Variable Speed at Alliant Energy

-- Consulting-Specifying Engineer, 10/1/2000

Using the basic design concepts employed at Harley-Davidson University (see page 28), an enhanced version was developed for the Alliant Energy World Headquarters located in Madison, Wisconsin. This 320,000-square-foot building currently under construction is a working model for the efficient energy-management policy of this utility company.

The building utilizes state-of-the-art lighting systems, high-performance glass and enhanced thermal characteristics, which are the first major step in maximizing overall building performance. Both chilled- and hot-water systems utilize primary/secondary variable-speed pumping.

The chiller plant consists of water-side economizer cooling in conjunction with a 3,240-ton-hour ice-storage chiller system. The overall building design cooling load is 640 tons, which is met by first reducing the chilled-water temperature with the chillers and then further lowering the temperature via the ice system.

Because the chillers are used to build ice, constant-flow pumps are used for the primary side. Variable-speed pumps are used in conjunction with plate-and-frame heat exchangers for the secondary side of the system. In addition to this, variable-speed pumps are used for the condenser water.

Condenser-water flow is maintained by sensing the differential pressure across the condenser and automatic valve. The two-way automatic control valve is modulated in response to changes in chiller-refrigerant head pressure.

Control limits are set to maintain the minimum flow at or above three feet per second to prevent fouling. The design intent is to provide the coldest water possible to the condenser, and this is accomplished via an outdoor wet-bulb sensor that resets the temperature controller for the variable-speed tower fans. In addition, the chiller compressor speed is controlled by variable-frequency drives. Anticipated chiller full- and part-load performance as determined by computer simulation are shown in the above Table.