Lehigh University in Bethlehem, PA built the five-story STEPS (Science, Technology, Environment and Policy Studies) Building in 2010. The building houses many of the University’s Chemistry and Biology labs. Laboratory buildings typically exhaust a great deal of air and bring substantial amounts of outside air to maintain safe indoor air quality. As such, the energy consumption for HVAC (heating and air conditioning) in labs is relatively high compared to traditional classroom and office buildings.
The STEPS building was designed to be LEED Gold rated. LEED (Leadership in Energy and Environmental Design) is an internationally recognized green building certification system aimed at improving building performance, including energy savings.
There are two large Air Handling Units (AHUs) located on the top floor of the STEPS building that provides heating and cooling to the building. These units are 100% outside air units, meaning that an equal amount of outside air is drawn into the building as an equal amount of inside air is discharged out of the building.
The air handlers were originally outfitted with energy recovery wheels, a competing technology to heat pipes; however, the wheels did not perform as expected, and not long after commissioning stopped spinning (the assumption is that this was due to mechanical issues with bearings, belts, and motors). At that point, the energy wheels were no longer recovering energy and were simply an obstacle to airflow, requiring more energy for the blowers to be able to supply the required amount of airflow.
The building maintenance and energy efficiency staff at the University were looking for an alternative to the energy recovery wheel that could be retrofit into the air handlers with minimal changes. They had considered heat pipe heat exchangers but ruled them out because the intake and exhaust air streams are stacked vertically. So, while a passive heat pipe heat exchanger would work when the lower (incoming) airstream was warm compared to the upper (exhaust) airstream, as is the case in the summer in Pennsylvania, it would not operate when the upper airstream was warmer, as occurs there in the winter when the potential energy savings are greatest.
ACT’s HVAC products sales representative company for the area, Energy Transfer Solutions out of Kennett Square, PA, met with the staff and introduced them to ACT’s pump-assisted air-to-air heat pipe heat exchanger (AAHX). This technology was available for retrofit and was determined to meet the energy recovery needs, as well as promising the energy recovered will pay back the cost of the system within 2 years of installation.
This advanced heat pipe heat exchanger works passively in the warmer months, evaporating refrigerant from the lower section and condensing it in the upper section with gravity return of the refrigerant. In the cooler months, small refrigerant pumps are used to deliver the liquid refrigerant to the upper half of the coil. The warm exhaust air evaporates the refrigerant and the vapor flows to the lower half of the coil, where it condenses and is ready to be pumped back to the top. Any excess liquid also returns to the bottom half of the coil by gravity.
ACT’s patented features within the pump-assisted AAHX include the coil manifold and refrigerant injection system which ensure uniform delivery of the refrigerant to each and every vertical finned tube. Additionally, the pumps run at a constant speed, so the control system is simply on-off, with no complex flow balancing or regulation required.
For this retrofit project, the heat pipe heat exchangers were designed and manufactured to fit through the existing doorways and slide into the air handlers. As such, the one air handler required three independent units, about 7 feet wide by 12 feet tall, and the other air handler required four independent units of the same size. The energy recovery wheels were disassembled and removed and the heat pipe heat exchanger units were slid into the same location. ACT supplied the units pre-wired including a prewired junction box that was mounted on the outside of the air handler. The junction box provided for a simple interface between the heat exchanger pumps and the air handler building management system.
The entire retrofit process, from removing the energy recovery wheels to installing the heat pipe heat exchangers and bringing the air handlers back online was only one week and was accomplished on schedule during the University’s winter break.
As an institute of higher education, Lehigh University has access to energy for both cooling and heating at around $.02/kWh. This is significantly lower than the typical energy costs of $.05 to $.10/kWh. Even with these low prices, it is estimated that the heat pipe heat exchangers will save nearly $100,000 per year in energy costs. For applications with more typical energy costs, the savings would be between $250K and $500K per year.
Two years after installation, ACT’s Pump-Assisted AAHX is operating as expected and delivering reliable energy recovery with 100% outside air, a great asset in airborne infectious disease management.