The impact of urban geometry on the radiant environment in outdoor spaces

Urban geometry, namely the quantitative relationship of building volumes and open spaces (i.e. built density) and their spatial configuration (i.e. urban layout), is a major modifier of urban microclimate. This paper presents the results of an ongoing research which explores the impact of urban geometry on the radiant environment in outdoor spaces, with direct implications for urban microclimate and outdoor thermal comfort. In particular, the research investigates the relationship between a set of urban geometric indicators (such as Built Density, Site Coverage, Mean building Height and Frontal Area Density) and Mean Radiant Temperature (Tmrt) at the pedestrian level, in different areas of London. Three representative areas of London were selected to be studied; in central, west and north London which are of high, medium and low built density, respectively. Each area was divided into squares of 500m x 500m size, with a total of 84 urban squares included in the study. The methodology comprises three stages: (i) A set of simple geometric indicators have been computed for all urban squares using special algorithms written and executed in Matlab software. (ii) Radiation simulations have been performed for 10 days of a typical year in London, with the use of SOLWEIG software. SOLWEIG simulates hourly, 3-D radiation fluxes, incoming to / outgoing from the ground, spatial variations of Tmrt, Ground View Factor (GVF) as well as Sky View Factor (SVF). Sunny and cloudy days have been considered, evenly distributed in the year in order for the effect of solar angles to be examined. (iii) Statistical tests have been conducted for investigating the correlation between urban geometry, as expressed by the geometric variables, and hourly, average values of Mean Radiant Temperature in the outdoor spaces of the urban squares. The simulation results show that at night-time and in fully overcast conditions, the outdoor spaces of central London’s urban squares are warmer than those of west and north London, due to greater longwave radiation emitted and reflected by building volumes. In contrast, on sunny days, average daytime Tmrt values have been found to be higher in North London’s urban squares due to the larger insolation of their outdoor spaces. Additionally, the statistical analysis has shown that in the absence of direct solar radiation, the correlation between the geometrical variables and average values of Tmrt is very high with an almost perfect linear relationship between the geometrical variables and average SVF values (r2= 0.980). In the presence of direct solar radiation, the strength of the correlation varies with the sun altitude angle; the higher the sun altitude angle, the higher the correlation. In particular, a threshold altitude angle of 20 degrees has been identified, above which the correlation of average Tmrt values with urban geometry approximates that of night-time / cloudy hours. Finally, further statistical tests showed that site coverage (built area over site area) and frontal area density (façades’ total area over site area) are the strongest indicators among those considered in the analysis

Urban Geometry, SVF and Insolation of Open Spaces: the case of London and Paris

The radiant environment in open spaces is very sensitive to the surrounding built form, which determines their openness to the sky and exposure to the sun. This paper presents the analysis of 132 urban forms in London and Paris, two cities at similar geographical latitude, but of different urban geometry, focusing on the relationship between urban geometry and insolation of open spaces at neighbourhood scale. The method consists of three stages: (1) the geometric analysis of the urban forms, (2) their solar access analysis and (3) the statistical exploration of the results. Special emphasis is on the sky view factor (SVF), which is employed as an integrated geometry variable and environmental performance indicator. The comparative analysis of the two cities underlines the significance of urban layout for modifying the outdoor radiant environment, and reveals temporal characteristics of the relation between urban geometry and insolation of urban forms, induced by the varying solar geometry. Indicatively, the average mean ground SVF (mSVF) was found to be primarily affected by the quantitative characteristics of the open space, and able to predict average daytime insolation on March 21 and June 21 (R2?>?0.8), in both cities

Sky view factor as predictor of solar availability on building façades

Solar availability on urban façades varies signifcantly, affected by obstructions by nearby buildings as well as orientation. A convenient way to evaluate their solar energy potential is deemed to facilitate the task of architects in increasing the use of photovoltaic systems and, thus solar energy generation in the urban environment. This study explores to what extent the sky view factor (SVF), a measure of the openness of a point to the sky, can be employed for evaluating solar irradiation of façades in complex urban scenes. For this purpose, extensive statistical analysis was performed testing the correlation of SVF with solar irradiances for 30 orientations, considering three European climates (i.e. Athens, London and Helsinki), and three periods (i.e. year, January and July). Special emphasis is put on global irradiance, which expresses the sum of three solar components, i.e. direct, diffuse and reflected. The study uses 24 urban forms - of 500 × 500 m area - in London for which SVF and solar irradiance simulations were performed for nine sky models (three locations by three periods). The results reveal a strong linear relationship (R2 > 0.8) between SVF and annual global irradiance in all orientations, at all three locations. In fact, as SVF was found to correlate well with both major solar components, direct and diffuse, it can be presumably used for predicting façades' annual solar irradiation at any location within the tested range of latitudes. With respect to monthly global irradiance, the relationship appears less consistent, affected by the increased sensitivity of the relationship of SVF with monthly direct irradiance to façade orientation and location's latitude, associated with the variations of solar altitude

Urban geometry and solar availability on façades and ground of real urban forms: using London as a case study

