Improving HVAC Efficiency in Commercial Buildings with Variable Frequency Drives
August 25, 2017
Automation
HVAC systems come in various configurations that differ significantly from each other. For example, an air conditioning system based on packaged rooftop units is drastically different from a chiller plant. However, HVAC installations have a common denominator: they use electric motors that are subject to variable loads. Regardless of whether these motors are used to drive compressors, water pumps or fans, there is a great opportunity to save energy by optimizing part-load operation.
In general, running a motor at partial speed is more energy-efficient than running it intermittently at full speed.
- Intermittent operation only provides linear savings. For example, a cooling tower fan with a duty cycle of 80% consumes 20% less energy than an identical fan operating full-time.
- Reducing motor speed provides cubic savings.. On the other hand, slowing down a fan to 80% speed reduces energy consumption by nearly 50%. The average airflow is the same as that of a fan running at full power 80% of the time, but savings are boosted significantly thanks to speed control.
Speed control can be accomplished with variable-frequency drives (VFD), and they have a broad range of applications in HVAC systems due to the cyclic nature of loads. It is important to note that VFDs are also known as variable-speed drives (VSD), adjustable-frequency drives (AFD) or adjustable-speed drives (ASD).
Fractional horsepower motors in small-scale HVAC systems can also achieve significant savings with speed control. The main difference here is that electronically commutated motors (ECMs) are more cost-effective than VFDs in these applications.
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VFD Applications in HVAC Systems
In most cases, VFDs are deployed in HVAC systems to control the speed of blower fans, pumps or compressors.
The main purpose of a cooling tower is removing the heat from a water circuit, which may be used in turn for process cooling or for a chiller plant. Cooling towers use fans to establish an airflow, improving heat removal through convection. These are open discharge fans, where there is no ductwork creating resistance to airflow; hence, the savings opportunity through speed control is significant.
When a VFD is deployed for a cooling tower fan, speed is normally controlled based on water temperature. Rather than cycling the fan on and off, it can be driven at reduced speed so that the water returning to the chiller or process is kept at a constant temperature – as previously explained, reduced-speed operation is far more efficient than intermittent operation at full speed.
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