materials Article
Design and Preparation of Carbon Based Composite Phase Change Material for Energy Piles Haibin Yang 1 , Shazim Ali Memon 2 , Xiaohua Bao 1, *, Hongzhi Cui 1 and Dongxu Li 1 1 2
*
College of Civil Engineering, Shenzhen University, Shenzhen 518060, China;
[email protected] (H.Y.);
[email protected] (H.C.);
[email protected] (D.L.) Department of Civil Engineering, School of Engineering, Nazarbayev University, Astana 010000, Kazakhstan;
[email protected] Correspondence:
[email protected]; Tel.: +86-755-8667-0364
Academic Editor: Thomas Fiedler Received: 26 January 2017; Accepted: 5 April 2017; Published: 7 April 2017
Abstract: Energy piles—A fairly new renewable energy concept—Use a ground heat exchanger (GHE) in the foundation piles to supply heating and cooling loads to the supported building. Applying phase change materials (PCMs) to piles can help in maintaining a stable temperature within the piles and can then influence the axial load acting on the piles. In this study, two kinds of carbon-based composite PCMs (expanded graphite-based PCM and graphite nanoplatelet-based PCM) were prepared by vacuum impregnation for potential application in energy piles. Thereafter, a systematic study was performed and different characterization tests were carried out on two composite PCMs. The composite PCMs retained up to 93.1% of paraffin and were chemically compatible, thermally stable and reliable. The latent heat of the composite PCM was up to 152.8 J/g while the compressive strength of cement paste containing 10 wt % GNP-PCM was found to be 37 MPa. Hence, the developed composite PCM has potential for thermal energy storage applications. Keywords: composite phase change materials; expanded graphite; graphite nanoplatelet; energy storage; energy piles
1. Introduction The rapid increase in global energy consumption has led to serious issues such as depletion of fossil fuels and degradation of the environment [1,2]. According to statistics, the world’s energy consumption will grow by 48% between 2012 and 2040 [3]. Hence, energy policy makers and researchers are paying a lot of attention to the building sector as it is responsible for around 30% of the total global energy consumption [4,5]. Energy piles—A fairly new renewable energy technique—use a ground heat exchanger (GHE) in the foundation piles to supply heating and cooling loads to the supported building. Figure 1 is a schematic drawing of energy piles application in building energy efficiency. Geothermal energy can sustainably be utilized with a ground-source heat pump, which takes advantage of the ground as an energy storage system [6,7]. In the energy piles system, the piles are used to absorb and transport thermal energy from the surrounding ground to buildings via fluid circulating in pipes placed within the piles. In fact, the thermal cycle can influence the loading of energy piles. Figure 2 displays the effect of thermal cycles of the energy piles system on pile stresses. Many studies have been carried out to investigate the geotechnical performance of piled foundations for ground-source heat-pump systems [8,9]. The main effects of temperature changes on pile behavior were assessed by the geotechnical numerical analysis method [10]. Applying thermal loads could induce a significant change in the stress–strain state of piles and the temperature change over the cross section of the piles should be designed to avoid high stress accumulated in piles [11,12]. Phase change materials are promising thermal energy shortage candidates that can be used to reduce the possible Materials 2017, 10, 391; doi:10.3390/ma10040391
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mismatch between thermal energy supply and demand. Applying phase change materials (PCMs) to the possible mismatch between thermal energy supply and demand. Applying phase change piles help inmismatch maintaining stable temperature withintemperature the and can then influence the axial thecan possible thermal energya supply and piles demand. Applying phase change materials (PCMs) to pilesbetween can ahelp in maintaining stable within the piles and can then load acting the on the PCMs, paraffin usually preferred it the is generally believed materials (PCMs) toload pilesAmong can help inthe maintaining aisstable temperature piles and can influence axialpiles. acting on piles. Among PCMs, paraffinwithin isasusually preferred asthen it is to begenerally chemically inert, show small volume changes during phase transition, innocuous, influencebelieved the axialnon-corrosive, load acting on the piles. Among PCMs, paraffin is usually preferred as it is to be chemically inert, non-corrosive, show small volume changes during phase generally believed to be chemically inert, non-corrosive, show small volume changes during phase inexpensive, and recyclable. However, it has low thermal conductivity, which in turn, limits transition, innocuous, inexpensive, and recyclable. However, it has low thermal conductivity, which its inexpensive, and recyclable. However, application ininnocuous, thermal energy storage [13]. intransition, turn, limits its application in thermal energy storage [13]. it has low thermal conductivity, which in turn, limits its application in thermal energy storage [13].
