Evaluation of the effects of greening and highly reflective materials from three perspectives

Transcription

1 Evaluation of the effects of greening and highly reflective materials from three perspectives - mitigation of global warming, mitigation of UHIs, and adaptation to urban warming - Akashi Mochida Professor Department of Architecture & Building Science, Tohoku University, Japan 1. Background 2. Outline of assessment system 3. Examples of total assessment 1) Effects of greening and highly reflective materials applied to vertical walls 2) Effects of roadside trees 3) summary of assessments 4. Evaluation of the effects of windows with heat ray retro reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection 2 1

2 Annual mean temperature [ºC] Annual mean temperature change in East Asian cities (Source: Japan Meteorological Agency & China Meteorological Administration) 1.3ºC/ 25years 2.8ºC/1years ºC/ 1years The temperature increases in East Asian cities are much more rapid than the pace of global warming. 3 Number of extreme hot days in Tokyo on which daily maximum temperature exceeds 35 o C [Year] Furthermore, the number of extremely hot days has been increasing and this caused the increase in health hazard risk in Japan. This graph was made using the data from website of Tokyo district meteorological observatory ( net.go.jp/tokyo/sub_index/tokyo/kikou/t_ts/t_ts.html) 4 2

3 Number of patients of heatstroke (hyperthermia: 熱中症 ) taken to hospital by ambulance service Tokyo Yokohama Nagoya Osaka 2 1 The number of heatstroke patients has increased sharply. This graph was made using the data from National Institute for Environmental Studies Bulletin Report on Heatstroke Patients ( 5 To improve such situation, various countermeasure techniques against urban warming have been adopted. Tree planting in urban area Increasing the solar reflectance of urban surface (High albedo surfacing) Introducing the wind from sea, river, planted park into the inside of city 3

4 In recent years, many studies have been done to evaluate the performance of these countermeasure techniques. But, in most of studies, performance of the techniques was assessed from different single viewpoint, i.e. 1) Energy savings, 2) The suppression of effluent sensible heat 3) The improvement of the thermal environment in pedestrian space, etc. Author Focused countermeasure Energy savings Effluent sensible heat Thermal environment in pedestrians spaces Akbari et al.(21) Cool surface 〇 Akbari, H. (22) Trees 〇 Ali Toudert & Mayer (2) Controlof building shapes 〇 Ichinose et al. (2) Roof greening 〇 〇 Sasaki et al.2) Greening & White painting 〇 Hataya et al. (27) Roadsidetrees 〇 Takebayashi & Moriyama (27) Greening & White painting 〇 Kondo et al. (28a,b) Highly reflective painting 〇 Hwang et al. (211) Controlof building shapes 〇 Shushua Bar et al.(211) Trees & grass 〇 Xuan et al. (2) Controlof pitch of buildings 〇 Allegrini et al. (2) Control of building shapes 〇 Saneinejad et al. (2) Evaporativecooling 〇 7 However, these aims often conflict with one another. For example, enhancing the solar reflectivity of vertical building walls has a positive impact on energy savings, but it has a negative impact on the outdoor thermal comfort of pedestrians, because reflected solar radiation from a building surface tends to be incident to pedestrians. Therefore, great difficulties still remain when policymakers and urban planners attempt to select proper countermeasure techniques, despite the enormous accumulation of knowledge Heat ray is reflected to pedestrian Cooling load Heat stress of pedestrian 8 4

5 To overcome the difficulties associated with the selection of proper countermeasures against urban warming, the purposes of such countermeasures should be considered well and defined clearly. First, there are two aspects to these countermeasures: mitigation and adaptation. Mitigation refers to the removal of the causes of the phenomenon Adaptation refers to the reduction in the effects of the phenomenon even though the magnitude of the phenomenon does not change. 9 Second, ongoing urban warming is being caused by both global warming and urban heat islands (UHIs). Global warming is caused by the rising concentration of greenhouse gases. A UHI is caused by a) the modification in the land-use from a natural environment into a built environment and b) the intensive energy consumption in urban area resulting in anthropogenic heat release Completely different countermeasures are needed to mitigate these two phenomena, global warming and UHIs, and to simultaneously adapt to urban warming. 1 5

6 Thus, we must recognize that there are three different perspectives of countermeasures for urban warming: Mitigation of global warming [ex. Reduce the CO 2 emission] Mitigation of urban heat island [ex. Increase the solar reflectance on the building surface] adaption to urban warming (global warming + UHI) [ex. Create the shade] However, their distinction remains unclear among researchers. 11 This study aimed to propose a total assessment method to assess the effects of the countermeasures to 1) mitigate global warming, 2) mitigate UHIs, and 3) adapt to urban warming simultaneously.

