Saving With Thermal Storage
As the facility continues to improve its energy efficiency, however, the ability to record additional energy savings is becoming increasingly more difficult and leading the Bromont facility to consider new technologies to achieve its goal.
One of the technologies the company adopted combines a Novanergy® thermal-energy-storage (TES) system and a YORK® variable-speed chiller from Johnson Controls.
According to those monitoring the $2.6 million project, the upgrades are yielding energy and power savings that translate to $350,000 annually. And the ‘green’ factor entered into the equation. The project offers environmental benefits as it reduces the plant’s refrigerant requirements by 50 percent and its greenhouse gas (GHG) emissions by 35 percent.
REPLACING AGING CHILLERSIBM Bromont is an 800,000- square-foot facility located near Montreal that assembles semiconductors from around the world on substrates to ultimately create microprocessors. These microprocessors are used in Game Cube, X Box and Playstation game consoles, as well as IBM servers.
In 2005, it was decided to replace two 25-year-old chillers that used R-12, a no longer produced CFC refrigerant. The chillers were part of a power plant that also included six 1,000-ton chillers and a 1,000-ton free-cooling heat exchanger that typically ran from Dec. 15 to March 15.
Together, the chillers and heat exchanger provided chilled water year-round to meet process-cooling demands. As plant personnel looked to replace 2,000 tons of cooling capacity, their desire to improve the overall efficiency of chilled-water production caused them to look at alternatives to the existing system.
Daniel Paré, advisory engineer at IBM Bromont, recalled an article he read about TES systems and the phase-change materials (PCMs) manufactured by another Canadian firm, Groupe Enerstat Inc. “I decided to call Enerstat, and I invited the company’s president, Dr. Stéphane Bilodeau, to visit our facility and determine whether their system might fit into our plant. It soon became apparent that the approach to chilled-water production offered the potential for significant energy savings, and we agreed to use the TES system.”
STORAGE SHIFTSThe system uses a chiller to charge its storage tanks at night, taking advantage of off-peak electricity rates. A thermal fluid circulates through the storage tanks and stores or draws Btus by changing the state of the PCM from liquid to solid and vice versa at prescribed temperature set points.
As the stored cold energy discharges, it provides cooling during daytime peak hours when electricity rates are typically higher. Benefits to the building owner include reduced energy usage and costs. The benefit to the power utility is shifting consumption from day to night, when there is less demand for electricity.
In addition, PCMs are able to store large quantities of energy in a smaller space than a traditional chilled-water-storage system, according to those familiar with the technology.
The project consisted of a 1,500-ton variable-speed chiller, two 1,600-ton-hour TES tanks, a 2,500-ton plate heat exchanger, a glycol loop and two pumps. Two separate PCMs offered melting points of 28°F for one tank and 40° for the second tank.
“The PCMs are excellent energy accumulators (better than ice) for faster energy transfer,” explained Pare. “And, unlike ice, PCMs do not expand during the phase change from liquid to solid, eliminating stress on the tank. In addition, because the tanks are sealed during manufacturing, there is also less risk of contamination.”
PARTIAL STORAGETo maximize the energy plant’s efficiency, Enerstat used a partial-storage approach in which the chiller operates to handle part of the cooling load in peak periods, and the TES tanks handle the rest of the demand. This enabled IBM to reduce the size of the chiller.
Because company officials wanted to periodically operate the system at temperatures below 32°, they opted to use ethylene glycol as the thermal fluid. They then searched for a chiller that would be able to operate efficiently in a secondary thermal storage loop, with variable tower-water and chilled-liquid temperatures.
TEMPERATURE FLEXIBILITYIn selecting a chiller for the system, IBM officials relied on the experience of André Paré, manager, commercial and industrial products with The Master Group, Boucherville, Québec. “What amazed me as I considered the appropriate chiller for this application was the range of operation of the centrifugal chiller with variable-speed drive. It allowed efficient operation at a wide range of off-design conditions,” he said.
In the TES system, tank MCP1 is situated before the chiller (in the secondary loop), and tank MCP2 is located after the chiller. Set at 40°, one storage tank operates continuously. It can be charged and discharged several times in a 24-hour period. The other tank is set at 28° and is used to support fluctuations with large amplitude. It is charged and discharged one or two times a day. According to Paré, “This project requires a lot of flexibility on the part of the chiller as it works with a variety of low-side and high-side temperatures. The excellent efficiency of the chiller at off-design conditions is a tribute to the variable-speed drive.”
Bilodeau added, “Typically, a drive like this would see only variations in the tower-water temperature and the building load. In this case, the drive also sees variations in the chilled-liquid temperature at the same time.”
TES SYSTEMThe project at the plant demonstrates the ability of a TES system to reduce not only energy consumption (kWh), but also the peak-power requirement (kW). By running a smaller chiller during the daytime, electricity demand is scaled back.
In a partial-storage TES system, the chiller typically runs at full capacity for 24 hours per day. When the building load is less than the chiller output, the surplus thermal energy is stored in the TES tanks. When the load exceeds the chiller output, the tanks satisfy the additional cooling requirement. On site, the reduction in kW demand is more than 1 MW.
To reduce energy consumption, the chiller operates efficiently at off-design conditions, with consumption ranging from 0.4 to 0.6 kW/ton compared to the performance of the former chillers of 0.6 kW/ton to 0.9 kW/ton. Additionally, operating the chiller at night with reduced tower-water temperature represents an improvement in chiller performance by as much as 15 percent.
The TES system also increases the capacity of the existing free-cooling heat exchanger (FCHE) from 750 tons to a maximum of 1,250 tons. When daytime ambient temperatures are too warm to use the FCHE, evening temperatures are often still low enough to charge the TES tanks. IBM can thus extend the free-cooling period from three months to nine months a year, adding more than 3,000 free-cooling hours per year and allowing IBM to recuperate more than 800 tons of free cooling and cut its power consumption by 900 kW. Annual energy savings amounted to 5,300 MWh.
According to Bilodeau, “The installation of a TES system that integrates synthetic phase-change materials with free cooling and a variable-speed-drive chiller was the first of its kind in North America. The project size was also a first. The storage tanks store more than 50 million Btu per charge.”
Enerstat also designed a control strategy for the project that involves the real-time computation of the thermodynamic balance and a predictive model of the coming peak loads.
ENVIRONMENTAL IMPACTBy replacing the two 1,000-ton CFC-12 chillers with one 1,500-ton chiller that uses HFC-134a, IBM reduced refrigerant requirements by 50 percent. The company also experienced a 35 percent reduction in greenhouse gas emissions for chilled-water production because of the energy-efficiency improvement.
The $2.6 million price tag for the project included $1 million already budgeted to replace the two chillers and $1.6 million for the TES system. With energy savings of 5,312 MWh, which at a low cost of $0.0274/kWh adds up to $145,500, and annual demand savings of $205,500, the folks at Bromont are looking at total annual savings of $350,000 and a payback period of seven years. This estimate does not include savings associated with upkeep, pumping costs and maintaining water temperature according to specifications during electrical fluctuations.
For more information, visit www.yorkupg.com.
Publication date: 05/07/2007