Air conditioning during warm daytime hours is one of the largest contributors to energy consumption in school operations. Johnston County Public Schools in North Carolina is saving big by using hybrid-cooling systems. Combining chillers and thermal-energy storage tanks, the school administrators have reduced energy cost by as much as 40 percent, even on days topping 100 degrees.

Challenge

- Operational budgets being cut

- Moisture damage during summer months

- Large maintenance budget reductions

Solution

To solve these issues, Johnston County Public Schools works with Brady Energy Services, a company that provides high-efficiency Earthwise HVAC systems and comprehensive building solutions for commercial, educational and industrial facilities across North Carolina. Brady engineers have recommended a program of retrofitting existing buildings with CALMAC IceBank technology. The IceBank tanks make and store ice at night for use in cooling the building during the daytime.

Princeton School in Johnston County, NC

How Does Ice Storage Work?

At night, the cold-water loop of the HVAC system, which contains 25 percent glycol, bypasses the air handlers and is cooled to 24 degrees by the chiller(s). It then circulates through plastic tubing in a water-filled storage tank. About 95 percent of water in the storage tank freezes. The next day, the chiller(s) cycles back to 52 degrees. A portion of the water circulates back through the plastic tubing in the storage tank. To achieve the optimal — 44 degrees at the air handlers — temperature-regulated valves mix water from the chillers with water that is cooled further by passing through the ice tank.

Daytime Savings

Randy Katz, general manager of Brady Energy Services, notes, “That a typical middle school can save as much as $1.2 million in operational costs over its 40-year lifespan.” Operating 44 schools, as well as administrative buildings, ice storage technology has helped trim the operating budget by more than $1 million annually.

Many electricity suppliers offer time-of-use rates that include a 20 to 90 percent reduction in energy prices at night. This means changing when a building draws the majority of its electricity can have a significant effect on the cost of operation. In a traditional HVAC system, the majority of the energy consumption occurs during the day at the highest time-of-use rates. Typically, the chillers work to lower the cooling-system water temperatures by 15 degrees or more during peak daytime hours. This means the system works hardest during the most expensive time of day. In addition, excessively warm days may exceed the ability of the chillers to cool the water sufficiently. This leads to uncomfortable classrooms, possible lower teacher retention, increased absenteeism and lower test scores.

With a hybrid ice-storage system, a portion of the energy-demand load is shifted to the lowest time-of-use rate periods. Typically, the chillers work about half as hard during the day as in a traditional system. The water returning from the air handler(s) only needs to be cooled approximately 8 degrees. The ice-storage tank cools the water the rest of the way. On excessively warm days, more of the water flows through the tanks to be cooled by the ice. The result, less daytime energy consumption, and consistent cooling to keep classrooms comfortable, and fostering a positive learning environment.

The IceBank field used to cool Princeton School

Humidity Control

Conserving energy by scaling back mechanical system operation when a building is unoccupied is always recommended. However, completely turning off systems can allow high humidity levels to become a breeding ground for molds and mildews. This can also lead to furniture and other materials warping or becoming damaged. With North Carolina summer temperatures exceeding 90 degrees and humidity levels ranging between 65 and 75 percent, moisture-related issues are a concern for the administrators of Johnston County Schools.

The ice storage system allows for scaling back during the summer by only running the chillers at night to make ice. During the day, the air-handler evaporator coils are cooled with the water circulating through the ice tanks. For Johnston County Public Schools, this means humidity control and damage prevention.

“Being able to keep conditioned air circulating in unoccupied buildings, as well as maintaining relative humidity below 50 percent can eliminate moisture related damage in the schools,” says Katz.

Cutting Maintenance Costs

“System standardization plays an important role in controlling operational expenses,” says Katz. “Maintenance crews at Johnston County Public Schools have developed the expertise to work on Ice Storage Systems and to keep them working optimally with confidence.”

Now in the tenth-year of using hybrid cooling, the school system specifies ice storage technology for new construction. “The cost of the storage tanks is more than offset by being able to specifying chillers that are 50 to 60 percent smaller and air-handling systems that are 20 to 40 percent smaller,” says Katz.

For Johnston County Public Schools, students in classrooms on days over 100 degrees, lower utility bills and less money spent on repairs have defined success.