7 1. Background 2. Outline of assessment system 3. Examples of total assessment 1) Effects of greening and highly reflective materials applied to vertical walls 2) Effects of roadside trees 3) summary of assessments 4. Evaluation of the effects of windows with heat ray retro reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection 13 The confusion between the countermeasures to mitigate global warming, to mitigate UHIs, and to adapt to urban warming arises because the focus regions for discussing urban warming differ among researchers. In this study, three domains for evaluating the effects of countermeasures for mitigating global warming, mitigating UHIs, and adapting to urban warming are set in an assessment domain. Assessment indices corresponding to each domain are calculated. Entire assessment domain Domain B Urban atmosphere Domain A Building interior Domain C Outdoor pedestrian space 14 7

8 Domain A for assessing the mitigation of global warming The cause of global warming is greenhouse gases, especially CO2. CO2 emission increases with the energy consumption. In order to assess the mitigation of global warming, domain A, which is the interior of buildings, is defined, and the energy consumption of its Heating, Ventilation, and Air Conditioning (HVAC) system is calculated. 15 In this study, the total energy consumption over the running time per day of HVAC system (Q HVAC_sum ) is used as an index for assessing the mitigation of global warming. _, jmax :Total number of buildings in assessment area Q HVAC, j :Energy consumption of HVAC of building j [W] Q HVAC_sum :The amount of Q HVAC for 24 h [MJ] The energy consumption per unit time of building j is calculated from the cooling load of building j(q in,j ) Q in, j : Amount of influent heat into building j( cooling load of building j)[w] COPj : Coefficient of performance of HVAC system of building j [ ] 1 8

9 Index for domain A : energy consumption of HVAC (=Q in /COP) Q in = amount of influent heat into building (cooling load) = amount of influent heat from outside (by convection, transmission through windows and ventilation) + amount of heat generated in building 17 The cooling load of building j is composed of influent heats by convection, transmitting radiation, and ventilation and heat generated inner building.,,, +,, +, +, Each component is expressed as follows:,,,,,,,,,,,,,,,,, imax Q in Q conv,inside Q r,trans Q vent Q inner :Total number of surface elements of buildings in assessment area :Amount of influent heat into building [W] :Influent heat by convection from building interior wall surface to indoor air [W] :Influent heat by radiation transmitting window [W] :Influent heat by ventilation [W] :Amount of heat generated inner building [W] 18 9

10 Domain B for assessing the mitigation of a UHI The cause of UHI is the increase in sensible heat in urban space due to the modification in land-use and anthropogenic heat release. To assess the mitigation of a UHI, domain B, which is the urban atmosphere, is defined, and the amount of effluent heat from the urban surface to the outdoor air is evaluated. 19 Domain B for assessing the mitigation of a UHI The total effluent heat over 24 h (Q out_sum ) is used as an index for assessing the mitigation of a UHI. _ Q out Q out_sum :Net effluent heat from urban surface to outdoor air [W] :The amount of Q out for 24 h [MJ] 2 1

11 Index for domain B: net effluent heat from urban surface to outdoor air (Q out ) Q out =net effluent heat from urban surface to outdoor air = effluent heat from urban surface by convection + wasted heat from HVAC system influent heat from outside to indoor = Qemi Qabs 21 Index for domain B: net effluent heat from urban surface to outdoor air (Q out ) The net effluent heat from an urban surface per unit time is estimated from the balance between the effluent heat (Q emi ) and the influent heat (Q abs ) from the urban surface. _ Q out Q out_sum Q emi Q abs :Net effluent heat from urban surface to outdoor air [W] :The amount of Q out for 24 h [MJ] :Effluent heat from urban surface to outdoor air [W] :Heat absorbed by urban surface [W] 22 11

12 Each component of the effluent heat (Q emi ) and the influent heat (Q abs ) is expressed as follows:,,,,,,,,,,, jmax :Total number of buildings in assessment area imax :Total number of surface elements of building and ground in assessment area Q HVAC :Energy consumption of HVAC [W] Q in :Amount of influent heat into building [W] Q r,trans :Influent heat by radiation transmitting window [W] Q vent :Influent heat by ventilation [W] Q emi :Effluent heat from urban surface to outdoor air [W] Q abs :Heat absorbed by urban surface [W] Q conv,outside :Effluent heat by convection from urban surface to outdoor air [W] 23 Domain C for assessing the adaptation to urban warming In this study, the effectiveness of countermeasures to adapt to the urban warming was assessed by thermal comfort of pedestrians. The pedestrian space within domain B is defined as another domain, domain C, to assess the adaptation to urban warming. 24

13 Index for domain C: acceptable volume ratio on the basis of SET* In this study, Standard Effective Temperature (SET*) was selected as thermal comfort index. The space within a height of 2m above ground is defined as pedestrian space. Spatial distributions of SET* within pedestrian space were simulated by 1) unsteady heat balance simulation at urban surface coupled with radiation and conduction computations, and 2) CFD simulation. 25 Index for domain C: acceptable volume ratio on the basis of SET* The acceptable volume ratio is used as an index for assessing the adaptation to urban warming. The acceptable volume is defined as the volume in which the value of SET* is less than its acceptable maximum limit. The target time is the hour when the maximum air temperature is reached. 2 13

