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Hygrothermal initial condition for simulation process of green building construction
Accepted Manuscript
Hygrothermal initial condition for simulation process of green building construction Marian ´ Vertaľ , Marek Zozulak ´ , Anna Vaˇskova´ , Azra Korjenic PII: DOI: Reference:
S0378-7788(18)30452-3 10.1016/j.enbuild.2018.02.004 ENB 8320
To appear in:
Energy & Buildings
Received date: Revised date: Accepted date:
3 March 2017 5 September 2017 5 February 2018
Please cite this article as: Marian ´ Vertaľ , Marek Zozulak ´ , Anna Vaˇskova´ , Azra Korjenic , Hygrothermal initial condition for simulation process of green building construction, Energy & Buildings (2018), doi: 10.1016/j.enbuild.2018.02.004
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Highlights Water content condition’s impact on green roof’s hygrothermal behaviour is shown. Impact of water content in substrate on roof membrane temperature is presented. Impact of water content in substrate on reducing heat flow through the roof is shown. Simulation tools application into green roof design process in Slovakia is emphasised.
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Hygrothermal initial condition for simulation process of green building construction Marián Vertaľa, Marek Zozulákb, Anna Vaškovác, Azra Korjenicd Department of Building Physics, Institute of Architectural Engineering, Civil Engineering Faculty, Technical University of Košice, Košice, Slovakia Civil Engineering Faculty, Technical University of Košice Vysokoškolská 4, 042 00 Košice, Slovakia Phone number: 00421 55 602 4167, E-mail address: [email protected]
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a,b,c
d
Vienna University of Technology, Institute for Building Construction and Technology, Research Centre of Building Physics and Sound Protection, Karlsplatz 13/206-2, 1040 Vienna, Austria Abstract:
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Since water is inextricably linked with life, its presence in green building constructions is an inevitable aspect in the process of hygrothermal behavior analysis. Water content in the substrate of green structures is a variable phenomenon throughout a year. During this period it has a strong influence on the building’s hygrothermal behavior.
The WUFI simulation tool analyzes hygrothermal behavior of the tested extensive green roof
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in the climatic conditions of East Slovakia (Košice). The presented simulation scenarios are aimed at assessing the influence of water in the green roof substrate on the roof membrane
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temperature and the fluctuation of heat fluxes through the roof during summer period (June 1 - August 31). Presence of water and its movement through capillary forces is an essential
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aspect of hygrothermal behavior and cannot be neglected in the simulation process. Water in the tested green roof substrate has reduced the average daily heat flow across the roof by 11%,
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which significantly contributed to lowering its summer overheating. Calculated water content in the substrate was influenced by the selected initial water content condition for up to two-
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thirds of the simulated period's length.
Key words: green roof, initial water content, heat flux, summer overheating, hygrothermal simulation
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Introduction Vegetation, as part of building structures, appeared as early as ancient times. Over the last 50 years, Europe has witnessed rediscovery of green structures. However, their application has primarily been of aesthetic significance. The importance of green roofs for rainwater retention and retention of water in the country is by far the main benefit (storm water management capacity), but in addition to that there are other important aspects of green building constructions getting into the spotlight. In recent years, green building structures have been
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subject to quite an intense talk in terms of the energy aspects of their existence. Among the most important benefits of green roofs are: transforming dead space into garden space, roof layers' protection and subsequently its life extension, reducing air-conditioning and winter heating costs, reducing storm water run offs, improving aesthetics, reducing "heat island" effect in cities, providing green space including new habitat for wild life and many other
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benefits. The increase of implemented green roofs across Western Europe goes hand in hand with increasing public awareness of their positive impact on the mankind, human society and the environment itself. In sharp contrast to that, green roofs in Slovakia are still vastly perceived as just an aesthetic element. When designing buildings in the project phase, green
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building constructions, especially green roofs, are usually a dominant architectural feature of such buildings. Nevertheless, in the implementation phase, they are overwhelmingly replaced
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by cheaper conventional roofs.
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Fig. 1 a few examples of students' works presented in an urban design competition named “Garden on the roof” - Refurbishment of roofs at Ťahanovce housing estate located in Košice (none of the designed green roofs has been implemented yet) Fundamental reason for doing so are firstly, investor's low awareness and secondly, architect's poor knowledge both being shown in disability to present huge benefits of green roofs and their technical, social, economic and environmental aspects. Wider use of green roofs in contemporary design in Slovakia is neither helped by current legislation, nor by traditional standard procedures regarding building structures assessment. When evaluating hygrothermal behaviour of green roofs using established stationary processes it is simply impossible to
ACCEPTED MANUSCRIPT quantify precisely the impact of growing layers on the roof's real behaviour and its further influence on the entire building. However, using simulation tools in designing green roofs opens up a possibility of presenting most of their benefits. Non-stationary simulation analyses present behaviour of building structures in real climatic conditions. Studies [1], [2] present long term in situ measurements results for various green roofs projects realized in German (Holzkirchen, Leibzig, Kassel) and Austrian (Vienna) climatic conditions. In these studies, WUFI simulation tools were tested and verified in the measurements of the
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green roofs' hygrothermal behaviour.
There have been several worldwide studies analysing hygrothermal behaviour and interaction of green roofs with surrounding environment for specific regions [3], [4], [5], [6], [7], [8], [9], [13], [16]. There is a number of works studying in detail hygrothermal behaviour of green roofs. Three main directions prevail: laboratory experiments, in situ experimental
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measurements and theoretical studies aimed at simulation of various green roof assemblies in non-stationary boundary conditions similar to real climatic conditions. Laboratory experiments carried out in the works [10], [11], [12] provide a detailed explanation on what is happening in green roofs under the influence of non-stationary
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conditions. Unlike in situ measurements, laboratory setup allows for more accurate quantification of substantial fluxes through green roofs. The influence of such aspects as water content, differences in temperature of vegetation and ambient air, as well as changes in
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air velocity on sensible and latent heat flux are analysed in laboratory setup [10]. The role of plants in the reduction of heat flux through green roofs is quantified in [10] by using an
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experimental apparatus called "Cold Plate". Conductive heat flux through the tested green roof and a subsequent quantified "additional" thermal resistance (0.76 - 0.84 (m2 / (WK)) was
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observed. Technical improvement of "Cold Plate" laboratory setup [12] has brought new information on evapotranspiration, and its strong dependency on leaf density (LAI), short-
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wave radiation and substrate water content. The influence of water content in the studied substrate on the heat flux was also quantified in laboratory conditions. Conductive fluxes for wet samples were around 16% lower than those for dry samples [12]. Assessing green roof with a simple quantifier such as thermal resistance, typical of conventional construction, is questionable. It results from the nature of physical definition of thermal resistance which is defined for the final number of homogeneous layers in steady state conditions. Inhomogeneity of green roof layers combined with dynamic heat and water transmission resulted in thermal resistance quantification over a wide range of values. This fact can be confusing for green roof designers.
ACCEPTED MANUSCRIPT In situ analyses of hygrothermal behaviour of green roofs on real [6], [13], [14], [15], [16] or experimental buildings [1], [2], [8] are often aimed at comparison of green roofs with conventional roofs. Reducing heat flux density through the roof by using vegetation, especially during summer period, is presented as the most important benefit. Depending on the roof's assembly, vegetation type and climate zone, reduction of an average heat flux across the roof with vegetation varies from 18-75% when compared to a conventional roof [14], [15], [16]. Frequently studied aspects are also temperature decreases within a roof assembly
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with vegetation as well as protection of the roof membrane layers from UV rays thus extending life cycle of such roof.
Sailor [17] implements a green roof module into the Energy Plus simulation tool, which then enables simulating real behaviour of buildings with vegetation-layer and also quantifying energy savings in such buildings.
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The simulation analyses in [18] define basic parameters of a green roof developed for an effective thermal protection in summer period. For elimination of heat fluxes through the green roof, following main parameters are defined: leaf area index (LAI) and leaf angle distribution as shadowing device, substrate thickness and its density, as well as water content.
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The model of green roof by Del Barrio [18] applies a detailed look at vegetation layer, but the roof assembly and substrate layer are simulated in a simplified manner. However, water storage and water transfer by capillary forces are not taken into account. It is both the water
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content in the substrate and its distribution that have major influence on hygrothermal
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behaviour of the studied green roof.
