- Email: [email protected]
Modelling solar effects on the heat and mass transfer in a street canyon, a simplified approach
Solar Energy 79 (2005) 10–24 www.elsevier.com/locate/solener
Modelling solar effects on the heat and mass transfer in a street canyon, a simplified approach Emmanuel Bozonnet *, Rafik Belarbi, Francis Allard Laboratoire d’E´tude des Phe´nome`nes de Transfert Applique´s au Baˆtiment, Universite´ de La Rochelle, Avenue Michel Cre´peau, 17042 La Rochelle Cedex 1, France Received 16 January 2004; received in revised form 15 July 2004; accepted 25 October 2004 Available online 2 December 2004 Communicated by: Associate Editor Matheos Santamouris
Abstract In urban areas, the climatic loads on buildings in summer conditions are largely affected by solar radiation. In this paper a modified simplified method for radiant interchange determination is used in a solar energy study. The good agreement with the radiosity method allows one to use this simplified method in the street canyon case. In a building pilot study, parametric analysis and building thermal behaviour can be assessed by simplified models which are useful for long-period simulation. Then this radiant interchange model is introduced in a zonal model of a canyon street and performed with a variable climatic conditions show case. The solar radiation is the only driving force in the street air movement. The interest of such approach for complex coupled phenomena studies is highlighted by obtained results and the assessment of variable climatic loads for different building zones can be considered with the model detailed herein. Future developments are planned in order to improve simulation accuracy by the addition of other local phenomena. 2004 Elsevier Ltd. All rights reserved. Keywords: Urban micro-climate; Solar radiation; Radiant interchange model; Zonal model
1. Introduction The solar irradiation is a noteworthy thermal load for buildings, especially in urban context where most surfaces have low albedo (Akbari et al., 2001). It is beside a significant factor in the phenomenon called ‘‘urban heat island’’ (Landsberg, 1979; Karl and Jones, 1989; Goodridge, 1992; Heino, 1999) which is critical in summer period. Due to the specificity of streets geo-
*
Corresponding author. Fax: +33 5 46 45 82 41. E-mail address: [email protected] (E. Bozonnet).
metry, reflected irradiations amplify this heat concentration. Alternative solutions for the thermal comfort in buildings are developed, such as natural ventilation, which reduce buildings energy consumption due to airconditioning systems (Kolokotroni et al., 2002) and the heat island contribution. Natural ventilation potential and wind flows were extensively studied in urban areas (Clifford et al., 1997; Allard and Santamouris, 1998; Carrilho da Graca et al., 2002; Jiang and Chen, 2002). However natural ventilation systems are tricky to design due to the low wind speed which contributes to the effect of urban heat island (Mihalakakou et al., 2002). This was pointed out during the European project
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.10.007
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
11
Nomenclature ai /e qi si Ei Edif Edir
short-wave absorption coefficient of the facet i (–) incoming short-wave flux (W) short-wave reflection coefficient of the facet i (–) short-wave transmission coefficient of the facet i (–) total irradiance of the facet i (W/m2) solar diffuse irradiation (W/m2) solar direct irradiation (W/m2)
URBVENT (Santamouris et al., 2001) in which wind speed measurements were carried out in different Athens streets. Specific wind models were developed for urban context (Hotchkiss and Harlow, 1973; Nicholson, 1975) but it appears to be restricted by local turbulence and thermal effects. Actually, in urban areas a turbulent sub-layer is created by the irregular obstacles and with the confinement effect thermal forces induced by solar irradiation have to be modelled. As a result, a local model taking into account the thermal and the mass transfers is necessary to define realistic climatic conditions near buildings. This study focuses on the street canyon case and solar impact on radiation trapping and buoyancy flows. The canyon shape is common in European cities and its confinement effect is significant. The final aim of our study is to assess the impact of thermal transfers between street and constructions for a full summer period assuming no wind effect in a first approach. Simplified approaches were chosen in order to analyse the impact of dynamic coupled climatic phenomena and with outlook of larger zone and parametric studies which can be helpful for urban planning.
2. The zonal model of a street canyon Solar energy can have high impact on indoor temperature in summer conditions, as shown a sensibility study made by Lauret et al. (2001). Solar radiance at atmosphere limits is about 1370 W/m2 and fluctuates during year (Cadiergues, 1998). A part of this energy is absorbed by atmosphere and at ground level solar energy results therefore from direct irradiation Edir and diffuse irradiation from sky Edif. In our study, the solar diffuse irradiation is calculated with an isotropic sky model and more accurate ones, taking into account the anisotropic characteristic of sky like the model of Perez et al. (1987), are not necessary for the required accuracy. Nevertheless some of these models are implemented in the solar irra-
Eie E0i Exi Exi0 Eiext Fji Qu R Si SFi
solar irradiance of the facet i (W/m2) interchange flux of the facet i (W/m2) total exitance of the facet i (W/m2) primary exitance of the facet i (W/m2) external irradiation of the facet i (W/m2) view factor between facet j and i (–) mass flow through a cell interface u (kg/s) radiative equilibrium remainder (W) surface of the facet i (m2) surface of the fictitious facet Fi (m2)
diation simulation software Solene (Miguet and Groleau, 2002) used in our study. Comparison of Solene results with experimental data for a street canyon revealed good agreement (Vachon, 2001). In this paper, as a first approach, a simplified street canyon (Fig. 1a) is considered. Low wind speed conditions are assumed so the thermal effect inside street cannot be neglected. This street is supposed long enough to neglect boundary effects (W/L 1 and H/L 1). Inside the studied zone solar effects and others heat and mass transfer are modelled for a summer period. In this kind of problem, due to the lack of accurate data about the boundary conditions, dynamic simulations are uneasy to achieve with classic fluid dynamics codes. Therefore, a zonal approach with empirical models integration was developed (Musy et al., 2001; Bozonnet et al., 2002). This meshless method produces accurate enough results for building energy consumption and comfort evaluation. The volume of the studied zone is partitioned into smaller entities called ‘‘cells’’ (Fig. 1b). For one cell there are six faces indexed u crossed by mass and heat fluxes in or out the cell volume V. With Qu the mass flow through the face u and, assuming that air is incompressible, the mass balance can be expressed by X Qu ¼ 0 ð1Þ The cell heat balance is expressed likewise, with q the cell air density, C the specific heat of air, T the air temperature, /u the heat transfer through interface u and /source the heat production into the cell: oT X þ /u ¼ /source qCV ð2Þ ot The cell air state parameters are supposed to be determined by the perfect gas law. At last the air mass flows are determined by the pressure gradient between cells (Musy et al., 2001) and density variation with temperature lead to a thermal buoyancy term in airflow calculation.
12
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 1. Studied zone in a street canyon with a W/H aspect ratio: (a) studied zone and (b) partition of street canyon into cells for zonal model.
Wall and ground conduction and heat storage are determined by a one dimensional model (modal reduction). The convective heat flux with surfaces is then included in cells energy balance. The long- and short-wave radiant interchange in buildings is then determined by the fictitious surfaces method (Walton, 1980; Musy et al., 2001), this method is detailed hereafter. Wall surfaces are partitioned into smaller area called ‘‘facets’’. These facets matching the surface of cell sides are about one square meter. The short-wave irradiation is calculated for each facet with • a view factor for mask effect, • a total primary irradiation Ei (W/m2) for the time and the considered location, including the direct and diffuse irradiation, • the walls radiant interchange. In this study, this approach is applied for outside walls subjected to solar irradiation.
