By Harry S. Kent, P.E.
Energy Consultant
Levittown, Pennsylvania

n 1992, J. Paskey, manager of facilities and engineering, and R. Mackie, maintenance and utilities manager, Rohm and Haas, Spring House Research Facility, requested an in-depth energy review of the entire facility, considering both long- and short-term goals. The purpose of the study was to identify various ways to reduce electrical cost and optimize chilled water usage.

The original chiller plant for Rohm and Haas, located in Spring House, Pennsylvania, was built 32 years ago. It started with 1,200 ton/hour installed capacity and served two buildings. By 1987, the plant had grown to 4,000 ton/hour installed capacity and served 13 buildings. The last research building was added in 1987. The buildings are all served by an underground loop header. By 1990, the available cooling capacity at peak ambient conditions could barely meet the peak demand. Setting supply chilled water temperature to 45°F from 50°F and hydraulically rebalancing the entire system permitted Rohm and Haas to meet cooling demands. Thus, more time was gained, permitting operation for the following two years; even 1991, which had unusually high ambient peak demands, presented only minor problems. This allowed for a total reevaluation and review of the major electric user. The chilled water plant uses almost 50 percent of the total electric demand.

Therefore, the energy utilization study was done in 1992.

Scope of the Project
The management at Rohm and Haas was very concerned with the environmental impact of the refrigerant that was to be used for cooling, since additional refrigeration capacity was required. The existing four chillers' refrigerant is either CFC-11 or R-114, which mandated replacement. Another concern was the ever-increasing electric summer peak demands, largely derived from the chillers and their auxiliaries, with the corresponding increase in operating cost.

Many options were evaluated, including absorption refrigeration, high-efficiency centrifugal chillers, and cogeneration.

After investigation and evaluations, a thermal energy storage system was chosen, which produces ice during off-peak hours and, if elected, chilled water during the peak hours. The conclusion was reached based on the capability to shave in excess of 50 percent or more of the electric peak demand resulting from operating the chillers and auxiliaries.

Peco Energy, the electric company serving Spring House, has a cooling thermal storage rider, which reduces the peak demand hours from 12 to 10 hours Monday through Thursday and 6 hours on Friday. In addition, the peak demand for each peak month is averaged, and this results in a lower annual peak billing demand. This provision is valid provided that the total cooling demand in reduced by 50 percent or more (preliminary results obtained during June and July 1995 indicate a reduction of about 70 percent).

In February 1994, Frank V. Radomski & Sons, Inc., a general contracting firm, was selected, and preliminary engineering and cost estimates were started. By September 1994, all engineering services were selected and all major equipment vendors were chosen. In September 1994, this job was put on a fast track and kept to a very rigidly enforced time schedule and cost control. Ground was broken in September, and the plant went into successful operation at the beginning of June 1995, exceeding all the peak demand shaving requirements during June and July.

Our goal was to shave a minimum of 1.6 MW. The payback calculations were based on 2.0 MW peak demand shaving and 10-month operation. We have achieved an average peak demand shaving of about 2.3 MW--a 15 percent increase.

From the time construction started in September 1994, all related disciplines and vendors were involved in weekly job progress meetings. Because of the complete cooperation of all team members, the progress was steady and timely. All skills from project management, architectural, electrical, and mechanical design and engineering, and plant operation were coordinated. In addition, monthly meetings were held with the major vendors to coordinate their efforts and assure system compatibility.

All final decisions were open to review and solved cooperatively. Everyone understood the objectives and technical requirements of the job and was thoroughly familiar with interrelated problems, including expected system performance. All this was achieved without causing delay. The result is a well-designed, architecturally attractive addition to the utility plant and the research facility. The system started on time with a minimum of field changes and operating problems.

Description of the System
The ice thermal energy storage system selected for the Rohm and Haas facility is an ice harvester-type system. A weekly load-shift strategy was incorporated in the system design to shift electric cooling load from the on-peak hours to the less expensive evening and weekend off-peak hours.

The system consists of four Mueller 250 ton/hour evaporators mounted on top of a rectangular poured-in-place concrete ice water storage tank. Four completely assembled units were shipped to the plant.

The system was chosen over other thermal energy storage technologies for the following reasons:

• The flexibility to optimize the system's efficiency under various load conditions.
  
