"Reducing the Summer Peak With Evaporatively Cooled Condensers" was the subject of a presentation by Gary Hogsett, P.E., energy management specialist, Tetra Tech EM Inc. "A problem in our part of the country [Kansas City] is the summer peak electric usage," said Hogsett. He did research on evaporatively cooled condensers to determine what electricity savings they may provide.
Hogsett said a sample rooftop unit at 75°F has a 12.1 kW draw. When the roof heats up to 105°, it has a 17.1 kW draw. When the roof heats to 125°, the draw is 21 kW - almost twice the draw at 75°.
With evaporative coolers, at higher temperatures, electric draw is much lower, he said. And with a typical air-cooled unit, performance drops off as temperature rises, he added. With evaporative coolers, performance does not drop off.
Using an evaporative pre-cooler on a rooftop unit, Hogsett said a savings of 11-12 percent can be achieved on cooling performance and much higher savings on peak demand.
He did testing on a rooftop unit at Oklahoma State University with and without an evaporative pre-cooler attached. Using the pre-cooler, he saw a 50 percent savings on peak demand and 12 percent savings on energy consumption. And Hogsett noted that evaporative pre-coolers have become less expensive and more reliable.
"It is important to plug up the holes and gaps in the building," said Woods. You can have significant gaps around the ducting penetrations in the roof.
"Air movement influences building performance," he said. Air pressure differences cause air to flow. This can be created by such things as stack effect, which exerts pressure on the upper walls and roof. The pressure sucks air up the building.
With these air pressure differences, unconditioned air leaks in and conditioned air out via roofs and exterior walls, roof and wall junctions, parapets, soffits, cladding intersections, vertical and horizontal pathways, or shafts such as elevator shafts.
To reduce uncontrolled air movement, Woods recommend-ed doing an assessment of the building using tools such as an air leak detector, smoke pencil, depressurization fan, infrared thermography, and energy-savings assessment software.
To get started, "prepare a scope of work on how you're going to tighten up the building," stated Woods. Address zones in order: top of building, bottom of building, vertical shafts, outside walls and openings, and compartmentalizing.
Explaining compartmentalizing, Woods said that the boiler room, for example, may have an opening for outside air. You will then need to separate this room from the rest of the building.
Fixes can be very cost-effective. "You can fix a window and tighten it up for one-tenth the cost of a new window," he said. He also noted, "You can go around a building and you probably won't find a single door that is properly weatherstripped."
The return on investment for a more energy-efficient building envelope upgrade is usually four to five years, said Woods. He cited one successful example where a school project in Syracuse, N.Y., is achieving estimated energy savings of $52,000 a year.
Deshaies said, "Data loggers can help save energy dollars and also improve IAQ." Data loggers are battery-operated measurement tools that will record unattended. They can have internal and/or external sensors.
These units can measure temperature, humidity, CO2, CO, ac current and voltage, and other parameters. They can be standalone or networked. Applications include IAQ, facilities management, and energy management.
Deshaies then described an IAQ case study. The application was a four-story, multiuse building where there was a concern about CO2 levels. Natural ventilation was used.
Temperature, humidity, and CO2 loggers were used to monitor the air of each floor of the building including the basement level. Several loggers were strung at 6-foot intervals in stairwells to measure temperature stratification.
The data revealed a high CO2 level when certain windows were closed and a range of temperatures among floors. The study recommended a number of ventilation corrections and reinforced the importance of balancing natural and HVAC ventilation.
Facilities management applications include HVACR trouble shooting and retrocommissioning. Building mechanicals and controls can drift over time; retrocommissioning "can get the building back to original operation," said Deshaies.
Energy management applications include submetering, lighting energy use, and heating and cooling energy use.
In a heating and cooling application at a South Dakota school district with 33 buildings, school officials made a $1,000 investment in loggers. Deshaies related that they received a $500,000 return on investment by just changing the setback temperature to operate HVAC systems only when needed.
Bhattacharjee said that successful chiller retrofits to reduce energy consumption can make use of chiller bank optimization, chiller sequencing, variable-flow chilled and condenser water pumping, application of a wet-side economizer, optimization of chilled-water use, reduction of chilled-water mixing, or installation of energy-efficient chillers. The viability of these retrofit options depends on variables such as system installed, chiller load profile, climate, economic factors, reliability issues, the process requirement, and others.
Bhattacharjee then described a case study at a plastics extrusion plant. Due to operational changes, the load on the chilled water plant was reduced, resulting in more part-load operation. The existing system did not include features to improve part-load operation, which contributed to higher energy cost.
A techno-economic analysis was carried out that looked at various energy conservation measures as mentioned above, such as sequencing of the chillers, utilizing variable-flow chilled and condenser water pumping, and installing a wet-side economizer to achieve "free" cooling (making chilled water with the chillers off).
Analysis of the different measures found that installing a wet-side economizer to attain free cooling provided the greatest savings. It resulted in energy savings of about 32 percent, for a cost savings of about $100,000. "It is clear that installation of a wet-side economizer can lead to maximum energy reduction in the chilled water plant," Bhattacharjee concluded.
Publication date: 10/23/2006