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Design and analysis of phase change material based floor heating system for thermal energy storage
Accepted Manuscript Design and analysis of phase change material based floor heating system for thermal energy storage Beom Yeol Yun, Sungwoong Yang, Hyun Mi Cho, Seong Jin Chang, Sumin Kim PII:
S0013-9351(19)30177-X
DOI:
https://doi.org/10.1016/j.envres.2019.03.049
Reference:
YENRS 8411
To appear in:
Environmental Research
Received Date: 30 November 2018 Revised Date:
18 March 2019
Accepted Date: 19 March 2019
Please cite this article as: Yun, B.Y., Yang, S., Cho, H.M., Chang, S.J., Kim, S., Design and analysis of phase change material based floor heating system for thermal energy storage, Environmental Research (2019), doi: https://doi.org/10.1016/j.envres.2019.03.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Design and analysis of phase change material based floor heating
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system for thermal energy storage
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Beom Yeol Yun, Sungwoong Yang, Hyun Mi Cho, Seong Jin Chang, Sumin Kim*
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Department of Architecture and Architectural Engineering, Yonsei University, Seoul 03722,
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Republic of Korea.
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* Corresponding author:
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E-mail: [email protected]
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Abstract
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Pleasant interior space is essential for modern people who spend considerably more time in the
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buildings than they did in the past. To achieve this, one aspect includes an ambient temperature that
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maintains the thermal equilibrium of the human body. The construction of wood framed buildings is
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becoming increasingly popular worldwide, and there have been recent trends toward constructing
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high-rise wooden houses. In this respect, heating methods appropriate for use in wooden buildings
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are being studied. Dry floor heating systems are predominantly used in wooden houses, but they
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provide a poor heat storage performance, which is not conducive to saving energy. In this study, the
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effects of thermal comfort and energy savings were analyzed after applying a phase change material
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(PCM) to floor heating, which can be used to reduce the peak temperature and contribute to energy
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savings. To enable shape stabilization, this study used Macro-Packed PCM (MPPCM), as shape
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stabilization is necessary when applying PCM. The heat storage performance was improved by
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applying MPPCM to a dry floor heating system. Paraffin-based PCMs, such as n-octadecane, n-
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eicosane, and n-docosane, were used to obtain a comfortable floor temperature range. Experimental
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temperatures ranged from 28°C to 35°C, with an entire temperature range of 7°C. Experimental
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results showed that the heat storage performance of MPPCM reduced the amount of energy used for
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heating by 43%, and n-eicosane was the most effective PCM for use in floor heating with respect to
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obtaining a comfortable floor temperature.
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Keywords
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Phase change materials; Heat storage; Latent heat; Dry floor heating system; Power
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consumption; Energy saving
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1. Introduction
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People spend a considerable more time in buildings nowadays than they did in the past
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(approximately 90% of time) (López-Pérez et al., 2019), and the internal comfort of a building has
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thus become more important (WHO, 1989; Park et al., 2016; Singh et al., 2010). The demand for
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electric power used in heating and cooling has recently increased dramatically throughout the world,
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and governments are strengthening policies to reduce the energy consumption of buildings.
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Therefore, research and development on efficient building energy systems are being conducted, and
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in particular, research relating to phase change materials (PCMs) is being actively conducted
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globally with respect to its use in reducing the heating and cooling energy load of buildings. Phase-
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change materials undergo phase changes with temperature and can accumulate and emit thermal
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energy by using latent heat when the phase changes from solid to liquid or from liquid to solid.
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Because latent heat has a better energy storage capacity than sensible heat, it saves heat and energy
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used in buildings more efficiently (Lee et al., 2017).
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Many studies have been conducted with the aim of achieving energy saving in buildings. For
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example, a simulation program conducted by Mi et al. (Mi et al., 2016) provided a 10% saving in
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heating energy, and the study of Lei et al. (Lei et al., 2016) enabled a reduction in the cooling load
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within the tropical climate of Singapore. Another study analyzed the effect of reducing the peak
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temperature using commercially available PCM products (Berardi and Soudian, 2019), and one
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study analyzed the reduction in cooling energy demand in a desert climate (where the demand for
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cooling energy is extreme) by using PCM within the walls (Hasan et al., 2018). In this regard,
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applying PCM to the buildings is the issue because it can reduce the building energy in various ways.
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Furthermore, studies are being conducted to reduce CO2 emissions, which are the main cause of
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global warming worldwide (Kahouli, 2018). It has been shown that the use of wood as a building
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material causes less CO2 to be released than when producing concrete (Nässén et al., 2012), and
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wooden houses using environmentally friendly and energy-efficient lightweight wooden building
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techniques are thus attracting attention. A technique for building high-rise wooden buildings has
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been developed based on the development of structural timber such as cross-laminated timber (CLT)
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(Brandner et al., 2016), and research on the construction of high-rise wooden buildings is being
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actively conducted globally (Ramage et al., 2017).
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In conventional floor heating, it is necessary to use three components: insulation, lightweight foam
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concrete, and mortar. However, when we consider the wooden buildings, there are few concerns
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about applying the conventional floor heating system (Shin et al., 2015). Therefore, the timber
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houses require a suitable type of heating system: simple to use and install lighter and proper heights.
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In a wooden building, it is thus preferable to use light dry floor heating system instead of wet floor
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heating for several reasons. The dry floor heating provides advantages in that there is a lighter load
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applied to the building than when using wet floor heating. Furthermore, this method does not need a
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thicker layer to fix heating, pipes and it takes more story heights. This is why Most wooden
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buildings adopt the electric panel heating system that meets the conditions; lighter and thinner.
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However, with dry floor heating, there are no thermal-energy storage media, such as mortar. For this
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reason, the dry floor heating system would more energy when the heating period.
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Therefore, this study applies PCM, which has an excellent heat storage performance, to the electric
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panel heating. The applied PCM types are divided to their melting temperature. Also, PCMs were
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fabricated macro-packed PCM (MPPCM) (Chang et al., 2017). To confirm the optimal thermal
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performance of the heating system, the power saving effect relating to MPPCM was analyzed by
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changing its composition.
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Studies on the use of PCM and the stabilization of phases have been conducted with respect to the
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inherent problems with PCM when applied to buildings: leakage in a liquid state and weakening due
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to phase changes. There are four ways to apply PCM to buildings. The first is to directly insert PCM
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into building materials; for example, a metal foam can be immersed in a PCM to form a PCM inside
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(Zhu et al., 2018), PCM can be placed in a concrete slab to reduce the peak temperature (Navarro et
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al., 2015), and free-cooling can be attained by applying PCM to lightweight buildings (Rouault et al.,
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2013). Second, PCM can be impregnated into pores by vacuuming building materials; for example,
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it has been applied to the gypsum board by vacuum impregnation (Jeong et al., 2016b). In addition,
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xGnP porous material has been impregnated with n-hexadecane and applied to concrete to enhance
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the thermal performance (Kim et al., 2014), and the thermal performance has been improved by
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impregnating expanded graphite with PCM (Ren et al., 2018; Tian et al., 2016; Zhong et al., 2014).
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Third, PMC can be inserted into small capsules, and studies on types of microencapsulated PCM and
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methods of applying it are progressing (Shannaq and Farid, 2015). For example, Alam et al. (2015)
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studied the phase stabilization of PCM through polymer coating; research has been conducted to
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stabilize PCM by wrapping it in silica, (Zhang et al., 2018); and studies using graphite as a phase
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stabilizing material have been conducted (Cui et al., 2015a). Finally, PCM can be packed into a
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packaging container in a process known as macro-packing. Macro-PCM has been applied to a roof to
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reduce the heat island phenomenon in an urban area (Yang et al., 2017), and thermal analysis of
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PCM in a processed spherical metal has been conducted (Cui et al., 2017). Furthermore, numerical
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simulations have been conducted with respect to the microencapsulation of mechanical properties
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(Cui et al., 2015b), and one study evaluated the thermal performance of a wood frame house by
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applying macroencapsulated PCM (Chang et al., 2017).
