STUDY ON THERMAL FUNCTION OF IVY-COVERED WALLS

Transcription

1 STUDY ON THERMAL FUNCTION OF IVY-COVERED WALLS Mr. Liao Zaii, Dr. Niu J.L Dept. of Building Services Engineering The Hong Kong Poltechnic Universit Hunghum, Kowloon, Hong Kong ABSTRACT This paper presents a mathematical model ielding a simplified representation of the thermal behaviors of iv-covered s. The model is integrated with a CFD program to implement simulation. Eperimental results have been used to form the boundar conditions of numerical simulations. A series of parametric sensitivit analses have been carried out for identifing the ke factors that affect ivcoverings potential for reduction on cooling load of the buildings. These analses indicate that ivcoverings can considerabl reduce the heat flu through the eternal s that its cover. Three ke parameters have been identified as relevant for ivcovered design: green densit (d), covering ratio (r), the supporting grid s geometrical characteristics. INTRODUCTION Over centuries have natural cooling techniques been emploed, but their application has been ver scarce in recent decades when mechanical cooling devices have become the standard alternative [1]. This results in a great energ ependiture and etra anthropogenic heat emission from buildings. To improve energ efficienc of buildings, researches have been focused on improving efficienc of devices and enhancing thermal insulation of buildings. Recentl, due to the increasingl serious energ shortage and global environmental pollution, requirement on building energ efficienc has been becoming higher. So application of natural cooling techniques becomes ver important and researchable. The potential of vegetation to significantl reduce building cooling loads has been reported in a number of studies. In 1980 s, Huang [6] [7] and his group in Lawrence Berkele Laborator conducted a stud on the potential of vegetation in reducing summer cooling loads in residential buildings. The results indicated that vegetation is beneficial to energ conversation, creating a potential for up to % of energ saving. However man technical details are still not well understood. This is reflected b the fact that there is a lack of technical information supporting landscaping that addresses energ and thermal performance issues. As a result of this gap in research, architects and planners have not full eploited the benefits of landscaping []. Iv-coverings on buildings, as a pleasant architectural feature, can be found throughout China, especiall in it s sub-tropical regions [8]. It is one of the most important stles of how vegetation eists in buildings in this region. A proper arrangement of iv-coverings on buildings not onl proves pschological effects but also improves unfavorable microclimatic conditions. Figure 1 presents a sample of ivcovered s in Hong Kong. The original design concern of this iv-covered was that the designers tried to evolve and to adopt eternal planting outside windows that has been a vernacular practice in Figure 1: An iv-covered in Hong Kong (source: Dept. of Architecture, CUHK, Hong Kong) urban dwellings in Hong Kong. However it is difficult, if not impossible, to quantitativel assess the actual effectiveness and figure out failures for future improvement because technical tool for such assessment is inadequate. When architects and planners design a thermal environment, the require the available data that can indicate the effects of each climatological use of plants. However, little such data has been available for the quantitative analsis of how planting techniques affects the building s thermal environment or how it produces energ savings in air-conditioned buildings. The value of iv-coverings as a technique to reduce airconditioning loads has neither been well understood nor documented. Ivies as pavement on buildings, can protect the eternal s from direct solar radiation and could cool it through enhanced evaporation. It is furthermore compatible with the functional, aesthetic and ecological criteria applied to design of the surroundings [1]. Ivies convert over 70% of solar energ that its absorb into bio energ via photosnthesis, without greatl increasing its temperature [8] [9]. This results in a much lower long- 1

