Effects of Thermophysical Properties of Wall Materials on ... - Core

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ScienceDirect Energy Procedia 75 (2015) 1850 – 1855

The 7th International Conference on Applied Energy – ICAE2015

Effects of thermophysical properties of wall materials on energy performance in an active building Linshuang Long, Hong Ye* Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, Anhui 230027, P.R. China

Abstract A high-performance building envelope is the prerequisite and foundation to a zero energy building. For the nontransparent part of envelope, the thermal conductivity and volumetric heat capacity are two thermophysical properties which can strongly influence the energy performance of the envelope. Although a lot of case studies have been performed on these two properties, the results could not give a comprehensive picture of the roles of thermal conductivity and volumetric heat capacity on the energy performance in an active building. In this work, a traversal study on the energy performance of a standard room with all potential wall materials was provided. These materials were differentiated by distinct thermal conductivity and volumetric heat capacity. It was revealed that the external wall requires a material with a low thermal conductivity and a high volumetric heat capacity, and the internal wall needs a material with a high volumetric heat capacity. These requirements for wall materials are consistent under various climate conditions. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute

Keywords: Building energy efficiency; wall materials; insulation performance; volumetric heat capacity; building energy simulation

1. Introduction The application of Zero Energy Buildings (ZEBs) has been perceived as a promising way to reduce both energy consumption and carbon dioxide emission [1-4]. Despite the accurate definition of ZEB is ambiguous [5], a high-performing building envelope is the prerequisite and foundation to a ZEB [6]. A building envelope includes two general parts: the transparent and non-transparent one. The nontransparent part of the envelope can be further divided into two types: the external envelopes in direct

* Corresponding author. Tel.: +86-551-63607281; fax: +86-551-63607281. E-mail address: [email protected].

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.161

Linshuang Long and Hong Ye / Energy Procedia 75 (2015) 1850 – 1855

contact with the outside environment (including solar radiation, outdoor air, etc.) and the internal envelopes. Widely investigated strategies to optimize the non-transparent envelope are to enhance their heat capacity [7, 8] as well as thermal insulation performance [9-11]. Most existing investigations were carried out experimentally or numerically with the method of case study. Case study is effective to give a direct comparison among particular cases. However, the meaning of the results is inevitably restrained: only a few types of materials or configurations of layers can be compared and evaluated. With these limited results, the effect of thermal insulation performance and heat capacity on energy performance can hardly be investigated comprehensively and there is always a lack of a big picture of the scene. Due to the fact that the insulation performance and heat capacity are inherent and concomitant properties of the envelopes, some questions remain to be addressed. How do these properties of a wall affect the energy performance of a building? How does the story diversify for different seasons or climate conditions? To answer these questions, one requires an overall concept about the roles of the insulation performance and heat capacity on the energy performance of the envelopes. From the aspect of engineering thermophysics, the insulation performance and the heat capacity of the envelopes can be characterized respectively by the thermophysical properties of the thermal conductivity and volumetric heat capacity of the materials. In this work, we performed a traversal study on the energy performance of a standard room with all potential wall materials. These materials were differentiated by distinct thermal conductivity and volumetric heat capacity. An overall concept on the energy performance can then be provided through a succinct approach.

2. Methods 2.1. Description of the room The standard room is assumed in a middle story of a multi-story apartment building. The room has internal dimensions of 4×4×4 m3 and contains only one external wall facing towards the south. The other walls, the ceiling and the floor are not directly exposed to the outdoor environment. The thickness of the exterior wall is 240 mm and that of the interior ones is 100 mm. For an actual room, a window is necessary and usually located in the external wall. However, windows and walls belong to the transparent and non-transparent parts of the building envelopes, respectively, leading to different effects on the energy performance. To exhibit the effect of the non-transparent envelopes, we at first removed the window from the external wall, and then investigated the room with a window to show its influence. In practical applications, a room without a window can be an analogy of a container or a freezer, which makes the study of such a special room also meaningful. The indoor temperature of the room is maintained with heating, ventilation and air conditioning (HVAC) facilities at 18 and 26 qC in the heating and cooling seasons, respectively. The room does not include any internal heat gain. The energy performance of the non-transparent envelopes with different materials will be calculated through a simulation tool.

