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ScienceDirect Procedia Engineering 118 (2015) 760 – 765

International Conference on Sustainable Design, Engineering and Construction

Simulating the Impact of Phase Change Material Embedded Building Envelopes on the Inter-Building Effect in Non-tropical Cities Yilong Hana, John E. Taylor a,* a

Charles E. Via, Jr. Department of Civil & Environmental Engineering, Virginia Tech, Blacksburg, VA, 24061

Abstract The built environment contributes significantly to rapidly growing world energy consumption. Along with urbanization, buildings continue to escalate this trend owing to their tighter spatial interrelationships and the influence of their surrounding micro-environment. The concept of the Inter-Building Effect (IBE) was introduced to understand complex mutual impact within spatially-proximal buildings. Disaggregate analysis further quantified shading effects and reflection effects separately from combined IBE interaction in a more nuanced way. Different from tropical cities where mutual reflection always shows a negative impression, recent research revealed more comp lex scenarios in non-tropical areas as the reflection or shading could become favorable month by month alternately according to climatological context s. The application of phase change materials (PCM s) has attracted attention due to its important characteristic to store and release heat within a certain temperature range. A variety of research has been conducted for building applications to improve energy conservation and thermal comfort both numerically and empirically. In this paper, we sought to explore and understand if PCM building envelopes could potentially mitigate negative thermal-energy impact within building canyons in a non-tropical area. Building upon previous IBE research and simulation models, we conducted several building network simulations under different climatological contexts of non-tropical cities. The results showed considerable improvements (up to 12%) of annual H VAC energy consumption when PCM -embedded building envelopes were used in the control building. The findings expand and deepen our understanding of the IBE, and may help minimize negative mutual influences among buildings that lead to increases in energy consumption in urban environments. Published by Elsevier Ltd.Published This is an open access article © 2015 The Authors. by Elsevier Ltd.under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of organizing committee of the International on Sustainable Design, of the International Conference onConference Sustainable Design, Engineering and Engineering Peer-review under responsibility of organizing committee and Construction Construction 2015 2015.

Keywords: Building Networks; Energy Efficiency; Inter-Building Effects; Phase Change Materials; Simulation

* Corresponding author. Tel.: +1 540 231 0972; Fax: +1 540 231 7532. E-mail address: [email protected].

1877-7058 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 organizing committee of the International Conference on Sustainable Design, Engineering and Construction 2015

doi:10.1016/j.proeng.2015.08.511

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1. Introduction Energy and its impact on the environment has become a central issue facing society. According to the latest energy data statistics from the International Energy Agency [1], annual global energy consumption has grown from 4000Mtoe in 1971 (million tonnes of oil equivalent) to nearly 9000Mtoe in 2011. Over the last 40 years, CO2 emissions have doubled. The rapidly growing world energy expenditure not only causes supply difficu lties and depletion of fossil energy resources, but also significantly influences the human liv ing environ ment by t r iggering climate deterioration locally (e.g. Urban Heat Island effects) and worldwide (e.g. global warming). The building sector contributes significantly to the rising trends of energy use by accounting for 32% of total final energy consumption and nearly 40% of primary energy consumption [2]. Modern developed countries generally consume substantial build ing-related energy. However, ongoing and emerging industrial developments and urbanization in fast developing countries and underdev eloped regions will intensify energy usage and may cause supply difficulties [3]. As a result, the energy and environmental issues initiated by buildings would become more challenging to the built environment itself and to society. Therefore, how to achieve a more sustainable built environment has become a grand challenge for engineers, building researchers, urban planners, and policy makers. 2. Background and research objectives In the presence of rapid urbanization, the relat ionship between building density and urban form has a ttracted wide interest, as it is expected that tighter and more co mplex building geo metries will be mo re prevalent in u rban areas in the following decades. Urban mo rphology, characterized by building density, size, height, orientation, and layout, could cause considerable variations in the local environ ment and microclimates. Mathematical and geometrical analyses have been conducted to study the issue concerning building height, plot rat io, orien tation, solar obstruction, etc., and early research generated insightful d iscoveries that urban microclimates and build ings are inextricably interwoven [4, 5]. It is difficult to predict one build ing’s energy performance accurately without considering the close proximity of other buildings and energy implication that could result. To understand the complex interactions that cause urban thermal-energy dynamics within spatially pro ximal building networks, the concept of the Inter-Building Effect (IBE) has been introduced and further investigated over the last several years [6, 7]. Research has demonstrated that the interrelationship between buildings within building networks results in substantial inaccuracies (up to 42% in su mmer, and up to 22% in winter) of energy consumption predictions for space heating, space cooling, and lighting. The research also demonstrates that, irrespective of the climatological context, the energy performance of one building can be meaningfu lly impacted by surrounding buildings through mutual reflection and mutual shading, the two primary c omponents that make up the IBE. In order to better understand and explore successful urban planning and building designs th at could mit igate negative IBE, researchers disaggregated and quantified the shading effect and reflection effect separately fro m the IBE. The more nuanced analysis found shading to contribute to increased heating loads while reflect ion increased cooling energy required for spatially-pro ximal buildings. The monthly analysis of several weather profiles of several U.S. metropolitan cit ies under different climate zones revealed that the contribution of reflection and shading varies on a month-to-month basis as the impact of IBE is closely related to geographical location and climatic conditions. Building energy performance in tropical cities with longer summers, longer daylight time and higher demand for cooling energy would benefit fro m mitigated reflection effect all year round. However, in non -tropical cities, the outcome of the IBE is rather complicated as the reflection or shading might turn out to be favorable or unfavorable month by month. Limited research has sought solutions to mitigate negatively thermal energy inter-building relationships. Although directional reflective build ing envelopes have been suggested to help lessen mu tually reflected solar radiat ion and mitigate Urban Heat Island effects within urban canyons in tropical/warmer areas, the solutions to address a similar

