Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 95 (2016) 175 – 180
International Scientific Conference “Environmental and Climate Technologies”, CONECT 2015, 14-16 October 2015, Riga, Latvia
Modeling of building envelope’s thermal properties by applying phase change materials Liene Kancane, Ruta Vanaga*, Andra Blumberga Institute of Energy Systems and Environment, Riga Technical University, Azenes iela 12/1, Riga, LV–1048, Latvia
Abstract All new buildings must be nearly zero-energy buildings (NZEB) by 2020, and all new buildings occupied and owned by public authorities must be NZEB after 2018. These targets are set in the European Member States according to the EU directive on the Energy Performance of Buildings [1]. It is a huge challenge is to achieve these goals in northern countries where traditional energy saving measures – insulation, window performance, heat recovery from ventilation systems - balance on the border of cost optimality. New solutions have to be implemented in the field of energy saving and storing. The biomimicry approach imitates processes found in nature to solve human problems that can provide conceptually new solutions. The example of northern mammals can serve as inspiration – fatty tissue functions as thermal insulator of the body, but phase change properties of the lipids are also used to store and release heat. Phase change materials (PCM) have received much attention as an alternative for energy saving and storing. The energy demand for heating the building and thus CO2 emissions can be reduced by the amount of latent heat stored in PCMs. Lately different concepts on PCM integration in the building envelope are emerging – built-in walls, ceilings, floors, as a thin layer or large storage tanks. In order to create new PCM integrated building components – a solar thermal façade system appropriate PCM has to be chosen. This paper reviews the selection of PCM in melting temperature range 21–22 °C. The Authors. Published by Elsevier Ltd. © 2016 2016Published © by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. Keywords: phase change materials; latent heat storage; solar thermal façade; biomimicry
* Corresponding author. Tel.: +371 29171132 E-mail address:
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1876-6102 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Riga Technical University, Institute of Energy Systems and Environment. doi:10.1016/j.egypro.2016.09.041
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1. Introduction The goal of the European Union is to cut its domestic GHG emissions by 80 % by 2050 compared to 1990 levels. The building sector consumes 35 % of all end energy consumption. The target has been set in the European Union Member States - according to the EU directive on Energy Performance of Buildings, all new buildings must be nearly zero-energy buildings (NZEB) by 2020, and all new buildings occupied and owned by public authorities must be NZEB after 2018. It is a great challenge to achieve these goals in northern countries where traditional energy saving measures – insulation, window performance, heat recovery from ventilation systems – balance on the border of cost optimality. Nevertheless industry and scientists are constantly searching for new insulation materials to be implemented in the market [2–6] as far as NZEB approach asks for conceptually new solutions. A nearly zero energy building is defined as the building that has a very high energy performance achieving low energy consumption and energy needed should be mostly produced by renewable sources produced on site or nearby. If planned accordingly, part of the energy demand can be covered by solar heat gains. Solar energy, however, is only available during daytime hours and the peak of energy gained is at noon [7]. Energy storage would allow shifting the peak load to the later hours. There are different types of energy storage – mechanical, electrical and thermal including sensible heat, latent heat and thermochemical energy storage. Following the example found in nature as the biomimicry suggests the principle of blubber integrated in outer layers of northern mammals can be observed to solve the problem of energy storing in outer layers. In order to create a new type of solar thermal facade system, phase change materials are chosen for energy storing in the building envelope [8]. It is known from literature that PCM can store large amounts of energy because of their ability of phase transition. The storage and release processes happen at an almost constant temperature which is called the melting and freezing temperature. The most widely used phase change materials are with solid-liquid transition. [9] Three types of PCMs are usually studied in literature for application in buildings: organic fatty acids and paraffins and inorganic salt hydrates. Currently there are a number of PCM products in the market which are designed for use in buildings – floor, wall and ceiling panels, glazing systems incorporated with PCM, aerated concrete bricks with PCM components, etc. However, particular attention must be paid to several material properties. One of the properties which should be considered is thermal conductivity which is rather low for most of the PCMs; therefore enhancement methods have to be evaluated. The right method for enhancing has to be chosen taking into account corrosion or sedimentation aspects. For a durable design, attention to the thermal cycles must be paid. The economic costs for PCMs incorporated into the building envelope could be rather high. But, as all technologies are developing very rapidly these days, PCM could stay more and more available due to sufficient research basis. The task set is to store in a PCM container the energy from sun radiation gathered on vertical prototype surfaces incorporated into the building wall. Different kinds of PCM and thermal conductivity enhancers were studied. 2. Choosing appropriate PCM for building thermal envelope The most important quality of PCMs is their high thermal storage capacity, which, for unit thickness (Fig. 1), is many times higher than conventional building materials like concrete. Various groups of PCMs have been studied in this paper, including organic compounds such as paraffins and fatty acids, and inorganic compounds such as salt and salt hydrates and polymeric materials.
