DESIGN RECOMMENDATIONS FOR NEW COURTYARD BUILDINGS

Transcription

1 DESIGN RECOMMENDATIONS FOR NEW COURTYARD BUILDINGS IN COMPACT HISTORICAL CENTRE OF HAVANA Abel Tablada 1 Department of Architecture, National University of Singapore, Singapore 4 Architecture Drive, Singapore Corresponding Author akiaetdl@nus.edu.sg, Tel: (65) , Fax: (65)

2 DESIGN RECOMMENDATIONS FOR NEW COURTYARD BUILDINGS IN COMPACT HISTORICAL CENTRE OF HAVANA ABSTRACT The Historical Centre of Havana in Cuba is a compact low-rise urban site declared World Cultural Heritage in 1982 by UNESCO. The Master Plan of the Historical Centre comprises the construction of residential buildings in the empty plots to allocate surplus population from overcrowded houses and from dilapidated and valuable colonial buildings. However, the new housing typologies, while increasing the gross floor area in comparison with previous colonial buildings, should also ensure proper environmental conditions and thermal comfort by maximising the potential for natural ventilation. The aim of this paper is to present preliminary design recommendations for new low-rise residential courtyard buildings inserted in the Historical Centre of Havana. The recommendations are based on previous studies on microclimatic measurements, comfort survey, Computational Fluid Dynamics simulations, thermal simulations and comfort analysis on a series of combinations of courtyard building prototypes. The recommendations aim to promote the design of thermally comfortable naturally-ventilated residential buildings in the Historical Centre of Havana in particular and in other compact lowrise urban areas in tropical-humid regions in general. Keywords: Courtyard buildings; Design recommendations; Natural ventilation; Thermal comfort; Tropical architecture. 1. INTRODUCTION The Historical Centre of Havana (Old Havana) in Cuba is a compact urban site declared World Cultural Heritage in 1982 by UNESCO. It is located on the west side of Havana s harbour at north and west, very close to the Tropic of Cancer as shown in Figure 1. It has a density of 30,000 inhabitants per km 2 (National Office of Statistics, 2011) plus a floating population of 37,000 per km 2 in an area of 2.14 km 2. Climatic conditions in the city are influenced by the sea with a combination of relatively high values of air temperature (August mean maximum: 31.4 C) and high values of relative humidity (August mean maximum: 91%). With a distinguished wet and dry season, Havana belongs to the Tropical savanna climatic zone according to Köppen s classification. Despite the comprehensive recovering plan that has been undertaken during more than 3 decades in the Historical Centre, there is still a significant amount of buildings in disrepair and empty plots inside the boundaries of the former intramural city (Fig. 1c). The Master Plan of the Historical Centre (Office of the City s Historian, 1998) comprises the construction of residential buildings in the empty plots to allocate surplus population from overcrowded houses and from dilapidated and valuable colonial buildings. However, the new housing typologies, while increasing the gross floor area in comparison with previous colonial buildings, should also ensure proper environmental conditions and thermal comfort by maximising the potential for natural ventilation. The application of natural ventilation strategies helps to prevent the use of air-conditioners in the new housing, contributing in this way to diminish the energy use and the effects of the urban heat island and the green-house gas emissions at local and global scales respectively. 2

3 Numerous studies have proposed design strategies for buildings in tropical and humid climates. However, most of the design recommendations (e.g. Koenigsberg et al., 1973; Lippsmeier, 1980; Givoni, 1998) are based on the assumption that cities in such climates have spread-out low-density urban environments. In recent years several studies have focused on urban design guidelines for tropical high density and high-rise urban environments, especially for East Asia (e.g. Ng, 2010; Cheung and Liu, 2011; Yuan and Ng, 2012). However, nor the spread-out neither the high-rise urban environments are representative of Old Havana and other tropical cities in Latin America, Africa and Asia. Figure 1: a) Location of Havana, b) typical street in Old Havana, c) satellite view of the Historical Centre of Havana (inside doted lines is the former intramural city). At the building scale, most studies related to courtyard buildings considered hot and dry climates and focused on their thermal performance rather than on the airflow conditions. Literature related to courtyard buildings in tropical humid contexts is scarce. Bittencourt and Peixoto (2001) and Rajapaksha et al. (2003) performed CFD simulations for a building and a house with a courtyard. In addition, Murakami et al. (2004) conducted a study on a porous-type building for a compact urban area of Hanoi. However, in these studies the buildings are fully or partially isolated and have openings in their exterior envelope in contrast with the situation of the present study in which the courtyard building is located in a very compact urban environment with openings mainly in the inner courtyard walls. The purpose of this paper is, therefore, to present preliminary design recommendations for future low-rise residential courtyard buildings inserted in the Historical Centre of Havana. The recommendations are based on microclimatic measurements and a comfort survey reported in Tablada et al. (2009) and on Computational Fluid Dynamics (CFD) simulations, thermal simulations and comfort analysis (Tablada et al., 2006) on a series of combinations of courtyard building prototypes. 3

