prediction of sensible heat flux from buildings and urban spaces using a detailed geometry model

PREDICTION OF SENSIBLE HEAT FLUX FROM BUILDINGS AND URBAN SPACES USING A DETAILED GEOMETRY MODEL OF A SUBSTANTIAL URBAN AREA - INTRODUCTION OF A PREDICTION MODEL OF ANTHROPOGENIC HEAT

INTO AN URBAN HEAT BALANCE SIMULATION MODEL - Takashi Asawa*, Shinji Yamamura**, Akira Hoyano*

*Tokyo Institute of Technology, Yokohama, Japan; **Nikken Sekkei Research Institute, Tokyo, Japan Abstract The present study improves an urban heat balance simulation model for urban micro-scale, so as to predict anthropogenic heat released from buildings, as well as surface sensible heat flux in an urban area. The proposed system combines the urban heat balance simulation with a building thermal simulation and a building energy simulation. The simulation system is applied to a substantial urban area in Tokyo, and the applicability of the proposed simulation model to the analysis of sensible heat flux in the area is examined. Key words: surface heat flux, anthropogenic heat, simulation model 1. INTRODUCTION Urban surfaces are covered with various materials, buildings with complex geometries, and vegetation. The nature of the surface and its temperature strongly affects the behavior of the lowest layers of the atmosphere. In addition, energy consumption in an urban area causes anthropogenic heat to be released into the atmosphere, and the heat and energy balance in the area are strongly related. The anthropogenic heat released from buildings depends on various factors, including building use, building thermal performance, and building equipment (such as HVAC systems). Estimating the sensible heat flux from buildings and urban spaces, taking into account the detailed conditions of urban surfaces and building energy consumption, is important when analyzing the urban thermal environment and micro-climate. In a previous study, the authors developed a heat balance simulation model that can predict the surface temperature distribution in urban blocks using a 3D CAD (computer-aided design) system (Asawa, 2008). The previous simulation model is a micro-scale model in an urban canopy using a detailed geometry model of an urban block. The present study improves the previous simulation model, so as to predict anthropogenic heat released from buildings, as well as surface sensible heat flux derived from convective heat transfer to/from building and ground surfaces. The proposed simulation model is expected to be applicable to the assessment of urban development and regeneration projects with respect to the urban and atmospheric thermal environments. 2. OUTLINE OF THE SIMULATION SYSTEM 2.1. Outline of the development The proposed simulation system can treat detailed geometry and materials that exist in an urban area and can calculate heat balance and heat transfer in the urban canopy (Fig. 1). The interaction between the outdoor heat balance and building energy consumption is also considered. The proposed system is constructed based on a 3D CAD system, which can integrate various properties with a geometry model and can translate geographic information system (GIS) data into the geometry model for simulation. The subject spatial scale of the proposed

simulation system is set to an ordinary urban block (within approximately 20 hectare). The applied mesh size is set to range from 200 mm to 400 mm, so as to correspond to the detailed geometry of buildings. Fig. 2 shows the structure of the proposed simulation system. 2.2. Simulation of surface temperature distribution and surface sensible heat flux Surface temperature distribution and surface sensible heat flux are calculated by heat balance and heat

Atmospheric radiation

Sky solar radiation

Direct solar radiation

Solar shadingLong wave radiation

Convective heat transfer

Solar reflection(Specular,Diffuse)

Heat conductionWater retentive pavement

Sensible heat

Latent heat

Solar transm-issionVentil

-ation

Cooling/Heating load

Anthropogenic heat flux

Energy consumption

EvapotranspirationEvaporation

Solar reflection(Specular,Diffuse)

Evaporation

HVAC system

Heat conduction

Heat emission

High reflective pavement

Surface heat flux

Fig. 1 Heat and energy balance in an urban canopy and the calculation items in the simulation system

