林业科学  2018, Vol. 54 Issue (11): 20-28   PDF    
DOI: 10.11707/j.1001-7488.20181104
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文章信息

邢靖晨, 周玉成, 虞宇翔, 李露霏, 常建民.
Xing Jingchen, Zhou Yucheng, Yu Yuxiang, Li Lufei, Chang Jianmin.
地采暖用脂肪酸相变地板储放热性能模拟
Simulation on Heat Storage and Release Performance of Fatty Acid Phase Change Floor Used for Ground with Heating System
林业科学, 2018, 54(11): 20-28.
Scientia Silvae Sinicae, 2018, 54(11): 20-28.
DOI: 10.11707/j.1001-7488.20181104

文章历史

收稿日期:2018-04-02
修回日期:2018-05-29

作者相关文章

邢靖晨
周玉成
虞宇翔
李露霏
常建民

地采暖用脂肪酸相变地板储放热性能模拟
邢靖晨1, 周玉成2, 虞宇翔1, 李露霏1, 常建民1     
1. 北京林业大学材料科学与技术学院 北京 100083;
2. 山东建筑大学信息与电气工程学院 济南 250101
摘要:【目的】建立传热数学模型,模拟探究一种以脂肪酸共晶混合物为相变材料的地采暖用相变储能地板,为新型地采暖储能地板的开发和应用提供理论基础。【方法】利用差示扫描量热法(DSC)和步冷曲线法分析脂肪酸共晶混合物的热性能,应用DSC和傅里叶红外(FTIR)分析其化学性质和热循环稳定性,确定脂肪酸共晶混合物对地采暖地面环境的适用性,选择脂肪酸共晶混合物作为相变储能地板用相变材料。建立并求解铺设脂肪酸相变地板的电加热地采暖系统传热数学模型,分析其节能性能和供热效果。具体过程为:首先,选择常用的典型房间理论模型,分析地采暖运行条件下室内热动态过程,并设定日夜间歇式地采暖运行模式;然后,建立房间传热模型,计算理论房间冬季室内供暖热负荷,确定相变材料用量以及地板表面与室内空气的换热关系;最后,建立地板传热模型,利用ANSYS软件模拟地采暖间歇式运行模式下脂肪酸相变地板的储放热过程,并观察室内空气温度和地板表面温度的变化规律,分析基于脂肪酸相变地板的电加热地采暖系统的节能性能与供热效果。【结果】脂肪酸共晶混合物相变温度在20~30℃之间,满足地采暖地面环境和人体舒适温度要求;脂肪酸共晶混合物熔程仅2~3℃,供热过程室内温度变化小;脂肪酸共晶混合物化学性质稳定,经过1 500次冻熔循环后熔点和潜热变化仅为0.27℃和1.7%,热循环稳定性突出,可满足在地采暖环境中长时间应用。以脂肪酸共晶混合物为相变材料的相变储能地板应用于地采暖环境中,可实现利用夜晚8 h储热、白天16 h放热的稳定供暖模式,将白天的用电负荷转移至夜晚,达到"移峰填谷"式节能用电目的,同时可将室内空气和地板表面温度控制在人体适宜范围内。【结论】脂肪酸相变地板用于电加热地采暖系统中,供暖和节能效果理想。通过改变脂肪酸共晶混合物的种类和配比,可灵活调整相变温度,使相变储能地板在不同气候地区及热负荷建筑设计的地采暖环境中都能发挥出良好的节能性能和供热效果,具有较大发展潜力。
关键词:地板采暖    相变储能    脂肪酸    传热模型    ANSYS模拟    
Simulation on Heat Storage and Release Performance of Fatty Acid Phase Change Floor Used for Ground with Heating System
Xing Jingchen1, Zhou Yucheng2, Yu Yuxiang1, Li Lufei1, Chang Jianmin1     
1. College of Material Science and Technology, Beijing Forestry University Beijing 100083;
2. School of Information and Electrical Engineering, Shandong Jianzhu University Jinan 250101
Abstract: 【Objective】In order to provide theoretical bases for the development and application of a new kind phase change energy storage floor used in ground with heating system, the thermal performance of phase change energy storage floor using fatty acid eutectic mixture as phase change material was investigated by a heat transfer mathematic model of ground with heating system in this study.