Influence of Plant and Substrate Characteristics of Vegetated Roofs on ...

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 78 (2015) 1171 – 1176

6th International Building Physics Conference, IBPC 2015

Influence of plant and substrate characteristics of vegetated roofs on a supermarket energy performance located in a semiarid climate Sergio Veraa,e,*, Camilo Pintoa, Felipe Victoreroa, Waldo Bustamanteb,e, Carlos Bonillac,e, Jorge Gironásc,e and Victoria Rojasd a

Department of Construction Engineering and Management, School of Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, 7820436, Chile b School of Architecture, Pontificia Universidad Católica de Chile, El Comendador 1916, Providencia, Santiago, 7520245, Chile c Department of Hydraulic and Environmental Engineering, School of Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, 7820436, Chile d VR+ARQ, Av. Francisco de Bilbao278, Providencia,, Santiago, 7510895, Chile e Center for Sustainable Urban Development (CEDEUS), Pontificia Universidad Católica de Chile, El Comendador 1916, Providencia, Santiago, 7520245, Chile

Abstract The aim of this paper is to study the influence of different parameters related to the vegetation and substrate of vegetated roofs (VRs) on the thermal and energy performance of a supermarket building. The main studied parameters of the substrate are density, thermal properties and moisture content while the evaluated vegetation parameters are plant height and leaf area index (LAI). This study was performed via thermal and energy simulations of a supermarket located in Santiago of Chile (semiarid climate) using Design Builder. The study concludes that VRs might significantly reduce the cooling energy consumption of a supermarket building and LAI is the main parameter that affects the magnitude of this influence. 2015The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license © 2015 Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL. Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL Keywords: Vegetated roofs; green roofs, substrate; leaf area index; semiarid climate; supermarket; energy consumption

1. Introduction Buildings are responsible for large energy consumption and green-house gas (GHG) emissions worldwide. In developed countries, the building sector accounts for 40% of the total energy consumption [1,2]. In Chile, a developing * Corresponding author. Tel.: +56-2-23544245; fax: +56-2-23544244. e-mail address: [email protected]

1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the CENTRO CONGRESSI INTERNAZIONALE SRL doi:10.1016/j.egypro.2015.11.089

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Sergio Vera et al. / Energy Procedia 78 (2015) 1171 – 1176

country, this sector is responsible for the 28.8% of the total energy consumption [3]. Regarding climate change mitigation, UNEP [4] emphasize that “the building has the largest potential for delivering long-term significant and cost-effective reduction of GHG emissions”. In order to reduce building energy consumption, significant technological advances in building envelopes have occurred in the last decades. One of these advances corresponds to vegetated roofs (VRs), also known as green roofs, which include plants to improve the roof performance [5]. VRs usually include several layers such as roof structure, insulation, waterproofing layer, root barrier, drainage layer, geotextile filter, growing media or substrate, and vegetation layer. 1.1. Impact of VRs on building energy consumption The impacts of VRs on buildings thermal performance have been widely studied. Table 1 summarizes the main heat and mass transfer mechanisms occurring in the VRs that might contribute to reduce buildings energy consumption [6-8]. Literature shows large variability about the impacts of VRs on roof’s thermal performance. Thermal performance results are commonly misinterpreted as energy savings, and studies that really estimate or measure energy savings are scarce [9]. Nichau et al. [10] estimated, using a very simplified building energy model, energy savings between 2% and 48% for well insulated and uninsulated buildings, respectively. On the contrary, De Nardo [11] found almost no annual energy saving differences due to the implementation of a VRs. Wong et al. [12] showed energy savings between 1 and 15% using a VRs in a five-story building in Singapore. In 2012, Sailor et al. [9] used the green roof model of Sailor [13], which has been implemented in EnergyPlus, to evaluate the impact of VRs in different U.S. climates. Similarly, Ascione et al. [14] evaluated the effectiveness of VRs in European climates. As expected, they concluded that the effectiveness of green roof to reduce building’s energy consumption is strongly associated to the climate. Ascione et al. [14] found that VRs allow annual energy savings between 1 and 11% in warm climates mainly due to significant cooling load reductions. In cold climates, VRs allow lower energy savings not only during the cooling season but also during the heating season. Table 1. Heat and mass transfer mechanisms in VRs. Heat and mass transfer mechanisms