2.2. Simulation tool The energy performance of the room is simulated using an energy modeling program called BuildingEnergy. Two primary simulation assumptions are made in BuildingEnergy: (1) the heat transfer

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across the building envelope is one-dimensional; and (2) the indoor temperature of the room and the adjacent rooms is the same, so the symmetric surfaces of the interior walls are adiabatic. BuildingEnergy has been validated using ANSI/ASHRAE Standard 140-2004 ("Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs") in our previous work [15]. We also validated the program through a series of experiments conducted in full-size rooms [16, 17]. 2.3. Ranges of the parameters The objective of this subsection is to rule the ranges of thermal conductivity and volumetric heat capacity to enable an overall investigation on all potential materials. The lower limit of the thermal conductivity of building components is reportedly achieved for a vacuum insulation panel (VIP), whose effective thermal conductivity can be as low as 0.002 W/(m·K) [18-20]. In practical applications, the thermal conductivity of some rocks is high in relation to other building materials except metals and can be set as the upper limit. For example, a quartzite (Sioux) has a conductivity of 5.38 W/(m·K) (adapted from Appendix A in [21]). In this study, we roughly set the thermal conductivity, k, in the range from 0.001 to 5 W/(m·K). The mass density of building materials is usually lower than 3000 kg/m3, and the specific heat capacity is usually less than 3000 J/kg (except phase change materials during their melting process). However, the materials with a high density generally have a low specific heat capacity and those with high specific heat capacity often have a low density. For instance, a marble (Halston) has a high density of 2680 kg/m 3, while a specific heat capacity of 830 J/kg; a yellow pine wood has a high specific heat capacity of 2805 J/kg, whereas a density of 640 kg/m3 (adapted from Appendix A in [21]). These facts make the product of the density and specific heat capacity, i.e., the volumetric heat capacity, approximately lower than 3000 kJ/(m3·K). Conservatively, the upper limit of the volumetric heat capacity is set as 5000 kJ/(m3·K). Regarding the lower limit, it is set as 50 kJ/( m3·K), referring to a polyurethane. Namely, the range of the volumetric heat capacity, CV, is from 50 to 5000 kJ/(m3·K). 3. Results and discussion 3.1. External walls The thicknesses of the external and internal walls were held at 240 and 100 mm, respectively. All potential materials of k and CV within the aforementioned ranges were computed in BuildingEnergy as the external or internal walls. The room was assumed to locate in Hefei, China, where the cooling/summer season is from June 15th to September 5th and the heating/winter season is from December 5th to March 5th of the following year. The climate data used in BuildingEnergy were the typical yearly meteorological data provided by the Chinese Architecture-specific Meteorological Data Sets for Thermal Environment Analysis. Fig. 1 shows the energy consumption contours for the external walls with different materials, in which materials of the internal walls were fixed as common bricks. As Fig. 1 depicts, both the conductivity and volumetric heat capacity of the external-wall materials can strongly impact the energy performance. The energy consumption varies extensively along with k and CV. A value of zero can be achieved with an extremely low k, also due to the absence of a window and internal heat source. For the summer


application (Fig. 1 (a)), generally, decreasing conductivity and increasing volumetric heat capacity of the materials cause a reduction in cooling energy consumption of the room. However, these properties have distinct significance according to ranges. Within the range of k Ә 0.25 W/(m·K), the contour lines are nearly horizontal, implying that the volumetric heat capacity of materials has negligible effect on the energy performance. As k increases, the slope of the contour lines also increase, namely, the significance of CV is increasing. When k is higher than 3.0 W/(m·K), the lines are roughly vertical, which means that the energy performance is predominantly affected by CV. For the winter application (Fig. 1 (b)), the general tendency is consistent with that in summer, but the slope of the contour lines is almost zero when CV ә2000 kJ/(m3·K), indicating that CV has a limited influence in winter.