1. Hygrothermal Simulation
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Hygrothermal behaviour of an extensive green roof in the climatic conditions of Kosice (city in Eastern Slovakia) was analysed. The purpose of the analysis is to determine water content
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in the tested extensive green roof substrate and at the same time to quantify its influence on the temperature on top of the roof membrane (under the substrate layer) as well as fluctuation of heat flux through the building structure during summer period. The analysis of hygrothermal behaviour of the extensive green roof during the summer period was performed using a simulation tool. A significant output of the simulation was the calculated water content in the substrate as well as spatial redistribution of water during the simulated time. Analysing the influence of precipitation on water content of a green roof substrate using a simulation tool is a challenging task. It requires certain changes and additional settings of the
ACCEPTED MANUSCRIPT simulation tool used as well as an inclusion of processes describing the movement of precipitation water in the substrate. The model used must be based on a detailed approach to the moisture balance assessment in the substrate layer. Relevant climatic data that characterizes the site to be analysed both sufficiently and accurately must become the simulation's boundary condition. Obviously, data on precipitations provides a necessary climatic parameter here as well. Unlike in conventional models which describe water transport only through vapour diffusion, the used simulation model must take into account the
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ability of the substrate to accumulate and transport liquid water by capillary forces. It is of a great importance to use a model that takes into consideration capillary activity of pores and respects the relevant drive potentials (usually relative humidity) in order to determine distribution of water in the substrate layer. Redistribution of water towards the outer surface of the substrate plays a crucial role in the evaporation process.
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An overview of hygrothermal numerical simulation tools applied to building physics is summarized in [19]. However, not all listed simulation tools are able to take into account the criteria [20], [21] for hygrothermal modelling of green roofs. Defined requirements are met by several simulation tools such as: MOIST [22], DELPHIN [23] or WUFI [24].
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The WUFI simulation model [24] was selected for further use for the following reasons: High accuracy of simulation results with measured values obtained in a long-term
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monitoring of hygrothermal behaviour in different types of green roofs exposed to real climatic conditions [2].
Optimized hygrothermal simulation model set-up [1] that expands the possibilities of
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WUFI's standard use by adding to the list the influence of vegetation layer, while taking into account rainwater reception and covering the effects of water drainage
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through the greenery layers. Existence of material parameters measured by laboratory procedures required for
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hygrothermal simulation of green roofs, including parameters that describe water storage, vapour diffusion and liquid water transport in the process of capillary suction and redistribution.
Compared to models [17], [18], the WUFI model is less complex in terms of plants modelling. Its neglecting of hysteresis is also a weakness. In the WUFI simulation model by Künzel, heat and moisture balance equations are formulated as [24]: dH T .(T ) hv.( p(. psat )) dT t
(3)
ACCEPTED MANUSCRIPT dw .(( D p(. psat )) (4) d t The liquid water transport coefficient is defined as:
D ( w) Dw ( w).
dw (5) d
where:
∂T/∂t (K/s) temporal change of the temperature
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dH/dT (J/(m3 K)) heat storage capacity of the moist building material,
dw/dφ (kg/m3) moisture storage capacity of the building material, ∂φ/∂t (1/s) temporal change of the relative humidity,
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φ (-) the relative humidity,
δ (kg/(m s Pa)) the water vapour permeability,
psat (Pa) the partial pressure of saturated water vapour in the air,
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hv (J/kg) the latent heat of evaporation of water, λ (W/(m.K)) the thermal conductivity,
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T (°C) is the temperature,
Dφ (kgm/s) liquid transport coefficient of the building material
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Dw (m2/s) liquid diffusivity
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2. Experimental green roof or the field study
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Monitoring of full-scale state structures in real climatic conditions is a current trend of building physics research [25], [26], [29]. An experimental green roof, which is an integral part of the laboratory's test chambers belonging to Faculty of Civil Engineering at Technical University in Kosice (Fig. 2), was used for a hygrothermal behaviour analysis in this study.
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Fig.2 Experimental cells of Technical University in Košice
Monitored structures, including the green roof, are exposed, from the outside, to real climatic conditions. The interior of the experimental cells is an internal environment and its parameters can be adjusted according to the nature of measurements. The scheme and description of the
Fig. 3 Experimental extensive green roof assembly
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test green roof used in this analysis is shown in Fig. 3.
The green roof layers were made in accordance with the instructions in the manufacturer's technical data sheet. The green roof is located on a test panel which is a standard roof system typical for buildings in Slovakia. Under the substrate layer lies a system element - a protective and storage mat (PM) which is fitted as recommended by the manufacturer. The test panel contains all the relevant layers necessary for a roof design in the climatic conditions typical of Slovakia. 100 mm thick mineral wool (TI-MW) as thermal insulation was applied between roof rafters complemented by an 80 mm thick PUR (TI-PUR) as an additional thermal insulation layer. Roof stiffening was carried out in the usual way by using OSB (Oriented
ACCEPTED MANUSCRIPT Strand Boards). The roof contains a hydro - insulation roof membrane (RM) as well as a vapour barrier layer. Basic material parameters used for simulating hygrothermal behaviour of the tested green roof are summarized in Tab 1. Tab. 1 Hygrothermal parameters of materials used for simulating hygrothermal behaviour of the tested green roof. S
PM
RM
OSB
Bulk density (kg/m3) Porosity (m3/m3) Specific heat capacity (J/(kg.K)) Thermal conductivity dry 10 °C (W/(m.K)) Water vapour diffusion resistance factor (-) Reference water content w80 (kg/m3) Free water saturation (kg/m3) Vapour barrier sd = 1500
912 0.65 1000 0.4 3.4 10.6 399
83 0.95 840 0.035 1 0.7 544
2500 0.001 0.5 0.5 20000 -
553 0.61 1700 0.12 134 76 610
TIPUR 40 0.95 1500 0.03 50 -
TIMW 60 0.95 850 0.04 1.3 -
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Experimental green roof
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The moisture retention curve (MRC) and dependence of thermal conductivity on temperature and water content for the substrate layer are shown in Fig. 4. Liquid transport coefficient for
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in Fig. 5.
Thermal conductivity (W/(m.K))
the process of capillary suction (Dws) and redistribution (Dww) for the substrate layer is shown
Temperature (°C)
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0
50
100
1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30
0.0
0.2
0.4
0.6
0.8
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Water content mwater / mtotal(-) Thermal conductivity, mouisture dependent Thermal conductivity, temperature dependent
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Fig. 4 Moisture retention curve MRC (on the left) and thermal conductivity dependent on temperature and water content for the substrate layer (on the right)
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Fig. 5 Liquid transport coefficient for the process of capillary suction (Dws) and redistribution (Dww) for the substrate layer The material parameters used in this study are part of the WUFI simulation software database
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and were developed within the research project "Zuverlässige Beurteilung der hygrothermischen und energetischen Auswirkungen von Gründächern" (SF-10.08.18.8 / II 3F20-10-1-100), funded by the research program "Zukunft Bau" of the Bundes institute für Bau-, Stadt- und Raumforschung (German Federal Ministry of Transport, Building and Urban
3. Boundary conditions
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development).
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When designing green roofs by using simulation methods, the choice of boundary conditions for calculation is a key aspect. A very important factor is the information on the amount and intensity of precipitation. Easily accessible and frequently used climatic data (TRY, IWEC,
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etc.), primarily dedicated to energy simulations, focus on typical weather in the region and generally do not contain quantitative rainfall data. To simulate the behavior of a green roof
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with relevant consideration of quantitative rainfall impact it is more appropriate to use data for hygrothermal modelling. Unfortunately, there is no such data for the territory of Slovakia.
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Therefore climatic parameters measured for the city of Košice (East Slovakia) were used as the boundary conditions for the hygrothermal behavior simulation of the experimental green roof. Measurements of meteorological parameters with mean 15-minute increments, including exterior air temperature, air humidity, atmospheric pressure, precipitation, wind speed and direction, incoming shortwave and long - wave radiation were compiled.
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Fig. 6. Environment data values measuring devices: a, b) weather station, c) pyranometer, d) indoor air temperature and relative humidity measuring sensor. For the simulation analyses, year 2013 was selected. It was a year characterized by high summer temperatures throughout Slovakia. However, as for East Slovakia, it was marked by significant rainfall as well. The summer of 2013 (June - August) in Slovakia is considered
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extreme in terms of temperature with a positive temperature deviation of +2 to +3 °C compared to the long term average (years 1961 - 1990). According to the SHMÚ (Slovak Hydro - Meteorological Institute), three periods of significant heat waves appeared in the summer of 2013, which culminated between June 16 - 23, July 28 - 29 and August 6 - 9. During the summer of 2013, several all-time highs were reached, regarding both temperature
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as well as the number of days with record high day and night temperatures. During the first heat wave between the 16th and 23rd June, the first super tropical night was
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recorded in Slovakia for the month of June since the history of measurements began in 1881. Super tropical night was even measured in each of the given year's summer months. The
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summer of 2013 became the 4th warmest ever recorded in Košice since the beginning of the weather monitoring (1881). The summer of 2013 precipitation records were marked by a very
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uneven rain distribution in many locations. Based on the SHMU analysis, in July 2013, many regions of Slovakia saw extraordinarily low to extremely low monthly precipitation sums
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having been recorded since the start of monitoring in 1881, with the exception of the areas located in the eastern part of the country. Average precipitation figures for West, Central and East Slovakia and the measured ones for the city of Košice can be found in table 2. Rainfall sums for the city of Košice during the summer of 2013 (June - August) correspond with the data measured in Eastern Slovakia (Tab. 2). The exception is the month of July in which Košice saw an increase in precipitations by 64 millimetres compared to East Slovakia average.