Figure1.1.Schematic Schematic drawing of of energy piles application ininbuilding energy efficiency [6]. Figure pilesapplication applicationin building energy efficiency Figure 1. Schematicdrawing drawing of energy energy piles building energy efficiency [6]. [6].
(a) (a)
(b) (b)
Figure piles system system on on pile pile stresses stresses[6]. [6].(a) (a)Ground Groundcooling cooling Figure2.2.Effect Effect of of thermal thermal cycles cycles of of the the energy energy piles Figure 2. Effect of thermal cycles of the energy piles system on pile stresses [6]. (a) Ground cooling reduces which can can cause cause tensile tensile stresses stressesininthe thepiles; piles; reduces stresses stresses along along the the cross cross section section of the piles which reduces stresses along the cross section of the piles which can cause tensile stresses in the piles; (b) section of of the thepiles. piles. (b)Heating Heatingcan cancause causeincreased increased stresses stresses along the cross section (b) Heating can cause increased stresses along the cross section of the piles.
Expanded (GNPs) are aresafe, safe,environmentally environmentallyfriendly, friendly, Expandedgraphite graphite(EG) (EG) and and graphene graphene nanoplatelets nanoplatelets (GNPs) Expanded graphite (EG) andthermal graphene nanoplatelets (GNPs) are safe, environmentally friendly, have low and conductivity. These are preferred preferred over metalmacro-scaled macro-scaled have lowdensity density and superior superior thermal These are over metal have low density and superior thermal conductivity. These are preferred over metal macro-scaled promoters, which have been used to increase PCMs thermal conductivity [14]. Sari and Karaipekli [15] promoters, which have been used to thermal conductivity [14]. Sari and Karaipekli [15] preparedwhich paraffin/expanded graphite PCM and that PCM 10 prepared paraffin/expanded graphite composite and found found that composite composite PCM with 10wt wt%%[15] promoters, have been used to increase PCMs thermal conductivity [14]. Sari andwith Karaipekli EGisisparaffin/expanded suitablethermal thermal energy energy storage With this mass fraction of composite, the ofofEG aasuitable storage candidate. With this massthat fraction ofEG EGin in composite, the prepared graphite composite PCM and found composite PCM with 10 wt % thermal conductivity improved bystorage approximately 273%With in pure etetal. improved by approximately in comparison comparison tofraction pureparaffin. paraffin. Wang al.[16] [16]the of thermal EG is a conductivity suitable thermal energy candidate. this massto of EGWang in composite, incorporated 90 wt wtimproved % of of polyethylene polyethylene expanded graphite that the incorporated 90 % glycol into expanded graphite and found thatWang thethermal thermal thermal conductivity by approximately 273% in comparison toand purefound paraffin. et al. [16] −1·K−1 −1−1 −1−1 −1 conductivity of polyethylene glycol (0.2985 W·m ) improved to 1.324 W·m ·K . It was shown conductivity90 of wt polyethylene glycol improved to 1.324and W·m ·K .that It was shown incorporated % of polyethylene glycol into expanded graphite found the thermal thatthe thecomposite composite PCM PCM is is aa promising promising candidate for latent heat storage applications. Xia etet al. − 1 − 1 − 1 − 1 that latent heat storage applications. Xia al. [17] conductivity of polyethylene glycol (0.2985 W·m ·K ) improved to 1.324 W·m ·K . [17] It was showedthat that by by incorporating incorporating 10 10 wt wt % of EG into paraffin, the composite PCM showed aa10-fold showed