3. Different methods for solar irradiance calculation Incident solar flow on each exterior surface is more or less reflected depending on surface colour or its radiant characteristics. Different approaches to calculate the short-wave radiant exchange between surfaces exist. At first in this study, the radiosity method which provides accurate results has been used. It needs the calculation of view factors between surfaces. This method can lead to many equations, which have to be solved simultaneously. The WaltonÕs simplified method (Walton, 1980), or fictitious surfaces method, seems then to be an interesting alternative for buildings simulations (Allard et al., 1985; Musy et al., 2001). The results of both methods are compared here for the studied street canyon. 3.1. Radiosity method Considering air transparent toward short-wave radiations, short-wave irradiance Ei (W/m2) on a facet i, sur-
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
13
Fig. 3. Fictitious facet Fi definition for the facet i. Fig. 2. Radiant parameters for the facet i surrounded by n 1 facets j. 2
face Si (m ), is the sum of the solar incident irradiance and the interchange flux E 0 i (Fig. 2): Ei ¼ Eie þ E0i
Eie
Exi ¼ Exi0 þ qi E0i
ð4Þ
and Exi0 ¼ qi Eie þ si Eiext
ð5Þ
qi and si are the reflection and transmission coefficients of facet i and Eiext (W/m2) is the short-wave irradiance of the other side of the facet (directed from the exterior of the studied zone). From (3) and (4) we obtain Ex ¼
Exi0
i
þ qi ðE
Eie Þ
ð6Þ
The interchange fluxes are also determined with view factors Fji between studied zone surfaces j and surface i by the relation: X X S i Ei ¼ S i Eie þ F ji S j Exj ¼ S i Eie þ F ij S i Exj ð7Þ j
i.e. Ei Eie ¼
X
X
F ij Exj
ð9Þ
j
Thus, the determination of each exitance needs these simultaneous equations solving. 3.2. Fictitious surfaces method
ð3Þ
For each facet i the total exitance1 Exi is the sum of reflected part of the interchange flux and the primary exitance Exi0 (W/m2) from this surface:
i
Exi ¼ Exi0 þ qi
F ij Exj
j
ð8Þ
j
For n facets i, n simultaneous linear equations are then obtained:
This simplified method from Walton (1980) permits a fast resolution of radiative balance for a closed domain. For each facet i a fictitious facet Fi is associated (Fig. 3), with a fictitious surface SFi (m2), a fictitious reflection coefficient qFi and a fictitious transmission coefficient sFi. The fictitious facet Fi takes the place of all facets j which contributed to the part of radiant exchange with facet i, this way j is not in the same plane that i for a parallelepipedal zone. The characteristics of Fi depend on facets j ones by the following relations: X SF i ¼ Sj ð10Þ j=j62P ðiÞ
P
j=j62P ðiÞ qj S j
qF i ¼
ð11Þ
S Fi P
ExF0 i ¼
j j=j62P ðiÞ Ex0 S j
ð12Þ
S Fi
The advantage of this method is to determine the exitance for each facet basing only on the fictitious surface characteristics. Then (Fig. 4) the total exitance of facet i can be expressed from primary exitances Exi0 and ExFi 0 (W/m2): qFi S i Exi0 þ S Fi ExF0 i S i Exi ¼ S i Exi0 þ qi F F ii ð13Þ 1 qF i ðqi F F ii þ F F iF i Þ i.e. ExF i
1
Radiant exitance: The radiant flux per unit area emitted from a surface (from http://www.photonics.com/dictionary).
i
Ex ¼
Exi0
þ qi F F ii
0 qF i Exi0 þ F F ii
1 qF i ðqi F F ii þ F F iF i Þ
ð14Þ
14
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 4. Interchange details for primary exitance from facet i and fictitious facet Fi, Exi0 and ExFi 0.
From Eqs. (6) and (14), we obtain ExFi 0
Ei ¼ Eie þ F Fii
qFi Exi0 þ F Fii 1 qFi ðqi F Fii þ F FiFi Þ
ð15Þ
Finally the irradiance of facet i is expressed only with primary exitances and surfaces characteristics. The simplification made by this method induces an imbalanced radiative equilibrium between incoming short-wave flux /e (W) inside the studied zone and the outgoing flux. The remainder called R is calculated from X /e ¼ ðai þ si ÞEi S i þ R ð16Þ i
In order to obtain a balanced equation, an iterative calculation is done. The remainder R is then redistributed in the outgoing fluxes (Walton, 1980; Musy et al., 2001), considering that the more a surface is absorbative or transparent for short-wave radiations, the more the corresponding part of the remainder is important. This is rendered by the new value of absorbed and transmitted flux for a facet i: ðai þ si ÞR ðai þ si ÞEi ¼ ðai þ si ÞEi þ P ðaj þ sj ÞS j
ð17Þ
j
The radiative equilibrium (16) is calculated, and the repartition of the remainder R is done again until the wanted precision is obtained.
In this approach, computing time and resources are significantly reduced, but fluxes repartition is losing accuracy, depending on view factors values. An evaluation of this method has been conducted.
4. Comparison of solar computation methods 4.1. Studied case Both methods exposed in the previous part are applied here to a canyon street slice as shown in Fig. 1. The street is 200 m long, 9 m width, 9 m high and the studied zone is 1 m depth, boundary effects are then neglected and the street is considered as ‘‘infinite’’. The street axe is supposed to be South/North and latitude of the site is 47 North. The studied volume inside street is shared in three vertical and three horizontal slices, see Fig. 1b. The exterior boundary of both walls and the ground is formed of nine rectangular facets (reflectance coefficient values are set to 0.8) and each facet is triangularized. The total primary incident solar irradiance is equal for the both studied methods. The mask factors and the direct and diffuse incident irradiance are determined with Solene software. Afterwards, the radiant interchange inside the studied zone is computed with both Solene, which is based on the radiosity method, and the fictitious surface method.
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
15
Fig. 5. Total solar fluxes incident and absorbed by walls in the studied zone, hours are in solar time: (a) before and after radiative interchange, (b) comparison between interchange computation with Solene and fictitious surfaces method (curve a and b).
16
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
4.2. The utilization of a modified fictitious surface method The results presented here (Fig. 5) for the summer solstice, show significant differences. Solar energy absorbed by walls after interchange on the whole day differs from about 28%. This gap can be explained by a great value of the radiative equilibrium remainder R which is misrepartitioned by the iterative procedure (17). In this case, the fictitious surfaces are very transparent to radiations due to open boundaries. Without any precise view factor determination, this energy loss
is overestimated and insufficiently corrected by the ratio used in Eq. (17). Then, with a modified repartition of R, Eq. (17) becomes ðai þ si Þ2 R ðai þ si ÞEi ¼ ðai þ si ÞEi þ P ðaj þ sj Þ2 S j
ð18Þ
j
The results obtained with this modification (Fig. 5b, curve b) show that solar energy absorbed by walls after interchange computed by Solene and by the modified fic-
Fig. 6. Solar absorbed flux after interchange computed with Solene and the modified fictitious surfaces method for one wall and ground, hours are in solar time: (a) West wall and (b) ground.
