investigation on the full-scale performance of PCM concrete set-ups was carried out by [26]. The study presented in [26] investigated the use of PCM in concrete floors. Four chambers of the same size were constructed; two with PCM concrete floor and two with ordinary concrete floor. The only heat source in each chamber was the solar irradiation through the windows. In the study, the PCM in the floor was activated by the direct sun irradiation and was well exposed. However, in practice, it would be very unusual to find concrete floor without any kind of covering such as wood, polyvinyl or tiles that would shelter the PCM from direct solar radiation. Therefore, although the research indicated that this technology could decrease the temperature fluctuations in the houses, it would be very difficult to implement it on a broader scale in practice. Similar research to the one presented in [26] was presented by [27]. The proposed experimental set-up consisted of two identical cubicles made of concrete. One cubicle was built of conventional concrete and one of new concrete with admixed 5% by weight of microencapsulated PCM. The obtained results revealed a decrease in temperature fluctuation in the room with PCM concrete and a shift of the temperature peak in the wall of 2 h. On the other hand, neither [26] nor [27] clearly distinguished how much of the decrease of temperature fluctuation was due to the reduced thermal conductivity and how much was due to the increase of the heat storage capacity of the concrete with PCM. Furthermore in [26] and [27], the diurnal indoor temperature fluctuated from very low to very high temperatures. These large temperature fluctuations would not be acceptable by any means in real buildings, and therefore the thermal mass of the cubicles would not be activated to the extent as in the presented experimental set ups. In [28], the authors continued the work presented in [27]. One of the main drawbacks discovered in [27] was a severe influence of high outdoor temperature peaks and solar radiation on PCM performance during the summer, which prevented its solidification during the night. As a result, PCM was not ready to undergo phase change the next day. The main objective of work presented in [28] was to overcome the problem and to increase the operation time of PCM and to improve thermal comfort. To decrease high temperatures in the investigated cubicles the special awnings were employed. Results indicated that the temperatures were slightly

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reduced and PCM remained active for at least 4% more time. Still the problem of too high temperatures was not entirely solved. The research presented by [29] took into account the challenge of measuring the specific heat capacity of the self-compacting PCM concrete. In reference [29] can be found some important remarks, for example, that DSC method should not be used to measure specific heat capacity of inhomogeneous materials since the samples suitable for DSC method is of only few milligrams and such small samples are not representative for the concrete materials. However, as presented in [29], the new method to determine the specific heat capacity of PCM concrete has its shortcomings. The heat flow used in the calculation is the one measured at the surface of the sample by the heat flux film sensor. In reality, the heat flux varies in the interior space of the sample due to the dynamic temperature at the specimen’s top and bottom boundaries. As a consequence, the paper proposes that the calculation method overestimates the heat flux transferred to the sample with respect to the internal temperature gradient over time and therefore, the calculated Cp(T) is also overestimated. What is more, the experimental set-up disregards the influence of the surrounding temperature on the heat transfer within the sample and this could also result in some inaccuracies. The method presented in [29] was improved by [30]. In reference [30] were proposed different methods to calculate the specific heat capacity of inhomogeneous PCM concrete materials and also improved experimental set-up was presented. Both imperfect linear methods and advanced non-linear inverse method are elaborated and obtained results are compared with each other. Further study on inverse method and various optimization algorithms to calculate the specific heat capacity of PCM concrete materials as a function of temperature Cp(T) was presented by [31]. A numerical study on the potential to increase the heat storage capacity in concrete material due to addition of PCM was presented by [32]. Numerical models were developed based on experimentally obtained thermal properties of PCM concrete presented in [30]. It was concluded that due to decreased thermal conductivity and density and only insignificant increase of specific heat capacity in PCM concrete the potential for improvement with regards to the ordinary concrete material is insignificant. The results presented in [32] were confirmed by the full-scale experimental validation published in [72]. To conclude, the realistic potential to increase dynamic heat storage capacity of concretes by incorporation of PCM is doubtful. Firstly, the thermal mass increase is not as high as expected and secondly, thermal conductivity decreases significantly due to addition of PCM to concrete. As a result, the energy from the air has difficulty in being transported to the inside of PCM concrete construction within daily realistic indoor temperature variations. Moreover, maximum amount of PCM in the concrete is not higher than 5–6% by weight (material is still workable), which means not much latent heat capacity can be introduced to sensible heat storage capacity. Consequently, 5–6% by weight of PCM corresponds to approximately 12–15% by volume of concrete, which means that the share of PCM in concrete is rather high and as a result, the price of the composite would be high due to rather high price of PCM. 3.3. PCM in bricks Almost no work has been done on brick constructive solutions. Until year 2008, only one study documented by [33] can be found. In [33], introduction of PCM in the brick was studied numerically and results indicated reduced heat flux entering the indoor space in the summer. However, the numerical model was not validated and no experimental work was done until the research presented by [34]. In research presented in [34], several cubicles using conventional brick and alveolar brick were built. Macroencapsulated PCM type RT-27 and SP-25 A8 was added in respectively one