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ΔT=15K
Heat capacity in kJ/kg
250 200 150 100 50 0
Water
Stone
Wood
Plastic
PCM
Fig. 1. Heat storage capacity by materials with the same volume [10].
Explanatory flowcharts and the classification of PCMs are described in several papers [11–13]. Two PCM groups (Fig. 2) are mainly used in applications for buildings: solid-liquid and solid-solid. Solid-liquid PCMs show larger latent heat of phase transition than solid-solid PCMs. However, solid-solid PCMs cannot leak and have smaller volume changes [14]. The application of gas-liquid and solid-gas PCMs is limited because of large volume changes, although these PCMs have a high phase transition latent heat.
Fig. 2. Classification of phase change materials [15].
Certain properties of PCM must be evaluated for successful integration in the building envelope. The material has to store a high amount of energy in a narrow temperature range. If a material fulfils this criterion, it should be further evaluated according to the following conditions: physical, chemical, technical and economic [16]. The essential physical properties include: x x x x x x x x
Phase change temperature must meet the system’s requirements; Minimal temperature range for melting-solidification and solidification-melting processes, no supercooling; High latent heat of transition; High specific heat capacity as addition to sensible heat absorption; Good thermal conductivity for storage and release of heat in a short time to gain effective cooling or heating; Small changes in volume during the phase changes; Stable melting and solidification cycles to maintain required properties as long as possible; Sufficient crystallization rate.
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The essential chemical properties include: x Chemically-stable material; x Non-degradable after many melting-solidification cycles; x Good compatibility with other building materials. The essential technical properties are: x Low steam pressure to avoid additional investment of material for container or cover development; x Safety considerations – PCM systems should provide adequate mechanical stability and it should be fireproof. The essential economic aspects are: x Suitable for reuse in the end of life cycle; x Competitive price. No material covers all of these requirements. Material is for the most part chosen due to its physical properties such as melting point temperature, supercooling and cyclic stability. For organic PCMs these properties depend on the substance’s purity – high purity gives narrow phase change transition and high latent heat but is found very expensive for use [15]. The most commonly used PCMs for application in buildings are paraffin, salt hydrates and fatty acids. Advantages and disadvantages of each group of PCM are summarized in Table 1. Table 1. Advantages and disadvantages of the most used PCM in buildings [17]. Advantages
Disadvantages Paraffin and fatty acids (organic PCM)
-
-
Wide melting temperature range High latent heat of fusion Phase transitions with little or no supercooling Self-nucleation properties Congruent melting No segregation
Low thermal conductivity Lower phase change enthalpy Lower density Flammable but possible to use flame retardants More expensive than salt hydrates Non-compatible with plastic Relatively large volume changes, but for some fatty acids small
Salt hydrates (inorganic PCM) - Good thermal stability - Low vapour pressure - Non-reactive, not dangerous - Compatible with conventional construction materials - Recyclable - Higher melting enthalpy - Higher latent heat of fusion - Low cost - Higher thermal conductivity - Non-flammable - Lower volume change - Compatible with plastics - Better to use salt hydrates than paraffin from -LCA point of view
-
Supercooling, poor nucleation In congruent melting and dehydration Segregation Not compatible with some building materials Corrosive to most metals and slightly toxic
3. Choosing the heat transfer enhancer In order to enhance heat transfer in PCM, a material of significantly higher thermal conductivity has to be embedded. In this study, the main focus was on metal enhancers with high thermal conductivity. It is very important to select a metal which is not corrosive in contact with PCM acids for long term building application.
