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DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor
Renewable Energy xxx (2015) 1e6
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor Guohui Feng a, b, *, Kailiang Huang a, b, Hailun Xie a, Huixing Li a, Xin Liu a, Shibo Liu a, Chihong Cao a a b
School of Municipal and Environmental Engineering, Shenyang Jianzhu University, Shenyang 110168, China Faculty of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 400030, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2015 Received in revised form 10 July 2015 Accepted 29 July 2015 Available online xxx
Reliable parameters of phase change material are essential for design and simulation of the PCM floor, which can be effectively used in heating system with non-continuous energy. This paper summarized latent heat, solidus temperature and liquids temperature of a typical PCM (capric acid) from reported tests based on differential scanning calorimeter (DSC). It is discovered that the results for the same PCM were significantly incongruent. Then, we arranged DSC tests with different procedures on capric acid and use the acquired parameters in simulations of a PCM floor, which has been reported with detailed experimental results in our former research. The aim is to present reliable latent heat, solidus temperature and liquids temperature of this common PCM and assess the impact of misinterpreted enthalpycapacity function on the simulated thermal storage and releasing effect of the PCM floor. Errors with 33％e883％ deviation for phase transition range of PCM were discovered from the improperly arranged tests. In the cases of simulation, a maximum difference of 20% was observed for the floor surface temperature. It means it is worthwhile setting standard DSC tests and ascertaining right effective capacity or enthalpy function of PCM in simulations related to PCM system design. © 2015 Elsevier Ltd. All rights reserved.
Keywords: DSC test Error Phase change material floor Simulation Thermal storage
1. Introduction Incorporation of phase change materials (PCMs) in buildings and heating systems is increasingly recommended as it is able to shift thermal/cooling load of air-conditioning system and solve the problem of unbalance of energy supply and requirement [1e3]. In this way, energy can be used more efficiently in the building area. From a realistic point of view, China is facing great pressure on building energy consumption now as it was already the world's second largest building energy user and first residential energy user [4]. Hence, application of phase change energy storage could be a potential method to push forward building energy efficiency and address global climate change. The present work focuses on the use of phase change energy storage in floor heating system. For the working principle of intermittent heating, large amount of heat is stored in one period and released in another period [3,5e10]. The phase transition of
* Corresponding author. School of Municipal and Environmental Engineering, Shenyang Jianzhu University, No. 9, Hunnan East Road, Shenyang 110168, China. E-mail address: [email protected] (G. Feng).
phase change material is characterized by negligible or small temperature change during the thermal storage and releasing process [1,2]. Thus, the PCM floor makes non-continuous energy, such as solar thermal energy, to be utilized more easily and efficiently. A kind of PCM floor including capillary plates and macropackaged PCM layer was put forward and established in our former research, and large-span intermittent heating was realized with the use of large amount of latent heat storage [3]. It helps to save space for water tank in the solar heating system and heat loss at night is effectively avoided. In addition, a PCM floor model has been developed by Mazo in simple building types. The results show when the heating system is fed by a heat pump which mainly operates during night time, it reduces the costs of electric energy consumption [10]. The simulation of PCM floor heating systems is necessary for the design and evaluation of its performance. The PCM parameters, such as latent heat, solidus temperature and liquids temperature, are often achieved by DSC tests. However, certain factors, like heating rate and amount of PCM sample, can influence the test results according to several reported researches [11e15]. In this study, a typical PCM (capric acid) used in our researched
http://dx.doi.org/10.1016/j.renene.2015.07.085 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: G. Feng, et al., DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.085
2
G. Feng et al. / Renewable Energy xxx (2015) 1e6
PCM floor was tested under professionally recommended [11,12], accustomed [3,16e21] and other procedures edited in the DSC to present more reliable results. Different simulation cases of the PCM floor, in which the variables included latent heat, solidus temperature and liquids temperature, were conducted and their results were compared. The aim is to assess the impact of misinterpreted effective capacity function on the simulated thermal storage and releasing effect of the PCM floor. 