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
titious method differs from about 6%. Moreover, there are slighter flux differences on different walls for the whole day. Considering each walls (Fig. 6), it appears that both evolutions are very similar. The west wall does not get direct sun light between 4 and 11 AM (true solar time), so the absorbed radiation is from diffuse and interchange fluxes. It appears that the modified fictitious surface method overestimates here the interchange flux absorbed by the wall. The same effect is observed for the ground during the morning and the evening, before 8 AM and after 16 PM (true solar time). The global absorbed energy is almost the same, so during the afternoon the modified fictitious surface method underestimate the west wall absorbed solar flux. It means that the primary incident solar flux absorbed by the wall is underestimated. Interchange fluxes are overestimated and this is mainly due to the lack of view factors determination. In this first approach interchange zone was limited to the studied zone. In order to be more realistic, interchange with side panels has to be considered. A Solene computation, see Fig. 7, for a larger interchange zone has been done considering 10 m both sides of studied zone, i.e. a 21 m depth street zone has been triangularized for the radiative interchange study.
17
The results obtained for the same 1 m zone show a greater part of absorbed flux, which mean that street ‘‘confine’’ the solar radiation. For the all day and all walls, this stands for about 23% more energy than in first study. Considering each wall, see Fig. 8a and b, the absorbed direct incident flux seems to be well assessed, but interchange flux computed by the modified fictitious surfaces method appears to be underestimated. This difference is more visible for the west-oriented wall, see Fig. 8a, and can be explained by view factor differences regarding sky and side panels contribution. This confinement effect incorrectly depicted here, amount to less loss by transparent panels, i.e. toward the outside of studied volume. Solar absorbed flux repartition details in west-oriented wall height, as shown in Fig. 9a, highlight this error localisation due to confinement. As confinement is important, F1 facet case shown by Fig. 9b, radiative interchange is predominant and absorbed fluxes are significantly underestimated, 44% for all day absorbed energy. For the middle height facet F2, see Fig. 9c, interchange is underestimated at unexposed to direct solar radiation hours, 10% for all day absorbed energy. For the top facet F3, see Fig. 9d, the same trend is observed in the morning, except for the afternoon as it appears that
Fig. 7. Total interchange solar flux absorbed by studied zone panels computed for studied zone interchange only, Solene and the modified fictitious surface method, and for a larger zone with Solene, hours are in solar time.
18
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
the modified fictitious surface method gives greater values than Solene ones. Overall results show same tendencies during the studied day, however locally errors can be no negligible. With a variation of geometry, for an aspect ratio W/H 5 1, the more the view factors are significant, the more the error made by simplified method is important. Moreover, the previous results appear imprecise for the confined facets. Although the results are precise for one wall, this model has to be refined for a better knowledge of interchange flux repartition. Finally, it appears that fluxes repartition is qualitatively well depicted by the results obtained with this im-
proved fictitious surface method. An improvement of the method is considered in order to take into account the confinement influence on interchange fluxes. Nevertheless parametric studies can be conducted on urban canyons knowing this systematic difference on computed solar fluxes. This method is also used in the next part in order to solve the long-wave radiant interchange, and low computing time is very valuable for the iterative procedure of surface temperature estimation. In urban pilot study, the benefit of assessing buildings and climatic phenomena interaction with parametric studies and simplified inputs can be considered with the proposed method.
Fig. 8. Solar absorbed flux after interchange in a 21-m depth street computed with Solene and the modified fictitious surfaces method for one wall and ground, hours are in solar time: (a) West wall and (b) ground.
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
5. Practical application for a street climate evaluation model Solar effects in a street are mixed with other effects such as convection and walls thermal inertia. For the study of a simplified street canyon, during one day (21 June), we have chosen a zonal approach described in part 2. This kind of model allows long-period studies for large volumes (Musy et al., 2001; Wurtz et al., 2003) and simulations can be run on a personal computer with low computing time. Besides, climatic conditions can be evaluated for different street zones. 5.1. Studied zone and boundary conditions At a regional scale, meteorological data give mean air conditions. However at a local scale, the studied street, there are a lot of local phenomena which have to be taken into account. The studied zone (Fig. 1a) is oriented South/North (a = 0), and it can be considered as bidimensional since there are symmetrical boundary conditions. The street
19
volume is partitioned into cells of 3 m · 3 m · 3 m as shown by Fig. 10. For the studied period, one day, no predominant wind occurs, so air circulation is only due to thermal effects, i.e. mainly solar effects. The solar primary irradiance is computed with Solene, as previously described, and then interchanges are determined by the modified fictitious surfaces method. Long-wave interchange is determined by the same method and the sky longwave radiation is determined with a supposed constant sky temperature Tsky in a first approach. Actually, the daily variation of this sky parameter depends on the air relative humidity and temperature (Ito and Miura, 1989). The inside buildings air temperature is supposed maintained at 20 C and walls energy exchange with cells are computed considering convection, radiation and conduction. Meteorological data give mean air temperature for the geographical zone, so this air temperature Text is maintained in the top part of the studied zone, where it is considered that local phenomena have no influences.
Fig. 9. Detail of solar absorbed flux after interchange for three facets of west-oriented panel in studied zone (a), facets F1 (b), F2 (c) and F3 (d)—curves b, c and d; hours are in solar time.
20
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 10. Canyon street partitioning and boundary conditions for zonal model.
5.2. Simulation results The studied day has been repeated seven times so an established variable regime is obtained. Actually air and wall temperatures are initialized at 20 C, but solicitations and walls inertia lead to a different established regime. Time step is 1-h long, which seems precise enough for observed phenomena. During the night period, as it can be seen in Fig. 11a, there is an air recirculation in the street due to convection exchange. Air is warmed up by walls, and the west-oriented panel is the warmer one at this time, 2 AM (mean time). So hot air is going up aside west wall, which induce an air movement from west to east in the street. Fresh air at 25 C from outside studied zone is then mixed to air from street, as a part of air rises out of street zone. Nevertheless it appears that a part of hot air from street is dragged again in the street zone, this phenomenon inhibits a bit the street night cooling. As walls are refreshing, convective fluxes decrease and in the morning, at 8 AM (mean time), see Fig. 11b, the main convection flux along west-oriented panel is lightened. The sun effect occurs as east panel is exposed, at 9 AM (mean time), see Fig. 12a, so a convective movement is induced in top left zone of the street, reversing the night air circulation. This convective
motion is strengthened during the morning as east panel, then street ground, are heated up by sun (see Fig. 12b and c). As hottest panel place change from left to right side of the street, during the afternoon, air circulation scheme changes (see Fig. 12d). The upper part of east oriented panel is still very hot, due to warm air convection, then there is a vertical rising air circulation near this panel. On the contrary, the lowest part of this panel is cooled. On the other side of the street, the west-oriented wall, sun mounted up heat by panel is delivered to air. So a secondary recirculation is induced in lower part of the street. During the afternoon, as west-oriented wall is warming up, this secondary air circulation increases (see Fig. 13 for 8 PM) amplified by cooling effect of convection on east oriented wall. Obtained results seem physically consistent. However temperature and air speed value are averaged for one hour and temporal variation is faded, but this time step looks acceptable for energy fluxes assessment between the building surfaces and the street. In these simulations, no three-dimensional effect was considered, which would have changed air circulation schemes, downward and upward air fluxes could have been observed in different places. However, it would have been necessary to study a greater domain, large
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
21
Fig. 11. Air temperature and speed for the studied zone: (a) at 2 AM (mean time) and (b) at 8 AM (mean time).
enough to include downward and rising air fluxes places. Another aspect has to be refined here: this is the thermal layer which is not specifically considered here. A more accurate representation of convection near walls, taking
into account roughness elements such as balconies, could give dissimilar results. It appears then that this simplified approach, considering sun effects on street temperatures and air circulation,
22
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 12. Air temperature and speed evolution for the studied zone: (a) at 9 AM (mean time), (b) at 10 AM (mean time), (c) at 4 PM (mean time), and (d) at 5 PM (mean time).
can give some consistent results which have to be compared with experimental measures.
