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Problem Statement

  1. The power drawn by the fans is huge and is approximately a quadratic function of the mass flow rate of air. Thus, large peaks in the air mass flow rate are undesirable.

  2. Conservative pre-cooling strategies fail to flatten the air mass flow rate sufficiently. Also, precooling well in advance of what is required increases energy consumption because the cumulative cooling load is larger than necessary.

  3. Currently, many commercial buildings in Singapore use a simple pre-cooling strategy. The cooling system is switched on at a fixed time (say 1 hour) before the work day begins (9 am). All zones are cooled to the same setpoint, irrespective of their cooling load and without regard to occupancy.

  4. Many HVAC control methods are centralized and involve sophisticated optimal control methods which aim to minimize the total energy consumption across all zones.

Background of Existing Air-Conditioning Systems

In the centralized Variable Air Volume (VAV) air-conditioning system, the water chiller produces chilled water at a fixed temperature (typically 4°C to 7°C in Singapore) which flows at a fixed flow rate. While receiving the chilled water, the Air Handling Unit (AHU) receives a mix of outside air and recirculated air at the same time. Outside air is used because it keeps carbon dioxide within mandatory levels; recirculated air is used because it has lower humidity and is already cooled. A heat exchanger uses the chilled water to cool the air supplied to the AHU to a preset temperature setpoint (typically 12°C to 14°C in Singapore). The cool air output is then forced by a supply fan into the duct network in the building. Fans are responsible for creating sufficient pressure differences to ensure that enough cool air is supplied to the cooling zones.

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A zone is an area inside a building that is controlled by a single thermostat. It could be part of a large room or might comprise of several small rooms. The user sets the temperature he/she requires on the thermostat in the room. The VAV box (i.e. controller) receives this information from the thermostat and changes the damper position by sending an electrical signal to the damper actuator.

Cool air comes in from the supply duct and mixes with the existing warmer air to cool the room. It then flows back to the AHU through the return duct. The damper actually controls the mass flow rate of the supply air into the zone by altering the cross-sectional area of the duct. When a user sets a lower temperature, the damper will open more to allow more cool air into the room; when a user sets a higher temperature, the damper will turn the other direction to close or to limit the amount of cool air into the room