By Jim Bererton, P. Eng. Stantec
WestJet Calgary CampusBuildings Building Mechanical & Electrical (HVAC) Systems Building Structure
Regional markets where natural gas costs much less than electricity challenge the commercial viability of conventional geo-exchange systems. There are few places where this is more true than in Albert...
Regional markets where natural gas costs much less than electricity challenge the commercial viability of conventional geo-exchange systems. There are few places where this is more true than in Alberta. Geo-exchange systems do require electricity and they only begin to reduce energy costs when the cost ratio of electricity over alternative heating energy is less than the seasonal performance coefficient.
In such a market, the annual energy savings of a conventional geoexchange system for the 30,200-m2 WestJet Airlines corporate campus near the international airport in Calgary, did not justify the upfront investment. It would have cost $1.4 million to install the large 200-bore-hole field that was necessary to meet the building loads.
As a result, designers had to use innovative thinking and sophisticated control algorithms to achieve the optimum efficiency necessary to make the geoexchange system worthwhile.
Conventional building design examines the worst-case weather conditions and sizes the boiler and chiller separately to meet the peak demands, which results in oversized equipment for more average conditions. As well, segregating the heating distribution network from the cooling distribution network forces designers to supply each building zone with capacity to meet peak heating and cooling demands. In larger commercial buildings like the West-Jet campus it is not uncommon to have simultaneous heating and cooling calls on the central HVAC plant.
The first and perhaps most effective design innovation to achieve optimum efficienct at the WestJet Campus was to dedicate two heat pumps to transfer heat directly from the cooling loop to the heating loop whenever simultaneous heating and cooling was required. When operating in this mode, the dedicated unit can achieve an effective COP of 7.0 because both the cooling energy extracted and the heating energy delivered are counted as useful energy produced. This direct energy-transfer method greatly reduced the size of the geoexchange borefield required to meet the load balance.
Incorporating geoexchange piping inside the structural piles also reduced the number of conventional boreholes. As the six-storey structure already required piles drilled over 20 metres in depth and up to 1.0 metres in diameter, the soil beneath the building could act as a shallow vertical borefield, with no need to drill down to bedrock.
The geoexchange piping loops were therefore attached inside the cylindrical rebar cages before they were lowered into the drilled piling hole. The loops were attached with high-strength tie wraps to ensure the piping did not separate from the cage as the concrete was poured. While there were some piping failures during the construction, using a tremie pipe would have avoided this risk.
Each piling loop was connected to a header system, forming a horizontal geo-exchange field beneath the parkade slab. In spaces where no headers were required, extra loops were installed to augment the horizontal field. When some of the piles located beneath the building failed, additional boreholes were drilled outside the building footprint.
Further examination of the building load profile during the design indicated that while many periods permitted the direct transfer from the cooling loop to the heating loop, there were also many days with heating only loads in the morning/evening and cooling only periods in the afternoon. The only way to use the heat rejected from the cooling loop was with thermal storage.
As a result, a large (13,000-L) water tank was installed in the basement mechanical room and piped in series with the ground loop. The tank stores the rejected heat from the cooling cycle for extraction during the next heating demand period. The concrete piles also act as thermal storage, but the rate the heat can be added or extracted from them is limited. The water tank has no such limitation; virtually all stored energy is immediately accessible by the heat pumps.
The design of the West Jet hybrid geothermal HVAC system involved many hundreds of model years to combine energy piles, conventional boreholes, a water thermal storage tank, heat pumps (dedicated to direct heat transfer, heating and cooling), a high-efficiency chiller, cooling towers, and high-efficiency boilers. Detailed models were developed using a TRNYS transient thermal analysis software to measure the size of each component and how they could best work in symphony. Cost functions were added to the model and a Hookes-Jeeves optimization algorithm was applied.
Compared to a conventional geoexchange design, the hybrid design saved over $800,000 in capital costs. Occupied last year, the building is projected to consume 67% less energy than a purely conventional design.
Mechanical-electrical consultants: Stantec (Jim Bererton, P. Eng.).
Architect, structural, landscape: Stantec.
Civil: Idea Group.
Coefficient of Performance Calculations
COP is determined as Useful Energy Output/ Electrical Energy input. For a heating example with 3 kWh of energy extracted from the ground and 1 kWh of energy supplied from electricity, 4 kWh of energy is delivered as useful heat. This gives 4 kWh Useful Heat/ 1 kWh electricity yielding a COP of 4.0. With 3 kWh of cooling energy extracted consuming 1 kWh of electricity, the COP is given by 3 kWh/1kWh resulting in 3.0. When the dedicated heat pumps perform these two functions simultaneously, there are 3 kWh of cooling + 4 kWh of heating delivered with the same 1 kWh of electricity. (3+4)/1 generates a COP of 7.0.]