Figure 1. The U.S. Patent and Trademark Office complex in Alexandria, VA, includes 2.5 million sq ft of space in five- to 10-story buildings. (Photo courtesy of Alan Karchmer.)

The challenge posed to Syska Hennessy Group by the owner, LCOR Alexandria, LLC, was to provide an energy efficient campus approach to a 2.5 million-sq-ft complex of five- to 10-story office buildings and two open parking structures, while retaining the flexibility of standalone office buildings. The solution from a mechanical engineering perspective was to develop a distributed central plant.

USPTO also wanted a more energy-efficient solution with greater backup and redundancy. The idea of a central plant was explored, but the site, a brownfield that had been an old train yard, posed its own challenges. First, the 10 acres proved too small to accommodate an independent central plant building. Any underground hazardous material cleanup would have increased the cost of the project further.

After a USPTO needs analysis was conducted, it was determined that a distributed central plant was the right answer. This distributed central plant links each 400,000-sq-ft building, two of which are connected by a 30,000-sq-ft, 170-ft-high atrium at the southernmost part of a quadrangle arrangement of buildings.

Figure 2. A “centralized performance solution” was implemented to connect each chiller plant to a common campus loop. (Photo courtesy of Kaveh Sardai.)

Designing for Maximum Efficiency

A variable primary pumping scheme was employed at USPTO to yield a smaller size mechanical room and long-term energy efficiencies. In addition, because of the demand for high reliability with concurrent financial constraints ruling out the simple addition of standby equipment, the project required an innovative adaptation of this pumping scheme.

The conventional application of a variable primary pumping scheme called for each building to have its own individual and independent chiller plant. Because redundant standby equipment could not be added to each building's decentralized chiller plant, a "centralized-performance solution" was created by connecting each of the six chiller plants to a common campus loop. This piping loop was routed through subterranean pedestrian walkways to eliminate the need for extensive excavation.

This pipe network allows any building's plant to feed the campus loop, much like a grid power distribution system. Furthermore, if required by scheduled maintenance, unforeseen equipment failures, or cooling demands, individual chillers and/or plants could be brought online to serve their respective building or the campus as a whole, or any partial combination.

Included in the mechanical penthouse of each structure are two Carrier chillers and a cooling tower. Four of the structures each have one Carrier 535-ton chiller and one Carrier 565-ton chiller. The two buildings linked by the atrium each use two 850-ton Carrier chillers as well as additional water-cooled Carrier chillers and also contain two air cooled chillers to provide 24/7 operation of USPTO's data center.

The system also includes plate-and-frame heat exchangers. Each plant has one unit for precooling. Each plant also uses two condenser water pumps with a 1,925 gpm capacity for the main USPTO building (Madison Building) plant and a 2,900 gpm capacity for each of the other four plants.

Figure 3. In the mechanical penthouse of each building are two chillers and a cooling tower. (Photo courtesy of Kaveh Sardai.)

Controls Optimize Efficiencies

In addition to cooling systems, the HVAC scheme incorporates 12 Simco heat recovery units to supply outside air for each building. Each unit recovers heat from exhaust airstreams from various parts of each building. The recovered heat is then used to preheat and/or precool airstreams used for ventilating the buildings.

The biggest challenge here was predicting the floor-by-floor demand for each building. To determine the demand, two terminal VAV units were installed on each floor, for a total of 120 VAV units. These provide accurate measurement of the air going into each floor and offer the ability to adjust the volumes through the BAS.

The air-handing systems operate in much the same fashion as the multiple chiller plants. Each air handler feeds a common duct loop. In the event of equipment failure or after-hours use, a single AHU may serve the entire floor loop.

Dedicated outside air is introduced to each AHU in a monitored and controlled fashion from the two energy recovery units. Virtually all of the exhaust and relief air is routed through these energy recovery units for substantial energy savings.

Figure 4. Condenser water pumps (under construction. (Photo courtesy of Kaveh Sardai.)

Stand Alone Capabilities

Dominion Electric Power is providing redundant 35 kV service feeders with an automatic "throwover" capability on the primary owner-furnished switchgear lineup. Each building contains a double-ended 2,500-kVA substation that is located at grade level, powering the mechanical and electrical systems. Each building also has an emergency generator ranging from 800 kW to 1,000 kW for life safety and critical loads.

The data center is supported by two dedicated 2,500-kVA substations that each power one of two 1,670- kVA rotary uninterruptible power supply (UPS) units, providing the mandated redundancy for 24/7 backup. In addition, there are a myriad of redundant feeders for the UPS for the critical data center loads. A dedicated 1,500-kVA standby generator supports the data center essential loads.

Included among these campus systems is a fire/life safety system consisting of a campus-wide, automatic wet-pipe sprinkler system and a voice-activated, multiplexed fire alarm system that connects all buildings. The system is designed for a high-rise and uses selective evacuation, allowing for evacuation of the floor where there is a fire as well as the floors above and below a fire. Each structure also has life safety egress and emergency lighting. The campus' two 900,000-sq-ft, five-level open parking garages are equipped with emergency lighting and backup power for the elevators.

A filtration system is also part of the design scheme for the complex. All of the intakes are located at the mechanical penthouse level of each building. They use a cartridge filter system that catches 65% of deleterious items coming through the filters. In addition, each floor in every building has a 65% filter and a 30% prefilter.

All the diverse systems are linked through a networked BAS, including filtration, life safety, electrical, chiller, and heat exchanger systems. However, the mechanical system and the electrical distribution system are separately monitored by computer. The network is made possible through a raceway of 24 4-in. conduits that allow connectivity of all the office buildings as well as the two parking structures.

The design of the USPTO complex provided an advanced, yet flexible approach for electrical, life safety, and filtration systems. All are successfully integrated into a combined complex/standalone scheme that satisfies the dual needs of the government agency tenant and private owner in a highly efficient and cost-effective way. ES