Description

Effect

Substrate evaporation and vegetation transpiration

This is known as evapotranspiration or latent heat transfer. It is a combined effect of water evaporation from the soils and transpiration of plants. Evapotranspiration is the main contributor to counterbalance the incident solar radiation on the roof

Evaporative cooling of the building surroundings. This contributes to reduce heat fluxes into the substrate, which turns in reduction of cooling loads

Shading provided by the canopy

The foliage reduces the amount of solar radiation that reaches the outer roof surfaces. This reduces the roof surface temperature in comparison with a traditional roof. Thus, heat flux into the roof also decreases.

Reduction of the substrate surface temperature and reduction of heat fluxes into the substrate, which turns in reduction of cooling loads.

Thermal inertia provided by soil

The growing media contributes with thermal mass that helps to stabilize indoor temperatures.

Reduction of peak loads, especially when VRs are implemented in lightweight constructions

Additional insulation provided by soil

The substrate adds thermal resistance to the roof which helps to reduce heat losses through the roof and heat gains into the roof.

Reduce heat fluxes through the roof. The influence of the additional thermal resistance depends on the level of roof insulation, type of soils and its moisture content.

1.2. Impact of vegetation and substrate properties on building energy consumption Table 1 shows that the main mechanisms of heat and mass transfer in VRs rely on how vegetation and substrate counterbalance the incident solar radiation on the roof by means of evapotranspiration and shading. These phenomena are taken into account by numerical models [6,13,15] based on the following canopy and substrate parameters: (1) the plant height; (2) leaf area index (LAI), which is the ratio between the surface of leaves and other transpiring parts of


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the plant and the soil surface below [16]; (3) stomatal resistance for water vapor transport; (4) substrate hygrothermal properties; and (5) substrate thickness, among others. Despite the crucial influence of plant and substrate properties on the heat and mass transfer across the VRs, there is a lack of studies about the influence of these parameters on the thermal performance and energy consumption of buildings. Wong et al. [12] showed building’s energy savings due to VRs between 1 and 15%. This large variation was mainly caused by vegetation type together with moisture and substrate thickness. Similarly, Theodosiou [17] concluded that LAI is the main parameter influencing the effect of VRs on surface temperature and heat fluxes, whereas, substrate thickness and plant height showed not to be significant. However, this study did not evaluate the impact of these vegetation and substrate parameters on the building energy consumption. The main objective of this study is to evaluate the influence of vegetation and substrate characteristics of VRs on the energy consumption of a supermarket placed in Santiago of Chile. This would help designers to understand better the influence of these parameters on the building energy performance, allowing them to maximize the benefits of VRs based on the proper selection of the plant-soil system. The study considered a one-story supermarket, in which greater energy savings are expected due to the larger influence of the roof area on the thermal performance of the building. Moreover, literature review shows that larger energy savings occur in warm climates, such as Santiago’s climate, which is characterized by a prolonged cooling season (6-8 months) due to high temperatures and solar radiation. 2. Methodology To perform this study, a 900 m2 one-story supermarket was simulated with Design Builder©, a user interface of EnergyPlus© which includes the Sailor green roof model [13]. The main inputs of the energy simulations are the followings: x Architecture and envelope: highly insulated concrete slab-on-grade (U-value = 0.47 W/m2·K), highly insulated lightweight walls (U-value = 0.27 W/m2·K), no fenestrations, lightweight and uninsulated steel roof deck. x Internal heat gains and schedules: internal heat gains were based on CIBSE Guide A [18] for retail stores. Illumination power density was 12 W/m2, while sensible and latent heat by people 16 and 12 W/m2, respectively. Supermarket indoor space was conditioned between 8 and 22 hrs. x HVAC system: The HVAC equipment used was a rooftop packaged DX unit. Thermostats were set to 21-23°C. The studied vegetation and substrate parameters are described below. The substrate thickness is 0.15 m. x Irrigation schedules: (1) no irrigation; (2) irrigation of 10 mm/h at 10:00, 14:00 and 18:00 h (30 mm/day in total); and (3) 20 mm/h during 24 hours (480 mm/day in total). The irrigation schedules were set throughout the year. The irrigation of 30 mm/day is reasonable for plants with medium irrigation requirements and substrates with low to medium moisture retention. However, the 480 mm/day over exceed the irrigation requirement of common vegetation used in VRs, thus this irrigation rate was set for testing how the model deal with extremely high irrigation values. x Plant height: 0.1; 0.2 and 0.3 m height. x Substrate type: the influence of two substrate studied by Sailor and Hagos [19] were investigated: (1) light and dry and (2) heavy and wet. The thermal conductivity, specific heat capacity, density and moisture content of these substrate are 0.208 W/m·K, 1100 J/kg· K, 730 kg/m3 and 0.1 m3/m3 for the light-dry substrate, and 0.85 W/m·K, 1800 J/kg·K, 1639 kg/m3 and 0.25 m3/m3 for the dense-wet substrate. The maximum moisture content at saturation for both substrates was 0.5 m3/m3. x LAI: 0.1 (bare soil), 2.7 and 5.0. 3. Results and analysis 3.1. Influence of plant height and irrigation on building energy consumption The influence of the plant height on the supermarket energy consumption were investigated for different substrate

