In Longueuil, QC, Canada, winters are bitter with temperatures as low as –24.4°C (–11°F). To efficiently heat and cool the Mountain Equipment Co-op (MEC), a 2100 m2 (22,600 ft2) single-story sporting goods store in Longueuil, the design team chose a radiant heating/cooling slab, ground source heat pump, and heat recovery devices. Annual energy savings using these strategies are 427,842 kWh.
The main ventilation unit, located on the roof of the building, has a capacity of 8510 L/s (18,000 cfm) to deliver 5.3 L/m2 (1 cfm/ft2), which is distributed by an underground duct network and supplied with floor-mounted displacement diffusers. The returns are located on the higher part of the sawtooth-shaped roof area, which is over the retail area, to maximize the displacement effect.
In normal operation, the unit modulates its flow with a variable frequency drive to maintain the desired conditions in the building. A dedicated fresh air unit of 2030 L/s (4,300 cfm) is connected to the distribution unit to provide fresh air. In normal operation, the system modulates with CO2 sensors to control the amount of fresh air required in the building. The fresh air unit incorporates a 75% total efficiency heat recovery module.
When the outdoor temperature permits the use of free cooling, a natural hybrid ventilation strategy (Figure 1) is used by opening the louvers located at the extremity of the sawtooth-shaped roof and by using only the supply fan to push air into the building. A maximum capacity of 8500 L/s (18,000 cfm) of natural hybrid ventilation is possible with the units. The louvers have been located downwind to maximize the natural pulling effect of the dominant wind.
A radiant heating/cooling slab is used to compensate the envelope losses or gains. The slab is fed with a propylene-glycol mixture connected to the building's four heat pump units. Each unit has a nominal capacity of 35 kW (10 tons) for a total of 140 kW (40 tons) of capacity. The propylene-glycol is also distributed to the ventilation units to serve their heating and cooling needs. The capacity of the geothermal system serves 100% of the building's heating and cooling needs.
In the winter, the incoming solar energy is accumulated in the concrete slab for “free” night heating of the store. During the heating season, the hot air is recovered in the upper part of the store and mechanically cooled. This heat is then injected in the 200 mm (8 in.) concrete floor slab for night heating of the store. Predictive controls manage the energy recuperation system.
The heat pumps always operate on the same mode, between two 1249 L (274 gallon) accumulation tanks. Those tanks dampen the energy needs or energy excess from the building. They also give the opportunity to simultaneously heat a specific zone with the heat extracted from another zone. When the needs are not perfectly balanced, the geothermal ground loop exchanger then operates as a heat sink or heat source, depending on the building’s needs to cool the hot tank or to heat the cold tank. A total of 10 vertical wells that reach a depth of more than 135 m (450 ft) have been installed on the building property to serve those needs. The wells connect individually to the collector inside the mechanical room of the building allowing for the testing of each well independently.
The 200 mm (8 in.) concrete floor slab thickness has been carefully calculated to accumulate the energy on a 12-hour basis. This thermal storage gives the opportunity to optimize the heat pump operation and maximize their efficiency. This concrete slab has an energy accumulation capacity of 555 kWh with a variation of 2.8°C (5°F) in heating or cooling mode (158 ton-hours). With the thermal storage, the number of heat pumps has been reduced from five to four, reducing the initial installation cost and footprint of the mechanical room. It also gives the building a more stable operation. Temperature sensors have been installed at different depths in the slab to monitor the status of thermal storage.
Citation: ASHRAE Journal, vol. 54, no. 6, June 2012 ©2012