Intelligent motor control to maximise HVAC efficiency

Because HVAC systems comprise a large amount of a building’s operating costs, it makes sense to ensure these systems are running as efficiently as possible. The key to identifying the energy-savings opportunities in HVAC systems is understanding the operating electrical profile of the system as it pertains to heating and cooling.

Most HVAC systems, particularly those over five years old, employ constant-volume air-handling units (AHU) positioned in different zones that use simple on/off controls. The building automation system (BAS) monitors temperatures in these zones and turns fans on or off as needed.

Constant-volume HVAC systems operate with the fans supplying air to conditioned spaces at flow rates designed to accommodate space heating and cooling requirements at 100% design loads. This doesn’t take into consideration the fact that most buildings typically operate at full load less than 5% of the time.

Since the supply of air is constant, the fan-energy use remains constant, regardless of the load on the HVAC system. Using dampers to mechanically adjust the airflow output into the appropriate rooms doesn’t save energy, since the fan is still running at full power, regardless of damper position. With fans often operating 18+ hours per day in many applications, they are a major component in a facility’s electrical energy consumption.

Constant to variable

While most buildings use a variety of simple controls to manage HVAC systems, many have yet to take advantage of new control upgrade options capable of wringing more savings out of operations. Getting the most out of controls requires systems that are capable of adjustment, such as variable-air-volume (VAV) systems, designed to regulate the volume of air delivered to individual zones in proportion to its current cooling or heating load.

Significant improvements have been made in energy-saving technologies that enable conversion of constant-volume HVAC systems to VAV control without changes to the existing mechanicals or BAS. It is primarily an electrical upgrade. These retrofit packages offer two core advantages: space temperatures can be controlled within acceptable limits over widely varying internal and external heat gains caused by weather conditions or sun exposure and, most importantly, energy consumption is reduced.

VAV systems regulate the airflow to conditioned spaces based on their requirements. As heating and cooling loads drop off, the fan system decreases the quantity of air being supplied. VAV systems can significantly reduce fan-energy requirements because most buildings run at 40-70% of full-load capacity and the amount of energy required by the fan is proportional to airflow.

As a rule, energy consumption in centrifugal loads, such as HVAC fans, will vary by the cube of the fan-motor’s speed. So, if the speed of a fan’s motor is decreased by 20%, the energy used to power it reduces by approximately 50%. In applications involving centrifugal loads, variable frequency drives can deliver significant energy savings when the HVAC system requires less than 100% usage.

Energy-saving packages

Converting a constant-flow HVAC system to VAV used to be complex and costly. That’s no longer the case, because new standalone upgrade packages are designed to deliver energy savings at a fraction of the cost of a traditional hardware retrofit.

Designed for easy installation as a self-contained unit, the core energy-saving package includes temperature and CO₂ sensors, microcontroller, high-performance variable speed drive (VSD) and touch-screen operator interface. The VSD adjusts fan speeds in response to heating or cooling needs and limits peak electrical demand. For many operations, the upgrade can pay for itself in energy savings in relatively short time frames, often under a year.

On constant-volume AHUs, fan speed is controlled through contactors, resulting in the fan being either on or off. The BAS provides the contact circuit that energises a contactor and starts the fan. The fan typically remains on most of the time and consumes 100% of the energy needed to rotate the fan at full speed, even if only a small change in air temperature is needed.

With VAV upgrades, temperature and CO₂ sensors are wired directly into the controller. Based on sensor feedback, the controller tells the VSD to speed up, slow down or turn a fan on or off. This eliminates the need to make changes to the existing BAS, simplifying the conversion process and minimising costs.

The energy-saving solution distributes heating and cooling BTUs in the same proportion as the original design, only using less fan energy. Through sensors tied to the controller, the system will adjust the speed of the supply fan, the return fan or both. This allows the HVAC system to minimise fan speed and energy use to reflect the actual amount of airflow required for the desired temperature change.

Simply put, when a minimal amount of heat or air conditioning is needed, the system minimises the speed of the fan and energy use. Conversely, when a large amount of heat or air conditioning is needed, the system increases fan speed to accommodate the need for heating or cooling transfer.

Figure 1: Variable speed power requirements.

Traditional HVAC design provides sufficient fresh air to deal with worst-case scenarios, assuming full occupancy at all times. By using sensors to measure CO₂ in the return air, the energy-saving package adjusts outside airflow to accommodate the estimated number of occupants. Reducing outside air intake means less cooling, heating, dehumidification and exhaust-fan speed, for further energy savings.

Reaping rewards

Some of the latest upgrade packages provide quantitative data that make it easier to calculate energy savings and determine potential payback. For example, Rockwell Automation’s FanMaster Energy Saving Package offers a web-based calculator that shows users their application-specific energy savings before they actually purchase it.

A package was installed in a food manufacturer’s facility, where potential savings of $311.45 were calculated over a 382-hour period (16 days), including an estimated $73.46 in savings associated with CO₂-demand ventilation. During the trial, fan-speed reductions of 35, 45, 55 and 100% were verified and extrapolated to an annual fan-speed reduction of 58.5%, equating to a potential yearly savings of $7000 based on $0.05/kWh. Notably, these savings could be achieved without incurring the expense of modifying the facility’s existing controls or mechanical system.

The payback period of a VAV upgrade can be 12 months or less, depending on the type and size of the system, and how much time the motor is operating at full speed versus how much flow is required to heat or cool the building. The life cycle of HVAC equipment in commercial buildings is typically 15-20 years, so a one- or two-year payback period can generate substantial savings over time.

Converting to a VAV system reduces equipment maintenance costs and downtime, while optimising and regulating airflow and temperature in occupant spaces. These benefits extend beyond occupant comfort levels and encompass other building spaces that may need regulated temperature control, such as warehouses and cleanrooms. Unlike a fan that runs at either full speed or off, VSDs can run at all speeds in between, allowing more flexible temperature control.

VSDs also help reduce long-term equipment wear. The drives provide a soft start instead of slamming motors on at full speed - so HVAC systems last longer, with less maintenance and fewer instances of unscheduled downtime.

As economic demands put increased pressure on organisations to find ways to reduce costs, effective energy management becomes a strategic business necessity. Advancements in energy-saving solutions that comprise intelligent motor control technologies help reduce energy costs and protect against the uncertainties of the power market.