Urban semi-enclosed spaces as climate moderators

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1 September 2004 Page 1 of 5 Urban semi-enclosed spaces as climate moderators ia Sinou 1 and Koen Steemers 2 The tin Centre for Architectural and Urban Studies, University of Cambridge, 6 Chaucer Road, CB2 2EB, Cambridge, s: ms378@cam.ac.uk, kas11@cam.ac.uk ABSTRACT: This paper identifies different types of urban semi-enclosed spaces and discusses their thermal implications. It demonstrates that semi-enclosed spaces create intermediate thermal conditions. These spaces were historically major urban design elements, which unfortunately today are set aside. The key architectural characteristic which identifies semi-enclosed urban spaces and their thermal performance as it has been defined in this research is that they are not open to the sky. The paper also presents some of the results obtained through the monitoring of case study areas and through the new model developed which predicts urban arcade type space temperatures. A result from the comparative study between the monitored and modelled data is also presented. Conference Topic: 3 Comfort and well-being in urban spaces Keywords: semi-enclosed spaces, temperature, air movement, thermal diversity INTRODUCTION Semi-enclosed spaces serve many purposes: (1) they create buffer zones in front of buildings reducing their energy demand, (2) they create microenvironments with intermediate thermal conditions, in general warmer than ambient in the winter and cooler in the summer, (3) they create spaces where people might find more comfortable conditions to relax, interact and engage in other activities, (4) they comprise a transitional space for people to thermally adjust when moving from interior to exterior and (5) they can create links between buildings and act as unification elements between different urban space components. Among the several purposes that semi-enclosed spaces serve, the fact that they shelter from the climate, as opposed to open spaces which are exposed, was the most significant in the course of this research. Such spaces are found in the whole of Europe, and create intermediate conditions both in summer and winter. Two case study areas one form northern Europe, the city of Cambridge and one form southern Europe, the settlement of Kastro, Siphnos, Greece, which accommodate a variety of semi-enclosed spaces, were used to monitor and investigate the thermal performance of semi-enclosed spaces [1]. Also, a model is developed predicting semi-enclosed spaces temperatures, which enabled the comparison between theoretical and actual monitored temperatures. 2. URBAN SPACE 2.1 Urban space Urban space is classified in (1) open and (2) semienclosed space. It could be noted that the relationship among the two is single sided: although open space can exist without semi-enclosed, the latter can not exist without open. These two groups of urban space can in turn be divided; open spaces into streets, squares and public parks, which can take various forms and semi-enclosed spaces into porches, arcades, porticos and hypostyle halls. The most significant distinction between open and semienclosed spaces for this research is that open spaces are exposed to outdoor climate whereas semienclosed are sheltered. Also, in terms of their uses both categories, open and semi-enclosed serve transitional and social purposes (Figure 1). Variety of forms Streets Square Parks Open Exposed microclimate Transitional Urban space Semienclosed Sheltered microclimate Social Porch Arcade Portico Hypost yle hall Figure 1: Classification of urban space and characteristics 2.2 Semi-enclosed space This section focuses on the specific geometry of semi-enclosed spaces and a typology developed through urban history. The paper aims to clarify the types of different urban spaces and also to explain the need for such a typology. Moreover, this typological classification is subsequently used to assess the environmental performance of urban form. The types of semi-enclosed spaces identified are: (1)

2 September 2004 Page 2 of 5 porch, (2) arcade, (3) portico and (4) hypostyle hall (Figure 2). according to the predominant air flow direction and the geometry of the space (Figure 5). Figure 2: Types of semi-enclosed outdoor spaces from left: porch, arcade, portico and hypostyle hall The environmental performance of each space type is illustrated in a simple diagrammatic form. The porch during winter is protected from rainfall and snowfall, whereas the welcome winter sun is allowed to penetrate, warming it up. In the summer the course of the sun does not allow solar radiation to enter the space, which remains cool. Furthermore, the air movement in the space is single sided and according to the wind direction it can either not affect the space significantly if the predominant air flow direction is parallel to the opening or create significant air movement and turbulence if it is normal or at an angle to the opening (Figure 3). Figure 3: Environmental implications of porch type The non-glazed arcade, which is the specific type in question in this research, is protected from rainfall and strong summer sun and yet allows the penetration of the winter sun. As opposed to the porch, the sun may enter within the space twice a day from the two openings depending on orientation and if unobstructed conditions are assumed. Furthermore, the wind is cross flowing into the space and according to the predominant wind direction it can either create significant air movement if it is blowing normal or at an angle to the openings, or not affect