Mechanical and electrical contractors are always looking for ways to save energy and money in buildings. Recent government regulations and the rising popularity of the green building concept have led to new construction and retrofit projects that achieve energy efficiency benefitting the building owner and improving occupant comfort. HVAC systems in particular are a large energy consumer and highly-efficient systems offer an opportunity for notable energy savings.
VARIABLE-SPEED DRIVESThe biggest key in identifying the energy savings opportunity of variable-speed drives in HVAC systems is to understand 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 only needs to work at full capacity on roughly the 10 hottest days and the 10 coldest days of the year. On the other 345 days, the HVAC system may operate at a reduced capacity. This is where a variable air volume system with variable-frequency drives (VFDs) can be used to match airflow to actual heating and cooling demands. A VFD can reduce the motor speed when full flow is not required, thereby reducing the power required and the electrical energy used.
If a building had a constant-volume air handling system with no variable-speed drives, the fan system would run at full speed all the time. In this type of system, VFD-controlled pumps can provide variable flow in place of energy wasting valve control methods. For variable airflow systems, yesterday’s technology of 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. VFD-controlled fan motors in this system will reduce energy costs.
HVAC equipment that can present opportunities for energy savings include centrifugal air handler fans, centrifugal exhaust fans, centrifugal chilled water pumps, centrifugal hot water pumps, cooling tower pumps, and cooling tower fans.
SAVING MONEYIt’s a general rule of thumb that a centrifugal pump or fan delivering 80 percent of its rated flow only requires 50 percent of the rated power. Consider the following example:
A 50-hp fan needs to supply air 10 hours per day for 250 days. The cost of running the fan at full speed would be:
50 hp x 0.746 kW/hp x 2,500 hrs x $0.08/kWhr = $7,460
Assume the fan does not have to run at full speed all the time, but instead runs on roughly the following schedule: 25 percent of time at 100 percent; 50 percent of time at 80 percent; and 25 percent of time at 60 percent. The cost of running the fan with a variable-speed drive would thus be:
50 hp x (1.0) 3 x 0.746 x 625 x $0.08 = $1,865
50 hp x (0.8) 3 x 0.746 x 1250 x $0.08 = $1,909.76
50 hp x (0.6) 3 x 0.746 x 625 x $0.08 = $402.84
Total = $4,177.60
POTENTIAL ANNUAL SAVINGS:
($7,460 - $4,177.60) = $3,282.40
In this example then, a variable air volume HVAC system could potentially save approximately $3,282 annually over a constant air volume system.
PAYBACK PERIOD, BENEFITSThe payback period on a VFD averages 18 to 24 months, but can be less than 12 months. The timeframe 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. With the life cycle of commercial building HVAC equipment typically registering more than 20 years, a one- or two-year payback period can generate a substantial return on investment.
VFDs also contribute to the overall building comfort level by optimizing and regulating air flow and temperature into the building occupant space. Rather than having a fan running either at full speed or off, a variable-speed drive can run at all speeds in between, allowing facilities managers and building occupants more control over adjusting the temperature. Schneider Electric’s Altivar® 61 ac drive takes occupancy 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. This helps the HVAC systems last longer, requiring less maintenance and causing fewer instances of unscheduled downtime.
CONSERVING ENERGYA variable air/water/refrigerant volume HVAC system controlled by VFDs can go a long way in helping a new or existing building achieve more effective energy management. Not only will HVAC systems run by VFDs save money, but they will 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 more green 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 conserves energy resources.