Variable-Refrigerant-Flow Heat Pumps, Energy-Recovery Ventilators Cut Carbon

Variable-Refrigerant-Flow Heat Pumps, Energy-Recovery Ventilators Cut Carbon

Aid efforts to eliminate net greenhouse-gas emissions

The University of Maine at Presque Isle (UMPI) was among the first universities to join the American College & University Presidents' Climate Commitment. As a signatory of the agreement in early 2007, President Donald N. Zillman committed UMPI to a rigorous plan to reduce its carbon footprint. A unique HVAC renovation at Folsom/Pullen Hall was among the first steps in the university's trek to carbon neutrality.

One of the primary educational buildings on the UMPI campus, Folsom/Pullen Hall includes classrooms, laboratories, and a lecture hall. It had not received any major mechanical upgrades since being built in 1966. For decades, students and faculty were subjected to excessive mechanical noise and wildly fluctuating indoor temperatures, while the university footed the bill for substantial inefficiencies.

An in-depth evaluation conducted by Mechanical Systems Engineers of Yarmouth, Maine, showed a heat loss of 1,604,188 Btuh for the Folsom wing (assuming 15 cfm per occupant) during occupied cycles. Steam-heated unit ventilators located on outside walls did not incorporate heat recovery. This, combined with insufficient sealing around the ventilators, accounted for 51 percent of the total heat loss when the units were in operation.

Replacement of the unit ventilators, along with replacement of an outdated oil-fired steam boiler serving the Folsom wing, was expected to go a long way in helping the university reduce its carbon footprint.

Kurt Magnusson, PE, of Mechanical Systems Engineers advocated a combination of a central energy-recovery ventilator (ERV) from SEMCO and a variable-refrigerant-flow system (variable-speed heat pumps). This would result in the best payback for UMPI while affording students and faculty the added benefit of air conditioning during summer.

A separately ducted ventilation system would eliminate most of the noise associated with conditioning the building. Also, it would allow maximum recovery of energy from exhaust air. A SEMCO FV-5000 ERV with electric preheat coil proved ideal for this cold-climate application.

The FV-5000 relies on SEMCO's EXCLU-SIEVE total energy-recovery wheel to recover both sensible and latent energy, not only reducing heating and cooling loads, but providing humidification during winter and dehumidification during summer. As the wheel rotates between the opposing supply- and return-air streams, higher-temperature air yields sensible energy to the wheel’s aluminum substrate. A molecular-sieve desiccant coating captures water vapor in the air stream and transfers it to where it is needed.

The FV-5000's energy wheel can operate at full capacity without risk of frost as long as outdoor conditions do not drop below -10°F at indoor design conditions of 70°F and 25-percent relative humidity. Because of Maine's extreme winters, however, an electric preheat coil was included on the unit. A silicon-controlled-rectifier controller modulates heat output according to outdoor-air temperature so that the coil operates only as needed to prevent frosting.

The variable-refrigerant system responds to indoor- and outdoor-temperature fluctuations by adjusting compressor speed to optimize energy use. Inverter technology allows the system to heat and cool simultaneously, basically transferring heat from the warmer side of the building to the colder side. The system is supplemented by operation of new oil-fired boilers during heating season.

"This approach reduced the total amount of ductwork we needed because we’re using smaller air handlers," Magnusson said. "That results in significant fan energy savings."

Fuel consumption dropped from 55,000 gal. of oil in 2007-08 to 22,000 gal. in 2008-09, surprising considering the HVAC renovation applies to only half of the total heating load. Both wings of Folsom/Pullen Hall share a boiler room, but only the Folsom side was renovated, while the other awaits financing. Although significant improvements to the thermal envelope of the Folsom wing were made, Magnusson attributes most of the fuel savings to the heat pumps and ERV.

According to David St. Peter, UMPI's director of physical facilities, electrical consumption increased by only $5,000, even though the Folsom wing is now almost entirely electric. That amount is expected to decrease with the completion of a campus windmill project.

"The building was a real energy hog before," Magnusson said. "Now, we've reduced its carbon footprint by 63 percent."


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