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types and LAIs without irrigation and minimum stomatal resistance of 50 s/m. Due to the length limitations of the paper, heating and cooling energy consumption are presented in Table 2 for heavy-wet substrate and LAI 5. As can be observed, the influence of plant height on heating and cooling energy consumption of the supermarket is negligible. Similar results were obtained for other LAIs with heavy-weight and light-dry substrates. Table 2. Energy consumption with different height of plants. Type of energy consumption (kWh/m2·year)

Plant height (m) 0.1

0.2

0.3

Heating

22.3

22.3

22.9

Cooling

11.1

10.8

10.7

The three irrigation approaches were evaluated for both substrate types, plant height of 0.3 m, LAI 5 and minimum stomatal resistance of 50 s/m. The heating and cooling energy consumptions for all cases are shown in Table 3. Increasing energy consumption with irrigation rate evidences that the model is able to capture that that the increment in moisture content cause higher conductivities and then larger heat losses through the roof, which generate higher heating energy consumptions and lower cooling energy consumptions. Comparing the results for both types of substrates, irrigation rate has larger impact on the energy consumption variation of heavy-wet substrate. This is an unexpected result that might evidence that the Sailor VRs model may not properly take into account the substrate moisture content and respective impact on their thermal properties. Further studies are required to clarify this, which are out the scope of this paper. Table 3. Energy consumption with different irrigation rates and schedules. Irrigation approach

Light-dry substrate

Heavy-wet substrate

Heating energy consumption (kWh/m2·year)

Cooling energy consumption (kWh/m2·year)



No irrigation

16.6

13.3

23.2

10.8

30 mm/day

17.0 (2.5% µ)

13.0 (2.4% ¶)

25.4 (9.5% µ)

8.3 (22.9% ¶)

480 mm/day

17.3 (4.3% µ)

12.3 (7.4% ¶)

26.1 (12.5% µ)

7.8 (28.0% ¶)

Note: Percentages between brackets indicate the increase or reduction in energy consumption in comparison with the case without irrigation.

