Elsevier

Energy and Buildings

Volumes 188–189, 1 April 2019, Pages 1-11
Energy and Buildings

Nano-encapsulation of phase change materials: From design to thermal performance, simulations and toxicological assessment

https://doi.org/10.1016/j.enbuild.2019.02.004Get rights and content

Abstract

The paper presents the results of an experimental activity aimed at producing and characterizing a nano-encapsulated PEG600 (PCMs) into a silica shell. The nano-encapsulation was meant to be useful to improve the material's suitability to integration in building components. The (300±15)nm nanoparticles that were produced underwent a full characterization of their thermal performances. An enthalpy of fusion as high as 66.24 kJ/kg, in a tight melting temperature range (20–21°C) was obtained, making the material suitable for thermal energy storage in buildings. In order to demonstrate the benefits of such as this technology on the reduction of heating and cooling demand of buildings, a concentration of 50% in weight of nanoparticles was, then, embedded into a gypsum plasterboard and used for all indoor plastered surfaces of a reference residential buildings. A saving of respectively up to 4.3% and up to 1.1% of heating and cooling energy demand was predicted in comparison to the ones of a building without PCM. Finally, the material underwent a full toxicological characterization exposing human alveolar basal epithelial cells to nanoparticles. The results showed that there were no toxic effects on cell morphology.

Introduction

In the last decade, the potential integration of PCMs in building components, as suitable latent thermal energy storage systems, has become a goal for several research activities. PCMs can store large amounts of latent heat in their phase transitions [1], achieving significant energy savings and comfort in buildings. If, on the one hand, sensible heat refers to heat that can be sensed by means of a thermometer, latent heat storage refers to the undetectable heat transfer, intrinsically associated with a phase transition [2]. PCMs can utilise their high latent heat storage, corresponding to the number of chemical bonds to be broken to activate the full isothermal phase transition at constant pressure. For this reason, within the tight temperature range in which the phase transition occurs, PCMs show higher efficiency than any other sensible heat storage material [3]. PCMs are classified in three classes: organic, inorganic and eutectic [3]: organic PCMs are paraffins, fatty acids, esters, and alcohols [4] while the most used inorganic PCMs are salt hydrates. Metallic PCMs, that are classified as inorganic, are rarely used in buildings due to their weight and high melting temperature. Generally, higher melting temperatures are reported for metals and inorganic PCMs, whereas they are lower in organic, salt hydrate and eutectic PCMs [5]. The main figures of merit affecting PCMs effectiveness are the melting temperature, the amount of latent heat of transition per unit weight, thermal conductivity (λ) and the specific heat. Shape-stabilized PCMs, by means of micro- and nano-encapsulation processes, avoid any leakage risk in the liquid phase, but also maximize heat transfer due to larger available surface area, compared to macrocapsules [6]. Micro-encapluation is a technology to encapsulate and shape-stabilize PCMs in spheres at microscale range (>1 µm); the current research trend is aimed at reducing the encapsulation size within the nanoscale range, so as to maximize size effects and surface area involved in heat transfer [7].

