Cool Deal By Terrance R. Liette, PE, LEED AP As printed in Consulting-Specifying Engineer, March 1998 Ohio School Districts Warms to Energy Savings Provided by Ice Thermal Storage For Kings Local School District in Kings Mills, Ohio, the first day of school in 1997 was particularly auspicious. A new 96,659-square-foot facility - Columbia Elementary School - was ready to open its doors to more than 650 fifth and sixth graders, helping relieve overcrowding in the Cincinnati suburb's existing elementary school system. And the new school incorporated some new technology - an air-conditioning system using a unique thermal ice-storage system, which promises to save the district thousands of dollars decades in utility costs and maintenance. Today's Challenges Along with many other public school systems in the United States, Kings Local School District faces a dilemma when it comes to maintaining comfortable learning environments for teachers, staff and students. Limited operating funds must be stretched constantly to meet a virtually unlimited range of needs. Administrators must ensure their building systems are efficient in both operation and energy use in order to keep utility bills in line with budget projections. In addition, because the buildings must be able to endure decades' worth of heavy use, the systems also must be designed for reliability and ease of maintenance. School environments can be full of surprises, but nobody wants an unanticipated, expensive air-conditioning system failure, especially when the mercury is streaking toward the 90-degree mark. That was the challenge presented to Fanning Howey, when the firm was selected to provide complete architecture and engineering services for Columbia Elementary School in 1995. To help the school meet its mission to provide a user-friendly, technology-rich educational environment for students, it seemed appropriate to take an innovative approach to designing the building's heating, ventilating and air-conditioning (HVAC) system. Thermal ice storage has become increasingly attractive to school officials because of its capability to shift power consumption to off-peak times. Columbia Elementary appeared to be a good candidate for the technology, but there were other alternatives and cost factors that required close study to ensure that the trade-offs would favor the school district. Although ice-storage systems generally have higher start-up and installation costs than conventional chiller systems, the approach to implementation has increased their effectiveness. One approach - partial storage, which combines ice storage with a smaller-capacity water chiller - has helped broaden the range of possible education-facility applications. The chiller can provide initial morning cooling, while ice-storage tanks come on duty as the load increases during the day. From experience, Fanning Howey has found that it is rare for a school to need all of the ice for a day's cooling. On a typical warm fall day, the system may use only about 35 percent of the ice to address the cooling load in the school building. Normally, it should require about eight hours to completely refreeze the ice tanks. But because of the efficiency of the ice and chiller system during the day, most nights require only a few hours of ice-making in order to be ready for the next day's cooling demand. The system also offers operation and maintenance advantages. The static ice-building tanks simply freeze and thaw, requiring no moving parts and only periodic fluid-level inspections. Electronic, automatic control systems further simplify operation and maintenance efforts. Long-term maintenance was the biggest concern of school officials, who aimed for a 50-year-plus life span for their new buildings. It was hoped any selected HVAC equipment would be equally long lived, with no more than routine repairs and replacements. Utility Assistance At the time the study for Columbia Elementary began, Ohio's power utilities were facing their own energy dilemma. Consumption was outpacing their ability to bring new generating plants on line. Many utilities were working actively with their high use customers to design systems that would shift consumption to off-peak demand periods, usually between 6 p.m. and 7 a.m. The Cincinnati Gas & Electric Company (CG&E, now known as Cinergy), which serves Kings Mills, offered a $215 rebate for every kilowatt (kw) that could be converted to off-peak hours. This rebate opportunity opened the door for several possibilities. Along with thermal ice storage, engineers also could consider a geothermal heat pump system as well as a conventional chiller system. It soon became apparent, however, that a geothermal system would not be suitable for this application. Although the earth's heat would provide a reliable source of energy for heating and cooling, heat pumps would still require electricity to operate their compressors any time the system was activated. The constant operation of the heat pumps likely would negate a portion of the peak-load demand-charge savings. School officials also were concerned about the noise generated by the heat pumps and its effect on the learning environment. Maintenance also was a concern: heat-pump compressors generally have a life cycle of 10-15 years, less than what officials were seeking. Thermal ice storage presented its own set of pros and cons. In addition to being expensive to install, ice storage can add a degree of complexity that many school systems don't want to deal with. Depending on the size of the building, its load characteristics and other factors, you easily reach a point where there is no payback. But as long as there is an opportunity to avoid utility demand charges over the long term, ice storage still can be an attractive alternative when properly used in conjunction with a conventional chiller system. The possibility that ice storage could benefit both energy provider and energy user spurred further research into the proposed system. CG&E agreed to fund part of the feasibility study for Columbia Elementary. If the technology could work here, the utility reasoned, it might be successful at other new and existing educational facilities throughout the utility's service area. Design Economy The chances of success for ice storage in this case were particularly good. The designers' layout provides one advantage. The one-story building is constructed of exterior masonry bearing walls with a steel structure at the interior and roof. A media center and computer labs form the hub for three classroom wings, with each wing housing nine to 10 classrooms, an extended learning area, a small group instruction/tutor room, storage areas, and restrooms. A fourth wing, located on an axis with the main entrance and media center, is designed to contain the cafeteria/commons area, gymnasium, music instruction rooms and administrative offices. Columbia Elementary is occupied fully during the September-June school year; only a quarter of the facility - including the gym, locker rooms, administrative area, and approximately a third of the classrooms - is used for summer school. Under Fanning Howey's design, the thermal ice storage system with an outdoor, air-cooled, rotary screw chiller serves the academic areas, gym, cafeteria, kitchen, and