4 2. MORPHOLOGY AND PLOT TYPES The urban morphology in the Historical Centre can be described as compact low-rise. The street pattern is semi-orthogonal and the parcel system shown in Figure 2a is one of shared party walls with elongated plots. The buildings occupy most of the plot area leaving only 15% to 20% of open space for inner courtyards and air/light shafts. The oldest residential buildings have one (4 5 m high) or two stories (8 10 m), while the apartment buildings have three (9 12 m) or four (12 15 m) storeys. Plot ratios vary between 1.5 and 2.5. The street canyons are about 7 10 m wide having width/height (W/H) aspect ratios from 1.2 to 0.5 (Fig. 1b). The compact nature of the urban structure and the presence of party walls for almost all buildings allowed limiting the natural ventilation and thermal comfort study to a selected number of generic building and courtyard configurations. In Tablada et al. (2009) a morphological subdivision of the Historical Centre was made and three typical plots were selected from two representative sectors. Afterwards, potential building layouts for each typical plot were explored by using a horizontal modular grid for rooms and inner courtyards (Fig. 2b). The horizontal modules are 3.5 m by 3 m and determine the room and courtyard dimensions. The width (W) of the courtyard is 3 m, 6 m or 9 m and the courtyard depth is 3.5 m, 7 m, or 10.5 m. In the selected configurations, the buildings have three floors with a total height of 9 m. Figure 2: a) Typical urban block with courtyard buildings in Old Havana, b) three representative plot types with modular subdivisions. For the CFD simulations, cases with a single or a double courtyard were considered. The aspect ratios of the courtyards were W/H = 0.33, 0.66 and 1.0. For the thermal simulations and comfort analysis, the combination of a single room with its adjacent courtyard(s) was considered. For single-side ventilated (SV) rooms, four window orientations were considered: east-northeast (ENE), south-southeast (SSE), westsouthwest (WSW) and north-northwest (NNW). These orientations coincide with the actual orientation of building blocks in Old Havana. For cross-ventilated (CV) rooms, the two possible orientations were considered: ENE-WSW and SSE-NNW. 3. SUMMARY OF RESULTS FROM CFD AND THERMAL SIMULATIONS In this section the results from the natural ventilation and comfort studies in the generic buildings three storeys with one or two consecutive courtyards- inserted in the compact urban environment of the Historical Centre of Havana are summarised. CFD 4

5 simulations (by Fluent Inc, 2003) were performed to obtain the values of indoor air speed and pressure coefficients required for the thermal and comfort simulations (by EnergyPlus, 2001). Thermal comfort was analysed by using the extended Predicted Mean Vote (PMV) index adapted to regions with warm conditions (Fanger and Toftum, 2002). For a detailed description of the simulations methodology and results the reader is referred to Tablada et al. (2006) NATURAL VENTILATION EVALUATION USING CFD SIMULATIONS The aspect ratio (W/H) of the courtyard influences the indoor air speed values, with the exception of the air speed inside the upstream rooms. A single courtyard with W/H = 0.66 provides higher indoor air speed than a narrow courtyard with W/H = Two consecutive wider courtyards (W/H = 0.66) provide higher indoor air speed than two courtyards of W/H = 0.33 for the central CV rooms. In general the SV rooms have very low indoor air speeds (< 0.1 m/s) while the presence of more than one courtyard can provide significantly better ventilation for the central CV rooms ( m/s). However, this improvement is different among the three floors and almost insignificant for the remaining SV rooms in the two-courtyard building THERMAL COMFORT ANALYSIS For cases with single-side ventilation, ground-floor rooms were always cooler than top rooms, independently of the orientation and shape of the courtyard. The slightly higher air speed of the top-floor SV rooms in comparison with the ground-floor SV rooms seems to be not enough to counteract the influence of the solar radiation on the roof and wall facade for these specific prototypes. On the other hand, the higher air speed values provided by cross-ventilation strategies improve thermal comfort under warm conditions even if the exterior air temperature is quite high, i.e. in the range of 26ºC to 32ºC. For rooms that are more protected from direct and indirect solar radiation, like ground-floor rooms, higher air speed values are not as crucial as for the top-floor rooms if cooking activity is not considered. The best thermal comfort on top-floor rooms is achieved by providing cross ventilation through wider courtyards, by the use of exterior louvers and by orienting the room towards the SSE and NNW. Therefore, with the application of these strategies thermal conditions during the summer improve in terms of PMV from 1.2 to 0.87 and in terms of Effective Temperature (ET*) from 32.4 C to 31 C which is close to the upper limit of Old Havana summer comfort conditions (ET* = 30.6 C) according to Tablada et al. (2009). 4. GENERAL DESIGN RECOMMENDATIONS FOR COURTYARD BUILDINGS In this section a summary of the general recommendations for the design of naturallyventilated courtyard buildings in Old Havana are presented. The recommendations are distilled from the previous studies which included field measurements and thermal sensation survey, CFD simulations and thermal comfort analysis (Tablada et al. 2006, 2009). Figure 3 and 4 illustrate several building layouts for each plot type out of around 200 analysed. Detailed recommendations for each of the three plot types and with several open space ratios will be presented in a future publication. 5

