Urban Environment: constraints and opportunities towards a sustainable quality

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Urban Environment: constraints and opportunities

towards a sustainable quality

A. FERRANTE*, A. BAROZZI*, C. MASOTTI*, A. MAVER* AND G. MIHALAKAKOU**

* University of Bologna, DAPT, Department of Architecture and Urban Planning, Faculty of Engineering, Viale Risorgimento 2, Bologna, ITALY

areffe@tin.it

** University of Athens, Department of Physics, Division of Applied Physics, Laboratory of Meteorology, University Campus, Build. Phys-V, Athens, GR157 84, GREECE

Abstract

To investigate the impact of bio-climatic techniques on energy efficiency in urban areas, a series of major factors influencing energy and buildings in urban environments imposing specific restrictions and priorities not encountered in non-urban areas has to be considered: urban historical and geometrical constraints, specific legislation regulating urban spaces, as well as the complex nature of the surfaces in the city lead to both conceptual and practical difficulties.

A new, integrated planning and design procedure for the environmental and climatic rehabilitation of urban areas has been worked out in the framework of a complex case study in the Ex-Staveco area which is a large urban area close to the city centre of Bologna consisting of a huge amount of historical industrial buildings (50.000 square meters) overlapping combined water and natural green systems, for a total densely-built urban zone of about 144.000 square meters.

The design methodology, performing the integration of the individual buildings’ design in the framework of a guide-line master plan defining urban, historical and bio-climatic goals, is essentially based on a threefold rehabilitation procedure considering:

decision-making and strategies of action aimed at a "night and day living" city where Office Buildings, Science and Technological Museums, Hotels, Green Sport Centres, Commercial Centres and Parking Areas ensure a multi-functional use of urban spaces and buildings throughout the different time periods;

large scale planning techniques related to the alternative use of natural resources in the public open areas (promotion of all forms of passive natural devices according to the potential effects of minimising localised pollution problems and enhancing urban microclimate);

passive technological measures involving the envelope of existing buildings as well as the design of new buildings (improvement of solar heating and natural cooling, minimising solar heat gains both in the open spaces and on the building’s facades).

The thermal behaviour of different buildings using passive heating and cooling techniques have been calculated. The present paper shows the thermal performance of one selected building by a transient building’s performance simulation program (TRNSYS, 1997).

Conference topics: case studies

Keywords: architecture, Energy, Green, Natural, Open, Passive, Simulation, Techniques, Temperature, Urban

INTRODUCTION

The present work, developed during the two-years Design Training Course for students of Engineering at the University of Bologna, was designed primarily to investigate the city as an integrated system, consisting of practical and theoretical issues on energy loading of buildings and vegetative organisms, and secondly to apply various important bio-climatic design techniques aiming at improving the microclimate as well as the overall qualitative conditions of a large urban area.

To investigate the impact of bioclimatic techniques on energy efficiency in urban areas, a series of major factors influencing energy and buildings in urban environments imposing specific restrictions and priorities not encountered in non-urban areas has to be considered: urban historical

and geometrical constraints, specific legislation regulating urban spaces, as well as the complex nature of the surfaces in the city lead to both conceptual and practical difficulties.

The study and application of natural passive systems on the city environment is a layered and multi-disciplinary process [1]: it is especially important to treat the subject in conjunction with other aspects of physical parameters involving passive and active components and techniques in relation to architectural and engineering design.

For these reasons the rehabilitation of the urban area has to be multi-dimensional in approach, encompassing enhancements to the physical environment, with a view to improving the quality of life for residents. Thus, the identity of an area should be enhanced, not destroyed, especially in

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terms of cultural heritage and preservation of the positive aspects of existing built and natural environments.

THE URBAN RENEWAL OF THE AREA "EX_STAVECO"

The design procedure

A new, integrated planning and design procedure for the environmental and climatic rehabilitation of urban areas has been worked out in the framework of the Ex-Staveco area which is a large urban area close to the city centre of Bologna consisting of a huge amount of historical industrial buildings (50.000 square meters) overlapping combined water and natural green systems, for a total densely-built urban zone of about 144.000 square meters.

The design procedure, considering the existing historical and urban constraints both deriving from urban conditions and natural-physical aspects to be enhanced, has lead to a general guide-line master plan defining urban, historical and bio-climatic goals: the restriction of the amount of possible solutions resulting from existing urban constraints has been thus combined with the first priority of achieving thermal comfort by natural means in urban spaces.

The significance of landscape and urban re-planning at this scale is undoubtedly evident, considering that the rehabilitation process of buildings cannot exist separately from the restoration process of the whole area. For the same reason the building design methodology has been conceived as an approaching process from the outside spaces to the buildings.

The influence of outdoor spaces on the architectural design procedure is described in detail [2] e [3]. The approach of conceiving the volumes to be built as the negative counterparts of the positive open spaces working from the outside inwards has been applied throughout the planning and architectural procedure of the present case study.