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Of these methods of use, the MPPCM method uses more PCM than the other methods. Of the
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characteristics of PCM, its latent heat capacity has the greatest effect on its energy storage
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performance. In methods other than MPPCM, only 80% of the PCM can actually be used; therefore,
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the amount of latent heat is relatively decreased (Huang et al., 2019). In contrast, the MPPCM
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method uses the most latent heat (approximately 98%), and it thus has a greater storage effect than
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other methods.
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Therefore, in this paper, PCMs were fabricated macro-packed PCM (MPPCM) (Chang et al., 2017).
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To confirm the optimal thermal performance of the heating system, the power saving effect relating
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to MPPCM was analyzed by changing its composition.
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2. Methodology
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2.1. Materials
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PCM is a substance that undergoes phase changes; it releases heat by accumulating or storing heat
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through physical changes (not chemical changes). There are three types of phase change materials:
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organic, inorganic, and eutectic (Jeong et al., 2016a; Vicente and Silva, 2014). Organic PCMs are
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chemically stable, and although they do not cause any phase separation or supercooling phenomena
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(examples are paraffinic PCMs, such as n-hexadecane, n-octadecane, and n-eicosane). Inorganic
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PCMs are comprised of a salt or metal salt, such as calcium chloride or sodium sulfate, and provide a
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high amount of latent heat per unit volume compared to organic PCMs; however, they can be
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affected by phase changes due to corrosion or supercooling (Safari et al., 2017), which is a distinct
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disadvantage. Eutectic PCMs are composed of a mixture of two or more components and have a
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rapid melting point, which makes it easy for phase changes to occur at particular temperatures.
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However, there is a current lack of basic research conducted on their thermal behavior and function.
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It is necessary to first select a PCM that provides an appropriate phase change temperature, prior to
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using it in a building. As PCMs use latent heat during phase changes, a phase change interval must
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be applied to fully provide the heat storage performance through latent heat. In addition, as the
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structure can be applied to the outer wall, inner wall, the ceiling, or (in the case of this study) to the
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floor, it is necessary to select an appropriate PCM that can provide the required temperature and
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phase change. For the floor of a building, this temperature range is 28–35°C (Kim, 1993). Therefore,
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the suitable phase change materials in this respect, according to the required phase change
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temperature, are paraffin-based n-octadecane (PARAFOL 18-97), n-eicosane (PARAFOL 20Z), and
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n-docosane (PARAFOL 22-95). These were thus used as the PCMs in the experiments in this study,
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and all PCMs used were obtained from Celsius Korea, South Korea. The properties of each PCM are
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described in Table 1.
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2.2. Preparation
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In manufacturing such MPPCM, the material for packaging the PCM should have excellent
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durability, resistance to corrosion, and minimal transformation in response to temperature changes.
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This study secured the phase stabilization and usability of PCM by employing nylon packaging in
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the MPPCM method. The manufacturing method and procedure used are shown in Figure 1 and are
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as follows. First, the PCM was heated to 60°C to allow it to completely melt, and 180 g of liquid
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PCM was then injected. The inside of the nylon package was then vacuum sealed using a vacuum
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packaging machine. The nylon package was divided into three compartments using a vacuum
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packaging machine to equalize the absorption and release of heat energy by splitting the PCM
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uniformly (without allowing it to deviate to one side). Finally, to make MPPCM of the same size and
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thickness, the MPPCM was fabricated by cutting the upper part to a size of 200 mm × 200 mm × 8
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mm (length × width × thickness) (Chang et al., 2017).
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2.3 Experimental set-up
In this study, the temperature of the electric panel was controlled and monitored via a temperature
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controller sensor; when the electric panel reached the target temperature, it was automatically turned
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off and the setback temperature was set to 7°C.
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Figure 2 shows the settings used in the experimental environment, which was a real environment
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where the floor was actually heated using the electric panel designed. The amount of heat released to
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the outside was reduced by using insulation, and the film was protected from damage by installing a
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film protection plate. A finishing material was also employed to ensure that the floor heat was spread
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evenly. In both experiments, the electric panel floor heating was constructed in the order of: heat
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insulation, electrical panel, film protection plate, and floor finish. The temperature was measured
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using a thermocouple sensor as shown in Figure 3, and data were recorded using a data logger (GL-
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840). Experimental conditions were set to heat for 12 h at a target temperature of 35°C, and the
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setback temperature was 28°C. The PCM was divided into an upper and lower part, both of which
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had contact with the electric film as a heat source.
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A second experiment was conducted to select which PCM provided the optimal performance in the
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electric panel heating system based on the results of the first experiment. Experimental conditions
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were based on the surface temperature of the layer at 28–35°C, which is a comfortable floor
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temperature range (Kim, 1993). In the experiment, the heating operation time was set to 12 h in each
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case, based on an average room residential time of 12 h, which was obtained from data of the Korean
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Statistical Information Service (KOSIS). The amount of electricity consumed was compared in
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experiments. In both experiments, the temperature of the experimental space was maintained at 25°C,
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and the temperature at the top and bottom of the heating film layer was simultaneously measured
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using the temperature sensor.