ACCEPTED MANUSCRIPT Tab. 2 Mean monthly areal precipitation total calculated by the isohyetal method from about 600 stations in Slovakia (SR) at the Slovak Hydro Meteorological Institute (SHMÚ) for West (W_SR), Central (C_SR) and East Slovakia (E_SR) regions supplemented with measured data for the city of Košice (millimetres R (mm) and percentages of normal R (%)) Percentages of normal (%) W_SR C_SR E_SR 124 62 22 14 21 55 112 122 152 137 185 167 44 43 67 235 204 207
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Precipitation R H2O (mm) C_SR E_SR Košice 57 19 13 21 53 117 121 135 114 159 125 122 27 36 38 110 87 85
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VIII.2013 VII.2013 VI.2013 V.2013 IV.2013 III.2013
W_SR 78 10 76 92 21 101
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Fig. 7 Weather data of Košice (Technical university of Košice campus) during simulation
4. Initial conditions and simulation scenarios definition In hygrothermal simulations, the initial condition of temperature and initial condition of water content in the structure represent the initial condition of construction. In long-term simulations, the effect of the initial temperature on the calculation result is negligible. For this reason, the initial temperature condition is simplified by setting a constant temperature across the structure to be analysed before starting the simulation. On the contrary to that, presence of water in the building structure has a long-lasting character. Presence of water in the structure
ACCEPTED MANUSCRIPT is a desired phenomenon, high occurrence of which is regularly repeated over the life cycle of the construction. Water in the structure changes the material parameters of the substrate and its presence greatly affects the evapotranspiration and thus the whole hygrothermal balance of the green roof. These are the main reasons why presence of water and its fluctuation in the green roof substrate cannot be excluded from the process of simulating its hygrothermal behaviour. Water content in the substrate in pre-simulation time is not usually known. Therefore, before starting the simulation, this input data needs to be set explicitly. In general,
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the initial condition can be entered based on exact data on the water content status, determined by a simulation tool or it can be estimated empirically. The most accurate way to determine initial water content at the beginning of a simulation is to achieve it directly or indirectly by measuring the water content in the substrate. Water content of building materials (such as soil) in a built-in state is measured by an indirect method, for example using TDR
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(time-domain reflectometry) [6]. It is possible to determine the value of the water content directly from the substrate sample by the gravimetric method [27]. Initial water content measurement provides accurate information on the presence of water in the substrate and reduces a possibility of a calculation error during the simulation. However, such
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measurements are expensive as well as time-consuming and are applicable in determining the initial condition for practical calculations only to a limited extent. It is much more practical to use a simulation tool in order to determine the initial water content when designing green
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roofs. Water content in the substrate is determined by a simulation by using relevant climatic parameters (Fig. 7). The least accurate way to determine water content in the substrate is to
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estimate the water content values empirically at the beginning of the simulation from the characteristic values of the material parameters of the substrate (wcap, w80 or MRC) and the
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nature of weather (e.g, long-term drought). Simulation in this study is performed for four calculation scenarios. The summary of the calculation scenarios characteristics is shown in
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Table 3. The scenarios differ from each other in the length of simulation time as well as in the initial condition of water content. The effect of the selected initial condition on the course of the calculated water content in the substrate during the simulated time can be observed on the analysed simulation scenarios.
ACCEPTED MANUSCRIPT Tab. 3 Characteristics of the analysed simulation scenarios Case Case 1 Case 2 Case 3 Case 4
Time 01.03. - 31.08. 25.05. - 31.08. 01.06. - 31.08. 01.06. - 31.08.
Initial condition Measured Dry Substrate (W.c. dry) Dry Substrate (W.c. dry) Wet Substrate (W.c. wet)
Average water content (kg/m3) 188 (March 1) 15 (May 25); 153 (June 1) 15 (June 1) 238 (June 1)
Case 1 is a reference scenario with a simulated time of 6 months (March 1 – August 31).
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Longer simulated time in Case 1 compared to the others served to observe the progression of
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water content in the pre-simulation time.
Fig. 8 Calculated and measured values of water content in the green roof substrate
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during simulated time (March 1 - August 31)
During this period (March 1 - August 31), at selected times, samples were taken from the
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green roof substrate throughout its whole profile (150 mm substrate thickness). Subsequently, water content at four different depths was measured gravimetrically from each of the samples
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taken. Water content measurement was performed by the Sartorius MA150 moisture analyser (Fig. 9 in the middle and on the right). The measurements showed uneven distribution of water in the substrate. Even after a long period without precipitation, the bottom layer of the substrate was wet (water content about 180 kg / m3), while the water content measured in the upper layer of the substrate reached only low values (20 kg / m3). Water content of the substrate calculated by using the WUFI simulation tool is shown in Fig. 8 by a black curve. The blue points represent the measured water content values (arithmetic water content average at four different depths of the substrate). The water content values measured at four depths throughout a 150 mm thick layer of dry substrate and a substrate layer following a
ACCEPTED MANUSCRIPT period of long and intense precipitation (W.c. wet) are shown in Fig 9 (on the left). These water content values represent the boundary intervals which a green roof substrate can
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oscillate in.
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Fig.9. Measured water content values of the dry substrate (W.C. dry), substrate after intense rain (W.C. wet) - on the left, green roof after taking samples to measure water content in the substrate - in the middle, moisture analyser - on the right
The initial condition for Case 1 reference scenario was determined by measuring at four
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different depths of the substrate (March 1). Simulation scenarios CASE 2, 3 and 4 are focused on the summer period (June 1 - August 31). For the simulation itself, the initial condition of
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dry substrate and wet substrate was used, Case 3 (W.C. dry) and Case 4 (W.C. wet) respectively, as shown in Fig. 9 on the left. For Case 2, a short pre-calculation of seven days was applied. At the beginning the initial condition (W.C. dry) was used. The boundary
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conditions for Case 2 simulation scenario are shown in Fig. 10.
Fig.10 Boundary conditions for Case 2 simulation scenario
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5. Discussion of calculated results Results of this study can be presented in two ways. The first one is to model water content in the substrate depending on precipitation. The second one is the impact of water content in the substrate layer on the roof membrane's temperature and heat flux changes in the summer period. The impact of precipitation water accumulated by the substrate reduces the heat flux
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through the roofing system. The WUFI simulation tool, which takes into account strong bond between heat and water transport processes, enables both ways to be analysed simultaneously. (The heat and moisture transport processes in buildings are usually strongly coupled). 5.1 Analysis of water in the substrate layer
The simulated course of water content in the tested green roof substrate obtained by a long-
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term calculation (March 1 - August 31), verified by numerous water content measurements (blue points), is defined as Case 1 reference simulation scenario and is shown in Fig. 11 by a black line. Other simulation scenarios present the course of water content in the substrate when using different initial conditions according to Table 3. When using the 7-day precalculation within Case 2 scenario (green line), water content increased from the starting
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value of 15 kg / m3 (may 25) to 150 kg / m3 (June 1) getting closer to the measured water content, in other words to Case 1 reference scenario. A sharp rise in pre-calculated water
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content was caused by intense storms in the pre-simulation time during which 85 mm of rain fell. Simulated water content courses with explicitly set initial water content for dry substrate
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(Case 3) and wet substrate (Case 4) are shown in Fig. 11 in orange and blue colour respectively. When using the initial water content of dry substrate (Case 3), the calculated
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water content equals the measured values, or Case 1 reference scenario, in approximately 2 months, while Case 4 simulation scenario reaches the measured values of water content (or
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Case 1) almost immediately.
