Heat pumps and filtered fume hoods can help achieve ZNE.


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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 Engineers

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 demand, 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 room exhaust, which allows the use of enthalpy wheel heat recovery without the concern for cross-contamination from the exhaust to the supply airstream.

The filtered fume hoods will continuously filter the room air. Combined with central air quality monitoring, this provides a “belt and suspenders” approach to safely reduce minimum air change rates in the laboratories from the typical 6 ACH to 4 ACH occupied, and 2 ACH unoccupied. For reference, the filter manufacturer, Erlab, maintains essentially zero air changes in their laboratory in France, relying solely on the fume hoods to contain contaminants and filter the air in the room.

Finally, to decouple the space conditioning from the ventilation, the design called for fan-coil units, so the exhaust and ventilation air change rate is never driven by the loads in the space.

The combination of the filtered fume hoods, enthalpy wheel heat recovery, reduced minimum air change rates with central air quality monitoring and fan-coil units resulted in dramatic energy savings. By reducing the air-handler capacity from 70,000 cfm to 24,000 cfm, reducing the stainless steel fume hood exhaust ductwork and cutting the venturi air valve count by half, significant cost savings were achieved.

Zoom out
Simultaneously, BCC entered into a power purchase agreement (PPA) to install a 3.2-MW PV array over an existing parking lot. This system is designed for near zero-net electricity for the entire campus. With the PPA, the array will be installed at no cost to BCC, and power will be provided at a lower cost than they currently pay, with no rate escalation.

Overall, the ZNE design eliminated fossil fuel consumption and reduced energy consumption by a predicted 70% compared to the original high-performance design, saving over $100,000 in energy cost per year. But the true test came when the detailed cost estimate was completed. The result: The ZNE solution was $200,000 lower than the high-performance design.

Beyond achieving ZNE at zero-net cost, the project was able to take advantage of nearly $1.15 million in grants and incentives. Combined with the capital, energy and maintenance cost savings, the result is a net-present savings of nearly $2.65 million over the first 20 years of the building’s life.