6. Conclusions The comparison of the fictitious surface method and a precise one, like radiosity method, shows that the simplified approach for a street canyon case had to be adjusted. A modification in the iterative process in order to equilibrate the radiative balance gave better results for the studied instance. Its low computation time ap-
pears to be beneficial also for the determination of long-wave radiation interchange and finally for the assessment and parametric study of urban microclimate impact on buildings. This modified method was implemented in a zonal simulation code for thermal and mass transfer study. In a first approach, the climatic phenomena were studied for a simplified street canyon during one day without any exterior wind effects. Solar radiation effect and convection were the only driving forces in the street volume. Results show different air circulation paths varying during the day, according to solar position and its thermal effect on walls. From a qualitative
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
23
Fig. 13. Air temperature and speed at 8 PM (mean time) for the studied zone.
point of view these results seem correct. Nevertheless this model has to be refined to take into account other phenomena such as major wind paths and thermal layer near walls. The first objective of this study was to assess an architectural solution toward climatic loads at the very beginning of building project. At this stage, building projects are not advanced enough for a precise knowledge of boundary conditions and the models described in this paper can be advantageously used to assess different building performances in an urban context. References Akbari, H., Pomerantz, M., Taha, H., 2001. Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy 70 (3), 295–310. Allard, F., Inard, C., Roldan, A., 1985. Caracte´risation the´orique et expe´rimentale du comportement thermique dÕune cellule dÕhabitation perturbe´e par des rayonnements de courtes et grandes longueurs dÕonde, Laboratoire e´quipement de lÕhabitat, INSA de Lyon, September 1985. Allard, F., Santamouris, M., 1998. Natural Ventilation in Buildings—a Design Handbook Altener DG XVII. James & James Ltd. Bozonnet, E., Wurtz, E., Belarbi, R., Allard, F., 2002. Simulation thermo-ae´raulique du microclimat urbain a` lÕe´chelle dÕune rue de type canyon. Proceedings of IBPSA, France, Paris.
Cadiergues, R., 1998. Receuil des de´marches et formules du guide AICVF N 2—Calcul des charges de climatisation et conditionnement dÕair. Carrilho da Graca, G., Chen, Q., Glicksman, L.R., Norford, L.K., 2002. Simulation of wind-driven ventilative cooling systems for an apartment building in Beijing and Shanghai. Energy and Buildings 34 (1), 1–11. Clifford, M.J., Everitt, P.J., Clarke, R., Riffat, S.B., 1997. Using computational fluid dynamics as a design tool for naturally ventilated buildings. Building and Environment 32 (4), 305– 312. Goodridge, J.D., 1992. Urban bias influences on long-term California air temperature trends. Atmospheric Environment. Part B. Urban Atmosphere 26 (1), 1–7. Heino, R., 1999. Homogeneity of the long-term urban data records. Atmospheric Environment 33 (24–25), 3879–3883. Hotchkiss, R.S., Harlow, F.H., 1973. Air pollution transport in street canyons. Technical Report EPA-R4-73-029, NTIS PB-233 252, Los Alamos National Laboratory, US Environmental Protection Agency. Ito, S., Miura, N., 1989. Studies of radiative cooling systems for storing thermal energy. Journal of Solar Energy Engineering 111 (3), 251–256. Jiang, Y., Chen, Q., 2002. Effect of fluctuating wind direction on cross natural ventilation in buildings from large eddy simulation. Building and Environment 37 (4), 379–386. Karl, T.R., Jones, P.D., 1989. Urban bias in area-averaged surface air temperature trends. Bulletin—American Meteorological Society 70 (3), 265–270. Kolokotroni, M., Watkins, R., Santamouris, M., Niahou, K., Allard, F., Belarbi, R., Ghiaus, C., Alvarez, S., SalmeronLissen, J.M., 2002. Passive ventilation cooling in urban
24
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
buildings: an estimation of potential environmental impact benefits. In: Proceedings of EPIC 2002 AIVC, Lyon, France, pp. 531–536. Landsberg, H.E., 1979. Atmospheric changes in a growing community (the Columbia, Maryland experience). Urban Ecology 4 (1), 53–81. Lauret, A.J.P., Mara, T.A., Boyer, H., Adelard, L., Garde, F., 2001. A validation methodology aid for improving a thermal building model: case of diffuse radiation accounting in a tropical climate. Energy and Buildings 33 (7), 711–718. Miguet, F., Groleau, D., 2002. A daylight simulation tool for urban and architectural spaces—application to transmitted direct and diffuse light through glazing. Building and Environment 37 (8–9), 833–843. Mihalakakou, G., Flocas, H.A., Santamouris, M., 2002. The impact of synoptic-scale atmospheric circulation on the urban heat island effect over Athens, Greece. In: Proceedings of EPIC 2002 AIVC, Lyon, France, pp. 301–306. Musy, M., Wurtz, E., Winkelmann, F., Allard, F., 2001. Generation of a zonal model to simulate natural convection in a room with a radiative/convective heater. Building and Environment 36 (5), 589–596.
Nicholson, S.E., 1975. A pollution model for street-level air. Atmospheric Environment (1967) 9 (1), 19–31. Perez, R., Seals, R., Ineichen, P., Stewart, R., Menicucci, D., 1987. A new simplified version of the Perez diffuse irradiance model for titled surfaces. Solar Energy 39, 221– 231. Santamouris, M., Papakonstantinou, K., Georgakis, C., 2001. Natural ventilation in urban-areas. URBVENT Report, Contribution to Task 1.1.1.–2.1.1. and 2.1.2., March 2001. Vachon, G., 2001. Transferts des polluants des sources fixes et mobiles dans la canope´e urbaine: e´valuation expe´rimentale. Nantes, Ecole Centrale de Nantes: 210. Walton, G.N., 1980. A new algorithm for radiant interchange in rooms loads calculations. ASHRAE Transactions 86 (II), 190–208. Wurtz, E., Deque, F., Mora, L., Bozonnet, E., Trompezinsky, S., 2003. Sim_Zonal: a software to evaluate the risk of discomfort: coupling with an energy engine, Comparison with CFD codes and experimental measurements. In: Proceedings of Eighth International IBPSA Conference, Eindhoven, Netherlands, pp. 1423–1428.
