Latest from Air Conditioning

Hpac 433 Design Solutions

Air-Cooled Chiller With 'Free Cooling' Looks to Be Right Buy for Best Buy

Jan. 1, 2007
Unit to save an estimated $18,720 a year in energy costs

In late 2006, Best Buy opened a store at Walden Galleria Mall, near Buffalo, N.Y. Like all Best Buy stores, this location is packed with demonstration electrical and electronic consumer goods, which generate a considerable heat load, in addition to lighting and personnel gains.

To reduce winter cooling costs, a direct air-side economizer was planned. The existing-building design and fan-coil/air-handling locations, however, prevented duct sizing sufficient enough to satisfy the cooling load. Major structural changes to the building were not an option, so another energy-efficient solution was required in a short period of time. That solution? A 120-ton “free-cooling” chiller from Motivair.

The MLC-FC 450 chiller supplies 42°F chilled glycol at a rate of 320 gpm to several variable-air-volume air handlers year-round via an integral pump and storage tank, all of which are contained within the chiller footprint outside of the store. Dual screw compressors in two independent refrigeration circuits provide the cooling capacity, while a standby pump with automatic change and alarm provides additional redundancy. An Ethernet connection for BACnet communication allows 24-hr remote monitoring and control.

Motivair free-cooling chillers are designed to produce 50-percent free cooling at ambient temperatures approximately 10°F below design chilled-water temperatures or 100-percent free cooling at approximately 20°F below chilled-water supply temperatures.

With a free-cooling chiller, a mechanical-refrigeration plant is not required to operate amid the lowest winter temperatures, avoiding low refrigerant head pressure and startup problems. Reducing the total number of operating hours and eliminating the most difficult operating conditions extends compressor life.

The free-cooling coils are located in front of the condenser coils, permitting the largest possible free-cooling-coil face area with low airflow resistance. This is important to minimizing required fan power.

The chillers’ programmable logic controller (PLC) continuously monitors outside ambient, return chilled-glycol, and design chilled-glycol-supply temperature. Once the ambient temperature is low enough to provide useful free cooling, the PLC automatically directs return glycol through the economizer coils, before it enters the evaporator. As the free-cooling effect increases, the refrigeration compressors unload sequentially until they no longer are required, at which point they remain in standby mode, and full cooling capacity is provided by condenser-fan power alone. Variable-frequency-drive (VFD) control of the fans allows precise control of head pressure, while the chiller provides full or partial mechanical cooling. When full free cooling is available and the compressors are stopped, the VFD maintains control of glycol temperature, maximizing energy savings at very low ambient temperatures, when less airflow is required to achieve 100-percent free cooling. All other aspects of mechanical and free-cooling operation, including three-way-glycol-valve operation and safety controls and alarms, are controlled by the PLC.

The change from mechanical to partial or full free cooling is both seamless and transparent to the operator. There are no temperature swings outside of the design temperature-control band, and the liquid-crystal display indicates when the free-cooling system is operational. The PLC prevents short cycling of the free-cooling controls and compressors. Compressor running hours are logged automatically by the PLC, so energy savings are available for verification at any time. Compressors and pumps are rotated automatically for even wear.

Buffalo spends approximately 1,532 hr at 32°F to 41°F (25-percent free cooling), 927 hr at 23°F to 32°F (50-percent free cooling), and 714 hr below 23°F (100-percent free cooling).

The MLC-FC 450 features two semi-hermetic screw compressors, each of which absorbs 75 kw. At a power cost of 8 cents per kilowatt-hour, Best Buy is estimated to save approximately $18,720 a year, compared with the costs associated with a standard winterized chiller.

A free-cooling chiller provides effectively free cooling by eliminating winter compressor operation, saving approximately 85 percent of the power cost normally associated with mechanical cooling during the coldest months.

The only requirements for free-cooling chillers are an outdoor location and the addition of glycol to the cooling loop. Installation cost is minimal; the only connections required are those for chilled glycol and electrical power. Multiple hermetic scroll compressors are used for 15- to 60-ton capacities, while dual, unloading semi-hermetic screw compressors are used for 75- to 250-ton capacities. Accessories include an integrated storage tank and dual circulation pumps. Options include Ethernet connection for remote monitoring and control via BACnet or LonWorks.

Generally, if a line were to be drawn from Norfolk, Va., to San Francisco, any location north would be considered ideal for free-cooling chillers. Typically, payback is one to two years.

Information and photograph courtesy of Motivair Corp.