Availability of solar radiation in the urban environment is determined to a great extent by urban geometry, namely how densely built-up an area is and how the given built volume is distributed spatially within the site. This paper explores relationships between urban geometry and solar availability on building façades and at the pedestrian level, with implications for buildings’ passive potential and outdoor thermal comfort, respectively. The study was based on the morphological and solar analysis of 24 urban forms of London, covering a wide range of built density values found across the city. Two aspects of solar availability were investigated at the neighbourhood scale, through statistical analysis: i) the relationships between urban geometry variables and solar availability indicators in different time periods, and ii) the seasonal solar performance of urban forms’ façades and ground. Apart from the strong, negative effect of density, the analysis revealed that solar availability on ground and façades is significantly affected by urban layout. Mean outdoor distance, site coverage, directionality and complexity were the most influential for the solar performance of open spaces; whilst building façades were mostly affected by complexity, standard deviation of building height and directionality. However, direct solar irradiance on ground and façades was found to be influenced by different variables in January and July, which is attributed to the different solar altitude angles. Related to that, urban forms have been identified that present higher irradiance values in January and lower in June when compared to others. Considering temperate climates, these examples highlight the potential for enhancing the seasonal solar performance of existing and future urban developments. Finally, the seasonal effect on solar availability appears to be much more pronounced for ground with its mean direct irradiance value increasing on average by a factor 15, from January to July, while for façades the increase is only by a factor 2.6

A simple tool for assessing solar and daylight access in urban canyons

A new online tool to assess solar and daylight access over the surfaces enclosing an urban canyon (i.e. building façades and street) is presented. From a limited series of parameters, users can easily describe the orientation and geometry of an urban canyon and precisely define an area of interest for which the calculations have to be performed. By implementing an original calculation method based on stereographic pictures, this tool aims both to demonstrate its capabilities and to serve for architectural and urban planning educational purposes. Its main advantage is that, beside providing numerical indicators (e.g. solar exposure, solar irradiation, sky view factor, daylight sky component) that can take inter-reflections into account, it also allows to visually interpret the results in a rather intuitive manner. The calculation method is available as an open source software package. It can serve to assess solar and daylight access in more complex urban forms

Sky view factor as predictor of solar availability on building façades ::using the case of London

Sky view factor (SVF) measures the openness of a point to the sky vault and as such, has been widely used in urban climatology and environmental design studies associated to various phenomena, including Urban Heat Island intensity and daylight availability. This study examines to what extent SVF can be also employed for predicting solar availability in the urban environment, with emphasis on building façades. SVF and solar irradiance simulations were performed for vertical façades in 24 urban forms -of 500x500m areain London; mean values were computed by urban form, and by façade orientation, considering 30 orientations at 12° azimuth intervals. The statistical analysis reveals a strong linear relationship (R2 >0.8) between SVF and annual global irradiance for all orientations. The models derived from linear regression tests were integrated into a graphical tool for predicting annual global irradiance on a façade in London as a function of its SVF and azimuth angle. Furthermore, the fact that SVF was found to correlate well with both major components of solar irradiation, namely direct and sky diffused irradiances, indicates that it can be used for predicting annual solar availability at latitudes similar to London, even for sunnier climates

An image-based method to evaluate solar and daylight potential in urban areas

Solar irradiance and illuminance are important renewable resources that can significantly increase buildings’ energy efficiency, associated to solar passive and active techniques and use of daylighting. In addition, it is widely acknowledged that the presence of natural light and some sunlight indoors is essential for inhabitants’ well-being. This paper presents a new method to assess solar and daylight availability in the built environment at different scales. The method is based on two types of images where the mutual obstruction between neighbouring buildings is represented over stereographic projections of the sky vault. The images can be used in two ways, either for the visual assessment of the examined surface(s) or, to be processed as to obtain a series of numeric performance indicators. In both ways, they can be combined with similar projections of the sun path or sky radiance/luminance distributions, for considering locations’ latitude and climate, respectively. To exemplify the use and relevance of the tools, especially at the early-design stages, the method is applied to compare the proposals submitted in a masterplan competition. The five finalists are examined in relation to the performance of their fac¸ades and roofs, as well as their impact on an existing fac¸ade. Last, a targeted analysis showed a good correlation between performance indicators, readily computed by the method, and predicted annual energy demands

Comparing the solar performance of urban forms in London

The paper explores the impact of urban geometry on solar availability in twenty-four urban forms of London. The morphological analysis of the urban forms was based on their digital elevation models (DEMs), for which density and six urban form descriptors were computed using image processing techniques. Solar radiation and mean radiant temperature (Tmrt) simulations were performed using PPF and SOLWEIG softwares, respectively. The statistical elaboration of the results reveals a strong correlation (r>-0.950) between density and mean ground and façades sky view factor (SVF) values. Furthermore, among the urban form descriptors considered, mean outdoor distance, site coverage and frontal area density were the most influential for the solar performance of open spaces; whilst solar availability on building façades was mostly affected by frontal area density and standard deviation of building height. The influence of the orientation of urban forms was found to vary with the sun’s altitude: the lower the altitude, the greater the influence. The seasonal performance of the urban forms in enhancing outdoor thermal comfort was evaluated through solar availability and Tmrt in open spaces. According to the climatic data of London, for 87.5% of daytime hours solar radiation would enhance outdoor thermal comfort. In this context, urban forms of lower density perform in general better allowing more solar radiation on the ground. However, as increased density is an objective for urban environmental sustainability, design solutions for enhancing solar availability in high/medium densities were explored. According to the findings, some general guidelines can be identified: i) lower coverage (and thus taller buildings) is preferable, ii) building forms should be designed with minimum undulations and iii) vertical and horizontal randomness is beneficial