As seen in ASHRAE Journal 2009 march edition
Project: 4200 St Laurent, Montreal office building
Motivated by the desire to implement green practices and faced with rising energy costs, the owners and managers of a thirteen story office building downtown Montreal have undertaken an ambitious project that drastically reduced its energy consumption. By using innovative technologies where conventional technologies were not applicable, a large part of the heat that the building was dissipating to the atmosphere is recovered. The recovered heat is used to heat the entire volume of fresh air that is introduced into the building, without compromising indoor air quality. Thanks to this new heat recovery system, no external energy source is required to heat the building’s fresh air. This amounts to very significant savings.
The building was the object of a detailed energy audit, by Ecovision Consulting, wherein a series of energy-saving measures were proposed with an overall payback of two years. The other energy-saving measures involved the building’s automated direct digital controls, the electrical systems, and the heating system of the building’s envelope. All measures have been operating for more than a year now, with remarkable results.
About the Building’s energy consumption
The building’s consumption of natural gas was high and expensive. A typical winter, before the energy-saving measures were installed, the consumption of natural gas for that winter amounted to 451007 m3, 182 856$ and 8386�F-days.
A typical year for electrical consumption before any of the electrical energy-saving measures were installed was 6,207,600 kWh, 385 634$, respectively. An energy management system was proposed, to the building managers, which would control peak electrical demand (by load shedding) so that they could increase the occupancy rate without worrying about excessive demand penalties.
A series of energy-saving measures were implemented. Automated direct digital controls, mechanical (HVAC) systems, and electrical measures. Careful attention was given to maintaining the existing air quality.
Mechanical measures were implemented such as installing two 3000 MBH condensing natural gas boilers to replace the existing four 4000 MBH atmospheric boilers. Also, installing a heat recovery unit that will transfer heat from the building’s exhaust air to its fresh air and diverting the water for the condensers of the a/c units into heat-exchangers in the fresh air supply. Heat that would otherwise be rejected to the atmosphere is transferred to the building’s fresh air.
The electrical measures are composed of improvements in the performance of the lighting fixtures and in the performance of the fan, pumps and compressor motors (using variable speed drives). 1600 T12 lighting fixtures were replaced with more efficient T8 lighting fixtures. The cooling-tower circuit was converted from a constant-volume to variable-volume system by installing a two-way valve at each a/c unit so the volume-flow-rate of the water in the cooling-tower circuit varies according to the actual need of the building (pressure sensors in the water are used for PID control of GPM).
Indoor Air Quality
The occupants were satisfied with the thermal comfort before our measures were implemented. The goal was therefore to maintain existing air quality. The fresh air unit supplies at least 15 cfm per person on every floor. After the energy-saving measures were installed CO2 levels and air velocity were verified throughout the building and found to be just as good as before. Each zone has a thermostat tied into the DDC controls, which assures thermal comfort.
The heat recovery unit that transfers heat from the exhaust air to the fresh air, consists of innovative technology (patent pending). It is a thermosiphon heat exchanger that uses an environment-friendly refrigerant. The thermosiphon operates like a heat-pipe in that it uses a condensing and evaporating refrigerant, but the liquid travels from the condenser coil to the evaporator coil by gravity and not by capillary action.
The second heat recovery unit consists of transferring the heat of the warm water from the a/c units’ condensers into the fresh air. The water is diverted from the cooling tower to a series of heat transfer coils in the fresh air ducts. Both heat recovery units together are capable of recovering enough energy to heat the fresh air all winter long.
Results and Benefits
The first complete winter, after the energy-saving measures were implemented, the total consumption in natural gas was 147 918 m3 for a cost of 68 008$ Canadian. The natural gas savings were in excess of 60% of the original consumption with a 60% reduction in cost as well.
During the twelve-month period the electrical consumption amounted to 5,469,600 kWh for a cost of 383,080$. That is 738,000 kWh (or 12%) less than the consumption during the year before the electrical measures were installed. However, during those months, the average occupancy rate had risen to 97% from 89.7%. When the increase in occupancy rate is taken into account, the electrical savings amount to at least 20%.
The total project cost added up to 527,600$CAN. The Canadian government and the local natural gas and electrical utility companies offered grants totalling 202,000$CAN. Expected savings were exceeded and, in all, 700 tons of equivalent CO2 emissions were eliminated from the atmosphere per year. The owners and managers of this building have taken a great leap forward in their pursuit of environmentally sustainable buildings.