Heat pumps and filtered fume hoods can help achieve
The 50,000-sf New Technology and Learning Center (NTLC) for Bristol Community College (BCC), Fall River, Mass., brings together disparate programs—chemistry, biology, medical and dental education—holding energy-dense uses, including 18 fume hoods,
high plug loads and specific ventilation and lighting requirements.
An initial basis of design called for a high-performance building with
numerous energy-conservation measures (ECMs) to meet the statutory
requirement of Massachusetts LEED Silver Plus, including a minimum
of 20% energy-cost reduction (compared to ASHRAE 90.1-2007).
While the project paused for funding, BCC intensified its American
College & University Presidents’ Climate Commitment (ACUPCC) to
carbon neutrality by 2050, initiating plans to develop a site-based solar
array. This new context presented an opportunity: reassess the original “high-performance” design, which, according to the energy model,
would not keep pace with BCC’s 2050 commitment. The team made a
strategic investment to develop a zero-net-energy (ZNE) design. With
few comparable built examples, the question was: how to achieve ZNE
for an energy-dense program in a cold climate?
A number of options were tested using simulations, calculations,
research and discussions with manufacturers of advanced building
technologies. Ultimately, a combination of technologies was developed,
including 50% lighting power density reduction, a high-performance
envelope and natural ventilation systems. Two key strategies with the
greatest impact are highlighted here.
Hybrid ground-source/air-source heat pump
ZNE buildings typically rely on renewable electricity for heating and
cooling by using a heat-pump system. This heat-pump approach often
includes a large ground-source well-field, designed to handle the peak
heating and cooling loads and annual demand for the building. For the
NTLC, this system would require about 80 closed-loop wells, each 500
ft deep. At $10 to $15 thousand per well, plus the cost of high-capacity
ground-source heat pumps, this is an expensive proposition. A more
cost-effective approach was required.
The amount of heat energy extracted from or rejected to a thermal mass
is a product of the thermal mass and the change in temperature. To reduce
the amount of thermal mass (well-field size), the seasonal temperature
swing in the ground was expanded. Therefore, after a summer of rejecting
heat from the building, the ground temperature may approach 90 F maximum; while after a winter of extracting heat from the ground, the ground
temperature may approach 30 F minimum.
In addition to expanding this range, the ground-source heat pump
system capacity was further reduced by designing it for the heating de-
mand, but not the full cooling demand. Instead, supplemental air-source
heat pumps were incorporated into the system. On peak cooling days,
running air-cooled heat pumps in lieu of a larger ground-source heat
pump system results in an energy penalty. But in September, with the
ground at maximum temperature, the air is often cooler: At this point,
air-source can out-perform ground-source.
Overall, strategic sizing of ground-source heat pumps results in a
first-cost savings that outweighs these energy penalties. Also vital to the
success of the hybrid heat pump plant is a control logic that optimizes
energy performance while maintaining an annual balance between heating and cooling in the ground, preventing the ground temperature from
creeping outside design limits from year to year.
Filtered fume hoods
While heat pumps can help avoid the use of fossil fuels, they don’t solve
the problem of high demand. And the budget couldn’t absorb the cost
of the ground-source system without savings elsewhere. Filtered fume
hoods were central to the solution.
A filtered fume hood has filters mounted on its top. These remove
chemicals from the airstream and release clean air back into the room.
Using three elements, this technology has achieved a level of safety and
flexibility that has allowed them to expand over the past 50 years into an
established market position.
The first is the filters themselves, which bind to contaminants at a molecular level. A redundant set of filters provides a backup to the primary
filters. When the primary filters approach saturation, they are removed
and the secondary filters moved to the primary position.
The second is a control system that includes air quality monitoring
and key card access, ensuring proper fume hood management and
timing of filter replacement. Although filter replacement is a safe, simple
task, third-party monitoring and replacement services are available.
The third element is a powder that’s contained in a layer just below the
carbon in the filter casing, which prepares the chemicals for the molecular
bonds needed for capture in the filter. Older filter technology had to be
tuned to chemical classes. Now they can operate nearly universally, able to
capture the range of chemicals used in a teaching laboratory setting.
The use of filtered fume hoods has a domino effect. By eliminating
most of the ducted hoods, the majority of the exhaust becomes general
Bristol Community College New Technology and Learning Center’s original high-performance design (left) and the new zero-net-energy design (right). Images: BR+A Consulting