By Joe Zwers
When the State of California needed a new central utility plant (CUP) to provide electricity, steam, chilled water, and compressed air to its 5.5-million-square-foot, 23-building campus in downtown Sacramento, the requirements were stiff.
The original CUP was sending as much as 15 million gallons of heated water per day to a spray header over the Sacramento River for discharge. In 2000 the EPA filed a complaint stating that this discharge was causing the river’s temperature to spike, adversely affecting the riverine wildlife. Since much of the equipment had reached its end of life, and the 40-year-old technologies could not approach the efficiencies of modern equipment and control systems, the state took the opportunity to cut its power costs and provide for anticipated growth.
In October 2007 the state signed a design/build contract with Skanska USA for a completely new facility. Groundbreaking for the $181 million plant took place in November 2007, and it began serving the campus the following August. The 78,000-square-foot design included a wide range of features to increase the value of the project.
The new plant features highly efficient gas-fired steam boilers along with cooling equipment outfitted with variable-speed electric chillers and cooling towers (figure 1). It also utilizes on-site renewable energy, including solar photovoltaic panels mounted in the parking area that generate power for the facility’s office and support areas, as well as domestic hot water.
Providing Emergency Power
The boilers also power a 2.9-MW Dresser Rand steam turbine generator to provide backup power in the event of a blackout. With quality and reliability as key requirements, rather than lowest cost, Skanksa selected RENTECH Boiler Systems to build the boilers with Coen low-NOx burners.
“RENTECH boilers were chosen based on quality and also due to their track record for a medium-pressure industrial steam plant,” Skanska senior project manager Ciaran Creighton said. “There were other technical aspects such as the burner package, turn-down ratio and emissions capabilities that factored into the decision.”
The CUP uses four RENTECH boilers, two that provide 81,000 lb of steam each per hour at 250 psi and two smaller units that provide 29,500 lb each per hour at the same pressure. Even during the summer, the boiler plant operates 24/7 to feed heat exchangers for building hot water and localized mechanical heating in the buildings. The steam plant was oversized so that it could meet demand for heat on even the coldest days, but this creates potential efficiency losses, higher emissions, and the chance of losing the flame. RENTECH’s high turndown ratio (6:1) was one of the critical factors.
The extra steam capacity also makes steam the best choice for power generation in an emergency. The CUP has three 4160v services coming from the Sacramento Municipal Utility District to feed the chillers. In the event all three are lost, the plant will fire up a 650-kW generator in manual mode, cross connect it to one of the boards, and then fire up the boiler plant. After half an hour, the steam turbine will be idling and after an hour the boiler plant will be stable enough to drive the steam turbine generator (STG) and start producing power. At that point, the STG replaces the 650-kW generator on the board and the operators can use it to feed primary pumps, distribution pumps, makeup pumps and the chillers themselves.
The STG was included in the original plan to ensure that the data center load would get the 3000 tons of critical cooling it needed. But since the thermal energy storage tank that was part of the final design holds enough water to cool the buildings for 7 hours on a 103F day, the electricity can do more than just run the chiller for the data center.
“The plant is extremely energy efficient,” Creighton said.
The original specs called for the plant to be built to LEED Silver Standards, but Skanska asked that the bar be raised.
“Originally when the RFP came out, it said LEED Silver, but we said it should be LEED Gold so they changed the RFP and made that a contractual requirement,” Creighton added.
By the time the plant was commissioned and certified, it had managed to achieve LEED Platinum. More important than the certification though, the state government has a more efficient, reliable and environmentally friendly source of heat, cooling and power to meet its growing needs.