2. DSC tests of the PCM and evaluation 2.1. Review of DSC tests of capric acid DSC test is a common method to acquire the properties of the PCM. The working principle is shown in Fig. 1, in which the temperatures of the sample and reference are controlled identical by using a delicately designed stove. The difference of power required to maintain this equality is recorded automatically and reflected in the DSC curves. Information deduced from the DSC curves includes latent heat, solidus temperature, peak temperature, liquids temperature, super-cooling degrees and specific heat capacity of the PCM. In this paper, only latent heat, solidus temperature, and liquids temperature were discussed as they are necessary for simulations and they are usually unreasonably unstable [3,16e21]. From the summary of DSC tests on capric acid (Table 1), we can see the results for the same PCM were significantly incongruent. The biggest deviation for solidus temperature, liquids temperature and latent heat were 2.5 C, 4.8 C and 19.2 kJ/kg, respectively. The minimum and maximum of phase transition range are 1.6 C and 6.8 C. Reasons that hinder the determination of properties can be attributed as follows: (1) The phase change materials applied in DSC tests were not usually pure substances; (2) Different test procedures were arranged (different heating rate and sample mass, for example); (3) the DSC instruments also have an influence on the DSC results. Table 1 showed the 5 C min1 is the common choice of reported researches. It complied with typical standards [22] used in DSC analysis, but it didn't follow the advices of Lazaro and Dumas [11,12]. Slow heating rate was recommended by them as the sample PCM was able to reach phase equilibrium both in thermal and chemical aspects. 2.2. Experimental uncertainty and the DSC test plan Capric acid with high purity (99.9％) was selected for the DSC tests so that it was able to neglect the influence of material impurity. The accuracy of the instrument itself was not mentioned in the manual description and this information was not seen in others' research, so it was also neglected in the error analysis and we focused on the principle and procedure of the DSC tests. The internal reason for the experimental uncertainty is due to lack of
phase equilibrium within the sample in a DSC. A kind of Round Robin Tests (RRTs) proposed by Lazaro was validated being able to avoid the common influential factors and show good agreement in enthalpy and in temperature [11]. It is based on two international networks: within the IEA (International Energy Agency), the ECES Imple-menting Agreement (Energy Conservation through Energy Storage IA) and SHC Programme (Solar Heating and Cooling) Task 42/Annex 24 ‘‘Compact Thermal Energy Storage e Material Development for System Integration’’ [23], and the COST Action TU0802 ‘‘Next generation cost effective phase change materials for increased energy efficiency in renewable energy systems in buildings (NeCoE-PCM)’’. In the present tests, the conditions of the “right” methodology are guaranteed and only two influencial factors, the heating rate and sample mass, were discussed by analyzing the test results. Heating rates of 10 C min1, 5 C min1, 1 C min1, 0.5 C min1 and 0.1 C min1 were arranged when the sample mass is 5 mg. Sample mass of 2 mg, 5 mg, 8 mg were arranged when the heating rate is 0.5 C min1 since no more than 10 mg were recommend by the DSC producer (PerkinElmer company). Except for heating rate and sample mass, other processes include: Temperature and enthalpy calibration measurements using Zn and indium. Maintaining the PCM at 10 C for 5 min. Increasing the temperature from 10 C to 50 C. Maintaining the PCM at 50 C for 5 min. Decreasing the temperature from 50 C to 10 C.
2.3. Test result and discussion Two different factors, heating rate and the sample mass, which may affect the DSC results, were arranged in the present tests. The test results were shown in Tables 2 and 3. Table 2 showed liquids temperature and latent heat are gradually increased with the increase of heating rate with the same quality. When the heating rate increased from 0.1 C min1 to 10 C min1, the biggest deviation for solidus temperature, liquids temperature and latent heat were 1.1 C, 6.4 C and 17 kJ/kg, respectively. In addition, the phase transition range is closely related to the heating rate. The minimum and maximum of phase transition range for the 5 cases were 0.6 C and 5.9 C. The heating rate of 0.1 C min1 was set as the correct reference as Lazaro recommend that extremely slow heating rate leads to the most accurate situation [11]. Errors with 33％e883％ deviation for phase transition range of PCM were discovered for the comparison of arranged tests. Table 3 showed that solidus temperature, liquids temperature and latent heat were similar when the sample mass was changed. The biggest deviation for solidus temperature, liquids temperature and latent heat were 0.1 C, 0.3 C and 1 kJ/kg. Thus, different heating rates and sample qualities have varying degrees of impact on the test results. From the comparison between Tables 2 and 3, it is revealed that the influence of heating rate on the result of PCM parameter is greater than sample quality. 3. Simulation of the PCM floor 3.1. The PCM floor depiction
Fig. 1. Scheme of the DSC head.
The schematic of the PCM floor was seen in Fig. 2. It was designed for solar water heating system by using the PCM layer to store heat in the daytime and release heat at night. The PCM floor contained 5 layers: insulating layer, double heating layers
Please cite this article in press as: G. Feng, et al., DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.085
G. Feng et al. / Renewable Energy xxx (2015) 1e6
3
Table 1 Summary of DSC tests on capric acid.
[3] [16] [17] [18] [19] [20] [21]
Solidus temperature ( C)
Liquids temperature ( C)
Latent heat (kJ/kg)
Heating rate ( C min1)
29.3 30.0 31.8 30.6 31.5 31.3 30.2
33.5 32.2 35.3 32.2 e e 37.0
162.0 156.4 146.5 155.5 165.7 149.1 164.6
e 5 5 e 10 5 5
From DSC curve From DSC curve
No DSC curve No DSC curve
Table 2 Value of the three PCM parameters under different heating rate. Heating rate ( C min1)
Solidus temperature ( C)
Liquids temperature ( C)
Latent heat (kJ/kg)
Sample mass (mg)
10 5 1 0.5 0.1
31.8 31.0 30.7 30.7 30.7
37.7 35.1 32.5 31.5 31.3
162.2 158.8 146.5 146.3 145.2
5 5 5 5 5
Table 3 Value of the three PCM parameters under different quality. Sample mass (mg)
Solidus temperature ( C)
Liquids temperature ( C)
Latent heat (kJ/kg)
Heating rate ( C min1)
2 5 8
30.7 30.7 30.8
31.4 31.5 31.7
146.2 146.3 147.2
0.5 0.5 0.5
vH v2 T v2 T ¼ ki ri þ vt vx2 vy2
! (1)
H is enthalpy, T is material temperature, k is the thermal conductivity and r is the density of the material. The enthalpy of PCM can be obtained as follows:
Fig. 2. Configuration of the novel PCM floor.