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negative effect is still largely unrealized in non-tropical areas where shading/reflection impact the IBE differently depending on the season. With the important characteristic to store and release heat within a certain temperature range, the use of phase change materials (PCMs) is considered as a suitable and promising solution for not only increasing indoor thermal comfo rt but also reducing the energy consumption of buildings [8]. Conventional building materials usu ally have sensible heat storage effects such that the heat is stored or released accompanied with temperature changes in the storage media. In contrast, PCMs have greater heat storage capacities with latent heat storage, where the heat is stored or released as heat of fusion/solidification during the phase change process of the storage. They can be divided into different subcategories based on their chemical co mposition, such as organic compounds, inorganic compounds and eutectics. Each material has its typical range of melting temperature and its range of melting enthalpy which could be employed for the thermal comfort context. PCMs have attracted attention by building researchers in the recent past, and several recent papers [8-10] reviewed the state-of-the-art on knowledge of PCMs today specifically fo r building application s including PCM selection, PCM thermal stability, impregnation of PCMs into construction materials (such as wallboards, walls , floors and ceilings), and parameters and calibration for PCM numerical simulat ion. Prev ious exploration demonstrated [8-10] that PCMs have different benefits depending on quantity and types, locations, and climates mostly discussed within the context of a single building. In this paper, we seek evaluate the applicat ion of PCMs fro m a perspective of inter-build ing relat ionships that extend beyond a stand-alone building scenario. The objective of this paper is to exp lore the impact of PCM technology in a dense urban building environment, and to study and understand if PCM -embedded building envelopes could serve as a possible solution to mit igate negatively thermalenergy impact of IBE in non-tropical regions. 3. Methodolog y For the purposes of front-end design and a more sustainable building lifecycle, building researchers have started to employ nu merical analysis. Simulat ion tools offer powerful functionalit ies to predict and improve building energy consumption for both research and design purposes. Of current main stream simulat ion environment and platforms, EnergyPlus [11], an energy analysis and thermal load simulat ion engine developed and distributed by the U.S. Depart ment of Energy, has become a popular build ing energy performance simu lation tool owing to its sophisticated and validated functions. It was also utilized for previous IBE research dynamic building analyses. Early IBE simu lation efforts were conducted based on a realistic physical urban block in the state of New Yo rk to study energy consumption of space cooling and space heating [7]. Later research was expanded to investigate the energy discrepancies in lighting and validated the IBE [6], e.g., mutual shading and mutual reflect ion, as an important effect to be modeled in situations where buildings are surrounded by other nearby buildings. Real -wo rld experimental work and empirical data were used to calibrate and verify the simu lation work. For the authenticity of the simulat ion results, we retained in formation about construction materials, temperature set -points, and schedules for lighting, equipment, and occupants from previous case studies. With that, we built a n urban block model with a network of nine buildings, and treated the middle building as the control building in which the energy and thermal properties were mon itored during the building energy simu lation. An essential geometric element for shading and reflect ion in the EnergyPlus environment, the shading surface, was used extensively to achieve our modelling objectives while avoiding excessive running times. The ability to simulate PCMs has been developed and incorporated in EnergyPlus using a one -dimensional conduction fin ite difference algorith m. Recently, Tabares -Velasco et al. [12] developed a procedure to verify and validate the PCM model using an approach as dictated by ASHRA E Standard 140, and also provide d several suggestions addressing the limitations of current EnergyPlus PCM models. In th is study, a PCM model was built referring to published data and validated experiment parameters as shown in Table 1. A micro-encapsulated PCM