PCMs can help customizing the redistribution of thermal loads in buildings. A recent review showed that generally PCMs are embedded in building elements and materials, especially in walls and floor elements, because they can provide energy storage by means of latent heat accumulation, resulting in higher heating storage with respect to typical sensible heat processes of building materials [8]. The improvement observed in PCM-embodying elements is due to this enhanced latent heating storage, even if no variation of specific heats takes place. For instance, the application of PCM capsules with paraffinic wax in lime plaster enhanced the apparent specific heat capacity, compared to the reference material [9]. PCM-enhanced plasters have been investigated as a suitable chance in the refurbishment of building envelopes, in the Mediterranean climate [10], in the hypothesis of adopting 3.0 cm thick plaster on all exposures and in different climatic conditions. The heat storage capacity of a special composite plaster was compared to a commercially produced lime-cement mortar, reporting an increase from 0.4 kJ/(kg·K) to about 2.1 kJ/(kg·K) after the addition of 24% PCM [11]. Pavlick et al. [12], in 2014, reported the enhanced performance of a PCM-modified plaster exhibiting specific heat capacity of 1.6 kJ/(kg⋅K) against 0.77 kJ/(kg⋅K) observed in the reference plaster. The integration of PCMs in lightweight building components was investigated by Fiorito [13], employing EnergyPlus for simulating the use of PCMs in a naturally ventilated test room. In that study, higher benefits were obtained by adding PCMs in walls or partitions, linearly with PCM thickness. Lee and Medina [14] simulated a frame wall embodying hydrated salt (melting and solidification temperatures between 27.6 and 29.6°C) macroencapsulated in containers larger than 1 mm. The aim was to reduce the cooling on-peak demand in California. Total energy saving reached values of 9.21 kWh/(m2·year) due to PCM-enhanced frame walls. Energy saving between 30 and 55% in the HVAC system were registered by Navarro et al. [15], who used an internal slab as a storage unit and as an active cooling supply in Spain. They used 52 kg of RT-21 paraffin macro-encapsulated in 1456 aluminium tubes of 12 mm diameter. Kenisarin and Mahkamov observed that further research activities on PCMs should, among the other objectives, achieve the narrowest possible temperature range for the phase change process in PCMs and reduce their costs [16]. Several research groups used paraffin, as reviewed by Zalba et al. [17] for its melting temperature (for instance, paraffin wax has a melting temperature of 28°C) and its high latent heat (244 kJ/kg), highly compatible with uses in constructions.

Nanotechnologies can help enhancing PCMs performance, as a natural evolution, since abrupt changes may occur, at nanoscale level, in thermophysical and physicochemical properties. Then nano-enhanced features of PCM materials could be suitably exploited and several research activities are currently working on this point, as reviewed by Parameshwaran and Kalaiselvam [18]. It has been observed that the inclusion of nanomaterials could improve some PCMs figures of merit, overcoming some of their limitations, such as a low value of thermal conductivity [19]. To this aim, several nano-enhanced PCMs have been proposed, embodying, for instance, copper, titania, alumina, silica and zinc oxide nanoparticles (NPs), thoroughly investigated by Teng and Yu [20]: they showed that titania NPs are more effective than other additives in order to enhance heat conduction and thermal storage capacity in paraffins, also affecting both melting onset temperature and the solidification temperature. A completely different route to nano-enhancement of PCMs consists in their encapsulation within a nano-shell [21] or a nanofiber [22]. In addition, nano-shells protect PCMs from the surrounding environment. Liu et al. [23] described the different routes to synthetize different kinds of nano-PCMs (sol-gel, miniemulsion, emulsion and in situ polymerization) and the respective advantages and disadvantages. Sari et al. [24] synthetized polystyrene and n-heptadecane micro-/nanocapsules adopting the emulsion polymerization route with capsule sizes ranging from 10 nm to 40 mm for a 1:2 ratio of polystyrene and n-heptadecane. However, among different materials to be used for encapsulation, amorphous silica shows high heat storage capacity and thermal conductivity [25]. In addition, it is biocompatible, nontoxic for living organisms and the environment [26], [27], and in its core it is easy to confine active molecules acting as reservoir [28]. Zhang et al. [29] synthesized silica spheres (7–16 µm) with n-octadecane core (melting temperature range of 23–28°C) using TEOS as an inorganic precursor by a sol-gel process, with different steps. The obtained nanomaterials showed a good thermal conductibility. Similarly, Belessiotis et al. [30] obtained silica spheres with paraffin core via sol gel method showing a latent heat of ∼ 156 kJ/kg. Latibari et al. [31] obtained nano-PCMs with palmitic acid core and silica shell, using multistep sol-gel method and investigated their thermal figures. The efficiency of encapsulation, defined as the percent ratio of the latent heat of the encapsulated PCM and that of the pure PCM, ranged from 83.25 to 89.55% (with a particle size between 183.7 and 722 nm) depending on pH of the chemical mix solution. Nevertheless, since palmitic acid has a melting temperature of 61°C, the application in buildings is quite difficult. The same limitation (high melting temperature ranging from 142.1°C to 166.2°C) concerned mannitol, chosen by Wu et al. [32] and Pethurajan et al. [33].