【Method】Differential scanning calorimetry(DSC) and step cooling curves were used to analyze the thermal properties of eutectic mixtures of fatty acids. The chemical properties and thermal cycling stability was tested by DSC and Fourier transform infrared(FTIR). The applicability of fatty acid eutectic mixture to the ground of ground with heating system was confirmed, and the fatty acid eutectic mixture was selected as the phase change material for the phase change energy storage floor. The energy saving performance and heating effect of phase change energy storage floor was analyzed by a heat transfer mathematic model of electric ground with heating system. Firstly, the theoretical model of a typical room was chosen and the thermal dynamic process of ground with heating system was described. The operation of ground with heating system was set to a batch-type. Then, the heat transfer model of the room was established, the heat load was calculated, and the amount of phase change material and the heat transfer relationships between floor surface and indoor were confirmed. The heat transfer model of floor was lastly established, and the heat storage and release process of phase change energy storage floor was analyzed using ANSYS software. The changes of indoor temperature and floor surface temperature were observed, and the energy saving performance and heating effect of the electric floor heating system based on the phase change energy storage floor of fatty acid were analyzed.【Result】The phase change temperature of the fatty acid eutectic mixture was between 20-30℃, which satisfies the temperature requirements of the floor circumstance for floor heating system and thermal comfort of human body. The melting range of fatty acid eutectic mixture was only between 2 and 3℃, which will reduce the temperature change of room during the indoor heating process. The chemical properties of fatty acid eutectic mixtures were stable and after 1 500 cycles of freeze-melting cycle, the changes of melting point and latent heat were only 0.27℃ and 1.7%, respectively. The thermal stability of the fatty acid eutectic mixture was outstanding, which can meet the requirement of long-term application in floor heating system. The phase change energy storage floor used in floor heating system can move the electricity from the peak to the valley and make the temperature of indoor and floor surface within the proper range of human body.【Conclusion】The heating effect and energy saving performance of phase change energy storage floor used in floor heating system were proved to be superior. By changing the type and ratio of the fatty acid eutectic mixture, the phase change temperature could be adjusted flexibly so that the phase change energy storage floor of fatty acid would play a good energy saving and heating effect in different climate areas and different heat load buildings which has great potential for development.
Key words: floor used for ground with heating system    phase change energy storage    fatty acid    heat transfer model    ANSYS simulation    

