Proven Benefits – Energy Management; Tower Engineering Group
March 1, 2009
By Canadian Consulting Engineer
The energy efficiency upgrade of the 12-storey office building at 125 Garry Street in downtown Winnipeg was more than just a chiller or boiler replacement. Such replacements are routinely performed th...
The energy efficiency upgrade of the 12-storey office building at 125 Garry Street in downtown Winnipeg was more than just a chiller or boiler replacement. Such replacements are routinely performed throughout Canada, sometimes even without engineers.
This upgrade project shows the success of the engineering method in achieving energy reductions in an ageing building as opposed to a simple turnkey replacement of equipment. Conditions inside the 35-year old structure have been greatly improved by applying readily available technology in innovative ways, all at an affordable cost. What’s more, the dramatic results (50% reduction in natural gas use and 15% reduction in electrical use) have been monitored and proven over the past four years.
The project did not start out as a systems upgrade but rather as a need to replace an aging chiller due to the phase-out of R-11 refrigerant. A number of supplier/contractors were very eager to replace the chiller with their products, but through the application of engineering methods, Tower Engineering won the trust of the owner, Brent Business Services, and convinced them to take a more comprehensive approach.
With an area of approximately 120,000 sq. ft. (11,150 m3), 125 Gerry Street was built with systems typical for its time: R-12 walls, R-20 roof, and double-pane, aluminum-frame windows. Heating was by two gas-fired boilers and cooling by one centrifugal chiller. Air supply was by a central variable air volume (VAV) system for floors 3 to 12, and a constant volume system with reheat for the first two floors and basement.
The building operated 16 hours a day on weekdays and 12 hours a day on weekends. This resulted in annual energy use of 2.47 gJ/m2 in the year prior to the system upgrades. Natural Resources Canada’s benchmark for similar buildings is 1.9 to 2.19 gJ/m2. Therefore, in its original state the building’s energy use was high. The owners and building managers were only too aware of the constantly increasing costs of maintenance and the low efficiencies of the aging equipment.
Tower’s first step, as opposed to just preparing drawings for the replacement of a chiller, was to conduct a detailed assessment of the existing major mechanical, electrical and structural systems. What became evident was that the solution proposed by the contractors was not the best for the building. Tower presented the owner with a detailed list of upgrade options, including construction costs, a payback analysis and the available grants from utilities and governments (see Table 1).
Modular System Of Chillers
Analysis showed that the best way to proceed was with a modular system of six chillers, each of 175 kW (50 tons) cooling capacity. This system would give a high level of redundancy. Also, it meant the rooftop mechanical room would not have to be upgraded to a Class T standard, which the building code would have required for a single new chiller due to refrigerant pressure issues.
Upgrading the mechanical room would have meant major work, including separating the chillers from the boilers and the addition of a vestibule entrance with an alarm. Also, the modules could be much more easily installed than a single chiller, since they could be carried upstairs and moved through doorways, rather than requiring to be hoisted in by crane.
The modular system resulted in a high part load efficiency because modules could be completely turned off. Before the retrofit, some of the building’s cooling energy was being wasted during the summer because certain areas needed to be reheated to make them comfortable. This reheating could not be accomplished without firing one of the large boilers –at significant cost.
With the modular system, five of the chillers would be used to cool the building directly, while the sixth chiller would be operated at a low temperature to sub-cool ventilation air in the summer. This method would change the way that fresh air was introduced into the building. It would also generate the reheat which would allow the boilers to be shut down in warm weather.
The new boilers were 85% efficient, near-condensing with fully modulating burners. As the existing heating system required high temperature water, there was no benefit in installing condensing boilers and thus their cost premium was saved.
Ventilation air sub-cooling and condenser water heat recovery
This HVAC option is often discussed but rarely implemented. The existing variable air volume (VAV) system would blend return and outside air then condition it to a temperature to meet the cooling requirements, about 13C (55F). Cooling air raises its relative humidity and dehumidifies it somewhat, but the air is in a saturated state. The new approach was to sub-cool the outside air to 7C (45F) and then blend it with the return air. The mixture would be at a much lower moisture level, requiring less additional cooling energy and resulting in a more comfortable environment in the cooling season. In this case, the reduction was almost 350 kW (100 tons).
The system was further designed so that condenser water from two chillers was pumped to the reheat coils that previously required water from the boilers. This water would otherwise be pumped through the cooling tower to dissipate the heat to the atmosphere. The process resulted, basically for free, in heating being available when cooling was operational and allowed the boilers to be shut down in summer.
Ventilation air heat recovery
By recovering the heat from the building’s exhaust air, it was estimated that the annual savings would be 19,000 m3 of natural gas for heating and about 42 kW (12 tons) for cooling. The electrical demand would also be reduced by about 6%.
In addition, variable speed drives (VFDs) were added to reduce the ventilation air system’s power needs. Also, a new direct digital control (DDC) system enables the building operators to monitor and optimize the systems. The DDC system reduces the building’s energy use by at least 10% or 2,000 gJ.
Success monitored over four years
Accounting for the installed cost and the annual savings, the simple payback was estimated at three years. However, once the building was upgraded, its hours of operation were increased to 24/7. Even with this 60% longer operating period, the upgrades resulted in building energy use of 1.83 gJ/m2 which is below the Natural Resources Canada benchmark! The actual results for four years ending 2008 have exceeded the estimates, as shown in Table 2.
The retrofit was so successful that none of the upgrade costs were passed on to the tenants. And remarkably, no money was spent on envelope upgrades. The energy saving results were achieved only through the mechanical systems.
Client: Brent Business Services
Prime consultant: Tower Engineering Group, Winnipeg. (Mike Houvardas, P. Eng., Greg Jorgensen, P. Eng.)
Mechanical & electrical contractors: Wescan, St. Vital Mechanical
Supplier: Multi-Stack -Hydronaire (chillers)
Engineers upgraded the HVAC system of a 35-year old office building and have documented dramatic benefits over four years.