Because of the growth of the school population, Gilbert looked for standard energy solutions that could be applied in multiple buildings. “System standardization is valuable because it means our maintenance crews can develop expertise in the systems and can work in many different buildings with confidence. It also means we can save money on maintenance supplies and spare parts.”

In North Carolina, much of the funding for local schools comes from the state, but the local school district is normally responsible for funding for facilities and physical operation and maintenance of the facilities. For that reason, holding down operating costs is seen as particularly important by the school administration. Energy costs are an important target.

STANDARD CHILLER PACKAGE

In 1996, the pace of school construction was accelerating and the need for improved classroom comfort was being emphasized. The district’s energy team and its construction department standardized on a school design featuring air-cooled screw chillers and ice storage tanks as an optimum solution for school cooling.

Since that time, 10 schools have been built with this system, and more are on the drawing boards.

The typical elementary school encompasses 82,000 square feet and is equipped with one 125-ton Trane air-cooled Series R™ screw chiller and six Calmac ICEBANK™ ice storage tanks. Middle schools of 111,000 square feet typically have two chillers and nine ice tanks. The high schools are 260,000 square feet and have three Trane air-cooled chillers and 12 Calmac tanks. Tanks are either the Calmac Model 1190 or the smaller Model 1060.

Trane’s Raleigh sales office worked with the school district to match up the components to meet the needs of each school type. According to sales engineer Jeff Mitchell from the Raleigh office, the Series R air-cooled chiller is a logical choice with ice storage. “Trane offers this product configured for this exact application. There are a lot of them out there in this type of service, and their reliability has been excellent.”

According to Gilbert, the air-cooled chiller approach with ice storage makes good sense for them. “We like the Trane Series R air-cooled chillers because of their reliability, and the fact that we don’t have to deal with condenser water. No water treatment and no tower maintenance were needed. This approach also eliminates water and sewer charges. Unattended operation is more practical.”

REDUCING DEMAND CHARGE

He points out that the primary advantage of ice storage for the school system is that it dramatically reduces electric demand charges, and also allows the schools to qualify for a favorable time of use electric energy rate from its electric supplier, CP&L. The chillers are operated on a primary loop with a 25% glycol solution at 24 degrees F going to the ice storage tanks.

Normally the schools make ice during the nighttime hours. In the morning, the chillers are used directly, through a Polaris heat exchanger, from 7 a.m. until 9 or 9:30 a.m., when the system is switched over to use the stored ice, which normally lasts through the day.

Chilled water from the secondary loop is circulated to air handlers and fan-coils throughout the building. The school system has standardized on Trane Modular Climate Changer™ air handlers, typically located in mezzanine locations in the buildings, for distribution of air to the larger spaces such as hallways, gyms, and cafeteria. These air handlers are directly ducted to ceiling diffusers in these large areas.

INDIVIDUAL CLASSROOM CONTROL

Classrooms are served by individual air handlers that have thumbwheel classroom controls. The classrooms are given a range from 68 to 76 degrees F. The air handlers typically receive chilled water at temperatures ranging from 46 degrees down to 41 degrees, depending on outdoor temperatures. For space heating and for air tempering purposes, the air handlers receive hot water from a gas-fired boiler. All of the air handlers are on a four-pipe system.

Gilbert noted that with the tight construction in newer school buildings, ventilation and humidity management are increasingly important. In most schools, there are humidistats which are set to initiate air handler operation in a dehumidifying mode when relative humidity levels exceed 60%.

Gilbert indicated that the school administration chose the air-cooled chiller/ice storage design because of its potential for reducing operating costs. His office has done analysis of the economic benefits of ice storage in specific school applications and has found the results impressive. He said, “We have found simple project paybacks of as little as two years, always less than four years. Over the anticipated life of the school of 40 years or more, we believe the total savings could be as great as $1.2 million.”

SMALLER CHILLER REQUIRED

Gilbert pointed out that the largest portion of the savings comes from reduced electric energy and demand charges. “Another element is that the chiller size can be smaller than if you had no storage, which partially offsets the cost of the ice storage system.”

He said that the Johnston County Schools have also learned as they’ve used the cooling plants. “We now specify ball valves rather than the standard gate valves and have gone to variable speed chilled water pumps to allow us to operate at lower levels. But the basic concepts of air-cooled chillers and ice storage have proven to be very effective.”

He mentioned that numerous other school systems have toured the Johnston County system, particularly those interested in air-cooled chillers and ice storage. “We suggest they do a rigorous evaluation of this and other options, paying particular attention to their local electric rate structure. That can make a real difference.”

With an average of two new schools being built every year, the scale of ice storage increases every year. According to Calmac southeastern sales manager Mark Johnson, the Johnston County Schools system in aggregate is already the largest ice storage operation in North Carolina, and one of the largest in the Southeast. The school system has demonstrated its commitment to keeping classrooms occupied every day of the school year.

For additional information, contact 651-407-3921 or www.trane.com.

Publication date: 09/09/2002