低温地板辐射采暖(简称地采暖)是一种新型的采暖方式,具有舒适、清洁、节能、环保、热稳定好等优点,正逐步替代传统散热器用于建筑采暖(Freestone et al., 1996Palmer et al., 2000)。地采暖供热介质主要为热水和电加热,相比于热水,电加热地采暖加工控制简单、热能利用高效且清洁环保,是更为理想的采暖方式(王子介,2004贾明岩,2011);但由于电力成本较高且供应紧张,纯电加热采暖方式难以推广。假如能利用电力峰谷差解决电力成本和负荷问题,则可为电加热地采暖的推广应用提供良好的解决方法。相变储能是利用物质相变过程中吸收或放出相变潜热进行能量存储与释放的技术,具有储能密度高、体积小巧、温度控制恒定、节能效果显著、相变温度选择范围宽和易于控制等优点,可解决热能在时间、强度及地点上不匹配的矛盾(Farid et al., 2004Khudhair et al., 2004Sharma et al., 2009)。因此,将相变储能技术与电加热地采暖系统相结合,利用电力峰谷进行“移峰填谷”式节能采暖,是理想的途径。

将相变储能技术与电加热地采暖系统相结合,国内外学者主要对将相变材料埋设于地采暖系统下的情况进行了探索,结合方式主要有将相变材料直接埋设于地下建筑结构(Barrio et al., 2004;冯国会等,2012)、与混凝土或水泥砂浆混合埋设(Yamagushi et al., 1997Farid et al., 2001颜家桃等,2010)、宏观封装成板条或块状模块进行埋设(周传辉,2002吴海涛等,2010陈思婷,2014)以及以塑料为支撑载体制备定型相变材料进行埋设(Lin et al., 2005Zhang et al., 2006Li et al., 2009)等,并利用传热数学模型或搭建试验房间的方法分析其节能性能和供热效果(李国建等,2007李建立等,2010徐小龙,2014)。结果表明,将相变材料用于电加热地采暖系统,具有显著的节能性能和良好的供热效果;但将相变材料埋设于地下的方式,热损耗较大,热控制困难,且加工方式复杂,不利于后期维护和更换,需要探索新的途径。

近年来,随着人们对节能环保的日益重视和低温定型相变材料的不断发展,将低温相变材料与基材尤其是木质基材复合制备相变储能板材成为了研究热点(Jeong et al., 2014Jin et al., 2014Cao et al., 2015苗扬等,20152016Jamekhorshid et al., 2016Guo et al., 2016郭玺等,2016陈永祥等,2017Yang et al., 2018),加之新型地采暖地面铺装材料不断出现(Seo et al., 2014;季翔等,2014王眈眈,2016Yi et al., 2017),使地采暖用相变储能地面铺装材料的开发具备了条件。目前,相变储能地板研究刚刚起步,对于其适用相变材料的选择及应用于地采暖环境的效果了解甚少。鉴于此,本研究自制脂肪酸共晶混合物,分析其热性能、化学性质和热循环稳定性,并通过建立传热数学模型,模拟脂肪酸相变地板在地采暖环境中的储放热性能,探究一种以脂肪酸共晶混合物为相变材料的地采暖用相变储能地板,以期为新型地采暖储能地板的开发与应用提供理论依据。

1 脂肪酸共晶混合物性能分析

对于地采暖相变储能地板的开发,选择适用于地采暖地面环境且符合人体舒适温度要求的相变材料极为关键。脂肪酸是低温有机相变材料的一种,取自于动植物油脂,廉价易得可再生,具有熔程短、潜热大、非过冷、性质稳定、无毒害且不易燃、不腐蚀等优点,在国内外得到了广泛关注(Sari et al., 2008Zhao et al., 2014Kahwaji et al., 2018Wen et al., 2018)。

考虑到地采暖地面平均温度限额和人体适宜温度范围,相变材料的相变温度应在20~30 ℃之间。而单一脂肪酸相变温度在30 ℃以上,且随着碳链变长相变温度以约10 ℃的间隔升高,因此需要通过配制脂肪酸共晶混合物,灵活调整相变温度,使其达到地采暖地面环境和人体舒适温度要求。本研究自制脂肪酸共晶混合物,利用差示扫描量热法(DSC)和步冷曲线法分析其热性能,应用DSC和傅里叶红外(FTIR)分析其化学性质和热循环稳定性,确定脂肪酸共晶混合物对地采暖地面环境的适用性,并为传热分析提供数据基础。

正癸酸-肉豆蔻酸(CA-MA)、正癸酸-棕榈酸(CA-PA)和正癸酸-硬脂酸(CA-SA)共晶混合物DSC曲线见图 1a,步冷曲线见图 1b。从图 1a可看出,3种脂肪酸共晶混合物的DSC曲线仅呈现一个具有单一熔点的平滑峰,说明只存在共晶反应,相变过程均匀稳定。CA-MA、CA-PA和CA-SA 3种脂肪酸共晶混合物的相变温度分别为20.62~24.33 ℃、23.25~26.57 ℃和26.61~29.78 ℃,在一定温度区间内几乎连续变化,对于多变的应用环境具有很强的适应性;相变潜热分别为168.82、171.04和172.68 J·g-1,储能密度较高。从图 1b可看出,在自然冷却条件下,3种脂肪酸共晶混合物只存在单个放热平台,相变过程仅为2~3 ℃,放热过程持续平稳,具有很好的供热性能。

图 1 正癸酸-肉豆蔻酸(CA-MA)、正癸酸-棕榈酸(CA-PA)、正癸酸-硬脂酸(CA-SA)共晶混合物DSC曲线(a)和步冷曲线(b) Figure 1 DSC curves(a) and cooling curves(b)of CA-MA, CA-PA and CA-SA eutectic mixture