3.2. Influence of substrate type and LAI The impact on energy consumption of both substrate type and LAIs were studied simultaneously for plant height of 0.3 m, without irrigation and minimum stomatal resistance of 50 s/m. Table 4 shows the cooling and heating energy consumption for light-dry and heavy-wet substrates. It can be observed that increasing LAI significantly reduces cooling energy consumption between 39.5% and 59.5%, while heating energy consumption increases less significantly (12.8%-21.1%). This comparison against the energy consumption of the bare substrate (LAI = 0.1) confirms the large cooling effect of VRs due to evapotranspiration of the foliage-substrate system and shading provided by the canopy, which turns in large reductions of heat fluxes into the roof. Figure 1 shows this effect for the heavy-wet substrate. During the cooling season in Santiago of Chile (October to March), the roof with bare substrate (LAI = 0.1) presents heat gains due to the lack of vegetation which increases cooling loads. In contrast, the vegetation with LAI 2.7 and 5.0 not only eliminates these cooling loads but contributes to dissipate internal heat of the supermarket through the roof because vegetation reduces the heat flux entering the substrate by shading and cooling the surroundings. Since Figure 1 presents monthly heat fluxes across the roof, daily variation cannot be observed. The latest can be seen in Figure 2 which displays the heat gains and losses through the roof for the three LAI values on January 14th (one of the hottest day of the year). Additionally, it is shown the heat fluxes for three different roof solutions without a VRs, insulated and uninsulated steel roof deck and uninsulated concrete slab. This figure shows that from 8 to 18 hrs heat losses occurred across the VRs with LAIs 2.7 and 5.0, whereas significant heat gains across the roof with bare substrate (LAI = 0.1) occur. This confirm that during higher ambient temperature and solar radiation hours, the

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vegetation allow converting solar radiation into plant evapotranspiration as well as shading the roof. As consequence cooling loads are significantly diminished. Also, it can be seen that the behaviour of the roof with bare substrate is similar to a roof composed of an uninsulated concrete slab. Both presents very high heat gains after midday and a delay in the peak heat gains due to the thermal mass of soil and concrete. However, the thermal mass effect of substrate disappears when vegetation is present (LAIs 2.7 and 5.0). Finally, it can be seen that the performance of the VRs with LAIs 2.7 and 5.0 is much better than that for the insulated steel roof deck, which also show high heat gains through the roofs. Table 4. Energy consumption with different LAIs. Leaf Area Index (LAI)

Light-dry substrate

Heavy-wet substrate





0.1 (bare soil)

12.7

23.2

16.6

27.5

2.7

14.7 (16.0% µ)

14.0 (39.5% ¶)

18.7 (12.8% µ)

13.0 (52.9% ¶)

5.0

15.3 (21.1% µ)

12.7 (45.4% ¶)

19.8 (19.2% µ)

11.1 (59.5% ¶)

Note: Percentages between brackets indicate the increase or reduction in energy consumption in comparison with the case of bare soil.

Fig. 1. Monthly heat fluxes across the VR for heavy-wet substrate.

Fig. 2. Hourly heat fluxes through the VR and outdoor air temperature on January 14th.

