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Lessons from the Ground Up

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©2016 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 58, no. 5, May 2016

Eric (Miao) Yang, P.E. BEAP, CPMP, HBDP, Member ASHRAE

About the Author
Eric (Miao) Yang, P.E., is an energy engineer at NORESCO in Fairfax, Va.

Virginia Beach City Public Schools is a national leader for integrating sustainability into buildings and extending it into classroom teaching. An energy retrofit project centered on a new ground-source heat pump (GSHP) system at Strawbridge Elementary School demonstrates how institutions can improve buildings, save energy, and create a culture of sustainability, financial stewardship, and classroom excellence.

 

Building Description

Strawbridge Elementary, an 84,948 ft2 (7892 m2) one-story facility constructed in 1991, accommodates approximately 750 students and 100 staff members. The project team, engaged through an energy savings performance contract as the financing vehicle, made three significant changes to the existing building: (1) replaced the heating and cooling systems at the school with a substantially more energy-efficient GSHP system; (2) installed more efficient lighting; and (3) upgraded the energy management control system (EMCS).

In the months since commissioning, it has become clear that this project not only achieves substantial energy savings but also provides an innovative new learning platform for students in a more comfortable learning environment.

 

Prior Heating and Cooling Systems

Until 1996, the school’s heating and cooling systems used a two-pipe water-source heat pump system for heating and cooling and several unitary direct-expansion (DX) rooftop units (RTUs) with electric heating for the gymnasium, library, cafeteria, and administrative areas.

As part of a renovation project in 1996–1997, the school closed outdoors air to all water-source heat pumps (WSHPs) in the classrooms. It also installed five outdoor air units (OAUs) with DX coils and heat-pipe energy recovery technology. Closing the outdoor air sections caused the existing WSHPs, each rated at 3.5 to 4 tons (12 to 14 kW), to become oversized for cooling.

Additional summer inefficiencies came from two aging closed-circuit fluid coolers that rejected heat from the condenser loop, using two 25 hp (19 kW) pumps (Figure 1a). During winter, two hot water boilers rated at 1,440,000 Btu/h (422 kW) output provided hot water to the WSHP system.

 

Heating and Cooling Retrofits

The heating and cooling retrofits, shown in Figure 1b, were installed in two phases to accommodate the school schedule. Between January and March 2013, the GSHP well-field was drilled (274 bore holes, 300 ft [91 m] deep) and a high-density polyethylene (HDPE) vault was buried in the large sports field behind the school (Photo 1). Between June and August 2013, the remaining GSHP components were installed, including two new 40 hp (30 kW) well pumps (lead-lag configuration) with variable frequency drives (VFDs). This new system was properly sized for the heating and cooling load of the entire school.

Additionally, a separate re-roofing project at the school replaced all existing RTUs and OAUs with new GSHP rooftop units. The five OAUs were replaced with five new units featuring high efficiency energy recovery wheels with VFDs. The unit serving the administrative areas was replaced with a variable refrigerant flow system, also with a dedicated outdoor air system coupled with an energy recovery wheel.

 

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