CA和CA-MA、CA-PA、CA-SA共晶混合物的FTIR曲线见图 2。由图 2可知,1 709 cm-1附近吸收峰为羧酸C=O特征峰;2 921、2 851和1 467 cm-1附近吸收峰对应—CH2不对称伸缩振动、对称伸缩振动和对称变形振动;1 350~1 180 cm-1特征吸收峰组为晶态长链羧酸—CH2面外摇摆振动峰;720 cm-1附近吸收峰对应4个及以上—CH2面外弯曲振动;2 957和2 868 cm-1附近吸收峰对应—CH3不对称伸缩振动和对称伸缩振动(回瑞华等,2001);约以3 000 cm-1为中心宽而散的吸收峰以及2 700~2 500 cm-1间的小峰由羧酸—OH伸缩振动及变形振动的倍频和组合频引起;930 cm-1附近特征性宽峰对应羧酸二聚体OH…O=面外变形振动(罗曼等,2007)。CA和CA-MA、CA-PA、CA-SA的特征峰基本一致,仅因碳原子数量不同使其吸收峰在位置和强度上略有差异,这说明脂肪酸在形成共晶混合物后,并未发生化学反应,化学性质稳定。

图 2 CA、CA-MA、CA-PA、CA-SA和CA-PA 1 500次热循环FTIR曲线 Figure 2 FTIR curves of CA and CA-MA, CA-SA eutectic mixture and CA-PA eutectic mixture before and after 1 500 freeze-thaw cycles

CA-PA共晶混合物冻熔循环后热性能见表 1,热循环后FTIR曲线见图 2。从表 1可看出,经500、1 000和1 500次冻熔循环后,CA-PA共晶混合物熔点变化为0.41、-0.18和0.27 ℃,潜热变化为-2.8%、-1.3%和1.7%,说明经历多次冻熔循环后,其熔点和潜热变化对于室内应用处于可接受范围,其变化原因可能为杂质影响、分析误差以及化学变化(Sari, 2006董珊珊,2012)。通过FTIR分析发现,冻熔循环1 500次与热循环的CA-PA共晶混合物红外曲线特征峰彼此匹配,说明未发生化学变化,热循环稳定性突出。

表 1 CA-PA共晶混合物冻熔循环后热性能 Tab.1 Thermal properties of CA-PA eutectic mixture after freeze-melt cycles

综上可知,脂肪酸共晶混合物相变温度适宜,相变潜热大,相变过程均匀且温差较小,化学性质稳定,热循环稳定性突出,满足地采暖地面环境应用要求,是相变储能地板的理想相变材料。

2 传热数学模型的建立

为了分析脂肪酸相变地板应用于电加热地采暖环境的节能性能和供热效果,本研究建立铺设脂肪酸相变地板的电加热地采暖系统传热数学模型,并利用ANSYS软件进行模拟求解。传热数学模型包括房间传热模型和地板传热模型2部分。利用房间传热模型,建立室内空气热平衡方程,计算理论房间冬季室内供暖热负荷,确定相变材料用量以及地板表面与室内空气的换热关系;利用地板传热模型,分析地板模块的储放热过程和温度变化,进而计算室内温度变化,确定脂肪酸相变地板的节能性能和供热效果。

2.1 问题描述

选择一处位于北京地区的典型房间理论模型(图 3a),房间宽4 m、长6 m、高3 m,除一面南向外墙,其他墙面均为内墙,外墙由240 mm水泥砂浆空心砖加20 mm聚乙烯保温板构成。南窗宽2 m、高2 m,由中空玻璃和塑料窗框组成,传热系数为2.77 W·m-2-1。房间每小时换气1次,室内人员、设备和灯光等换热量为15 W·m-2。参考脂肪酸与相变储能板材的物理性质,设定相变储能地板物性参数。相变储能地板和墙体材料物性参数见表 2

图 3 房间理论模型(a)与室内热动态过程(b) Figure 3 Theoretical model(a) and thermal dynamic process(b)of room
表 2 相变储能地板和墙体材料物性参数 Tab.2 Physical parameters of phase change heat storage floor and wall materials