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Fig. 11 Calculated and measured (blue points) values of water content in the green roof substrate during simulated time may 25; June 1 - August 31 for the analysed simulation scenarios 5.2 Simulated temperature and heat flux courses under substrate layer
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Figs. 12 to 14 show the outdoor air temperature, solar radiation and calculated temperatures at the border between the substrate and the roof layer in a place indicated as P2 (Fig. 3) for all defined simulation scenarios. Simulated scenarios (Case 1, 2, 3 and 4) used black, green, orange and blue colours. The identical colour lines (in form of dashed lines) show the
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calculated water content in the green roof substrate. From Figs. 12-14, substrate layer's presence and its influence on the temperature course of P2 is clearly visible. The calculated
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daily temperature peak under the substrate layer (P2) is shifted by more than 7 hours compared to the daily temperature peak. This phenomenon is observed throughout the entire simulated time. Influence of different water content in the substrate on the calculated
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temperature course at P2 is observable especially at the beginning of the simulated time (June 1 - June 16). The biggest differences are observable when comparing Case 3 simulation
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scenario with Case 1 or Case 4 reference scenario (Fig. 12).
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Fig. 12 Surface temperature courses under the substrate layer at P2 and the average
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water content in the substrate calculated for 4 simulation scenarios and the period of June 2013
At the beginning of the simulated time (June 1 – June 3), the temperature of the entire
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simulated roof profile, including the substrate layer, is balanced, since the roof temperature for Case 1 and 2 was set by using the simulation tool (June 1) while the initial temperature for
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Case 3 and 4 was explicitly set at 21° C for the entire roof profile. Decrease in outdoor air temperature along with lower solar radiation intensity (June 3-5) caused reduction in heat
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exchange between the building structure and the surrounding environment. This decrease was projected into the gradual equalization of calculated surface temperatures at P2 for all
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simulation scenarios. The arrival of warmer days without precipitation (June 6) again increased the heat exchange between the experimental cell and the outdoor environment. This phenomenon was immediately manifested in an increase of the calculated surface
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temperatures at P2. During the period of June 6-9, the calculated surface temperature at P2 increased and reached values similar to exterior air temperature. This period was particularly interesting in terms of differences in calculated surface temperatures at P2 among individual simulation scenarios. Simulation scenarios with high water content in the substrate (Case 1 and 4) showed significantly lower surface temperatures at P2 compared to Case 3 simulation scenario (dry substrate). Differences in daily peaks of the calculated surface temperature at P2 were between 3 and 4 K. For Case 2 simulation scenario using a seven day pre-calculation, the calculated daily peak surface temperature at P2 was 1-2 K higher than Case 1 reference
ACCEPTED MANUSCRIPT scenario. Due to climatic changes (especially precipitation and drying) the water content becomes more or less balanced during the simulated time, thus eliminating the influence of initial water content on the calculated surface temperature under the substrate at P2 site. At the end of June, the difference in calculated surface temperature peaks at P2 for Case 1 and Case 3 simulation scenarios is approximately 1 K, and this surface temperature difference is
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repeated throughout nearly entire month of July (Fig. 13).
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Fig. 13 Surface temperature courses under the substrate layer at P2 and the average water content in the substrate calculated for 4 simulation scenarios and the period of
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July 2013
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Following a period of precipitation July 16 - July 30, the calculated water content reaches approximately same value for all simulated scenarios analysed. The influence of the initial water content on the hygrothermal behaviour of the green roof is negligible from this moment
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on (Fig. 14).
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Fig. 14 Surface temperature courses under the substrate layer at P2 and the average
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water content in the substrate calculated for 4 simulation scenarios and the period of August 2013
The maximum roof membrane surface temperature values calculated are summarized in Tab. 4. The highest roof membrane temperature was calculated for Case 3 scenario while the
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lowest one was calculated for Case 1 reference scenario. The simulation result confirmed how significant substrate and water content inside it is when used in a green roof assembly and
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how strong its influence on the roof membrane temperature reduction can be. Due to lower thermal stress and protection against UV radiation, life cycle of waterproof layers as well as
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life cycle of the entire roof is prolonged. Tab. 4 Calculated maximum surface temperatures of the roof membrane under the substrate
Maximum temperature P2 24.35 °C 26.13 °C 27.75 °C 24.70 °C
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Case Case 1 Case 2 Case 3 Case 4
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layer during summer period (June - August) for the simulated Case 1 - Case 4 scenarios
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Fig. 15 Course of heat flux under the substrate layer at P2 site calculated for 4
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simulation scenarios for days of June 6 and July 29
The influence of the substrate layer on hygrothermal behavior of the green roof can also be observed on heat flux simulation under substrate layer at P2. In Fig. 15 on the left, June 6 heat flux course is displayed, while the right side shows July 29 heat flux. Different initial water content in the substrate for the simulation scenarios at the beginning of the simulation (June 6)
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affects the fluctuation of heat flux through the analyzed roof. At the end of July, the influence of the initial water content on heat flux is negligible. Roof protection against summer
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overheating by applying vegetation with a substrate layer can be interpreted by a quantification of heat fluxes. The heat flow through the roof was obtained by integrating the
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heat flux over time. Average daily heat flows calculated through the tested green roof over a period of three summer months are summarized in Fig. 16. Average daily heat flows for Case 1, 2 and 3 simulation scenarios are compared. The biggest difference of average daily heat
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flow was observed in June between Case 1 and Case 3 scenario. Higher water content in the substrate (Case 1) reduces the heat flow through the green roof by 11% compared to Case 3
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scenario. The use of pre-calculation (Case 2) reduces the average daily heat flow in June by 7% compared to Case 3 scenario. As for the month of July the difference in the average daily heat flow between Case 1 and Case 3 scenarios is just 2%, while in August the impact of the initial water content on this parameter is completely negligible.
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Fig. 16 The average daily heat flow through the tested green roof calculated for different initial water contents represented by Case 1, 2 and 3 simulation scenarios. Summer
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period (June - August) for Košice (East Slovakia) was simulated
Conclusion
The simulation analysis showed significant influence of water content in green roof substrate on hygrothermal behavior of such roof. The choice of the initial water content used in the
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presented simulation scenarios reflects the real moisture conditions of the particular type of substrate in dry and wet state. The use of specific climatic parameters aimed to analyze
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hygrothermal behavior of the tested green roof in the most accurate context possible. Analysis has shown that using a sufficiently long pre-calculation can provide reliable information on water content in the substrate depending on climatic parameters (especially precipitation).
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However, for a more precise comparison of the accumulated water content, but especially its redistribution in the substrate's profile, a more accurate measurement is required. Applying
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TDR sensors at different depths of the substrate allows for real-time monitoring of water content profiles.
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Use of a short-term pre-calculation can be questionable. In Case 2 simulation scenario, use of a 7-day pre-calculation resulted in a more accurate simulation owing to " suitable weather" at pre-simulation time when 85 mm of rain fell to the substrate, which significantly increased water content of the substrate and brought it closer to the real moisture state. The unpredictability of the precipitation and its uneven distribution in the region was also demonstrated by the difference between the average monthly precipitation sums in the region of East Slovakia and the city of Košice in the month of July 2013 (Table 2). Simulation analysis demonstrated that:
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Water content in substrate layer is crucial to hygrothermal behavior of a green roof. In the simulation, the amount and distribution of water in the studied substrate profile is a significant factor.
The influence of water in the substrate layer was reflected on the roof membrane's temperature. The highest calculated temperature differences between the dry and wet substrate simulation scenario were around 3 K. Considering high temperatures of roof membranes in conventional roofs, this might not look like a high number, but it can be
increasing the roof structure's life cycle.
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said that water content participates in reducing the membrane temperature thus
The impact of water in the substrate was shown in reducing the heat flux across the roof with a 150 mm coarse layer of substrate, which in the summer period participates in protecting the building from overheating. For analyzed wet and dry substrate
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scenarios, the average daily heat flow reduction is 11%.
An important conclusion of the thesis is the applicability of a suitably chosen simulation tool in green roof design. The technically justified benefits of green roofs obtained by such calculation and later reflected into economic aspects, present a
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stimulus for investors and building owners that can hardly be overlooked. Making building constructions that surround us greener to much bigger extent would
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fundamentally change such buildings’ behavior, would bring undoubtable benefits in terms of their energy consumption, and last but not least would improve the quality of our life. We
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believe that these arguments will contribute to a more massive incorporation of green structures design in Slovakia hand in hand with utilizing the sophisticated hygrothermal
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simulations assessment.
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Acknowledgements This work was supported by the Slovak Scientific Grant Agency (VEGA) in collaboration with Slovak Ministry of Education, Science, Research and Sports (MESRS) and Slovak Academy of Sciences (SAS) under Grant number 1/0835/14.