consisting of capillary plates, PCM layer and the surface layer. Foursquare concrete skeletons were designed for the PCM layer, in which skeletons make the container stable and the left cavities are for macro-encapsulating PCM. Detailed information was depicted in our former research [3]. 3.2. Mathematical model and simulation In Fig. 3, “bc” and “ad” represent upper floor surface and lowsurface of the floor; “ab” and “cd” are adiabatic surfaces. For a typical composite PCM unit, it is a symmetrical structure along the capillary flow direction if one side of cavity skeleton is assumed to be PCM. A mathematical model was established with some simplifications: (1) heat loss at wall boundary of PCM floor was neglected; (2) the boundary below the insulation material was thermally insulated; (3) natural convection of PCM during melting process and supper cooling effect during freezing process were ignored; (4) temperature of hot water in capillary plates was considered to be the average value of inlet water and outlet water. Governing equation for the floor:
H ¼ CPs T
ðT Ts Þ
Hm ðT Tl Þ H ¼ Hs þ Tl Ts
ðTs < T < Tl Þ
H ¼ Hl þ Cpl ðT Tl Þ
ðT Tl Þ
(2)
Hs is the enthalpy of initial melting, Hl is the enthalpy of full melting, Ts is the temperature of PCM in initial liquid state, Tl is the temperature of PCM in full liquid state, Cps is specific heat of PCM in solid state and Cpl is the specific heat of PCM in liquid state. The boundary conditions on the floor surfaces are as follows:
vT hcom Tind Tf ;up ¼ k vy y¼L1 ;x¼bc
(3)
where Tind is the temperature of indoor air, Tf,up is the temperature of the upper floor surface, hcom denotes the combined convection and radiation heat transfer coefficient between the indoor air and the floor's upper surface, and it can be acquired in research of Manuel [24].
k
vT ¼0 vy y¼0;x¼ad
vT k x ¼ 0; y ¼ ab ¼ 0 vx x ¼ L2; y ¼ cd
(4)
(5)
The boundary conditions on the inner surfaces of the tubes are:
Please cite this article in press as: G. Feng, et al., DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.085
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G. Feng et al. / Renewable Energy xxx (2015) 1e6
Fig. 3. Heat storage and heat releasing process in the simulation.
vT k ¼ hinn Tave Tj vr r¼R x
3.3. Simulation results and discussion
2 jþ12
vT k ¼ hinn Tave Tj vr r¼R x
n
n
3.3.1. Validation of the simulation In the cycle of 24 h, the heating capillary plates worked for 8 h and then it became insulated at the internal circles to reflect the thermal releasing period. The heat storage and heat releasing
2
jþ12
(6)
þðyc1 Þ2 ¼R2
þðyc2 Þ2 ¼R2
ðfor heating periodÞ
(7)
j ¼ 1; 2; 3…8
where hinn is the heat transfer coefficient of inner surface of capillary pipe and it can be calculated in research of Yang [25], R is the radius of capillary pipes, r is the length along radius of capillary pipes, c1 and c2 are the coordinate positions for centers of upper and low capillary plates.
kinn
vT ¼0 vr r¼R
ðfor non heating periodÞ
(8)
The finite volume method was used to solve the mathematical model with the help of CFD method, which allows taking the PCM as solid material by using the effective capacity method.
simulation results were shown in Fig. 3. With the extension of heating time, the PCM melted gradually. The place near the cement skeleton got fast melted as the concrete material is superior in thermal conductivity. In the heat releasing period, PCM temperature gradually decreased with its releasing heat to the room. When the temperature dropped to freezing point, PCM gradually solidified. The temperature of two points, locating at PCM area next to concrete skeleton and the floor surface, were monitored and compared with experimental results. The curves of temperature variation were presented in Fig. 4. The temperature change trend of simulation and experiment is alike and it reflects the heat storage
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5
as the increasing of phase change temperature range. Even though the average floor surface temperatures for Case 2, Case 3 and Case 4 were close to Case 1, their influences on indoor thermal environment could be quite different as higher temperature in the daytime is unnecessary and lower temperature means poor heating effect in the PCM releasing period. Consequently, error of the PCM parameters would lead to serious misunderstanding the characteristics of the PCM floor. 4. Conclusions
Fig. 4. Temperature comparison between simulation and experimentation.
and releasing rule of the PCM floor. The PCM floor surface temperature drops slowly and maintains above 20 C for 16 nonheating hours. In addition, a sudden drop of floor surface temperature appears when the heating pump is turned off. The deviation for the simulated and measured results are within1.5 C, hence the simulation method is reliable and can be used for further analysis. 3.3.2. Influence of the PCM parameters Latent heat, solidus temperature and liquids temperature of the PCM were taken as the influencing factor in the simulation. Case 1, Case 2, Case 3 and Case 4 represent the DSC results with heating rate of 0.5 C min1, 1 C min1, 5 C min1 and 10 C min1. Information of the influencing factor can be checked in Table 2. In the 4 cases of simulation, floor surface temperature was monitored and used for comparison. We assume the case 1 as the reference and check how the DSC test error would influence the results of PCM floor simulation. The comparison results were seen in Fig. 5. It was found that maximum difference of 20% was observed for the floor surface temperature. Furthermore, the deviation extent increased
Fig. 5. Surface temperature with different PCM parameters.