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3. Results and Discussion
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3.1. Experiments to determine layer configuration An experiment was conducted to determine the best layer composition, different experiments were
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conducted for the various cases shown in Table 2, and the temperature of the MPPCM was measured
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at the top of the electric panel (UP_Case) and at the bottom (DOWN_Case). Table 3 shows the time
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delay of the UP_Case based on the DOWN_Case. The delay in time until the lower temperature limit
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of 28°C was obtained is defined as the heating delay, and the cooling delay is the delay in time until
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a temperature of 28°C was reached during non-heating. Figure 5 shows the temperature changes
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during 20 h for UP and DOWN of Octadecane (Case_n-octadecane), Eicosane (Case_n-eicosane),
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and Docosane (Case_n-docosane), respectively. Figure 4 shows results for the sample without PCM
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(ref), where there is no temperature difference between the upper and lower parts of the electrical
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panel, and the temperature changes are the same during heating and cooling, which shows that the
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thermal energy applied to the top and bottom of the panel is the same. The top graph in Figure 5
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shows the temperature change for Case_n-octadeane. The heating delay was increased by 127 min,
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and the cooling delay was decreased by 7 min. For the cases of n-octadecane_UP and n-
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octadecane_DOWN, it is possible to confirm the phase change interval at the beginning of heating,
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and the difference in surface temperature at the time when the phase change occurred, which shows
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that heat was transferred to the upper surface when the phase change occurred. In n-
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octadecane_DOWN, the surface temperature reached a maximum of 33.4°C, even during the phase
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change, whereas n-octadecane_UP did not reach 28°C. It can be seen that the MPPCM based on n-
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octadecane stores heat in the form of latent heat through the phase change, and the time taken to
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reach a comfortable temperature range is delayed when MPPCM is located at the upper part of the
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electric panel. The middle graph in Figure 5 shows the temperature change of Case_n-eicosane. The
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heating delay was increased by 17 min, and the cooling delay was decreased by 11 min. For n-
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eicosane_UP, the surface temperature continuously increased from 30°C, while it was maintained for
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n-eicosane_Down after reaching 35°C. The phase change zone was confirmed during cooling, and
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the surface temperature was maintained at 29°C for more than five hours. The temperature change
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graph of Case_n-docosane is shown in the bottom graph of Figure 5. The heating delay was
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increased by 12 min, and the cooling delay was decreased by 4 min. In addition, there was no phase
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change interval, because the temperature of the electrical panel did not reach the melting point of n-
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docosane. It was thus confirmed that heat was stored and transferred only in the form of sensible
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heat. This experiment confirms that the DOWN_Case was faster than the UP_Case with respect to
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the time taken to reach a comfortable temperature range. The surface temperature difference between
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the UP_Case and the DOWN_Case during heating is plotted in Figure 6, according to time. The
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maximum temperature difference is measured to be 7.3°C during heating, and the temperature
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difference is approximately 1°C during cooling (these differences are shown cumulatively in Figure
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7 to analyze the heat energy). In all cases, there was a continuous increase in the accumulated
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temperature difference during heating. However, the greatest increase was found for Case_n-
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octadecane, which had the largest temperature difference, and the smallest increase was seen in
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Case_n-docosane, which had no phase change interval. It is considered that an increase in the
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accumulated temperature difference indicates a more efficient use of thermal energy, this means that
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they can be kept at a higher temperature when the same energy is used. In summary, an increase in
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the accumulated temperature difference shows an equivalent efficiency that results in less energy
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being used. Therefore, these experiments prove that forming a layer by placing the PCM under the
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electric panel reduces the heating delay, shortens the time taken to reach a comfortable temperature,
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and increases energy efficiency by increasing the surface temperature. It is thus evident that the most
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efficient positioning of MPPCM is to construct the layer under the electric panel. It is also shown
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that the PCM needs to be located away from the direction in which heat is to be transferred, and this
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will be evidenced in further studies related to heat transfer. However, if the PCM is not located close
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enough to the heat source, as in the present experiment, it is necessary to study the complex concept
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of heat transfer and heat preservation when heat is to be conserved (de Gracia and Cabeza, 2015).
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3.2. Experiment to determine efficiency during and after heating As a result of the experiment described in the previous section, the MPPCM was placed under the
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electric panel. Experiments were performed under the same conditions as in the previous experiment,
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and the measurement results for a total of 20 h are shown in Figure 8. Analysis was conducted
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during heating and after heating; first, the heating time during heating and the cooling time during
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heating were analyzed, and experimental results are shown in Figure 9, where the heating time
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shown in the graph represents the time taken to reach the set temperature of 35°C after heating, and
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the cooling time represents the setback temperature of 28°C after cooling. A comparison of heating
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times shows that ref was 4.69 min and Case_n-docosane was 4.16 min. This comparative reduction
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in heating time means that less energy required for heating is consumed through the heat storage
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performance of the PCM. Therefore, n-docosane-based MPPCM, which has the shortest heating time,
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is considered to be the most effective for use when heating. The cooling time of ref was measured as
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8.54 min, and the shortest cooling time of 7.79 min was recorded for Case_n-octadecane. The
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longest cooling time was 13.82 min for Case_n-docosane. The reason why the cooling time of
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Case_n-octadecane is shorter than that of ref is that the PCM n-octadecane used in Case_n-
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octadecane has a melting point of 27.5°C, and it is considered that the energy behavior is only in the
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form of sensible heat. Case_n-docosane shows a 62% increase in cooling time compared to ref, and
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the increase in cooling time indicates that PCM uses stored energy in the form of latent heat through
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phase change. Therefore, MPPCM based on n-docosane is considered to be most effective for
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cooling, because it has the longest cooling time. In this work, we conducted a different analysis from
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the ones that did heat analysis after non-heating followed by heating. In previous studies, the thermal
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effects of PCM liquification and solidification were analyzed one-cycle after the end of heating
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(Panayiotou et al., 2016). Moreover, in the same way, as in the previous study, it was performed
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thermal analysis while heating was in progress (Novais et al., 2015). However, previous studies have
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shown that PCM is an inadequate method for evaluating the overall thermal performance of PCM,
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including the specific heat and latent heat. Therefore, this study analyzed the effect of PCM during
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heating. Heating time refers to the time required for heating, and the amount of heat required to heat
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the PCM can be analyzed over time, cooling time means that the PCM can dissipate the heat energy
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it possessed as latent heat to reduce the time required for heating (Krishna et al., 2017).
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An experiment was also conducted to confirm one cycle of heating on and off when heating. This
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was analyzed by dividing the time from the start of heating to the end into two hour periods, and
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Figures 10 and 11 show temperature graphs for 0 2 h and 10 12 h after the start of heating,
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respectively. n-eicosane showed the lowest number of heating times between 0 and 2 hours
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immediately after heating, whereas in the final 10 12 h of heating, n-docosane showed the lowest
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number of heating times. Therefore, the use of Case_n-eicosane is considered to be more effective
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when using only sensible heat, and when using latent heat and sensible heat at the same time,
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Case_n-docosane is more effective. In this experiment, the relative efficiency was examined by
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comparing the heating and cooling times.
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To quantitatively measure the actual heating energy used in heating, the amount of heating energy
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used was derived from the measured power. The amount of electric energy used was measured using
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an electric power measuring device and measured in real time, and the standby power and
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consumption power were measured as 0.3W and 139W, respectively. Standby power refers to the
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amount of power used during cooling time, and the amount of power used is that used during heating
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time. Therefore, the power consumption through the measured cooling time and heating time can be
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calculated using Eq. (1) as follows,
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Wh = ∗ + ∗ ,
(1)
Where Wh is power consumption; and are the heating time and cooling time, respectively; and
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and are the standby power and consumption power. The power used is the instant power
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consumption, which is the same as the standby power and the power used as a result of this
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experiment. Figure 12 shows the numerical values obtained using this equation and the power
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consumption. To compare the calculated value with the actual electric energy used, the total
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operation time of the electric film (15 h) was measured. The calculated power consumption was
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741.92 Wh for ref, 734.99 Wh for Case_n-octadecane, 510.76 Wh for Case_n-eicosane, and 487.64
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Wh for Case_n-docosane, respectively. There were differences of up to 27% between the calculated
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amount of electricity and the measured consumed, and power consumption was decreased in
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Case_n-octadecane, n-eicosane, and n-docosane compared to ref. The power consumption
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measurements were as follows: ref consumed 1030 Wh of power, Case_n-octadecane consumed 943
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Wh, Case_n-eicosane consumed 674 Wh, and Case_n-docosane consumed 595 Wh. Due to the
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shortened heating time from the heat storage effect, Case n-octadecane, Case_n-eicosane, and
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Case_n-docosane showed reductions in power consumption of 9%, 35%, and 43% compared to ref,
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respectively, and this decrease occurred because energy was stored and discharged by latent heat
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through heat storage and phase changes.
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It is also considered that the power consumption decreased because there was a decrease in the
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operation time compared to the total amount of time that it could have been used. Although n-
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docosane is considered to be the most efficient when only power consumption is considered, Figure
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8 shows that only n-eicosane satisfies the cooling range required to provide a comfortable floor
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temperature. Therefore, n-eicosane is considered to be the most efficient, if both power consumption
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and floor comfort temperature are considered, and the results of this analysis are shown in Table 4.
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In this work, there are differences between the results of calculations and measurements; however,
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the tendency is consistent. Nevertheless, differences from predicted results mean that other variables
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are involved, and further studies are thus needed to determine which additional variables should be
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considered when employing a numerical approach. In addition, both thermal comfort and the amount
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of energy used need consideration. Energy can only be said to have been optimized when less energy
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is used and thermal comfort is provided in the human indoor environment.