Fig. 1 Energy consumption contours related to the external walls with various materials in Hefei.

3.2. Internal walls Now we consider the energy performance of the internal-wall materials. Similar contour lines are presented in Fig. 2, in which the external-wall materials were maintained as common bricks. It can be observed that the energy consumption decreases as k increases when k Ә 0.5 W/(m·K). For materials with k higher than 0.5 W/(m·K), the contour lines are horizontal so the energy performance is influenced exclusively by the volumetric heat capacity. The increase in CV causes the reduction in both cooling and heating energy consumptions. Note that the energy consumption ranges in summer and winter are 7.2 ~ 8.3 and 35.88 ~ 36.28 kWh/m2, respectively. Nevertheless, the corresponding ranges in Fig. 1 are 0 ~ 22.5 and 0 ~ 87.2 kWh/m2. The much larger ranges imply a more significant role of the external wall on the energy performance, meanwhile, a greater potential for improvement.

Fig. 2 Energy consumption contours related to the internal walls with various materials in Hefei.

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3.3. Climate conditions The foregoing discussion was established in the city of Hefei, which has a climate of hot summer and cold winter. To examine the effect of the climate conditions, we also simulated the situations for Beijing with a cold climate, and Guangzhou with a climate of hot summer and warm winter. The results shows that the trends of influences of the conductivity and volumetric heat capacity on the energy consumption are utterly the same as those in Hefei, excluding the impact of climate on optimizing of wall materials. The difference occurs exclusively in the energy consumption ranges: the rooms in Guangzhou perform higher cooling consumption, and those in Beijing perform higher heating consumption. 3.4. Windows As previously declared, we have ignored the potential effect of the window until now. Here we reconsider the performance of a room with a window. The single-glazed window, which locates in the center of the external wall, has a size of 1.5×1.5 m2 and a solar transmittance of 77 %. Comparing the results with and without a window, it is discovered that the existence of the window only raises the cooling energy consumption, and does not change the trend of the influence of wall material on the energy performance. The lowest energy consumption that can be obtained through the improvement in the external wall was zero (Fig. 1 (a)), while the corresponding value is 11.4 kWh/m2. The gap between the lower limits was generated by the transparent part of the envelope, i.e., the window, and may be eliminated through the development of the window, revealing that a high-performance building envelope should be achieved by the simultaneous improvement in the transparent and non-transparent parts. 4. Conclusions In this study, the thermal conductivity and volumetric heat capacity of the wall materials were investigated to elucidate the roles of thermal insulation performance and heat capacity on the energy performance of the external and internal walls in an active building. Through a traversal study, we can reach the following conclusions. y For an external wall, the thermal conductivity of the materials should be low, and the volumetric heat capacity should be high. y For an internal wall, the volumetric heat capacity plays a dominate role on the energy performance and should be as high as possible. The thermal conductivity should also be high, but a valve higher than 0.5 W/(m·K) is unnecessary. y These requirements of the materials hold whatever the climate conditions. Acknowledgements This work was funded by the National Basic Research Program of China (Grant No. 2009CB939904). References [1] Baetens R, De Coninck R, Van Roy J, Verbruggen B, Driesen J, Helsen L, et al. Assessing electrical bottlenecks at feeder level for residential net zero-energy buildings by integrated system simulation. Appl Energ. 2012;96:74-83.


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Biography Dr. Hong Ye is an associate professor at department of thermal science and energy engineering, University of Science and Technology of China. Dr. Ye’s research interests are in the areas of heat transfer, application of solar energy, building efficiency, measurement of thermal properties and thermophotovoltaic (TPV) system.

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