This paper summarized latent heat, solidus temperature and liquids temperature of a typical PCM from reported tests based on differential scanning calorimeter. It was found that for the same PCM, the detected results were significantly incongruent. Repeated DSC tests were arranged to discover the influence of heating rate and sample mass on the detected PCM parameters. Errors with 33％e883％ deviation for phase transition range of PCM were discovered for the improperly arranged tests. These parameters were used in the PCM floor simulation and a maximum difference of 20% was observed for the floor surface temperature, which greatly influenced the prediction of the simulation. The research show the importance of setting standard DSC tests and ascertaining right PCM parameters in simulations related to PCM system design. Acknowledgments We are grateful for the financial support of Natural Science Fund of China (No. 51308352), project of College Innovation Team of Liaoning Province (LT2013013) and project of Technology Bureau of Shenyang (F13-160-9-00). References [1] Francis Agyenim, Neil Hewitt, Philip Eames, Mervyn Smyth, A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS), Renew. Sustain. Energy Rev. 14 (2010) 615e628. [2] A. Pasupathy, R. Velraj, R.V. Seeniraj, Phase change material-based building architecture for thermal management in residential and commercial establishments, Renew. Sustain. Energy Rev. 12 (2008) 39e64. [3] Kailiang Huang, Guohui Feng, Jianshun Zhang, Experimental and numerical study on phase change material floor in solar water heating system with a new design, Sol. Energy 105 (2014) 126e138. [4] IEA, in: Energy Balances, IEA, Paris, France, 2012. [5] M. Farid, W.J. Kong, Underfloor heating with latent heat storage, Proc. Inst. Mech. Eng. 215.5 (2001) 601e609. [6] K.P. Lin, Y.P. Zhang, X. Xu, Hongfa Di, Rui Yang, Penghua Qin, Experimental study of under-floor electric heating system with shape-stabilized PCM plates, Energy Build. 37 (2005) 215e220. [7] Li Guojian, Feng Guohui, Zhu Neng, Yanjun Hu, Experiment of the new phase change heat storage electric heating floor system, J. Shenyang Jianzhu Univ. Sci. 22.2 (2006) 294e298 (in Chinese). [8] Jianli Li, Ping Xue, Hong He, Wenying Ding, Jinmin Han, Preparation and application effects of a novel form-stable phase change material as the thermal storage layer of an electric floor heating system, Energy Build. 41.8 (2009) 871e880. [9] qunli Zhang, hongfa Di, kunping Lin, Yinping Zhang, Simulation on the thermal performance of hydraulic floor heating modular with shape-stabilized phase change material for thermal energy storage, J. Eng. Thermophys. 27.4 (2006) 641e643 (in Chinese). [10] Javier Mazo, Monica Delgado, Jose Maria Marin, Belen Zalba, Modeling a radiant floor system with phase change material (PCM) integrated into a building simulation tool: analysis of a case study of a floor heating system coupled to a heat pump, Energy Build. 47 (2012) 458e466. ~ alosa, , [11] Ana Lazaro, Conchita Pen Aran Sole Gonzalo Diarce, n Zalba, Stefan Gshwander, Luisa Thomas Haussmann, Magali Fois, Bele F. Cabeza, Intercomparative tests on phase change materials characterization with differential scanning calorimeter, Appl. Energy 109 (2013) 415e420. phane Gibout, Laurent Zalewski, Ke vyn Johannes, [12] Jean-Pierre Dumas, Ste phane Lassue, Jean-Pierre Be de carrats, Pierre Tittelein, Erwin Franquet, Ste de ric Kuznik, Interpretation of calorimetry experiments to characterise Fre phase change materials, Int. J. Therm. Sci. 78 (2014) 48e55. n, E. Günther, H. Mehling, S. Hiebler, L.F. Cabeza, Determination of [13] C. Castello
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[14]
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[17]
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the enthalpy of PCM as a function of temperature using a heat-flux DSC-A study of different measurement procedures and their accuracy, Int. J. Energy Res. 32 (13) (2008) 1258e1265. E. Günther, S. Hiebler, H. Mehling, R. Redlich, Enthalpy of phase change materials as a function of temperature: required accuracy and suitable measurement methods, Int. J. Thermophys. 30 (2009) 1257e1269. ASTM Standard E793, Standard Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry, 2006, http://dx.doi.org/ 10.1520/E0793-06. PA; 2006. Ahmet Sarı, Ali Karaipekli, Preparation, thermal properties and thermal reliability of capric acid/expanded perlite composite for thermal energy storage, Mater. Chem. Phys. 109 (2008) 459e464. Yi Zhang, Jingyan Zhang, Bin Huang, Dongxu Li, Thermal conductivity test on fatty acid phase change materials and numerical simulation of heat transfer process of phase change, J. Funct. Mater. 14.13 (2012) 1950e1959 (in Chinese). Shilei Lv, Zhu Neng, Feng Guohui, Eutectic mixtures of capric acid and lauric acid applied in building wallboards for heat energy storage, Energy Build. 38 (2006) 708e711.
