Lowering the cost of keeping it cool

The use of VFDs in Heating, Ventilation and Air Conditioning Systems (HVAC) in Shopping Centres & Commercial buildings.

How can the energy efficiency of heating ventilation and air-conditioning systems be improved?

Hotter summers and longer winters mean that commercial heating ventilation and air-conditioning (HVAC) systems are seeing more run-time than ever before, and in commercial structures such as shopping centres and office buildings, this means greatly increased energy expenditure and associated costs.

Normally, most HVAC systems are designed to operate at near peak, switching on for short periods throughout the year – summer and winter. These peaks cause huge spikes in energy consumption, and with increasing demand on energy sources – due to longer, hotter spells in the summer – operational budgets can be seriously dented.

In order to lower costs and improve efficiency, the use of latest-generation variable frequency drives (VFDs) across HVAC systems is an easy prefit or retrofit option for most existing air-handling units, fans, pumps and compressors. In any HVAC system equipped with VFDs, the drives adjust motor speeds based on the system load requirements, and the building’s opening/closing times and operational schedules. The result is dramatic reductions in energy consumption across the board. Such drives can modulate motor speeds very smoothly within a broad range, with any reduction in motor speeds providing a significant reduction in power usage.

For instance, in the United States, HVAC and refrigeration applications consume 93% of motor-driven energy use in the commercial sector: therefore, reducing unnecessary energy consumption in these systems will have a major impact on energy efficiency. With the introduction of VFDs, energy savings for pumps and fans can be around 30–50% over conventional speed applications, and around 35% on average for compressors.

The impact of VFDs in HVAC systems

Installing VFDs into a new system, existing HVAC system, or components used in those systems, will increase the initial investment.

But the reduced future replace/repair costs from VFDs’ soft-start capability (giving a reduction of mechanical stress on the motor and increased motor reliability) – combined with improved energy savings – results in a much swifter payback period and an almost immediate return on investment. Other operational benefits include step-less regulation for motor speed, reduced motor power consumption and metering and monitoring of system performances, allowing for greater, more precise control – ever more vital in commercial buildings where indoor air quality is under increasing focus.

Source control, filtration and the use of ventilation in buildings, is, therefore, an area over which operations teams should have maximum control. Poor indoor air quality has been linked to ‘sick building syndrome’, reduced productivity and even impaired learning in schools.

There are several types of compressors associated with refrigeration within HVAC systems suitable for an upgrade through VFD application: screw, scroll, reciprocating and centrifugal.

In industry, air compressors are a common power source for pneumatic control systems; and refrigerant compressors for air-conditioned systems. Office and residential buildings tend towards air-handling units (AHU), or air-con ‘chillers’. In short, any operations teams looking to improve energy efficiency would be well advised to assess the entire infrastructure of a buildings’ HVAC systems.

Also, drives featuring more compact dimensions can be easier to install (more flexible to mount) and are less conspicuous, so save valuable space. These modern classes of drive combine energy savings with greater comfort for those occupying the building.

Another way a drives upgrade can impact the ‘bottom line’ is if they are introduced to equipment pre-installation, as part of a separate component of the overall HVAC system. For example, Ireland’s leading manufacturer of a wide range of standalone and integrated air handling units (AHUs) and heaters for commercial and industrial premises, Mark Eire BV, was searching for a compact, yet powerful, drive to match with their high throughput AHUs.

After much research they turned to Control Techniques’ (CT) Commander drive ranges – after discovering other drives with the required specifications could be up to 50% larger by volume, with panel footprints over 40% larger. The sheer size of these drives made them commercially prohibitive as they were simply too outsized to be accommodated.

Paraic Ó Conaola, purchasing manager at the Mark Eire BV plant, explains: “The Commander SK drives are so compact that they can be integrated into the air handling unit itself, eliminating the need for a separate cubicle.” 

The increase in the power-to-size ratio of the drives was achieved by rethinking the drives’ design – with much of the space savings and efficiencies being down to new technologies and composite materials used in the drive’s enclosure. The drives improved the energy efficiency of the Mark Eire AHUs, precisely matching fan speed to the actual air demand.

Using the ‘autotune’ feature, where the drive sets up the basic parameters itself, the setting up of the drives proved quick an easy (simply setting the minimum speed, usually 38Hz, and the maximum, 50Hz). The display shows the diagnostics. Final set-up was carried out on-site since air volumes are rarely exactly as quoted.

Drive technology has long been integral to optimising energy consumption: the new challenge is to present that technology in a condensed and unobtrusive form. Future engineering prowess will be as much about the importance of square footage to the shopkeeper as it is about the automation itself.