14 Assessment Procedure Indices corresponding to each domain are derived from the results of unsteady heat balance simulation at urban surface coupled with radiation and conduction computations and non-isothermal CFD simulation. Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio Background 2. Outline of assessment system 3. Examples of total assessment 1) Effects of greening and highly reflective materials applied to vertical walls 2) Effects of roadside trees 3) summary of assessments 4. Evaluation of the effects of windows with heat ray retro reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection 28 14

15 Simulation Target In the 1 st example, the assessment system was applied to the ideal town block model where the height and width of buildings were set to be identical. Building coverage ratio: 25% Building height, road width: 2m Building use: Business Office Meteorological conditions of Otemachi in Tokyo were used as boundary conditions. 29 Details of the office building Building Structures HVAC System Settings Related to Indoor Heat Balance Number of stories [-] 5 Floor height [m] 4 Ceiling height [m] 2.8 Rentable area ratio [-].8 Building volume available for work 439 purposes [m 3 ] Ventilation rate [m 3 /s] 2.18 COP [-] 3 Air temperature set point [ C] 28 Running time [h] (7-21) Interior heat generation Given by the guidelines for calculating building energy Heat capacity of materials inside the consumption in Japan building, such as furniture and documents (NILIM and BRI, 213) 3 15

16 Calculation Cases Three situations were simulated using different physical properties for the vertical wall surfaces of the town block model: (1) concrete, (2) highly reflective materials, and (3) green For all the cases compared, the physical properties of the roof and road surface were identical. Case Albedo Emissivity Moisture Surface Window Material name [-] [-] availability wetness[-] [ ] ratio [-] Concrete Concrete Highref Highly reflective material Greening Greening Albedo >Solar reflectance (1)concrete (2)highref (3)green 31 Calculation conditions for unsteady radiation and conduction Simulation Calculation date August 3th (Related to solar altitude) Calculation period For 48 h from : a.m. on August 29 th Meteorological Data data for air conditioning design [3] was used Mesh Number (x y z) Domain size (x[m] y[m] z[m]) Boundary conditions Ground Solid surface Vertical gradient of soil temperature was set to zero in.5 m of earth In: Total heat transfer coefficient, =9[W/m 2 K], Out: Convective heat transfer coefficient, c =[W/m 2 K] was imposed. Expanded AMeDAS Weather Data for air conditioning design ( for typical summer condition ) was used as meteorological data. 32 1

17 Non-isothermal CFD Simulation In this simulation, 7 7 buildings were modeled, and the result for the central area was used for calculating the assessment index. Target time was 2 p.m., Inflow conditions at this time wind direction: south wind speed: vertical profile was given by power law, 2.7m/s at a height of.5m air temperature: 33.9 o C absolute humidity:.18[kg/kg ] Assessment area S W 33 E N Calculation conditions for CFD Simulation Calculation time 2 p.m. Mesh Number (x y z) Domain size (x[m] y[m] z[m]) Turbulence model Durbin type revised k model Discretization scheme of convective teams of transport equations First order upwind scheme 34 17

18 Boundary conditions of CFD Simulation Inflow wind direction South Wind velocity z u ( z) U s zs a=.27 [4],z s =.5[m], U s =2.7[m/s] [3] Turbulence energy and dissipation rate The vertical distributions are given in accordance with [4] Air temperature 33.9[ C] [3] Absolute humidity.18[kg/kg ] [3] Solid surface Outflow and lateral Upper Velocity Temperature Absolute humidity <u>, <v>, <w>, k, ε, q, q:zero gradient <u>, <v>, k, ε, q, q: zero gradient, <w>= The generalized log law was applied Surface temperatures of each mesh are given by the result of unsteady radiation and conduction simulation. Sensible heat flux from solid surface h w : hw c w p a c =[W/m 2 K], q p : The air temperature of the first mesh from solid surface Latent heat flux from solid surface L w : Lw w fw f p a w =α c. 1 [W/m 2 K], b: Surface wetness, f p : The water vapor pressure of the first mesh from solid surface 35 Simulation Results 3 18

19 Simulation results (1) Unsteady radiation and conduction simulation -> Unsteady heat balance simulation coupled with radiation and conduction computations Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 37 Heat balance components considered in the coupled analysis We considered the multi reflections of short and long wave radiation by calculating Gebhart factors. S i : Solar radiation [W] R i : Longwave radiation [W] H i : Sensible heat flux [W] C i : Heat gain by heat conduction [W] LE i : Latent heat flux [W] Monte Carlo simulation Hi Ci Ci Hi LEi Ri Ci Ci Ci Si Ri Si Ci Ri LEi Hi LEi Hi Ci Ci Ci 38 19