conventional brick and one alveolar brick cubicle and thermal behavior was studied. In total, 5 cubicles were built and they were located in Puigverd de Leida in Spain (typical Spanish continental climate with cold winters and warm summer and significant temperature oscillations between day and night). • Reference cubicle (Reference): only conventional hollow bricks and no insulation. • Polyurethane cubicle (PU): 5 cm of spray polyurethane between two layers of brick. • PCM cubicle (RT27 + PU): 5 cm of spray polyurethane between two layers of brick and additional layer of RT-27 paraffin. • Reference cubicle (Alveolar): Only alveolar bricks and no insulation. • PCM cubicle (SP25 + Alveolar): CSM panels with SP25 A8 hydrate salt are located inside the cubicle between alveolar bricks and plaster board. The PCMs tested were designed for cooling application. During the experiment, two different set-ups were performed: • Free-floating temperature: no cooling system is used and temperatures in the cubicles with and without PCM are compared. • Controlled temperature: heat pump is used to set a constant temperature (24 ◦ C) inside the cubicles. The energy consumption for running the heat pumps in the cubicles is compared. For the free-floating experiments, the peak temperatures in the cubicles with PCM were reduced up to 1 ◦ C. It was also discovered that under the free-floating condition, some problems with solidification of PCM occurred. It was concluded that a cooling strategy (natural or mechanical) should be defined to improve performance of PCM. For controlled temperature experiments, the accumulated energy for the heat pump was significantly higher for the reference cubicle than that of the remaining four cubicles. The RT27 + PU cubicle achieved a reduction of 15% compared to PU cubicle, while the SP25 + Alveolar cubicle reached 17% of energy savings compared to Alveolar one. On the other hand, ratio of external area to volume of the cubicles is very high comparing to any real full-scale building. As a consequence, in the real building ratio of the PCM area to the floor area would be much smaller and probably as a result also possible improvement would be reduced. Another parameter that could cause temperature peak reduction and energy use reduction in cubicles with PCM is the very low thermal conductivity of paraffin, which could simply work as additional insulation. Therefore, it would be recommended to have a reference cubicle without PCM that had the same thermal resistance of constructions as one with PCM. Recently, [35] has performed and documented the experimental tests on the hollow brick with PCM. The aim of the study was to reveal PCM potential as a passive solution, by decreasing the internal temperature fluctuations and by increasing the time delay between the external and internal conditions. Experimental results were also used to validate the developed numerical model of such a construction element. The PCM chosen in the experiment was paraffin RT18 with melting temperature at 18 ◦ C. Two wall specimens made of a typical clay brick were built one with macroencapsulated-PCM and other without PCM. The wall specimens were tested using the climatic chamber, where one side of the specimen was imposed by exterior boundary condition and on the other side was left free-floating condition. For the first, the time delay for the brick wall with PCM was delayed by 3 h with respect to the conventional brick wall. For the second, the thermal amplitude was reduced from 10 ◦ C (wall specimen without PCM) to 5 ◦ C (wall specimen with PCM).

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Although experimental researches have proven that PCM helps to decrease indoor temperature fluctuations and increase the time delay due to its application in the brick walls, the challenge to efficiently activate PCM in the brick walls would be their relatively low thermal conductivity. Voids in the bricks are filled with standing almost still air and therefore have very good insulation properties. Moreover, this technology would be applicable in rather warm climates (Mediterranean climate) where no additional external insulation is applied on the external side of the fac¸ade, as it is done in colder climates.

3.4. PCM in composites and other materials The investigation of the thermal properties of the novel composite of palmitic acid (PA) and expanded graphite (EG) was documented by [36]. The maximum mass fraction of PA in EG was found as 80% without leakage of PA in the melted state and therefore, the composite was characterized as a form-stable PCM. The thermal conductivity of the new composite and pure PA was measured using a thermal properties analyzer KD-2. An analysis of the thermal conductivity as a function of amount of PA in EG was conducted. It was discovered that the thermal conductivity significantly increases with increased amount of EG. For 20% by weight of EG, the thermal conductivity increased to 0.6 W/m K from 0.17 W/m K measured for pure PA. Latent heat, melting and solidification temperatures were measured with use of DSC method. Due to relatively high melting and freezing temperatures that were measured respectively at 60.88 ◦ C and 60.81 ◦ C, this material could be applied in the buildings probably only in solar systems or in domestic hot water storage systems. Another PCM graphite composite was tested by [37]. Authors have tested porous graphite filled with PCM for thermal energy storage (TES) application. Plates with PCM alone and plates with PCM embedded in the graphite matrix were tested in the specially designed thermal energy storage set-up. Compared with pure PCM plates, the composite helped significantly to reduce time for charging (50% of time). As a consequence, the power consumption for fans used in the test was reduced by 50%. Moreover, due to increased thermal conductivity of the graphite PCM composite, the tested plates could have been thicker of 70% for the same time of response. A tile with PCM for building use was experimentally tested by [38]. The paper presents the design and development of the tile and the experimental method to test its performance. The developed tiles were tested over a period of 60 days in one side of the solar house placed in Madrid that had door-window toward south. It was observed, that high effectiveness was achieved for tiles close to the door-window where the direct solar radiation hit the tiles. The contribution of the tiles in the deeper location, further from the window, was very small. Therefore, the key conclusion was that the scheme should be limited to the portion of the floor that can receive the direct solar radiation. It should be kept in mind, that mixing PCM materials with materials representing only sensible heat has always two consequences. On one hand, if the admixed material has high thermal conductivity, the activation of the thermal mass of such a composite is faster and more efficient, which is usually positive for TES. On the other hand, the volumetric heat storage capacity is reduced due to replacement of part of the PCM by sensible heat storage. Therefore, mix design and proportion of new composites with PCM has to be always tailored to their application and designed with respect to desired effect or energy savings.

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4. PCM in construction (active) Thermally activated panels made of combined gypsum and microencapsulated PCM was presented in [39]. The prototypes were developed to handle heat gains of up to 40 W/m2 due to latent heat incorporation. The inserted capillary tubes were used to remove absorbed heat in the panel. The testing results indicated promising results within the realistic temperature variations. It is worth highlighting, that during the development process it was discovered that the thermal conductivity of gypsum drops significantly after addition of low thermal conductive PCM. As a remedy to that, aluminum fins were proposed to counteract increased thermal resistance. In the paper, however, a remark can be found that further improvement with regards to fire retardation is needed to broaden the system applicability. In the paper, it was also not elaborated on the necessary energy required to discharge the panels. The system anyway seems to be an interesting solution to increase the thermal mass of light weight buildings without necessity to redesign construction to handle heavy additional elements. A recently published paper [40] presented the development of the radiant floor panels with granulated PCM with incorporated pipes for heating and cooling. The initial results from the paper indicated that the newly developed panels with PCM might be advantageous during summer condition (cooling) but their initial design performance was not beneficial during winter condition (heating). The estimation of the performance of the new panels was performed with regards to the reference floor made of mortar screed. The worse performance during heating condition was explained due to increased resistance between pipes and the melted granular PCM. As a result of that, an inefficiency of heat exchange from the pipe level to the surrounding environment was observed. In the paper it can be found, that in order to decrease the thermal resistance of granular PCM, a special optimized steel matrix was introduced that increased heat exchange from the pipe level to the surrounding. After optimization of the layer with PCM with special custom steel matrix, a numerical simulation of performance for winter, mid-season and summer season was performed. The conclusion was drawn, that in the summer condition the amount of cooling water to maintain a comfortable temperature was reduced by 25% with regards to mortar screed floor. In the mid-season, PCM floor with active system turned off reduced the floor temperature peak of up to 3.5 ◦ C with regards to the mortar screed floor. In the winter, performance of both floors was the same. In the paper, however, end-user savings of energy due to use of the new PCM floor were not evidently presented. What is more, authors pointed out that further experimental tests should be carried out to validate the developed numerical models and to further improve and test the real living performance of the panels. Moreover, authors indicated that if the performance of the panel appears to be commercially attractive, the development of a special transfer function that could help to simulate the new floor in the whole building analysis software should be considered. In recently published [41], a numerical analysis of the thermally activated floor with two layers of PCM, for cooling and heating, can be found. The aim of the paper was to find the optimal melting temperatures for two layers of PCM, one with the purpose to cool and the other to heat, when the thermally activated system is turned on during the off peak electrical hours. In that manner, high latent heat storage can be charged during off peak hours and can release heat or cold during the peak electrical hours. The numerical analysis indicated that the fluctuations of the floor surface temperatures and heat fluxes can be reduced by using PCM. Though in both cases reduction of the fluctuations is caused due to the latent heat capacity of PCM but also due to application of the additional high resistance of two layers of low thermally conductive PCM. The additional latent heat storage capacity is positive whereas