media center - spaces that usually are unoccupied after 4 p.m. Administrative offices are served by an independent air-handling unit with a direct-expansion cooling coil and an outdoor, air-cooled condensing unit. The system was sized for an indoor temperature of 75° F and an outdoor design temperature of 92° F from April to September. Using these space and occupancy parameters, the project team calculated the parameters of the proposed system using sophisticated energy-modeling software, which enabled them to evaluate a variety of factors such as building load, load profiles, energy consumption and energy costs. They also assessed the cost of the ice-storage system's components, comparing them to geothermal heat pumps and a conventional central chiller plant. The thermal ice storage cooling plant and conventional chiller plant were both used with a fan-powered, variable-volume air system. The cost analysis revealed the partial ice-storage system would cost approximately $240,000 or $58,000 more than a conventional chiller approach. However, modeling also indicated that partial ice storage would shift 99 kw of energy to off-peak time. With CG&E's $215 per kw rebate, the school district could reduce the premium by $21,285. But the real icing on the cake was the model's prediction of the long-term savings achieved by shifting 40 percent of peak-demand cooling to off-peak ice generation. Using this configuration, the partial ice-storage system would save the school district approximately $12,290 each year in electric costs, providing a three-year pay back. Fanning Howey generally considers five years to be an optimal pay-back period. Even without CG&E's rebate program, the ice-storage system's efficiency still would have resulted in favorable pay-back results. Although the partial ice-storage system cost $37,400 more than a conventional central-chiller design, long-term utility savings and low maintenance costs were incentive enough to secure additional school district funding to make up the difference. And the pay back in maintenance was projected to be as good as lower utility costs. System Design The new school's central cooling plant consists of an air-cooled water chiller located outside the building's mechanical room. Ice tanks are located outdoors next to the chiller and are piped together to an indoor plate-and-frame heat exchanger, forming the ice-loop water system. This loop contains a minimum 25 percent ethylene-glycol mixture to prevent potential water freeze-ups. The chilled-water piping system continues to cooling coils in the main air-handling unit located on an interior mechanical mezzanine. Every night, the loop water-piping system is chilled to 23° F to make ice. Two pumps - an ice-loop water pump for daytime cooling and nighttime ice-making, and an adjacent pump to circulate chilled water through the school to meet daytime cooling needs - are also included. The complexity of partial ice-storage posed few additional challenges to the new school's electrical system. The only additional electrical consideration was the addition of a 20-kw heating circuit to prevent outside tanks from freezing during the winter. And while a standard condensing unit would have required a 600-amp switch, the ice-storage option required only a 400-amp switch. This helps the system provide the necessary cooling capacity, but with less energy. Diversity of Applications Partial ice storage also may have applications for retrofit projects. Many large high schools, designed 10 to 15 years ago, likely have chillers with excess capacity. Such facilities simply could add an ice-storage system and make use of the extra capacity without buying new chillers or changing energy-use patterns. The variety of applications and benefits makes this technology a strong contender in school projects, according to Kings Mill personnel familiar with the project. "Our goal all along was to find a system that would provide the greatest level of comfort for our students and teachers at the least cost to he district," says Richard Baldwin, the district's building and grounds director. "From what we've seen so far, the ice-storage system certainly meets that need." A Chilly Future for Ice Storage? Not long after Columbia Elementary School and Williamsburg new Middle/High School received their rebates from Cincinnati Gas & Electric, the utility discontinued its off-peak incentive program. While the schools will continue to save on the demand charges by producing ice during nighttime hours, the rebate that helped defray a significant portion of the additional expense necessary for ice storage systems will no longer be available to school systems in southwest Ohio. Such incentives, along with the opportunity to save on peak-demand charges, are often prime motivators for adopting ice storage. Some worry that these incentives will disappear as more states follow the lead of California and deregulate their utility industries. With users able to select from several energy service providers, will there be room for incentive programs that make ice storage feasible? "The differential between peak and off-peak demand is still going to be there," says Tom Niquette, a spokesman for the Montana Power Group, which recently began offering electric supply and management services to California businesses and industries. "On the other hand, customers will play a more active role in determining how they purchase their power. They'll have to carefully compare whatever pricing options are available, plus any value-added services, to determine what provider is best for them." Sharing a Good Idea Computer modeling of the potential energy savings that ice storage could provide at Columbia Elementary School in Kings Mills, Ohio, proved so favorable, that local utility Cincinnati Gas & Electric (CG&E) decided to consider other applications. A new middle/high school being designed by Fanning Howey in nearby Williamsburg, Ohio, seemed a good candidate. The utility funded a feasibility study to determine possible peak-use energy reductions. Although this project was to be larger - 106,891 square feet - and have a second story, models proved favorable. The main difference was the amount of the rebate and projected cooling load, requiring a five-year simple pay-back period. The results also were encouraging for the Williamsburg Local School District which, like Kings Mills, was eager to see a low-maintenance energy-efficient system installed in the new school. Installation proceeded smoothly at both schools. Systems were ready for operation in September 1997. Although school officials had few opportunities to fully gauge the equipment's effectiveness during the fall, they expect performance and efficiency to meet projections as spring weather brings warmer days and higher demands on the building's cooling system. The two projects illustrate how partial ice storage systems are an attractive alternative for educational facilities. Although school buildings increasingly are required to serve both educational and community uses, most will still have large areas unoccupied after school hours. In such cases, smaller, dedicated chillers could cool community spaces, similar to the configuration of the units for the administrative areas at Columbia and Williamsburg.