The design methodology is essentially based on a threefold rehabilitation procedure considering:

• decision-making and strategies of action aimed at a "night and day living" city where Office Buildings, Science and Technological Museums, Hotels, Green Sport Centres, Commercial Centres and Parking Areas ensure a multi-functional use of urban spaces and buildings throughout the different time periods;

• large scale planning techniques related to the alternative use of natural resources in the public open areas (promotion of all forms of passive natural devices according to the potential effects of minimising localised pollution problems and enhancing urban microclimate);

• passive technological measures involving the historical existing buildings' facades as well as the design of new buildings (improvement of solar heating and natural cooling, minimising solar heat gains both in the open spaces and on the building’s facades).

Climatic and environmental rule of natural components and passive techniques

Plants have a strong effect on climate: trees and green spaces can help cooling our cities and save energy. Trees are able to provide solar protection to individual houses during the summer period while evapotranspiration from trees can reduce urban temperatures. Trees also help mitigate the greenhouse effect, filter pollutants, mask noise, prevent erosion and calm their human observers. Moreover, evapotranspiration from soil - vegetation systems can remarkably reduce urban temperatures. Shading from trees is an effective way for reducing significantly the energy for cooling purposes.

Results of computer simulations aimed at studying the combined effect of shading and evapotranspiration of vegetation on the energy use of several typical one-storey buildings in US cities have showed that by adding one tree per house, the cooling energy savings varied from 12 to 24 %, while adding three trees per house can reduce the cooling load between 17 to 57 percent. According to this study, the direct effects of shading account for only 10 to 35 % of the total cooling energy savings. The remaining savings result from temperatures lowered by evapotranspiration.

The evaporative and transpiration cooling effects of plants are used to pre-cool ventilation air, for buildings and open spaces. Dense vegetation furthermore helps to filter particles from air.

The air temperature and the relative humidity are reduced as the air passes through the foliage covering the roof. The plants, because of the biological functions of photosynthesis, respiration, transpiration and evaporation, absorb a significant proportion of solar radiation, creating a protective layer and thus reducing temperature during summer.

The climatic conscious design of outdoor spaces and the appropriate use of natural components are key elements to reduce the outcome of unsound evolution of urban zones such as the Ex-Staveco Area, where large surfaces of vegetation can improve the thermal performance of the whole area, while offering a public refreshing area close to the city centre.

The main objectives identified in the overall rehabilitation procedure of the Ex-Staveco area can be summarised as follows:

• Promotion of all forms of passive natural devices according to the potential effects of minimising localised pollution problems and enhancing urban microclimate;

• Increase of permeable surfaces in the outdoor spaces;

• Improvement of solar heating and natural cooling, minimising solar heat gains both in the open spaces and on the building’s facades (allowing protection for south-east and south-west facades by means of shading devices and “natural filters”).

• Ensure CO2 absorption by plants and urban pollutant

dispersion by natural ventilation and night cooling. To achieve the above objectives the following strategies have been proposed:

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• Ensure "openings" from existing green parks outside the area (where a higher portion of fresher and cleaner air is available) towards public open spaces and buildings’ facades by means of proposed pedestrian green ways (climatic corridors) considering the direction of prevailing summer breeze crossing the area Ex-Staveco;

• Increase as much as possible permeable surfaces on the pavements thus enhancing the water-green-climate interactions;

• Provide shading and filtering for the buildings’ facades (especially the south-west oriented ones during the warm period of the year) by means of vegetation, water ponds and water river extension in pools eventually equipped with macrophitic treatment of waste water;

• Provide spatial separation between polluted areas and open public spaces;

• Ensure openings from the pedestrian areas towards the buildings’ facades;

• Provide shading and filtering for the buildings’ facades (especially on the south-west orientation in warm summer season and north-east ones in cold winter season) by means of selected passive techniques such as winter gardens, pergolas, deciduous trees and curtains.

The urban area has therefore been restored by means of various design previsions such as:

• Definition of potential ecological networks (green belts and bio-climatic corridors as "widespread quality" pattern) where existing (or designed) open green areas inside and outside the area boundaries are support links in a system of quality interconnected elements;

• Partial demolition of the industrial buildings without urban and historical significance presenting very high levels of unhealthy conditions;

• Re-balance the environmental processes aimed at the climatic improvement in the relatively free open spaces (existing or in prevision);

• Urban rehabilitation of the main crossing central routes towards the historical centre as pedestrian galleries crossing the commercial, social and public activities;

• Insertion of selected passive systems such as atrium between existing historical buildings along the high circulation streets intimately connected to the whole urban rehabilitation.

Figure 1. Schematic aereal view of the new urban plan in the Ex-Staveco area of Bologna

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Figure 2. Schematic 3-D section of the new underground railway station used for the thermal performance simulation by TRNSYS.

THE SUNSPACE AS A PASSIVE HEATING/COOLING SYSTEM

Sunspaces and atriums as architectural techniques have been throughout employed in the design and rehabilitation of the main buildings in the Ex-Staveco area.

Sunspaces represent acceptable ecological solutions contributing to the reduction of the thermal loads of the building’s envelope and to the improvement of microclimatic conditions of public urban spaces.

