Thermal storage system offers comfort plus savings
Weichler is referring to a thermal storage system installed to serve two new buildings on the ETS campus.
ETS creates, administers, and evaluates a wide range of educational placement and achievement tests including the SAT®, the Graduate Record Examination (GRE®), and the Test of English as a Foreign Language (TOEFL®). National headquarters for ETS is on a 370-acre rural site near Princeton. More than 1,600 employees work on the site comprised of 15 buildings that include offices, a library, training facilities, production areas, a conference center, and administrative facilities.
ETS moved to this site in 1956 and has an experienced facilities hvac staff. The buildings are served by a variety of comfort systems. ETS has been a pioneer in advanced hvac technology, including early double-wall air handlers and an early building automation system application in 1983. Recently, the ETS organization chose to install a thermal storage chiller plant in its two newest buildings on the site.
These new facilities, referred to as the L and N Buildings, are mirror images and are adjacent to each other. The two-story buildings each encompass 105,000 sq ft and were completed in July and December of 1996, respectively. They house the ETS Assessment Division and various administrative offices.
While the buildings were in the design stage, an evaluation was made of various options for building comfort cooling.
Weighing options“We felt it would be a chilled-water system,” said Weichler. “Our in-house staff is experienced in this type of operation. But we wanted to look at various options for the type of system.”
The evaluation was managed by Wayne Evans, senior engineering designer at Hillier Clarks Summit, an engineering consulting firm. Four primary types of system were evaluated for total lifecycle cost:
1. High-efficiency electric centrifugal chillers;
2. High-efficiency electric centrifugal chillers with ice storage;
3. Direct-fired absorption chillers; and
4. Electric rotary screw chillers with ice storage.
The evaluation included a 15-year lifecycle cost analysis based on a computer projection of the energy requirements and expected occupancy cycles of the two buildings.
Based on the analysis, the choice was made to go with rotary screw chillers with ice storage. This option was favored, in part, because of its potential to reduce the impact of demand charges and lower off-peak energy rates from the electric utility, Public Service Electric and Gas Co. (PSE&G).
Specifically, the system includes two Trane Company (La Crosse, WI) 235-ton Model RTHB water-cooled helical-rotary chillers, 16 Calmac size 190 (190-ton) ice storage tanks, and a double-cell, induced-draft cooling tower. The system also includes primary and secondary chilled-water and glycol-solution pumps and condenser water pumps. The chillers and ice tanks for both buildings are located on the basement level of Building L.
On a design cooling day, the stored ice provides more than 20% of the ton-hours of necessary cooling. On cooler days, that contribution rises as high as 50%. Ice making is normally scheduled between 10 p.m. and 6 a.m., depending on the level of depletion. During the day, the chiller operates at up to 100% capacity; the excess requirement is met by melting ice.
The glycol/chilled-water system provides water at 37ÞF to the four Trane Modular Climate Changer® air-handling units in each building. These, in turn, feed fan-powered vav boxes with electric resistance heating coils throughout each building. At the vav boxes, this low-temperature air is mixed with 78º plenum air to provide 56º air to the conditioned spaces.
Outside inThe system includes a provision for supplying outside air to the system through the air handlers. The outside airflow is controlled using Trane Traq™ dampers, based on the ventilation needs of the critical zone vav box.
In the design process, ventilation air requirements were verified using the manufacturer’s VentAir 62™ design program. The building is slightly pressurized to prevent infiltration of unfiltered air.
The two facilities are managed using a Tracer Summit® building automation system. The system optimizes the operation of the chiller and ice system as well as the building’s ventilation requirements. The system manages the duct reheat as well as perimeter baseboard heating for optimum comfort and efficiency. Chiller operation and ice making are adjusted to match the anticipated occupancy levels of the buildings.
The advantages of the system, according to Weichler, are a high level of operational flexibility, increased reliability because of the ice inventory, and the ability to operate the chillers at their most efficient load levels by managing stored ice use.
He notes that the employees housed in the facility demand high comfort levels and that these demands can be met completely, even at the peak of the cooling season.
The installation of the hvac equipment was on a tight schedule, as the timetable called for building occupancy 11 months after excavation began.
System start-up was comparatively uneventful. This smooth start-up Weichler attributed to a good design, good equipment, and a staff that already has considerable experience with chilled-water systems.
ETS expects savings of $2.5 million over the life of the facility through the use of ice storage.