8800DB0601
08/2006
Raleigh, NC, USA
Data Bulletin
Boosting the Energy Efficiency of HVAC Systems
with Variable Speed Drives
Combatting Rising Energy Costs
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. Recent government regulations and the rising popularity
of the “green building” concept have led to new construction and retrofit
projects that achieve energy efficiency to benefit the building owner and
occupants. Since HVAC systems in particular are huge energy consumers,
highly efficient systems offer the opportunity for great energy savings.
The Influence of Government
After four years of work by 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% for federal
buildings beginning in 2006 and extending through 2015—an additional 20%
energy reduction on top of the 15% previously mandated. Therefore, facilities
managers for government buildings must find ways to reduce the cost of
operating the buildings—for example, by reducing the run time of the air
conditioning system and using lower wattage light ballasts. The bill also
promotes efficient construction by offering a tax deduction for a property more
efficient than a building designed to ASHRAE®/IESNA1 standard 90.1-2001.
The United States government—with all its office buildings, schools,
hospitals, and military bases—is the single largest user of energy in the
country. According to the National Electrical Manufacturers Association
(NEMA), 45% of the energy consumed by the government is used to heat,
ventilate, and cool buildings (see Figure 1). Most of these HVAC systems
could operate more efficiently.
Figure 1:
Annual Energy Use in Government Buildings2
1
American Society of Heating, Refrigerating and Air-Conditioning Engineers /
Illuminating Engineering Society of North America
2
Source: NEMA
1
Boosting the Energy Efficiency of HVAC Systems with Variable Speed Drives
The Role of Variable Speed Drives in HVAC Systems
The Role of Variable Speed Drives
in HVAC Systems
8800DB0601
08/2006
According to the U.S. Department of Energy, electric motors powering
centrifugal pumps and fans account for 64% of the electricity used by
industrial systems. That equals $33 billion of electricity per year.3 Therefore,
using variable speed drives within HVAC systems is a major component of
the energy-efficiency and cost-savings game.
Key to identifying the energy savings opportunities of variable speed drives
in HVAC systems is understanding the operating cycle of the system versus
the heating and cooling needs actually required. Most HVAC systems are
designed to keep the building cool on the hottest days and warm on the
coldest days. Therefore, the HVAC system needs to work at full capacity
only on the 10 or so hottest days and the 10 or so coldest days of the year.
On the other 345 days, the HVAC system can operate at a reduced
capacity. This is where a variable air volume system with variable speed
drives (also called variable frequency drives, or VFDs) can be used to
match air flow 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 variable
speed drives, the system runs at full speed all the time. Using dampers to
mechanically adjust the air flow 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
Cooling tower fans
Figure 2:
How Much Energy Can Be Saved?
Generally, a centrifugal pump or fan delivering 80% of its rated flow requires
only 50% of the rated power. Figure 3 demonstrates the relationship
between flow and power requirements for a centrifugal pump or fan.
3
2
Using VFDs in an HVAC system
Source: Energy Center of Wisconsin.
© 2006 Schneider Electric All Rights Reserved
8800DB0601
08/2006
Boosting the Energy Efficiency of HVAC Systems with Variable Speed Drives
Payback Period of Variable Speed Drives
Figure 3:
Flow vs. Power Requirements for a
Centrifugal Pump or Fan
A = % flow
B = % pressure
C = % power required
Consider the following example:
A 50 hp fan needs to supply air 10 hrs/day for 250 days.
The cost of running the fan at full speed would be:
50 hp x 0.746 kW/hp x 2500 hrs x $0.08/kWhr = $7,460.00
Assuming the fan does not have to run at full speed all the time:
25% of time at 100%; 50% of time at 80%; 25% of time at 60%
The cost of running the fan with a variable speed drive 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 1250 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
Potential Annual Savings: $7460.00 – $4177.60 = $3,282.40
A variable air-volume HVAC system using a VFD could potentially save
$3,282.40 annually over a constant air volume system.