2 wave radiation between foliages and the surfaces of eternal s that are shaded underneath ivies. Vu and Asaeda [2] found that temperature of green leaves could be 23 C lower than the asphalt and concrete surfaces under same level of solar radiation. Liao [8] in his site measuring has found a similar result. The preliminar result indicates that iv-covered s can reduce solar loads b up to 30% [8]. In 1980s, Akira Hoano conducted an eperimental stud on effects of an iv sunscreen [3] covering a west. He reported that iv-coverings (full) could reduce heat flu through the eternal s b 3 quarters [3]. He has also figured out the highl inverse correlation between solar transmittance and iv growth conditions (including foliage geometric characteristics and covering ratio) [3]. However, the technical information supporting a technical iv-covering treatment that can optimize the climatological effects of iv-coverings is still scarce. This stud aims to develop a mathematical model for simulating thermal behaviors of ivcovered s and assessing the potential for savings in cooling energ. The model in detailed represents the mechanism of how iv-coverings interact with the to create a microclimate. The ke relevant parameters that greatl affect the thermal function of iv-coverings will be addressed to identif the criteria for iv-covering design and treatment. MATHEMATICAL MODEL Figure 2a and 2b show a sample of iv-covered (ICW). No etra supporting structures are built to hold ivies. The ivies just naturall climb upward on the. Obviousl, three major components can be distinguished in an ivcovered : the iv canop (leaves), the root grid, and the. The foliages basicall form Figure 2a: An Iv-covered Wall the canop that shades the that would otherwise be eposed directl to the sun and the Figure 2b: A Close-up of ICW surroundings. The root grid, climbing upward on the, links the leaves and the together. In this stud, an etra structure enhancing such linkage is proposed: a metal grid supporting roots. It is embedded into buildings eternal, providing grids that stabilize ivies climbing on the. So the ICW model, as illustrated in figure 3, is derived from three models: iv canop, supporting grid (SG), and eternal. In this paper, the ICW is supposed to be horizontall large enough to assum horizontal homogeneit. Therefore a 2-dimentional model will be developed. Iv Canop Model The iv canop is basicall composed of the leaves and the air within the leaf cover. The compleit of a canop as a sstem of sources and sinks Foliage Root Grid Wall of heat and mass Figure 3a Structure of ICW is such that an eact description Solar rad of its phsical Solar rad Free behavior is leaf air almost conv TIR impossible. In this paper, a conv canop is treated conv conv as one conv conv homogeneous SG laer, which is Canop characterized b: Figure 3b Representation of the model (1) one identical value of leaves temperature; (2) and one identical value of temperature and moisture content of the air within the leaf cover; The iv canop is geometricall characterized b the following factors that determine the growth condition of ivies: (1) covering ratio (r) : the percentage of the surface covered; (2) green densit (d): the surface area of leaves within the control volume. For a certain level of covering ratio, a higher green densit means a better growing condition of the iv. (3) leaf shapes: simple leaf, dissected leaf, and compound leaf [4]. The major processes contributing to the determination of the canop s thermal state include: (1) solar radiation absorbed b the leaves; (2) long-wave radiative echange (TIR) between the leaves and the sk and the surroundings; (3) convective heat transfer between the free air (outside the canop) and the air within the canop; (4) convective heat transfer between the leaves and the air (both free and within the canop); (5) transpiration in the leaves; 2

3 (6) photosnthesis that converts solar energ absorbed into bio-energ. So, the heat balances of the leaves and the air in the canop look like: ( rδd) ( cρ) leaf ( cρ) leaf H leaf air trans, leaf air air H air SG canop r iv covering ratio (%); d green densit (m 2 /m 3 ), defined as the surface area of leaves within controlled volume; δ average thickness of iv leaves (m); ρ densit (kg/m 3 ); c specific heat (J/kg.K); t temperature ( C); τ time (sec); H canop height of canop (m); leaf leaves of the iv; air air within the iv cover; ψ rad, sol the amount of absorbed solar radiation (W/m 2 ); ψ rad,tir the net long-wave radiation on leaves (W/m 2 ); ψ leaf-air the convective heat flu between the leaves and the surrounding air (W/m 2 ); ψ trans,leaf-air the heat flu due to leaves transpiration (W/m 2, negative); ψ photo the energ consumed for photosnthesis (W/m 2 ); ψ air-free the convective heat flu between the free air and the air within the canop (W/m 2 ); ψ air-sg the convective heat flu between the air within the canop and the air within the supporting grid (W/m 2 ). The heat transfer flu, ecept ψ photo, including ψ trans,leaf-air, in the right side of the equations can be calculated based on the method described b E. P. Del Barrio [1]. ψ photo is treated as proportional to the absorbed solar radiation [9]. The inputs of the canop model are solar radiation flu, sk temperature, surface temperature of the eternal, and outside temperature. The air in canop section but not covered b leaves moves into the supporting grid. So the gaps formed b leaves act as air inlets of the SG. Supporting Grid Model canop dt air = ψ dτ dt leaf dτ photo leaf air = ψ rad, sol air free rad, TIR The supporting grid basicall is composed of the metal grid where the roots of ivies climb, roots, and (1) (2) the air within it. In this stud, the impact of the thermal masses and conductivit of the metal grid and roots is ignored. So the SG model just presents the behaviors of the air within it.the SG is characterized b the height of the grid. A time averaged turbulance transport model, k-ε, is emploed to model the air within a SG. The resulting mean conservation equations of momentum can be written in the 2-dimensional formula: ( ρυ ) υ = ( µ ρ υ υ ) + S τ ( ρυ ) υ = ( µ ρ υ υ ) + S τ S P υ υ S = + ( µ e ) + ( µ e ) (6) P pressure (Pa); µ e effective molecular viscosit (Pa.S); ν ν air velocit in and direction (m/s). The energ equation can be written in the following formula: Pr Prandtl number q Heat flu per unit volume (W/m 3 ). The boundar conditions are two folds: inlets among the canop, and the surface. The heat transfer between air within a SG and the can be written as: Q = h P = + ( µ e h convective heat transfer coefficient on the surface (W/m 2. C); t temperature of the surface ( C); A surface area of the concerned (m 2 ). Eternal Wall (EW) Model υ υ ) + ( µ e ) ( ρt) µ t = ( ρ t υ ) + q" / c τ Pr A ( t t ) (3) (4) (5) (7) (8) Although it is not necessar, to simplif the problem, the is assumed as a homogeneous laer of a solid material. This laer has constant thermophsical properties. The heat transfer equation is in two dimensions: ( ρc) λ 2 2 t t t = τ (9) 3