the space significantly if the direction is parallel to the openings (Figure 4). Figure 5: Environmental implications of portico type The environmental performance diagram of the hypostyle hall type shows that the space is protected from rainfall and summer sun whereas the winter sun is allowed to enter from every side. Furthermore, the wind flows unobstructed in the space regardless of its predominant direction, possibly creating significant air movement (Figure 6). Figure 6 Environmental implications of hypostyle type The main geometrical difference, related to this research, between open and semi-enclosed urban spaces is that semi-enclosed spaces always have an overhead plane. Thus, semi-enclosed spaces receive significantly less solar radiation than an open space of the same floor area would have received. In winter when the sun is low, the incident radiation is more than in the summer. This heat is absorbed by the surfaces and then released through convection and longwave radiation. The enclosure of the space and the fact that heat escapes to the open sky only through the vertical openings results in trapping the heat in the space, which tends to warm up more than an open space would. In the same way, at night heat is released more slowly to the outside, than from an open space, creating an intermediate temperature warmer than the outside. In summer when the sun is high, less solar radiation enters the space, which causes a slower rise in temperature. Consequently, the space stays generally cooler than the outside. Wind speed and direction play an important role in the heat transfer of these spaces, since a significant part of heat transfer occurs through convection. Air movement within the space is strongly determined by the orientation of the open areas and the urban context in relation to the wind speed and direction. Figure 7 shows in a simple way the heat transfer that occurs in semi-enclosed spaces during day and night. Figure 4: Environmental implications of arcade type Regarding the environmental performance of the portico, the space is protected from rainfall and summer sun and allows the winter sun to enter, similar to the porch. Furthermore, the wind flow is single sided and can result in various patterns Figure 7: Heat transfer in a semi-enclosed space

3 September 2004 Page 3 of 5 3. TEMPERATURE RESULTS 2.1 Temperatures recorded during peak and trough ambient conditions Globe temperatures are recorded in both areas in winter and summer for a week, monitoring: one open space, eight semi-enclosed spaces and one house. The case areas were clearly very different however many similarities were also identified which made the comparisons between them feasible [2]. This section investigates the temperatures recorded in semi-enclosed spaces at the same time as peak and trough air meteorological and globe ambient temperatures were monitored. This demonstrates the wide range of conditions recorded simultaneously in different semi-enclosed urban spaces located in the same area. For the case of Cambridge the temperatures recorded in an open unobstructed location, the Parkside, and for the case of Kastro the Square, are used as globe ambient temperatures in order to undertake a more realistic comparison than air temperatures would, with the semi-enclosed globe temperatures. In winter, the lowest air meteorological temperature was recorded on 12 ch at 7 measuring 0.4 o C whereas the lowest Parkside garden was recorded on 3 ch at 1 measuring 0.8 o C (Table 4). In the first case, the semi-enclosed space temperatures were much higher and varied from 3.1 o C in the tin Centre porch to 9.0 o C in the Sidney arcade, whereas in the second case they varied from 2.7 o C in the Trinity hypostyle hall to 4.9 o C in the Queens porch. On 7 ch the highest winter air and Parkside globe temperatures were recorded at 3 and 2 respectively. In the first case semi-enclosed space temperatures varied from 12.6 o C in the Sidney arcade to 13.8 o C in the tin Centre porch, whereas in the second they varied from 12 o C in the Trinity hypostyle hall to 17.4 o C in the tin Centre porch (Table 1). It can be observed that when the trough ambient temperatures are recorded, very early in the day, semi-enclosed space temperatures are much higher, whereas in the afternoon when the peak ambient temperatures are recorded semi-enclosed spaces show slightly lower temperatures in most of the cases. Table 1: Cambridge temperatures in semi-enclosed spaces at peak and trough ambient date time house air ambient Parkside globe abbient queens entrance porch trinity wheewels arcade Sidney arcade arcade queens cloister portico trinity nevilles hypostyle emmanuel portico martin centre porch sidney cloister portico These significant winter temperature differences between ambient and semi-enclosed spaces, demonstrate the variety of conditions that are experienced in a northern European city during winter. They also express a strong indication that semi-enclosed outdoor spaces can be used as essential urban elements that shelter not only from strong summer sun and light, which is usually the case in southern climates, but also from winter cold and rainfall in northern climates. The equivalent result from Kastro in summer is presented in Table 2. During the summer monitoring the lowest ambient temperatures were recorded very early in the morning measuring 16 o C and 16.8 o C for air meteorological and Square globe temperatures respectively. In the first case the lowest semienclosed temperature recorded was in the Central Loggia measuring 18 o C and the highest in the Steadi porch