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4. Conclusions
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In this study, PCM was fabricated in the form of MPPCM with the aim of reducing the amount of
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energy required for heating and improving the heat storage performance of dry floor heating, which
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is mainly used in wooden houses. Organic PCMs, paraffinic n-octadecane, n-eicosane, and n-
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docosane, were employed with respect to their phase change temperatures of 28°C to 35°C.
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Experimental conditions were adapted to measure the heat storage performance relating to latent
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heat according to phase changes and the reduction in the amount of heating energy consumed. Two
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experiments were conducted to derive the results. The results of each experiment are as follows.
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1) Layout test results for the DOWN_Case, where PCM is located lower than in the UP_Case (where
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MPPCM is located on the upper part of the electrical panel), show a short and long heating and
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cooling delay, respectively; therefore, instantaneous heating occurs by minimizing the heat transfer
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time. The results also mean that most DOWN_Case can increase the efficiency of the heating energy.
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Therefore, MPPCM should be constructed by placing the layer at the bottom of the tile, as in the
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DOWN method, and a layer efficiency experiment was performed in this respect for each case.
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2) Case_n-docosane was shown to be the most efficient in the layer efficiency test, and heating and
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cooling times were analyzed through experiments. The efficiency of using sensible heat, sensible
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heat and latent heat simultaneously, and only sensible heat immediately after electric film operation
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was compared and analyzed, and actual usage values were compared. The energy was found to be
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used most efficiently with a shorter (longer) heating (cooling) time. For case n-octadecane, the result
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showed a 9% decrease in power consumption due to the heat transfer of PCM only in the form of
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sensible heat. Case_n-eicosane and Case_n-docosane had melting points of 36.4°C and 41.6°C,
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respectively. The PCM used in this study was found to be capable of storing and discharging heat in
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the form of latent heat, which was reduced heating energy by 43%.
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This study shows that using PCM can reduce the amount of energy consumed in heating buildings,
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and by applying MPPCM, the power consumption can be reduced from 8% to 43%. The largest
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power reduction was seen when using n-docosane in Case_n-docosane. However, n-eicosane is
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considered to be the most efficient when considering both power consumption and the floor comfort
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temperature. Its use can reduce the amount of heating energy required because it improves the
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otherwise poor storage capacity of dry floor heating, which is mainly used in wooden houses, and it
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maintains the maximum temperature of the floor, thereby enabling more efficient use of energy. It is
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considered both practical and feasible to employ n-eicosane in dry floor heating within wooden
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buildings.
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Acknowledgement
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This research was supported by Basic Science Research Program through the National Research
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Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning
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(2016R1A1A1A05921937). This research was supported by the Yonsei University Research Fund of
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2018 (2018-22-0193).
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Lists of Tables and Figures
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Table 1. PCM thermal properties.
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Table 2. Case definition of MPPCM.
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Table 3. Comparison of heating delay and cooling delay of UP and DOWN of each MPPCM.
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Table 4. Experimental results used to determine suitable MPPCM for use in dry floor heating.
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Figure 1. MPPCM preparation process.
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Figure 2. Schematic diagram of dry floor heating composition. Units: [mm]
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Figure 3. Measuring position of thermocouple.
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Figure 4. Comparison between temperatures of UP and DOWN surfaces of ref. sample during
485
heating and cooling.
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Figure 5. Comparison between the UP and DOWN temperatures during heating and cooling
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of each case: (a) case n-octadecane, (b) case n-eicosane, (c) case n-docosane.
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Figure 6. Temperature difference between UP and DOWN for each case.
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Figure 7. Accumulation of temperature difference.
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Figure 8. Melting and solidification processes occurring when heating each case: (a) case n-
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octadecane, (b) case n-eicosane, (c) case n-docosane.
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Figure 9. Comparison between heating and cooling times for each case.
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Figure 10. Temperature change during first two hours.
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Figure 11. Temperature changes between the 10th and 12th hours.
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Figure 12. Calculated and measured energy use of each case.
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Table 1. PCM thermal properties. PCM
Peak melting point(°C)
Latent heat(kJ/kg)
n-octadecane
27.5
244
n-eicosane
36.4
247.3
n-docosane
41.6
243.2
499
Table 2. Case definition of MPPCM. Case
PCM Ref
without PCM
Octa
n-octadecane
Eico
n-eicosane
Doco
n-docosane
Ref
without PCM
Octa DOWN Eico
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n-eicosane
n-docosane
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n-octadecane
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Table 3. Comparison of heating delay and cooling delay of UP and DOWN of each MPPCM. Heating delay (min)
Cooling delay (min)
ref
-
-
n-octadecane
127
7
n-eicosane
17
11
n-docosane
12
4
511 512
28
16
PCM
Energy use (W)
Reduction ratio (%)
Comfortable Time (min)
Order of dry floor heating suitability
n-octadecane
943
9
15
3
n-eicosane
674
35
183
1
n-docosane
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Table 4. Experimental results used to determine suitable MPPCM for use in dry floor heating
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Solid PCM
Liquid PCM
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Put in PCM in Package
Sealing by machine
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Put in PCM in Package
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Figure 1. MPPCM preparation process.
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Heating film
Film shield
(a) Layer component A
A
B
1000
1000
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(c) Three dimensional array
Figure 2. Schematic diagram of dry floor heating composition. Units: [mm].
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(c) Cross section A-A'
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Finish
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Without_PCM_UP Without_PCM_DOWN
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Time(h)
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Figure 4. Comparison between temperatures of UP and DOWN surfaces of ref. sample during
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Octa_UP Octa_DOWN
(a) n-octadecane
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Temperature(℃)
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Eico_UP Eico_DOWN
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(c) n-docosane
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Cooling
Heating 5
10
15
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Time(h)
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Figure 5. Comparison between the UP and DOWN temperatures during heating and cooling of each
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Octa Eico Doco
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Time(h)
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Octa Eico Doco
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Time(h)
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Figure 7. Accumulation of temperature difference.
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P_1 bottom P_2 P_3 P_4 top
44 (a) n-octadecane 42
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comportable zone
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P_1 bottom P_2 P_3 P_4 top
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comportable zone
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comportable zone
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Cooling
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10
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Figure 8. Melting and solidification processes occurring when heating each case: (a) case n-
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5.0
Heating time Cooling time
16 14
4.8
10 8
4.4
6
Time(min)
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4.2
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12
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4.0
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545 546
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Figure 9. Comparison between heating and cooling times for each case.
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Time(h)
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Figure 10. Temperature change during first two hours.
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Ref
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12.0
Figure 11. Temperature changes between the 10th and 12th hours.
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Measured Energy Use(Wh) Calculated Energy Use(Wh)
1000
9%
1030
600
943 741.92
734.99
400 674
510.76
200
595
0
559 560
Octa
Eico
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Doco
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Energy Use(Wh)
35% 800
Figure 12. Calculated and measured energy use of each case.
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HIGHLIGHTS ▶ The energy saving effect was analyzed when applied to the dry floor heating of PCM. ▶ The heating method using electric energy was described and the energy used was expressed
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in kWh. ▶ The Cace_n-Doco thermal ability in dry floor heat is using reduce heating energy by 43%. ▶ The Cace_n-Eico thermal ability in dry floor heat is shown that the heating effect was more
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than 3 hours through latent heat.