[19] Shaokun Song, Lijie Dong, Zhengyao Qu, Jing Ren, Chuanxi Xiong, Microencapsulated capricestearic acid with silica shell as a novel phase change material for thermal energy storage, Appl. Therm. Eng. 70 (2014) 546e551. [20] Yaguang Yuan, Yanping Yuan, Nan Zhang, Yanxia Du, Xiaoling Cao, Preparation and thermal characterization of capricemyristicepalmitic acid/expanded graphite composite as phase change material for energy storage, Mater. Lett. 125.15 (2014) 154e157. [21] Yeliz Konuklua, Halime O. Paksoyb, Murat Unalb, Suleyman Konukluc, Microencapsulation of a fatty acid with poly(melamineeureaeformaldehyde), Energy Convers. Manag. 80 (2014) 382e390. [22] ASTM Standard E793, Standard Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry, 2006, http://dx.doi.org/ 10.1520/E0793-06. PA; 2006. [23] http://www.iea-shc.org/task42/ (05.08.12). s-Chicote, Ana Tejero Gonza lez, Eloy Velasco Go mez, Francisco [24] Manuel Andre Javier Rey Martínez, Experimental study on the cooling capacity of a radiant cooled ceiling system, Energy Build. 54 (2012) 207e214. [25] Shiming Yang, Wenquan Tao, Heat Transfer, fourth ed., Higher Education Press, Beijing, 2006 (in Chinese).
Please cite this article in press as: G. Feng, et al., DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.085
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor Guohui Feng a, b, *, Kailiang Huang a, b, Hailun Xie a, Huixing Li a, Xin Liu a, Shibo Liu a, Chihong Cao a a b
School of Municipal and Environmental Engineering, Shenyang Jianzhu University, Shenyang 110168, China Faculty of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 400030, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 March 2015 Received in revised form 10 July 2015 Accepted 29 July 2015 Available online xxx
Reliable parameters of phase change material are essential for design and simulation of the PCM floor, which can be effectively used in heating system with non-continuous energy. This paper summarized latent heat, solidus temperature and liquids temperature of a typical PCM (capric acid) from reported tests based on differential scanning calorimeter (DSC). It is discovered that the results for the same PCM were significantly incongruent. Then, we arranged DSC tests with different procedures on capric acid and use the acquired parameters in simulations of a PCM floor, which has been reported with detailed experimental results in our former research. The aim is to present reliable latent heat, solidus temperature and liquids temperature of this common PCM and assess the impact of misinterpreted enthalpycapacity function on the simulated thermal storage and releasing effect of the PCM floor. Errors with 33％e883％ deviation for phase transition range of PCM were discovered from the improperly arranged tests. In the cases of simulation, a maximum difference of 20% was observed for the floor surface temperature. It means it is worthwhile setting standard DSC tests and ascertaining right effective capacity or enthalpy function of PCM in simulations related to PCM system design. © 2015 Elsevier Ltd. All rights reserved.
Keywords: DSC test Error Phase change material floor Simulation Thermal storage
1. Introduction Incorporation of phase change materials (PCMs) in buildings and heating systems is increasingly recommended as it is able to shift thermal/cooling load of air-conditioning system and solve the problem of unbalance of energy supply and requirement [1e3]. In this way, energy can be used more efficiently in the building area. From a realistic point of view, China is facing great pressure on building energy consumption now as it was already the world's second largest building energy user and first residential energy user [4]. Hence, application of phase change energy storage could be a potential method to push forward building energy efficiency and address global climate change. The present work focuses on the use of phase change energy storage in floor heating system. For the working principle of intermittent heating, large amount of heat is stored in one period and released in another period [3,5e10]. The phase transition of
* Corresponding author. School of Municipal and Environmental Engineering, Shenyang Jianzhu University, No. 9, Hunnan East Road, Shenyang 110168, China. E-mail address: [email protected] (G. Feng).