measuring 19.2 o C. In the second case the temperatures varied from 17.7 o C in the Second Loggia to 19.9 o C in the Steadi porch. On the other hand, the highest summer ambient temperatures recorded were 25.8 o C and 38 o C for air meteorological and Square globe respectively. In the first case, the variation among the semi-enclosed spaces temperatures was from 23.1 o C in the Second Loggia to 27.2 o C in the Steadi porch. Finally, in the second case the semi-enclosed temperatures recorded varied from 24.4 oc in the Central Loggia to 27.2 oc in the Steadi porch. It can be observed that when the trough temperatures are recorded semienclosed spaces are higher that ambient but within the comfort range, whereas when the peak ambient temperatures are recorded the space temperatures are in most of the cases much lower. Table 2: Kastro temperatures in semi-enclosed spaces at peak and trough ambient date time house air ambient square globe open second loggia central loggia steadi porch theol. passage arcade passage arcade In order to demonstrate the intermediate thermal environment that semi-enclosed spaces create as well as the variety of thermal conditions that such spaces with different characteristics result in, an example from the Kastro monitoring undertaken during the pilot study is presented. This monitoring undertaken at an early stage of the research while walking around the settlement, focused on recording air temperature with an air thermistor and globe temperature with a black globe thermometer and logging the data to a Squirrel data logger at intervals of two minutes. It should be noted that the globe temperature had a time lag of about five minutes in

4 September 2004 Page 4 of 5 relation to the air temperature, which means that in some cases where the stay in a place was very short the globe thermometer did not record the value whereas the air thermometer recorded it. The route followed was the same and was repeated several times during the week. One set of results are presented in Graph 4 in order to show: (1) the variety of thermal conditions that are experienced while walking in the settlement through open and semi-enclosed spaces and (2) the recorded differences between air and globe temperature values. On 5 June the monitoring was continuous from 12 8 The results show how diverse the temperatures were in different outdoor spaces and the house in the course of one day (Graph 4). Although the monitoring in each space lasted only for a short length of time, it can be observed that in some of the spaces, for example the North Loggia, the temperatures tend to be constant, whereas in others like the Passage and the Streets they tend to fluctuate more. Also, in terms of temperature values the maximum difference recorded among the spaces is approximately 10 o C. The graph also shows that in most of the cases globe temperature was approximately 1 o C to 2 o C higher than air temperature, which was an expected outcome of the radiation exchange between hot summer surfaces and the thermometer s globe Globe and air temperatures in Kastro during pilot study, 5 June 2001 passagecent ral loggia main st reet t avern globe temperature museum st reet nort h loggia air temperature house square Graph 1: Globe and air temperature monitoring while walking around Kastro The monitoring results of the study showed that in Cambridge, semi-enclosed spaces remain warmer in the winter during most of the day, allowing for longer and more comfortable occupancy. In the summer semi-enclosed spaces remain cooler during the day and slightly warmer during the night. The analysis of the peak and trough semi-enclosed temperatures experienced while the ambient peak and trough temperatures occurred, showed that a wide variety of temperatures can be experienced within semienclosed spaces. Even in the summer, where one would think that in a northern climate the sun is always favoured, occasionally temperature exceed significantly comfort levels, therefore the cooler semienclosed spaces could provide more comfortable urban spaces. The analysis of the Kastro data showed that in summer, semi-enclosed spaces stay cooler for most of the day and approximately 2 o C warmer during the night. In winter semi-enclosed spaces remain cooler during the day and slightly warmer during the night. The peak and trough temperatures showed a wide range of values, whereas the lowest diurnal average temperatures were recorded in the North Loggia, which was the most enclosed semi-enclosed space, and the highest diurnal average temperature in the Steadi porch, which was a highly obstructed space with northeast orientation. The temperature variation has demonstrated that semi-enclosed outdoor spaces create thermal intermediate conditions and therefore become valuable urban elements that shelter not only from sun and light, which is the case in Mediterranean climates, but also from cold and rainfall in northern climates. 