When comparing EC fans and variable speed drives, it is easy to assume that the solution with the highest upfront efficiency will work out the cheapest. However, this risks missing the bigger picture, says Carl Turbitt, HVAC Drives – UK Sales Manager, ABB.
It’s no secret that data centres consume a lot of energy. Aside from the actual computing, cooling systems need to be run around the clock to ensure optimal performance. With energy prices spiralling in recent years, ensuring optimal efficiency of equipment is paramount to try and reduce costs wherever possible.
However, focusing on efficiency alone is a false economy, as high efficiency can cost you in other ways. Take air handling units (AHUs) for instance. Powering an AHU typically uses one of two competing technologies: electronically commutated (EC) fans, and motor/drive arrangements. EC fans are self-contained configurations that integrate a motor, drive and fan into a single package. Conversely, a motor and drive are installed separately and then connected together.
Competing technologies
The difference in efficiency between the two solutions is very small, so the differentiating factor can often be price. EC fans are typically the cheaper option based on upfront cost, but this doesn’t tell you the whole story. While the CAPEX may be lower, the OPEX of an EC fan compared to a drive and motor can often be higher.
Another factor to consider is how the AHU is actually run. EC fans typically reach higher efficiencies at peak loads, while a separate motor and drive perform better at partial loads. In fact, the vast majority of fan applications are very rarely run at peak loads, if at all.
This is for a number of reasons. Because EC fans are integrated devices, they are not as effective at dissipating the heat they generate. Heat is the number one common cause of premature failures, resulting in the breakdown of the electronics within the drive component, as well as the insulation of the motor. In contrast, a separate drive and motor can be situated away from one another. Where an EC fan can fail after only a few years, the motor and drive can often last decades with only minimal maintenance required.
Whole life cost
Whilst it may seem convenient on the face of it to have one device rather than two, this again is a false economy. There are nine units in a typical EC fan array, generally oversized to allow for redundancy and reduced speed. When one unit fails, this not only affects the amount of airflow the unit can manage, but it can also be very difficult to fix. When two units fail, the whole array may need to be replaced.
EC fans are often customised to individual applications, and as such will not always be sized to traditional IEC dimensions. If the whole array needs replacing, then it will have to be replaced with an array of similar or the same dimensions, which may not be easy to source at short notice. Conversely, separate motors and drives are very often interchangeable, and in many cases can be mixed and matched between different manufacturers.
Having to replace a device more often will typically result in a higher lifetime cost, particularly as the difference in efficiency between EC fans and drive/motor configurations is so small. This is also bad for sustainability, as any old devices will need to be disposed of. In addition, whereas an EC fan will typically have a drive and motor for each fan in an array, a single drive can control multiple fan motors at the same time, reducing the amount of hardware required. It can also deliver far more power. EC fans are limited to 5.5 kW each, which means you need a lot of them to reach the required airflow. A single motor and drive can easily deliver up to 250 kW.
Connectivity
Connecting an EC fan to wider plant control and building management system (BMS) can also be more difficult, requiring additional protection devices for each motor within an EC fan array, as well as extra wiring. Modern standalone drives increasingly have in-built connectivity, and can be controlled and/or monitored from an external system. The latest drives also feature other benefits, such as in-built harmonic mitigation.
Finding the right solution
EC fans are a perfectly good solution in certain circumstances. For small, single cell applications which typically operate at peak loads, they can provide excellent efficiency at a relatively low cost. However, whilst efficiency is of course important, it should not come at the cost of reliability and versatility. Data centres require vast amounts of cooling, and avoiding downtime is critical. Whilst EC fans often have a lower upfront purchase cost, in the long run, a separate motor and drive can often work out as the more cost-effective solution.
