Reliable onsite power supports critical research projects at ASU
2006 case study from Combined Cycle Journal
Deregulation and the spiraling cost of energy have focused many large electricity consumers — such as municipalities and universities — on the benefits of self generation.
In the last few years, several municipalities have added small, efficient combined cycles in an attempt to stabilize power prices for native-load customers. Universities, which typically have both large thermal and electrical loads, often can reduce their energy bills by installing cogeneration plants.
Self generation is not new, of course. Economics forced it into hibernation when regulated utilities with large portfolios of coal-, nuclear-, and/or hydro-based generation had a price advantage. But that may no longer be true in some areas of the country. Often there are other reasons for installing onsite power.
Arizona State University, for example, opted for a combined heat and power (CHP) plant to guarantee a reliable supply of electricity, steam and chilled water for planned research and dormitory facilities. Staffing and funding for research projects would have been jeopardized if ASU could not ensure continuity of energy supply.
The university in Tempe hired APS Energy Services Co. of Phoenix, an unregulated subsidiary of Pinnacle West Capital Corp., also the parent of the local regulated utility Arizona Public Service Co., to design and construct a CHP plant with the following capabilities at full build-out:
- 15 MW of onsite generating capability.
- 24,000 tons of chilling.
- 160,000 lb/hr of steam.
- 4 MW of emergency power.
- 30 MW from APS’ new South Substation.
Layout. You can drive by the new CHP facility without ever thinking it’s a powerplant (figure 1). Perhaps that’s to be expected given the university setting. Originally, thought was given to a single-level arrangement, like most powerplants in the West, but sufficient land was not available. The only way to go was up.
The building is divided, with chiller hall adjacent to the generating equipment. The former is designed to accommodate twelve 2000-ton electric chillers; chilled water and circulating pumps are on the floor below. The gas and steam turbines (GT, ST) are arranged inline at grade (figure 2); two 2-MW emergency diesel/generators also are located in the turbine hall. Condenser and condensate storage tanks are on the ground floor as well.
The valve station for routing boiler steam to the campus heating system, steam turbine and/or condenser is on the mezzanine level, along with air compressors and circulating-and cooling-water pumps. Heat-recovery steam generator (HRSG) and cooling towers are on the roof and hidden from view by building walls that extend way beyond the roof level.
Phased construction. The project is being built in phases. The first phase, completed in 2005, included building construction plus installation of three chillers as well as electrical service from the South Substation. Second phase, which began commercial operation in May 2006, included installation of two more chillers and one combined cycle. Additional chillers and a duplicate power train are scheduled for later phases as needed.
New buildings for nanotechnology research are just north of the CHP facility; dormitories are being built on the opposite side. Site assessments for air quality, vibration and noise migration, and electromagnetic interference guided building design to ensure compatibility with the nearby laboratories and dormitories.
Nanotechnology research facilities are particularly sensitive to vibration, according to Kevin Nissley, manager of construction and safety for APS Energy Services. Thick concrete walls and heavy structural members were so effective in dampening vibration, he continued, none could be detected outside the building’s walls.
Powerplant details. GT is a Taurus 70, supplied by Solar Turbines Inc. of San Diego, rated a nominal 7.5 MW at ISO conditions. Output on an average day in Tempe is about 6.5 MW. Air is cooled before entering the compressor via chilled water coils in the inlet duct. The Taurus 70 now typically operates at half load because all of the buildings that the energy facility eventually will service have not yet been built.
A natural-gas pretreatment skid, complete with booster compressor, is provided in case pipeline pressure drops below that required by the GT. The gas supplier expects low-pressure conditions a couple of times annually. The remainder of the time, gas quality and pipeline pressure are within specifications, and the skid is bypassed.
GT is of the single-shaft type with a 14-stage axial compressor turning at 15,200 rpm. Compression ratio is 16:1, inlet air flow at rated conditions is 57.7 lb/sec, case is vertically split. The axial-flow turbine has three stages; combustion system is of the dry, low-NOx type with 12 fuel injectors. Generator is driven through a reduction gear and operates at 1800 rpm.
GT exhaust gas (213,840 lb/hr at 910F when the unit is operating at full-load ISO conditions) flows to the rooftop HRSG (figure 3), built by Rentech Boiler Systems of Abilene. It is rated 32,000 lb/hr of 150-psig saturated steam with duct burners off. When firing natural gas, the boiler can produce up to 80,000 lb/hr.
Kevin Slepicka of Rentech Boilers said the unit features a water-cooled furnace. The boiler manufacturer’s scope of supply included ductwork, burner, SCR (selective catalytic reduction) system for NOx control, CO catalyst, economizer, and stack.
Boiler makeup is relatively simple given the low-pressure saturated steam produced. Softened city water is processed through a reverse-osmosis system and stored in a makeup tank. A mixed bed demineralizer is installed in the unlikely event further treatment is required. The critical nature of the CHP facility demands that it be equipped to operate under a variety of off-normal conditions.
A Murray steam turbine/generator is located in front of the GT (generator end) and produces about 2 MW at full capability. Murray turbines now are manufactured by Dresser-Rand, which also owns the Terry, Moore, Worthington, Whiton, Coppus and Nadrowski ST lines.
Electrical system. The CHP facility would operate in an island mode should APS be unable to serve the university. At full output, the plant is able to carry the critical electrical and thermal loads for all seven research buildings planned, according to energy manager Rick Becker of APS Energy Services. Under normal conditions, the GT and ST generators operate in parallel with the 12.47-kV at Butte Substation. Insuring such operational flexibility required a battery of electrical studies and a sophisticated interconnection agreement.
APS Energy Services, through its Northwind™ Phoenix subsidiary, will operate and maintain the CHP facility for 20 years. Operations supervisor at the plant is Frank Cosentino.