ACCEPTED MANUSCRIPT References [1]
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Wärme- und Feuchtetransports in Bauteilen mit einfachen Kennwerten. Dissertation Universität Stuttgart, 1994. [25]
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296,.
Hygrothermal initial condition for simulation process of green building construction Marian ´ Vertaľ , Marek Zozulak ´ , Anna Vaˇskova´ , Azra Korjenic PII: DOI: Reference:
S0378-7788(18)30452-3 10.1016/j.enbuild.2018.02.004 ENB 8320
To appear in:
Energy & Buildings
Received date: Revised date: Accepted date:
3 March 2017 5 September 2017 5 February 2018
Please cite this article as: Marian ´ Vertaľ , Marek Zozulak ´ , Anna Vaˇskova´ , Azra Korjenic , Hygrothermal initial condition for simulation process of green building construction, Energy & Buildings (2018), doi: 10.1016/j.enbuild.2018.02.004
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Highlights Water content condition’s impact on green roof’s hygrothermal behaviour is shown. Impact of water content in substrate on roof membrane temperature is presented. Impact of water content in substrate on reducing heat flow through the roof is shown. Simulation tools application into green roof design process in Slovakia is emphasised.
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Hygrothermal initial condition for simulation process of green building construction Marián Vertaľa, Marek Zozulákb, Anna Vaškovác, Azra Korjenicd Department of Building Physics, Institute of Architectural Engineering, Civil Engineering Faculty, Technical University of Košice, Košice, Slovakia Civil Engineering Faculty, Technical University of Košice Vysokoškolská 4, 042 00 Košice, Slovakia Phone number: 00421 55 602 4167, E-mail address: [email protected]
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a,b,c
d
Vienna University of Technology, Institute for Building Construction and Technology, Research Centre of Building Physics and Sound Protection, Karlsplatz 13/206-2, 1040 Vienna, Austria Abstract:
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Since water is inextricably linked with life, its presence in green building constructions is an inevitable aspect in the process of hygrothermal behavior analysis. Water content in the substrate of green structures is a variable phenomenon throughout a year. During this period it has a strong influence on the building’s hygrothermal behavior.
The WUFI simulation tool analyzes hygrothermal behavior of the tested extensive green roof
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in the climatic conditions of East Slovakia (Košice). The presented simulation scenarios are aimed at assessing the influence of water in the green roof substrate on the roof membrane
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temperature and the fluctuation of heat fluxes through the roof during summer period (June 1 - August 31). Presence of water and its movement through capillary forces is an essential
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aspect of hygrothermal behavior and cannot be neglected in the simulation process. Water in the tested green roof substrate has reduced the average daily heat flow across the roof by 11%,
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which significantly contributed to lowering its summer overheating. Calculated water content in the substrate was influenced by the selected initial water content condition for up to two-
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thirds of the simulated period's length.
Key words: green roof, initial water content, heat flux, summer overheating, hygrothermal simulation
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Introduction Vegetation, as part of building structures, appeared as early as ancient times. Over the last 50 years, Europe has witnessed rediscovery of green structures. However, their application has primarily been of aesthetic significance. The importance of green roofs for rainwater retention and retention of water in the country is by far the main benefit (storm water management capacity), but in addition to that there are other important aspects of green building constructions getting into the spotlight. In recent years, green building structures have been
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subject to quite an intense talk in terms of the energy aspects of their existence. Among the most important benefits of green roofs are: transforming dead space into garden space, roof layers' protection and subsequently its life extension, reducing air-conditioning and winter heating costs, reducing storm water run offs, improving aesthetics, reducing "heat island" effect in cities, providing green space including new habitat for wild life and many other
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benefits. The increase of implemented green roofs across Western Europe goes hand in hand with increasing public awareness of their positive impact on the mankind, human society and the environment itself. In sharp contrast to that, green roofs in Slovakia are still vastly perceived as just an aesthetic element. When designing buildings in the project phase, green
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building constructions, especially green roofs, are usually a dominant architectural feature of such buildings. Nevertheless, in the implementation phase, they are overwhelmingly replaced
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by cheaper conventional roofs.
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Fig. 1 a few examples of students' works presented in an urban design competition named “Garden on the roof” - Refurbishment of roofs at Ťahanovce housing estate located in Košice (none of the designed green roofs has been implemented yet) Fundamental reason for doing so are firstly, investor's low awareness and secondly, architect's poor knowledge both being shown in disability to present huge benefits of green roofs and their technical, social, economic and environmental aspects. Wider use of green roofs in contemporary design in Slovakia is neither helped by current legislation, nor by traditional standard procedures regarding building structures assessment. When evaluating hygrothermal behaviour of green roofs using established stationary processes it is simply impossible to
ACCEPTED MANUSCRIPT quantify precisely the impact of growing layers on the roof's real behaviour and its further influence on the entire building. However, using simulation tools in designing green roofs opens up a possibility of presenting most of their benefits. Non-stationary simulation analyses present behaviour of building structures in real climatic conditions. Studies [1], [2] present long term in situ measurements results for various green roofs projects realized in German (Holzkirchen, Leibzig, Kassel) and Austrian (Vienna) climatic conditions. In these studies, WUFI simulation tools were tested and verified in the measurements of the
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green roofs' hygrothermal behaviour.
There have been several worldwide studies analysing hygrothermal behaviour and interaction of green roofs with surrounding environment for specific regions [3], [4], [5], [6], [7], [8], [9], [13], [16]. There is a number of works studying in detail hygrothermal behaviour of green roofs. Three main directions prevail: laboratory experiments, in situ experimental
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measurements and theoretical studies aimed at simulation of various green roof assemblies in non-stationary boundary conditions similar to real climatic conditions. Laboratory experiments carried out in the works [10], [11], [12] provide a detailed explanation on what is happening in green roofs under the influence of non-stationary
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conditions. Unlike in situ measurements, laboratory setup allows for more accurate quantification of substantial fluxes through green roofs. The influence of such aspects as water content, differences in temperature of vegetation and ambient air, as well as changes in
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air velocity on sensible and latent heat flux are analysed in laboratory setup [10]. The role of plants in the reduction of heat flux through green roofs is quantified in [10] by using an
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experimental apparatus called "Cold Plate". Conductive heat flux through the tested green roof and a subsequent quantified "additional" thermal resistance (0.76 - 0.84 (m2 / (WK)) was
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observed. Technical improvement of "Cold Plate" laboratory setup [12] has brought new information on evapotranspiration, and its strong dependency on leaf density (LAI), short-
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wave radiation and substrate water content. The influence of water content in the studied substrate on the heat flux was also quantified in laboratory conditions. Conductive fluxes for wet samples were around 16% lower than those for dry samples [12]. Assessing green roof with a simple quantifier such as thermal resistance, typical of conventional construction, is questionable. It results from the nature of physical definition of thermal resistance which is defined for the final number of homogeneous layers in steady state conditions. Inhomogeneity of green roof layers combined with dynamic heat and water transmission resulted in thermal resistance quantification over a wide range of values. This fact can be confusing for green roof designers.
ACCEPTED MANUSCRIPT In situ analyses of hygrothermal behaviour of green roofs on real [6], [13], [14], [15], [16] or experimental buildings [1], [2], [8] are often aimed at comparison of green roofs with conventional roofs. Reducing heat flux density through the roof by using vegetation, especially during summer period, is presented as the most important benefit. Depending on the roof's assembly, vegetation type and climate zone, reduction of an average heat flux across the roof with vegetation varies from 18-75% when compared to a conventional roof [14], [15], [16]. Frequently studied aspects are also temperature decreases within a roof assembly
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with vegetation as well as protection of the roof membrane layers from UV rays thus extending life cycle of such roof.
Sailor [17] implements a green roof module into the Energy Plus simulation tool, which then enables simulating real behaviour of buildings with vegetation-layer and also quantifying energy savings in such buildings.
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The simulation analyses in [18] define basic parameters of a green roof developed for an effective thermal protection in summer period. For elimination of heat fluxes through the green roof, following main parameters are defined: leaf area index (LAI) and leaf angle distribution as shadowing device, substrate thickness and its density, as well as water content.
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The model of green roof by Del Barrio [18] applies a detailed look at vegetation layer, but the roof assembly and substrate layer are simulated in a simplified manner. However, water storage and water transfer by capillary forces are not taken into account. It is both the water
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content in the substrate and its distribution that have major influence on hygrothermal
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behaviour of the studied green roof.