* 4259 Nagatsuda-cho, Midori-ku, Yokohama, Kanagawa 226-8502, Japan

The seventh International Conference on Urban Climate, 29 June - 3 July 2009, Yokohama, Japan

transfer simulation in an urban canopy using an urban heat balance simulation model we developed in a previous study (Asawa, 2008). The surface sensible heat flux in an urban area is estimated by summing the convective heat flux for all surfaces included in the area. The total (area-averaged) heat flux is an index for the influence of urban surfaces on the atmospheric thermal environment. The authors

also proposed the heat island potential (HIP), which is expressed as the measure of a temperature, for the evaluation of surface sensible heat. The heat exchange between the urban canopy and the atmosphere above the canopy will be examined in a future study. 2.3. Simulation of building energy consumption and anthropogenic heat in an urban block Energy consumption and anthropogenic heat in an urban block are calculated by coupled simulation of the urban heat balance simulation model and the building thermal simulation model. The calculations are implemented for each building, taking into account the building use, building thermal performance, building equipment, and surrounding conditions. This coupled simulation model considers the actual conditions of a substantial urban block and

calculates the total energy consumption and anthropogenic heat in the area. 3. NUMERICAL SIMULATION MODEL 3.1. Urban heat balance simulation Heat balance in an outdoor space affects not only the formation of the outdoor thermal environment but also the building cooling/heating load. The urban heat balance simulation model calculates heat balance and heat transfer (direct solar radiation, sky solar radiation, reflected solar radiation, atmospheric radiation, exchange of longwave radiation with the surroundings, convective (sensible) heat transfer, latent heat transfer, conductive heat transfer) for all surfaces in an urban area (Table. 1). The surface temperature distribution and surface sensible heat flux are output successively during the target days. Air temperature data, which is derived from meteorological data, and the convective heat transfer coefficient are uniformly applied to the all surfaces for heat balance simulation and estimation of the surface sensible heat flux. This is a main approximation in this simulation. The convective heat transfer coefficient is estimated by the Jurges formula. The effect of this approximation on the simulation results for surface temperature and surface sensible heat flux has been examined. Heat transfer simulation models for rooftop greening and wall greening are incorporated into the urban heat balance simulation model. These heat transfer simulation models use the evaporation ratio as the parameter for evaporation and evapotranspiration in the estimation of latent heat flux on green-covered surfaces. 3.2. Building thermal simulation Radiant flux data on external surface meshes (cells), calculated in the previous step (Section 3.1), are summed for each surface object. The radiant flux data are then translated to the building thermal simulation model and set for the boundary condition data of the calculation point on each surface object. At the calculation points of

Urban heat balance simulationDirect solar radiation, Sky solar radiation,

Atmospheric radiation, Long wave radiation(Surroundings) , Convective heat transfer, Latent heat transfer, Heat conduction

Building thermal simulationThermal transmission load, Solar radiation load, Ventilation load,Indoor heat emission

Building energy simulation

Spatial component DB

Meteorological data

Energy demand data

Equipment performanceCOP (Coefficient of Performance)

Input (3D-CAD) Solver Output

Sensible heat load

Latent heat load

Energy consumption

CO2 emission

Surface temperatureHeat Island Potential (HIP)Surface heat flux ＋Anthropogenic heatMean Radiant Temp.(MRT)

Reflective solar radiation (Specular, Diffuse)3D-MeshModel

Material DB

Lighting, Electrical appliance, Hot-water supply, Kitchen equipment, HVAC system, others

DataBase

Geom-etric

Inform-ation

Radiant fluxBoundary condition for building surface

Building

Building

Outdoor

Indoor air temp.