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additional resistance between pipe level and surrounding is not. This, however, is not distinguished in the paper. All in all, for the analyzed set up, the optimal melting temperature of PCM for heating and cooling were respectively 38 ◦ C and 18 ◦ C. In the paper, authors claim that the floor with PCM increased the energy release during the peak period by approximately 41% and 38% respectively for heating and cooling with regards to the floor without PCM. It should be kept in mind that results are based on simulations and were not validated with experimental tests. The investigation of use of the off-peak electrical heating to heat up buildings with the latent heat storage located in the floor was also published earlier in [42]. The numerical and experimental analysis of the performance potential indicated that the system could be promising as long as the phase change temperature and thickness of the air under the floor is properly chosen. Lastly, relatively simple design method and control strategy to obtain satisfactory indoor temperatures was presented in the paper.

5. PCM in glazing, shadings, blinds (windows, slats, shutter) While research on PCM integrated in the opaque constructions is very common and numerous publications resulting in various product development have been documented during the last decades, very few studies have been accomplished on PCM in the transparent materials and shading components. From the energy use and thermal perspective, windows and glazing represent weak link between internal and external condition in buildings. In the cold climates, windows are responsible for significant heat losses from the indoor to the outdoor. On the contrary, in the hot climates excessive heat that enters through the glazing causes increased energy use for cooling. Still, in the newly designed and constructed buildings the tendency is to keep ratio of glazed to opaque area in the fac¸ades on high level. This, however, is going to be changed due to more and more strict legislations on energy use in buildings. Therefore, to keep a high ratio of glazed to opaque areas in the fac¸ades, these types of external envelopes should be able to switch their thermal and optical properties and provide the best possible response to varying outdoor conditions. As a result, the solar gains that must be blocked from entering the building through the transparent envelopes during the summer days should be maximized in the winter. One of the recently investigated technologies is to combine PCMs in the glazing constructions and solar shading components. Phase change materials represent high latent heat capacity and when directly exposed to the direct solar irradiation can overcome phase transition and absorb part of the solar heat before it enters the room. Moreover, some of PCMs are translucent and therefore light can be transmitted to the indoor space. The experimental and numerical investigation on the prototype external wall-system for solar space heating and daylighting that was composed of transparent insulation material (TIM) and translucent PCM was documented in [43]. The PCM used, was commercially available salt hydrate with melting interval of 24–29 ◦ C that were packed in the glass containers. Although authors claim that the energy gains with the prototype wall are promising, there is lack of results for the reference construction that could be used as baseline for the performance of the prototype wall. Moreover, in [43] authors present a rather thorough analysis of optical properties of PCM trapped in between two layers of glass. Firstly, a Monte Carlo technique was used to simulate optical behavior of PCM. By using random numbers and probability distributions and according to optical laws and material properties, it was counted weather photons are absorbed, reflected or transmitted. The computational analysis was followed by measurements of spectral transmittance of solid and liquid PCM in spectrophotometer. Measured results

indicated that transmittance of solid PCM is low compared to liquid state and this can be observed especially for visible wavelength, whereas for longer waves the transmittance is almost the same regardless of PCM phase. Authors concluded that the disadvantage of chosen PCM was backscattering of solar radiation in the solid state that leaded to reduction of heat and light gains into the room. However, for some climates this does not necessarily have to be a disadvantage. The authors of [44] did the experimental and numerical investigation on the thermally efficient double glass windows with PCM and air in between. The number of glass sheets with different thicknesses and gaps between them were also investigated. Results revealed that less energy is transmitted through the windows with PCM than through the windows with air between panes. On the other hand, U-value W/(m2 K) of the windows with PCM was higher than the one for the windows with air and this could result in higher heat losses in the cold climates. The optical properties, transmittance and reflectance, of different combinations of pane thickness, spacing between panes, and filling material (air, PCM, PCM with color) were tested by spectrophotometer in wavelength range of 300–2800 nm. In double glass panels, the influence of PCM compared to air filling was detected for wavelength higher than 1000 nm. Regarding colored PCM, pure ones had the highest transmittance and measured blue and green PCM had significantly lower transmittance in wavelength approximately between 400 and 700 nm. In the paper, however, it is not indicated whether measurements were conducted for PCM in solid or liquid state and this could have decisive influence on the obtained results. A numerical comparison between thermal efficiency of two glass windows one filled with absorbing gas and the other with PCM was presented by [45]. The investigated window was exposed to solar radiation in the hot climate (Brazil). Results indicated that PCM filters out the thermal radiation and by the process of phase change reduces the amount of penetrating heat until it is totally melted. Like in [43], the U-value W/(m2 K) of windows increased due to addition of PCM in the gap between panes. The authors of [46] performed the computational and experimental analysis of PCM-fac¸ade-panel for day lighting and room heating. The double glazed windows combined with PCM were investigated and compared to the double glazed fac¸ade without PCM. Heat losses through the PCM fac¸ade panel were lower of about 30% in the south oriented fac¸ade and the solar heat gains were reduced by about 50% compared to the conventional window. Contrary to the results presented in [44] and [45], the calculated U-value W/(m2 K) of window with PCM was lower than for the window without PCM and this is because panel with PCM was added as additional layer to the window instead of replacement of air void between glass panes. The tests were done on windows with three types of PCM: RT25, S27 and L30. It was concluded that the PCM panels are good supplements to the conventional windows in places where no visual contact to the environment is needed. The panels provided homogeneous illumination and thermal performance with very low heat losses. In the article, it was mentioned that there was observed leakage from the containers, especially when salt hydrate PCMs were used. The solar shading system with integrated PCM was investigated by [47]. The vertical slats filled with salt hydrate PCM with melting range between 26 ◦ C and 30 ◦ C were mounted and tested in the office rooms. The rooms with the PCM slats were compared to the corresponding rooms equipped with the conventional interior vertical blind. In the summer months, in the offices facing westward, the air temperature was 1–2 K lower in the room with PCM blinds than in the room with conventional blinds. Moreover, it was observed that during the winter season, the PCM blinds did not interact with radiators and did not increase power for heating. The major problem discovered was the regeneration of PCM blinds