20 Surface Temperatures of Exterior Building Surface - Results of Radiation and Conduction Simulation - The surface temperatures of the four vertical surfaces were averaged. The surface temperatures of the greening and highly reflective materials were lower than that of concrete during daytime. Temperature [ C] concrete highref green airtemp 24 : 4: 8: : 1: 2: : 39 Simulation results (2) Estimation of Mean Radiant Temperature (MRT) using the results of unsteady radiation and conduction simulation Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 4 2

21 MRT is Mean Radiant Temperature (MRT) is the uniform surface temperature of a black enclosure with which an individual exchanges the same heat by radiation as the actual environment considered. In outdoor space, MRT indicates the radiant heat (both solar (short wave) radiation and longwave radiation) coming from sky, building walls, ground and absorbed by human body. 41 Horizontal Distribution of Mean Radiant Temperature (MRT) (2 p.m., h=1.25m) - Results of Radiation and Conduction Simulation- The MRT values around the highly reflective material surface were higher in the overall assessment area than those for concrete, while the MRT values around the greening were lower than those for concrete

22 Simulation results (3) CFD simulation Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 43 Horizontal Distribution of Wind Vector and Air Temperature (2 p.m., h=1.25m) - Results of Non-isolated CFD Simulation- The air temperatures for the greening case was lower than those of the concrete and highly reflective material cases

23 Comparison of absolute humidity and air temperature in greening case (2 p.m., h=1.25m) N N W E W E S S Wind velocity and air temperature Absolute humidity The absolute humidity is high in the same area of low air temperature. 45 Simulation results (4) Estimation of thermal comfort index SET* Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 4 23

24 1) Wind velocity 2) Temperature 3) Radiation (MRT) 4) Humidity 5) Clothing ) Metabolism given from the results of CFD and heat balance simulation coupled with radiation and conduction computations assumed Human thermal comfort index SET * (Standard Effective Temperature) A.P.Gagge, J.A.J.Stolwijk, Y.Nishi: 1971, An effective temperature scale based on a simple model of human physiological regulatory response, ASHRAE Transactions, 77, pp , 1977 Horizontal Distribution of SET* (2 p.m., h=1.25m) The SET* values in the case of the highly reflective material is the highest because of the worsening radiant environment. The SET* values for the greening case increases in the area near the west wall of the building because of the increase in the humidity

25 Assessment Results 49 Indices for each domain From now, assessment results using these simulation results are shown. Domain A was set to assess the mitigation effect of global warming: Index: the energy consumption of HVAC Domain B was set to assess the mitigation effect of a UHI: Index: the effluent heat from urban surface Domain C was set to assess the adaptation effect to urban warming: Index: the acceptable volume ratio on the basis of SET* Entire assessment domain Domain B Urban atmosphere Domain A Building interior Domain C Outdoor pedestrian space 5 25

26 Assessment results (1) Index of domain A Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 51 Assessment of Global Warming Mitigation ( Domain A) - total energy consumption of HVAC system over running time per day- In the cases of the greening and highly reflective materials, the energy consumptions were lower than in the concrete case. The total energy consumption of HVAC over running time [MJ/day] Qinner/COP Qintial/COP Qvent/COP Qconv,inside/COP concrete highref green 52 2

27 Assessment results (2) Index of domain B Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 53 Assessment of UHI Mitigation (Domain B) - the total effluent heat from the urban surface over 24 h - In the case of the greening, the amount of net effluent heat decreased significantly because of the reduction in the effluent heat by convection from the wall surface (Q build ). The total effluent heat over 24 h [MJ/day] Qvent Qan Qroad Qbuild concrete highref green 54 27

28 Assessment results (3) Index of domain C Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 55 Assessment of Adaptation to Urban Warming (Domain C) - the frequency distribution of SET* within pedestrian space (2 p.m.) - In the case of the highly reflective material, the volume where SET* exceeded 35 o C increased overall, and the acceptable volume ratio in the highly reflective material case was 7%, while that of the concrete case was 84%. In the greening material case, the acceptable volume ratio was 79%. Volume ratio [ ] Acceptable Unacceptable concrete highref green the maximum acceptable SET* value (SET* max ) 35 C. SET* [ C] 5 28

29 Summary of assessment results ( Example 1 ) In the conditions assumed in this study, greening and highly reflective material have good impact on mitigating global warming and UHIs. However, in terms of adapting to urban warming, greening was not effective and highly reflective material had obviously negative impact. Energy consumption Net effluent sensible heat Acceptable volume ratio Case of HVAC [MJ/day] from urban surfaces [MJ/day] evaluated on the basis of SET* (<35 o C) [Domain A] [Domain B] [Domain C] Concrete [%] Highref [%] Greening [%] Background 2. Outline of assessment system 3. Examples of total assessment 1) Effects of greening and highly reflective materials applied to vertical walls 2) Effects of roadside trees 3) summary of assessments 4. Evaluation of the effects of windows with heat ray retro reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection 58 29