Relative humidity

Calcul-ation

Inform-ation

Fig. 2 Structure of the simulation system

Table 1 Heat balance formula on each surface


openings in buildings, incident solar radiation data (direct solar radiation, sky solar radiation, and reflected solar radiation) are also summed, so as to estimate the quantity of solar transmission into the indoor space. Solar radiation transmitted through the opening is absorbed by the walls and the floor. The building thermal simulation is implemented using the radiant flux data allotted to all external surfaces, and the cooling/heating load in the building is then calculated. One-dimensional unsteady heat conduction in building members, such as walls and roofs, is calculated by the backward-difference method. Each floor has one zone, or is divided into several zones, and calculation points of air temperature and cooling/heating load are set for each zone. 3.3. Building energy simulation The energy consumption elements calculated in the present study include HVAC systems, lighting, electrical appliances, hot-water supplies, and kitchen equipments. The energy consumption by a building HVAC system is calculated using the cooling/heating load data estimated by the building thermal simulation (Section 3.2) and the coefficient of performance (COP) of the HVAC system. The total energy consumption for lighting and electrical appliances is estimated by the unit demand schedule data in the database for each building-use type and the floor area data, which is directly obtained from the building 3D CAD model. Energy simulation that includes district heating and cooling (DHC) plants will be considered in the future. 3.4. Calculation of anthropogenic heat released from buildings Anthropogenic heat released from buildings consists of heat emission from heat source equipment, outdoor units of HVAC systems (such as cooling towers), and mechanical ventilation and hot-water supply systems (e.g., Ashie, 2002, kametani, 1996). These quantities are estimated based on the energy consumption data calculated in the previous section (Section 3.3). The proportion of sensible heat to latent heat differs in each HVAC system (the dependence on gas supply or electrical supply) as well as with the type of outdoor unit (air-cooled or water-cooled). In the present study, anthropogenic heat generated by automobile traffic is not considered. 4. APPLICATION RESULTS 4.1. Subject urban area and calculation conditions The proposed simulation model was applied to the prediction of total sensible heat flux from the buildings and outdoor spaces in a substantial urban area. A business/residential area with massive and tall buildings in the center of Tokyo was selected as the subject site. The specifications of the urban block and buildings are listed in Table.2 and Table. 3. The unit demand data for lighting, electrical appliances, and hot-water supplies per building floor area for each building-use type were set according to measurement data (e.g. Ojima laboratory, 1995). The heat source equipments of HVAC systems are an absorption refrigerator (with a cooling tower) and air-cooled heat pump chiller for office buildings and a room air conditioner for residential buildings. The indoor air temperature of air-conditioned room was 26 °C. The meteorological conditions used in the calculation were for a summer day with a clear sky (August 5th). Each simulation was continued for five days in order to obtain a periodic steady-state solution for the subject day.

4.2. Simulation results Figure 3 shows the simulation results for the surface temperature distribution of the urban block at 3 p.m. The surface temperatures of the western wall of the

Build. No.

Use type

HVAC system

B1 O AR B2 O AR B3 O HC B4 R RA B5 R RA

Site area[m2] 11760 Floor area

ratio [%] 420

Building coverage

ratio [%]

45

O: Office, R: Residential AR: Absorption refrigerator

with a cooling tower HC: Air-cooled heat pump

chiller RA: Room air conditioner

Table 3 Building info.Table 2 Urban block

N N

気温25 30 40 50

表面温度(℃)

554535

Surface temp. [°C]

Air temp. 31.7 °C

B1

B2 B3

B4

B5

Fig. 3 Simulation results of surface temperature distribution (3 p.m., clear sky day in summer, August 5th, Tokyo)

Build. No. (HVAC)

Sensible heat [kW/m2]

Latent heat [kW/m2]

B1 (AR) 0.45 1.69 B2 (AR) 0.43 1.70 B3 (HC) 1.11 0.060 *1 (AR) 0.427 0.979 *2 (HC) 1.10 0.061

AR: Absorption refrigerator HC: Air-cooled heat pump chiller *1, *2: Office building, Ashie (2002)