S0013-9351(19)30177-X
DOI:
https://doi.org/10.1016/j.envres.2019.03.049
Reference:
YENRS 8411
To appear in:
Environmental Research
Received Date: 30 November 2018 Revised Date:
18 March 2019
Accepted Date: 19 March 2019
Please cite this article as: Yun, B.Y., Yang, S., Cho, H.M., Chang, S.J., Kim, S., Design and analysis of phase change material based floor heating system for thermal energy storage, Environmental Research (2019), doi: https://doi.org/10.1016/j.envres.2019.03.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Design and analysis of phase change material based floor heating
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system for thermal energy storage
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Beom Yeol Yun, Sungwoong Yang, Hyun Mi Cho, Seong Jin Chang, Sumin Kim*
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Department of Architecture and Architectural Engineering, Yonsei University, Seoul 03722,
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Republic of Korea.
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* Corresponding author:
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E-mail: [email protected]
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Abstract
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Pleasant interior space is essential for modern people who spend considerably more time in the
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buildings than they did in the past. To achieve this, one aspect includes an ambient temperature that
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maintains the thermal equilibrium of the human body. The construction of wood framed buildings is
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becoming increasingly popular worldwide, and there have been recent trends toward constructing
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high-rise wooden houses. In this respect, heating methods appropriate for use in wooden buildings
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are being studied. Dry floor heating systems are predominantly used in wooden houses, but they
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provide a poor heat storage performance, which is not conducive to saving energy. In this study, the
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effects of thermal comfort and energy savings were analyzed after applying a phase change material
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(PCM) to floor heating, which can be used to reduce the peak temperature and contribute to energy
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savings. To enable shape stabilization, this study used Macro-Packed PCM (MPPCM), as shape
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stabilization is necessary when applying PCM. The heat storage performance was improved by
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applying MPPCM to a dry floor heating system. Paraffin-based PCMs, such as n-octadecane, n-
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eicosane, and n-docosane, were used to obtain a comfortable floor temperature range. Experimental
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temperatures ranged from 28°C to 35°C, with an entire temperature range of 7°C. Experimental
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results showed that the heat storage performance of MPPCM reduced the amount of energy used for
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heating by 43%, and n-eicosane was the most effective PCM for use in floor heating with respect to
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obtaining a comfortable floor temperature.
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Keywords
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Phase change materials; Heat storage; Latent heat; Dry floor heating system; Power
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consumption; Energy saving
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1. Introduction
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People spend a considerable more time in buildings nowadays than they did in the past
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(approximately 90% of time) (López-Pérez et al., 2019), and the internal comfort of a building has
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thus become more important (WHO, 1989; Park et al., 2016; Singh et al., 2010). The demand for
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electric power used in heating and cooling has recently increased dramatically throughout the world,
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and governments are strengthening policies to reduce the energy consumption of buildings.
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Therefore, research and development on efficient building energy systems are being conducted, and
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in particular, research relating to phase change materials (PCMs) is being actively conducted
45
globally with respect to its use in reducing the heating and cooling energy load of buildings. Phase-
46
change materials undergo phase changes with temperature and can accumulate and emit thermal
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energy by using latent heat when the phase changes from solid to liquid or from liquid to solid.
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Because latent heat has a better energy storage capacity than sensible heat, it saves heat and energy
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used in buildings more efficiently (Lee et al., 2017).
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Many studies have been conducted with the aim of achieving energy saving in buildings. For
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example, a simulation program conducted by Mi et al. (Mi et al., 2016) provided a 10% saving in
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heating energy, and the study of Lei et al. (Lei et al., 2016) enabled a reduction in the cooling load
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within the tropical climate of Singapore. Another study analyzed the effect of reducing the peak
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temperature using commercially available PCM products (Berardi and Soudian, 2019), and one
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study analyzed the reduction in cooling energy demand in a desert climate (where the demand for
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cooling energy is extreme) by using PCM within the walls (Hasan et al., 2018). In this regard,
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applying PCM to the buildings is the issue because it can reduce the building energy in various ways.
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Furthermore, studies are being conducted to reduce CO2 emissions, which are the main cause of
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global warming worldwide (Kahouli, 2018). It has been shown that the use of wood as a building
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material causes less CO2 to be released than when producing concrete (Nässén et al., 2012), and
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wooden houses using environmentally friendly and energy-efficient lightweight wooden building
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techniques are thus attracting attention. A technique for building high-rise wooden buildings has
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been developed based on the development of structural timber such as cross-laminated timber (CLT)
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(Brandner et al., 2016), and research on the construction of high-rise wooden buildings is being
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actively conducted globally (Ramage et al., 2017).
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In conventional floor heating, it is necessary to use three components: insulation, lightweight foam
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concrete, and mortar. However, when we consider the wooden buildings, there are few concerns
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about applying the conventional floor heating system (Shin et al., 2015). Therefore, the timber
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houses require a suitable type of heating system: simple to use and install lighter and proper heights.
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In a wooden building, it is thus preferable to use light dry floor heating system instead of wet floor
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heating for several reasons. The dry floor heating provides advantages in that there is a lighter load
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applied to the building than when using wet floor heating. Furthermore, this method does not need a
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thicker layer to fix heating, pipes and it takes more story heights. This is why Most wooden
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buildings adopt the electric panel heating system that meets the conditions; lighter and thinner.
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However, with dry floor heating, there are no thermal-energy storage media, such as mortar. For this
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reason, the dry floor heating system would more energy when the heating period.
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Therefore, this study applies PCM, which has an excellent heat storage performance, to the electric
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panel heating. The applied PCM types are divided to their melting temperature. Also, PCMs were
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fabricated macro-packed PCM (MPPCM) (Chang et al., 2017). To confirm the optimal thermal
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performance of the heating system, the power saving effect relating to MPPCM was analyzed by
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changing its composition.
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Studies on the use of PCM and the stabilization of phases have been conducted with respect to the
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inherent problems with PCM when applied to buildings: leakage in a liquid state and weakening due
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to phase changes. There are four ways to apply PCM to buildings. The first is to directly insert PCM
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into building materials; for example, a metal foam can be immersed in a PCM to form a PCM inside
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(Zhu et al., 2018), PCM can be placed in a concrete slab to reduce the peak temperature (Navarro et
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al., 2015), and free-cooling can be attained by applying PCM to lightweight buildings (Rouault et al.,
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2013). Second, PCM can be impregnated into pores by vacuuming building materials; for example,
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it has been applied to the gypsum board by vacuum impregnation (Jeong et al., 2016b). In addition,
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xGnP porous material has been impregnated with n-hexadecane and applied to concrete to enhance
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the thermal performance (Kim et al., 2014), and the thermal performance has been improved by
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impregnating expanded graphite with PCM (Ren et al., 2018; Tian et al., 2016; Zhong et al., 2014).
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Third, PMC can be inserted into small capsules, and studies on types of microencapsulated PCM and
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methods of applying it are progressing (Shannaq and Farid, 2015). For example, Alam et al. (2015)
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studied the phase stabilization of PCM through polymer coating; research has been conducted to
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stabilize PCM by wrapping it in silica, (Zhang et al., 2018); and studies using graphite as a phase
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stabilizing material have been conducted (Cui et al., 2015a). Finally, PCM can be packed into a
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packaging container in a process known as macro-packing. Macro-PCM has been applied to a roof to
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reduce the heat island phenomenon in an urban area (Yang et al., 2017), and thermal analysis of
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PCM in a processed spherical metal has been conducted (Cui et al., 2017). Furthermore, numerical
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simulations have been conducted with respect to the microencapsulation of mechanical properties
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(Cui et al., 2015b), and one study evaluated the thermal performance of a wood frame house by
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applying macroencapsulated PCM (Chang et al., 2017).