1. Hygrothermal Simulation
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Hygrothermal behaviour of an extensive green roof in the climatic conditions of Kosice (city in Eastern Slovakia) was analysed. The purpose of the analysis is to determine water content
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in the tested extensive green roof substrate and at the same time to quantify its influence on the temperature on top of the roof membrane (under the substrate layer) as well as fluctuation of heat flux through the building structure during summer period. The analysis of hygrothermal behaviour of the extensive green roof during the summer period was performed using a simulation tool. A significant output of the simulation was the calculated water content in the substrate as well as spatial redistribution of water during the simulated time. Analysing the influence of precipitation on water content of a green roof substrate using a simulation tool is a challenging task. It requires certain changes and additional settings of the
ACCEPTED MANUSCRIPT simulation tool used as well as an inclusion of processes describing the movement of precipitation water in the substrate. The model used must be based on a detailed approach to the moisture balance assessment in the substrate layer. Relevant climatic data that characterizes the site to be analysed both sufficiently and accurately must become the simulation's boundary condition. Obviously, data on precipitations provides a necessary climatic parameter here as well. Unlike in conventional models which describe water transport only through vapour diffusion, the used simulation model must take into account the
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ability of the substrate to accumulate and transport liquid water by capillary forces. It is of a great importance to use a model that takes into consideration capillary activity of pores and respects the relevant drive potentials (usually relative humidity) in order to determine distribution of water in the substrate layer. Redistribution of water towards the outer surface of the substrate plays a crucial role in the evaporation process.
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An overview of hygrothermal numerical simulation tools applied to building physics is summarized in [19]. However, not all listed simulation tools are able to take into account the criteria [20], [21] for hygrothermal modelling of green roofs. Defined requirements are met by several simulation tools such as: MOIST [22], DELPHIN [23] or WUFI [24].
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The WUFI simulation model [24] was selected for further use for the following reasons: High accuracy of simulation results with measured values obtained in a long-term
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monitoring of hygrothermal behaviour in different types of green roofs exposed to real climatic conditions [2].
Optimized hygrothermal simulation model set-up [1] that expands the possibilities of
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WUFI's standard use by adding to the list the influence of vegetation layer, while taking into account rainwater reception and covering the effects of water drainage
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through the greenery layers. Existence of material parameters measured by laboratory procedures required for
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hygrothermal simulation of green roofs, including parameters that describe water storage, vapour diffusion and liquid water transport in the process of capillary suction and redistribution.
Compared to models [17], [18], the WUFI model is less complex in terms of plants modelling. Its neglecting of hysteresis is also a weakness. In the WUFI simulation model by Künzel, heat and moisture balance equations are formulated as [24]: dH T .(T ) hv.( p(. psat )) dT t
(3)
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D ( w) Dw ( w).
dw (5) d
where:
∂T/∂t (K/s) temporal change of the temperature
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dH/dT (J/(m3 K)) heat storage capacity of the moist building material,
dw/dφ (kg/m3) moisture storage capacity of the building material, ∂φ/∂t (1/s) temporal change of the relative humidity,
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φ (-) the relative humidity,
δ (kg/(m s Pa)) the water vapour permeability,
psat (Pa) the partial pressure of saturated water vapour in the air,
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hv (J/kg) the latent heat of evaporation of water, λ (W/(m.K)) the thermal conductivity,
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T (°C) is the temperature,
Dφ (kgm/s) liquid transport coefficient of the building material
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Dw (m2/s) liquid diffusivity
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2. Experimental green roof or the field study
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Monitoring of full-scale state structures in real climatic conditions is a current trend of building physics research [25], [26], [29]. An experimental green roof, which is an integral part of the laboratory's test chambers belonging to Faculty of Civil Engineering at Technical University in Kosice (Fig. 2), was used for a hygrothermal behaviour analysis in this study.
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Fig.2 Experimental cells of Technical University in Košice
Monitored structures, including the green roof, are exposed, from the outside, to real climatic conditions. The interior of the experimental cells is an internal environment and its parameters can be adjusted according to the nature of measurements. The scheme and description of the
Fig. 3 Experimental extensive green roof assembly
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test green roof used in this analysis is shown in Fig. 3.
The green roof layers were made in accordance with the instructions in the manufacturer's technical data sheet. The green roof is located on a test panel which is a standard roof system typical for buildings in Slovakia. Under the substrate layer lies a system element - a protective and storage mat (PM) which is fitted as recommended by the manufacturer. The test panel contains all the relevant layers necessary for a roof design in the climatic conditions typical of Slovakia. 100 mm thick mineral wool (TI-MW) as thermal insulation was applied between roof rafters complemented by an 80 mm thick PUR (TI-PUR) as an additional thermal insulation layer. Roof stiffening was carried out in the usual way by using OSB (Oriented
ACCEPTED MANUSCRIPT Strand Boards). The roof contains a hydro - insulation roof membrane (RM) as well as a vapour barrier layer. Basic material parameters used for simulating hygrothermal behaviour of the tested green roof are summarized in Tab 1. Tab. 1 Hygrothermal parameters of materials used for simulating hygrothermal behaviour of the tested green roof. S
PM
RM
OSB
Bulk density (kg/m3) Porosity (m3/m3) Specific heat capacity (J/(kg.K)) Thermal conductivity dry 10 °C (W/(m.K)) Water vapour diffusion resistance factor (-) Reference water content w80 (kg/m3) Free water saturation (kg/m3) Vapour barrier sd = 1500
912 0.65 1000 0.4 3.4 10.6 399
83 0.95 840 0.035 1 0.7 544
2500 0.001 0.5 0.5 20000 -
553 0.61 1700 0.12 134 76 610
TIPUR 40 0.95 1500 0.03 50 -
TIMW 60 0.95 850 0.04 1.3 -
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Experimental green roof
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The moisture retention curve (MRC) and dependence of thermal conductivity on temperature and water content for the substrate layer are shown in Fig. 4. Liquid transport coefficient for
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in Fig. 5.
Thermal conductivity (W/(m.K))
the process of capillary suction (Dws) and redistribution (Dww) for the substrate layer is shown
Temperature (°C)
-50
0
50
100
1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30
0.0
0.2
0.4
0.6
0.8
1.0
Water content mwater / mtotal(-) Thermal conductivity, mouisture dependent Thermal conductivity, temperature dependent
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Fig. 4 Moisture retention curve MRC (on the left) and thermal conductivity dependent on temperature and water content for the substrate layer (on the right)
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Fig. 5 Liquid transport coefficient for the process of capillary suction (Dws) and redistribution (Dww) for the substrate layer The material parameters used in this study are part of the WUFI simulation software database
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and were developed within the research project "Zuverlässige Beurteilung der hygrothermischen und energetischen Auswirkungen von Gründächern" (SF-10.08.18.8 / II 3F20-10-1-100), funded by the research program "Zukunft Bau" of the Bundes institute für Bau-, Stadt- und Raumforschung (German Federal Ministry of Transport, Building and Urban
3. Boundary conditions
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development).
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When designing green roofs by using simulation methods, the choice of boundary conditions for calculation is a key aspect. A very important factor is the information on the amount and intensity of precipitation. Easily accessible and frequently used climatic data (TRY, IWEC,
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etc.), primarily dedicated to energy simulations, focus on typical weather in the region and generally do not contain quantitative rainfall data. To simulate the behavior of a green roof
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with relevant consideration of quantitative rainfall impact it is more appropriate to use data for hygrothermal modelling. Unfortunately, there is no such data for the territory of Slovakia.
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Therefore climatic parameters measured for the city of Košice (East Slovakia) were used as the boundary conditions for the hygrothermal behavior simulation of the experimental green roof. Measurements of meteorological parameters with mean 15-minute increments, including exterior air temperature, air humidity, atmospheric pressure, precipitation, wind speed and direction, incoming shortwave and long - wave radiation were compiled.