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in the night. Only the combination of mechanical ventilation and tilted windows for natural ventilation was sufficient to completely regenerate the PCM in monitored rooms in the night. The parametric numerical study on use of PCM in the window shutter to reduce solar heat gain was presented by [48]. The reference shutter was made of a foam filled aluminum. The results indicated that the magnitude of PCM melting temperature and its quantity in the shutter have a significant effect on the thermal performance of the PCM shutter. The melting point of the PCM in the shutter should be close to the upper temperature limit of the windows during the daytime. The PCM should be prevented from completely melting during the working hours and its amount should be sufficient to absorb large amount of heat during the daytime. Numerical results indicated that about 23% heat gain through the window can be reduced with 3 cm thick PCM shutter with regards to the same shutter made of foam and aluminum.

6. PCM in HVAC components/heat exchangers In this chapter, some of the research documented on the performance of ventilation and air conditioning systems will be listed, combined with various latent heat storage concepts that were documented during approximately last decade. In 2000, a paper [49] on novel ventilation cooling system consisting on latent heat storage and heat pipe was published. The theoretical model over-predicted heat transfer rate by about 100%, but predicted heat pipe surface temperature within 2 ◦ C. Authors indicated that the major reason for the discrepancy between the theoretical calculation proposed and experimental results was due to the longer time of melting of PCM than initially predicted. What is more, measurements in the specially designed experimental rig showed that a large temperature difference between air and PCM (more than 15 ◦ C) was needed to cause the phase change within practical time span between 7 h and 10 h. If the temperature difference dropped to more reasonable value of 5 ◦ C, the heat transfer between air and PCM would drop and charging and discharging time would be extended to 19 h. In that case, the number of units able to cover heat gains in the ordinary office room would be large and difficult to install in practice. In [50], three computer models have been developed to evaluate the performance of the thermal storage system with PCM for air conditioning purpose. It was concluded that the better performance could be obtained for smaller air gaps and thinner PCM slabs. On the other hand, this will cause some drawbacks, such as: increase of PCM containers (higher cost), increase of volume of the system and higher pressure across the storage. The purpose of the storage system would be to shave up peak loads and also to find the optimum design and mass of PCM for different climatic conditions. Although authors claim that experimental validation of numerical results will follow, it was not found in the literature. What is also important, is the fact that computer models were solved with some strong initial assumptions which if changed might have influenced obtained results. Another concept combining ventilation and latent heat storage was presented in [51]. In the presented concept, PCM was incorporated in the rock wool ceiling boards used for suspended ceilings. The thermal mass of suspended ceiling was then cooled down during the night by air blown from the air handling unit (AHU). During the day, air was extracted from the room but from the plenum above suspended ceiling which was cooled during the night. As a result, warm air from the room was cooled down before it returned to AHU. The potential of the concept was studied by solving numerical model located in Tokyo. The thermal capacity of the suspended ceiling with PCM used in the model was determined by experimental manner in the specially designed chamber. The results from the simulation have indicated that the thermal load of the room space

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with PCM was increased with regards to the room without PCM. What is more, running costs were reduced because much cheaper night electrical tariffs were used to run AHU. On the other hand, authors pointed out that not all flammability issues were solved and these have to be solved before product can be commercialized. To sum up, in [51] one major problem has to be specified. The experimental set up and the design of the chamber used to determine thermal capacity of the boards with PCM is not correct to conduct dynamic tests since heat loses to the surrounding are not controlled and, what is more, heat is absorbed not only in the tested boards but also by all of the constructions of the chamber. In that manner, the absolute thermal capacity of the boards cannot be derived and only relative capacity with regards to ordinary boards without PCM can be determined. As a consequence, the numerical models may have overestimated the performance of the suspended ceiling with integrated latent heat storage. In [52], a numerical investigation of the shape-stabilized phase change material (SSPCM) plates combined with night time ventilation for the summer condition was presented. The plates were located on the internal side of walls and ceiling. The SSPCM can store cold transported to the room by using night time ventilation. Numerical results indicated that high temperature peaks could be reduced by up to 2 ◦ C with regards to room without PCM. Authors pointed out that the air change rate during the night hours should be as high as possible. Moreover, correct melting temperature, heat of fusion, thermal conductivity and thickness of SSPCM shall be considered according to climatic condition. For example, for the presented case (Beijing) thickness shall not be higher than 20 mm, ACH during night shall be as high as 40 h−1 , ACH during day shall not be too high (approximately 1 h−1 ), melting temperature should be around 26 ◦ C, heat of fusion shall be 160 J/kg and thermal conductivity should be equal or higher 0.5 W/(m K). Recently published work [53] was the continuation of the work presented in [52]. The night time ventilation considered in [52] was replaced for the same room with SSPCM located in Beijing by the decentralized mechanical ventilation with one common extract for all rooms. The ventilation is decentralized because inlet air was provided individually to each room and a fan was located in the fac¸ade. In that manner, no ducting was necessary and specific fan power (SPF) can be improved. The ACH was as suggested in [52] during night at 40 h−1 and during day at 1 h−1 . Obtained results indicated that the indoor thermal-comfort was improved and 76% of the daytime energy use was saved compared with the case without SSPCM and night ventilation. However, within the 76% of energy saving the 66.4% is saved due to night time ventilation and only remaining 9.6% is saved thanks to SSPCM. To conclude, the PCM installation with ventilation strategies has indicated that to discharge heat accumulated in PCM construction during the day, a high ACH is required during the night, else PCM is not ready to accumulate peak loads the next day. Moreover, to obtain significant improvement in reduction of peak temperatures a major part of the internal finish has to be covered with PCM materials or PCM composites. As a result, the issue of flammability of such materials has to be considered and required test carried out to determine if the material could be used as a building material.