The glass covering systems between the coupled buildings fronting the high circulation street, for example, lead to a thermally compact building during the winter period where heat losses from the buildings are reduced, while the passive gained solar energy can be very useful.

In the new underground railway station the atrium contributes significantly to the energy balance of the office buildings as well as to the thermal performance of the open public space itself. To achieve an improved thermal performance in the winter, that is to gain heat from solar radiation, the atrium has been assumed closed by two glazed entrances, while for avoiding the undesirable overheating during the summer period of the year the following bioclimatic techniques have been considered:

• the openings of glazed entrances should facilitate ventilation techniques during the summer;

• additionally, the openings at the top have the important task of extracting the hot air.

• accordingly, the outdoor air is allowed to enter inside the atrium and remove the stored heat which is trapped during the day;

• also, the air movement from underground tubes and volumes provides a considerable increase of heat dissipation from the ground towards the atrium itself as well as the building materials, and the warmer air is exhausted into the low temperature atmospheric heat sink.;

• insertion of curtains as additional selected passive systems addressing the cooling need of the buildings and the atrium during summer. These shading devices can be useful for avoiding the overheating process, by reducing direct solar gain.

CALCULATION OF THERMAL PERFORMANCE

The thermal behaviour of the railway station was calculated using the TRNSYS environment (*). TRNSYS is a transient system program with a modular structure which facilitates the addition to the program of mathematical models not included in the standard TRNSYS library. The whole building was considered to be one thermal zone, with floor on the ground. The building is connected, on its south side, with an atrium, (Fig. 2). Simulations have been performed using an hour as the time interval taking into account that all measurements were taken on an hourly basis. Hourly values of the following climatic parameters for the city of Bologna, have been used for the calculations:

Ambient air temperature, (°C) Global solar radiation, (W/m2)

Diffuse solar radiation (W/m2)

Relative humidity, (%)

Figure 3 shows the ambient air temperature values for one representative day of July as well as the indoor air temperature values calculated inside :

1. the building when it is not connected with the atrium 2. the same building connected with the atrium.

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Figure 3. Thermal performance for a representative summer day.

From Figure 3, it can be seen that the ambient air temperature values varied between 14.1 and 35.2°C, the indoor temperature inside the building not connected with the atrium fluctuated between 13.8 and 33.2°C, while the indoor temperature of the building connected with the atrium varied in the range of 11.5 and 28.2°C.

Accordingly, Figure 4 shows the ambient air temperature values for one representative day of January as well as the indoor air temperature values calculated inside:

• the building when it is not connected with the atrium

• the same building connected with the atrium.

As shown, the ambient air temperature values varied in the range of 3.2 and 10.2°C, the indoor temperature inside the building not connected with the atrium fluctuated between 3.9 and 10.9°C, while the indoor temperature of the building connected with the atrium varied in the range of 4.7 and 13.5°C.

Figure 4. Thermal performance for a representative winter day.

CONCLUDING REMARKS

A new, integrated planning and design procedure for the environmental and climatic rehabilitation of urban areas is presented in this paper. The methodology is essentially based on a rehabilitation procedure considering both large scale landscape planning techniques related to the alternative use of natural resources such as water and green and passive technological measures involving the envelope of historical buildings. The overall basis of the architectural measures proposed in the present case study lies in the value of green, water and selected passive techniques to improve microclimate in the urban sites.

Furthermore, passive selected techniques such as sunspaces have been throughout employed in the design and rehabilitation of the main buildings in the Ex-Staveco area.

An extensive validation procedure has been followed describing the thermal performance of the atrium and one representative building during the summer and winter period of the year.

From the obtained results it is found that:

As expected, the air temperature inside the atrium is satisfyingly higher than the external ambient temperature in the winter and similarly lower during the summer, thus demonstrating the gallery to be a pleasant public open space. The thermal performance of the connected building is therefore strictly influenced, governed and improved by the presence of the atrium itself.

ACKNOWLEDGMENTS

Figure 1, representing the plan-volumetric view of the area, was designed by A. Ferrante and drew by F. Giulianelli. Fig. 2 has been designed by F. Giulianelli and revised by A. Ferrante. The authors would like to express their gratitude to F. Giulianelli for his valuable help.

REFERENCES

Santamouris, M., N. Papanikolaou, I. Livada, I. Koronakis, C. Georgakis, A. Argiriou and D. Asimakopoulos (1998), On the impact of urban climate on the energy consumption of buildings,

Sol. Energy.

Tombazis A.N. (1995) The design of exterior spaces in relation to their effect on the indoor climate of buildings, Proceedings of the International Symposium Passive Cooling of Buildings, Athens, Greece, pp. 83-88.

Bitan A. (1992), The high climatic quality of the city of future, Atmospheric Environment 26B, pp. 313-329.

Goulding J., Lewis J.O., and Steemers T.C. (1993), Energy in Architecture - The European Passive Solar Handbook, E.U. Publication, Batsford, London.

(*) TRNSYS 14 (1997), A transient system simulation programme developed from Solar Energy Laboratory, University of Wisconsin, Madison, and Transsolar, Stuttgart.

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