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Passive cooling strategies and honing them with dynamic simulation models

David McEwan

Post the COP27 summit and with doubts cast over the viability of the +1.5°rise in global temperatures, passive cooling is as much a priority as heating, says David McEwan of IES

IES’ Virtual Environment model image of St Sophia’s Primary School.
IES’ Virtual Environment model image of St Sophia’s Primary School. Credit: IES

In the UK, cooling strategies are a relatively new priority. Historically, the country’s building stock has been geared more towards keeping warmth in during cold weather than ensuring occupants are cool when it’s hot outside. More modern buildings retain heat well as a result of legislation, demanding well insulated, tightly sealed structures.

However, following this summer’s heatwaves, which saw temperatures breach the 40° mark for the first time, the need to keep buildings cooler has become stark. A warming climate and current regulations have the potential to work against each other in terms of energy efficiency and thermal comfort. Winter heating loads may be lower in new or recently retrofitted buildings, but cooling loads may not have been understood, which could leave buildings uncomfortable or even uninhabitable for periods as summers get warmer.

For many, air conditioning would be the go-to cooling measure in well-insulated buildings to ensure optimal temperatures, but it’s not necessarily the most effective, and could be costly in capital and running costs. That’s not to say that we should shun air conditioning altogether – in some instances it will be necessary – but it should be a last resort after building designers have explored passive cooling methods that can improve occupant comfort without the same high level of energy use and subsequent carbon emissions.

Passive cooling uses natural and non-energy-intensive techniques. Here, it’s crucial to note that there’s no silver bullet that can keep all buildings cool while using minimal energy. Every building and architectural project is different – this is why the use of performance modelling technology is vital in the planning process. Performance modelling of passive cooling measures is most effective when done from the earliest stages, to ensure the correct decisions are made before it’s too late to make effective changes. Modelling should be carried out at each subsequent design stage, to help hit energy and cost targets and mitigate the risk of a performance gap occurring between planned energy use and actual operational use.

The guide covers how dynamic simulation modelling closes the performance gap, and achieves net zero and operational energy goals

Dynamic simulation modelling for energy efficiency

To overcome the challenges presented by variations between buildings, consultants at IES have developed a dynamic simulation modelling guide built around the eight RIBA Plan of Work stages. The guide was commissioned as part of the Scottish Net Zero Public Buildings Standard, however, the common sense modelling techniques explained in the guide can – and should – be applied to every architectural project as best practice in performance measurement.

Designed to be used by the entire project team, the guide gets everyone, including designers, contractors and facility managers, into a mindset of accurate energy planning. It also gives architects the right information at each RIBA stage to make confident design decisions. Project teams can consider as many options as they wish in order to make sure they select the most appropriate and effective measures in specific buildings.

The guide covers how dynamic simulation modelling closes the performance gap, and achieves net zero and operational energy goals, as well as how to train project teams in modelling and managing buildings effectively. It also outlines several modelling techniques, including operational energy modelling, BIM and model geometry, solar and daylighting studies, metering, design analysis considerations, weather files and passive design.

Weather files for climate future-proofing

When using dynamic simulation modelling, weather file selection depends on the purpose of the analysis and therefore needs to be carefully considered as part of each stage before simulation. CIBSE offers a range of current and future year weather files that can be considered for use.

When investigating thermal comfort, future weather files should be used as part of ‘future proofing’ building performance, particularly where passive ventilation and cooling solutions are being tested. It’s recommended to use a range of future weather files to cover the potential spread of projections due to the impacts of global warming on thermal comfort. The process of creating future weather files needs to be set out by modelling experts, especially for specific locations and places with microclimates.

From energy models to digital twins

Using a process known as calibration, energy models can be evolved from a static representation of building(s) into a fully-fledged, living digital twin, which mirrors the behaviour and dynamics of its real-world counterpart.

By capturing and feeding real-world operational data from buildings back into the digital twin, and combining this with the power of physics-enabled simulation, machine learning and AI, a highly accurate digital asset can be created which evolves with the building itself.

This pushes the boundaries of traditional energy modelling to provide real-time insights and opportunities to measure and optimise performance, learn from historical performance and plan for the future of your buildings.

Passive design features

Passive design is most effective when considered early in the design process, and ideally before the planning application phase – for example, assessing external shades on buildings or comparing alternative shapes and orientations of the building. The modelling guide produced by IES outlines examples of features to consider in energy modelling.

Building orientation: Facing the longest axis of a structure in an east to west direction will benefit that area of the building with maximum solar gains during the winter, while also being advantageous to summer cooling by limiting the longest axis to morning and afternoon sunlight.

Floor layout planning: Placing the most frequently used areas, or those that may contain equipment or machinery that raises internal temperatures, on the east to west axis will help to regulate temperatures in these rooms. Facades with lower solar gains can be assessed using solar studies.

Self-shading and external shading features: Overhangs and shadings, especially on southern facades, help to reduce overheating in the summer months, but these need to be tested so they are correctly sized and shaped to work in balance with daylighting strategies. This can be done with modelling technology that calculates the size and angles necessary to block and allow the sun based on the exact positioning of the building and time of year.

Window to wall ratio: The ideal wall to window ratio depends on the floor area, latitude and climate. This largely refers to south side windows, where most solar gain enters buildings.

Whole building ventilation strategies: In passive design, ventilation strategies that are modelled through the whole building allow for heat to be recovered for warmth, and natural ventilation to be used to cool. For cooling, this would involve ensuring windows are positioned on the side of the prevailing wind and that the air can then circulate once inside the building. Fully natural ventilation, a mixed-mode hybrid approach, and mechanical ventilation and cooling can all be considered.

Thermal mass: This is where building materials can absorb, store and later release significant amounts of heat. This is crucial in passive design as it allows heat control from solar gains and moderates temperature fluctuations.

There’s no silver bullet that can keep all buildings cool while using minimal energy

Case study – St Sophia’s Primary School, East Ayrshire

The renovation of St Sophia’s Primary School with architect Hamson Barron Smith was one of the Pathfinder Projects for the ‘Net Zero Public Building Standard’. IES used the dynamic simulation modelling guide to predict the operation of the 1950s school building in order for it to become EnerPHit certified. This is a Passive House standard intended for refurbishments with more flexibility to accommodate common retrofitting challenges. East Ayrshire Council aimed for the refurbishment to achieve high operational energy performance in fabric design and high quality internal environments.

Initial observations made during the dynamic simulation modelling revealed that classrooms now show temperatures below 25°C; an improvement from the original analysis. This takes into account the installation of louvred vents in four classrooms, leading to a free area of airflow of 34% when open, and opening any windows in those four classrooms would further benefit passive ventilation.

Applying the modelling guide to the school’s refit has identified a predicted 71% reduction in energy use each year and is informing the next stage of design to allow the school to reach EnerPHit status while maintaining comfortable summertime temperatures.

David McEwan is consultancy director at IES


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