Faced with 16- to 22-year-old cooling equipment that required repeated compressor replacements and couldn't meet the cooling demands of its school buildings, the Humble Independent School District (ISD) in Humble, TX, suffered from repeated failures, which meant purchasing several new compressors each year. Because the equipment couldn't keep up with the demands of the hot, humid Texas climate, it translated into discomfort for the school's students and teachers.

Additionally, some of the air chilled screw chillers were generating complaints about noise. The district wanted to replace the cooling systems in eight schools (serving over 4,000 students) with equipment that was more reliable and energy efficient to provide comfortable and productive learning environments. It also sought extensive lighting retrofits.

The district hired ACR Engineering (Austin, TX) to assist with an energy conservation plan for schools in the district. The recommendation was to replace the original air cooled equipment with a system that used thermal energy storage (TES) tanks to provide chilled water storage for off-peak air conditioning. Reliable chillers were the key to making the thermal storage project work.

In February 1999, Humble ISD ordered 10 Carrier (Farmington, CT) Model 30GTN chillers for seven elementary schools (averaging about 70,000 sq ft each) and a 140,000-sq-ft middle school in the district. The multiple, smaller compressors used on the 30GTNs are designed to provide more flexible staging for constantly varying load conditions, resulting in maximized efficiency and substantial cost savings, according to Bruce Orcutt, system sales consultant for Carrier South Texas.

One of the main challenges in installing the chillers, the TES tanks, and the rest of the system was the quick turnaround demanded by the district. The equipment had to be on site by mid-May when school was out for the summer. And the contractor, Northwest Mechanical, (Houston) had two-and-a-half months to complete the installation of the 10 chillers at eight different schools prior to school starting in the second week of August. The installation proceeded smoothly, according to all parties involved.

Out With The Old, In With The Cold

The new equipment is controlled by an Automated Logic (Kennesaw, GA) computerized energy management system and was designed so that it could be reset for thermal storage. The equipment runs in two different modes, operating at 44 degrees F water temperature during the day, and 40 degrees during thermal storage. The school district also elected to install all-new primary pumps, new piping, and automated valving to go with the thermal storage. The automated valves were necessary to divert water to the chillers under different operating scenarios, according to Allan Scott, Humble's district engineer.

"These chillers use the new ComfortLink control system, which is a real benefit because it communicates well with the computerized energy management system," says Scott. "We went from a start-stop system to a fully automated system on each school."

One obstacle the project faced arose in the form of objections to the 38-ft-tall storage tanks from homeowner groups. In some cases, the school principals met with groups to assure them the tanks would be painted in pleasing colors, and/or accompanied by landscaping.

"Security can also be an issue with these tanks, because the kids try climbing up on top of them, but we modified the ladders and the housings of the cages around the ladders to minimize those concerns," Scott says.

Energy Costs Slashed By As Much As 28%

In the first four months of cooling, the new chillers reduced the district's energy costs by 13% overall, with one school saving as much as 28%, translating into district-wide savings of more than $100,000 annually. The TES has also had a most significant impact. The above savings refer strictly to the chillers, but Scott said that once the 26 storage tanks went on-line, "We have seen some great savings. It's tough to gauge because we experienced tremendous rate increases in 2001 in kilowatt-hours, but in 1999 to 2000 there was a drastic decrease - 38% in energy costs."

"Also energy consumption per square foot is down 18% from '98-'99, and despite a 35% increase in electricity cost during the '00 -'01 school year, the district spent 4% less on a square-foot basis in 2001 compared to '98-'99," he adds.

Comfort has also improved substantially. "We couldn't get our chilled water temperatures down where we needed them with the old equipment," says Howard Kucera, Humble ISD maintenance manager. "Combine that with 100 degrees Texas summers with 98% relative humidity, and people were uncomfortable. Being able to put good, cold water into these buildings has really helped our comfort issue."ES