Urban layout and façade solar potential: a case study in the Mediterranean Region

ACE 21 URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE MEDITERRANEAN REGION ALESSANDRA CURRELI AND HELENA COCH ROURA ACE: Architecture, City and Environment = Arquitectura, Ciudad y Entorno [en línea]. 2013, Año 7, núm. 21 Febrero P. 117-132 ISSN: 1886-4805 Website access: http://www-cpsv.upc.es/ace/Articles_n21/articles_pdf/ACE_21_SA_14.pdf UPCommons Access: http://hdl.handle.net/2099/13010 ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 117 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE MEDITERRANEAN REGION CURRELI, Alessandra 1 COCH ROURA, Helena 2 Remisión inicial: 04-01-2013 Remisión definitiva: 16-1-2013 Key words: Urban building block, street network orientation, urban canyon aspect ratio, vertical solar potential Abstract The access of direct radiation on building façades and the consequent opportunities for the passive use of solar energy mostly depend on the geometry of the urban canyon. The main factors which affect the influence of surrounding obstructions are the height to width ratio of the street section and the orientation of the urban pattern. This study aims to investigate the relationship between the two afore-mentioned parameters with regard to the solar energy gains on the different sides of a courtyard block which is taken as a typological case study. The concept of vertical solar potential is a useful tool to compare and assess the response of different urban layouts to the thermal and lighting requirements of a Mediterranean climate, in both quantitative and qualitative terms. The results of the analysis are expected to provide relevant information when formulating early design solutions to improve the solar performance and energy efficiency of urban patterns. 1. Introduction To date, studies and research on the energy efficiency of contemporary cities mainly focused on the contrast between compact and dispersed models, spotlighting on the concept of density. This is a quantitative and measurable indicator which provides a neutral characterisation of a generic entity and allows immediate and objective comparisons between different situations. 1 Alessandra Curreli: Architectural Building Department, School of Architecture of Barcelona– Polytechinic University of Catalonia (UPC), Av. Diagonal 647, 08028 Barcelona, Spain. Email contact: [email protected] 2 Helena Coch Roura: Architectural Building Department, School of Architecture of Barcelona– Polytechinic University of Catalonia (UPC), Av. Diagonal 647, 08028 Barcelona, Spain. Email contact: [email protected] ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 118 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura Despite its rigorous scientific character, in a real urban context density itself becomes a fairly relative concept which has strong cultural and geographical roots: there is not an ideal density value since human preferences can change according to temporal and spatial coordinates. Furthermore, density does not exhaustively describe the urban experience, because it does not take into account the qualitative attributes of the spatial structure that daily supports the inhabitants (Lynch, 1990) and affects individuals’ perceptions (Rapoport, 1975). Several older and more recent studies (Martin, 1972; Cheng, 2010; Berghauser Pont and Haupt, 2010) demonstrate that the same building intensity value may exhibit very different spatial organisations. The form of the city is therefore understood as a physical environment that expresses the quality of the space through the design and the structure of the urban pattern and is manifested in its typological and morphological characteristics in the 3-D space. The site layout and the distribution of the built volumes effectively affect the comfort and the environmental behaviour of indoor and outdoor urban spaces, as previous research has demonstrated (Ratti et al., 2003; Robinson, 2006; Cheng et al., 2006), and the concept of building intensity is not exhaustive enough to fully assess and seek these features. Instead, a qualitative and formal approach might provide a more relevant contribution and play a crucial role in improving the energy efficiency of a building cluster, particularly with regard to its ability to exploit solar energy, in terms of technical, individual and social use (Kaiser, 1996). This study precisely examines the relationship between the morphological structure of a homogeneous urban pattern (in terms of street network orientation and urban canyon geometry) and the potential solar access on the building envelope. The attention is specifically focused on the vertical surfaces, considering their possibilities to receive and passively use direct radiation. The selection of a representative existing case study and the analysis of its different configurations can provide interesting topics to discuss regarding appropriate design solutions, in an effort to improve and regulate the solar gains depending on the climatic needs. 2. Street network orientation and urban canyon geometry Within his studies concerning the efficiency of land use, Martin (1972) identifies the grid as the main element generating the urban structure, a framework of reference for the development of the city and of its internal relationships. Actually, the establishment of the street network has always been one of the first acts when founding new settlements. In the several examples of vernacular villages as well as in the ancient Roman and Greek foundational cities, the planning of the grid was most often guided by direct experience and by religious and strategic factors, but the intentionality and awareness of a specific solar orientation are not completely certain (De Pascali, 2008). More conscious and specific studies concerning the relationship between the planning and design choices and the solar performance of an urban pattern have been developed since the 20th century thanks to the previous knowledge accumulated coupled with the availability of modern techniques to measure and calculate solar radiation. ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 119 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura During the twenties, Rey et al. (see Montavon, 2010) developed a theory based on the relationship between the daylight hours and the corresponding air temperature; more specifically, the heliothermic axis was determined as the direction which is supposed to bring the maximal solar thermal contribution and which is defined, therefore, as the optimal orientation of the urban pattern 3 . About 15 years later, Vinaccia (1943) proposed a different approach aimed at providing an equal amount of sunshine to all four façades of a cubic building. The author demonstrated that to achieve this condition in temperate regions the street network orientation should match the direction of the sunrise on the summer’s solstice. Based on the empirical demonstration by Heiligenthal, Gropius (1977) claimed that a northsouth orientation is more desirable than a 45° tilted one because it allows a lower value of height to width ratio (1.5 versus 2), ensuring the same conditions of solar exposure to the façades (2 hours of direct radiation on the winter solstice) while occupying less land. The experiments were implemented and assessed with regard to the city of Frankfurt (50° North latitude), where the analysis refers to a specific type of urban block, namely the row, and the main priority was to get the maximum solar gains throughout the whole year. Later, in the eighties, Knowles (1981) stressed the importance of the street orientation as one of the main components in the framework for future urban growth aimed at improving the use of solar energy and the quality of life in cities. By comparing the Jeffersonian and Spanish grids in Los Angeles (34° North latitude), the author claimed that the solar envelope 4 over a city block oriented on the cardinal points would contain more developable volume than a block built over a diagonal orientation, but that the latter would ensure a more homogeneous distribution of solar radiation within the urban pattern. Most of the later studies and handbooks generally recognise the N-S orientation, with small deviations of +/- 25-30° (Chrisomallidou, 2001; Littlefair, 2000, as optimal because it provides a large south-facing surface which receives the most solar radiation during the cold season and is easy to shade in summer. This is true in general terms, but what about the condition of the other building façades, such as the east and west facing ones in temperate and hot regions? In this regard, Olgyay (1998) embarked upon a meticulous study of the incident radiation on a vertical surface, considering different orientations at intervals of 30 degrees, in order to assess the optimal exposure (summer/winter) at different latitudes. Another important consideration to bear in mind is that, in a building cluster, the effective solar gains on a vertical surface are also affected by the presence of facing obstructions. The cast shadowing, in fact, can change significantly according to the spatial configuration of the surrounding streets and buildings along with the variable position of the sun. In other words, not only the street network orientation but also the urban canyon geometry have a significant influence on a façade’s effective solar potential. The strict and complex interconnection between these two parameters is a key topic worth studying and analysing. 3 The work quoted by Montavon is: A. Rey, J. Pidoux and C. Bardet (1928). La science des plans de villes. Ses applications à la construction, à l'extension, à l'hygiène et à la beauté des villes, orientation solaire des habitations. 4 Knowles defines the solar envelope as a “container to regulate development within limits derived from the sun’s relative motion. Development within this container will not shadow its surround during critical periods of the day”. ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 120 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura Compagnon (2004) proposed an interesting method to assess the potential exposure of differently-oriented façades by means of a polar diagram that displays the amount of vertical surface area in each direction weighted by its corresponding sky view factor. The orientation rose is an useful tool to evaluate and compare the influence of the obstructions with regard to diffuse radiation, even if the direct solar contribution from different cardinal points is not taken into account. More recent studies (Strømann-Andersen and Sattrup, 2011; Van Esch et al., 2012) discuss the effects of the urban canopy design on solar access in northern European regions, highlighting the complexity and relevance of this topic with regard to a building’s passive energy use. This study fits in with this branch of research and aims to assess the solar potential of the building façades within a homogeneous urban fabric, taking into account the joint influence of the grid orientation and the urban canyon aspect ratio, the latter being the proportional relationship between its height and its width, as defined by Erell et al. (2011). The purpose of the experimental analysis is to find a tendential relationship between these two parameters in order to identify common building performances and formulate tentative guidelines for solar urban design, with particular attention to the Mediterranean climate’s seasonal requirements. More specifically, the main objectives of this study can be summarised as follows: - To study the growth/decline rate of a building façade’s solar potential with regard to its progressive rotation, in the absence of obstructions. - To evaluate the relative and changing impact of the height-to-width ratio (in terms of percentage of solar radiation loss due to obstructions) according to the different orientations of an urban canyon. 3. Case study and methodology This research deals with the Mediterranean climatic region and the peculiar pattern of the Eixample district in Barcelona, Spain (41° 23’ north latitude) which has been selected as a representative example of the courtyard block prototype (Figure 1). ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 121 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura Figure 1. Bird’s eye view of the Eixample district in Barcelona Source: http://www.bing.com/maps/ The configuration of the Eixample is characterised by quadrangular chamfered blocks (illas) closed along all four sides and accessed by streets. The average height of the front façade along the perimeter of the block is about 20 m. Originally, the courtyard (pati) was supposed to be used as a semi-public garden, but the pressures from private owners and building speculation during the sixties led to the construction of one-storey structures in the interior space. The regular layout of the blocks defines a network of streets 20 m wide with a 45° north orientation. The street canyon geometry is fairly regular in all directions and it is characterised by an average height-to-width ratio of about one. This parameter, as well as the other average dimensions of the urban pattern, were determined according to the Spacecalculator. The latter is defined as a system of variables calculated using mathematically related experimental data (Berghauser Pont and Haupt, 2005) that is a very useful tool in describing the built space. The numerical values are summarised in the Table 1. Table 1. Main dimensional features of the Eixample urban fabric Variables Units Symbol Average value Floor Area Ratio - FAR 3,12 Side Length m L 113 Front Façade Height m H 20 Street Width m W 20 Height-to-Width Ratio - H/W 1 Source: Authors’ own. In order to avoid all the specificities and morphological irregularities found in a real urban context, which might make the understanding of its solar behaviour difficult, a 3-D digital mockup (a cluster of nine blocks occupying an area measuring 400x400 m2) was built with reference to the typological structure and average measurements of the selected sample (Figure 2). ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 122 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura Figure 2. Axonometric representation of the layout extrapolated Source: Authors’ own. This research is based on a comparative analysis implemented by means of Heliodon2, a simulation software which provides potential data about the cumulative distribution of solar energy collected by the building envelope, taking into account the influence of surrounding obstructions (Beckers and Rodríguez, 2009). In this case, only direct radiation on the external vertical envelope is considered. In this case study, the street network orientation was modified, while keeping its height-to-width ratio constant. Two different configurations of the same urban pattern were compared: - Configuration A: the actual diagonal orientation (N-E/S-W; N-W/S-E) - Configuration B: a hypothetical orthogonal orientation (N-S; E-W) Within the selected 3x3 layout, the solar gains for the central block were assessed and its performance was evaluated in both summer (from 21/06 to 20/09) and wintertime (from 21/12 to 20/03). In the first phase of the analysis [1], the simulation was implemented in the absence of obstructions, while, in the second step [2], the surrounding blocks were taken into account in order to estimate the weight of their cast shadows on the façades of the central block. In summary, the analysis was extended to eight case studies which are listed in Table 2. ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 123 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura Table 2. Summary of cases being studied A B Case study Layo ut Temporal interval Obstructions 1.AS A Summer No 2.AS Yes 1.AW Winter No 2. AW Yes 1.BS B Summer No 2.BS Yes 1.BW Winter No 2. BW Yes Source: Authors’ own. Three parameters are chosen to assess and compare the influence of the grid orientation and of the H/W ratio on the façades’ solar potential: 1) The Sun factor - Sf (kWh/m2) which is the ratio of the direct incoming radiation on a façade and its surface area. The Sf is used to express the vertical solar potential in each direction. 2) The influence of obstructions - Io (%) which expresses the losses in potential solar energy due to the shadows cast by the neighbouring blocks. 3) The energy gains - Es (MWh) and their distribution on the different front façades of the block. 4. Results We focused our attention on the long sides of the block (identified as F.1 - F.4) as the chamfers overlook the intersections, central spaces which cannot be described or considered as urban canyons (Erell et al., 2011). Furthermore, the solar collection on the lateral façades is much more considerable than on the chamfers, thanks to the side façades’ larger exposure surface (1.694 m2 each one). The overall numerical results are reported in Table 3. ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 124 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura Table 3. Overall results: Winter (left) and summer (right) season Parameter Case study F.1 F.2 F.3 F.4 Parameter Case study F.1 F.2 F.3 F.4 Orientation NE SE SW NW Orientation E S W N Sun factor (Sf) 1.AW 9 219 219 9 Sun factor (Sf) 1.AS 108 249 249 108 2.AW 7 145 145 7 2.AS 86 227 227 86 Energy gains (Es) 1.AW 15 371 371 15 Energy gains (Es) 1.AS 184 422 422 184 2.AW 13 246 246 13 2.AS 146 385 385 146 Influence of obstructions (Io) 14 34 34 14 Influence of obstructions (Io) 14 34 21 21 Sun factor (Sf) 1.BW 88 297 88 0 Sun factor (Sf) 1.BS 222 215 222 16 2.BW 64 199 64 0 2.BS 164 215 164 15 Energy gains (Es) 1.BW 150 504 150 0 Energy gains (Es) 1.BS 376 364 376 27 2.BW 108 337 108 0 2.BS 278 364 278 25 Influence of obstructions (Io) 28 33 28 - Influence of obstructions (Io) 28 33 26 0 Source: Authors’ own. The outcomes of the analysis are summarised and displayed through a polar diagram model which is divided into eight circular sectors (q.1-q.8) with the same angle extension of 45 degrees and corresponding to different solar orientations (Figure 3). Figure 3. The polar diagram model Source: Authors’ own. 4.1 The vertical solar potential trend As expected, during the winter season, the vertical solar potential has a non-linear cyclical variation. Within the quadrants q.1, q.2, q.3 and q.4, the value of Sf progressively grows from 0 ACE© AÑO VII núm.21, FEBRERO 2013 | URBAN LAYOUT AND FAÇADE SOLAR POTENTIAL: A CASE STUDY IN THE 131 MEDITERRANEAN REGION Alessandra Curreli, Helena Coch Roura CHENG, V. et al. Urban form density and solar potential. In: PLEA2006 - The 23rd Conference on Passive and Low Energy Architecture Conference Proceedings, Vol. 1, ed. R.Compagnon, P.Haefeli; W.Weber. (23rd, Geneva, Switzerland). 2006, pp: 1701-1706. CHRISOMALLIDOU, N. 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