Stratified Thermal Energy Storage Tank
Nease High School
St. Augustine, Fla.

In 1998, collaborating companies completed construction on a $1 million mechanical project that involved designing and building a stratified thermal energy storage (also referred to as "chilled water storage") tank for Allen D. Nease High School in St. Augustine, Fla. Estrella Engineering Inc. designed the project, which involved providing a new 480,000-gallon chilled water storage tank system and boosting the capacity of the school's existing cooling system for planned expansions. As part of this project Estrella Engineering also designed the addition of new chillers, cooling towers, boilers and direct digital controls, and administered the mechanical construction contract. This project was part of a master plan for Allen D. Nease High School. Estrella Engineering provided mechanical engineering services for this master plan from 1993 through 2000.

After six years of operation, the Nease High School thermal storage tank
project was a success and continues to save the St. Johns County Public
Schools a projected $ 45,930 each year in energy and operating costs, or 20 percent of the schools annual cost (1996 figures). The project achieves energy savings by generating and storing chilled water for cooling during hours of reduced electrical rate charges. Then it depletes the tank's cooling capacity during hours of peak electrical rate charges.

"With the new thermal storage tank, we do no operate our chillers during peak hours," said Jody Hunter, coordinator for school-based maintenance and custodial services for the St. Johns County School System. Instead, the chillers operate at night during off-peak hours when energy rates are less expensive. "With the system we had before, we were running for longer lengths of time and during peak hours. That's a big savings."

Florida Power & Light of Juno Beach and Atlanta-based Heery Consulting
developed the economic analysis needed to justify the addition of the thermal storage tank. The study predicted a three-and-a-half-year simple payback for the school district. As an incentive to reduce power consumption, FPL funded 48 percent, or $150,000, of the cost of the thermal storage tank installation. The thermal energy storage tank was pre-engineered, installed and commissioned by Tampa Tank.

Larry Estrella, principal and senior design engineer for Estrella Engineering, located in Orange Park, credits the project's success to the collaboration between experts in various functions of the project. After reviewing examples of the technology's historical successes and failures, Estrella Engineering approached Tampa Tank for the design and construction of the thermal storage tank. Tampa Tank was specified as an approved manufacturer based on their experience and successful track record in the building, installation and commissioning of stratified
thermal storage tanks.

"My goal was to deliver a successful project, not to be a pioneer in tank design," said Estrella, who was familiar with the stratified thermal storage technology but had yet to design or implement a project of that nature.

This technology, first developed during the early 1980s, employs the natural stratification principal, in which hot and cold water remain separated simply by the natural layering of varying water densities, rather than physical boundaries. Earlier methods involved building multiple, smaller tanks, which proved too costly and cumbersome; flexible membranes that often tore or interfered with pump suction,
causing costly damage; or moveable diaphragms, which often jammed in position or failed structurally. Stratified thermal energy storage technology grew out of efforts to eradicate these problems, said John Andrepont, president of The Cool Solutions, consultants specializing in thermal and district cooling, based in Lisle, Ill.

"It's a tank full of water with no moving parts, so there's nothing to break," added Dr. William Bahnfletch, associate professor, department of architectural engineering at Pennsylvania State University. Important operations "can be done with fairly simple controls, so it shouldn't need a lot of attention."

"If a tank is designed and utilized correctly, the naturally separated regions of hot and cold water will not mix," Estrella said. "Water must enter and exit the tank in a very slow, non-turbulent fashion in order to avoid breaking the natural barrier (thermocline) between the hot and cold water." Maintaining the thermocline allows the tank design and construction to be a relatively simple storage vessel, thereby reducing construction cost and maintenance.

Also, in many cases of failure, "the system operation can be the weak link," Estrella said. Many of those hired to operate and maintain HVAC plants are not familiar with chilled-water storage or ice-storage technology. Chilled-water storage provides a significantly simpler control system as compared to ice storage for example. Chilled-water storage and ice storage are the two predominant types used today. Ice-storage tanks require much less space, but feature more complex
designs, which typically involve frequent maintenance. Therefore, compared to other alternatives, its application on this project was much more likely to be successful for the school district and staff to own, operate, and maintain.

"With 486,000 gallons of water that we didn't have before, the chemical treatment is somewhat more frequent," Hunter said. "But as far as creating more work, the new system hasn't at all. It's been a very smooth operation."