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The literature study on metal impact was carried out for all three types of PCMs – paraffins, fatty acids and salt hydrates. Studies of [18, 19] showed that copper and carbon steel do corrode with fatty acid, but aluminium and stainless steel are more resistant and therefore can be used as thermal conductivity enhancers for fatty acids. For paraffins commonly used particle dispersed are from graphite, Al, Cu. Although salty hydrates have higher thermal conductivity than organic PCMs, still enhancers should be used to speed up charging or discharging process. Therefore at first compatibility with other materials choosing enhancer must be studied. Literature review is carried out only on corrosion tests in this study while there are metal enhancers used in experiment. . An experiment prepared by [18] where salt hydrate SP21E from “Rubitherm” was tested with five metal sheets – aluminium, carbon steel, copper, stainless steel 304 and stainless steel 316. Results show that this PCM is not compatible with aluminium and carbon steel. But aluminium and both stainless steel are suitable for long term use with this salt hydrate. For another study [20] five salt hydrates (CaCl2, Na2S, CaO, MgSO4, and MgCl2)were tested with common metals: aluminium, stainless steel 316, carbon steel and copper. It was an immersion corrosion test under humidity and temperature defined conditions. Results show that Stainless steel 316 shows no degradation and is resistant to all salt hydrates. Na2S is not recommended with copper and aluminium, whereas copper is resistant to CaCl2 and MgCl2. 4. Result and discussion As a result of this study, three systems were chosen for further research: x Paraffin RT21HC with copper thermal enhancers; x Salt hydrate SP21E with copper thermal enhancers; x Eutectic mixture of fatty acids with aluminum thermal enhancers. The next step is to design solar thermal system with selected PCM and enhancers incorporated, to do numerical study and make an actual prototype. Acknowledgements The work has been supported by the National Research Program “Energy efficient and low-carbon solutions for a secure, sustainable and climate variability reducing energy supply (LATENERGI)”. References [1] EU directive. Available: http://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32010L0031&from=en [2] Ojanen T, Seppa IP, Nykanen E. Thermal Insulation Products and Applications - Future Road Maps. Energy Procedia 2015;78:309–314. [3] Lorenzati A, Fantucci S, Capozzoli A, Perino M. The Effect of Different Materials Joint in Vacuum Insulation Panels. Energy Procedia 2014;62:374–381. [4] Muizniece I, Lauka D, Blumberga D. Thermal conductivity of freely patterned pine and spruce needles. Energy Procedia 2015;72:256–262. [5] Muizniece I, Blumberga D, Ansone A. The Use of Coniferous Greenery for Heat Insulation Material Production. Energy Procedia 2015;72:209–215. [6] Safikhani T, Abdullah AM, Ossen DR, Baharvand M. Thermal Impacts of Vertical Greenery Systems. Environmental and Climate Technologies 2014;14:5–11. [7] Weinläder H, et al. PCM-facade-panel for daylighting and room heating. Solar Energy 2005;78:177–186. [8] Vanaga R, Blumberga A. First Steps to Develop Biomimicry Ideas. Energy Procedia 2015;72:307–309. [9] Pielichowska K, Pielichowski K. Phase change materials for thermal energy storage. Progress in Materials Science 2014;65:67–123. [10] Glossary: PCM's in Thermal Energy Storage Applications. Retrieved: http://www.rubitherm.de/english/ [05.03.15]. [11] Cabeza LF, et al. Materials used as PCM in thermal energy storage in buildings: A review. Renewable and Sustainable Energy Reviews 2011;15:1675–1695. [12] Rathod MK, Banerjee J. Thermal stability of phase change materials used in latent heat energy storage systems: A review. Renewable and Sustainable Energy Reviews 2013;18:246–258. [13] Memon SA. Phase change materials integrated in building walls: A state of the art review. Renewable and Sustainable Energy Reviews 2014;31:870–906.
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