Cooling system

Chilled ceiling panels and Phase Change Material

Overview

A key consideration in the design of office buildings was how to cool the space. Even in Melbourne’s winter, cooling is required. Heat load is generated from two main sources:

heat loads 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. Typically, air chilled to about 13°C is introduced into the office at a high level and velocity in order to mix with the existing hot air, to create the desired temperature across the office floor.

The air entering CH2 is approximately 20°C (the lower end of the comfort range). The process of refreshing the air approximately twice every hour means that air leaving CH2 removes 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. This is done in two ways:

Through the use of the thermal mass of the exposed concrete ceilings. The concrete absorbs the heat from the rising air which is later removed from the ceiling at night with a 'night purge'
By using the chilled ceiling panels to circulate chilled water. The chilled water absorbs the heat and transports it to tanks in the basement containing Phase Change Material (PMC). The PCM tanks store any heat collected during the day, which is then removed at night through evaporative cooling by cooling towers on the roof.

On particularly hot days the cooling towers might be used during the day, but this is kept to a minimum for energy efficiency.

Radiant cooling

CH2’s comfortable temperature is achieved primarily by radiant cooling rather than by cooling the spaces with chilled ventilation air.

Radiant cooling is based on the principle that individuals are primarily cooled by body heat being radiated towards cooler surfaces. In CH2’s case, cooler surfaces are provided in the form of exposed concrete ceilings and chilled ceiling panels. In contrast to conventionally air-conditioned buildings, CH2 is not cooled via the influx of large volumes of cold air. The CH2 system recognises that air temperature is not the only way to measure and achieve thermal comfort within a building. CH2 acknowledges that humans sense a combination of environmental conditions such as temperature, humidity and draughts to gauge their thermal comfort. Feeling cold is not based on air temperature alone.

Physiologically, humans possess cooling systems that are sensitive to air humidity, air movement and surface temperatures. Also, 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 21ºC-23ºC, 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 or ‘stack’ effect, 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

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). CH2’s PCM’s are a salt suspension which freezes at 16°C.

The water in the tanks is chilled by the frozen PCM balls to about 16°C and is then pumped around the building to the chilled ceiling panels when cooling is required. The water that returns from this circulation is usually about 2-3 degrees warmer. Heat from the water is transferred to the 16°C PCM balls re-chilling the water. 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. When the PCM has melted into liquid phase, and 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, cool nights are used to dissipate the heat contained in the water. Water is put through the cooling towers on the roof. Using a trickle evaporative cooling process heat is dissipated to the night air, and cool water 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

Located on the buildings south façade, CH2’s shower towers comprise 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 is used to assist 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 0.5-0.7°C is removed). This ‘pre-cooling’ of the water before it enters the phase change tanks assists the phase change material to last longer before melting.