The mixed-use of the 800-foot-tall building required different systems for the residential and commercial areas.
The office floors feature a dedicated outdoor delivery system on every floor, supplying air to fan-powered terminal units that provide preconditioned and dehumidified air to the fan-powered terminal units installed at each zone.
“The outdoor air delivery system contains a high-temperature chilled water coil that precools the outdoor air in the summer and a secondary DX coil that will provide additional cooling and dehumidification,” said Alan Davis, design engineer on the project. “This also increases efficiency for the chilled water plant, because it allows the chilled water supply temperature to be higher than what would normally be required for dehumidification.”
This system eliminates the need to overcool and reheat supply air during part-load cooling conditions, as is typical of a central variable-air volume system, making it much more efficient.
Meanwhile, residential units are conditioned and ventilated by four-pipe fan coil units that have direct access to fresh air intake through the façade of each condominium.
“Additional energy savings also come about because of the reduced need for fans, since only ventilation air is distributed throughout the building,” Davis said.
The chilled water plant consists of three variable-speed, water-cooled centrifugal chillers, two cooling towers with variable speed fans and two waterside economizers that provide free cooling to office and residential chilled water loops when the outdoor conditions allow.
Additionally, a mini natural gas-fueled cogeneration combined heat and power turbine with 65kW electricity and 118 kW heat generation capacity provides electricity for electrical base loads and heating to meet hot water demand. Due to the utility rate structure in San Francisco, operating the cogeneration turbine results in utility cost savings.