地采暖运行条件下室内热动态过程见图 3b。可以看出,室内热源主要为人员、设备、灯光和电加热层,其中,电加热层通过地板表面辐射和对流与室内空气换热;室内热消耗主要为外围结构与室内空气换热以及空气渗透或通风与室内空气换热。室内热动态过程受室内和室外计算温度影响,根据《室内空气质量标准》 (GB/T 18883—2002)和《民用建筑供暖通风与空气调节设计规范》 (GB 50736—2012),本研究室内计算温度取20 ℃,室外计算温度取北京地区供暖室外计算温度-7.6 ℃。

基于脂肪酸相变地板的电加热地采暖系统,自下而上分为基础层、保温层、电加热层和相变储能地板层。电加热层加热时间为夜晚23时至清晨7时共8 h,此期间电加热层向室内供热并给相变储能地板提供储热热能;电加热层停止加热时间为清晨7时至夜晚23时共16 h,此期间由相变储能地板向室内供热。

2.2 房间传热模型

根据室内热动态过程,建立室内空气热平衡方程:

$ {\rho _{\rm{a}}}V{C_{\rm{a}}}\frac{{\partial {T_{\rm{n}}}}}{{\partial t}} = {Q_1} + {Q_2} + {Q_3} + {Q_4}。$ (1)

式中:ρa为空气密度(kg·m-3);V为房间容积(m3);Ca为空气比热容(J·kg-1-1);Tn为室内空气温度(℃);Q1为外围结构与室内空气的换热量(W);Q2为地板表面与室内空气的换热量(W);Q3为室内热源与室内空气的换热量(W);Q4为空气渗透或通风与室内空气的换热量(W)。

2.2.1 外围结构与室内空气的换热量

设内墙和上下楼板绝热,只有外墙与外窗换热。利用外围结构传热系数公式计算外墙传热系数(W·m-2-1):

$ K = \frac{1}{{\frac{1}{{{\partial _{\rm{n}}}}} + \sum {\frac{\delta }{{\partial \lambda \cdot \lambda }} + {R_{\rm{k}}}} + \frac{1}{{{\partial _{\rm{w}}}}}}}。$ (2)

式中:n为外围结构内表面换热系数(W·m-2-1);w为外围结构外表面换热系数(W·m-2-1);δ为外围结构各层材料厚度(m);λ为外围结构各层材料导热系数(W·m-1-1);λ为材料导热修正系数;Rk为封闭空气间层热阻(m2·℃W-1)。

参考《民用建筑供暖通风与空气调节设计规范》 (GB 50736—2012),取外墙内、外表面换热系数分别为8.7和23 W·m-2-1,计算外墙传热系数。利用外墙和外窗传热系数计算外围结构基本耗热量(W):

$ Q = \alpha \cdot F \cdot K \cdot \left( {{T_{\rm{n}}} - {T_{{\rm{out}}}}} \right)。$ (3)

式中:α为外围结构温差修正系数;F为外围结构面积(m2);Tn为冬季室内计算温度(℃);Tout为供暖室外计算温度(℃)。

外围结构耗热量包括基本耗热量和附加耗热量,附加耗热量(P)按其占基本耗热量的百分率确定:

$ {Q_1} = Q \times P。$ (4)

外围结构附加耗热量主要包括朝向修正率、风力修正率和外门附加率。本研究房间理论模型坐北朝南,不处于大风环境且无外门,因此只计算南向修正率。

2.2.2 地板表面与室内空气的换热量

地板表面与室内空气的换热方式包括对流换热和辐射换热(周兴红,2004王海霞,2005),计算公式如下:

$ {Q_2} = S \times \left( {{q_{\rm{f}}} + {q_{\rm{d}}}} \right)。$ (5)

其中,

$ {q_{\rm{f}}} = 4.98 \times {10^{ - 8}}\left[ {{{\left( {{T_{\rm{f}}} + 273} \right)}^4} - {{\left( {{T_{\rm{r}}} + 273} \right)}^4}} \right]; $ (6)
$ {q_{\rm{d}}} = 2.13{\left( {{T_{\rm{f}}} - {T_{\rm{n}}}} \right)^{1.31}}。$ (7)