7. Thermal properties determination: thermal conductivity and specific heat capacity 7.1. Measurements of specific heat capacity The correct design of the building or storage system with integrated PCMs requires correct knowledge of the thermal properties of the PCMs used. For example, the single data points, the phase change enthalpy at the melting temperature or the heat of fusion

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do not describe PCM properties with sufficient accuracy to perform dynamic simulations of a room or a whole building containing PCM. The phase change occurs in a temperature range and not at a constant temperature level, and therefore specific heat capacity or enthalpy of this type of material has to be known as a function of temperature. Literature review indicated that the following methods are most often used to measure specific heat capacity of pure PCMs and their composites: differential scanning calorimetry (DSC) and T-history method, and the review of these methods will be presented in this section. Regarding DSC method, two modes can be distinguished: dynamic mode and isothermal step mode. In number of publications, the dynamic mode is used to determine the specific heat capacity of pure PCMs and PCM composites, however, in the majority of them there is lack of understanding of the methodology and therefore results obtained present only part of the whole picture. The shortcomings and some sensitivity analysis of dynamic DSC measurements are discussed and presented in [8]. Authors point out, that measurements with different heating rates and different sample masses gave results that differ considerably from each other. In the article, an explanation for the discrepancy of the results can also be found. Finally, the conclusion that in context of PCM, the dynamic mode is not the proper approach can be found and instead of the dynamic mode an isothermal step mode or T-history method should be used. In the article, it is also stressed that DSC in general is not suitable for heterogeneous materials. This conclusion should be considered with special care. Another method, the T-history method has been proposed in reference [9]. T-history method proposed in reference [9] has the following advantages: a large sample size of organic, inorganic, encapsulated or composed PCM can be measured, ranges of heating and cooling rates and temperatures are sufficiently large for various PCM applications, the instrumentation and experimental set-up is simple and inexpensive. The method has been improved later by [54,55]. Still, although DSC with isothermal step mode and T-history methods are well-developed, they share the same shortcoming – the sample tested has to be homogeneous. In the recent publication [30], a new experimental set-up and various calculation methods to determine the specific heat capacity of inhomogeneous concrete with microencapsulated-PCM was suggested. Among four proposed methods, an inverse method based on nonlinear sequential quadratic programming (SQP) algorithm was defined as the most accurate. In another publication [31], using experimental data from [30] authors go one step further and compare three different optimization algorithms to define which one is the most effective. From this investigation, SQP method is found to be with the highest accuracy and the least complexity compared to the Particle Swarm Optimization and Genetic Optimization method. 7.2. Measurements of thermal conductivity Proper knowledge of thermal conductivity is the second key thermal parameter that has to be known to properly design latent heat storage systems or to correctly simulate dynamic models with PCMs. In the literature, the following methods are most often used to experimentally determine thermal conductivity of materials: hot-wire method, T-history method, hot plate measurements, hot box measurements. Each listed method is shortly explained and relevant reference is given in this chapter. The hot-wire method allows measuring the thermal conductivity utilizing a particular heat conduction equation that is valid for a linear heat source in a homogenous and isotropic medium at uniform initial temperature. As proposed by Carslaw and Jaeger’s theory [56], the method is established for an infinitely small heat source and an infinite mass. According to this theory, the heat

delivered by the electric current flow diffuses through the system along orthogonal directions with respect to the hot wire. The hot wire method is very easy to proceed with simple equipment. However, according to the national standards (GB/T 10297-1998), this method is suggested to measure thermal conductivities lower than 2 W/m K of isotropic materials. The hot-wire method was used, for example, in [57,58] to investigate the thermal conductivity of pure PCM and PCM with different amount of graphite fibers. The T-history method proposed by Zhang et al. [9] can also be used to determine thermal conductivity properties of PCMs. However, in order to ensure that the tube contains the melted PCM whose temperature is uniform, the following conditions have to be fulfilled: • The ratio of the length to diameter of a tube should be larger than 15. • The Biot number should be less than 0.1. • The Stefan number should be between 0 and 0.5. This method can only be applied to measure the thermal conductivity of the PCMs whose phase-change process is one clear interface between two phases (in fact, for some salt hydrates, this condition cannot be met). Thermal conductivity of PCM composite materials that are always in the solid state can be performed according to the standardized steady-state procedures, for example, with hot plate or hot box apparatus. For example, thermal conductivity of PCM gypsum and PCM concrete materials can be determined. In [59], an experimental investigation with use of a guarded hot plate apparatus was proposed to determine the thermal conductivity of the PCM concrete composite incorporating up to 6% by weight of microencapsulated paraffin. Measurements were conducted for the temperatures below, within and above melting temperature range of used PCM and it was concluded that the thermal conductivity is almost the same regardless if PCM in microcapsules is in liquid or solid state. 8. Heat transfer enhancement in PCM The most of PCMs suffer from low thermal conductivities, being around 0.2 W/(m K) for paraffin and 0.5 W/(m K) for salt hydrates. Such low thermal conductivities extend charging and discharging periods of TES systems. In buildings, charging and discharging periods are imposed usually by day (charging) and night (discharging) cycles which are closed within 24 h. As a consequence of the low thermal conductivity of PCMs, not all latent heat might be utilized or the PCM material would not be fully discharged and would not be ready to absorb heat the day after. Therefore, a heat transfer enhancement strategy in PCMs is required. In the past, numerous activities have been documented on the heat transfer enhancement in PCMs and these activities can be grouped in two sub-activities: thermal conductivity enhancement and heat transfer enhancement on the surface. Regarding heat transfer enhancement on the surface, for example, extension of the surface area by fins or application of PCM in the porous materials has already been proposed. Regarding thermal conductivity increase, several studies have been proposed so far and they suggested combining PCM with some highly conductive materials such as copper, graphite and metals. The latent heat storage capacity of phase change materials (PCM) is considerable but the limitation factor to activate this heat is their very low thermal conductivities that prevent from efficient utilization of these materials, for example [60]. Numerous studies have been conducted on enhancement of thermal conductivity of phase change materials. For example, [61] has investigated