paraffin, the composite PCM showed 10-fold shown that the composite PCM is a promising candidate for latent heat storage applications. increasein in thermalconductivity conductivity over over pure pure paraffin. paraffin. Mill et thermal et al. al. [18] [18]improved improvedthe thermalconductivity conductivity Xiaincrease et al. [17]thermal showed that by incorporating 10 wt %Mill of EG into paraffin, thethe composite PCM showed paraffinby bytwo twoorders orders of of magnitude magnitude using using porous porous graphite ofofparaffin graphite matrices matriceswith withparaffin. paraffin.Zeng Zengetetal. al.[19] [19] a 10-fold increase in thermal composite conductivity over pure paraffin. Mill et al. [18]toimproved the thermal prepared Tetradecanol/EG PCM and showed that in comparison pure tetradeconal prepared Tetradecanol/EG composite PCM and showed that in comparison to pure tetradeconal conductivity paraffin two orders of magnitude using porous graphite matrices with paraffin. −1), the by (0.433 W·m W·mof ·K thermal conductivity of −1−1 −1 (0.433 ·K ), the thermal conductivity of the the sample sample with with 77 wt wt % % of of EG EG increased increased toto Zeng et al. [19] prepared Tetradecanol/EG composite PCM and showed that in comparison to pure tetradeconal (0.433 W·m−1 ·K−1 ), the thermal conductivity of the sample with 7 wt % of EG increased
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to 2.76 W·m−1 ·K−1 . The optimum weight percentage of EG in Tetradecanol/EG composite PCM was suggested as 20 wt %. In the recent past, graphene nanoplatelets (GNPs), which have high thermal conductivity and specific surface area, have been used and found to be suitable for PCM applications [20]. In order to enhance the composite PCM’s thermal conductivity, Mehrali et al. [21] used GNPs with different surface areas in palmitic acid (PA). The maximum percentage of PA absorbed by GNPs without any sign of leakage was found as 91.94 wt %. Moreover, GNPs with a surface area of 750 m2 /g improved composite PCM’s thermal conductivity 10-fold with respect to pure palmitic acid. GNPs were used by Tang et al. [22] to improve the thermal conductivity of palmitic acid/high density polyethylene composite PCM. With 4 wt % of GNPs, the thermal conductivity was nearly 2.5 times higher than pure form-stable composite PCM. Silakhori et al. [20] showed that GNPs with 1.6 wt % improved the thermal conductivity of palmitic acid/polypyrole with an increase of 34.3%. In this research, two kinds of carbon-based composite PCMs (expanded graphite-based PCM and graphite nanoplatelet-based PCM) were prepared for potential application in energy piles. Thereafter, a systematic study was performed and different characterization tests were carried out on two composite PCMs. 2. Materials and Methods 2.1. Materials A technical grade paraffin, which is generally believed to be chemically inert, non-corrosive, show small volume changes during phase transition, innocuous and inexpensive, was used for this research [23]. As far as a carrier for PCM is concerned, expanded graphite (supplied by Qingdao Teng Sheng Da Carbon Machinery Co., Ltd., Qingdao, China) with an expansion ratio of 300 and technical-grade graphene nanoplatelets (supplied by Chinese Academy of Sciences Chengdu Organic Chemical Co., Ltd., Chengdu, China) were used. The properties of EG and GNPs are enlisted in Table 1. Table 1. Properties of expanded graphite (EG) and graphene nanoplatelets (GNPs). Type
Particle Size
Diameter
Expanded Time
Nanosheet Number
Expanded graphite Graphene
~0.5 mm /
/ 5–7 µm
300 /
/
