Heating, ventilation and cooling (HVAC) systems are among the largest consumers of energy in a facility. According to the U.S. Department of Energy, HVAC systems consume between 40 and 60 percent of a facility’s power. With the pursuit of energy efficiency targets by all sectors of the building industry, HVAC systems are an obvious starting point to reduce consumption.
Designed to keep buildings ventilated and comfortable, HVAC systems are often sized to operate at peak heating and cooling load conditions, even though in typical building applications, those conditions occur only 1 to 5 percent of the time. As a result, pump and fan motors associated with HVAC systems use more energy than necessary, even during off-peak operating hours.
Proper selection of pumps, motors and controls is essential to ensure HVAC systems operate efficiently and reliably. Additionally, energy management tools like variable frequency drives (VFDs) can be applied to HVAC systems to realize substantial energy savings.
Like the throttle on a car, a VFD adjusts the speed of the HVAC fan or pump motor to meet the ever-changing demands put on the system. Just as conditions when driving require different speeds, a fluctuating building environment requires an adjustable flow of energy. By leveraging VFDs, an HVAC system runs only as much as it needs to, downshifting when desired temperatures are reached.
VFDs can be installed in nearly any HVAC system in residential, commercial and institutional buildings. VFDs are advantageous because they ensure that HVAC systems perform properly, reliably and efficiently during conditions that occur off-peak load hours, which is 95 to 99 percent of the time. By operating at a reduced capacity, otherwise known as part-load demand, VFDs can match air flow to actual heating and cooling demands. A VFD can reduce the motor speed when full flow is not required, thereby reducing the power and electrical energy used.
By matching system capacity to the actual load, major savings in system motor energy use are achieved. Energy consumption in centrifugal fan and pump applications follows Affinity Laws, which means that flow is proportional to speed, pressure is proportional to the square of speed, and horsepower is a cubical function of speed. Pumps that operate at higher efficiencies will save horsepower and use less electricity, reducing operating costs. For example, if an application only needs 80 percent flow, the fan or pump will run at 80 percent of rated speed and only require 50 percent of rated power. In other words, reducing speed by 20 percent requires only 50 percent of the power.
Benefits beyond energy savings
Reducing energy consumption is not the only benefit of installing VFDs on HVAC systems. VFDs are also effective tools for cutting operating costs and improving overall system performance.
VFDs can improve occupant comfort by optimizing and regulating air flow and temperature within different areas or zones of a building. Unlike a fan that runs either at full speed or is off, a VFD can run at any speed, giving facilities managers and building occupants more control over adjusting temperatures.
The installation of VFDs in the new U.S. Bank Stadium in Minneapolis, home of the National Football League’s Minnesota Vikings, ensures fan comfort – a necessity in a region known for extreme climate. With seating for up to 70,000, managing a wide variety of ambient conditions is essential. A VFD-based booster pump system provides precise climate control and reduces the stadium’s energy consumption.
Another benefit of VFDs is reduced long-term wear and tear on equipment. A motor without a VFD operates at a constant speed. On typical startup, a constant-speed motor is subject to high torque and electrical surges that can reach up to 10 times the full current load. When full power is not needed, the motor can cycle on and off frequently in an attempt to match the load. This creates unnecessary wear and reduces equipment life. VFDs, on the other hand, have a soft-start capability that gradually ramps up a motor’s operating speed. The soft-start greatly reduces the stress on the motor and related components so HVAC systems last longer. This decreases maintenance costs and prolongs equipment life.
Advantages over control valves
Most conventional building HVAC applications are designed to operate fans and pumps at constant speeds. Building loads, however, are anything but constant. In a conventional system, a mechanical throttling device like a control valve can be used to reduce water or air flow in the system. The drive motor, however, continues to operate at full speed, using nearly the same amount of energy regardless of the heating or cooling load on the system. While mechanical throttling devices can provide good control, they are not efficient and they waste mechanical and electrical energy.
As a result, throttled systems create mechanical stresses such as excessive pressure and temperature on the pump system, which can lead to premature seal or bearing failures. More importantly, pump systems that use control valves consume substantial amounts of energy.
VFDs eliminate the need for expensive and energy-wasting throttling mechanisms like control valves. Unlike control valves, which simply restrict the amount of air or fluid being delivered, VFDs can control motor speed and stabilize pump motor performance.
The flow control provided by VFDs ultimately improves the efficiency of the motor and the application, resulting in higher efficiency and significant cost savings. Ultimately, those cost and energy savings easily offset any capital costs associated with installing a VFD on a new HVAC system or retrofitting an existing system.
Vertical inline pumps were selected for the retrofit from an electric heating system to hydronic distribution inside the Twin Parks housing development in the Bronx, New York. The pumps, which incorporate a variable speed drive and combine sensorless technology with the energy savings of variable flow, react to the system demands by varying speed, significantly reducing electrical costs compared to a full-speed operation.
More features than ever before
VFDs today are more efficient than ever. The next generation currently on the market can reduce energy consumption in a facility by as much as 70 percent.
Now loaded with expanded capabilities, including modular design and advanced communication systems, VFDs provide users streamlined ways to achieve higher energy efficiencies.
VFDs can easily handle multipump configurations like office buildings and hotels. Designed with multimaster functionality, these drives can allow individual pumps to take control if one or more units or sensors are inactive. This ensures reliable operation and helps end users obtain cost and energy savings while achieving optimal performance.
Equipped with an intelligent control system that accurately adapts to demands, the next generation of VFDs only uses power as needed, resulting in lower operational costs. These VFDs also come standard with communications protocols like BACnet and Modbus for seamless Building Management System integration, which allows for better control and monitoring of HVAC systems. Optional Wi-Fi for remote connection to the unit via any mobile device, laptop or PC is also available for easy system upkeep.
Another innovative feature of modern VFDs is modular unit design. Pumps can be configured in virtually any arrangement depending on need, whether it is a single pump or a combination of pumps, which saves space, reduces installation time and facilitates easy maintenance.
Additional cost savings
Not only can VFD installation reduce energy expenses, but in many cases it may help businesses and organizations qualify for energy savings programs like utility rebates and tax credits. Rebate levels vary widely within the U.S.; However, the American Council for an Energy-Efficient Economy estimates that rebates usually cover from 15 to 35 percent of installed cost. By combining these types of incentives, returns on investment for VFD installations can be realized in as little as six months.
Kyle Schoenheit is a global product manager, Americas, for Xylem‘s Applied Water Systems business unit. To determine potential energy use and cost savings associated with installing a VFD, access Xylem’s downloadable energy savings calculator at hydrovar.com.