Modelling solar effects on the heat and mass transfer in a street canyon, a simplified approach Emmanuel Bozonnet *, Rafik Belarbi, Francis Allard Laboratoire d’E´tude des Phe´nome`nes de Transfert Applique´s au Baˆtiment, Universite´ de La Rochelle, Avenue Michel Cre´peau, 17042 La Rochelle Cedex 1, France Received 16 January 2004; received in revised form 15 July 2004; accepted 25 October 2004 Available online 2 December 2004 Communicated by: Associate Editor Matheos Santamouris
Abstract In urban areas, the climatic loads on buildings in summer conditions are largely affected by solar radiation. In this paper a modified simplified method for radiant interchange determination is used in a solar energy study. The good agreement with the radiosity method allows one to use this simplified method in the street canyon case. In a building pilot study, parametric analysis and building thermal behaviour can be assessed by simplified models which are useful for long-period simulation. Then this radiant interchange model is introduced in a zonal model of a canyon street and performed with a variable climatic conditions show case. The solar radiation is the only driving force in the street air movement. The interest of such approach for complex coupled phenomena studies is highlighted by obtained results and the assessment of variable climatic loads for different building zones can be considered with the model detailed herein. Future developments are planned in order to improve simulation accuracy by the addition of other local phenomena. 2004 Elsevier Ltd. All rights reserved. Keywords: Urban micro-climate; Solar radiation; Radiant interchange model; Zonal model
1. Introduction The solar irradiation is a noteworthy thermal load for buildings, especially in urban context where most surfaces have low albedo (Akbari et al., 2001). It is beside a significant factor in the phenomenon called ‘‘urban heat island’’ (Landsberg, 1979; Karl and Jones, 1989; Goodridge, 1992; Heino, 1999) which is critical in summer period. Due to the specificity of streets geo-
*
Corresponding author. Fax: +33 5 46 45 82 41. E-mail address: [email protected] (E. Bozonnet).
metry, reflected irradiations amplify this heat concentration. Alternative solutions for the thermal comfort in buildings are developed, such as natural ventilation, which reduce buildings energy consumption due to airconditioning systems (Kolokotroni et al., 2002) and the heat island contribution. Natural ventilation potential and wind flows were extensively studied in urban areas (Clifford et al., 1997; Allard and Santamouris, 1998; Carrilho da Graca et al., 2002; Jiang and Chen, 2002). However natural ventilation systems are tricky to design due to the low wind speed which contributes to the effect of urban heat island (Mihalakakou et al., 2002). This was pointed out during the European project
0038-092X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2004.10.007
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
11
Nomenclature ai /e qi si Ei Edif Edir
short-wave absorption coefficient of the facet i (–) incoming short-wave flux (W) short-wave reflection coefficient of the facet i (–) short-wave transmission coefficient of the facet i (–) total irradiance of the facet i (W/m2) solar diffuse irradiation (W/m2) solar direct irradiation (W/m2)
URBVENT (Santamouris et al., 2001) in which wind speed measurements were carried out in different Athens streets. Specific wind models were developed for urban context (Hotchkiss and Harlow, 1973; Nicholson, 1975) but it appears to be restricted by local turbulence and thermal effects. Actually, in urban areas a turbulent sub-layer is created by the irregular obstacles and with the confinement effect thermal forces induced by solar irradiation have to be modelled. As a result, a local model taking into account the thermal and the mass transfers is necessary to define realistic climatic conditions near buildings. This study focuses on the street canyon case and solar impact on radiation trapping and buoyancy flows. The canyon shape is common in European cities and its confinement effect is significant. The final aim of our study is to assess the impact of thermal transfers between street and constructions for a full summer period assuming no wind effect in a first approach. Simplified approaches were chosen in order to analyse the impact of dynamic coupled climatic phenomena and with outlook of larger zone and parametric studies which can be helpful for urban planning.
2. The zonal model of a street canyon Solar energy can have high impact on indoor temperature in summer conditions, as shown a sensibility study made by Lauret et al. (2001). Solar radiance at atmosphere limits is about 1370 W/m2 and fluctuates during year (Cadiergues, 1998). A part of this energy is absorbed by atmosphere and at ground level solar energy results therefore from direct irradiation Edir and diffuse irradiation from sky Edif. In our study, the solar diffuse irradiation is calculated with an isotropic sky model and more accurate ones, taking into account the anisotropic characteristic of sky like the model of Perez et al. (1987), are not necessary for the required accuracy. Nevertheless some of these models are implemented in the solar irra-
Eie E0i Exi Exi0 Eiext Fji Qu R Si SFi
solar irradiance of the facet i (W/m2) interchange flux of the facet i (W/m2) total exitance of the facet i (W/m2) primary exitance of the facet i (W/m2) external irradiation of the facet i (W/m2) view factor between facet j and i (–) mass flow through a cell interface u (kg/s) radiative equilibrium remainder (W) surface of the facet i (m2) surface of the fictitious facet Fi (m2)
diation simulation software Solene (Miguet and Groleau, 2002) used in our study. Comparison of Solene results with experimental data for a street canyon revealed good agreement (Vachon, 2001). In this paper, as a first approach, a simplified street canyon (Fig. 1a) is considered. Low wind speed conditions are assumed so the thermal effect inside street cannot be neglected. This street is supposed long enough to neglect boundary effects (W/L 1 and H/L 1). Inside the studied zone solar effects and others heat and mass transfer are modelled for a summer period. In this kind of problem, due to the lack of accurate data about the boundary conditions, dynamic simulations are uneasy to achieve with classic fluid dynamics codes. Therefore, a zonal approach with empirical models integration was developed (Musy et al., 2001; Bozonnet et al., 2002). This meshless method produces accurate enough results for building energy consumption and comfort evaluation. The volume of the studied zone is partitioned into smaller entities called ‘‘cells’’ (Fig. 1b). For one cell there are six faces indexed u crossed by mass and heat fluxes in or out the cell volume V. With Qu the mass flow through the face u and, assuming that air is incompressible, the mass balance can be expressed by X Qu ¼ 0 ð1Þ The cell heat balance is expressed likewise, with q the cell air density, C the specific heat of air, T the air temperature, /u the heat transfer through interface u and /source the heat production into the cell: oT X þ /u ¼ /source qCV ð2Þ ot The cell air state parameters are supposed to be determined by the perfect gas law. At last the air mass flows are determined by the pressure gradient between cells (Musy et al., 2001) and density variation with temperature lead to a thermal buoyancy term in airflow calculation.
12
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 1. Studied zone in a street canyon with a W/H aspect ratio: (a) studied zone and (b) partition of street canyon into cells for zonal model.
Wall and ground conduction and heat storage are determined by a one dimensional model (modal reduction). The convective heat flux with surfaces is then included in cells energy balance. The long- and short-wave radiant interchange in buildings is then determined by the fictitious surfaces method (Walton, 1980; Musy et al., 2001), this method is detailed hereafter. Wall surfaces are partitioned into smaller area called ‘‘facets’’. These facets matching the surface of cell sides are about one square meter. The short-wave irradiation is calculated for each facet with • a view factor for mask effect, • a total primary irradiation Ei (W/m2) for the time and the considered location, including the direct and diffuse irradiation, • the walls radiant interchange. In this study, this approach is applied for outside walls subjected to solar irradiation.