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three different thermal conductivity enhancement methods: longitudinal fins, lessing metal rings of 1 cm diameter and steam bubbles in the cylindrical sample of paraffin wax. The experimental results indicated that the heat transfer enhancement with fins and lessing rings was appreciable for solidification enhancement and bubble agitation was applicable to enhance melting. In [62], the heat enhancement of thermal conductivity of the paraffin wax by embedding the metal matrix structure that is known under the trade name Explofoil has been investigated. It was found that the improvement in the thermal conductivity due to the introduction of an aluminum strip matrix was linearly related to the volumetric fraction of the metal in the wax. For example, for 1.6% fraction of the metal matrix, the thermal conductivity was 4 times higher than for the pure wax. The research presented by [63] investigated the paraffin wax RT58 with and without embedded copper foam. The addition of copper foam can increase the overall heat transfer rate by 3–10 times (depending on the metal foam structures and material). Another experimental investigation on thermal conductivity enhancement of PCMs was elaborated in [64]. The article indicated that addition of the graphite fractions to the shape-stabilized PCM resulted in a great increase of thermal conductivity. For 20% mass fraction of graphite, the thermal conductivity was 221% higher than the original. The [60] presented experimental investigation showed that the thermal conductivity enhancement of the paraffin wax was due to the presence of the aluminum powder of particles of only 80 ␮m. In the investigation, the mass fraction of the aluminum was 0.5 and the resulting decrease of charging time of the composite was reduced by 60% with regards to the pure paraffin wax. Another method to combine PCMs with highly conductive materials was to immerse them in the porous structures that are characterized by a high thermal conductivity. An effort to impregnate the porous aggregate with PCM can be found in [65]. The aggregate was impregnated with liquid PCM under vacuum condition and then after was used to cast thermal energy storage concrete. Authors did indicate that under vacuum condition more PCM can be impregnated in the porous aggregate than by simple immersion but they did not investigate the thermal conductivity of aggregate before and after PCM incorporation. Recent numerical and experimental work documented in [66] investigated the heat transfer in PCM embedded in the porous metals. In the paper, authors pointed out that the metal foam sample with larger relative density (smaller porosity) has better heat transfer performance than the one with smaller relative density. What is more, simulation results indicated that the metal foams with smaller pore size can achieve better heat transfer performance than those with larger pore size. From the comparison of samples having metal foams embedded into PCM with the pure PCM sample, it was found that the addition of metal foams can increase PCM heat transfer performance by approximately 10 times and effectively transfer heat from metal skeleton to the PCM. To sum up, various investigations indicated that in order to increase the low thermal conductivity of PCMs a material of very high thermal conductivity has to be added. There are also numerous publications on the surface heat transfer enhancement. For example, the research on internally finned tube where PCM was located in the annular shell space around the tube was presented in [67]. By introducing the internal fins, the convective heat transfer was enhanced. It was documented that the melting volume fraction of PCM was significantly increased with regards to plain pipe. In that case, the melting fraction can be increased by increasing the thickness, height and number of fins. In [68], numerical and experimental investigation on finned tubes was presented with the objective of using them in TES with PCM. It was concluded that the number of fins, fin length, fin thickness, the degree of super heat and the aspect ratio of the annular spacing are found to influence the time for complete solidification, solidified

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mass fraction and the total energy. In [69], the study on the heat transfer characteristics has been determined for the circular finned and not finned-tube units during the freezing of PCM. The ratio of outside heat transfer coefficient in the finned to the non-finned tube system was determined at 3.5 within the finned section, and this ratio decreased gradually far from the finned section with an increase in crystal volume. Another study on the heat enhancement by increase of the surface and roughness of the tube was presented in [70]. The study illustrated that the patented Vipertubes increased the overall heat transfer by more than 100% with regards to the conventional plain tubes. Another research on the grooved tubes was presented in [71]. For different geometric grooved shapes: circular, trapezoidal and rectangular, the heat transfer enhancement was obtained respectively up to 63%, 58% and 47% in comparison with the smooth tube at the highest investigated Reynolds number Re = 38,000. Although, all the presented studies extension of the surface area resulted in the enhanced heat transfer amount, none of the examples apply to the building enclosures such as walls or ceilings. The challenge of examination of the heat transfer enhancement due to extended surface area of the ceiling element was recently documented in [72]. The ceiling element was the prefabricated concrete hollow core deck with specially designed tails made of cement paste that were integrated and attached to the bottom of the decks. In total, 4 different types of tails were designed, where 2 kinds were flat and 2 with saw cuts finish giving an area factor of 2.7 with respect to the flat finish. Additionally, one deck with flat and one with saw cuts was with PCM and remaining 2 decks, one with flat and one with saw cuts were without PCM. The tiles with PCM contained 6% by weight of microencapsulated paraffin with melting point temperature at 23 ◦ C. All decks were tested in the specially designed modified hot box chamber and with the slot diffuser positioned and designed to blow air on the ceiling. The experiments were conducted for all the decks maintaining the same periodic steady-state boundary condition and the energy balance was logged to determine the dynamic heat storage capacity of each deck type. From the obtained results, it was concluded that neither presence of PCM nor the extension of the surface area resulted in the increase of the dynamic heat storage capacity of the decks. The experimental investigation presented in [72] is the first of its kind and therefore, more similar test should be done to validate the obtained results.

