Plant Efficiency at Alliant Energy
A Wisconsin utility's home office, the latest central plant project from February author Pete Zak, P.E., is a testament to both operational and energy efficiency.
CSE's February 2002 issue featured an article on chiller-plant efficiency by Peter Zak, P.E., Zak Engineering, Mequon Wis. The article focused on a recent project at the General Mitchell International Airport in Milwaukee, which chose to save energy, increase performance and plan for future expansion by replacing its existing central plant.
Along with this story, Zak provided a recount of another recent project (nearing completion) which demonstrates the principles of central plant efficiency: Alliant Energy World Headquarters in Madison, Wis.
Alliant Energy World Headquarters, a new 320,000-sq.-ft. office complex, is a showcase for the efficient energy-management policy of this regional utility company, and the installed HVAC system offers a testament to this commitment. From the beginning, this complex "demonstration" project was undertaken with efficiency in mind.
As a result, the first step in the design of Alliant's high-performance HVAC system was the inclusion of complementary design components such as enhanced thermal characteristics, high-performance glass and a state-of-the-art lighting system. The centerpiece of the system, however, is the central plant, with its flexible operational capabilities and precise controls.
The chiller plant features waterside economizer cooling coupled with a 3,240-ton hour ice-storage system. The 640-ton cooling load is met by first reducing the return water temperature using the waterside economizer (if available), then further lowering the chilled water temperature with the chillers and, finally, utilizing the ice system.
The heating system uses high-efficiency, sealed combustion boilers. Provisions have been made for the addition of a heat exchanger, which will recover heat produced by four microturbines. Whenever the microturbines are operating during the heating season, the heat exchanger will operate as the lead boiler, followed by additional boilers as the hot-water demand increases.
While constant-speed pumps are used on the glycol side of the cooling plant, variable-speed chilled-water pumps modulate to meet the system flow requirements as sensed by any of the seven differential-pressure transmitters located at the air-handling units.
In addition, condenser water flow is modulated in response to the differential pressure transmitter positioned across the condenser and automatic control valve. This automatic flow-control valve is modulated in response to the refrigerant head pressure.
Cooling tower fan speed is also modulated using variable-frequency drives, with the condenser water leaving the towers reset to the lowest possible temperature using a wet bulb controller. To save fan energy, the operating sequence utilizes the surface area of both towers prior to using fan energy. The tower fans are modulated upon a rise in the leaving water temperature, and to further enhance the efficiency of the system, variable-speed drives are used to control the compressor speed.
Hot-water distribution is accomplished by a primary/secondary pumping system, where the secondary pumps are controlled by variable-frequency drives that respond to the flow requirements sensed by multiple differential-pressure transmitters.
The anticipated full- and part-load performance of the chillers is indicated in the following table:
To provide a much flexibility to meet the building operational profile, eight operating scenarios were developed for the cooling-system control schemes. Each scenario has a different combination of the economizer, chillers and ice tanks operating.
To aid in the development of the control system, schematics were prepared indicating sensing points, valve locations and fluid flow. In addition to this, tables were prepared for each operating scenario that indicated the position of each valve.
Several methods are used to interface the building automation system (BAS) with the heating- and cooling-system equipment, and the end result allows the facility operator to monitor and control the entire system.
The chillers, for example, utilize a BACNET interface, and the chilled-water system control package is monitored by the BAS via a modbus interface software. The facility operator can reset chilled-water setpoints, initiate start/stop functions, set demand limit requirements, etc. In addition, safety controls for the chillers can be monitored, but not modified or controlled through the interface. And while the BAS will read an alarm status, any corrective action must take place at the chiller control panel, as having the operator physically at the unit when correcting an alarm condition prevents accidental equipment damage.
The chilled-water variable-speed pumping system, once started, will control the pump speed in response to the system differential pressure transmitters and select the optimal number of pumps to operate using a wire-to-water efficiency calculation.
The chiller-plant "operational mode," which sets the valve positions, is also controlled through the BAS, as are the cooling towers. The cooling-tower control system uses a zero-10 volt reference signal to start and stop the fans in response to the reset water temperature sensor.
The hot water variable-speed pumping system is controlled in the same manner, but with one additional control feature. The pumping package also controls the boiler start/stop operation. Once again all points are monitored by the BAS via the Modbus interface software.
The Alliant Energy World Headquarters was designed as a working model for energy efficient building systems, and the efficient HVAC system plays a key role. The building is currently being completed and occupied, and future considerations include applying for an Energy Star rating and monitoring the chiller-plant operations as part of an ASHRAE demonstration project.