High Performing Buildings - Spring 2009 - (Page 22) The horizontal runs of the geothermal heat exchanger run below the underground parking slab. The horizontal runs were encased in concrete before final backfill and the pouring of the slab. The topmost layer of PEX piping, which makes up the solid thermal energy storage, is shown before the final backfill. The installation was done from “within” the building, before the slab on grade was poured. outside air. If there are not enough chilled water consumers, the geothermal exchanger acts as a virtual consumer. Supplied chilled water creates an additional load on the chiller to maximize the heat generated in heat pump operation. The boilers are used if needed. During the cooling season, the geothermal heat exchanger acts as a heat sink to reject the heat produced by the building, using the ground for seasonal energy storage. During the winter, the solid thermal energy storage is used to preheat the returning heating water during the day, acting as an energy producer, whereas it is recharged at night with the water-cooled chiller (acting as a consumer). The reverse happens during the summer. The charging and discharging of the thermal storage is controlled in order to prevent overloading the main chiller when charging and to avoid discharging it too quickly. This allows the system to benefit from the peak load shaving long enough to pass through the day. The main objective is to avoid starting a boiler or a backup air-cooled chiller. 22 HigH Performing Photo © Pageau morel Vertical geothermal exchanger Originally, the heating and cooling energy of the building was to be mainly produced by a geothermal heat exchanger coupled with two 60-ton chillers intended to work as heat pumps. It called for a geothermal heat exchanger made of 100 450-foot boreholes. However, there was no open space left on the construction site to locate those boreholes. The only remaining available space was located under the building, but that space couldn’t accommodate 100 boreholes while respecting recommended distances from the foundation elements and with the required spacing between boreholes. Knowing that the peak loads on the geothermal exchanger are typically responsible for a sizeable portion of the total borehole length required, the building team sought to reduce peak loads, thereby reducing the size of the geothermal exchanger. However, the overall heat balance had to be maintained to obtain the desired building energy efficiency target. Thus, thermal energy storage needed to be used. It was designed to completely replace one of the spring 2009 original 60-ton chillers. The storage had to be capable of producing 60 tons during a period of at least eight hours after being charged. The peak loads perceived by the geothermal exchanger were reduced by half. With this new data, calculations determined that 60 boreholes were required, which could be drilled below the building. solid Thermal energy storage This building benefited from a particular situation. A portion of land beneath one of the existing buildings was contaminated and needed to be excavated; however, it later had to be backfilled. It was decided that the backfilled space would be used as thermal profiles in thermal storage Buildings
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