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Modelling tool helps create comfortable outdoor spaces amid heatwaves

Farah Husayni

With the UK recording its highest-ever temperatures, building consultancy XCO2 has used parametric environmental modelling to suggest measures for improving outdoor thermal comfort when designing new cities

Credit: XCO2

Last July, the UK saw its highest recorded temperature, reaching 40.3°C for the first time in history. This trend has also been observed across other European countries in recent years. Such heatwaves exacerbate the urban heat island effect with potentially detrimental impacts to human comfort, health and performance.

The frequency and intensity of these extreme weather events are a source of growing concern for how we design our cities. Architecture and urban planning play a key role in designing more comfortable spaces with passive mitigation strategies.

Achieving adequate thermal comfort levels, both indoors and outdoors, is crucial. We can evaluate thermal comfort inside buildings with the aid of the various simulation methods and tools available. The question is, however, how do we gauge comfort in outdoor spaces using a comprehensive approach? Assessing outdoor conditions is technically complex due to their changeability.

More importantly, a multitude of variables come into play. In an outdoor environment, metrics such as wind speed, airflow patterns, surface temperatures, detailed radiation analysis, and heat exchange need to be analysed. Consequently, there is a growing need for outdoor thermal assessments in order to understand, evaluate and create comfortable urban microclimates.

At XCO2, we have developed an integrated approach to assess these multiple variables and increase comfort conditions across urban areas in masterplans. We use parametric environmental modelling and the Universal Thermal Climate Index (UTCI), an established metric to measure outdoor thermal comfort. UTCI was developed to provide an equivalent temperature. This describes how the human body responds physiologically to specific outdoor environments, allowing us to interpret the index of comfort values on a familiar scale (eg ‘moderate cold stress’ or ‘strong heat stress’). It is an equivalent temperature (°C) for a given combination of wind speed, radiation, humidity and air temperature as well as non-meteorological parameters as detailed in the figure below. The corresponding assessment scale comprises 10 categories that range from ‘extreme cold stress’ to ‘extreme heat stress’.

UTCI calculation elements/components
UTCI calculation elements/components

We start by using a modelling workflow within Grasshopper, which links various components to account for local wind conditions on site. This then feeds into the localised thermal comfort model. After the 3D geometry is built, we use several environmental plug-ins to develop the scripts. Ladybug is used to connect meteorological data and specify analysis periods. Butterfly is used to run the computational fluid dynamics (CFD) simulation using the OpenFOAM engine. Honeybee is used to run the energy simulation through the EnergyPlus engine to model surface temperatures and then bespoke view factors are calculated accounting for shortwave and longwave radiation exchange on site. ParaView is used to visualise wind velocity plots from the Butterfly simulation and export results into CSV format which are then post-processed to get wind velocities in the proposed masterplan for each point of the grid for each hour of the analysis period. Final outputs are then linked to build an outdoor microclimate map simulation using Honeybee. 

Visualisation examples - Top views
Visualisation examples - Top views

The iterative simulations then suggest measures to improve the thermal comfort of public spaces. These might include rearrangement of building orientations and massing to allow for wind channels where needed, retained and proposed tree locations and densities, urban shading elements to minimise solar impacts, and structures to redirect the wind flowing towards amenity areas.

It is expected that we will see more unprecedented heatwaves in the years to come. We, therefore, need immediate actions to adapt well-designed urban areas. Assessing urban microclimates at early design stages with an overarching approach helps design teams implement innovative solutions to combat extreme heat and ensure comfortable environments.

Farah Husayni is senior sustainability consultant at XCO2


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