Drive Technology: Boosting Energy Efficiency of HVAC Systems

May 5, 2008
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Two VFDs in a supermarket application.

The challenge of saving energy and reducing energy-related costs is on the minds of both consumers and business owners today. Recent situations such as China’s expanding energy consumption, events in the Middle East, environmental protection concerns, electrical transmission grid failures, government mandates, and Hurricane Katrina have elevated the energy-efficiency discussion and brought energy costs and energy efficiency to the front page.

Building owners and facility managers continually look for ways to save energy and money. Since HVAC systems in particular are huge energy consumers, highly efficient systems offer the opportunity for great savings.

And savings are achievable with variable-frequency drives (VFDs).

GOVERNMENT INFLUENCES

The U.S. government is taking steps to help push energy savings. After four years in Congress, the Energy Policy Act of 2005 (EPAct) was signed into law in August 2005. While the act contains many elements related to energy sources, production, transmission, and use, one section of the bill establishes an annual energy-reduction goal of 2 percent for federal buildings, extending through 2015 - an additional 20 percent energy reduction on top of the 15 percent previously mandated. Therefore, facilities managers for government buildings must find ways to reduce the cost of operating the buildings.

The United States government, with all its office buildings, schools, hospitals, and military bases, is considered the single largest user of energy in the country. According to the National Electrical Manufacturers Association (NEMA), 45 percent of the energy consumed by the government is used to heat, ventilate, and cool buildings. Most of these HVAC systems could operate more efficiently.

ROLE OF VFDS

According to the U.S. Department of Energy (DOE), electric motors powering centrifugal pumps and fans account for 64 percent of the electricity used by industrial systems. That equates to $33 billion of electricity a year, according to the Energy Center of Wisconsin. Therefore, using VFDs within HVAC systems is a major component of the energy-efficiency and cost-savings game.

Key to identifying the energy-savings opportunities of VFDs in HVAC systems is the understanding of the operating cycle of the system versus the heating and cooling needs actually required. On all but the hottest and coldest days of the year, the HVAC system can operate at a reduced capacity, otherwise referred to as part-load demand. This is where a variable-speed volume system with VFDs can be used to match airflow to actual heating and cooling demands. The VFD can reduce the motor speed when full flow is not required, thereby reducing the power and the electrical energy used.

If a building uses a constant-volume air-handling system with no VFDs, the system runs at full speed all the time. Using dampers to mechanically adjust the airflow output into the appropriate rooms does not control the speed of the motor and does not save energy.

HVAC equipment offering opportunities for energy savings includes centrifugal air handler fans, centrifugal exhaust fans, centrifugal chilled water pumps, centrifugal hot water pumps, cooling tower pumps, and cooling tower fans.

PAYBACK PERIOD

According to most manufacturers, the payback period of a VFD averages 18 to 24 months, but can be less than 12 months. The period depends largely on the type and size of the system and how much time the motor is operating at full speed versus how much flow is actually required to heat or to cool the building space. The life cycle of HVAC equipment in commercial buildings is typically 15-20 years, so a one- or two-year payback period can generate a substantial return on investment.

OTHER BENEFITS

VFDs also contribute to the building’s overall comfort level by optimizing and regulating airflow and regulating airflow and temperature into the building’s occupant space. Unlike a fan that runs either full speed or off, a VFD can run at all speeds in between, allowing facilities managers and building occupants more control to adjust the temperature. Some drives are capable of taking occupant comfort one step further with accurate flow control and a modulating output switching frequency to reduce noise pollution caused by motor operation.

In addition, VFDs reduce the long-term wear and tear on equipment. The drives provide a soft start instead of slamming motors on at full speed - so HVAC systems last longer, requiring less maintenance, and causing fewer instances of unscheduled downtime.

GOING GREEN

In addition to government regulations, the rising popularity of the green building movement is increasingly motivating mechanical (and electrical) contractors, engineers, and consultants to create more efficient HVAC systems. The U.S. Green Building Council (USGBC) administers the LEED (Leadership in Energy and Environmental Design) Green Building Rating System to define green building by establishing a common standard of measurement.

A significant component of the LEED rating system for new buildings includes improving energy efficiency. More specifically, an ASHRAE prescriptive strongly recommends the following:

• HVAC systems with total fan power greater than 5 hp use variable-air-volume fan control.

• Individual, variable-air-volume fans with motors greater than or equal to 15 hp use VFDs.

CONSERVING THE EARTH

A variable air/water/refrigerant volume HVAC system controlled by VFDs can go a long way in helping a new or existing building achieve greater energy efficiency. HVAC systems run by VFDs not only save money, they also increase the comfort of the building and reduce equipment maintenance costs and downtime. Plus, meeting the requirements of the EPAct and achieving a greener system through LEED certification can offer more money-saving opportunities if the building is eligible for state and local government incentives. Ultimately, more efficient HVAC systems create more energy-efficient buildings, which in turn conserve energy resources across the United States and the world.

Sidebar: How Much Can Be Saved?

Generally, a centrifugal pump or fan delivering 80 percent of its rated flow requires only 50 percent of the rated power.

Consider the following example:

If: A 50-horsepower (hp) fan needs to supply air 10 hours a day for 250 days.

Then: The cost of running the fan at full speed would be:

50 hp x 0.74 kW/hp x 2,500 hrs x $0.08/kWhr = $7,460

If: (Assuming the fan does not have to run at full speed all of the time:) 25 percent of time at 100 percent; 50 percent of time at 80 percent; 25 percent of time at 60 percent.

Then: The cost of running the fan with a VFD would be:

50 hp x (1.00)3 x 0.746 x 625 x $0.08 = $1,865.00

50 hp x (0.80)3 x 0.746 x 1,250 x $0.08 = $1,909.76

50 hp x (0.60)3 x 0.746 x 625 x $0.08 = $402.84

Total = $4,177.60

Therefore, a variable air volume HVAC system using a VFD could potentially save $3,282.40 annually over a constant air volume system.

Publication date: 05/05/2008

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