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Of these methods of use, the MPPCM method uses more PCM than the other methods. Of the
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characteristics of PCM, its latent heat capacity has the greatest effect on its energy storage
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performance. In methods other than MPPCM, only 80% of the PCM can actually be used; therefore,
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the amount of latent heat is relatively decreased (Huang et al., 2019). In contrast, the MPPCM
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method uses the most latent heat (approximately 98%), and it thus has a greater storage effect than
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other methods.
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Therefore, in this paper, PCMs were fabricated macro-packed PCM (MPPCM) (Chang et al., 2017).
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To confirm the optimal thermal performance of the heating system, the power saving effect relating
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to MPPCM was analyzed by changing its composition.
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2. Methodology
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2.1. Materials
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PCM is a substance that undergoes phase changes; it releases heat by accumulating or storing heat
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through physical changes (not chemical changes). There are three types of phase change materials:
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organic, inorganic, and eutectic (Jeong et al., 2016a; Vicente and Silva, 2014). Organic PCMs are
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chemically stable, and although they do not cause any phase separation or supercooling phenomena
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(examples are paraffinic PCMs, such as n-hexadecane, n-octadecane, and n-eicosane). Inorganic
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PCMs are comprised of a salt or metal salt, such as calcium chloride or sodium sulfate, and provide a
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high amount of latent heat per unit volume compared to organic PCMs; however, they can be
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affected by phase changes due to corrosion or supercooling (Safari et al., 2017), which is a distinct
125
disadvantage. Eutectic PCMs are composed of a mixture of two or more components and have a
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rapid melting point, which makes it easy for phase changes to occur at particular temperatures.
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However, there is a current lack of basic research conducted on their thermal behavior and function.
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It is necessary to first select a PCM that provides an appropriate phase change temperature, prior to
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using it in a building. As PCMs use latent heat during phase changes, a phase change interval must
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be applied to fully provide the heat storage performance through latent heat. In addition, as the
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structure can be applied to the outer wall, inner wall, the ceiling, or (in the case of this study) to the
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floor, it is necessary to select an appropriate PCM that can provide the required temperature and
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phase change. For the floor of a building, this temperature range is 28–35°C (Kim, 1993). Therefore,
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the suitable phase change materials in this respect, according to the required phase change
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temperature, are paraffin-based n-octadecane (PARAFOL 18-97), n-eicosane (PARAFOL 20Z), and
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n-docosane (PARAFOL 22-95). These were thus used as the PCMs in the experiments in this study,
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and all PCMs used were obtained from Celsius Korea, South Korea. The properties of each PCM are
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described in Table 1.
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2.2. Preparation
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In manufacturing such MPPCM, the material for packaging the PCM should have excellent
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durability, resistance to corrosion, and minimal transformation in response to temperature changes.
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This study secured the phase stabilization and usability of PCM by employing nylon packaging in
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the MPPCM method. The manufacturing method and procedure used are shown in Figure 1 and are
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as follows. First, the PCM was heated to 60°C to allow it to completely melt, and 180 g of liquid
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PCM was then injected. The inside of the nylon package was then vacuum sealed using a vacuum
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packaging machine. The nylon package was divided into three compartments using a vacuum
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packaging machine to equalize the absorption and release of heat energy by splitting the PCM
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uniformly (without allowing it to deviate to one side). Finally, to make MPPCM of the same size and
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thickness, the MPPCM was fabricated by cutting the upper part to a size of 200 mm × 200 mm × 8
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mm (length × width × thickness) (Chang et al., 2017).
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2.3 Experimental set-up
In this study, the temperature of the electric panel was controlled and monitored via a temperature
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controller sensor; when the electric panel reached the target temperature, it was automatically turned
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off and the setback temperature was set to 7°C.
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Figure 2 shows the settings used in the experimental environment, which was a real environment
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where the floor was actually heated using the electric panel designed. The amount of heat released to
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the outside was reduced by using insulation, and the film was protected from damage by installing a
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film protection plate. A finishing material was also employed to ensure that the floor heat was spread
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evenly. In both experiments, the electric panel floor heating was constructed in the order of: heat
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insulation, electrical panel, film protection plate, and floor finish. The temperature was measured
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using a thermocouple sensor as shown in Figure 3, and data were recorded using a data logger (GL-
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840). Experimental conditions were set to heat for 12 h at a target temperature of 35°C, and the
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setback temperature was 28°C. The PCM was divided into an upper and lower part, both of which
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had contact with the electric film as a heat source.
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A second experiment was conducted to select which PCM provided the optimal performance in the
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electric panel heating system based on the results of the first experiment. Experimental conditions
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were based on the surface temperature of the layer at 28–35°C, which is a comfortable floor
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temperature range (Kim, 1993). In the experiment, the heating operation time was set to 12 h in each
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case, based on an average room residential time of 12 h, which was obtained from data of the Korean
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Statistical Information Service (KOSIS). The amount of electricity consumed was compared in
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experiments. In both experiments, the temperature of the experimental space was maintained at 25°C,
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and the temperature at the top and bottom of the heating film layer was simultaneously measured
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using the temperature sensor.
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3. Results and Discussion
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3.1. Experiments to determine layer configuration An experiment was conducted to determine the best layer composition, different experiments were
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conducted for the various cases shown in Table 2, and the temperature of the MPPCM was measured
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at the top of the electric panel (UP_Case) and at the bottom (DOWN_Case). Table 3 shows the time
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delay of the UP_Case based on the DOWN_Case. The delay in time until the lower temperature limit
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of 28°C was obtained is defined as the heating delay, and the cooling delay is the delay in time until
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a temperature of 28°C was reached during non-heating. Figure 5 shows the temperature changes
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during 20 h for UP and DOWN of Octadecane (Case_n-octadecane), Eicosane (Case_n-eicosane),
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and Docosane (Case_n-docosane), respectively. Figure 4 shows results for the sample without PCM
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(ref), where there is no temperature difference between the upper and lower parts of the electrical
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panel, and the temperature changes are the same during heating and cooling, which shows that the
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thermal energy applied to the top and bottom of the panel is the same. The top graph in Figure 5
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shows the temperature change for Case_n-octadeane. The heating delay was increased by 127 min,
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and the cooling delay was decreased by 7 min. For the cases of n-octadecane_UP and n-
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octadecane_DOWN, it is possible to confirm the phase change interval at the beginning of heating,
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and the difference in surface temperature at the time when the phase change occurred, which shows
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that heat was transferred to the upper surface when the phase change occurred. In n-
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octadecane_DOWN, the surface temperature reached a maximum of 33.4°C, even during the phase
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change, whereas n-octadecane_UP did not reach 28°C. It can be seen that the MPPCM based on n-
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octadecane stores heat in the form of latent heat through the phase change, and the time taken to
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reach a comfortable temperature range is delayed when MPPCM is located at the upper part of the
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electric panel. The middle graph in Figure 5 shows the temperature change of Case_n-eicosane. The
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heating delay was increased by 17 min, and the cooling delay was decreased by 11 min. For n-
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eicosane_UP, the surface temperature continuously increased from 30°C, while it was maintained for
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n-eicosane_Down after reaching 35°C. The phase change zone was confirmed during cooling, and
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the surface temperature was maintained at 29°C for more than five hours. The temperature change
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graph of Case_n-docosane is shown in the bottom graph of Figure 5. The heating delay was
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increased by 12 min, and the cooling delay was decreased by 4 min. In addition, there was no phase
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change interval, because the temperature of the electrical panel did not reach the melting point of n-
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docosane. It was thus confirmed that heat was stored and transferred only in the form of sensible
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heat. This experiment confirms that the DOWN_Case was faster than the UP_Case with respect to
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the time taken to reach a comfortable temperature range. The surface temperature difference between
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the UP_Case and the DOWN_Case during heating is plotted in Figure 6, according to time. The
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maximum temperature difference is measured to be 7.3°C during heating, and the temperature
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difference is approximately 1°C during cooling (these differences are shown cumulatively in Figure
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7 to analyze the heat energy). In all cases, there was a continuous increase in the accumulated
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temperature difference during heating. However, the greatest increase was found for Case_n-
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octadecane, which had the largest temperature difference, and the smallest increase was seen in
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Case_n-docosane, which had no phase change interval. It is considered that an increase in the
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accumulated temperature difference indicates a more efficient use of thermal energy, this means that
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they can be kept at a higher temperature when the same energy is used. In summary, an increase in
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the accumulated temperature difference shows an equivalent efficiency that results in less energy
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being used. Therefore, these experiments prove that forming a layer by placing the PCM under the
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electric panel reduces the heating delay, shortens the time taken to reach a comfortable temperature,
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and increases energy efficiency by increasing the surface temperature. It is thus evident that the most
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efficient positioning of MPPCM is to construct the layer under the electric panel. It is also shown
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that the PCM needs to be located away from the direction in which heat is to be transferred, and this
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will be evidenced in further studies related to heat transfer. However, if the PCM is not located close
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enough to the heat source, as in the present experiment, it is necessary to study the complex concept
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of heat transfer and heat preservation when heat is to be conserved (de Gracia and Cabeza, 2015).
