ABSORBERS INCREASE OVERALL EFFICIENCY

When the mall was originally conceived, much of the area was still undeveloped. Access to a bulk electric power supplier was difficult and costly. For that reason, the building owners chose to install an independent power system for the facility.

A cogeneration concept was woven into the design from the beginning. The plan was to increase the efficiency of the facility by using the heat recovered from the water-cooled natural gas electric generators to supply absorption cooling equipment. The plan also called for electric centrifugal chillers as a backup for the absorption chillers.

According to the property managers, the concept of on-site electric generation with heat recovery for absorption cooling has been periodically reviewed. The property managers say it continues to be seen as the best system for this property. With additions and upgrades, the electric generation plant today consists of seven Waukesha diesel units with a total capacity of 5.8 MW. The engines operate at 1,200 rpm and can meet the full electrical requirement of the mall, with at least one engine always in reserve.

The combined electric generation and chiller plant is located within the mall building itself. The normal fuel for the engines is natural gas purchased under contract from City Gas Corp. of Florida, a division of the NUI Corp. Because of the need for uninterrupted electric service, the mall also maintains a three-day backup supply of propane gas.

EXHAUST HEAT RECOVERY INCREASES COOLING CAPACITY

The engines are cooled by a natural circulation method. The more recently upgraded machines have heat recovery units on the engine exhaust. At full load, the engines produce a heat output of approximately 2.5 million Btu. As the engines have been upgraded through the years, and with the addition of exhaust heat recovery, the plant now has a useful heat output nearly twice as great as when the plant was originally installed, according to the property managers.

The engines receive a complete overhaul each 40,000 operating hours, which, in this application, works out at about seven to eight years. At the time of the engine overhauls, the units are commonly upgraded with the most recent improvements for heat recovery and capacity enhancement.

The hot water from the engine plant goes to two Trane absorption chillers. These consist of a 550-ton Trane Model ABSC single-stage absorption unit installed in 1986 and a Trane Horizon®, Model ABSD single-stage absorption unit, installed in 2000. These are the primary chillers for the mall and at least one unit is in use year-round.

The units receive water from the engines at approximately 245?F (118?C), using it to generate chilled water at about 42?F (7?C). The cooling tower design calls for condenser water going to the tower from the chillers at 102?F (39?C) and returning at 85?F (29?C).

CENTRIFUGAL UNIT FOR BACKUP, PEAKING

In addition to the absorption units, the plant includes two electric centrifugal chillers for use when cooling requirements exceed the capacity of the absorption machines. One Trane Model CVHA CenTraVacTM chiller rated at 550 tons and a newer R-123 Trane Model CVHF CenTraVac rated at 750 tons are used. The newer unit replaced an earlier Model CVHA. The mall electrical generation plant can completely supply these units, as needed.

Four Marley cooling towers, located on the roof of the structure, serve the four chillers. The towers, rated at 700 tons each, are on a common loop. Makeup water for the cooling towers is of generally good quality and is supplied by the city of Cocoa’s system. A local firm provides routine makeup water treatment services.

According to chief engineer, Warren Meissner, the base-loaded absorption chillers require relatively little maintenance. "Generally we inspect and clean the condenser tubes annually. Other than that, there have been very few outages for the absorbers. And for much of the year, they run around the clock."

The combined control room for the electric generation facility and the chiller plant is staffed around the clock, seven days a week. The operators are instructed to ensure that the hot water flowing to the absorbers remains within the required range and that the cooling towers are performing as needed. "We have operators on duty because of the electrical plant," said

Meissner, "so they operate the chiller plant too. Otherwise we could have designed the chiller plant controls for unattended operation."

Two 100-hp chilled water pumps distribute the chilled water from the plant. The water is pumped to air handlers throughout the mall that discharge conditioned air through ductwork. Because of its Florida location, the facility requires chilled water year-round. This long operating season

contributes to the overall energy efficiency of the cogeneration application.

POTENTIAL FOR ADDITIONAL RECOVERY

According to Meissner, the mall has sufficient electric generation capacity to meet its needs well into the future. Eventually he hopes to update all of the engines and install additional heat recovery in the exhaust systems. "The more heat we can recover," he noted, "the more efficient the chiller operation becomes." He also indicated that he is particularly pleased with the newer Horizon absorption unit. "It's even more efficient, so we get more cooling out of every unit of heat input." Meissner observed that if the system were to be designed all over again, he would recommend building a chilled water storage tank to extend the economic benefits of absorption cooling. "And I would recommend designing an electric system to sell surplus power back on line. We have engines idle quite a bit of the time." Operations manager Golden emphasized the success of the system. "We've been able to meet all of our air conditioning commitments but, until we got the newest absorption chiller, it was always a concern. Now we're going into the season with a comfortable surplus." He said that in the retail real estate market, it is important to hold costs for tenants. "Our efficient cooling plant gives us a tool to make tenants happy."

Publication date: 11/26/2001