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Fig. 6. Environment data values measuring devices: a, b) weather station, c) pyranometer, d) indoor air temperature and relative humidity measuring sensor. For the simulation analyses, year 2013 was selected. It was a year characterized by high summer temperatures throughout Slovakia. However, as for East Slovakia, it was marked by significant rainfall as well. The summer of 2013 (June - August) in Slovakia is considered
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extreme in terms of temperature with a positive temperature deviation of +2 to +3 °C compared to the long term average (years 1961 - 1990). According to the SHMÚ (Slovak Hydro - Meteorological Institute), three periods of significant heat waves appeared in the summer of 2013, which culminated between June 16 - 23, July 28 - 29 and August 6 - 9. During the summer of 2013, several all-time highs were reached, regarding both temperature
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as well as the number of days with record high day and night temperatures. During the first heat wave between the 16th and 23rd June, the first super tropical night was
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recorded in Slovakia for the month of June since the history of measurements began in 1881. Super tropical night was even measured in each of the given year's summer months. The
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summer of 2013 became the 4th warmest ever recorded in Košice since the beginning of the weather monitoring (1881). The summer of 2013 precipitation records were marked by a very
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uneven rain distribution in many locations. Based on the SHMU analysis, in July 2013, many regions of Slovakia saw extraordinarily low to extremely low monthly precipitation sums
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having been recorded since the start of monitoring in 1881, with the exception of the areas located in the eastern part of the country. Average precipitation figures for West, Central and East Slovakia and the measured ones for the city of Košice can be found in table 2. Rainfall sums for the city of Košice during the summer of 2013 (June - August) correspond with the data measured in Eastern Slovakia (Tab. 2). The exception is the month of July in which Košice saw an increase in precipitations by 64 millimetres compared to East Slovakia average.
ACCEPTED MANUSCRIPT Tab. 2 Mean monthly areal precipitation total calculated by the isohyetal method from about 600 stations in Slovakia (SR) at the Slovak Hydro Meteorological Institute (SHMÚ) for West (W_SR), Central (C_SR) and East Slovakia (E_SR) regions supplemented with measured data for the city of Košice (millimetres R (mm) and percentages of normal R (%)) Percentages of normal (%) W_SR C_SR E_SR 124 62 22 14 21 55 112 122 152 137 185 167 44 43 67 235 204 207
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Precipitation R H2O (mm) C_SR E_SR Košice 57 19 13 21 53 117 121 135 114 159 125 122 27 36 38 110 87 85
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VIII.2013 VII.2013 VI.2013 V.2013 IV.2013 III.2013
W_SR 78 10 76 92 21 101
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Fig. 7 Weather data of Košice (Technical university of Košice campus) during simulation
4. Initial conditions and simulation scenarios definition In hygrothermal simulations, the initial condition of temperature and initial condition of water content in the structure represent the initial condition of construction. In long-term simulations, the effect of the initial temperature on the calculation result is negligible. For this reason, the initial temperature condition is simplified by setting a constant temperature across the structure to be analysed before starting the simulation. On the contrary to that, presence of water in the building structure has a long-lasting character. Presence of water in the structure
ACCEPTED MANUSCRIPT is a desired phenomenon, high occurrence of which is regularly repeated over the life cycle of the construction. Water in the structure changes the material parameters of the substrate and its presence greatly affects the evapotranspiration and thus the whole hygrothermal balance of the green roof. These are the main reasons why presence of water and its fluctuation in the green roof substrate cannot be excluded from the process of simulating its hygrothermal behaviour. Water content in the substrate in pre-simulation time is not usually known. Therefore, before starting the simulation, this input data needs to be set explicitly. In general,
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the initial condition can be entered based on exact data on the water content status, determined by a simulation tool or it can be estimated empirically. The most accurate way to determine initial water content at the beginning of a simulation is to achieve it directly or indirectly by measuring the water content in the substrate. Water content of building materials (such as soil) in a built-in state is measured by an indirect method, for example using TDR
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(time-domain reflectometry) [6]. It is possible to determine the value of the water content directly from the substrate sample by the gravimetric method [27]. Initial water content measurement provides accurate information on the presence of water in the substrate and reduces a possibility of a calculation error during the simulation. However, such
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measurements are expensive as well as time-consuming and are applicable in determining the initial condition for practical calculations only to a limited extent. It is much more practical to use a simulation tool in order to determine the initial water content when designing green
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roofs. Water content in the substrate is determined by a simulation by using relevant climatic parameters (Fig. 7). The least accurate way to determine water content in the substrate is to
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estimate the water content values empirically at the beginning of the simulation from the characteristic values of the material parameters of the substrate (wcap, w80 or MRC) and the
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nature of weather (e.g, long-term drought). Simulation in this study is performed for four calculation scenarios. The summary of the calculation scenarios characteristics is shown in
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Table 3. The scenarios differ from each other in the length of simulation time as well as in the initial condition of water content. The effect of the selected initial condition on the course of the calculated water content in the substrate during the simulated time can be observed on the analysed simulation scenarios.
ACCEPTED MANUSCRIPT Tab. 3 Characteristics of the analysed simulation scenarios Case Case 1 Case 2 Case 3 Case 4
Time 01.03. - 31.08. 25.05. - 31.08. 01.06. - 31.08. 01.06. - 31.08.
Initial condition Measured Dry Substrate (W.c. dry) Dry Substrate (W.c. dry) Wet Substrate (W.c. wet)
Average water content (kg/m3) 188 (March 1) 15 (May 25); 153 (June 1) 15 (June 1) 238 (June 1)
Case 1 is a reference scenario with a simulated time of 6 months (March 1 – August 31).
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Longer simulated time in Case 1 compared to the others served to observe the progression of
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water content in the pre-simulation time.
Fig. 8 Calculated and measured values of water content in the green roof substrate
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during simulated time (March 1 - August 31)
During this period (March 1 - August 31), at selected times, samples were taken from the
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green roof substrate throughout its whole profile (150 mm substrate thickness). Subsequently, water content at four different depths was measured gravimetrically from each of the samples
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taken. Water content measurement was performed by the Sartorius MA150 moisture analyser (Fig. 9 in the middle and on the right). The measurements showed uneven distribution of water in the substrate. Even after a long period without precipitation, the bottom layer of the substrate was wet (water content about 180 kg / m3), while the water content measured in the upper layer of the substrate reached only low values (20 kg / m3). Water content of the substrate calculated by using the WUFI simulation tool is shown in Fig. 8 by a black curve. The blue points represent the measured water content values (arithmetic water content average at four different depths of the substrate). The water content values measured at four depths throughout a 150 mm thick layer of dry substrate and a substrate layer following a
ACCEPTED MANUSCRIPT period of long and intense precipitation (W.c. wet) are shown in Fig 9 (on the left). These water content values represent the boundary intervals which a green roof substrate can
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oscillate in.
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Fig.9. Measured water content values of the dry substrate (W.C. dry), substrate after intense rain (W.C. wet) - on the left, green roof after taking samples to measure water content in the substrate - in the middle, moisture analyser - on the right
The initial condition for Case 1 reference scenario was determined by measuring at four
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different depths of the substrate (March 1). Simulation scenarios CASE 2, 3 and 4 are focused on the summer period (June 1 - August 31). For the simulation itself, the initial condition of
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dry substrate and wet substrate was used, Case 3 (W.C. dry) and Case 4 (W.C. wet) respectively, as shown in Fig. 9 on the left. For Case 2, a short pre-calculation of seven days was applied. At the beginning the initial condition (W.C. dry) was used. The boundary
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conditions for Case 2 simulation scenario are shown in Fig. 10.
Fig.10 Boundary conditions for Case 2 simulation scenario
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5. Discussion of calculated results Results of this study can be presented in two ways. The first one is to model water content in the substrate depending on precipitation. The second one is the impact of water content in the substrate layer on the roof membrane's temperature and heat flux changes in the summer period. The impact of precipitation water accumulated by the substrate reduces the heat flux
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through the roofing system. The WUFI simulation tool, which takes into account strong bond between heat and water transport processes, enables both ways to be analysed simultaneously. (The heat and moisture transport processes in buildings are usually strongly coupled). 5.1 Analysis of water in the substrate layer
The simulated course of water content in the tested green roof substrate obtained by a long-
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term calculation (March 1 - August 31), verified by numerous water content measurements (blue points), is defined as Case 1 reference simulation scenario and is shown in Fig. 11 by a black line. Other simulation scenarios present the course of water content in the substrate when using different initial conditions according to Table 3. When using the 7-day precalculation within Case 2 scenario (green line), water content increased from the starting
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value of 15 kg / m3 (may 25) to 150 kg / m3 (June 1) getting closer to the measured water content, in other words to Case 1 reference scenario. A sharp rise in pre-calculated water
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content was caused by intense storms in the pre-simulation time during which 85 mm of rain fell. Simulated water content courses with explicitly set initial water content for dry substrate
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(Case 3) and wet substrate (Case 4) are shown in Fig. 11 in orange and blue colour respectively. When using the initial water content of dry substrate (Case 3), the calculated
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water content equals the measured values, or Case 1 reference scenario, in approximately 2 months, while Case 4 simulation scenario reaches the measured values of water content (or
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Case 1) almost immediately.
