Duke University studied emergency power aesthetics, noise, emissions, and maintenance costs. The result—a central generator plant.
Mark Demana, PE, RMF Engineering, Raleigh, N.C.
The Durham, N.C., location of Duke University opened in 1892 as Trinity College and was renamed Duke University in 1924. Many of the original buildings are still used today. The 1,300-acre academic campus includes 175 buildings, many of which require a large enough emergency power demand to warrant a local generator. More than 30 stationary generator sets are distributed around the east, central, and west campuses, ranging in size from 30 to 1,500 kW.
The gensets consist almost entirely of diesel-fueled engine prime movers, with a sufficient quantity of fuel stored at each location to meet code requirements (NFPA 110: Standard for Emergency and Standby Power Systems). The total tank capacity of these units is more than 14,500 gal. To ensure they will operate when called upon and meet code requirements, each genset must be inspected weekly and exercised. Code requires testing for Level 1 systems at 30% load for a minimum of 30 minutes monthly-or exercising monthly with the available load and annually with supplemental loads at no less than 50% of the nameplate kilowatt rating for 30 continuous minutes and at no less than 75% of the nameplate kilowatt rating for 1 continuous hour for a total test duration of no less than 1.5 continuous hours. Typical maintenance includes checking and replacing fluids, belts, hoses, and batteries (as needed) and servicing the fuel by polishing, chemically treating, and/or replacing it.
Maintaining the many units is only one of the challenges of owning and operating standby gensets. Architects and campus planners work to hide the units from view for best campus aesthetics; the noise emitted when the units are running can disturb building occupants and passersby; and exhaust fumes from the gensets occasionally are drawn into building ventilation systems, causing false alarms and complaints. Except when needed, the standby genset is a necessary evil.
In 2012, the director of Duke University's facilities management department commissioned a study to determine the feasibility of centralizing the emergency power. Unlike the common centralized chilled water and the steam and hot-water systems, the very limited amount of run time on an emergency-only power generator would not result in energy savings. Instead, the study focused on aesthetics, noise, emissions, and maintenance costs.
The study captured the size, location, age, and actual load of all the distributed gensets. Replacement and maintenance costs were annualized over 30 years and compared against the estimated cost of installing and operating a central generator plant (CGP). The result of the cost analysis showed a 10-year payback. The load analysis concluded that replacing the 11.5 MW of distributed generation could be accomplished with two central plants, each with two 3.25-MW units, to be installed in phases as the demand required.
The first genset at CGP-1 was installed in October 2014 near a steam plant. The second unit at CGP-1 will be installed to meet campus growth following the execution of the campus master growth plan. Each CGP is modeled as two outdoor enclosed gensets rated at 3.25 MW each at 12.47 kV, connected to paralleling switchgear and associated underground distribution. A pad-mount transformer would replace local generators. The cost of converting existing buildings is spread across the 30-year study based on their age versus their expected life. Despite the relatively long payback period, this strategy allowed for the improvement of aesthetics and reduction in noise and exhaust infiltration.
Site selection for the CGP and the impacts of noise and exhaust were studied carefully. Airflow modeling helped determine the orientation and confirmed the design of combining the radiator cooling air with the exhaust in an upward discharge to minimize any odors near the site. Specifications called for 70 dB maximum at 25 ft, which the manufacturer exceeded, meaning measured sound levels are at or below the requirements.
Central generator plant
Immediately after the installation of the first large genset at CGP-1 in October 2014, the connection of first-priority buildings commenced. There were challenges in this process-emergency power is one of the most detailed systems for code compliance. Meeting with the authority having jurisdiction (AHJ) to explain the system was paramount. Confusion about the system had to be resolved including separation of emergency circuits from normal, the ability of a remote transfer switch to start the genset, and the ability to energize loads in the required 10 seconds or less. Once the AHJ was satisfied with the design, the project could move forward with implementation. The outage of emergency power to a building meant it could not be occupied during the transition. Additionally, the interface to older transfer switches was problematic and had to be handled carefully. The design team recognized the need for system monitoring to ensure all buildings would have emergency power available. A campus-wide SCADA system is used to verify all transfer switches have emergency power and that the CGP generators are healthy.
Several new buildings at Duke University have been designed and built that have taken advantage of the CGP system for emergency power. The improved aesthetics, as well as the relief of not having to worry about the exhaust and noise, satisfied both the design team and the university.
Executing a CGP at Duke University has been a success. The university is happy with the clean, small footprint of the pad-mounted transformer that now serves buildings instead of a diesel gensets. As the existing aging generators are phased out, the appearance and noise reduction are immediately apparent. Costs associated with maintenance, repair and testing the individual gensets are eliminated, with an average savings of approximately $300,000 per year. CGP-1 is located near a steam plant, keeping major utilities consolidated and minimizing the campus impact. The genset provides backup power to the steam plant as well as the first phase of building generator replacements. CGP-2 is located near a chiller plant and similarly provides backup for the chiller plant as well as a series of new and veteran buildings. Delayed starting of the chillers and boilers ensures the life safety power to occupied buildings will meet the 10-second requirement. The economy of scale and the improvements to the campus have created a win-win for the university.
Mark Demana is a project manager with RMF Engineering. He is an electrical engineer with more than 30 years of experience in electrical design and project management.