2.1 Relationships between spatial characteristics and temperature The relationship between spatial characteristics and temperature fluctuation of semi-enclosed spaces were also investigated in this project. This analysis focused on the examination of one characteristic at a time. Clearly in a monitoring study it is difficult to investigate a single parameter by comparing different spaces and at the same time ensure that other determining factors of these spaces remain the same. One of the semi-enclosed types, the arcade, was selected in order to be thoroughly investigated; a model was created exploring the type s relationships between architectural characteristics and thermal performance. The results obtained are here briefly summarised. The new model, Arcade, is based on the Admittance Method [3] and calculates peak and mean to peak dry resultant temperatures of arcade type spaces. Its accuracy was tested by inputting extreme values and also a parametric study was undertaken. The results from the latter showed that a key geometrical characteristic introduced by this study, the degree of enclosure, DoE, defined as the ratio of the total to the open surfaces of a space, is inversely related to the mean to peak swing temperature. Also, in spaces of the same proportions and therefore DoE, larger volumes result in higher mean to peak swing temperatures. The volume and floor area to volume investigation showed negative relationships with the temperature swings; higher values of volume and floor area to volume result in lower temperature swings. Moreover, it was demonstrated that among groups of different spaces with the same DoE, spaces that have lower DoE result in a wider range of temperature swings, whereas spaces with high DoE tend to result in low range of temperature swings (the temperature swings resulted tend to be almost the same). With regards to orientation it was demonstrated that north-south orientation result in the lowest temperature swings whereas east-west in the highest. Also, it was shown that orientation can have a significant impact on the temperature fluctuation of a space regardless its DoE. In terms of air movement the study demonstrated that air speed is a mostly influential parameter on the thermal performance of a semi-enclosed space having a greater effect in spaces with lower DoE. Namely, the more exposed a

5 September 2004 Page 5 of 5 space is to the climate the more effected it will be by air movement. The relationships between architectural characteristics of semi-enclosed spaces and their diurnal temperature fluctuations, which were defined as the thermal diversity indicator among different spaces, were investigated through the monitoring results as well. The monitoring study investigated the link between architectural characteristics and their thermal performance and demonstrated that DoE, floor to volume ratio, dimensions and proportions of the openings, orientation, building obstructions, air speed and direction and landscape formation are parameters related to temperature fluctuation of semienclosed urban spaces. Among those parameters that were studied the study verified that the DoE seems to be the most influential and that it has a strong negative relationship with the temperature fluctuation of semi-enclosed spaces. Both the monitoring analysis and the analysis of the new model Arcade showed that the most significant of the parameters explored wit regard to the temperature fluctuation of a space is the DoE [4]. Graph 1 shows a comparison between the modelled and the monitored results. The scatter diagram demonstrates the relationship between DoE and temperature fluctuation of semi-enclosed spaces. Every point in the graph represents either a monitored space or in the case of the modelled data a hypothetical space. The upper line is the product of the model temperatures showing extreme results, since the input climate data were the peak of the day in question. According to the model, these theoretical temperature fluctuations are the highest fluctuations that can be obtained. The lower four data sets are the monitored results form both locations and seasons. It can be said that both the monitoring as well as the modelled results demonstrated that the DoE is inversely closely related to the DoE. 4. CONCLUSION This paper presented a classification between open and semi-enclosed spaces, which was the basis of this research. Also, it identified the different types of semi-enclosed spaces and discussed their thermal performance. It was shown that semi-enclosed spaces create thermally intermediate micro-environments which often more comfortable than open spaces. Moreover, it demonstrated that semi-enclosed spaces result in a variety of thermal conditions which together with open spaces may create a thermally diverse environment. The exploration of spaces relationships between key architectural characteristics and their thermal performance was another part of the research. It is shown that the DoE seems to be the most influential parameter with regard to the temperature fluctuation in semi-enclosed spaces. The result from both the model developed and the monitoring undertaken showed that the two parameters have a strong negative relationship. The introduction of urban semi-enclosed spaces of different types, which in contemporary design can take many forms, could create more variable and comfortable urban environments. REFFERENCES [1] M.Sinou and K. Steemers, Proc. PLEA 2003, Santiago, Chile, (2003). [2] M.Sinou and K.Steemers, Proc. International Conference of Sustainable Planning and Development, Wessex Institute of Technology, Skiathos, Greece, (2003), [3] CIBSE Guide A, Environmental Design, London: Chartered Institution of Building Services Engineers, (1999). [4] K.Steemers, M.Ramos, and M.Sinou, Urban thermal diversity. Journal of Chongqing University, 2(Special Issue), (2003), DoE and globe DT from monitoring (both locations and seasons) and theoretical 'Arcade' temperatures DT(oC) DoE Kastro winter Kastro summer Cambridge summer Cambridge winter theoretical Power (Kastro winter) Power (Kastro summer) Power (Cambridge summer) Power (Cambridge winter) Power (theoretical ) Graph 2: Relationship between DoE and temperature fluctuations in Arcade model and monitored spaces