6 Figure 3: Schematic representation of some possible building layouts (out of 160 considered) for plot type 1 (a, b, c) and plot type 2 (d, e, f) with open space ratio = 0.3. The modules in grey represent the courtyard area and the modules with lines represent the rooms with cross ventilation by direct contact with two courtyards or through an adjacent room. Figure 4: Schematic representation of some possible building layouts (out of 32 considered) for plot type 3 with open space ratio = 0.3. The modules in grey represent the courtyard area and the modules with lines represent the rooms with cross ventilation by direct contact with two courtyards or through an adjacent room GENERAL RECOMMENDATIONS 1. Building geometry Open space ratios > 0.25 are recommended. Two types of courtyards are recommended: (1) a wide courtyard (0.6 W/H 1.0) to ensure enough daylight and ventilation inside main rooms (living rooms, dining rooms and bedrooms), to provide some greenery, circulation and access to apartments and (2) narrow courtyards (0.3 W/H 0.5) for services rooms and as a secondary opening for the main rooms to ensure CV. Ventilation shafts may also be a third option to ensure CV if space is limited. An interconnection between the street and the courtyards and among the main courtyards through a circulation path on ground floor is recommended. For plot type 1 and 2 (Fig. 3), buildings with courtyards equivalent to two modules should preferably have less than around 9 m height (1 to 3 floors). For plot type 3 (Fig. 4), buildings with a wide courtyard (W > 6 m) could have a maximum height of 11 m (3-4 floors). If one or two wide courtyards (W 7 m) are linked with the street, the maximum height of the building could be 14 m. 6

7 2. Orientation The orientation of the courtyard s main axis should prioritize the climatic factors (prevailing winds, solar radiation) rather than following the actual plot orientation (e.g. in Figure 3, case c and f, and in Figure 4, case d ). Windows orientation on top-floor rooms should be taken into account according to the function of the room. Daytime rooms like living-rooms should preferably be oriented towards the SSE-NNW. On the contrary, top-floor bedrooms should preferably face ENE for SV cases but for CV cases they should face SSE-NNW in order to avoid solar radiation on the WSW orientation. Kitchens and bathrooms can be oriented to WSW. If oblique courtyard walls and windows are possible, then orientations ranging clockwise from NNW to NNE and from SSE to SSW are preferable. For plot type 1 (Fig. 3), cases a and b are preferable for buildings with ENE and WSW facade orientations. Case c is better for NNW and SSE. For plot type 2 (Fig. 3), case e is preferable for buildings with ENE and WSW facade orientations. Case f is better for NNW and SSE facade orientations. For plot type 3 (Fig. 4), the longest courtyard s facades should preferably be oriented towards NNW and SSE. 3. Ventilation and solar protection Cross ventilation is recommended for the main daytime-use rooms like the living-room and dining-room. Cross ventilation on bedrooms is more effective and necessary on top-floor than on ground and middle-floor bedrooms. A connection between SV rooms by internal openings or by sliding walls is recommended in order to provide cross ventilation when required. Wider courtyards (0.6 W/H 1.0) should have proper sun shadings over windows and courtyard walls (e.g. exterior horizontal louvers or pergolas with wines). The use of movable louvers as part of the window system is recommended for the main rooms on every floor. Sun shadings should protect, at least, the top-floor rooms facing the narrow courtyards (0.3 W/H 0.5). The use of transitional spaces around courtyards like galleries and balconies should be promoted in order to avoid direct solar radiation and brusque changes in radiant temperature and daylight values. 5. CONCLUSIONS In this paper, general design strategies to favour natural ventilation and thermal comfort inside residential buildings in Old Havana have been recommended. The design strategies are based on previous studies in which field measurements, thermal comfort survey, CFD simulations, thermal simulations and a comfort analysis have been conducted for different generic courtyard buildings. The positive effect of the air movement on thermal comfort in warm and humid conditions has been given priority for the elaboration of the recommendations. Higher air speed values provided by cross ventilation strategies improve thermal comfort for the rooms which are more exposed to solar radiation like top-floor rooms and rooms oriented towards ENE and WSW. The cooling sensation of air movement is effective even if the exterior air temperature is in the range of 26º C and 32º C. In the context of Old Havana, three main design strategies should be promoted: (1) to provide cross ventilation in the majority of rooms by means of a sequence of different sizes courtyards linked to the street through a ground-floor circulation path; (2) to use a higher open space ratio than the minimum required by the current urban regulations in 7