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4. Conclusions This paper discusses the influence of vegetation and substrate parameters on the energy consumption of a supermarket located in semiarid climate with a prolonged cooling season. The studied vegetation-substrate system parameters are foliage height, LAI, irrigation rates, and substrate types. To carry out this study, energy simulations were performed in Design Builder©. The main conclusions that can be drawn from this research are: x Vegetation plays a key role to reduce cooling loads due to canopy evapotranspiration and shading on VRs. These phenomena contribute to significantly decrease heat gain through the roof due to reduction of incident solar radiation and evaporative cooling of the building surroundings. The vegetation contributes to dissipate internal heat gains through the supermarket’s roof during summer, which significantly reduces cooling energy consumption up to 45% and 60% for light-dry and heavy-wet substrates, respectively. Even though, the performance of VRs with medium and high LAI exceeds the performance of an insulated roof (without VRs). x Leaf area index, LAI, is the most important parameter that influences the supermarket energy consumption in a semiarid climate. On the other hand, plant height does not impact the energy consumption of the supermarket. x Irrigation contributes to reduce cooling loads and increase heating loads because thermal conductivity of soil is increased. However, the results evidences that the Sailor’s VRs model implemented in EnergyPlus© might not properly take into account the effect of increasing moisture content on the thermal properties of the substrate. x Thermal inertia of the substrate does not influence the thermal performance of the VRs with LAIs 2.7 and 5.0. Acknowledgements This work was funded by CORFO under the grant INNOVAChile 12IDL2-13630 and supported by the National Commission for Science and Technological Research (CONICYT) under the research projects FONDECYT 1150675 and CONICYT/FONDAP 15110020. References [1] DECC, Energy consumption in the UK, in, Department of Energy & Climate Change, 2013. [2] DOE, Building Energy Data Book, in, U.S. Department of Energy, 2013. [3] MinEnergia, Balance Energético 2012, in, Ministry of Energy of Chile, 2013. [4] UNEP, Buildings and Climate Change: Summary for Decision-Makers, in, United Nations Environment Programme, Sustainable Buildings & Climate Change, Paris, 2009. [5] E.C. Snodgrass, L. McIntyre, The green roof manual: a professional guide to design, installation, and maintenance, Timber Press, 2010. [6] P.C. Tabares-Velasco, J. Srebric, A heat transfer model for assessment of plant based roofing systems in summer conditions, Building and Environment, 49(0) (2012) 310-323. [7] H.F. Castleton, V. Stovin, S.B.M. Beck, J.B. Davison, Green roofs; building energy savings and the potential for retrofit, Energy and Buildings, 42(10) (2010) 1582-1591. [8] U. Berardi, A. GhaffarianHoseini, A. GhaffarianHoseini, State-of-the-art analysis of the environmental benefits of green roofs, Applied Energy, 115(0) (2014) 411-428. [9] D.J. Sailor, T.B. Elley, M. Gibson, Exploring the building energy impacts of green roof design decisions – a modeling study of buildings in four distinct climates, Journal of Building Physics, 35(4) (2012) 372-391. [10] A. Niachou, K. Papakonstantinou, M. Santamouris, A. Tsangrassoulis, G. Mihalakakou, Analysis of the green roof thermal properties and investigation of its energy performance, Energy and Buildings, 33(7) (2001) 719-729. [11] J.C. DeNardo, Green roof mitigation of stormwater and energy use, Pennsylvania State University, State College, PA, 2003. [12] N.H. Wong, D.K.W. Cheong, H. Yan, J. Soh, C.L. Ong, A. Sia, The effects of rooftop garden on energy consumption of a commercial building in Singapore, Energy and Buildings, 35(4) (2003) 353-364. [13] D.J. Sailor, A green roof model for building energy simulation programs, Energy and Buildings, 40(8) (2008) 1466-1478. [14] F. Ascione, N. Bianco, F. de’ Rossi, G. Turni, G.P. Vanoli, Green roofs in European climates. Are effective solutions for the energy savings in air-conditioning?, Applied Energy, 104(0) (2013) 845-859. [15] S.-E. Ouldboukhitine, R. Belarbi, I. Jaffal, A. Trabelsi, Assessment of green roof thermal behavior: A coupled heat and mass transfer model, Building and Environment, 46(12) (2011) 2624-2631. [16] V. Novák, Evapotranspiration in the Soil-Plant-Atmosphere System, Springer, 2012. [17] T.G. Theodosiou, Summer period analysis of the performance of a planted roof as a passive cooling technique, Energy and Buildings, 35(9) (2003) 909-917. [18] CIBSE, CIBSE Guide A: Environemnatl Design, in, The Chartered Institution of Building Services Engineers, London, 2006. [19] D.J. Sailor, M. Hagos, An updated and expanded set of thermal property data for green roof growing media, Energy and Buildings, 43(9) (2011) 2298-2303.