式中:S为房间面积(m2);qf为单位地面面积辐射传热量(W·m-2);qd为单位地面面积对流传热量(W·m-2);Tf为地板表面平均温度(℃);Tr为室内非加热表面的面积加权平均温度(℃)。

2.2.3 室内热源与室内空气的换热量

计算公式如下:

$ {Q_3} = S \times N。$ (8)

式中:N为室内热源单位地面面积换热量(W·m-2)。

2.2.4 对空气渗透或通风与室内空气的换热量

计算公式如下:

$ {Q_4} = {C_{\rm{a}}}{\rho _{\rm{a}}}V \cdot \left( {{T_{\rm{n}}} - {T_{{\rm{out}}}}} \right) \cdot n/3\;600。$ (9)

式中:n为房间换气次数(h-1)。

利用空气热平衡方程,确定地板表面与室内空气的换热关系,并求得冬季室内计算温度下的室内供暖热负荷,得出保持16 h室内计算温度的相变材料用量,为脂肪酸相变板的设计提供参数。

2.3 地板传热模型

地板传热模型为脂肪酸相变地板的一维相变传热。本研究利用焓法求解相变传热问题,假设相变材料在熔化和凝固时的比热容与温度呈正比关系且连续变化(Farid et al., 2001叶宏等,2004)。为突出物理本质,合理简化问题以便于计算,作如下假设(林坤平等,2006张群力等,2006冯国会等,2012):1)传热只沿地板厚度方向;2)忽略相变材料融化时的自然对流和凝固时的过冷效应;3)除相变区等效比热外,相变材料在固态和液态的物性相等且为常数;4)室内温度与冬季室内计算温度相同;5)保温层无漏热损失。相变储能地板控制方程如下:

$ \frac{{\partial H}}{{\partial t}} = \lambda \frac{{{\partial ^2}T}}{{\partial {x^2}}}。$ (10)

相变储能地板的焓与温度的关系如下:

$ \begin{array}{*{20}{c}} {{H_{\rm{s}}} = {c_{\rm{s}}} \times T,T \le {T_{\rm{s}}};}\\ {{H_{\rm{m}}} = {H_{\rm{s}}} + \left[ {\left( {{c_{\rm{s}}} + {c_{\rm{1}}}} \right)/2 + \Delta H/\left( {{T_1} - {T_\text{s}}} \right)} \right] \times }\\ {\left( {T - {T_{\rm{s}}}} \right),{T_{\rm{s}}} < T \le {T_1};}\\ {{H_1} = {H_{\rm{s}}} + {H_{\rm{m}}} + {c_1} \times \left( {T - {T_1}} \right),T \ge {T_1}。} \end{array} $ (11)

式中:H为焓;c为比热容;T为温度;下标s、m和l分别代表固态、相变态和液态;ΔH为相变潜热。

地板传热模型边界条件和初始条件如下:

$ 地板上表面:\lambda \frac{{\partial T}}{{\partial t}} = {q_{\rm{d}}} + {q_{\rm{f}}}; $ (12)
$ 地板下表面: - \lambda \frac{{\partial T}}{{\partial t}} = {q_{{\rm{power}}}}; $ (13)
$ 初始条件:t = 0,T = {T_{{\rm{init}}}}。$ (14)
3 模拟计算与结果分析

本研究利用ANSYS软件求解传热数学模型,ANSYS分析可概括为创建模型、施加载荷和输出结果3个步骤。

施加载荷时,由于地板上表面包括对流和辐射2种换热方式,且换热系数随地板表面平均温度和室内温度变化,不能设定为常数。因此本研究根据空气热平衡方程及式(15)、(16),将对流和辐射换热系数换算为地板上表面温度的函数再进行施加:

$ {T_{\rm{f}}} = {T_{\rm{n}}} + 9.82 \times {\left( {\frac{{{q_{\rm{x}}}}}{{100}}} \right)^{0.969}}; $ (15)
$ h = \frac{{{q_{\rm{d}}} + {q_{\rm{f}}}}}{{{T_{\rm{f}}} - {T_{\rm{n}}}}}。$ (16)