4 λ the thermal conductivit of the (W/m 2. C) The inside ambient air is assumed of a constant temperature. Therefore the boundar condition for this side is: L h in t λ L = hin [ t( L, τ ) t in ] = thickness of the (m); convective heat transfer coefficient on the inside surface (W/m 2. C). Iv-covered Wall (ICW) Model The ICW model is composed of the iv canop model, supporting grid model, and eternal model described above, together with the coupling models. Such models represent the boundar conditions at the canop-sg and SG-EW interfaces, satisfing the phsical constraints of continuit for the state variables and the flu. Canop-SG coupling model Continuit of the state variables at the Canop-SG interface implies: ψ air SG = hair SG ( t air, SG t canop ) (11) SG-EW coupling model Continuit of the heat transfer flu at the SG-EW interface implies: dt d ψ TIR,leave-EW ψ SG-EW () SG EW = ψ SG EW TIR, leave EW dt d SG EW = ψ SG EW + rad, TIR (12-1) the net long-wave radiative heat transfer between leaves and the eternal. the convective heat transfer between the air in the SG and EW. Equation 12-1 applies to the iv-covered section, 12-2 applies to the section directl eposed to solar radiation. The geometric characteristics of the ICW mode include: (1) foliage covering ration (r); (2) green densit (d); (3) average leaf thickness (δ); (4) and the height of SG (H). ψ (12-2) These parameters define the thermal performance of an ICW. This stud aims to obtain an understanding of how these variables impact on the function of an ICW. Such are treated as constants in a simulation case. To figure out the impacts of these parameters, a series of simulation are needed to establish a parametric sensitivit analsis. NUMERICAL SIMULATION As the first stage of the stud, this paper concerns with stead-state cases. Numerical method is developed to calculate the thermal performance of the eternal. In this paper, for simplification of the problem, the construction of EW is a 370mm of cla brick. The thermal properties of the is listed as the following: Densit: kg/m 3 Thermal conductivit: 0.81 W/m.K Specific heat: 800 J/kg.K The boundar conditions of EW are: (1) The indoor side: temperature of inside ambient air is treated as a constant, or C. The convective heat transfer coefficient is 8.33 W/m 2.K. (2) The outside surface essentiall consists of two parts: one shaded b ivies and the other directl eposed to solar radiation. The shaded one is treated as a convective and radiative boundar. The convective heat transfer coefficient of this part is 16.6 W/m 2.K. Given different combinations of (r, d, H), the Iv Canop model and SG model are emploed to calculate the environmental temperature that is used to determine radiative heat echange. The eposed one is treated as a normal eterior surface of eternal. It is also a convective and radiative boundar. But the long-wave radiation of this part is ignored because direct solar radiation is much greater. Numerical method is also emploed to perform simulation of SG model. This step establishes the boundar conditions of EW described above. The boundar conditions of SG are listed as the following: (1) The EW side: shaded and eposed alternativel located. The ratio of shaded one to eposed one is calculated b the following equation: shaded d = r (13) ep osed d 0 d 0 : reference green densit. (2) The Iv Canop side: iv is treated as a solid object that echanges heat with air within iv 4