30 Simulation target In the 2nd example, the effects of roadside trees in an actual urban area were evaluated. The thermal environment around wide intersections is often considerably worse. In this study, an area around a wide intersection in Shinbashi, typical business district in Tokyo, was selected as the simulation target Shinbashi Sta. 新橋駅 This image was extracted from Google earth 59 Computational domain Two situations with and without roadside trees were simulated. Tree crown height [m] Tree height [m] Tree crown width [m] Pitch of tree planting [m] Leaf area density [m 2 /m 3 ] Surface Wetness [ ]

31 Various effects of trees considered by tree canopy model (Yoshida, Ooka, Mochida, Murakami, Tominaga (2)) (1) Aerodynamic effects of the planted tree (2) Thermal effects of the planted tree: a) Shading effects on solar radiation and long wave radiation, b) Generation of water vapor from tree canopy. (a) solar radiation (b) (c) (d) (turbulence) (penetration) longwave radiation (a) aerodynamic effects of tree canopy (b) latent heat from tree canopy (c) shading effect on long wave radiation (d) shading effect on short wave radiation 1 Canopy model for reproducing aerodynamic effects of Tress In order to reproduce the aerodynamic effects of stationary small scale obstacles that are smaller than the grid size, such as trees and small buildings, various models have been developed based on the methodology of canopy flow modeling 2 31

32 Outline of canopy model (1) In Canopy model, we consider the situations where small obstacles (solid) are included in the computational grid, and fluid and solid are coexisting. Tree Canopy 3 Outline of canopy model (2) Instead of reproducing the configurations of small obstacles by computational grids, the model equations used in CFD are modified to include the extra terms expressing their effects. Tree Canopy 4 32

33 33 5 k model with tree canopy model decreases in velocity increases in turbulence increases in dissipation i i x u i i j j i t j i j j i i F x u x u x k p x x u u t u 3 2 k k j t j j j F P x k x x k u t k F C P C k x x x u t k j t j j j 2 1 j i i j j i t k x u x u x u P [Continuity equation] [k transport equation] [ transport equation] [Momentum equation] : fraction of the area covered with trees C f : drag coefficient for canopy a : leaf surface area density C p1 : model coefficient for F -F i : extra term added to the momentum equation + F k : extra term added to the transport equation of k + F : extra term added to the transport equation of Fi Fk i ui F F k C p F k 2 j i f u u a C a F i F k F ε 2 j i f u u a C u i F i parameters to be determined according to the real conditions of trees, a, C f : Expressions of extra terms F i, F k, F in tree canopy model i F i u C k : fraction of the area covered with trees a : leaf surface area density C f : drag coefficient for canopy

34 Leaf surface are density a 1 (leaf surface area) a = 2 Volume of tree crown area of one side leaf surface Tree Crown( 樹冠 ) 7 Expressions of extra terms F i, F k, F C : a model coefficient in turbulence modeling, which should be optimized, for prescribing the time scale of the process of energy dissipation in canopy layer, a, C f : parameters to be determined according to the real conditions of trees F i C f a u i u j 2 F k u i F i F ε C k u i F i 8 34

35 Comparison of vertical velocity profiles behind tree : measurement : CFD with type B model a=1.17[m 2 /m 3 ] C f =.8[-] (x 1/H=1) (x 1/H=2) (x 1/H=3) (x 1/H=4) (x 1/H=5) (x 1/H=1) (x 1/H=2) (x 1/H=3) (x 1/H=4) (x/h=5) (x 1/H=1) (x 1/H=2) (x 1/H=3) (x 1/H=4) (x 1/H=5) (x/h=1) (x/h=2) (x/h=3) (x/h=4) (x/h=5) (1) C= (4) C=1.8 (x 1/H=1) (x 1/H=2) (x 1/H=3) (x 1/H=4) (x/h=5) (2) C= (5) C=1.9 (x/h=1) (x/h=2) (x/h=3) (x/h=4) (x/h=5) (3) C= () C=2. 9 Comparison of vertical velocity profiles behind tree (C pe1 =1.8) measurement Simulation (C =1.8)

36 Various effects of trees considered by tree canopy model (Yoshida, Ooka, Mochida, Murakami, Tominaga (2)) (1) Aerodynamic effects of the planted tree (2) Thermal effects of the planted tree: a) Shading effects on solar radiation and long wave radiation, b) Generation of water vapor from tree canopy. (a) solar radiation (b) (c) (d) (turbulence) (penetration) longwave radiation (a) aerodynamic effects of tree canopy (b) latent heat from tree canopy (c) shading effect on long wave radiation (d) shading effect on short wave radiation 71 Shading effects of solar and long-wave radiations The present model is based on the following assumptions: 1. Only the effect of tree crown is modelled. The effects of stem and branches are assumed to be negligibly small. 2. The ratio of absorbed radiations to the total incident radiation on the tree crown is given by the function 1 exp ka x, x, 1 2 x3 (1) Distance through the tree crown l [m] (2) Leaf area density a [m 2 /m 3 ] (3) Absorption coefficient k [-] (here, k =.) ll [m] Tree crown Leaf area density a [m 2 /m 3 ] Absorption coefficient k [-] Tree crown= 樹冠 3