式中:qx为单位地面面积所需散热量(W·m-2);h为地板上表面综合换热系数(W·m-2-1)。

输出结果时,以地板内部温度分布云和地板上表面温度曲线的形式观察模拟结果。

脂肪酸相变储能地板在地采暖环境中运行第1天的储放热过程见图 4。可以看出,相变储能地板能够持续稳定地在夜晚吸收热能、在白天放出热能,将白天的用电负荷转移至夜晚,实现“移峰填谷”式采暖,节能性能突出。脂肪酸相变储能地板在地采暖环境中连续运行3天的地板表面和室内温度变化见图 5。可以看出,在相变储能地板相变过程中,地板表面和室内温度变化明显减缓,储放热过程均匀规律,尤其是放热过程,相变材料全部凝固用于供热,热利用效率高,且室内温度平稳舒适,温差仅3 ℃左右,供热效果理想。

图 4 脂肪酸相变储能地板内部温度分布随时间的变化 Figure 4 Variation of temperature distribution in fatty acid phase change heat storage floor with time
图 5 脂肪酸相变储能地板表面温度(a)和室内温度(b)随时间的变化 Figure 5 Variation of temperature in fatty acid phase change heat storage floor surface(a) and indoor (b) with time

另外,从模拟结果还可发现,脂肪酸相变储能地板相变温度在23.25~26.57 ℃时,室内温度保持在19~22 ℃,考虑人体舒适温度为16~24 ℃,因此相变储能地板用相变材料的相变温度范围可在20~28 ℃之间。本研究只是针对一处北京地区典型地采暖环境进行了模拟分析,不同气候地区、热负荷建筑设计和地采暖工况等都可能影响相变储能地板的适宜相变温度,可以通过改变脂肪酸共晶混合物的种类和配比,灵活调整脂肪酸相变地板的相变温度,使其在各种情况下都能发挥出良好的节能性能和供热效果。

4 讨论

基于脂肪酸优异的热性能和稳定的理化性质,以脂肪酸为相变材料的相变储能板材作为地采暖地板应用时具有理想的节能性能和供热效果。常用脂肪酸共晶混合物的相变温度可在10~70 ℃之间灵活调整,因此除了地采暖环境外,脂肪酸相变储能板材还具备用在其他适宜节能领域的潜力,如太阳能、地热能的收集应用以及废热余热的回收再利用等;另外,脂肪酸相变材料的相变温度十分适用于建筑室内,因此除了地面应用外,还可用于墙面、吊顶和家具等室内装饰部分,以减少建筑内外的热流波动,降低室内热量需求,进而降低室内供暖设施的工作量,在实现节能的同时提高室内舒适度。

脂肪酸相变储能板材具有多样的应用方式和广阔的应用前景,未来需要充分挖掘其在各种应用环境中的节能潜力,为新型节能材料的开发与应用提供参考。

5 结论

本研究建立了铺设脂肪酸相变地板的电加热地采暖系统传热数学模型,通过对传热模型的模拟和分析发现,相变材料的热性能是影响相变储能地板节能性能和供热效果的关键因素。脂肪酸共晶混合物热性能分析表明,其相变温度在20~30 ℃之间,并可通过改变混合物的种类和配比,灵活调整相变温度,对于多变的应用环境具有很强的适用性;相变熔程仅2~3 ℃,供热过程平稳且温差小;相变潜热在170 J·g-1左右,储能密度大。加之稳定的化学性质与突出的热循环稳定性,脂肪酸共晶混合物是相变储能地板理想的相变材料。

在相变材料足量且相变温度适宜的条件下,脂肪酸相变储能地板用于一般的地采暖环境中,可以实现夜晚8 h储热、白天16 h放热的“移峰填谷”式稳定供暖,节能性能突出,同时可将室内空气和地板表面温度控制在人体适宜范围内,供热效果理想,为新型地采暖储能地板的开发和应用提供了理论基础。

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