5 canop and free air, as described b equations (1) and (2). The gap between iv foliages is treated as air inlet. Similarl, covering ratio (r) is emploed to size the air gap. Eperimental data is emploed to determine temperature of the foliages. Under a certain solar radiation, a normall growing leaf of green color can remain much lower temperature than an building materials. Temperature of the shined surface can be as low as just 2 to 3 C higher than the ambient air. This stud emplos this simplified relationship to calculate temperature of leaves. All leaves are treated as homogenous thermal object, or of an identical temperature. Thus, given climatic condition, temperature of leaves can be determined. Overall, the following three steps are involved in implementation of the ICW model: (1) Determine climatic condition and temperature of leaves. (2) Construct mesh grid of air within SG according to (r, d, H). Then emplo numerical method to performance simulation of SG. Figure 4 presents the profile of air movement within the SG under the conditions: solar radiation intensit is 900 W/m 2, temperature of free air is 30 o C, velocit of free air is 1.5 m/s. (3) Using the outputs of step2, assign the boundar conditions of EW model. The mesh grid is predefined according to (r, d). Figure 5 presents temperature pattern in the eternal under different covering ratios. Given different combinations of (r, d, H), the heat flu from the eternal into indoor air can be respectivel calculated. To analze impacts of one variable on the thermal performance of an ICW, the other two variables are kept constant so that a sensitivit analsis of this variable can be established. Such analsis can answer the questions that this paper rises up in the beginning. RESULTS and SENSITIVITY ANALYSIS Heat flu transferred into the indoor air is selected as an indication of the thermal performance of an ICW. Under static condition, it can be calculated b the following equation: Wall Leave Air gap between leaves Figure 4 Air movement within SG 5a: r=0%, d=2 5b: r=%, d=2 5c: r=50%, d=2 = h t t ) (14) in ( Inside in t inside : temperature of an ICW s inside surface. Given a combination of (r, d, H), can be calculated accordingl. The following parametric analses are established under such common conditions: (1) : 370mm cla brick; (2) indoor temperature: C; (3) outdoor air temperature: 34 C; (4) solar radiation flu: 800 W/m 2 ; 5d: r=90%, d=2 Figure 5: temperature pattern in an iv-covered 5

6 (5) reflectance of eternal surface: Three group of parametric sensitivit have been carried out as: (1) versus Covering Ratio (r) Case condition: r : 0, %, 30%, 50%, 70%, 90%, 0%. d : 1, 2, 3 H : 5,, (mm) Case d=1 d=2 D=3 H=5mm * * * H=mm * * * H=mm * * * Simulation result is presented in Figure 6. d=1 d=2 d=3 35 H=5mm 5 0% % 40% 60% 80% 0% Covering Ratio (%) d=1 d=2 d=3 35 H=mm 5 0% % 40% 60% 80% 0% Covering Ratio (%) d=1 d=2 d=3 35 H=mm 5 0% % 40% 60% 80% 0% Covering Ratio (%) Figure 6 Relationship between and Covering Ratio (r) The figure shows that for a certain height of SG, decreases considerabl as covering ratio (r) increases. It can be eplained because an increment of r results in a bigger portion of surface that can benefit from shading of ivies. However, it is also found that when r is less than 30%, is ver closed to the situation of a bare. This is because sectional heat conduction in the will thus omit the benefit benefits of ivies. This criteria value, 30% for brick, varies with the thermal properties of s. Between 40% to 90%, has strongest inverse correlation with covering ratio. The three figures together also show that an increment of d value can slightl reduce. This is because for a certain value of r, bigger the value of d is, bigger portion of the eternal s surface will be actuall shaded. Green Densit is determined b the growing condition of ivies. A better planting results in a higher green densit. Therefore, to obtain the best performance of ICWs, It is needed to grow ivies above a certain level of covering ratio, for brick, the covering ratio should not be less than 30%. (2) versus Green Densit (d) Case condition: d : 1, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 r : 30%, 60%, 90% H : 5,, (mm) Case r=30% r=60% r=90% H=5mm * * * H=mm * * * H=mm * * * Figure 7 presents the simulation result. The figures show that for a certain level of covering ratio, Green Densit has considerable impacts on while d is less than 2.5. Once d becomes greater than 2.5, has ver small dependenc on d value. It can be eplained because once d is greater than 2.5, shading coefficient can no longer be improved considerabl b enlarged area of leaves. Therefore, in case green densit reaches a certain level, for instance 2.5 for brick, it is important to give higher priorit to achieve bigger covering ration rather than to increase green densit. (3) versus Height of SG (H) Case condition: H :5,,, (mm) d : 1, 2, 3 r : 30%, 60%, 90% Case d=1 d=2 D=3 r=30% * * * r=60% * * * r=90% * * * Figure 8 presents the simulation result. The figure shows that for a certain level of green densit, has a considerable dependenc on H 6