37 Generation (transpiration) of water vapor and heat balance at leaf surface The heat balance equation at leaves that compose the tree crown S P SP R DP HP LEP (1) LE P S P R DP H P LE P : Absorbed solar radiation [W] : Absorbed long-wave radiation [W] : Sensible heat [W] : Latent heat [W] R DP (2) HP APcTaP TP LEP LAPWPf ap fsp (3) Using Eqs. (1), (2) and (3), leaf surface temperature T P is obtained. H P, LE P and T P are used as boundary conditions for CFD computation. H P Simulation Results 74 37

38 Horizontal Distribution of wind velocity (2 p.m., h=1.5m) H=75m N W E W H=75m H=27m S H=27m 5 [m/s] Wind direction (1)Without trees H=27m There is an area of high wind speed around front corner of the tall building. 75 Comparison of wind velocity distributions of the cases with and without trees W H=75m H=75m N W E S H=27m H=27m (1)Without trees (2)With trees 5 [m/s] The areas of high wind speed around the tall building are seen in the both cases. In the east part of the domain, wind speed is relatively low. 7 38

39 Horizontal Distribution of Mean Radiant Temperature (MRT) (2 p.m., h=1.5m) W H=75m H=75m N W E S H=27m H=27m (1) Without trees (2) With trees 3 5 [] Effects of tree shade is clearly seen on the north side of the road. 77 Horizontal Distribution of SET* (2 p.m., h=1.5m) W H=75m H=75m N W E S H=27m H=27m (1)Without trees (2)With trees [] In the north west part of the domain, SET* is low in both cases due to high wind speed. In the east part, SET* in the case with trees is lower than that in the case without trees. This difference was mainly caused by the difference in MRT values

40 Assessment Results 79 Indices for each domain From now, assessment results using these simulation results are shown. Domain A was set to assess the mitigation effect of global warming: Index: the energy consumption of HVAC Domain B was set to assess the mitigation effect of a UHI: Index: the effluent heat from urban surface Domain C was set to assess the adaptation effect to urban warming: Index: the acceptable volume ratio on the basis of SET* Entire assessment domain Domain B Urban atmosphere Domain A Building interior Domain C Outdoor pedestrian space 8 4

41 Assessment results (1) Index of domain A Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 81 Assessment of Global Warming Mitigation ( Domain A) - total energy consumption of HVAC system over running time per day- The difference between the cases is very small. The influent heat by transmission through windows (Qr, trans) is slightly smaller in the case with roadside trees. The total energy consumption of HVAC over running time [GJ/day] Without roadside tree With roadside tree Qinitial/COP Qr,trans/COP Qinner/COP Qvent/COP Qconv,inside/COP 82 41

42 Assessment results (2) Index of domain B Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 83 Assessment of UHI Mitigation (Domain B) - the total effluent heat from the urban surface over 24 h - In the case with roadside trees, the amount of net effluent sensible heat decreases by 5 %. This is caused by the reduction in the effluent sensible heat convected from the road surface in the tree shade. 7 The total effluent heat over 24 h [GJ/day] Without roadside tree With roadside tree Qan Qconv (from road) Qconv (from building) Qconv (from tree) Qvent Qr,trans 84 42

43 Assessment results (3) Index of domain C Building form Location Non isothermal CFD simulation Three dimensional distribution of velocity, air temperature, and absolute humidity SET* calculation Meteorological data Unsteady radiation and conduction simulation Surface temperature at ground and buildings Surface temperature at Amount of exhaust ground and buildings heat from HVAC MRT calculation Three dimensional distribution of MRT Influent heat by conduction Influent heat by radiation Influent heat by ventilation Evaluation index of domain A Amount of energy consumed by HVAC Effluent heat by convection from surfaces Exhaust heat from HVAC Evaluation index of domain B Amount of net effluent heat from urban surface Evaluation index of domain C Acceptable volume (SET*<Acceptable maximum limit) ratio 85 Assessment of Adaptation to Urban Warming (Domain C) - the frequency distribution of SET* (2 p.m.) - In the case with trees, the area where SET* is less than the acceptable limit (35 o C) increases. The acceptable volume ratios considering the entire pedestrian space in both cases are very high Volume ratio [%] Acceptable Without roadside trees With roadside trees Unacceptable SET* [ C] the maximum acceptable SET* value (SET* max ) 35 C. 8 43