• The ability to operate as a chiller as well as an ice maker.
  
• The ability to maintain consistent low water temperature from the ice storage, thus providing lower water temperatures to the existing air handling units. This allows an increase in heat transfer and compensates for increased loads for some of the units and thus saves in replacement costs.

American Industrial Refrigeration (AIR) furnished, assembled, engineered, and coordinated the high-side refrigeration package system, including a four-cell evaporator condenser furnished by Evapco, two screw compressor packages, and a PC-based control system was supplied by FES. The control system includes the integration and programming of the ice system's controls, compressor package controls, refrigeration system controls, and the ice and chilled water system controls.

This system is enclosed in an insulated double-wall enclosure, 42 feet long by 28 feet wide. The refrigeration high-side package, including all electrical switchgear, wiring, and pipe insulation, was completely fabricated at AIR's shop facilities. This also included the installation of the compressor packages. It was trucked from Minnesota to the Rohm and Haas plant site. The package was designed and built to be split into two halves and reassembled at the job site. The structure was designed to support the evaporative condensers, which were shipped directly in two packages. The condensers were placed and piped at the job site, using a preassembled structural steel cat walk assembly and piping. The on-site construction time for the assembly of these packages and evaporative condenser was four weeks.

The refrigeration system is an HCFC-22 liquid recirculation system with a capacity of 1,280 tons of refrigeration during the ice making mode and 1,720 tons of refrigeration during the water chilling mode. Since the intent of this project was to reduce energy consumption, every consideration was given to achieve this aim with economic justification.

For example, the compressor packages were provided with oversized oil separators and suction valve assemblies to reduce pressure losses and allow operation at higher suction pressures and lower discharge pressures. Thermosyphon oil cooling was selected for further economy of operation. The system utilizes the economizer cycle available on screw compressors. A flash-type economizer vessel is used to subcool the HCFC-22. The flash gas goes to a sideport connection on the screw compressor. This further increases the overall efficiency of the system. The evaporative condenser was oversized to allow overall operation at lower condensing temperatures. All equipment was selected with zero negative performance allowance. All electric motors were selected for high efficiency. The evaporator fan motors and one of the chilled water pumps have variable speed drives.

All operating functions are automatically controlled by the PC-based control system, which controls the ice harvesters and the water-side system. Compressors, condenser fans, condenser water pumps, refrigerant pumps, and control valves can all be operated on local control. In addition to its general features, such as dual pressure relief valves and high- and low-level alarms, the refrigeration high-side package also includes refrigerant detectors and an oxygen detector. All of the alarm signals are sent to the control system for operator display and acknowledgment.

The ice water storage tank includes a spray distribution system at the top of the tank to provide for an even melting of the ice. This produces low temperatures at the suction header.

The suction channel is an 18 by 18 inch formed channel in the bottom of the tank. This channel is covered by a 1/2 inch galvanized plate that has perforations to draw water evenly across the bottom of the tank.

The mechanical equipment room is located between the ice storage tank and the existing utility building and is 25 by 80 feet and 23 feet high. Located in the mechanical equipment room are three ice water pumps nominally rated at 2,500 gpm each, circulating ice water through a single plate-type heat exchanger rated for 7,400 gpm and 15°F or 4,625 ton/hour back to the ice harvesters. There are also three chilled water pumps rated at 2,500 gpm. One of the chilled water pumps has a variable speed drive. The electrical switchgear is located on the second floor.

The ice water storage tank is an above-ground concrete tank, poured in place to hold more than 45,000 ton/hour of latent cooling. The tank's nominal internal dimensions are 90 feet long by 60 feet wide with a usable height of 22 feet. The tank floor is 12 inches thick and the walls are 18 inches. Galvanized structural steel, fully welded to wall channels, is anchored to the 18-inch concrete tank walls. The tank top is designed to support the four 250 ton/hour harvester-type evaporators and two future evaporators. The tank interior is coated with a commercial industrial membrane, which is liquid applied urethane coating. All masonry exposed walls are insulated with 3 inches of polyisocyanurate insulation and 4 inches of split face block exterior. The roof insulation has 3 inches of polyisocyanurate insulation, covered by Carlisle ballasted EPDM system, with 18 by 18 inches concrete pavers for ballast.