7 value. Bigger H value is, higher is. This is because a bigger H results in more freedom for outside free air, that is hotter in cooling season, to enter SG, which in turn increases heat convection from air to the. Therefore, it is important to alwas minimize the height of SG. The figure also shows that higher covering ratio is, bigger this dependenc will be. While designing an ICW, SG should be configured as small as possible H=5 H= H= Green Densit r=30% H=5 H= H= Green Densit r=60% H=5 H= H= Green Densit r=90% Figure 7 Relationship between and Green Densit (d) CONCLUSION and FUTURE WORKS This paper introduces a model framework for simulating the thermal behaviors of iv-covered s. The model is then simplified and the numerical method has been emploed to implement simulations. A series of parametric sensitivit analses have been carried out to identif the ke factors as relevant for technical iv treatment with an aim of optimizing the climatological effects of iv-covered s in buildings. The ke finding are highlighted as: (1) An ICW is essentiall consisted of three ke components: iv canop, supporting grid, and eternal. 30 r=30% r=60% r=90% 5 H (mm) d= H (mm) d=2 30 r=30% r=60% r=90% r=30% r=60% r=90% 5 H (mm) d=3 Figure 8 Relationship between and H (2) The ke factors that significantl affect the thermal function of an ICW include: green densit (d), covering ratio (r), and geometrical characteristics of the supporting grid (H). (3) Given certain d and r, heat flu transferred into indoor () increases as H becomes bigger while H remains within the critical point. (4) Given H and d, increases as r becomes smaller. It can be eplained because a smaller r means less percentage of the eternal is shaded from direct solar radiation. The covering ratio (r) has most significant effect on the thermal function of an ICW. (5) Given H and r, slightl increases as d becomes smaller. The percentage of the eternal surface that is shaded actuall is determined not onl b covering ratio, but also the green densit. While the densit is 1, this percentage equals to covering ratio. While d increases, this percentage slightl becomes bigger. 7

8 (6) Therefore, in case that construction requirement can be satisfied, the height of the supporting grid should be as small as possible. And covering ratio should be as big as possible. becomes significantl higher while r is less than 30%. An ICW of r value lower than that has similar thermal performance of a bare. Therefore, to take advantage of iv-coverings, the covering ratio must greater that this value. Compared with a bare, a 0% covered could reduce solar gain b up to 37%. To simplif implementation of simulation, a steadstate model has been emploed. To stud the thermal performance of ICWs in more detailed, it is necessar to simulate the time-dependent cases in the future stud. An eperimental sstem will also be built up to produce essential data that can verif the accurac of the simulations. Since a complete rigorous numerical simulation is too complicated, this paper presents the mathematical model and the framework for such a would-be simulation. In this paper, simplification is made to preliminaril investigate the ke parameters of ICWs. Due to the simplification, the effect of some parameters, such as H, could not be well established. A conjunct heat-transfer simulation will be performed further to re-evaluate the effect of this design parameter. Residential Heating and Cooling Requirements Winter ASHRAE Meeting, Atlanta, GA, Februar -14, Lbl Y.J. Huang, H.Akbari. The Potential of Vegetation in Reducing Summer Cooling Loads in Residential Buildings. Lbl Liao Zaii, Tsou Jineu. Impacts of Greenbelts on the Thermal Environment of Residential Communities. The First Conference on Architectural Design and Technolog for Sub- Tropical Climates. Guangzhou, China, Liu Qia. Heat Transfer in Vegetated Rooftop of Urban Residential Buildings. The First Conference on Architectural Design and Technolog for Sub-Tropical Climates. Guangzhou, China, Alan K. Meier. Strategic Landscape and Airconditioning Savings: A Literature Review. Energ and Buildings, Volume -16 (1990/91), P REFERENCES 1. Elena Palomo Del Barrio. Analsis of the green roofs cooling potential in buildings. Energ and Buildings Volume 27 (1998) P V.Thanh C., T, Asaeda, E. M. Abu. Reductions in air conditioning energ caused b a nearb park. Energ and Buildings, Volume 29, Number 1 (1998), P A. Hoano. Climatological Uses of Plants for Solar Control and the Effects on the Thermal Environment of a Building. Energ and Buildings, Volume 11 (1988), P S.H. Raza, M.S.R. Murth, O. Bhaga, and G. Shlaja. Effect of Vegetation on Urban Climate and Health Urban Colonies. Energ and Buildings, Volume -16 (1990/91) P Y. Harazono. Effects of Rooftop Vegetation using Artificial Substrates on the Urban Climate and the Thermal Load of Buildings. Energ and Buildings, Volume -16 (1990/91), P Y. J. Huang, H. Akbari, and H. Taha. The Wind- Shielding and Shading Effects of Trees on 8