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Energy Efficiency from the Ceiling (Fan) to the Floor

February 12, 2013 By Christian Taber

Christian Taber

Energy Efficiency from the Ceiling (fan) to the Floor

With the goal of being the most energy efficient high rise in North America, architects and engineers built a 21 story, 695,000 ft² office tower which serves as the home of Manitoba Hydro Power, located in downtown Winnipeg [Ontario, Canada]. Temperature swings from -22 F to 86 F challenged the design team to build an energy efficient building that uses passive, solar energy to heat the facility. The incorporated features include a double façade building envelope that traps the sun’s heat, the use of radiant heating and cooling, displacement and natural ventilation, a high-efficiency heating and cooling system, large diameter, low speed fans and an immense geothermal well system all allowing for increased energy efficiency.

As we move towards initiatives like AHSRAE (American Society of Heating, Refrigerating, and Air-conditioning Engineers) Vision 2020 and the AIA  (American Institute of Architects) 2030 Commitment, which offer guidance and strategies to help sequence the development of net zero-ready buildings, we must reevaluate the role that typical building components play in a facility’s energy efficiency. Recent changes to Appendix G of ASHRAE Standard 90.1 (Energy Standards for Buildings Except Low-Rise Residential Buildings) allow the inclusion of energy savings through elevated air speed in energy simulations. In this paradigm, large overhead fans can save up to 35% on energy bills and have been incorporated in many net-zero ready buildings worldwide.

Partnering with HVAC

One of the largest contributors to energy use in commercial buildings is associated with central air systems. Large diameter, low speed fans help with the air distribution of central HVAC systems, reducing supply fan energy, increasing ventilation effectiveness and reducing the load on heating and cooling equipment.

…in the winter

In facilities that must contend with cool to cold winter temperatures, heating system efficiency is drastically improved through destratification, circulating warm air trapped at the ceiling level down to room occupants. Heated air from a forced air system (85-125° F at ceiling level) is less dense than the ambient air (65-75° F) and naturally rises to the ceiling. Running a large diameter overhead fan to 10-30 percent of its maximum rotations per minute (RPM) in the forward direction redirects warm air from the ceiling to occupant level, increasing comfort and reducing the amount of heat lost through the building envelope.

…in the summer

On the other extreme, large diameter, low speed fans are used in warmer months to circulate the air in a facility at elevated speeds, offering a cooling effect to the occupants. Although large overhead ceiling fans do not lower the air temperature in a space, the perceived cooling effect can make occupants feel up to 10 F cooler. Each degree the set point is raised reduces HVAC related energy usage by 2-3%; for example, by raising the thermostat from 74 F to 81 F, one could expect to save 15-20% on annual cooling.

Fanning the towers

Back at the Manitoba Hydro Power complex, during the long, cold winter months, the outside air for the atria is heated by a geothermal heat pump system with wells driven 400 ft. underground, circulating glycol which is heated by the ground source heat exchanger. (This same glycol is then cooled in the summer to maintain an appropriate thermal comfort level for occupants.)

Due to the high air temperatures at the ceiling caused by stratification and the resulting lack of thermal comfort for those located on the upper floors near the atria, Manitoba Hydro’s options were to either exhaust the heat out the operable windows or recirculate the heat from the ceiling down to the floor level. Large diameter, low speed fans were installed to push the heat down to the floor level.

Because of the high ceilings within the space, the fans are run at relatively higher speeds to move the heated air to the floor, without causing a draft. With the fans running close to full speed, the temperature discrepancy is now 5 degrees less. “The large overhead fans mix the air, allowing us to minimize the heating and cooling demand in the space,” said Mark Pauls, hydro energy engineer with Manitoba Hydro. The tower consumes 88 kWh/m²/a, compared to 400 kWh/m²/a for a typical large scale North American office tower located in a more temperate climate.

The other extreme

The progressive thinkers at DPR Construction transformed a former retail shop into their new Phoenix, Ariz., regional headquarters, turning it into a showplace for sustainable building practices.

This net-zero office houses four shower towers made of inexpensive plastic tubing and dual shower heads, each providing evaporative cooling at peak operating conditions. It does this by pulling outside air across the shower and misting heads before it is transferred into the building via dampers at the tube’s base.

The other end of the building boasts Arizona’s largest solar chimney, an 87-ft. long structure clad in zinc to absorb the sun’s heat. By heating the air within it, the solar chimney utilizes natural convection to pull warm air out of the building, which in turn draws cooler air up and creates airflow in the space below.

This cooled air mixes with outside air brought in through 81 operable windows hooked into the building’s energy management system, where it’s all thoroughly circulated by twelve 8-ft. diameter overhead fans. “The large [diameter] overhead fans are a huge component to our passive cooling comfort system. It would not work without them,” said Andy Hill, preconstruction manager and sustainability specialist.

A full house and a final thought

Even though energy conservation is a focal point in many construction projects, employee comfort and satisfaction are of equal significance. Considering cost associated with a workforce, said Manitoba Hydro’s Mark Pauls, all employee costs were considered in the development of the tower. “Because our employee costs are roughly 100 times our utility bill,” he explains, “if we improve productivity and decrease absenteeism by 1% each that dwarfs any energy savings we would ever see.”

Christian Taber is Senior Applications Engineer (LEED AP BD+C, HBDP, BEMP, CEM) for Big Ass Fans.



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