phase change material is characterized by negligible or small temperature change during the thermal storage and releasing process [1,2]. Thus, the PCM floor makes non-continuous energy, such as solar thermal energy, to be utilized more easily and efficiently. A kind of PCM floor including capillary plates and macropackaged PCM layer was put forward and established in our former research, and large-span intermittent heating was realized with the use of large amount of latent heat storage [3]. It helps to save space for water tank in the solar heating system and heat loss at night is effectively avoided. In addition, a PCM floor model has been developed by Mazo in simple building types. The results show when the heating system is fed by a heat pump which mainly operates during night time, it reduces the costs of electric energy consumption [10]. The simulation of PCM floor heating systems is necessary for the design and evaluation of its performance. The PCM parameters, such as latent heat, solidus temperature and liquids temperature, are often achieved by DSC tests. However, certain factors, like heating rate and amount of PCM sample, can influence the test results according to several reported researches [11e15]. In this study, a typical PCM (capric acid) used in our researched
http://dx.doi.org/10.1016/j.renene.2015.07.085 0960-1481/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: G. Feng, et al., DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.085
2
G. Feng et al. / Renewable Energy xxx (2015) 1e6
PCM floor was tested under professionally recommended [11,12], accustomed [3,16e21] and other procedures edited in the DSC to present more reliable results. Different simulation cases of the PCM floor, in which the variables included latent heat, solidus temperature and liquids temperature, were conducted and their results were compared. The aim is to assess the impact of misinterpreted effective capacity function on the simulated thermal storage and releasing effect of the PCM floor. 2. DSC tests of the PCM and evaluation 2.1. Review of DSC tests of capric acid DSC test is a common method to acquire the properties of the PCM. The working principle is shown in Fig. 1, in which the temperatures of the sample and reference are controlled identical by using a delicately designed stove. The difference of power required to maintain this equality is recorded automatically and reflected in the DSC curves. Information deduced from the DSC curves includes latent heat, solidus temperature, peak temperature, liquids temperature, super-cooling degrees and specific heat capacity of the PCM. In this paper, only latent heat, solidus temperature, and liquids temperature were discussed as they are necessary for simulations and they are usually unreasonably unstable [3,16e21]. From the summary of DSC tests on capric acid (Table 1), we can see the results for the same PCM were significantly incongruent. The biggest deviation for solidus temperature, liquids temperature and latent heat were 2.5 C, 4.8 C and 19.2 kJ/kg, respectively. The minimum and maximum of phase transition range are 1.6 C and 6.8 C. Reasons that hinder the determination of properties can be attributed as follows: (1) The phase change materials applied in DSC tests were not usually pure substances; (2) Different test procedures were arranged (different heating rate and sample mass, for example); (3) the DSC instruments also have an influence on the DSC results. Table 1 showed the 5 C min1 is the common choice of reported researches. It complied with typical standards [22] used in DSC analysis, but it didn't follow the advices of Lazaro and Dumas [11,12]. Slow heating rate was recommended by them as the sample PCM was able to reach phase equilibrium both in thermal and chemical aspects. 2.2. Experimental uncertainty and the DSC test plan Capric acid with high purity (99.9％) was selected for the DSC tests so that it was able to neglect the influence of material impurity. The accuracy of the instrument itself was not mentioned in the manual description and this information was not seen in others' research, so it was also neglected in the error analysis and we focused on the principle and procedure of the DSC tests. The internal reason for the experimental uncertainty is due to lack of
phase equilibrium within the sample in a DSC. A kind of Round Robin Tests (RRTs) proposed by Lazaro was validated being able to avoid the common influential factors and show good agreement in enthalpy and in temperature [11]. It is based on two international networks: within the IEA (International Energy Agency), the ECES Imple-menting Agreement (Energy Conservation through Energy Storage IA) and SHC Programme (Solar Heating and Cooling) Task 42/Annex 24 ‘‘Compact Thermal Energy Storage e Material Development for System Integration’’ [23], and the COST Action TU0802 ‘‘Next generation cost effective phase change materials for increased energy efficiency in renewable energy systems in buildings (NeCoE-PCM)’’. In the present tests, the conditions of the “right” methodology are guaranteed and only two influencial factors, the heating rate and sample mass, were discussed by analyzing the test results. Heating rates of 10 C min1, 5 C min1, 1 C min1, 0.5 C min1 and 0.1 C min1 were arranged when the sample mass is 5 mg. Sample mass of 2 mg, 5 mg, 8 mg were arranged when the heating rate is 0.5 C min1 since no more than 10 mg were recommend by the DSC producer (PerkinElmer company). Except for heating rate and sample mass, other processes include: Temperature and enthalpy calibration measurements using Zn and indium. Maintaining the PCM at 10 C for 5 min. Increasing the temperature from 10 C to 50 C. Maintaining the PCM at 50 C for 5 min. Decreasing the temperature from 50 C to 10 C.
2.3. Test result and discussion Two different factors, heating rate and the sample mass, which may affect the DSC results, were arranged in the present tests. The test results were shown in Tables 2 and 3. Table 2 showed liquids temperature and latent heat are gradually increased with the increase of heating rate with the same quality. When the heating rate increased from 0.1 C min1 to 10 C min1, the biggest deviation for solidus temperature, liquids temperature and latent heat were 1.1 C, 6.4 C and 17 kJ/kg, respectively. In addition, the phase transition range is closely related to the heating rate. The minimum and maximum of phase transition range for the 5 cases were 0.6 C and 5.9 C. The heating rate of 0.1 C min1 was set as the correct reference as Lazaro recommend that extremely slow heating rate leads to the most accurate situation [11]. Errors with 33％e883％ deviation for phase transition range of PCM were discovered for the comparison of arranged tests. Table 3 showed that solidus temperature, liquids temperature and latent heat were similar when the sample mass was changed. The biggest deviation for solidus temperature, liquids temperature and latent heat were 0.1 C, 0.3 C and 1 kJ/kg. Thus, different heating rates and sample qualities have varying degrees of impact on the test results. From the comparison between Tables 2 and 3, it is revealed that the influence of heating rate on the result of PCM parameter is greater than sample quality. 3. Simulation of the PCM floor 3.1. The PCM floor depiction
Fig. 1. Scheme of the DSC head.
The schematic of the PCM floor was seen in Fig. 2. It was designed for solar water heating system by using the PCM layer to store heat in the daytime and release heat at night. The PCM floor contained 5 layers: insulating layer, double heating layers
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Table 1 Summary of DSC tests on capric acid.