3. Different methods for solar irradiance calculation Incident solar flow on each exterior surface is more or less reflected depending on surface colour or its radiant characteristics. Different approaches to calculate the short-wave radiant exchange between surfaces exist. At first in this study, the radiosity method which provides accurate results has been used. It needs the calculation of view factors between surfaces. This method can lead to many equations, which have to be solved simultaneously. The WaltonÕs simplified method (Walton, 1980), or fictitious surfaces method, seems then to be an interesting alternative for buildings simulations (Allard et al., 1985; Musy et al., 2001). The results of both methods are compared here for the studied street canyon. 3.1. Radiosity method Considering air transparent toward short-wave radiations, short-wave irradiance Ei (W/m2) on a facet i, sur-
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
13
Fig. 3. Fictitious facet Fi definition for the facet i. Fig. 2. Radiant parameters for the facet i surrounded by n 1 facets j. 2
face Si (m ), is the sum of the solar incident irradiance and the interchange flux E 0 i (Fig. 2): Ei ¼ Eie þ E0i
Eie
Exi ¼ Exi0 þ qi E0i
ð4Þ
and Exi0 ¼ qi Eie þ si Eiext
ð5Þ
qi and si are the reflection and transmission coefficients of facet i and Eiext (W/m2) is the short-wave irradiance of the other side of the facet (directed from the exterior of the studied zone). From (3) and (4) we obtain Ex ¼
Exi0
i
þ qi ðE
Eie Þ
ð6Þ
The interchange fluxes are also determined with view factors Fji between studied zone surfaces j and surface i by the relation: X X S i Ei ¼ S i Eie þ F ji S j Exj ¼ S i Eie þ F ij S i Exj ð7Þ j
i.e. Ei Eie ¼
X
X
F ij Exj
ð9Þ
j
Thus, the determination of each exitance needs these simultaneous equations solving. 3.2. Fictitious surfaces method
ð3Þ
For each facet i the total exitance1 Exi is the sum of reflected part of the interchange flux and the primary exitance Exi0 (W/m2) from this surface:
i
Exi ¼ Exi0 þ qi
F ij Exj
j
ð8Þ
j
For n facets i, n simultaneous linear equations are then obtained:
This simplified method from Walton (1980) permits a fast resolution of radiative balance for a closed domain. For each facet i a fictitious facet Fi is associated (Fig. 3), with a fictitious surface SFi (m2), a fictitious reflection coefficient qFi and a fictitious transmission coefficient sFi. The fictitious facet Fi takes the place of all facets j which contributed to the part of radiant exchange with facet i, this way j is not in the same plane that i for a parallelepipedal zone. The characteristics of Fi depend on facets j ones by the following relations: X SF i ¼ Sj ð10Þ j=j62P ðiÞ
P
j=j62P ðiÞ qj S j
qF i ¼
ð11Þ
S Fi P
ExF0 i ¼
j j=j62P ðiÞ Ex0 S j
ð12Þ
S Fi
The advantage of this method is to determine the exitance for each facet basing only on the fictitious surface characteristics. Then (Fig. 4) the total exitance of facet i can be expressed from primary exitances Exi0 and ExFi 0 (W/m2): qFi S i Exi0 þ S Fi ExF0 i S i Exi ¼ S i Exi0 þ qi F F ii ð13Þ 1 qF i ðqi F F ii þ F F iF i Þ i.e. ExF i
1
Radiant exitance: The radiant flux per unit area emitted from a surface (from http://www.photonics.com/dictionary).
i
Ex ¼
Exi0
þ qi F F ii
0 qF i Exi0 þ F F ii
1 qF i ðqi F F ii þ F F iF i Þ
ð14Þ
14
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 4. Interchange details for primary exitance from facet i and fictitious facet Fi, Exi0 and ExFi 0.
From Eqs. (6) and (14), we obtain ExFi 0
Ei ¼ Eie þ F Fii
qFi Exi0 þ F Fii 1 qFi ðqi F Fii þ F FiFi Þ
ð15Þ
Finally the irradiance of facet i is expressed only with primary exitances and surfaces characteristics. The simplification made by this method induces an imbalanced radiative equilibrium between incoming short-wave flux /e (W) inside the studied zone and the outgoing flux. The remainder called R is calculated from X /e ¼ ðai þ si ÞEi S i þ R ð16Þ i
In order to obtain a balanced equation, an iterative calculation is done. The remainder R is then redistributed in the outgoing fluxes (Walton, 1980; Musy et al., 2001), considering that the more a surface is absorbative or transparent for short-wave radiations, the more the corresponding part of the remainder is important. This is rendered by the new value of absorbed and transmitted flux for a facet i: ðai þ si ÞR ðai þ si ÞEi ¼ ðai þ si ÞEi þ P ðaj þ sj ÞS j
ð17Þ
j
The radiative equilibrium (16) is calculated, and the repartition of the remainder R is done again until the wanted precision is obtained.
In this approach, computing time and resources are significantly reduced, but fluxes repartition is losing accuracy, depending on view factors values. An evaluation of this method has been conducted.
4. Comparison of solar computation methods 4.1. Studied case Both methods exposed in the previous part are applied here to a canyon street slice as shown in Fig. 1. The street is 200 m long, 9 m width, 9 m high and the studied zone is 1 m depth, boundary effects are then neglected and the street is considered as ‘‘infinite’’. The street axe is supposed to be South/North and latitude of the site is 47 North. The studied volume inside street is shared in three vertical and three horizontal slices, see Fig. 1b. The exterior boundary of both walls and the ground is formed of nine rectangular facets (reflectance coefficient values are set to 0.8) and each facet is triangularized. The total primary incident solar irradiance is equal for the both studied methods. The mask factors and the direct and diffuse incident irradiance are determined with Solene software. Afterwards, the radiant interchange inside the studied zone is computed with both Solene, which is based on the radiosity method, and the fictitious surface method.
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
15
Fig. 5. Total solar fluxes incident and absorbed by walls in the studied zone, hours are in solar time: (a) before and after radiative interchange, (b) comparison between interchange computation with Solene and fictitious surfaces method (curve a and b).
16
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
4.2. The utilization of a modified fictitious surface method The results presented here (Fig. 5) for the summer solstice, show significant differences. Solar energy absorbed by walls after interchange on the whole day differs from about 28%. This gap can be explained by a great value of the radiative equilibrium remainder R which is misrepartitioned by the iterative procedure (17). In this case, the fictitious surfaces are very transparent to radiations due to open boundaries. Without any precise view factor determination, this energy loss
is overestimated and insufficiently corrected by the ratio used in Eq. (17). Then, with a modified repartition of R, Eq. (17) becomes ðai þ si Þ2 R ðai þ si ÞEi ¼ ðai þ si ÞEi þ P ðaj þ sj Þ2 S j
ð18Þ
j
The results obtained with this modification (Fig. 5b, curve b) show that solar energy absorbed by walls after interchange computed by Solene and by the modified fic-
Fig. 6. Solar absorbed flux after interchange computed with Solene and the modified fictitious surfaces method for one wall and ground, hours are in solar time: (a) West wall and (b) ground.