9. Heat transfer in the room Considering thermal activation of thermal mass, both the heat transfer within construction, but as well heat transfer on the surface of the construction has to be taken into account. Within construction, heat is transferred by conduction and the thermal properties, that are describing how fast and how much heat is transferred, are included within expression for thermal inertia (TI) of the materials which the construction is made of. Thermal inertia depends on materials density, specific heat capacity and thermal conductivity. The methods to determine the last two thermal properties of PCM of PCM composites were presented in the previous chapter in this paper and density can be relatively simply determined. The challenge, however, arise when trying to determine heat transfer on the surface of the construction. In the room, when determining surface heat transfer coefficient on the construction, convection and radiation have to be taken into consideration and these two ways of heat transfer are the key parameters that influence how much heat is transfer from the space to the construction of the building. Very often these two are considered as one and then it is referred as the total heat transfer coefficient (THTC). The higher the radiant and convective heat

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transfer coefficient is, the smaller the thermal resistance is on the surface and the easier heat/cold (energy) is transferred from the air to the building construction. Therefore, in the buildings where temperature typically fluctuates according to day–night schedule to efficiently activate the high thermal mass in the construction, the highest possible THTC is appreciated. As stated in [73], temperature levels of the internal surfaces of the constructions are relatively similar and do not drop below 20 ◦ C and rise above 30 ◦ C. What is more, their emissivity values are also similar and are usually within 0.8–0.9. In [73], a sensitivity analysis with various emissivity values was conducted and indicated that if using constant radiant heat transfer coefficient of 5.5 W/(m2 K), one do not commit error higher than 4%. In [73], an experiment on floor radiant cooling was also conducted and the measured data was used to calculate convective heat transfer coefficient (CHTC) on the cooled floor. In total, 4 different calculation methods were presented where each one led to different result. The reason for all differences was due to differences in the reference temperature used in the space. Nevertheless, regardless of the reference temperature used, the THTC calculated in [73] varied from approximately 7–10 W/(m2 K). It has to be highlighted that in the test on radiant cooling floor, no ventilation in the chamber was implemented. Additional ventilation of the chamber could result in higher convective heat transfer due to increased air velocity. In [74], standard THTCs vary between 5.9 and 10 and depend on direction of heat transfer flow. Standard values for convective heat transfer coefficient given in [74] are 2.5 W/(m2 K) for vertical walls, 5.0 W/(m2 K) for upward heat flow and 0.7 W/(m2 K) for downward heat flow. The higher convective heat transfer can be expected due to increased air flow rate and as a result of that increased air velocities close to the surface of heat transfer. The experimental investigation on convective heat transfer for night cooling case was documented in [75]. The average convective heat transfer for all surfaces in specially designed chamber was calculated for different air change rates (ACH). The average convective heat transfer coefficient varied from 0.28 W/(m2 K) for ACH of 2.3 h−1 to 1.36 W/(m2 K) for ACH of 13 h−1 .The analysis presented in [75] was continued with respect to the convective heat transfer on each surface in the chamber separately and results of that analysis can be found in [76]. For example, it was documented that local convective heat transfer on the ceiling depends strongly on ACH and on the local condition. Local convective heat transfer was observed to be significantly higher in the location where air jet enters the room and where jet attaches to the surface of the ceiling. A ratio of 1–5 can be observed between the lowest and the highest convective heat transfer close to the inlet, where the lowest convective heat transfer coefficient is calculated at approximately 2 W/(m2 K) and the highest at 10 W/(m2 K). The convective heat transfer coefficient can be also derived from convective heat flux, see Eq. (1). qconv = h · Tsurface-reference

(1) W/m2 ,

h – convective heat transqconv – convective heat flux, fer coefficient, W/m2 K, TSurface-Reference – temperature difference between surface temperature and reference temperature, K. However, in such case h depends strongly on chosen reference temperature which varies depending on the type of the flow. For natural convection, Treference is usually chosen to be the temperature of the air in the room. For forced convection, Treference is usually chosen to be the inlet air temperature. For mixed convection, a commonly used technique is presented in [77]. The method interpolates two independent limiting solutions: h

n

hmixed = hnatural + hforced

n

(2) W/m2

K, hforced – forced hnatural – natural heat transfer coefficient, heat transfer coefficient, W/m2 K, hmixed – mixed heat transfer coefficient, W/m2 K, n – blending coefficient.

To conclude, average THTC on the building construction in the ventilated room is expected not to be higher than 10–11 W/(m2 K) and not lower than 5.5 W/(m2 K) and for the THTC in that range, potential of the thermal mass in the buildings should be investigated.

10. Discussion A review of TES using PCM with focus on the building application has been carried out. The information gathered is divided with respect to different application of PCM. From the investigation, it can be concluded that PCM application for passive solutions in construction materials has been studied by many researchers. It was documented that the gypsum materials can be combined by up to 45% by weight of PCM when reinforcing the structure with some additives and up to 60% by weight in the wall board composites. On the other hand, in the concrete materials only up to approximately 6% (by weight) of PCM could have been implemented. Furthermore, with the present knowledge, thermal properties of homogeneous and inhomogeneous materials with PCM can be experimentally determined as a function of temperature. Moreover until present, several full-scale experimental investigations have been carried out to determine the potential of the passive application of PCM materials in the building construction in order to decrease indoor temperature fluctuations or the number of hours with overheating. Results, however, indicated various potential for improvement depending on the tested PCMs, their configuration and amount in the test room, materials they are integrated with and boundary condition set in the tests. Generally, it can be observed that PCMs can help increase thermal storage of light weight buildings, resulting in decreasing high temperature peaks by up to 3 ◦ C, whereas for heavy constructions improvement is not that obvious. On the other hand, real case studies are not available so far and performance of PCM gypsum and PCM boards is not documented for the real full-scale operation condition. With regards to the thermally activated building constructions with PCM, it can be concluded that the largest drawback and limitation documented was significant drop of thermal conductivity due to addition of PCM, and as a result worse cool and heat transport from pipe level to the surrounding. In the available publications, authors indicate that the remedy to that problem can be integration of high conductive meshes in the layers of construction that incorporate PCM. However, no financial estimation and calculations are available on feasibility of such technologies. With regards to PCM in the glazed envelopes, only few researches can be found and much less work has been documented comparing to the opaque constructions with PCM. Based on the reviewed studies, it can be concluded that PCM in glazing, shading and shutters might be an interesting addition to the building envelope in order to minimize solar heating loads. Application of PCM in the glazed surfaces shall be carefully designed, since on one hand, it can help reduce solar thermal loads during the hot season, but on the other hand, it can decrease thermal resistance of windows and by that increase heat losses during the cold season. Therefore, potential of PCM in the glazed envelopes should be carefully studied with respect to the climate condition. What is more, still some technical problems with leakages of PCM from the window frames can be found reported. Research on combination of the ventilation systems and strategies with latent heat and cold storage indicated that this concept might be promising with respect to the energy use reduction and improvement of indoor comfort. Latent cold storage combined with ventilation can be compared to night cooling and latent heat storage can be used to utilize night cheaper tariffs. Still, due to