Payback Period of Variable Speed
Drives
The payback period of a VFD averages 18–24 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 cool the building space. The life cycle of
HVAC equipment in commercial buildings is typically 15–20 years, so a oneor two-year payback period can generate a substantial return on
investment. Visit www.SquareDLeanTools.com and use our Drives
Calculator to determine the potential for energy savings in your application.
Other Benefits of Variable Speed
Drives
Variable speed drives also contribute to the building’s overall comfort level
by optimizing and regulating air flow and temperature into the building’s
occupant space. Unlike a fan that runs either full speed or off, a variable
speed drive can run at all speeds in between, allowing facilities managers
and building occupants more control to adjust the temperature. The
Telemecanique® Altivar® 61 AC drive from Schneider Electric takes
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.
© 2006 Schneider Electric All Rights Reserved
3
Boosting the Energy Efficiency of HVAC Systems with Variable Speed Drives
Data Bulletin
Going Green
In addition to government regulations, the rising popularity of the “green
building” movement is increasingly motivating mechanical and electrical
engineers and consultants to create more efficient HVAC systems. The U.S.
Green Building Council® (USGBC) administers the LEED® Green Building
Rating System™ to define “green building” by establishing a common
standard of measurement. According to the USGBC, the LEED system
(Leadership in Energy and Environmental Design) was also created to:
•
•
•
•
•
Environmental Consciousness
Purchasing building products, mechanical
systems, and electrical components that are
designed and built to optimize energy efficiency
and conserve resources can contribute to your
green building and your environmental
conscience. Electrical distribution and
automation and control manufacturers such as
Schneider Electric have responded to this
market need by designing products and
solutions that are energy efficient and
environmentally friendly.
Schneider Electric seeks to reduce the
environmental impact of its products and
solutions over their entire life cycle—especially
by optimizing their energy and natural resource
consumption and by proposing recycling
solutions at the end of their life.
8800DB0601
08/2006
Promote integrated, whole-building design practices
Recognize environmental leadership in the building industry
Stimulate green competition
Raise consumer awareness of green building benefits
Transform the building market
Essentially, the LEED system is a voluntary standard for designing,
constructing, operating, and certifying green buildings, or those that
significantly reduce and even eliminate the negative impact of buildings on
the environment and their occupants. Several certification programs are
available, including those for new construction (LEED-NC) and existing
buildings (LEED-EB).
A significant component of the LEED rating system for new buildings
includes improving energy efficiency. For example, one prerequisite of the
LEED-NC Energy & Atmosphere component is meeting both the mandatory
provisions and prescriptive/performance requirements of ASHRAE 90.12004, which sets minimum requirements to promote the principles of
effective, energy-conserving design for buildings and building systems.
Mandatory requirements for HVAC systems include mechanical equipment
efficiencies, controls, construction, insulation, and completion. More
specifically, the ASHRAE prescriptive strongly recommends the following:
•
HVAC systems with total fan power greater than 5 hp use variable airvolume fan control
•
Individual, variable air-volume fans with motors greater than or equal to
15 hp use variable speed drives
LEED certification alone has its benefits. In addition to saving energy costs,
it also allows the building owner to take advantage of state and local
government incentives, and makes the building project more marketable to
tenants seeking energy-efficient and sustainable facilities.
Conserving the Earth’s Energy
For more information, refer to data bulletin
8800PD9401, Application Guide for Using AC
Drives in HVAC Systems.
Schneider Electric USA
8001 Knightdale Blvd.
Knightdale, NC 27545
1-888-SquareD (1-888-778-2733)
www.us.SquareD.com
www.us.Telemecanique.com
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
Energy Policy Act of 2005 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 U.S. and the world.
Electrical equipment should be installed, operated, serviced, and maintained only by
qualified personnel. No responsibility is assumed by Schneider Electric for any
consequences arising out of the use of this material.
© 2006 Schneider Electric All Rights Reserved