Table 4 Comparison ofanthropogenic heat between the simulation results and previously reported values (quantity per unit floor area)


buildings B1 and B2 increase to over 40 °C due to strong direct solar radiation from the west, as well as the temperature increase of the flat roofs and asphalt-paved road. The cooling loads of these buildings are larger than that of building B3, which is shaded by these tall buildings during the afternoon. The surface temperatures of the window panes are relatively low due to the air cooling in the building and the low solar absorptivity of the glass surface. On numerous glass surfaces, the sensible heat flux is a negative value. Outdoor parking lots and parks are shaded by neighbor buildings and tall trees, and the ground surface temperature ranges from approximately 30 °C to 40 °C. At night (8 p.m.), the surface temperatures of the buildings and roads remain higher than the air temperature as a result of heat storage during the daytime. Figure 4 shows the results for anthropogenic heat released from each building. The buildings with cooling tower (water-cooled) release a large quantity of latent heat compared with sensible heat. The amount and proportion of anthropogenic sensible heat obtained in the present estimation correspond approximately to previously reported values by Ashie (2002) (Table 4). For latent heat flux, the calculated values in this study are larger than that of the previously reported values. Figure 5 shows the results for area-averaged sensible heat flux including surface heat flux and anthropogenic heat flux. The peak value of anthropogenic sensible heat flux from buildings is almost equal to the surface sensible heat flux in this area. The high density of building energy consumption in the area cause the quantity of the anthropogenic sensible heat flux, although the office buildings have cooling towers for heat release and latent heat flux occupies a significant portion of the total heat flux. At night, the surface sensible heat flux remains over 30 W/m2 due to heat storage in buildings and roads. These results show the applicability of this simulation model to the analysis of sensible heat flux from an urban area, taking into account the spatial geometry, materials, and building-use types in the area. The examination using a broader range of examples of urban areas will be conducted in a future study. 5. CONCLUSION A simulation system to predict surface sensible heat flux and anthropogenic heat from buildings in an urban area was developed for the assessment of urban development with respect to the urban and atmospheric thermal environments. The proposed system combines the urban heat balance simulation with the building thermal simulation and the building energy simulation. The simulation system was applied to a substantial urban area in Tokyo, and the applicability of the proposed simulation model to the analysis of sensible heat flux in the area was examined. Acknowledgement The present study was conducted as a part of collaborative study with A&A Co., Ltd, to whom the authors would like to express their gratitude. References Asawa, T., Hoyano, A., Nakaohkubo, K., 2008, Thermal design tool for outdoor space based on heat balance simulation using a 3D-CAD system, Building and Environment, 43, 2112-2123. Asawa, T., Hoyano, A., 2004, Development of heat transmission simulation of building considering the effects of spatial form and material in outdoor space (in Japanese), Journal of environmental engineering (Transactions of AIJ), 639, 47-54. Ashie. Y., Tanaka. M., Yamamoto. T., Taguchi. A., 2002, A study of the regional heat discharge from air conditioning systems and related devices based on the composition ratio of cooling heat source devices (in Japanese), Transactions of the Society of Heating,Air-Conditioning and Sanitary Engineers of Japan, 86, 77-86. Ojima Laboratory, 1995, Unit Demand Data of Power, Heat and Water in Various Buildings for Tokyo area, Waseda University Press, written in Japanese. Kametani. S., Mizuno. M., Shimoda. Y., Kuzuhara. H., Nishi. T., 1996, Study on the heat environmental load from a building equipped with the air-conditioning system Part 1 Examination on the waste heat character of a building with differences of air-conditioning systems, Transactions of the Society of Heating,Air-Conditioning and Sanitary Engineers of Japan, 62, 1-11.

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(A: Site area [m2]dS: Unit surface area [m2])

Fig. 4 Simulation result of anthropogenic heat flux from each building (clear sky day in summer) Fig. 5 Simulation result of area-averaged sensible heat

fluxes in the site (quantity per unit site area)


prediction of sensible heat flux from buildings and urban spaces using a detailed geometry model

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