Energy Wheel Puts College on Efficiency Roll
When Haverford College in Haverford, Pa., decided to build its new Integrated Science Center, school administrators wanted the facility to fit the campus' classical look. But nostalgia for architectural style didn't carry over to the HVAC. In fact, school officials made it clear to designers from Princeton, N.
When Haverford College in Haverford, Pa., decided to build its new Integrated Science Center, school administrators wanted the facility to fit the campus' classical look. But nostalgia for architectural style didn't carry over to the HVAC. In fact, school officials made it clear to designers from Princeton, N.J.-based CUH2A that they wanted the 187,000-sq.-ft. building to have state-of-the-art HVAC systems.
The science center houses a variety of spaces: public areas, classrooms, faculty offices and a library. However, approximately 40% of the building is dedicated to laboratory space, creating challenges in achieving the desired energy-efficiency standard. Total fume hood count is 110.
Traditionally, lab system design includes a variable-air-volume fume hood system with a VAV supply-air system enhanced by sensible heat recovery. Air is delivered to the lab space through VAV reheat terminal boxes that would have the air quantity coordinated with the variable exhaust quantity of the fume hood system.
These VAV systems have complicated operational schemes to provide for both comfort and fume hood exhaust operation, but they still have major thermal inefficiencies due to the following operating characteristics:
Summertime cooling of the laboratory uses conditioned outside air. When humidity levels are high, providing air conditioning with outside air can easily require three times the refrigeration energy of recirculated air.
The air distributed for the air-handling systems must be cooled to approximately 55
When the supply-air quantity required to match the exhaust air is not enough to provide sufficient summertime cooling to match the space load, the system will again react in an inefficient mode.
In the winter, heat is provided by using outdoor air that must first be heated to 70
A simpler solution
It was apparent to the design team that the standard laboratory system would be neither effective nor efficient. Rather than forcing one HVAC system to meet all the requirements, the solution would be separate make-up air and comfort-conditioning systems.
The make-up air unit for the science center was designed to produce essentially room-temperature, humidity-neutral air to efficiently replace the exhausted air. This also allowed for the elimination of reheat energy.
At the heart of this system is a total energy heat wheel. "The purpose of the enthalpy wheel, as opposed to a lot of other heat recovery devices, is to recover latent loads—moisture and humidity loads—as well as temperature-sensible loads," says Philip Bartholomew, P.E., CUH2A's lead mechanical engineer on the project.
In addition to the enthalpy wheel, the system incorporates a sensible heat wheel. "Sensible wheels are only used for free reheat," says Bartholomew. "It doesn't have the same coating [as the enthalpy wheel] and is aluminum with a urethane finish. It doesn't absorb moisture." The primary purpose of the sensible heat wheel is to eliminate the need for dehumidification reheat and also assist in the unit's heat recovery efficiency.
Individual room heating and cooling fan coil units provide space comfort conditioning. This approach allows for the efficient cooling and heating provided by full return air to the unit. A return air unit for multiple spaces would not be appropriate because of the cross-contamination of fumes from lab to lab.
Four possible system types were modeled operating under the same summertime conditions:
100% outside-air AHU
100% outside-air AHU with sensible heat recovery
Sensible recovery make-up air unit
Total heat recovery make-up air unit
CUH2A decided to go with a total energy heat wheel exchange device because it would increase system efficiency substantially—up to 69% savings.
The heat wheel allows an extremely small amount of fume hood exhaust-air stream to be returned to the space by the supply-air stream. If applied correctly, this heat wheel cross-contamination is considerably less than that caused by the typical arrangement of fume hood discharge stack and air-conditioning system fresh air intake. Although this is not a hazard for most applications, a thorough evaluation of hazard levels is required.
A critical factor is the proper selection of the hygroscopic media of the wheel. For ideal operation, only the water molecule should be allowed to be absorbed into the media. The three angstrom molecular sieve approaches this ideal.
Additionally, the purge operation may be affected by the variable airflows, and compensation might be required. Also, designers must investigate the possibility of severe winter frosting on the media.
There are several relatively simple remedies for this. In the Haverford project, the conditioning of make-up air to room-neutral conditions allows air to be delivered to the space without concern for dumping and drafts. This, in turn, allows for an approach with several advantages: cost savings in supply-air ductwork distribution; quieter operation; simpler control, balancing and startup; and easier operation and maintenance.
Using the corridor as a supply/make-up air plenum, air is delivered to the laboratory area hallway ceiling. From there, it is passively drawn into the space by the action of the fume hood exhaust. A fabric backdraft damper in the partition above the corridor ceiling maintains the desired pressure of -0.03 in.
The damper opening is self-adjusting to the fume hood exhaust quantity and naturally maintains a fume hood containment air velocity at the transfer opening to the space. An added benefit of this approach is that the backdraft damper will instantly shut with the opening of a lab door, which provides a gentle inward fume containment velocity. This containment velocity will typically be more effective than that of a constant supply-to-exhaust air differential provided by traditional laboratory systems.
Incorporation of heat recovery devices into the make-up air units will nearly double the initial equipment cost as compared to a simpler heating and cooling 100% outside AHU. This cost, however, is more than offset by three major areas of construction savings: the reduced plant size, reduced supply-air ductwork and a simpler control system.
The system has been in operation for a few years now and has proved to be stable and efficient under all types of conditions. One drawback has been the required filter changeout for the high number of fan coil units. However, this added maintenance cost is insignificant when compared with the benefits: lower construction costs, quieter laboratories and more efficient energy use.