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                    Heating and cooling systems

                    View of office space with undulating ceiling panels
                    Energy-efficient heating and cooling systems help CH2 maintain thermal comfort in both summer and winter.

                    There are times throughout the year when CH2 does not need supplementary heating or cooling to maintain thermal comfort. Air entering the building through floor vents, usually at a temperature of around 20 degrees, provides a basic ambient temperature control.

                    ​Heating system

                    Additional heating is provided by hot water through an underfloor hydronic system located around the perimeter windows. Given that air supplied to CH2’s office spaces is already heated to about 20°C, when heat is required, it is designed to be supplied at the points where heat loss is concentrated – the windows.

                    Hot water pipes are located in the underfloor space along the north and south walls, while beneath each window in the floor is a timber grille supplying radiant heat from the hydronic system. There are also small wall-mounted radiators along the south wall to assist with heat to areas restricted by full-height partitions.

                    The heat from the grilles under the forms a warm air barrier around the perimeters, which rises into the space naturally using buoyancy, not fans.

                    Cooling system

                    Even in Melbourne’s winter, office buildings require cooling. Heat is generated from two main sources:

                    • heat load from people, lighting, computers and other equipment
                    • heat gain or loss at windows or through the fabric of the building.

                    Conventional air-conditioned buildings deal with this heat load by re-chilling recirculated air. In CH2 the air is refreshed twice an hour, removing around 40 per cent of the heat load from the building.

                    The remaining 60 per cent of the heat load is stored during the day and removed at night:

                    • The thermal mass of the exposed concrete ceilings absorb the heat from the rising air during the day. The heat is later removed at night via a 'night purge'.
                    • During the day, the chilled water circulating in the chilled ceiling panels absorbs heat and transports it to storage tanks in the basement. At night the heat is removed through evaporative cooling by cooling towers on the roof.
                    • On particularly hot days, the cooling towers may be used, but this is kept to a minimum for energy efficiency.

                    Heating and cooling features

                    Radiant cooling

                    In contrast to conventionally air-conditioned buildings, CH2 is not cooled via the influx of large volumes of cold air but primarily by radiant cooling.

                    Radiant cooling is based on the principle that individuals are primarily cooled by body heat being radiated towards cooler surfaces such as CH2’s exposed concrete ceilings and chilled ceiling panels. Human skin is more responsive to the cooling or heating effects of radiant surfaces within a room than to direct contact with the air surrounding the body.

                    CH2 is designed to maintain the office at a temperature of 21°C to 23°C, which is the mean of air and radiant temperatures. To control the indoor comfort level, the ceilings are kept cooler in summer than in winter by regulating the temperature of the concrete ceilings and the operation of the chilled ceiling panels.

                    Thermal mass and the night purge

                    At night, when the external temperature has fallen below that of the internal concrete ceilings, windows beneath the low points of the vaulted ceiling automatically open. Cool night air flows 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, assisted by the roof-mounted wind-driven turbines (when wind conditions are right). This process is known as the ‘night purge’.

                    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.

                    If the outside temperature is at least two degrees lower than the temperature of the concrete an effective night purge can occur. This occurs on a floor-by-floor basis, meaning that some floors may have a night purge while others do not.

                    Whether a floor conducts a night purge is also determined by the temperature of the concrete ceilings. If the temperature of the concrete falls below a set level (usually about 20°C in summer) the windows will close and cease the night purge. This is to prevent the ceilings becoming too cold. In winter, this set point is raised to approximately 24°C.

                    In very high-wind situations, the purge windows on one side of the building remain shut and the wind-driven turbines maintain the purging air flow.

                    Chilled ceiling panels and phase change material

                    Phase change material (PCM) ball In the most passive mode, the chilled water to the ceiling panels is supplied by three large tanks in the basement. Each of the tanks contains nearly 10,000 small stainless steel balls filled with a form of phase change material (PCM), a salt suspension which freezes at 16°C.

                    The water in the tanks is chilled by the frozen PCM and then pumped around the building to the chilled ceiling panels when cooling is required. The water that returns from this circulation is usually about two to three degrees warmer. Heat from the water is transferred to the PCM balls re-chilling the water. The PCM balls continue to absorb heat until they melt.

                    By this process, the PCM acts as a thermal storage battery. When the PCM can no longer absorb heat, the PCM system is shut down and the chillers on the roof are used to chill the water required to run the cooling system. This occurs more frequently in summer.

                    As with the night purge process, the heat contained in the water is dissipated through the cooling towers on the roof at night through a trickle evaporative cooling process. Cool water is then 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.

                    Shower towers

                    Close-up of one of CH2's shower tower tubes.Located on the building's south facade, CH2’s shower towers are five tubes of durable lightweight fabric, 13 metres tall and 1.4 metres in diameter, inside which a water shower induces air movement and cooling. The ensuing evaporative cooling process cools both air and water.

                    From a simple shower rose at the top of the tower, water falls through the three-story tube, pulling air in from openings at the top. Both the water and the air are then cooled by this evaporative cooling process.

                    The cool air is fed into the ground floor lobby, shops and arcade to assist with the cooling of these spaces.

                    The cool water assists with the cooling of the office spaces by pre-cooling water returning from the chilled ceiling panels and improving the performance of the phase change material. Some of the heat absorbed by the water circulating through the chilled ceiling panels is dissipated through the shower towers (about 0.5 to 0.7°C). This ‘pre-cooling’ of the water before it enters the phase change tanks assists the phase change material to last longer before melting.

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