[3] [16] [17] [18] [19] [20] [21]
Solidus temperature ( C)
Liquids temperature ( C)
Latent heat (kJ/kg)
Heating rate ( C min1)
29.3 30.0 31.8 30.6 31.5 31.3 30.2
33.5 32.2 35.3 32.2 e e 37.0
162.0 156.4 146.5 155.5 165.7 149.1 164.6
e 5 5 e 10 5 5
From DSC curve From DSC curve
No DSC curve No DSC curve
Table 2 Value of the three PCM parameters under different heating rate. Heating rate ( C min1)
Solidus temperature ( C)
Liquids temperature ( C)
Latent heat (kJ/kg)
Sample mass (mg)
10 5 1 0.5 0.1
31.8 31.0 30.7 30.7 30.7
37.7 35.1 32.5 31.5 31.3
162.2 158.8 146.5 146.3 145.2
5 5 5 5 5
Table 3 Value of the three PCM parameters under different quality. Sample mass (mg)
Solidus temperature ( C)
Liquids temperature ( C)
Latent heat (kJ/kg)
Heating rate ( C min1)
2 5 8
30.7 30.7 30.8
31.4 31.5 31.7
146.2 146.3 147.2
0.5 0.5 0.5
vH v2 T v2 T ¼ ki ri þ vt vx2 vy2
! (1)
H is enthalpy, T is material temperature, k is the thermal conductivity and r is the density of the material. The enthalpy of PCM can be obtained as follows:
Fig. 2. Configuration of the novel PCM floor.
consisting of capillary plates, PCM layer and the surface layer. Foursquare concrete skeletons were designed for the PCM layer, in which skeletons make the container stable and the left cavities are for macro-encapsulating PCM. Detailed information was depicted in our former research [3]. 3.2. Mathematical model and simulation In Fig. 3, “bc” and “ad” represent upper floor surface and lowsurface of the floor; “ab” and “cd” are adiabatic surfaces. For a typical composite PCM unit, it is a symmetrical structure along the capillary flow direction if one side of cavity skeleton is assumed to be PCM. A mathematical model was established with some simplifications: (1) heat loss at wall boundary of PCM floor was neglected; (2) the boundary below the insulation material was thermally insulated; (3) natural convection of PCM during melting process and supper cooling effect during freezing process were ignored; (4) temperature of hot water in capillary plates was considered to be the average value of inlet water and outlet water. Governing equation for the floor:
H ¼ CPs T
ðT Ts Þ
Hm ðT Tl Þ H ¼ Hs þ Tl Ts
ðTs < T < Tl Þ
H ¼ Hl þ Cpl ðT Tl Þ
ðT Tl Þ
(2)
Hs is the enthalpy of initial melting, Hl is the enthalpy of full melting, Ts is the temperature of PCM in initial liquid state, Tl is the temperature of PCM in full liquid state, Cps is specific heat of PCM in solid state and Cpl is the specific heat of PCM in liquid state. The boundary conditions on the floor surfaces are as follows:
vT hcom Tind Tf ;up ¼ k vy y¼L1 ;x¼bc
(3)
where Tind is the temperature of indoor air, Tf,up is the temperature of the upper floor surface, hcom denotes the combined convection and radiation heat transfer coefficient between the indoor air and the floor's upper surface, and it can be acquired in research of Manuel [24].
k
vT ¼0 vy y¼0;x¼ad
vT k x ¼ 0; y ¼ ab ¼ 0 vx x ¼ L2; y ¼ cd
(4)
(5)
The boundary conditions on the inner surfaces of the tubes are:
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G. Feng et al. / Renewable Energy xxx (2015) 1e6
Fig. 3. Heat storage and heat releasing process in the simulation.
vT k ¼ hinn Tave Tj vr r¼R x
3.3. Simulation results and discussion
2 jþ12
vT k ¼ hinn Tave Tj vr r¼R x
n
n
3.3.1. Validation of the simulation In the cycle of 24 h, the heating capillary plates worked for 8 h and then it became insulated at the internal circles to reflect the thermal releasing period. The heat storage and heat releasing
2
jþ12
(6)
þðyc1 Þ2 ¼R2
þðyc2 Þ2 ¼R2
ðfor heating periodÞ
(7)
j ¼ 1; 2; 3…8
where hinn is the heat transfer coefficient of inner surface of capillary pipe and it can be calculated in research of Yang [25], R is the radius of capillary pipes, r is the length along radius of capillary pipes, c1 and c2 are the coordinate positions for centers of upper and low capillary plates.
kinn
vT ¼0 vr r¼R
ðfor non heating periodÞ
(8)
The finite volume method was used to solve the mathematical model with the help of CFD method, which allows taking the PCM as solid material by using the effective capacity method.
simulation results were shown in Fig. 3. With the extension of heating time, the PCM melted gradually. The place near the cement skeleton got fast melted as the concrete material is superior in thermal conductivity. In the heat releasing period, PCM temperature gradually decreased with its releasing heat to the room. When the temperature dropped to freezing point, PCM gradually solidified. The temperature of two points, locating at PCM area next to concrete skeleton and the floor surface, were monitored and compared with experimental results. The curves of temperature variation were presented in Fig. 4. The temperature change trend of simulation and experiment is alike and it reflects the heat storage
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as the increasing of phase change temperature range. Even though the average floor surface temperatures for Case 2, Case 3 and Case 4 were close to Case 1, their influences on indoor thermal environment could be quite different as higher temperature in the daytime is unnecessary and lower temperature means poor heating effect in the PCM releasing period. Consequently, error of the PCM parameters would lead to serious misunderstanding the characteristics of the PCM floor. 4. Conclusions
Fig. 4. Temperature comparison between simulation and experimentation.
