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How BPS thinking added localised, lateral and lifecycle innovation to Treehouse

Building Performance By Joe Tang, Director, Building Services – 07 April 2022


Joe Tang in a grey suit and pale shirt with blurred green park background

Joe Tang

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Designing a net zero building is not just about what goes into the construction drawings – it requires thinking about the entire lifecycle, including strategies for ongoing building performance management.

In developing the Advancing Net Zero Treehouse design, we had an opportunity to incorporate every possible energy saving strategy, from biophilia and passive design through to next-gen edge computing infrastructure. Every decision was considered through the twin lenses of energy-efficiency and human comfort and tested through modelling to ensure optimal performance can be maintained and the building systems also be adaptive to future change.

Building Performance Services (BPS) is often thought of as the engineering expertise that adds value after completion. However, for robust and innovative integrated services design, bringing BPS in from the outset made sense. BPS brings all the engineering disciplines together and has specific expertise in ensuring buildings work optimally for people.

What people within a building want is comfort – they don’t want to be too hot, and they want the lighting and the space to suit their purposes. As BPS engineers, we also need to make sure the building is healthy, safe and works as efficiently as possible minimises its impact on the environment.

For Treehouse the technical challenge for the mechanical, electrical, and plumbing design was to ensure building systems would deliver the required comfort while meeting the parameters for net zero energy and net zero emissions.

Firstly, we leveraged what the local climate can deliver. Hong Kong is hot and humid, and the site is affected by the Urban Heat Island effect due to surrounding buildings. We turned this constraint into an opportunity through designing in a thermal chimney that sends hot air upward, effectively flushing the building continuously with fresh air from ground level which is pre-cooled through an artificial wetland that is irrigated from the building’s grey water.

Treehouse, Hong Kong, China

CGI of the Treehouse, the surrounding neighbourhood and the harbour

The thermal chimney was possible because we worked with the architect to fine-tune the structural design to locate the core on the south side. This is the most heat exposed side of the building, so locating the core here reduces internal heat gain.

As the site is located at the base of a mountain, there is a seasonal downdraft of cooler air, and the Treehouse incorporates wind scoops that capture that air and use it as part of the cooling and ventilation strategy.

The wetland also contributes to the cooling further through the air drawn into the building at ground level passing over HVAC system heat exchange pipes that transfer some heat into the water – this helps the algae and microorganisms that will clean the water to flourish.

Treehouse also harvests seasonal rain and the captured water is used for cooling. Reducing potable water use is a major priority for Hong Kong because most potable water is imported. So, the building uses a seawater cooling system. We have also designed in a phase change material cooling system in the basement that means the chillers can operate at night when the ambient air is cooler in any case, and the chillers can store coolth in the phase change material which can then release it during the warmth of the day.

The design also reduces the height water needs to be pumped to as part of the energy-minimisation strategy. The architect placed plant further down the building to make this possible.

Spatial planning focused on creating a more granular space, so individuals can find the space that has the temperature, light levels, and humidity levels that suit them best. This enables the indoor climate control to be finely tuned and reduces the volume of spaces being conditioned.

The workplace concept of the “nomad” was the focus – enabling people to move around to find the space they prefer. An edge computing infrastructure supports this, as it removes the cabling that anchors people to specific workspaces in more conventional designs. Another benefit of the edge computing is it drastically reduces the energy use and the heat output associated with personal computer use. The software is all hosted on the server, not the device, so the energy and heat output from each device is a fraction of that associated with a stand-alone laptop. Less heat output further reduces the energy required for cooling.

The result of our approach and the fully integrated, highly localised thinking is a building designed to use around one quarter of the energy of a conventional building, all of which can be supplied by photovoltaics and energy storage.

BPS added enormous value to the design task, because we look at every aspect of how a building operates and ask, ‘why can’t we do that a different way?’