Innovations in natural ventilation can help deliver more sustainable and comfortable commercial buildings, says Chris Twinn, sustainability group head at Arup.
Worldwide interest in natural ventilation for commercial buildings has been rapidly increasing, largely prompted by the growing understanding of its potential to reduce carbon emissions. This marks quite a turnaround from ten years ago when air-conditioning was seen as the sole direction of travel for these building types.
While natural ventilation has always had a role in the milder climate of the UK, it is in other countries with less benign conditions where the baton of interest and development has now been taken up. Contributions to the expanding knowledge come from Australia, USA, East Asia and beyond. Take for example Singapore, which has a hot and almost continuously humid climate, but also has one of the most advanced and pragmatic sets of design guidance on natural ventilation embedded into its Green Mark building environmental assessment method. Indeed, I am writing this alongside an openable window on the 39th floor of a tower in Shanghai. Here in China the newly 3-Star building environmental assessment method gives credits for providing openable windows. Other technological developments are set to push the extent of natural ventilation applications still further. Electric vehicles bring pollution- and noise-free streets, greatly increasing the proportion of buildings where it becomes feasible. Cities like Shanghai are showing the way with fully operational electric bus routes alongside demonstration electric cars and many thousands of electric scooters. Across the world, city planners are plotting their future carbon trajectories, mindful that this development in transport can as much as halve building energy use compared with air-conditioned alternatives. With the relatively long life of buildings, this has even prompted requests for new buildings to be future-proofed for natural ventilation beyond their initial few years operating as sealed air-conditioned boxes.
Designed by Morphosis with engineer Arup, the San Francisco Federal Building’s shape and orientation maximise airflow for cooling and ventilation. The first five levels, with high concentrations of people and equipment, are fully air-conditioned. Above the fifth floor, windows automatically adjust, allowing fresh air directly into the building for ventilation and cooling. A ‘living skin’ allows air through openings on the windward side and venting through the leeward wall. The Building Automation System (BAS) opens and closes windows, vents and sunscreens in response to internal and external conditions. At night, the BAS opens windows to cool the building’s exposed concrete columns, shear walls and wave-form ceilings. The window wall features manually-operated windows and includes a heating system integrated into the mullions. A few central, enclosed offices and meeting rooms are served by local, supplemental cooling units to accommodate higher density occupancies. A perforated metal sunscreen protects the south-east facade from solar gain; fixed translucent sunshades attached to a catwalk shade the north-west elevation.
Rapid IT changes are also causing us to question the previously assumed limitations of natural ventilation. The new generation of mobile personal computing and communications equipment, as well as remote cloud computing, significantly reduces office energy and cooling needs. Conventional large mechanical cooling capacities may be unnecessary when passive cooling can suffice. This trend is likely to accelerate as more countries introduce occupant carbon taxes similar to the UK’s Carbon Reduction Commitment to incentivise building users to reduce energy consumption. Similarly a move towards LED desk task lights, coupled to more modest background lighting levels, offers the potential to halve the installed power capacities for lighting and associated cooling.
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| Above: Dinghe Mansion high-rise office, Shenzhen, China, by architect and engineer Arup Associates. The cruciform building plan is designed to allow cross ventilation across the entire floor area, whatever the wind direction. |
This expanding range of built best practice shows us some clear trends in the direction of travel. First, the use of passive cooling to increase the proportion of the year when natural ventilation provides sufficient cooling – normally by way of cost-effective fully-exposed concrete floor soffits coupled to night-flush ventilation. Even in sub-tropical Hong Kong, this could mean that outdoor air can provide free cooling for 50 per cent of hours in a year.
Also evident is a general preference for cross-ventilation. This avoids the problem of providing large stack ventilation voids that bridge different tenancies in terms of acoustics, fire and security.
It is increasingly clear that the devil is in the detail – for example the need for well-designed window furniture to allow for easy fully-adjustable controls, down to small crack openings. Similarly, modest window sizes are designed for good quality perimeter daylighting and room surface illumination to provide the perception of good daylight without excessive glare deeper into the building – as well as low summer solar heat gains needed for natural ventilation.
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| Jessop West Building, Sheffield University, by architects Sauerbruch Hutton |
Alongside these trends we are seeing large reductions in capacity and complexity for any backup mixed-mode mechanical systems. There are also designs that haven’t performed to their intended potential and we need to learn that a good airtight envelope is essential to ensure occupants can properly control air flows and avoid hot or cold infiltration when unintended.
Furthermore, keeping air paths simple and controls intuitive for the occupants is critical – complex air routes require far more user understanding and education than is ever achieved in practice. Normal control should be delegated to individual occupants because automatic controls find it difficult to judge parameters, such as when more local air movement is wanted by occupants. Many examples adopt a mixed-mode approach and struggle to achieve changeover as intended. Passive systems need room-condition, gentle daily swings to achieve full capacity, while standard HVAC controls seek fixed conditions and tend to default to ‘on’. Likewise automatic temperature sensors are generally unable to identify thermal mass radiant temperature benefits. Consequently controls need careful configuration. The most successful natural ventilation applications are those in which the mechanical systems are designed to fully switch off, instead of attempting to operate half-and-half across a floor.
Considerable thought is going into understanding how to extend the range of applications. One of the most interesting is developing natural ventilation for high-rise and super high-rise projects. This involves designing building plans that improve the consistency of cross-ventilation routes for all wind directions but avoid the need to cross sub-tenancy partition lines – learning from Swiss Re feedback. Allied to this is designing slim facades and window solutions that harness wind pressures manageably so they can drive ventilation.
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| performance of natural ventilation in high rise buildings. |
One of the key research areas is establishing a better understanding of trickle vents and ‘time averaged’ fresh air rates. This makes use of the room reservoir of fresh air gathered overnight such that reduced trickle rates are sufficient over the following day. This directly reduces peak cooling needs as well as improving winter room humidities compared with normal air-conditioning.
Another interesting area of research is in exploiting heat recovery with natural ventilation, something normally presumed impossible. This would employ room-exposed thermal mass, as is used for summer passive cooling, but instead using it in mid-season and winter for heat recovery. In essence this involves storing excess heat gathered during a winter’s day and then re-emitting it into the room when the temperature starts to fall overnight, in turn reducing the need for morning boost heating. This feature becomes effective as the envelope reaches super-insulation standards.
An area ripe for research is a better understanding of how single-sided ventilation works under fluctuating wind pressures through a single window. Building physicists will tell you they can predict ventilation in terms of stack- and cross-ventilation, but there is a further alternative. This uses the continually fluctuating wind pressure to create air movement in and out through a single opening into a room. This offers the potential to achieve good room ventilation but using fewer window openings.
So natural ventilation has a significant and growing role in delivering more sustainable and comfortable commercial buildings. The technical knowledge and experience is developing rapidly. To fully deliver, we must not only reduce the systems our buildings need, but they should cost less to build and to operate. A truly ‘triple bottom line’ win-win-win.
Chris Twinn is a director at Arup and head of its Building Engineering Sustainability Group; he is currently based at Arup’s Shanghai office
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