DALLAS, TX — According to the Department of Energy (DOE) and the Environmental Protection Agency (EPA), the most energy-efficient building in the United States lies in the heart of downtown Dallas.

The Centex Building at the Harwood International Center earned an unprecedented score of 99 points (out of a possible 100) in the EPA/DOE Energy Star Building rating system. The Energy Star program rates energy performance based on actual use, making it possible to compare the overall efficiency of buildings nationwide.

The Centex Building, engineered and developed by Harwood International, has 176,384 sq ft of office space and a 180,000-sq-ft parking garage.


A key component of the building’s efficiency is its cooling system. The system combines Calmac Manufacturing Corp.’s Ice BankÒ thermal energy storage tanks with a Trane screw-type water-cooled chiller to significantly reduce the building’s peak electrical load.

The system is constantly monitored by Energy Manager Jon Stringer and Chief Engineer Anthony Salvato. Stringer explained the rudiments of the system.

“In the morning we run both chillers to cool the building,” he said. “During off-peak times we make ice in all of our fourteen thermal energy storage tanks.” The tanks are located in the basement of the building, adjacent to the parking garage.

“At 11:45 a.m. we drop one chiller offline and limit the other one to 85% capacity. At noon, the thermal storage system goes online and provides cooling to the entire building (with the aid of one chiller) during the peak demand times between noon and 8 p.m.”

The process of shifting the system from chiller to thermal storage — known as “load shedding” —reduces the kW demand.

“The load shift makes it easy to keep up with the demand,” added Stringer. “And the need for manpower is greatly reduced.”

According to Trane service account manager Larry Wiser, the two Trane 325-ton helical rotary screw chillers have only three moving parts, making them “very low maintenance.”

The Centex Building system is known as a “partial” ice storage system, since it works in tandem with a chiller. (A full storage system would work totally off of 28 storage tanks.) Three miles of polyethylene tubing are contained in each 8- by 8-ft tank. Ice is frozen at night (off peak) utilizing a glycol solution, which is run through the tubes, freezing the fresh water around the tubes. The 14 tanks provide 280 tons of cooling capacity per hour. There are no moving parts in the tanks, which makes the job easier for the six engineers on staff.

“Once the system is brought up to the most efficient level of operation, system maintenance is minimal,” added Stringer. “The only maintenance on the tanks is to check water levels and fill them” to compensate for evaporation.

Both Stringer and Salvato can monitor the system with laptop computers or from their homes. Only one person can have remote access to the system at a time and only Stringer and Salvato have authorized access.

Stringer said that he is constantly monitoring the system in order to find ways to be even more energy-efficient. “Our ultimate goal is to reduce peak demand,” added Stringer. “If our energy uses approach last year’s peak demand, I get an alarm. At that point I scramble to shut down some non-essential things like lights.

“So far we are staying below last year’s levels. Calmac and Trane are always giving us hints on making the ice storage system more efficient. We talk with our building tenants to learn about their electrical usage and work with them to conserve energy. We can’t tell tenants how to occupy their space — we have to be flexible.”

Based on energy efficiency and reduced costs, Jim Morton, Calmac’s southwestern regional manager, said, “A partial storage system like this can generate a five-six year payback.”

Stringer said his company, Harwood International, looked at a lot of systems before settling on the Calmac thermal energy system. He said the choice was pretty easy.

“Everybody wins with this system,” he said. “Owners and tenants win because energy costs, which are usually passed on to tenants, have been reduced. And having an efficient system adds value for the building owner.”

“We are very proud that the thermal energy storage’s capabilities were utilized in this exciting building,’ said Calmac president Mark MacCracken. “The Energy Star Building Label is a significant achievement for many facilities, but we are particularly delighted to know our Ice Bank tanks helped create one of the highest scores ever achieved in the rating system.”

Publication date: 03/25/2002