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    Keeping it cool: how Melbourne’s Council House 2 took advantage of the night

    Geraldine Chua

    A study released by the City of Melbourne a number of years ago points out that offices fitted with conventional HVAC systems do not necessarily provide healthy working environments. 

    Instead, established “links between health, productivity and increased fresh air use”, and the ventilation of buildings with natural forces, can lead to improvements in occupant health and productivity, as well as a significant reduction in energy requirements.

    This natural ventilation philosophy is no less valid almost a decade on, continuing to prove its worth in the success of Council House 2 (CH2), offices of the Melbourne council staff and focus of said study.

    Located on Little Collins Street, CH2 was conceived because new accommodation was needed for staff. However, this need also presented an opportunity to set new standards for workplace design that the City simply could not pass on.

    CH2, designed by DesignInc in collaboration with Melbourne City Council, was piloted to lead the charge towards Melbourne’s aims of zero emissions by 2020. It includes many ESD initiatives to improve its energy efficiency, including basing its design on biomimicry – the first urban example in Australia – and the employment of a clever ventilation strategy.

    The brief had outlined that, as far as possible, the building should rely on passive energy systems without compromising on comfort and aesthetics, and that 100 per cent fresh air should be provided to all occupants, with one complete air change every half hour.

    For CH2, fresh air was delivered by a mechanical ventilation system during the day – an under-floor air supply technique that distributes filtered air drawn from the outside without any recycling of air – and natural ventilation at night using the stack effect, enhanced by turbine ventilators.

     

    Image: Manufacturing of the automated timber windows. Source: City of Melbourne

    Some areas, such as the bathrooms, rely only on direct natural ventilation through vents and windows.

    At the same time, the team also wanted to demonstrate that cooling, often a hindrance to the energy efficiency of many offices, can be delivered in a sustainable way. Instead of using conventional air-conditioning systems, air is refreshed approximately twice every hour, allowing air leaving the building to remove about 40 per cent of the heat load.

    The remaining 60 per cent stored during the day is removed at night via the use of thermal mass, night purging, and chilled ceiling panels.

    Keeping cool at night

    According to the City of Melbourne, spaces are not cooled by large volumes of cold air, but by radiant cooling, which is when individuals are cooled by body heat being radiated towards cooler surfaces.

    In CH2, the concrete ceilings are left exposed and kept cooler in summer than in winter, enhanced by chilled ceiling panels. These concrete ceilings absorb heat from the rising air in the day, which is then removed from the ceiling at night via night purging.

    When external temperatures fall at night, becoming lower than the internal ceiling temperatures, windows located beneath the low points of the vaulted ceiling automatically open. This allows cool night air to flow in and across the ceiling’s underbelly, removing the previous day’s heat by cross ventilation and by being drawn up through the exhaust air shafts.

    “Exhaust air in the flues is propelled upwards by the chimney or ‘stack’ effect, assisted by roof-mounted wind-driven turbines (when wind conditions are right),” the CH2 website states.

    “The night purge is controlled by CH2’s computerised building automated system (BAS). Using information gathered from temperature sensors in the concrete at two locations on each floor and combining this with an external temperature reading (from the weather station on the roof), the BAS automatically opens the windows at the coolest part of the night – usually between 2am and 6am.”

    Night purging only occurs when external temperatures are at least two degrees lower than the concrete temperature; otherwise the process is ineffective. Night purging for CH2 therefore works on a floor-by-floor basis, and if the concrete ceilings are cold enough, the windows will remain closed.

    Another cooling measure is the use of chilled ceiling panels. In its most passive mode, the chilled water is supplied by three large tanks, each containing nearly 10,000 small stainless steel balls filled with a salt suspension Phase Change Material (PCM) which freezes at 16 degrees.

    The water in the tank is chilled by the frozen PCM balls before circulating around the building to the chilled ceiling panels when cooling is required. It returns to the tank about 2-3 degrees warmer, and heat from the water is transferred to the PCM balls which re-chill the water. The council website explains:

    “The PCM balls continue to absorb heat (which is energy) enabling the material to have enough energy to break-down the molecular bonds and move from solid into liquid phase. Essentially, the balls absorb heat until they melt. By this process, the PCM acts as a thermal storage battery.

    “As with the night purge process, cool nights are used to dissipate the heat contained in the water, with water being put through the cooling towers on the roof. Using a trickle evaporative cooling process, heat is dissipated to the night air and cool water is brought back down to the basement.

    “In winter, when the night air is very cool, the water that returns to the basement is cold enough to re-freeze the PCM balls without the need for chillers. In warmer months the chillers (in the rooftop plant room) provide chilled water to the basement to freeze the PCM balls.”

    CH2 is designed to maintain the office at a temperature of 21-23 degrees. It was completed in 2006, and is the first purpose-built building in Australia to achieve and exceed a Six Star Green Star rating by the Green Building Council of Australia (GBCA).

    To find out more about CH2 and its various sustainability innovations, please click here.

    Photography by Dianna Snape and David Hannah. Source: archdaily.com

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