©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 12, December 2013.
By Louise Smith, P.Eng., Associate Member ASHRAE
About the Author
Louise Smith, P.Eng., is a project manager at AME Consulting Group in Victoria, BC, Canada. She is a member of ASHRAE’s Vancouver Island Chapter.
When Vancouver Island University (VIU) replaced its existing Cowichan Campus building in Duncan, BC, Canada, its focus was on sustainability. Key features of the project include a high-efficiency, centralized mechanical system and a high-performance building envelope and lighting design. The building also uses natural and displacement ventilation strategies to reduce the demand on the central plant.
The 44,500 ft2 (4134 m2) building contains teaching and library spaces, computer and science labs, a health lab, offices, boardrooms and a cafeteria. The new Cowichan Campus building was completed in June 2011.
VIU's sustainability policy mandates that all new developments achieve a LEED Canada—New Construction Gold certification. Therefore, the new building was designed to significantly exceed ASHRAE/IESNA Standard 90.1-2004.
The central heating and cooling plant for the new building is a modular 60 ton (211 kW) high-efficiency water-to-water heat pump, which is connected to a vertical closed loop geo-exchange field. The ground source heat pump system provides heat in the winter and is used as a heat sink in the summer to provide cooling to the building while recharging the geo-exchange field. The near constant temperature of the ground, coupled with the loop field design, provide a steady source of heat and a sink for heat rejection throughout the year.
For the Cowichan Campus building, there are three 20 ton (70 kW) modules each with dual compressors that allow efficient staging under part-load conditions. Formation thermal conductivity testing was performed during the schematic stages of the project design to confirm the ground conditions were suitable and provide essential data for the loop field design. The Cowichan Campus geo-exchange field consists of 96 boreholes spaces at 15 ft (4.5 m) centers, each drilled to a depth of 115 ft (35 m) and located under the landscaped wetlands behind the building.
The geo-exchange field was sized to provide 100% of the peak design building cooling load, which was equal to 67% of the peak design heating load. Analysis of the annual heating and cooling degree days for this climatic location, indicated that this would be sufficient to heat the building for more than 90% of the year, which provides an optimum payback period. The additional heating load is supplied by two high-efficiency condensing boilers.
To provide additional energy savings, the central water-to-water heat pumps were piped for simultaneous heating and cooling, without the use of the geo-exchange field. This occurs twice a year during shoulder seasons when the north-facing spaces require heating, but the south-facing spaces need cooling. This design strategy uses a single heat pump unit to satisfy both the heating and cooling load by transferring the excess heat from one zone to another. This is the most efficient mode of operation for the water-to-water heat pump with a coefficient of performance of 7.1.
Throughout the building, in-floor radiant piping distributes the heating and cooling from the central plant. This is a quiet and efficient method of delivering thermal comfort to the building and its occupants. The radiant system utilizes low temperature hot water, approximately 110°F (43°C), for the heating season and relatively high temperature chilled water, approximately 65°F (18°C), for the cooling season. This complements the ideal operating temperatures of the ground source heat pump system. By using the radiant slab as the primary source of heating and cooling, the size of the air-distribution system was significantly reduced, which provided both capital cost and fan energy savings.
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