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The current Air Handling Unit (AHU) based Variable Air Volume (VAV) systems using a large central AHU for both sensible cooling and dehumidification have several disadvantages. All these will incur penalties on both capital and operating costs as they increase first cost on chiller plant, AHU, VAV boxes and associated auxiliaries on added cooling load, and operating cost for the extra sensible cooling of the air.

To solve these problems, Air T&D has developed the “New Generation of Energy Efficient ACMV Systems”, which adopts several innovative technologies. As chiller consumes the largest amount of energy (over 50%), the ideal supply air temperature to the space is around 16oC~18oC, and the Coefficient Of Performance (COP) is almost linearly dependent on the chilled water supply temperature. As chillers consume most of the energy in ACMV systems and the Coefficient of Performance (COP) is almost linearly dependent on the chilled water supply temperature. Higher chilled water temperature not only increases chiller COP but also improve IEQ. However, the higher chilled water temperature reduces the capability of dehumidification.

In this project, It was proposed to develop a prototype of low cost, energy efficient, easy to be implemented and operated solution for general building dehumidification applications, “Dual Cycle Air Dehumidification Systems” (DCADS) for DOAS and PAHU applications. The product will be able to overcome the drawbacks of the existing dehumidification schemes and achieving substantial energy savings.

DCADS uses higher temperature central plant chilled water or VRF refrigerant to precool the air and then use a high efficiency internal heat pump to dehumidify the air to the required humidity ratio and reheat the air before discharge. The heat pump system allows for condensing the steam and lowering water contents in the air. To control the supply air temperature, part of the compressed refrigerant (in gaseous state) passing through a bypass condenser (coaxial heat exchanger), where the cooling medium is the outlet chilled water from the primary cooling coil. Since the pressure drop in the bypass condenser is much smaller compared with the main condenser, the supply air temperature is controlled by regulating the refrigerant going into the bypass condenser through a control valve.

The “New Generation of ACMV system” differentiates from the existing ones in terms of its independent control of indoor temperature, humidity and ventilation (varying one parameter will not affecting others), so that lower energy cost and good Indoor Environment Quality (IEQ) can be achieved.

For the new system, DCADS can integrated with chilled water system and Passive Displacement Cooling (PDC) system to achieve notably energy savings for the whole ACMV system. For the existing system, DCADS can be integrated to the building flexibly.  For some buildings with lower RH level, DCAD is able to achieve within lower power consumption.

The project main deliverables are:

• DCADS with design tools consisting of reliable design data, systematic design and installation guidance that addresses the local climate conditions.
• New methodologies for system integration including DCADS, chilled water system and PDC air distribution loops.
• A fully automated, operational, and commercially ready system which is energy-efficient and reliable achieving energy savings of 20% compared with the current technologies.