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3.2. Experiment to determine efficiency during and after heating As a result of the experiment described in the previous section, the MPPCM was placed under the
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electric panel. Experiments were performed under the same conditions as in the previous experiment,
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and the measurement results for a total of 20 h are shown in Figure 8. Analysis was conducted
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during heating and after heating; first, the heating time during heating and the cooling time during
235
heating were analyzed, and experimental results are shown in Figure 9, where the heating time
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shown in the graph represents the time taken to reach the set temperature of 35°C after heating, and
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the cooling time represents the setback temperature of 28°C after cooling. A comparison of heating
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times shows that ref was 4.69 min and Case_n-docosane was 4.16 min. This comparative reduction
239
in heating time means that less energy required for heating is consumed through the heat storage
240
performance of the PCM. Therefore, n-docosane-based MPPCM, which has the shortest heating time,
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is considered to be the most effective for use when heating. The cooling time of ref was measured as
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8.54 min, and the shortest cooling time of 7.79 min was recorded for Case_n-octadecane. The
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longest cooling time was 13.82 min for Case_n-docosane. The reason why the cooling time of
244
Case_n-octadecane is shorter than that of ref is that the PCM n-octadecane used in Case_n-
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octadecane has a melting point of 27.5°C, and it is considered that the energy behavior is only in the
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form of sensible heat. Case_n-docosane shows a 62% increase in cooling time compared to ref, and
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the increase in cooling time indicates that PCM uses stored energy in the form of latent heat through
248
phase change. Therefore, MPPCM based on n-docosane is considered to be most effective for
249
cooling, because it has the longest cooling time. In this work, we conducted a different analysis from
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the ones that did heat analysis after non-heating followed by heating. In previous studies, the thermal
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effects of PCM liquification and solidification were analyzed one-cycle after the end of heating
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(Panayiotou et al., 2016). Moreover, in the same way, as in the previous study, it was performed
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thermal analysis while heating was in progress (Novais et al., 2015). However, previous studies have
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shown that PCM is an inadequate method for evaluating the overall thermal performance of PCM,
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including the specific heat and latent heat. Therefore, this study analyzed the effect of PCM during
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heating. Heating time refers to the time required for heating, and the amount of heat required to heat
257
the PCM can be analyzed over time, cooling time means that the PCM can dissipate the heat energy
258
it possessed as latent heat to reduce the time required for heating (Krishna et al., 2017).
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An experiment was also conducted to confirm one cycle of heating on and off when heating. This
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was analyzed by dividing the time from the start of heating to the end into two hour periods, and
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Figures 10 and 11 show temperature graphs for 0 2 h and 10 12 h after the start of heating,
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respectively. n-eicosane showed the lowest number of heating times between 0 and 2 hours
263
immediately after heating, whereas in the final 10 12 h of heating, n-docosane showed the lowest
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number of heating times. Therefore, the use of Case_n-eicosane is considered to be more effective
265
when using only sensible heat, and when using latent heat and sensible heat at the same time,
266
Case_n-docosane is more effective. In this experiment, the relative efficiency was examined by
267
comparing the heating and cooling times.
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To quantitatively measure the actual heating energy used in heating, the amount of heating energy
269
used was derived from the measured power. The amount of electric energy used was measured using
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an electric power measuring device and measured in real time, and the standby power and
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consumption power were measured as 0.3W and 139W, respectively. Standby power refers to the
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amount of power used during cooling time, and the amount of power used is that used during heating
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time. Therefore, the power consumption through the measured cooling time and heating time can be
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calculated using Eq. (1) as follows,
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(1)
Where Wh is power consumption; and are the heating time and cooling time, respectively; and
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and are the standby power and consumption power. The power used is the instant power
278
consumption, which is the same as the standby power and the power used as a result of this
279
experiment. Figure 12 shows the numerical values obtained using this equation and the power
280
consumption. To compare the calculated value with the actual electric energy used, the total
281
operation time of the electric film (15 h) was measured. The calculated power consumption was
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741.92 Wh for ref, 734.99 Wh for Case_n-octadecane, 510.76 Wh for Case_n-eicosane, and 487.64
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Wh for Case_n-docosane, respectively. There were differences of up to 27% between the calculated
284
amount of electricity and the measured consumed, and power consumption was decreased in
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Case_n-octadecane, n-eicosane, and n-docosane compared to ref. The power consumption
286
measurements were as follows: ref consumed 1030 Wh of power, Case_n-octadecane consumed 943
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Wh, Case_n-eicosane consumed 674 Wh, and Case_n-docosane consumed 595 Wh. Due to the
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shortened heating time from the heat storage effect, Case n-octadecane, Case_n-eicosane, and
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Case_n-docosane showed reductions in power consumption of 9%, 35%, and 43% compared to ref,
290
respectively, and this decrease occurred because energy was stored and discharged by latent heat
291
through heat storage and phase changes.
292
It is also considered that the power consumption decreased because there was a decrease in the
293
operation time compared to the total amount of time that it could have been used. Although n-
294
docosane is considered to be the most efficient when only power consumption is considered, Figure
295
8 shows that only n-eicosane satisfies the cooling range required to provide a comfortable floor
296
temperature. Therefore, n-eicosane is considered to be the most efficient, if both power consumption
297
and floor comfort temperature are considered, and the results of this analysis are shown in Table 4.
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In this work, there are differences between the results of calculations and measurements; however,
299
the tendency is consistent. Nevertheless, differences from predicted results mean that other variables
300
are involved, and further studies are thus needed to determine which additional variables should be
301
considered when employing a numerical approach. In addition, both thermal comfort and the amount
302
of energy used need consideration. Energy can only be said to have been optimized when less energy
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is used and thermal comfort is provided in the human indoor environment.
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4. Conclusions
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In this study, PCM was fabricated in the form of MPPCM with the aim of reducing the amount of
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energy required for heating and improving the heat storage performance of dry floor heating, which
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is mainly used in wooden houses. Organic PCMs, paraffinic n-octadecane, n-eicosane, and n-
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docosane, were employed with respect to their phase change temperatures of 28°C to 35°C.