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layer with thickness of 0.037 meter was embedded to building exterior walls (between external brickwork and concrete block) inherited fro m previous ly published IBE models. The temperature-enthalpy profile of PCMs for this study was as shown in Figure 1. To avoid simulat ion errors, the time step of the simu lation was set to three minutes as suggested by the program developers. Monthly and annual simulation outputs were reported in the results. T able 1. Material properties of PCM-embedded layers T hickness (m)

0.037

Conductivity (W/m-K)

0.2

Density (kg/m3)

235

Specific Heat (J/kg-K)

1970

Fig. 1. T emperature-Enthalpy profile of PCM-embedded layers

Previous IBE research has found that IBE effects vary by climatological context. To investigate the impact of PCM bu ild ing envelopes to the IBE in non-tropical reg ions, two typical U.S. cit ies, Minneapolis, MN and Washington, D.C. were selected for a preliminary study. Minneapolis, MN represents an extreme climate condition as the coldest metropolitan city in the United States, while Washington, D.C. has a distinctive and roughly equal length seasons as a typical temperate city. Under the weather prof ile of each city, the impact of PCM was tested through a comparative study by setting the building envelopes of the control building either conventional or PCM embedded, and the energy perfo rmance of the control build ing was mon itored and reported over the course of simulation period. 4. Analysis and results Annual results of control building energy consumption including space heating, space cooling, and total primary energy are contained in Table 2 for Minneapolis, MN and Washington, D.C.. The left colu mns of the table indicate the simulat ion results fro m the conventional IBE when regular construction materials were used for the control building; the numbers on the right report the control building’s energy performance when PCMs were embedded in exterior walls. Variation percentages were also calculated according to the baseline conventional IBE value s. A “-” sign means that savings would occur for that particular situation, indicating less energy is being used.

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T able 2. Cross-regional analysis of the control building’s energy consumption with different building envelopes Conventional Building Envelopes PCM-embedded Building Envelopes Heating (kWh) Cooling (kWh) T otal (kWh) Heating (kWh) Cooling (kWh) T otal (kWh) Minneapolis, MN

35871

12033

47904

30676

11477

42153

-

-

-

(-14.5%)

(-4.6%)

(-12.0%)

Washington, D.C.

16620

16489

33109

13660

15511

29171

-

-

-

(-17.8%)

(-5.9%)

(-11.9%)

Our first observation from Table 2 is that PCM bu ild ing envelopes lead to energy conservation in all three studied categories, space heating consumption, space cooling consumption, and total HVA C energy consumption, under an dense urban IBE environment, as the results of variation percentages stay negative for both Washington, D.C. and Minneapolis, MN. The control building consumed nearly the same amount of heating and cooling in the temperate city of Washington, D.C. , while the cooling energy consumption was almost three times compared to heating in the coolest city in the United States (Minneapolis, MN). Heating consumption showed greater improvement in both Washington, D.C. (-17.8%) and Minneapolis, MN (-14.5%). The numbers for cooling energy (-5.9% and -4.6%) were less substantially impacted. Overall, total primary energy use for HVAC systems decrease considerably in both cities by around 12%. 5. Discussion Due to economic boo ms, urban sprawl and population migration, build ings and the urban built environ ment have contributed significantly to and will continue to exacerbate global energy concerns. Previous research has demonstrated that one building’s energy performance is influenced by its nearby microenvironment creat ing InterBuilding Effects (IBE) within spatially-pro ximal building networks. Researchers disaggregated mutual shading and mutual reflection for a more nuanced analysis of the comp lex IBE and found inconsistent and complicated trends to deal with the IBE in non-tropical areas. The research presented in this paper contributes to the explorat ion of solutions to mitigate the negative impact by the IBE in non-tropical dense urban settings. With the capabilities to shift and decrease peak loads, PCMs have been considered as a promising technology to both improve human living co mfort and reduce build ing energy consumption without substantial increase in the weight of the construction material. Prev ious exp loration in the built environ ment demonstrate that PCMs have different benefits depending on quantity and types of PCM selection, locations of imp lementation and climates. Th is paper expands the discussion of the use of PCM technology fro m one single building analysis to a more realistic inter-building environ ment. We found the application of PCM embedded building envelopes to contribute to decreasing loads in both heating and cooling despite the different climat ic co nditions in non-tropical regions, and the total primary energy consumption for HVA C reduced by up to 12%. Th is is due to the fact that PCMs raise the building inertia to solar light wh ich in itiates mutual shading and mutual reflect ion of IBE, and may help to minimize negative mutual influences between buildings that lead to increases in energy consumption in urban environments. The purpose of this research was to examine the impact of PCM-embedded building envelopes to thermal-energy dynamics within an inter-building environ ment, and to quantify the influence to the negative IBE impact in nontropical areas. Utilizing advanced building energy simu lation, the results provide us a more exp licit understanding of the dynamic response of PCM technology to inter-building relationships. The simu lation results also suggest that PCM -embedded building envelopes could serve as possible solutions to mit igate unfavorable thermal-energy IBE outcomes. Nevertheless, the modeling and simulation efforts also result in some limitations. The build ing network model used for this study contains only nine buildings to keep the research scope reasonable, it is possible that a more co mplicated outcome due to larger urban scale may exist and, if so, was neglected in the research. Future research should address the limitations to understand the impact of PCM s fro m a larger scale analysis , incorporate

