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High-Rise Energy Efficiency

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©2014 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 56, no. 7, July 2014.

By Shengwei Wang, Ph.D., Member ASHRAE; and Wai-Keung Pau, Member ASHRAE

About the Authors
Shengwei Wang, Ph.D., is a chair professor and the director of the Building Energy and Automation Research Laboratory, Department of Building Services Engineering, The Hong Kong Polytechnic University. Wai-Keung Pau was the deputy project director at Sun Hung Kai Properties Limited in Hong Kong.

Reducing building energy consumption while also providing occupants a satisfactory indoor environment was the major challenge for those working on the 118-story International Commerce Centre (ICC) super high-rise in Hong Kong. Since HVAC system design in 2005 and the first stage of occupancy in 2008, the author’s team has worked to develop innovative solutions to enhance the HVAC system’s energy efficiency and improve environmental performance. Solutions include improved system configurations, optimized control strategies and proper maintenance.

The ICC has a gross floor area of approximately 3,455,215 ft2 (321,000 m2), not counting the hotel that occupies the skyscraper’s top 17 floors. The building serves as a commerce center with commercial offices (primarily international financial firms), shopping malls, and the hotel.

Cooling is provided by a central chilling system on the eighth floor that consists of six identical centrifugal chillers (each with cooling capacity of 7230 kW [2,056 ton]) and 11 indoor cooling towers (each designed with motor power of 150 kW). Plate heat exchangers deliver cooling from lower floors to higher ones to avoid extremely high static pressure.

 

Improved System Configuration

Energy and operation performance of the HVAC system was modeled with different configuration/equipment options using local weather conditions and simulated operating conditions. Energy benefits from alternative designs were assessed and optimized designs were created and implemented.

 

Simplification of Secondary Water Loop Systems

The original primary-secondary pumping paradigm for the third and fourth zones in the building’s upper stories used primary constant speed pumps at the secondary side of each heat exchanger. The major function of these primary pumps is to provide the circulation force of chilled water to overcome the pressure drop of heat exchangers. However, under part load, especially under light load, these primary chilled water pumps would contribute a large portion of the total energy used in the third and fourth zones.

To reduce initial cost and save operating energy, the primary pumps were eliminated. The simplification of the secondary water loop required no extra investment and the site assessment shows that the annual energy savings using the optimized design is more than 1.0 million kWh.

 

Cooling Tower System Selection and Operation

The cooling tower system's original design used two-speed (two-stage) fans (later replaced by variable speed fans with the minimum allowed speed of 37 Hz). To maximize energy savings, the fans were changed to variable speed fans using variable frequency drives (VFD). And, the minimum operating frequency of the fans was reduced from 37 Hz to 20 Hz. Based on the commissioning test results, such use of VFDs and lower operating frequency can provide an annual energy savings up to 2.36 million kWh.

 

Daylighting Design

In the ICC, daylighting is incorporated by using curtain walls in offices and the lobby. The design allows maximum daylight access, lowering the demand on artificial lighting.

 

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