TAMPA — Classrooms are empty when the air-conditioning system at Roland Park Elementary and Middle goes on at 9 p.m. The 3 miles of tubes spiraling inside each of eight ice tanks on campus will freeze solid by 5 a.m.
In the morning when students arrive, a mix of glycol and water flows quietly through the tubes, and the ice outside them maintains the internal temperature. Glycol allows the water to drop to 19 degrees without freezing. Meanwhile, a heat exchanger collects the warmth from indoors and channels it through the ice tanks. Then fans blow over the cooled water, creating cold air that brings each classroom to a comfortable 76 degrees.
Roland Park is one of five schools where Hillsborough County schools have installed the systems, called thermal energy storage. Hunters Green, Schwartzkopf, Bing and Boyette Elementary schools also got the systems.
They're made by Calmac, which has been building ice skating rinks for 30 years. The company discovered the same technology could be used to cool buildings, according to its Web site. Instead of the tubes lying flat, like on an ice skating rink, they spiral inside 7- by 7-foot tanks that contain 17,000 gallons of water.
Most of the energy is spent during off-peak hours, which saves the district money, says Bob Wegmann, general manager of operations for the Hillsborough County School District.
The new systems — which cost $585,708 to install — are projected to save $50,000 to $75,000 a year in those five sites alone.
"It's like buying a Prius. You have to put more money up front, then analyze if it will return soon enough. This technology should pay for itself in six to 10 years," Wegmann said.
If the technology lives up to expectations, Tampa Electric Co. will give the district a rebate for an estimated $132,000, according to utility spokesman Rick Moreira.
"This estimated rebate amount is one of the largest we've paid," said Moreira.
At Roland Park, the new equipment works in tandem with an existing air system, cutting air-conditioning needs in half.
All of the systems installed are partial storage systems, designed to meet half of a site's air-conditioning needs. But three schools have been running them at higher capacity, getting even more cooling power.
"For about a six- to eight-month period, we can operate partial storage efficiently as full storage, which is less expensive. It's during the hotter months, June, July, etc., that we use them as partial storage," said Wegmann.
A study completed last year by the Army Corps of Engineers determined that off-peak thermal cooling saves as much as 50 percent on energy bills.
"The more you can reduce those peaks, the more advantageous it is for the customer, because peak energy is more expensive," Moreira said.
Hillsborough's school system tried similar technology about 20 years ago but found it ineffective.
"The technology then was so far behind where it needed be in order to function," Wegmann said. "The systems were maintenance intensive, broken all the time, and eventually Hillsborough had to remove all the systems."
Two more schools may get the new technology in the summer, Wegmann said.
Arielle Stevenson can be reached at firstname.lastname@example.org or (813) 226-3321.