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emp irical studies that could calibrate simulat ion analysis, as well as seek out other measures that could be used to mitigate the negative IBE impacts. 6. Conclusion The research in th is paper built upon and extended previous approaches that studied energy predictions in a dense urban building network. As the negative impact of IBE shows complicated and dynamic results in non -tropical cities, a procedure to examine the impact of PCM build ing envelopes to IBE was developed and conducted in this paper. Two typical cities were chosen for the preliminary investigation in the dynamic EnergyPlus simulat ion environment. By using different materials in building envelopes in the comparative study, the findings of energy consumption in the control building demonstrated considerable improvements and consistent trends when PCM technology is imp lemented in building exterior walls. This research expands our understanding of the use of PCMs fro m a stand-alone building scenario to an urban dense inter-building environ ment, and the results suggest PCM building envelopes as possible solutions to mit igate negative inter-building influences and improve energy efficiency within urban building networks in non-tropical cities. Acknowledgements This material is based in part upon work supported by the National Science Foundation under Grant No. 1142379. Any opinions, findings, and conclusions or reco mmendations expres sed in this material are those of the authors and do not necessarily reflect the view of the National Science Foundation. References [1] Hui SCM. Low energy building design in high density urban cities. Renew Energ 2001;24:627-40. [2] International Energy Agency. Energy Efficiency. . [3] Architecture2030. Architecture 2030 Will Change the Way You Look at Buildings. 2013. . [4] Zhao H-X, Magoulès F. A review on the prediction of building energy consumption. Renew S Energ Rev 2012;16:3586-92. [5] Li B, Yao R. Urbanisation and its impact on building energy consumption and efficiency in China. Renew Energ 2009;34:1994-8. [6] Davis K. T he urbanization of the human population. In: Richard TL, Frederic Stout, edtitors. The City Reader, Taylor & Francis; 2011. [7] United Nations, Department of Economic and Social Affairs. World Urbanization Prospects, the 2011 Revision. . [8] Yao R, Steemers K. Urban Microclimates and Simulation. In: Yao R, editor. Design and Management of Sustainable Built Environments, Springer; 2013. p. 77-97. [9] de la Flor FS, Domıń guez SA. Modelling microclimate in urban environments and assessing its influence on the performance of surrounding buildings. Energ Build 2004;36:403-13. [10] Yang X, Grobe L, Stephen W. Simulation of reflected daylight from building envelopes. International IBPSA Conference2013. [11] Conceição António CA, Monteiro JB, Afonso CF. Optimal topology of urban buildings for maximization of annual solar irradiation availability using a genetic algorithm. Appl Therm Eng 2014;73:422-35. [12] Krüger E, Pearlmutter D, Rasia F. Evaluating the impact of canyon geometry and orientation on cooling loads in a high-mass building in a hot dry environment. Appl Energ 2010;87:2068-78. [13] Pisello AL, Taylor JE, Xu X, Cotana F. Inter-building effect: Simulating the impact of a network of buildings on the accuracy of building energy performance predictions. Build Environ 2012;58:37-45. [14] Pisello AL, Castaldo VL, T aylor JE, Cotana F. Expanding Inter-Building Effect modeling to examine primary energy for lighting. Energ Build 2014;76:513-23. [15] Ellis PG, T orcellini PA. Simulating tall buildings using EnergyPlus. International IBPSA Conference2005. p. 279-86. [16] Crawley DB, Lawrie LK, Winkelmann FC, Buhl WF, Huang YJ, Pedersen CO, et al. EnergyPlus: creating a new-generation building energy simulation program. Energ Build 2001;33:319-31. [17] U.S. Department of Energy. EnergyPlus engineering reference: the reference to EnergyPlus calculations. 2013.