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The benefits of an Outdoor Comfort Assessment

By Harshad Joshi on Monday 5 November 2018

Generally, the analysis of any built environment is limited to carrying out analysis of all the spaces inside buildings. Dynamic Simulation Modelling (DSM) is an excellent tool to assist in designing equipment that controls the indoor environment. 

However, external spaces are equally important in today’s dynamic world. Outdoor spaces like parks and seating areas of cafes and restaurants provide a good exposure to sunshine and fresh air and allow a break from the monotony of a desk. They also bring the benefit of increased profitability for such businesses by increasing footfall without increasing floor space. External thermal comfort analyses carried out so far are often limited to a specific date and time, such as the summer/winter solstice, which doesn’t provide a meaningful indication of what the results look like throughout the whole year.

IES has developed a new and innovative approach for annual microclimate modelling to conduct outdoor thermal comfort assessments at the district level. This is especially useful for advising Masterplanning teams, enabling them to make informed decisions right from the start of their new projects. Our approach takes into account the impact of massing based on computational fluid dynamics (CFD) modelling of multiple wind speeds and directions, and then extrapolated to each hour of the year based on data from the weather file. The overall outdoor thermal comfort assessment accounts for the annual variation in external thermal comfort by combining the hourly effect of radiation, local air speeds from the wind comfort results, air temperature, relative humidity and physiological variables such as clothing and metabolic rate.

The results are then reported using the Universal Thermal Comfort Index (UTCI); which is predictive of external comfort and is equally applicable across a wide range of warm and cool climate types.

This approach includes the following analyses:
 

  • Sky View and Shading Analysis – The radiation received by the site will have an impact on the heat gain through facades of buildings, bus shelters and possible discomfort for pedestrians. This approach can recommend shading strategies and inform design of optimum walking routes for pedestrians. Figure 1 below shows the impact of shading on received radiation and the associated reduction in radiation experienced by people within the space. 

Figure 1: Example of Radiation Distribution and Impact of Shading

 

  • Wind Comfort Analysis –CFD analyses are frequently based primarily on predominant wind directions and speeds or use fixed values or a defined set of wind speeds and directions. However, this approach does not take into account the high variability of wind, particularly in urban environments. Our approach undertakes an annual CFD analysis accounting for each hour of the year, we can measure the impact on wind comfort and safety, referenced by the Dutch Standard NEN 8100 which identifies how often in the year the wind speed at any point on the site exceeds 5 m/s. This illustrates how often in a year pedestrians will be comfortable at various levels of activity. Figure 2 illustrates the annual percentage of hours with wind speed > 5 m/s for an example district.



Figure 2: Example of wind comfort analysis 

 

  • Design improvement

    •    The blue zones shown in Figure 2 illustrate the spaces that would be suitable for sedentary or active use; as wind speeds are amenable to comfort for more than 95% of the year.

    •    The areas in green are better suited for active use and the areas in yellow are best suited for transitory spaces.

    •    The areas in orange and red indicate that high wind speeds occur in these spaces, often enough to create discomfort at best and in the worst case scenario active harm to occupant safety.

    •    These would represent ideal locations to implement localised solutions such as buffers and wind breaks. This assessment is impossible to undertake with a conventional CFD model as it is typically agnostic of the wind environment throughout the year. 
     
  • Impact on Thermal Comfort – All of the analyses described above can then be combined to generate the occupant experience on outdoor thermal comfort. While there are multiple metrics to measure outdoor comfort, UTCI has proven to be the most reliable predictor of thermal comfort in climate types ranging from cooler climates (e.g. Denmark) to warmer and humid climates (e.g. Malaysia). Figure 3 shows the comparison between undertaking the traditional ‘snapshot’ comfort analysis; in this instance the summer solstice, and how it varies significantly from the comfort range experienced throughout the rest of the year. The summer solstice analysis indicates comfort throughout the site, but an annual overview indicates this is a cool climate where outdoors is thermally uncomfortable for most of the year.

Figure 3: Example of outdoor comfort optimization with simulated vegetation types

 

  • Passive Design Performance – The thermal comfort analysis can be made at a more granular level to look at variations in comfort across the site; to undertake a quantitative analysis of how often each point on the site falls within the comfort zone and what percentage of time occupants experience hot or cold discomfort throughout the year.  Figure 4 shows an example of this type of quantitative overview. This approach can be extended to test the efficacy of passive design interventions (covered walkways, vegetation cover, and wind barriers) by quantifying their impact on thermal comfort experienced throughout the site. From this iterative process the optimal comfort level for outdoor spaces can be identified. 

Figure 4: Example of Quantified Thermal Comfort Results
 

This approach will be primarily very useful to shopping centre owners, commercial estate landlords, and urban planners. They can use this service to prove to potential tenants how they will be able to maximise the use of space in and around their units. Urban planners and architects can use this service to help gain planning permission as this addresses the effect of the sun and wind via a single service and allows for the before and after comparison.

To find out more about the benefits of this service, and how IES Consulting can help you contact us via http://www.iesve.com/consulting/contact