Twin Tower Cogen

Cogeneration systems, one might argue, are on the rise. That hypothesis, however, gains significant credence when the nation's largest publicly held office building owner and manager—Equity Office Properties (EOP)—jumps on the cogen bandwagon with a twin tower installation in San Francisco.


Cogeneration systems, one might argue, are on the rise. That hypothesis, however, gains significant credence when the nation's largest publicly held office building owner and manager—Equity Office Properties (EOP)—jumps on the cogen bandwagon with a twin tower installation in San Francisco.

Further adding weight to the argument is the fact that the office giant is embarking on a second cogen project by the bay—a decision sure to catch the attention of other developers and owners.

EOP's decision to implement the technology, according to Paulino Tiña, an associate at Glumac International, the consulting A/E on the cogen beta project—One Market Plaza—was not that difficult given Calfornia's volatile energy market. In fact, he says, a number of buildng owners besides EOP are considering cogen given the urgent need to help relievethe state's utility grid while improving system reliability and taking advantage of alternate energy at lower rates.

The cogen system in One Market Plaza is the first of its kind in San Francisco. And it's also among the first on-site power systems to be interconnected to the San Francisco utility grid in full compliance with Pacific Gas & Electric's (PG&E) Rule 21 Interconnection Standards. But making the system work for the twin high-rise office complex—Spear and Steuart Towers—took a true building team effort that included construction manager Venture Builders and Northern Power Systems.

In contemplating cogen, the process by which steam—in this case, for building heating—is generated from the waste heat of engine generator cooling water and exhaust, there were a number of obstacles, particularly those associated with trying to install distributed generation (DG) equipment and its associated systems into an existing high-rise building. But according to Tiña, the technology was never really in doubt. "A review of utility trend data, billing history and installed mechanical and electrical systems in the building clearly indicated that the building would be an excellent choice for distributed generation," says the engineer.

At One Market Plaza, the cogen system, which operates in parallel with the utility, provides much of the power for the building's electricity demands. But to provide the greatest amount of electrical efficiency, according to members of the Northern Power team, the system needs to operate at near capacity. However, the waste heat generated in this process allows the building operator to displace the need for natural gas to fire the building's boiler.

Specifically, the gensets offset the steam produced by the building boiler via heat-recovery steam generators. An added benefit is that the installed system pollutes less due to the nature and efficiency of the reciprocating engines. Therefore, in peak heating season, according to Northern Power, the system can provide a fuel efficiency of 78%—a number more than twice that of the 30% figure typically delivered with grid power. In all, the cogeneration system can deliver approximately 30% of One Market Plaza's yearly electricity and 85% of its yearly steam demand, notes the on-site power specialist company.

Installation challenges

The main challenges of installing a cogen system in an existing commercial facility, according to Tiña, are a lack of centralized plant and the issue of available physical space. "Ideally, cogen equipment would be installed in 'common,' non-tenant areas that typically don't generate revenue for the building owner," he says.

For One Market Plaza, adjacent parking space was procured to house said equipment. But eliminating this parking area meant the loss of a needed revenue-generating asset for the owner. As a result, the design team was sensitive to reduce the installation's impact on other revenue-generating spaces. This was no simple task as space had to be allocated for piping, switchgear and auxiliary equipment. Adequate space for maintenance and egress clearances also had to be accommodated.

As far as the equipment itself, three 500-kW natural gas-fired engine generators make up the heart of the 1.5-megawatt system. Following a survey of the building, it was determined that a former 1,200-sq.-ft. generator room in the basement proved ideal for the new gensets. The space would also house electrical control switchgear. However, limited vertical space posed a problem for installation of the corresponding heat-recovery steam generators (HRSGs). The solution, according to Tiña, was to install the HRSGs in a 400-sq.-ft. space one floor above the genset room. A new gas meter room was also constructed adjacent to the heat exchanger room.

To ventilate these spaces, a horizontal fresh-air intake shaft was run along the south side of the building for combustion air. The exhaust from the engines, says Tiña, exits the generator room and routes to the spill-air shaft, located approximately 50 ft. away, up to and slightly past the seventh floor. The spill-air shaft also houses an auxiliary cooling radiator and steam condenser. The three heat-recovery steam generators and the steam generators that connect to the building boiler system are both located in the basement.

As noted, the new DG system is paralleled with the existing electrical distribution system in One Market Plaza and consists of nine electrical services. Tiña says each is 4,000 amps, 480/277 volts, serviced from two separate underground utility transformer vaults with redundant transformers connected by their secondaries. Six services are fed from the Spear Tower vault, three services from the Steuart Tower vault.

The Spear Street service is located approximately 250 ft. from the new generator room, and the Steuart Street service is located approximately 300 ft. north of the room, one floor above. "We performed a load profile to determine which services would best interface with the new cogeneration plant in terms of economics and anticipated load trend," says Tiña.

Based on the study, three services qualified, two in Spear Tower and one in Steuart.

As far as the gensets, each is connected via additional distribution equipment to a main service entrance switchboard. "Upon power failure, the engine generators will shut down, since they are not able to support the total load of the two towers," says Tiña.

The cogen plant has been commissioned and is operating. It's success, Tiña hopes, will push more owners to look at installing distributed generation in their facilities for efficiency and reliability.

"In San Francisco, the electrical utility infrastructure has been strained due to continued demand for more power. And it's been difficult due to [lack of] funding and limited space for additional substations," says Tiña.

Bright outlook

The good news, however, is that he believes DG and cogen equipment will become more and more efficient and, hence, operating costs per kilowatt will decrease.

"As equipment becomes more efficient, more advanced technologies, such as fuel cells, will become more attractive to building owners," says Tiña.

In the case of One Market Plaza, the dramatically increased fuel efficiency qualifies EOP's new system for an incentive rebate from the Self-Generation Incentive Program of the California Public Utility Commission (CPUC), a program to encourge on-site generation.

The ultimate test of the techology's success, however, is the fact that EOP already has plans for a cogen installation at a second San Francisco property—201 Mission Street.

Energy Efficiency for a Second San Fran Building

Equity Office (EOP), developer of San Francisco's One Market Plaza, is following up its first cogen success story in the city with plans for a turnkey, grid-connected on-site combined heat and power (CHP) system for its 201 Mission Street office building.

The new CHP system will meet a significant portion of 201 Mission Street's electricity demand and will be fully interconnected to, and operate in parallel with, the Pacific Gas & Electric utility's downtown network grid. This enables the utility grid to meet only the power needs above and beyond what the on-site system can supply and, as a result, generated electricity on-site will not be burdened by the losses and additional costs associated with electricity transmission and distribution.

"The CHP system reduces the building's draw on the city's overburdened power supply and further exemplifies EOP's commitment to making our office buildings energy efficient and environmentally friendly," says Frank Frankini, EOP senior vice president-engineering and energy operations. "The addition of the CHP system furthers 201 Mission's qualifications as a potential recipient of the EPA's Energy Star award for energy efficiency."

Designed to deliver improved energy efficiency and qualify the office building for California's Self-Generation Incentive Rebate, the 750-kW system features two 375-kW natural gas-fired engine generator sets that will feed two of the building's three electrical services. Waste heat from the engines and exhausts will be used to produce hot water for the building's heating system.

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