8 the Historical Centre, preferably > 0.25 in order to allow wider inner courtyards with 0.6 W/H 1.0; and (3) to provide an efficient solar protection for the wider courtyards including vegetation- in order to reduce the solar heat gain. The results of this and previous studies indicate that adequate design solutions can provide summer indoor thermal conditions within or close to the limits of the comfort zone proposed for Old Havana. The recommendations of this study can be useful for the extension and/or improvement of the current urban regulations of the Historical Centre of Havana. They can also guide the decisions of practitioners working in other compact low-rise urban areas in tropical humid regions. Detailed recommendations for each of the three plot types with a larger number of design options will be presented in a future publication. Further work is needed to evaluate thermal, daylight and energy performance of optimal prototypes for each plot and facade orientation. 6. REFERENCES Bittencourt, L, Peixoto, L.,2001. The influence of different courtyard configurations on natural ventilation through low-rise school buildings. Paper presented at the Seventh International IBPSA Conference. Rio de Janeiro. Cheung, J.O.P., Chun-Ho Liu., CFD simulations of natural ventilation behaviour in highrise buildings in regular and staggered arrangements at various spacings. Energy and Buildings 43, EnergyPlus Energy Simulation Software, DOE Energy Efficiency & Renewable Energy, Building Technology Program, Available from: energy.gov/buildings/energyplus. Fanger, P.O., Toftum, J Extension of the PMV model to non-air-conditioned buildings in warm climates. Energy & Buildings, 34, Fluent Inc Fluent 6.1 User Manual. Givoni, G Climate Considerations in Building and Urban Design. Van Nostrand Reinhold, New York. Koenigsberg O.H., Ingersoll, T.G., Mayhew, A., Szokolay, S.V Manual of Tropical Housing and Building. Longman, London. Lippsmeier, G Building in the Tropics. Callwey, München. Murakami, S., Kato, S., Ooka, R., Shiraishi, Y., Design of a porous-type residential building model with low environmental load in hot and humid Asia. Energy and Buildings, 36, National Office of Statistics (2011). Available from: ]. [Accessed 20 March Ng, E., Designing for Urban Ventilation, In: Ng, E. (ed) Designing High-Density Cities for Social and Environmental Sustainability, Earthscan, London. Office of the City s Historian, Plan de desarrollo integral. Plan Maestro de la Oficina del Historiador, Havana. Rajapaksha, I., Nagai, H. Okumiya, M A ventilated courtyard as a passive cooling strategy in the warm humid tropics. Renewable Energy, 28, Tablada, A, Blocken, B, Carmeliet, J, De Troyer, F, Verschure, H., On natural ventilation and thermal comfort in compact urban environments the Old Havana case. Building and Environment, 44(9), Tablada, A, Blocken, B, Carmeliet, J, De Troyer, F, Verschure, H., Airflow conditions and thermal comfort in naturally-ventilated courtyard buildings in a tropical-humid climate. Paper presented at the 6th International Conference on Urban Climate, Gothenburg, June. Yuan, C., Ng, E., Building porosity for better urban ventilation in high-density cities: A computational parametric study. Building and Environment, 50,