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Experimental conditions were adapted to measure the heat storage performance relating to latent
311
heat according to phase changes and the reduction in the amount of heating energy consumed. Two
312
experiments were conducted to derive the results. The results of each experiment are as follows.
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1) Layout test results for the DOWN_Case, where PCM is located lower than in the UP_Case (where
314
MPPCM is located on the upper part of the electrical panel), show a short and long heating and
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cooling delay, respectively; therefore, instantaneous heating occurs by minimizing the heat transfer
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time. The results also mean that most DOWN_Case can increase the efficiency of the heating energy.
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Therefore, MPPCM should be constructed by placing the layer at the bottom of the tile, as in the
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DOWN method, and a layer efficiency experiment was performed in this respect for each case.
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2) Case_n-docosane was shown to be the most efficient in the layer efficiency test, and heating and
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cooling times were analyzed through experiments. The efficiency of using sensible heat, sensible
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heat and latent heat simultaneously, and only sensible heat immediately after electric film operation
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was compared and analyzed, and actual usage values were compared. The energy was found to be
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used most efficiently with a shorter (longer) heating (cooling) time. For case n-octadecane, the result
324
showed a 9% decrease in power consumption due to the heat transfer of PCM only in the form of
325
sensible heat. Case_n-eicosane and Case_n-docosane had melting points of 36.4°C and 41.6°C,
326
respectively. The PCM used in this study was found to be capable of storing and discharging heat in
327
the form of latent heat, which was reduced heating energy by 43%.
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This study shows that using PCM can reduce the amount of energy consumed in heating buildings,
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and by applying MPPCM, the power consumption can be reduced from 8% to 43%. The largest
330
power reduction was seen when using n-docosane in Case_n-docosane. However, n-eicosane is
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considered to be the most efficient when considering both power consumption and the floor comfort
332
temperature. Its use can reduce the amount of heating energy required because it improves the
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otherwise poor storage capacity of dry floor heating, which is mainly used in wooden houses, and it
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maintains the maximum temperature of the floor, thereby enabling more efficient use of energy. It is
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considered both practical and feasible to employ n-eicosane in dry floor heating within wooden
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buildings.
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Acknowledgement
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This research was supported by Basic Science Research Program through the National Research
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Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning
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(2016R1A1A1A05921937). This research was supported by the Yonsei University Research Fund of
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2018 (2018-22-0193).
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Lists of Tables and Figures
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Table 1. PCM thermal properties.
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Table 2. Case definition of MPPCM.
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Table 3. Comparison of heating delay and cooling delay of UP and DOWN of each MPPCM.
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Table 4. Experimental results used to determine suitable MPPCM for use in dry floor heating.
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Figure 1. MPPCM preparation process.
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Figure 2. Schematic diagram of dry floor heating composition. Units: [mm]
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Figure 3. Measuring position of thermocouple.
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Figure 4. Comparison between temperatures of UP and DOWN surfaces of ref. sample during
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heating and cooling.
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Figure 5. Comparison between the UP and DOWN temperatures during heating and cooling
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of each case: (a) case n-octadecane, (b) case n-eicosane, (c) case n-docosane.
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Figure 6. Temperature difference between UP and DOWN for each case.
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Figure 7. Accumulation of temperature difference.
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Figure 8. Melting and solidification processes occurring when heating each case: (a) case n-
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octadecane, (b) case n-eicosane, (c) case n-docosane.
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Figure 9. Comparison between heating and cooling times for each case.
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Figure 10. Temperature change during first two hours.
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Figure 11. Temperature changes between the 10th and 12th hours.
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Figure 12. Calculated and measured energy use of each case.
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Table 1. PCM thermal properties. PCM
Peak melting point(°C)
Latent heat(kJ/kg)
n-octadecane
27.5
244
n-eicosane
36.4
247.3
n-docosane
41.6
243.2
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Table 2. Case definition of MPPCM. Case
PCM Ref
without PCM
Octa
n-octadecane
Eico
n-eicosane
Doco
n-docosane
Ref
without PCM
Octa DOWN Eico
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n-eicosane
n-docosane
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n-octadecane
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Table 3. Comparison of heating delay and cooling delay of UP and DOWN of each MPPCM. Heating delay (min)
Cooling delay (min)
ref
-
-
n-octadecane
127
7
n-eicosane
17
11
n-docosane
12
4
511 512
28
16
PCM
Energy use (W)
Reduction ratio (%)
Comfortable Time (min)
Order of dry floor heating suitability
n-octadecane
943
9
15
3
n-eicosane
674
35
183
1
n-docosane
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Table 4. Experimental results used to determine suitable MPPCM for use in dry floor heating
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∆t
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Solid PCM
Liquid PCM
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Put in PCM in Package
Sealing by machine
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Put in PCM in Package
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Figure 1. MPPCM preparation process.
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Heating film
Film shield
(a) Layer component A
A
B
1000
1000
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(c) Three dimensional array
Figure 2. Schematic diagram of dry floor heating composition. Units: [mm].
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(c) Cross section A-A'
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Finish
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Figure 3. Measuring position of thermocouple.
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Without_PCM_UP Without_PCM_DOWN
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Temperature(℃)
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Time(h)
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heating and cooling.
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Figure 4. Comparison between temperatures of UP and DOWN surfaces of ref. sample during
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Octa_UP Octa_DOWN
(a) n-octadecane
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Temperature(℃)
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Eico_UP Eico_DOWN
(b) n-eicosane
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(c) n-docosane
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Cooling
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10
15
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Time(h)
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Figure 5. Comparison between the UP and DOWN temperatures during heating and cooling of each
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case: (a) case n-octadecane, (b) case n-eicosane, (c) case n-docosane.
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Octa Eico Doco
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Figure 6. Temperature difference between UP and DOWN for each case.
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Octa Eico Doco
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Sum of temperature difference / S? T
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Time(h)
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Figure 7. Accumulation of temperature difference.
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P_1 bottom P_2 P_3 P_4 top
44 (a) n-octadecane 42
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comportable zone
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P_1 bottom P_2 P_3 P_4 top
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Temperature(℃)
3 hours
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comportable zone
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Cooling P_1 bottom P_2 P_3 P_4 top
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comportable zone
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Cooling
Heating 5
10
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Figure 8. Melting and solidification processes occurring when heating each case: (a) case n-
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Heating time Cooling time
16 14
4.8
10 8
4.4
6
Time(min)
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4.2
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Time(min)
12
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4.0
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545 546
Octa
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Figure 9. Comparison between heating and cooling times for each case.
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Time(h)
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Figure 10. Temperature change during first two hours.
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Ref
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12.0
Figure 11. Temperature changes between the 10th and 12th hours.
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Time(h)
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Measured Energy Use(Wh) Calculated Energy Use(Wh)
1000
9%
1030
600
943 741.92
734.99
400 674
510.76
200
595
0
559 560
Octa
Eico
487.64
Doco
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Energy Use(Wh)
35% 800
Figure 12. Calculated and measured energy use of each case.
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HIGHLIGHTS ▶ The energy saving effect was analyzed when applied to the dry floor heating of PCM. ▶ The heating method using electric energy was described and the energy used was expressed
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in kWh. ▶ The Cace_n-Doco thermal ability in dry floor heat is using reduce heating energy by 43%. ▶ The Cace_n-Eico thermal ability in dry floor heat is shown that the heating effect was more
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than 3 hours through latent heat.