44 The acceptable volume ratios considering the entire pedestrian space in both cases were very high. -> This is because the high-wind-speed in the area around a front corner of tall building. The modification of wind flow by a tall building, and the resulting high wind speed strongly affected the reduction of SET* under the conditions assumed in this calculation. Horizontal distributions of wind velocity (at 1.5 m height) In the east part of the computational domain, the wind speed is relatively low. In this part, SET* in the case with tress is lower than that without tress. The acceptable volume ratios: 87%(without tress) 97% (with tress) This result indicates that planting roadside tree is effective for adaptation to urban warming especially in areas of low wind speed. Horizontal distributions of SET* (at 1.5 m height) 44

45 Summary of assessment results ( example 2 ) Under the conditions assumed in this study, the presence of roadside trees had positive effects on the mitigation of UHIs and for adaptation to urban warming, but had little effect on mitigation on global warming. Case Energy consumption of HVAC [GJ/day] [Domain A] Net effluent sensible heat from urban surfaces [GJ/day] [Domain B] Acceptable volume ratio evaluated on the basis of SET* (<35 o C) [Domain C] Entire pedestrian East side space Without trees [%] 89[%] With trees [%] 97[%] 1. Background 2. Outline of assessment system 3. Examples of total assessment 1) Effects of greening and highly reflective materials applied to vertical walls 2) Effects of roadside trees 3) summary of assessments 4. Evaluation of the effects of windows with heat ray retro reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection 9 45

46 1) Most previous studies that compared effects of countermeasure techniques were aimed at a single perspective. 2) However, these aims often conflict one another. 3) Therefore, great difficulties still remain when policymakers and urban planners attempt to select proper countermeasure techniques, despite the enormous accumulation of knowledge. 4) To overcome the difficulties, a new total assessment method was proposed to assess the countermeasures for urban warming from the three different viewpoints, i.e. a) mitigating global warming, b) mitigating UHIs, and c) adapting to urban warming, and two examples of the assessment using the proposed method were shown. 91 2) Green and highly reflective surfaces had a positive impact on the mitigation of global warming and UHIs. However, in terms of adapting to urban warming, greening was not very effective and the highly reflective material had a clearly negative impact under the conditions assumed in this study. 3) Roadside trees had positive impacts on the mitigation of UHIs and the adaptation to urban warming, but had little effect on mitigation of global warming under the conditions assumed in this study. 92 4

47 Next target of this study -evaluation of the effects of countermeasure techniques for window- Single float glass (no special techniques) Single float glass with heat shading film (mirror reflection) Single float glass with heat ray retroreflective film ( 再帰性反射 ) 1. Background 2. Outline of assessment system 3. Examples of total assessment 1) Effects of greening and highly reflective materials applied to vertical walls 2) Effects of roadside trees 3) summary of assessments 4. Evaluation of the effects of windows with heat ray retro reflective film on the outdoor thermal environment using a radiant analysis method considering directional reflection 94 47

48 Recent years, in order to reduce the cooling load of building, heat ray reflective glass or heat ray reflective film for window have been widely adopted. (for example, heat shading films( 遮熱 film)) But they usually reflect solar radiation to pedestrian space. So, using these modifications have a negative impact on thermal comfort of pedestrian. Heat ray is reflected to pedestrian outdoor thermal environment Cooling load 95 To avoid the negative impact on pedestrian, heat ray retro-reflective film ( 熱線再帰性反射 film) has been developed. Outdoor space retro reflection ( 再帰性反射 ) Incident direction Indoor space Single float glass Heat ray retroreflective film 9 48

49 Heat ray retro-reflective film( 熱線再帰性反射 film) has been developed. It is expected that adopting this film will have positive impacts on both reducing indoor cooling load and mitigating thermal environment in outdoor space. Heat ray is reflected to incoming direction Cooling load outdoor thermal environment In this study, to evaluate the effect of heat reflective film on outdoor thermal environment, a) a new method of radiation simulation which can consider directional reflection was developed, and b) radiant analysis of outdoor thermal environment in Shinbashi ( 新橋 ), Japanese typical business district, was performed

50 Outline of revised method for radiant computation 99 Problem of previous radiation computation method Many researchers have developed and used radiant simulation to evaluate the effects of radiation on outdoor thermal environment. However, in most of these methods, each surface in the computational domain is assumed to be a perfectly diffusively reflecting surface. Incident direction ex) concrete wall 1 5

51 Problem of previous radiation computation system Therefore, most of the existing methods can not evaluate the radiant field that is strongly affected by the directional reflection, such as the radiant field around a window with a heat ray retro-reflective film. ex) concrete wall ex) retro reflective film 11 In this study, to consider the directional reflection, radiant heat exchanges between urban surfaces were calculated by a method proposed by Yoshida (University of Fukui) et al. (214). This method revised the progressive radiosity method extended to the directional radiant computation by Ichinose (Tokyo Metropolitan University) et al. (25) for outdoor space. For indoor space For outdoor space 51