The cost-effectiveness of PCMs inclusion in building envelopes was investigated by Kosny et al. [34], who found that the commercial cost of a PCM with a latent heat as high as 116 kJ/kg, while produced commercially, can be projected to be 4.4–6.6 USD/kg. On the other hand, PCMs for building applications should be produced by means of environmental-friendly processes and raw materials. Among the possible PCMs, paraffin is inflammable and it is classified as a doubtful carcinogen (source: Sigma-Aldrich), while PEG is an inert inexpensive and versatile polymer for customizing nanostructured materials due to its intrinsic biocompatibility and water solubility [35]. It is also widely used in the biomedical field and it has been approved by Food and Drug Administration for many applications [36]. In addition, the range of melting temperature is between 17 and 22°C (Source: Sigma-Aldrich), that is well within the range of comfortable indoor environment temperatures and, for this reason, its application in buildings is preferable.

In this work, we obtained (300±15) nm SiO2@PEG600NPS by means of a one step, easy and reproducible synthetic route. The obtained nanostructures were then fully characterized, before undergoing a toxicological assessment. The SiO2@PEG600NPS showed a good thermal performance, with an enthalpy of fusion as high as 66.24 kJ/kg, in the tight melting temperature range (20–21°C): such feature makes it a good candidate for thermal energy storage in building applications, especially to reduce energy uses during winter season HVAC. As will be seen hereafter, it is precisely during the winter season that the incorporation of the PCM inside the plaster performs its effectiveness. On the contrary, during the summer season, it shows no particular benefit, mainly because the temperature of the internal surface of the vertical walls is almost always above the Tm of the PCM. Following the experimental design and synthesis activities of the nanostructures incorporating PCMs, we performed dynamic simulations able to show the possible extent of energy savings obtained by integrating a certain percentage of SiO2@PEG600NPS (50%) in building gypsum plasters, comparing a reference case, devoid of PCM, with another one, containing the proposed material applied over all internal vertical plastered surfaces. It is the case to specify that the choice of PEG600 as a suitable PCM was carried out following a tight comparison aimed at identifying a material having, at the same time, different specificities. Firstly, the compatibility with a low-cost synthesis mode of the hosting SiO2 shell; secondly, it represented a biocompatible PCM and, finally, a series of preventive simulation activities (not reported in the text) provided an ideal range of melting temperatures to make the maximum contribution during the winter season, associated to the maximum benefit in terms of energy saving. After all these considerations and activities, we decided to adopt the PEG600 as a material able to guarantee a satisfactory compromise.

Section snippets

Materials and methods

In this experimental activity we have reported the results of a cross-disciplinary design and the full characterization of a novel nanostructured material, showing the advantage of achieving full shape stabilization of a biocompatible PCM (PEG600) within the nanoscale, inside a non-toxic amorphous SiO2 shell. The main properties of this specially designed material were fully characterized after the chemical synthesis. To this aim, at first microscopy characterization was carried out to observe

PCM properties

SiO2@PEG600 nano-particles were firstly characterized in water by TEM in order to analyze their morphology, showing that NPs were spherical and monodispersed with a size of (300 ± 15) nm (Fig. 3a). DLS measurements confirmed the size of NPs (Fig. 3b) showing a uniform size distribution. SEM-EDS analysis confirmed the morphology and smooth surface of NPs and further corroborated the presence of confined PCM (PEG600) in the SiO2 NPs core. Indeed, the silicon, oxygen and carbon element peaks

Conclusions

The nano-encapsulation of a PCM for potential use in the construction sector was investigated in this paper. In particular, in order to enhance the potential of the PCM to contribute to energy saving and thermo-regulation of the indoor temperature in buildings PEG600 was chosen because its melting temperature is close to 20°C. In addition, in order to embody this material in plasters and other building mixtures, the PCM was nano-encapsulated in a silica shell which may also contribute to

Acknowledgments

A.C. kindly acknowledges the Action co-founded by Cohesion and Development Fund 2007–2013 – APQ Research Puglia Region ‘‘Regional programme supporting smart specialisation and social and environmental sustainability – FutureInResearch”.

Conflict of interest

The authors declare that they have no conflict of interest.

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