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Fig. 11 Calculated and measured (blue points) values of water content in the green roof substrate during simulated time may 25; June 1 - August 31 for the analysed simulation scenarios 5.2 Simulated temperature and heat flux courses under substrate layer
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Figs. 12 to 14 show the outdoor air temperature, solar radiation and calculated temperatures at the border between the substrate and the roof layer in a place indicated as P2 (Fig. 3) for all defined simulation scenarios. Simulated scenarios (Case 1, 2, 3 and 4) used black, green, orange and blue colours. The identical colour lines (in form of dashed lines) show the
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calculated water content in the green roof substrate. From Figs. 12-14, substrate layer's presence and its influence on the temperature course of P2 is clearly visible. The calculated
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daily temperature peak under the substrate layer (P2) is shifted by more than 7 hours compared to the daily temperature peak. This phenomenon is observed throughout the entire simulated time. Influence of different water content in the substrate on the calculated
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temperature course at P2 is observable especially at the beginning of the simulated time (June 1 - June 16). The biggest differences are observable when comparing Case 3 simulation
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scenario with Case 1 or Case 4 reference scenario (Fig. 12).
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Fig. 12 Surface temperature courses under the substrate layer at P2 and the average
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water content in the substrate calculated for 4 simulation scenarios and the period of June 2013
At the beginning of the simulated time (June 1 – June 3), the temperature of the entire
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simulated roof profile, including the substrate layer, is balanced, since the roof temperature for Case 1 and 2 was set by using the simulation tool (June 1) while the initial temperature for
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Case 3 and 4 was explicitly set at 21° C for the entire roof profile. Decrease in outdoor air temperature along with lower solar radiation intensity (June 3-5) caused reduction in heat
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exchange between the building structure and the surrounding environment. This decrease was projected into the gradual equalization of calculated surface temperatures at P2 for all
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simulation scenarios. The arrival of warmer days without precipitation (June 6) again increased the heat exchange between the experimental cell and the outdoor environment. This phenomenon was immediately manifested in an increase of the calculated surface
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temperatures at P2. During the period of June 6-9, the calculated surface temperature at P2 increased and reached values similar to exterior air temperature. This period was particularly interesting in terms of differences in calculated surface temperatures at P2 among individual simulation scenarios. Simulation scenarios with high water content in the substrate (Case 1 and 4) showed significantly lower surface temperatures at P2 compared to Case 3 simulation scenario (dry substrate). Differences in daily peaks of the calculated surface temperature at P2 were between 3 and 4 K. For Case 2 simulation scenario using a seven day pre-calculation, the calculated daily peak surface temperature at P2 was 1-2 K higher than Case 1 reference
ACCEPTED MANUSCRIPT scenario. Due to climatic changes (especially precipitation and drying) the water content becomes more or less balanced during the simulated time, thus eliminating the influence of initial water content on the calculated surface temperature under the substrate at P2 site. At the end of June, the difference in calculated surface temperature peaks at P2 for Case 1 and Case 3 simulation scenarios is approximately 1 K, and this surface temperature difference is
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repeated throughout nearly entire month of July (Fig. 13).
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Fig. 13 Surface temperature courses under the substrate layer at P2 and the average water content in the substrate calculated for 4 simulation scenarios and the period of
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July 2013
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Following a period of precipitation July 16 - July 30, the calculated water content reaches approximately same value for all simulated scenarios analysed. The influence of the initial water content on the hygrothermal behaviour of the green roof is negligible from this moment
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on (Fig. 14).
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Fig. 14 Surface temperature courses under the substrate layer at P2 and the average
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water content in the substrate calculated for 4 simulation scenarios and the period of August 2013
The maximum roof membrane surface temperature values calculated are summarized in Tab. 4. The highest roof membrane temperature was calculated for Case 3 scenario while the
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lowest one was calculated for Case 1 reference scenario. The simulation result confirmed how significant substrate and water content inside it is when used in a green roof assembly and
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how strong its influence on the roof membrane temperature reduction can be. Due to lower thermal stress and protection against UV radiation, life cycle of waterproof layers as well as
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life cycle of the entire roof is prolonged. Tab. 4 Calculated maximum surface temperatures of the roof membrane under the substrate
Maximum temperature P2 24.35 °C 26.13 °C 27.75 °C 24.70 °C
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Case Case 1 Case 2 Case 3 Case 4
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layer during summer period (June - August) for the simulated Case 1 - Case 4 scenarios
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Fig. 15 Course of heat flux under the substrate layer at P2 site calculated for 4
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simulation scenarios for days of June 6 and July 29
The influence of the substrate layer on hygrothermal behavior of the green roof can also be observed on heat flux simulation under substrate layer at P2. In Fig. 15 on the left, June 6 heat flux course is displayed, while the right side shows July 29 heat flux. Different initial water content in the substrate for the simulation scenarios at the beginning of the simulation (June 6)
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affects the fluctuation of heat flux through the analyzed roof. At the end of July, the influence of the initial water content on heat flux is negligible. Roof protection against summer
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overheating by applying vegetation with a substrate layer can be interpreted by a quantification of heat fluxes. The heat flow through the roof was obtained by integrating the
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heat flux over time. Average daily heat flows calculated through the tested green roof over a period of three summer months are summarized in Fig. 16. Average daily heat flows for Case 1, 2 and 3 simulation scenarios are compared. The biggest difference of average daily heat
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flow was observed in June between Case 1 and Case 3 scenario. Higher water content in the substrate (Case 1) reduces the heat flow through the green roof by 11% compared to Case 3
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scenario. The use of pre-calculation (Case 2) reduces the average daily heat flow in June by 7% compared to Case 3 scenario. As for the month of July the difference in the average daily heat flow between Case 1 and Case 3 scenarios is just 2%, while in August the impact of the initial water content on this parameter is completely negligible.
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Fig. 16 The average daily heat flow through the tested green roof calculated for different initial water contents represented by Case 1, 2 and 3 simulation scenarios. Summer
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period (June - August) for Košice (East Slovakia) was simulated
Conclusion
The simulation analysis showed significant influence of water content in green roof substrate on hygrothermal behavior of such roof. The choice of the initial water content used in the
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presented simulation scenarios reflects the real moisture conditions of the particular type of substrate in dry and wet state. The use of specific climatic parameters aimed to analyze
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hygrothermal behavior of the tested green roof in the most accurate context possible. Analysis has shown that using a sufficiently long pre-calculation can provide reliable information on water content in the substrate depending on climatic parameters (especially precipitation).
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However, for a more precise comparison of the accumulated water content, but especially its redistribution in the substrate's profile, a more accurate measurement is required. Applying
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TDR sensors at different depths of the substrate allows for real-time monitoring of water content profiles.
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Use of a short-term pre-calculation can be questionable. In Case 2 simulation scenario, use of a 7-day pre-calculation resulted in a more accurate simulation owing to " suitable weather" at pre-simulation time when 85 mm of rain fell to the substrate, which significantly increased water content of the substrate and brought it closer to the real moisture state. The unpredictability of the precipitation and its uneven distribution in the region was also demonstrated by the difference between the average monthly precipitation sums in the region of East Slovakia and the city of Košice in the month of July 2013 (Table 2). Simulation analysis demonstrated that:
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Water content in substrate layer is crucial to hygrothermal behavior of a green roof. In the simulation, the amount and distribution of water in the studied substrate profile is a significant factor.
The influence of water in the substrate layer was reflected on the roof membrane's temperature. The highest calculated temperature differences between the dry and wet substrate simulation scenario were around 3 K. Considering high temperatures of roof membranes in conventional roofs, this might not look like a high number, but it can be
increasing the roof structure's life cycle.
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said that water content participates in reducing the membrane temperature thus
The impact of water in the substrate was shown in reducing the heat flux across the roof with a 150 mm coarse layer of substrate, which in the summer period participates in protecting the building from overheating. For analyzed wet and dry substrate
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scenarios, the average daily heat flow reduction is 11%.
An important conclusion of the thesis is the applicability of a suitably chosen simulation tool in green roof design. The technically justified benefits of green roofs obtained by such calculation and later reflected into economic aspects, present a
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stimulus for investors and building owners that can hardly be overlooked. Making building constructions that surround us greener to much bigger extent would
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fundamentally change such buildings’ behavior, would bring undoubtable benefits in terms of their energy consumption, and last but not least would improve the quality of our life. We
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believe that these arguments will contribute to a more massive incorporation of green structures design in Slovakia hand in hand with utilizing the sophisticated hygrothermal
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simulations assessment.
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Acknowledgements This work was supported by the Slovak Scientific Grant Agency (VEGA) in collaboration with Slovak Ministry of Education, Science, Research and Sports (MESRS) and Slovak Academy of Sciences (SAS) under Grant number 1/0835/14.
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