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flammability risk, all products with PCM shall be tested with respect to fire retardation and fulfill necessary fire codes and standards. Regarding measurements of thermal properties of PCMs and their composites, several experimental methods are presently available. For thermal conductivity measurements, hot wire method, T-history method, and measurements with hot plate and hot box apparatus can be used, keeping in mind that for the pure PCMs hot plate and hot box methods would not be recommended. With regards to the specific heat capacity measurements, DSC isothermal step mode and T-history methods can be recommended. On the contrary, results from DSC dynamic mode might be dependent on sample size and temperature ramp. For the inhomogeneous PCM composites, such as, PCM concrete composites another approach than for homogeneous materials should be used, see [30,31]. Due to the low thermal conductivities of PCMs, several studies have documented on the heat transfer enhancement. In most of the cases, it was proposed to mix PCM with some high conductive materials in order to be able to activate deeper layers of the thermal mass. The best results have been obtained with additives such as copper and graphite characterized by a very high thermal conductivity. On the other hand, mixing PCMs with materials representing sensible heat capacity decreases their volumetric heat capacity, which is not appreciated in TES. Also several papers can be found on the heat transfer enhancement of PCM located in specially finned pipes. Extension of the surface area helped to decrease the time of activation of the thermal mass and increased the volume of the thermally activated thermal mass. The experimental research on concrete ceiling with extruded surface area and PCM did not indicate heat transfer enhancement with regards to flat ceilings. The other factor that should always be kept in mind when specifying the potential for heat and cold storage in PCM materials is the heat transfer coefficient on the surface. On the internal room surfaces in the building, the total heat transfer coefficient (include heat transfer due to convection and radiation) is usually not lower than 5.5 W/(m2 K) and not higher than 11 W/(m2 K). For the room case, it is the convective heat transfer part that determines if the total heat transfer is closer to minimum or maximum value within given range because radiant part is not varying much and is usually between 5 and 5.5 W/(m2 K). 11. Concluding remarks In this chapter, some of the key issues regarding PCM applications in the building, which should be taken into account when studying performance of PCM application, are listed. The issues listed illustrate key observations and conclusions made during preparation of this review. • When studying the potential of PCM products, boundary condition (temperature fluctuation, heat transfer on the surface) and heating loads in experimental set ups and in simulations have to represent realistic condition, else their performance will be either over or underestimated. • Heat storage potential of PCM technologies should always be analyzed taking into account thermal properties of PCMs/PCM composites but also heat transfer condition on the surface. • Proper thermal properties of PCMs and their composites, such as, thermal conductivity and specific heat capacity have to be determined as a function of temperature to correctly determine dynamic performance and potential for the whole energy storage system. • Information about PCM content, ratio of surface with PCM to total surface of the room, location and thickness should always be reported for the studied case.

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• Potential of latent heat application should always be compared to realistic and relevant reference systems/constructions. • When studying the potential and performance of PCMs in external envelopes (both transparent and opaque) of the building, the climatic condition has to be taken into account. • The improvement potential due to application of PCM technology in the building should be studied with respect to the relevant time span of performance: day, month, year. • Heat stored in the PCM during high temperature periods (days) should be discharged during low temperature periods (nights) to be able to perform on the consecutive day. Therefore, it is recommended to combine PCM technologies designed for room application with, for example, night cooling, natural ventilation, TABS with free cooling. • It can be concluded, that it is advantageous to expose construction elements to direct solar radiation. Therefore, PCM materials should be located close to the transparent building envelopes. For future work, it can be recommended to focus more on: • Economic feasibility of PCM application in the building, as this matter is not covered in scientific publications. • Measurements on real buildings with PCM technologies. Real case studies are essential to document performance and potential of PCM products in real operation conditions. Based on the measurements in the real full-scale buildings with PCM, calibrated numerical models could be developed and results between models with and without PCM compared. • Flammability of pure PCM but also of PCM composites. For example, it would be of great value to determine to what ratio PCM could be combined in some typical building materials and still fulfill building fire codes and standards. • Development of simple but validated dynamic simulation tools capable to model PCM performance. The tool should be developed for architects and consultants to broaden the knowledge and understanding of PCM potential and still give accurate results. Acknowledgements This work is collectively sponsored by the Danish Agency for Science, Technology and Innovation and the Ministry of Science and Technology of P.R. China in the Sino-Danish collaborated research project: “Activating the Building Construction for Building Environment Control” (Danish International DSF project No. 09-71598, Chinese international collaboration project No. 2010DFA62410). References [1] Y. Hong, G. Xin-shi, Preparation of polyethylene-paraffin compound as a formstable solid–liquid phase change material, Solar Energy Materials and Solar Cells 64 (2000) 37–44. [2] J. Cho, A. Kwon, C. Cho, Microencapsulation of octadecane as a phase-change material by interfacial polymerization in an emulsion system, Colloid and Polymer Science 280 (2002) 260–266. [3] C. Liang, X. Lingling, S. Hongbo, Z. Zhibin, Microencapsulation of butyl stearate as phase change material by interfacial polycondensation in a polyurea system, Energy Conservation and Management 50 (2009) 723–729. [4] A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials, Solar Energy 30 (4) (1983) 313–332. [5] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (2003) 251–283. [6] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Conservation and Management 45 (2004) 263–275. [7] Y. Zhang, G. Zhou, K. Lin, Q. Zhang, H. Di, Application of latent heat thermal energy storage in buildings: State-of-the-art and outlook, Building and Environment 42 (2007) 2197–2209. [8] E. Günther, H. Mehling, Enthalpy of phase change materials as a function of temperature: required accuracy and suitable measurement methods, International Journal of Thermophysics 30 (2009) 1257–1269.

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