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
titious method differs from about 6%. Moreover, there are slighter flux differences on different walls for the whole day. Considering each walls (Fig. 6), it appears that both evolutions are very similar. The west wall does not get direct sun light between 4 and 11 AM (true solar time), so the absorbed radiation is from diffuse and interchange fluxes. It appears that the modified fictitious surface method overestimates here the interchange flux absorbed by the wall. The same effect is observed for the ground during the morning and the evening, before 8 AM and after 16 PM (true solar time). The global absorbed energy is almost the same, so during the afternoon the modified fictitious surface method underestimate the west wall absorbed solar flux. It means that the primary incident solar flux absorbed by the wall is underestimated. Interchange fluxes are overestimated and this is mainly due to the lack of view factors determination. In this first approach interchange zone was limited to the studied zone. In order to be more realistic, interchange with side panels has to be considered. A Solene computation, see Fig. 7, for a larger interchange zone has been done considering 10 m both sides of studied zone, i.e. a 21 m depth street zone has been triangularized for the radiative interchange study.
17
The results obtained for the same 1 m zone show a greater part of absorbed flux, which mean that street ‘‘confine’’ the solar radiation. For the all day and all walls, this stands for about 23% more energy than in first study. Considering each wall, see Fig. 8a and b, the absorbed direct incident flux seems to be well assessed, but interchange flux computed by the modified fictitious surfaces method appears to be underestimated. This difference is more visible for the west-oriented wall, see Fig. 8a, and can be explained by view factor differences regarding sky and side panels contribution. This confinement effect incorrectly depicted here, amount to less loss by transparent panels, i.e. toward the outside of studied volume. Solar absorbed flux repartition details in west-oriented wall height, as shown in Fig. 9a, highlight this error localisation due to confinement. As confinement is important, F1 facet case shown by Fig. 9b, radiative interchange is predominant and absorbed fluxes are significantly underestimated, 44% for all day absorbed energy. For the middle height facet F2, see Fig. 9c, interchange is underestimated at unexposed to direct solar radiation hours, 10% for all day absorbed energy. For the top facet F3, see Fig. 9d, the same trend is observed in the morning, except for the afternoon as it appears that
Fig. 7. Total interchange solar flux absorbed by studied zone panels computed for studied zone interchange only, Solene and the modified fictitious surface method, and for a larger zone with Solene, hours are in solar time.
18
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
the modified fictitious surface method gives greater values than Solene ones. Overall results show same tendencies during the studied day, however locally errors can be no negligible. With a variation of geometry, for an aspect ratio W/H 5 1, the more the view factors are significant, the more the error made by simplified method is important. Moreover, the previous results appear imprecise for the confined facets. Although the results are precise for one wall, this model has to be refined for a better knowledge of interchange flux repartition. Finally, it appears that fluxes repartition is qualitatively well depicted by the results obtained with this im-
proved fictitious surface method. An improvement of the method is considered in order to take into account the confinement influence on interchange fluxes. Nevertheless parametric studies can be conducted on urban canyons knowing this systematic difference on computed solar fluxes. This method is also used in the next part in order to solve the long-wave radiant interchange, and low computing time is very valuable for the iterative procedure of surface temperature estimation. In urban pilot study, the benefit of assessing buildings and climatic phenomena interaction with parametric studies and simplified inputs can be considered with the proposed method.
Fig. 8. Solar absorbed flux after interchange in a 21-m depth street computed with Solene and the modified fictitious surfaces method for one wall and ground, hours are in solar time: (a) West wall and (b) ground.
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
5. Practical application for a street climate evaluation model Solar effects in a street are mixed with other effects such as convection and walls thermal inertia. For the study of a simplified street canyon, during one day (21 June), we have chosen a zonal approach described in part 2. This kind of model allows long-period studies for large volumes (Musy et al., 2001; Wurtz et al., 2003) and simulations can be run on a personal computer with low computing time. Besides, climatic conditions can be evaluated for different street zones. 5.1. Studied zone and boundary conditions At a regional scale, meteorological data give mean air conditions. However at a local scale, the studied street, there are a lot of local phenomena which have to be taken into account. The studied zone (Fig. 1a) is oriented South/North (a = 0), and it can be considered as bidimensional since there are symmetrical boundary conditions. The street
19
volume is partitioned into cells of 3 m · 3 m · 3 m as shown by Fig. 10. For the studied period, one day, no predominant wind occurs, so air circulation is only due to thermal effects, i.e. mainly solar effects. The solar primary irradiance is computed with Solene, as previously described, and then interchanges are determined by the modified fictitious surfaces method. Long-wave interchange is determined by the same method and the sky longwave radiation is determined with a supposed constant sky temperature Tsky in a first approach. Actually, the daily variation of this sky parameter depends on the air relative humidity and temperature (Ito and Miura, 1989). The inside buildings air temperature is supposed maintained at 20 C and walls energy exchange with cells are computed considering convection, radiation and conduction. Meteorological data give mean air temperature for the geographical zone, so this air temperature Text is maintained in the top part of the studied zone, where it is considered that local phenomena have no influences.
Fig. 9. Detail of solar absorbed flux after interchange for three facets of west-oriented panel in studied zone (a), facets F1 (b), F2 (c) and F3 (d)—curves b, c and d; hours are in solar time.
20
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 10. Canyon street partitioning and boundary conditions for zonal model.
5.2. Simulation results The studied day has been repeated seven times so an established variable regime is obtained. Actually air and wall temperatures are initialized at 20 C, but solicitations and walls inertia lead to a different established regime. Time step is 1-h long, which seems precise enough for observed phenomena. During the night period, as it can be seen in Fig. 11a, there is an air recirculation in the street due to convection exchange. Air is warmed up by walls, and the west-oriented panel is the warmer one at this time, 2 AM (mean time). So hot air is going up aside west wall, which induce an air movement from west to east in the street. Fresh air at 25 C from outside studied zone is then mixed to air from street, as a part of air rises out of street zone. Nevertheless it appears that a part of hot air from street is dragged again in the street zone, this phenomenon inhibits a bit the street night cooling. As walls are refreshing, convective fluxes decrease and in the morning, at 8 AM (mean time), see Fig. 11b, the main convection flux along west-oriented panel is lightened. The sun effect occurs as east panel is exposed, at 9 AM (mean time), see Fig. 12a, so a convective movement is induced in top left zone of the street, reversing the night air circulation. This convective
motion is strengthened during the morning as east panel, then street ground, are heated up by sun (see Fig. 12b and c). As hottest panel place change from left to right side of the street, during the afternoon, air circulation scheme changes (see Fig. 12d). The upper part of east oriented panel is still very hot, due to warm air convection, then there is a vertical rising air circulation near this panel. On the contrary, the lowest part of this panel is cooled. On the other side of the street, the west-oriented wall, sun mounted up heat by panel is delivered to air. So a secondary recirculation is induced in lower part of the street. During the afternoon, as west-oriented wall is warming up, this secondary air circulation increases (see Fig. 13 for 8 PM) amplified by cooling effect of convection on east oriented wall. Obtained results seem physically consistent. However temperature and air speed value are averaged for one hour and temporal variation is faded, but this time step looks acceptable for energy fluxes assessment between the building surfaces and the street. In these simulations, no three-dimensional effect was considered, which would have changed air circulation schemes, downward and upward air fluxes could have been observed in different places. However, it would have been necessary to study a greater domain, large
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
21
Fig. 11. Air temperature and speed for the studied zone: (a) at 2 AM (mean time) and (b) at 8 AM (mean time).
enough to include downward and rising air fluxes places. Another aspect has to be refined here: this is the thermal layer which is not specifically considered here. A more accurate representation of convection near walls, taking
into account roughness elements such as balconies, could give dissimilar results. It appears then that this simplified approach, considering sun effects on street temperatures and air circulation,
22
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
Fig. 12. Air temperature and speed evolution for the studied zone: (a) at 9 AM (mean time), (b) at 10 AM (mean time), (c) at 4 PM (mean time), and (d) at 5 PM (mean time).
can give some consistent results which have to be compared with experimental measures.