and releasing rule of the PCM floor. The PCM floor surface temperature drops slowly and maintains above 20 C for 16 nonheating hours. In addition, a sudden drop of floor surface temperature appears when the heating pump is turned off. The deviation for the simulated and measured results are within1.5 C, hence the simulation method is reliable and can be used for further analysis. 3.3.2. Influence of the PCM parameters Latent heat, solidus temperature and liquids temperature of the PCM were taken as the influencing factor in the simulation. Case 1, Case 2, Case 3 and Case 4 represent the DSC results with heating rate of 0.5 C min1, 1 C min1, 5 C min1 and 10 C min1. Information of the influencing factor can be checked in Table 2. In the 4 cases of simulation, floor surface temperature was monitored and used for comparison. We assume the case 1 as the reference and check how the DSC test error would influence the results of PCM floor simulation. The comparison results were seen in Fig. 5. It was found that maximum difference of 20% was observed for the floor surface temperature. Furthermore, the deviation extent increased
Fig. 5. Surface temperature with different PCM parameters.
This paper summarized latent heat, solidus temperature and liquids temperature of a typical PCM from reported tests based on differential scanning calorimeter. It was found that for the same PCM, the detected results were significantly incongruent. Repeated DSC tests were arranged to discover the influence of heating rate and sample mass on the detected PCM parameters. Errors with 33％e883％ deviation for phase transition range of PCM were discovered for the improperly arranged tests. These parameters were used in the PCM floor simulation and a maximum difference of 20% was observed for the floor surface temperature, which greatly influenced the prediction of the simulation. The research show the importance of setting standard DSC tests and ascertaining right PCM parameters in simulations related to PCM system design. Acknowledgments We are grateful for the financial support of Natural Science Fund of China (No. 51308352), project of College Innovation Team of Liaoning Province (LT2013013) and project of Technology Bureau of Shenyang (F13-160-9-00). References [1] Francis Agyenim, Neil Hewitt, Philip Eames, Mervyn Smyth, A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS), Renew. Sustain. Energy Rev. 14 (2010) 615e628. [2] A. Pasupathy, R. Velraj, R.V. Seeniraj, Phase change material-based building architecture for thermal management in residential and commercial establishments, Renew. Sustain. Energy Rev. 12 (2008) 39e64. [3] Kailiang Huang, Guohui Feng, Jianshun Zhang, Experimental and numerical study on phase change material floor in solar water heating system with a new design, Sol. Energy 105 (2014) 126e138. [4] IEA, in: Energy Balances, IEA, Paris, France, 2012. [5] M. Farid, W.J. Kong, Underfloor heating with latent heat storage, Proc. Inst. Mech. Eng. 215.5 (2001) 601e609. [6] K.P. Lin, Y.P. Zhang, X. Xu, Hongfa Di, Rui Yang, Penghua Qin, Experimental study of under-floor electric heating system with shape-stabilized PCM plates, Energy Build. 37 (2005) 215e220. [7] Li Guojian, Feng Guohui, Zhu Neng, Yanjun Hu, Experiment of the new phase change heat storage electric heating floor system, J. Shenyang Jianzhu Univ. Sci. 22.2 (2006) 294e298 (in Chinese). [8] Jianli Li, Ping Xue, Hong He, Wenying Ding, Jinmin Han, Preparation and application effects of a novel form-stable phase change material as the thermal storage layer of an electric floor heating system, Energy Build. 41.8 (2009) 871e880. [9] qunli Zhang, hongfa Di, kunping Lin, Yinping Zhang, Simulation on the thermal performance of hydraulic floor heating modular with shape-stabilized phase change material for thermal energy storage, J. Eng. Thermophys. 27.4 (2006) 641e643 (in Chinese). [10] Javier Mazo, Monica Delgado, Jose Maria Marin, Belen Zalba, Modeling a radiant floor system with phase change material (PCM) integrated into a building simulation tool: analysis of a case study of a floor heating system coupled to a heat pump, Energy Build. 47 (2012) 458e466. ~ alosa, , [11] Ana Lazaro, Conchita Pen Aran Sole Gonzalo Diarce, n Zalba, Stefan Gshwander, Luisa Thomas Haussmann, Magali Fois, Bele F. Cabeza, Intercomparative tests on phase change materials characterization with differential scanning calorimeter, Appl. Energy 109 (2013) 415e420. phane Gibout, Laurent Zalewski, Ke vyn Johannes, [12] Jean-Pierre Dumas, Ste phane Lassue, Jean-Pierre Be de carrats, Pierre Tittelein, Erwin Franquet, Ste de ric Kuznik, Interpretation of calorimetry experiments to characterise Fre phase change materials, Int. J. Therm. Sci. 78 (2014) 48e55. n, E. Günther, H. Mehling, S. Hiebler, L.F. Cabeza, Determination of [13] C. Castello
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Please cite this article in press as: G. Feng, et al., DSC test error of phase change material (PCM) and its influence on the simulation of the PCM floor, Renewable Energy (2015), http://dx.doi.org/10.1016/j.renene.2015.07.085