Case study: University heat-recovery system
The Vanderbilt University Engineering Science Building (VU ESB) contains dry and wet research labs, a clean room, and classrooms. The labs and clean room are the predominant building uses and most energy-intensive spaces. A primary goal established at the beginning of design was to have a flexible HVAC system that would minimize reheat.
The base HVAC system used for comparison was a chilled-water (CHW) variable air volume (VAV) system with hot-water (HW) reheat. Because the labs had a minimum air-change requirement and the clean room had a minimum air-recirculation requirement, fan energy was the highest energy end use rather than heating; however, it was a close second.
An energy model was used to approximate the annual energy use of the building with the base VAV system and active chilled beams (ACB) for the labs. ACBs allow for zone heating and cooling control, which reduces reheat. The only reheat that occurs with an ACB is when the zone has no need for cooling and the primary air must be reheated. This same approach is being applied in some health care facilities by reducing primary air-change rates.
The clean room is served by a make-up air unit (MAU) dedicated to maintaining strict humidity limits coupled with recirculation units to continuously filter the air. The MAU takes preconditioned outside air (OA) from the lab dedicated outside-air system (DOAS), lowers the dew point to 40°F, and reheats it to 65°F. The typical brute-force method would require a dedicated glycol chiller to produce extra-cold CHW to reduce the dew point and then reheat it with HW.
Instead, the team elected to use a desiccant wheel in series with a CHW coil on the primary 42°F CHW loop. This allowed for the elimination of the dedicated glycol chiller and reduced the heating and cooling energy needed to maintain the clean room conditions.
The smaller classroom and gathering areas are served with a standard VAV system with return air. The lab areas of the building are 100% OA with ACB. Since the need for reheat in the labs had already been minimized via system selection, the focus for employing waste-heat recovery moved to precooling/heating the large volume of required OA.
When designing air-side heat-recovery systems for labs and hospitals, there is always concern about what exhaust air is safe for heat recovery. In hospitals, there is potentially hazardous air from isolation rooms, soiled storage, and emergency waiting rooms, to name a few. No one wants to potentially contaminate the OA by recovering heat and/or moisture from those spaces. In a lab building like the VU ESB, air from the exhaust hoods is the major airstream of concern. It was decided that the hood exhaust would not go through the heat-recovery device. Beyond the fear of contamination, certain chemicals could also damage the heat-recovery media.
An enthalpy wheel was chosen for the main lab DOAS because of its ability to precool and dehumidify the OA in the summer months while preheating and humidifying the OA in the winter months, and any fume hood exhaust would not be available to the heat-recovery system. Being in Climate Zone 4A, the enthalpy wheel would have a positive effect year-round. The main piece of analysis was to determine whether a dual-wheel system would yield lower energy consumption than a single-wheel arrangement.
ACB systems are often designed with the primary air delivered at a neutral temperature, one that provides little heating or cooling effect because it is close to the space’s set points. In cooling mode, a dual-wheel arrangement would allow the DOAS to reheat the primary air with recovered heat using a second sensible-only wheel instead of HW.
The trade-off to adding an air-side heat-recovery device is always whether the amount of energy saved via preheating/cooling is enough to offset the increased fan energy that results from the increased pressure drop, which happens when moving the air through the heat-recovery equipment. An economizer bypass allows the air to bypass the heat-recovery device when outdoor conditions are suitable, which reduces wasted fan energy. The energy trade-off to make air-side heat recovery financially viable hinges on the utility rates. Where electricity rates are lower and gas rates are higher, the increase in fan energy due to the added static makes less of a cost impact than the reduction in heating energy. The opposite is also true, especially when high-efficiency boiler systems are used.
Through energy modeling, it was determined the single enthalpy wheel energy savings were above and beyond the increase in fan energy. Interestingly, the determining factor as to whether the second sensible-only wheel would provide enough savings was the needed primary air temperature. The second wheel would only provide benefit if cooling effect from the primary air was not needed.
The other negative factor of a potential second wheel is it only provides benefit in cooling mode. The second wheel doesn’t turn when the unit is in heating mode, but the pressure-drop penalty still exists. A bypass could be incorporated to remove the wheel from the airstream to remove the pressure-drop penalty when the wheel wouldn’t be effective; however, for large systems like those in the VU ESB, a bypass is very costly. Some smaller systems have bypasses as a standard feature, which can make the needed energy savings easier to achieve because the fan penalty only has to be paid when energy can be recovered. Even though the energy model showed lower energy consumption with the dual-wheel arrangement supplying neutral air, the final design used one wheel to assure there was enough flexibility in the cooling capacity of the labs without increasing the number of ACB.
The lab DOAS heat-recovery system resulted in an estimated savings of $47,200 and 7.85% energy per year as compared with the same system without heat recovery. The clean room desiccant heat-recovery system also saved a substantial amount of energy, and its first cost was offset against the cost of the now unnecessary glycol chiller.
Waste-heat recovery systems may be incorporated in many ways. Energy codes are pushing more designs to require waste-heat recovery, whether it be via air, water, or refrigerant. At the end of the day, waste-heat recovery is effective because heat that would otherwise be squandered is being used to reduce energy consumption.
Cory Duggin is the energy-modeling wizard at TLC Engineering for Architecture Inc., providing building-performance simulation efforts across the 375-plus-person firm through both direct project involvement and by supporting project teams on specific and unique modeling issues. He is a member of the Consulting-Specifying Engineer editorial advisory board.