52 The equations of the radiant computation considering directional reflection The equations of the extended radiosity method are as follows, κ ρ (1) κ, ρ (2) R i(j) E i(j) ρ ki(j) κ ki hemi(k,i) : the radiosity per unit solid angle of surface element i intercepted by a surface element j [W/sr] : the radiation per unit solid angle emitted from surface i to surface j [W/sr] : the fraction of the radiosity reaching surface j from surface k via surface i per unit solid angle [1/sr] : the correction coefficient of the distribution of the reflected radiosity from surface k to surface i : the reflectivity measurement value from surface k via surface i to the surroundings Element k Element j Element i Element k 13 The equations of the radiant computation considering directional reflection κ ρ (1) κ, ρ (2) 14 52

53 Radiant analysis of outdoor thermal environment in Shinbashi ( 新橋 ) district 15 Simulation target In this study, an area around a wide intersection in Shinbashi, typical business district in Tokyo, was selected as the simulation target Shinbashi Sta. 新橋駅 This image was extracted from Google earth 1 53

54 Computational domain Wide intersection 4m 1m 4m 1m N 4m W E 新橋駅 4m H=24m H=4m S H=32m H=1m In terms of reduction of calculation cost, several buildings located in the same town block were modeled as a lumped building. 17 Calculation cases 4m 1m H=24m H=32m H=4m H=1m 1m 4m 4m 新橋駅 W N E S 4m Two cases of simulations with two different films put on the western surface of a building are compered here

55 Calculation cases H=24m H=32m 4m 4m H=4m H=1m W N E One is the case where the retro reflective film( 熱線再帰性 film) is put on the western surface of a building. S The other is the case where the conventional heat shading film( 遮熱 film) is put on the same surface. 19 Meteorological conditions Meteorological data Japan Meteorological Agency in Tokyo The target date 13:,14:,15:,1: July 23, 21 Thermal environment on a particularly hot summer day was simulated

56 Meteorological condition on July 23, 21 July 23 was sunny day. Maximum temperature was 35. Air temperature[ ] Global solar radiation[w/m2] The amount of solar radiation absorbed by ground surface (July 23, 21) N 13: W S 9 E 14: [W/ m2 ] Retro reflective film Conventional heat shading film 1 5

57 The amount of solar radiation absorbed by ground surface (July 23, 21,13:) 9 13: N W E S [W/ m2 ] Retro reflective film Conventional heat shading film In the case with Heat shading film, the high peak value of solar radiation is observed around the western surface of the building 113 The amount of solar radiation absorbed by ground surface (July 23, 21,14:) N 9 14: W E S Retro reflective film [W/ m2 ] Conventional heat shading film The peak is not observed in the case with Retro reflective film at 14:

58 The amount of solar radiation absorbed by ground surface (July 23, 21) 15: N W E S 9 1: [W/ m2 ] Retro reflective film Conventional heat shading film 115 The difference of the amount of solar radiation absorbed by ground surface (Retro reflective film Conventional heat shading film) : 14: 15: 11 [W/ m2 ] 1: The amount of solar radiation absorbed by ground surface in the case with the heat ray retro reflective film is lower by up to 11 W/m 2 than that with the conventional heat shading film

59 Conclusions The effect of a heat ray-retro reflective film on the thermal environment in outdoor space is evaluated using a numerical simulation on the basis of the radiant analysis method considering directional reflection. In this study, the radiant environments around a building in Shinbashi district, with heat ray retroreflective film( 熱線再帰性反射 film) and heat shading film ( 遮熱 film), were simulated. The result indicated that the amount of solar radiation absorbed by ground surface in the case with the heat ray retro-reflective film was lower by up to 11 W/m 2 than that with heat shading film. 117 References 1) Yumino S., Uchida T., Sasaki K., Kobayashi H., Mochida A. Total assessment for various environmentally conscious techniques from three perspectives: mitigation of global warming, mitigation of UHIs, and adaptation to urban warming, Sustainable cities and society, 19, (215), ) Yumino S., Uchida T., Mochida A., Kobayashi H., Sasaki K. Evaluation of greening and highly reflective materials from three perspectives, Proceedings of I 9th International Conference on Urban Climate (ICUC9) (215) 3) Shinji Yoshida, Saori Yumino, Taiki Uchida, Akashi Mochida Effects of windows with heat ray retro reflective filim on outdoor thermal environment and building cooling load, Journal of Heat Island Institute International, 9(2), (214), ) Shinji YOSHIDA, Saori YUMINO, Akashi MOCHIDA, Taiki UCHIDA, An evaluation of the effects of heat ray reflective film on the outdoor thermal environment using a radiant analysis method, Proceedings of the 9th International Conference on Urban Climate (ICUC9) (215) 59