6. Conclusions The comparison of the fictitious surface method and a precise one, like radiosity method, shows that the simplified approach for a street canyon case had to be adjusted. A modification in the iterative process in order to equilibrate the radiative balance gave better results for the studied instance. Its low computation time ap-
pears to be beneficial also for the determination of long-wave radiation interchange and finally for the assessment and parametric study of urban microclimate impact on buildings. This modified method was implemented in a zonal simulation code for thermal and mass transfer study. In a first approach, the climatic phenomena were studied for a simplified street canyon during one day without any exterior wind effects. Solar radiation effect and convection were the only driving forces in the street volume. Results show different air circulation paths varying during the day, according to solar position and its thermal effect on walls. From a qualitative
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
23
Fig. 13. Air temperature and speed at 8 PM (mean time) for the studied zone.
point of view these results seem correct. Nevertheless this model has to be refined to take into account other phenomena such as major wind paths and thermal layer near walls. The first objective of this study was to assess an architectural solution toward climatic loads at the very beginning of building project. At this stage, building projects are not advanced enough for a precise knowledge of boundary conditions and the models described in this paper can be advantageously used to assess different building performances in an urban context. References Akbari, H., Pomerantz, M., Taha, H., 2001. Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas. Solar Energy 70 (3), 295–310. Allard, F., Inard, C., Roldan, A., 1985. Caracte´risation the´orique et expe´rimentale du comportement thermique dÕune cellule dÕhabitation perturbe´e par des rayonnements de courtes et grandes longueurs dÕonde, Laboratoire e´quipement de lÕhabitat, INSA de Lyon, September 1985. Allard, F., Santamouris, M., 1998. Natural Ventilation in Buildings—a Design Handbook Altener DG XVII. James & James Ltd. Bozonnet, E., Wurtz, E., Belarbi, R., Allard, F., 2002. Simulation thermo-ae´raulique du microclimat urbain a` lÕe´chelle dÕune rue de type canyon. Proceedings of IBPSA, France, Paris.
Cadiergues, R., 1998. Receuil des de´marches et formules du guide AICVF N 2—Calcul des charges de climatisation et conditionnement dÕair. Carrilho da Graca, G., Chen, Q., Glicksman, L.R., Norford, L.K., 2002. Simulation of wind-driven ventilative cooling systems for an apartment building in Beijing and Shanghai. Energy and Buildings 34 (1), 1–11. Clifford, M.J., Everitt, P.J., Clarke, R., Riffat, S.B., 1997. Using computational fluid dynamics as a design tool for naturally ventilated buildings. Building and Environment 32 (4), 305– 312. Goodridge, J.D., 1992. Urban bias influences on long-term California air temperature trends. Atmospheric Environment. Part B. Urban Atmosphere 26 (1), 1–7. Heino, R., 1999. Homogeneity of the long-term urban data records. Atmospheric Environment 33 (24–25), 3879–3883. Hotchkiss, R.S., Harlow, F.H., 1973. Air pollution transport in street canyons. Technical Report EPA-R4-73-029, NTIS PB-233 252, Los Alamos National Laboratory, US Environmental Protection Agency. Ito, S., Miura, N., 1989. Studies of radiative cooling systems for storing thermal energy. Journal of Solar Energy Engineering 111 (3), 251–256. Jiang, Y., Chen, Q., 2002. Effect of fluctuating wind direction on cross natural ventilation in buildings from large eddy simulation. Building and Environment 37 (4), 379–386. Karl, T.R., Jones, P.D., 1989. Urban bias in area-averaged surface air temperature trends. Bulletin—American Meteorological Society 70 (3), 265–270. Kolokotroni, M., Watkins, R., Santamouris, M., Niahou, K., Allard, F., Belarbi, R., Ghiaus, C., Alvarez, S., SalmeronLissen, J.M., 2002. Passive ventilation cooling in urban
24
E. Bozonnet et al. / Solar Energy 79 (2005) 10–24
buildings: an estimation of potential environmental impact benefits. In: Proceedings of EPIC 2002 AIVC, Lyon, France, pp. 531–536. Landsberg, H.E., 1979. Atmospheric changes in a growing community (the Columbia, Maryland experience). Urban Ecology 4 (1), 53–81. Lauret, A.J.P., Mara, T.A., Boyer, H., Adelard, L., Garde, F., 2001. A validation methodology aid for improving a thermal building model: case of diffuse radiation accounting in a tropical climate. Energy and Buildings 33 (7), 711–718. Miguet, F., Groleau, D., 2002. A daylight simulation tool for urban and architectural spaces—application to transmitted direct and diffuse light through glazing. Building and Environment 37 (8–9), 833–843. Mihalakakou, G., Flocas, H.A., Santamouris, M., 2002. The impact of synoptic-scale atmospheric circulation on the urban heat island effect over Athens, Greece. In: Proceedings of EPIC 2002 AIVC, Lyon, France, pp. 301–306. Musy, M., Wurtz, E., Winkelmann, F., Allard, F., 2001. Generation of a zonal model to simulate natural convection in a room with a radiative/convective heater. Building and Environment 36 (5), 589–596.
Nicholson, S.E., 1975. A pollution model for street-level air. Atmospheric Environment (1967) 9 (1), 19–31. Perez, R., Seals, R., Ineichen, P., Stewart, R., Menicucci, D., 1987. A new simplified version of the Perez diffuse irradiance model for titled surfaces. Solar Energy 39, 221– 231. Santamouris, M., Papakonstantinou, K., Georgakis, C., 2001. Natural ventilation in urban-areas. URBVENT Report, Contribution to Task 1.1.1.–2.1.1. and 2.1.2., March 2001. Vachon, G., 2001. Transferts des polluants des sources fixes et mobiles dans la canope´e urbaine: e´valuation expe´rimentale. Nantes, Ecole Centrale de Nantes: 210. Walton, G.N., 1980. A new algorithm for radiant interchange in rooms loads calculations. ASHRAE Transactions 86 (II), 190–208. Wurtz, E., Deque, F., Mora, L., Bozonnet, E., Trompezinsky, S., 2003. Sim_Zonal: a software to evaluate the risk of discomfort: coupling with an energy engine, Comparison with CFD codes and experimental measurements. In: Proceedings of Eighth International IBPSA Conference, Eindhoven, Netherlands, pp. 1423–1428.