Massachusetts Building Opts For Hybrid Plant
Built in 1975, the 800,000-square-foot McCormack Building was originally designed with a steam absorption chiller system consisting of two 600-ton, single-stage, steam absorption chillers. Steam was purchased to drive the chillers and provide space heat and domestic hot water to the building.
As part of an energy-saving program in the mid-1980s, the steam absorption system was replaced with three 450-ton electric centrifugal chillers. Unfortunately, there was not enough electricity available, so just two of the three chillers actually went on-line, providing 900 tons of cooling capacity to the building. Since that time, the demands on the building's electrical system have continued to increase significantly.
A new cooling system was needed, and it was decided that a hybrid chiller plant would fit the building's needs. A mix of direct-gas-fired absorption chillers and electric-drive centrifugal chillers was ultimately installed in 2003 to provide cooling to the office building. This combination allows the state to take advantage of less expensive gas utility rates to condition the building efficiently and economically.
Changes Were NeededOne of the biggest issues in the building was the fact that the electrical system was originally designed to support a steam absorption system. The addition of electric chillers in the 1980s and the further demands for electricity created by the explosion of office automation soon exceeded the supply of electricity to the building.
To meet the demand, the building relied on an electric feed from a building under construction nearby to supplement its electrical supply.
"We knew the feed was a temporary solution to a long-term problem," stated Bijan Mohammadipour, principal engineer with BSOB. "At the same time, the cost of electricity was increasing, as was the cost of steam, all at a time when our operating budget was cut. So, we needed to come up with a method that allowed us to reduce our energy costs without compromising the comfort level in the building."
To that end, J.F. White Contracting Co., Framingham, Mass., embarked on the design-build project with design engineer VAV International Inc., Woburn, Mass. The company installed a hybrid chiller plant that included two York YPC two-stage, direct-fired, natural gas absorption chiller-heaters in the McCormack Building. The company also moved two electric centrifugal chillers (one 550-ton and one 650-ton) from an adjacent building undergoing renovation into a new hybrid plant. One of the 450-ton chillers from the 1980s renovation remained in place as an emergency backup chiller.
The gas-fired chillers function as lead chillers, able to meet the building's 900-ton cooling load without increasing the demand on the building's electrical service. They also take advantage of lower gas rates during the cooling season, when electric rates are typically much higher, reducing energy costs and the dependency on electricity.
Jeff Rosen, mechanical division manager, J.F. White Contracting Co., noted that the chiller-heaters can also heat hot water off the cooling process. "That hot water is able to be run through heat exchangers and basically heat the domestic hot water system during the summer, so the gas-fired boilers can virtually be shut down during the summer if the owner chooses," he said.
The cooling capacity in the building has been increased by 300 tons while the electrical demand has decreased by 300 kW. When the gas-fired chillers are not running, the demand is reduced further, which is one of the benefits of a true hybrid system: The electric chillers can be run on hot nights to keep the building cool. During the daytime, when electricity costs are higher, the gas-fired chillers can be run.
Aerial InstallationThe McCormack Building is located in downtown Boston, on top of Beacon Hill right behind the State Capitol. The slopes of the adjacent streets are such that crane access would not be possible, so a helicopter was needed to take out the old chillers and put in the new ones.
"We had to cut a hole in the roof and dismantle two of the three chillers that were already in place to remove them with the helicopter. Then we lowered in the two electric centrifugal chillers taken from another state building and then the two gas chillers. We moved about 133,000 pounds of equipment in nine lifts, and it was under two-and-a-half hours," said Rosen.
The streets close to the building had to be closed off in order for the helicopter lifts to take place. "We had to have all the equipment lined up, in order, and ready to be in-stalled. That required quite a bit of planning on our part."
Once the chillers were in, the rest of the installation was fairly straightforward.
The main mechanical room is located on the 22nd floor, and that is where the boilers and chillers were installed. Quite a few revisions had to be made to make room for the new chillers, pumps, and variable-frequency drives. The seventh-floor mechanical room only required new heat exchangers to be installed. No ductwork was changed during the renovation.
According to preliminary studies conducted by Mohammadipour, the move to a hybrid plant will save Massachusetts approximately $530,000 annually in utility costs. "That figure increases when you add manpower savings that are associated with newer, more efficient equipment," he said.
In addition to these savings, the gas company provided $258,000 in energy conservation rebates. Together, the savings and rebates have enabled Mohammadipour and his team to easily meet the governor's recent directive to reduce operating costs of government buildings by 10 percent.
Rosen noted that the entire job was a positive experience because the tenants in the building were not disturbed. "The tenants weren't interrupted or moved," he said. "We were able to complete a major mechanical job in a building without having one complaint from the tenants with respect to the building being cold or hot or somebody having to be displaced. That was great."
And, for the first time in a long time, the tenants are working in a comfortable environment.
Publication date: 07/12/2004