My first exposure to an “industrial” uninterruptible power supply (UPS) was in 1993, but it wasn’t as elegant of a solution as you are probably visualizing.

While attending community college, our programmable logic controller (PLC) class took a field trip to a packaging plant. When looking in the controller cabinet, I noticed that the PLC’s power leads originated from a DeWalt 24 V cordless drill battery pack sitting in its charger. If power was lost, the PLC continued to run from the cordless drill battery for long enough to save its data and shut down, as opposed to just power off abruptly and lose critical data.

That was not the most elegant solution, to be sure, but for that engineer and application, it filled the bill. Today, we have more sophisticated options, but, as this example from more than 30 years ago showed, when power goes away, a PLC doesn’t necessarily need to keep running for hours. Sometimes it just needs enough power to come to a controlled shutdown.

Battery-based UPS systems have been successful because they can store substantial amounts of energy and provide backup power for minutes or even hours. However, many factory power interruptions are much shorter. A machine may experience voltage dips or brief interruptions that require only seconds of backup power. Repeatedly cycling a battery for these events can eventually create a maintenance concern.

This highlights an important distinction between energy storage and power delivery. Batteries are excellent at storing energy for extended periods, but not every automation application requires extended backup.

For a graceful controller shutdown, there may only need to be a couple of seconds of power required. This is where capacitors come in. In the not-so-distant past, a conventional capacitor did not exist that could do the job. I remember the ongoing joke in some of my engineering classes of sending a new student to get a 1 Farad (1F) capacitor. Inevitably, one of our professors would do a calculation in front of the class for how physically large a 1F capacitor would be—approximately the size of a large refrigerator. It certainly wasn’t something you would solder onto a circuit board. It wasn’t until the development of the supercapacitor that a short-duration capacitor-based UPS became practical for industrial controllers.

Supercapacitors approach energy storage differently. Instead of storing energy primarily through a chemical reaction like a battery, they store electrical energy electrostatically. This gives them characteristics useful for short-duration power backup.

One major advantage is cycle life. Batteries gradually wear as they undergo repeated charge and discharge cycles. Supercapacitors can tolerate dramatically more cycles, making them attractive for machines that experience frequent power disturbances. They can also charge and discharge very quickly, allowing a supercapacitor-based UPS to recover its stored energy rapidly after a short interruption.

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However, there is a tradeoff. Supercapacitors generally provide high power density rather than high energy density. They can deliver substantial power quickly but aren't necessarily the best choice when a load must operate for a long time. The question isn't whether supercapacitors can replace batteries everywhere. It's whether the application actually requires the characteristics of a battery.

Factory lines contain numerous loads where short-duration backup can be valuable. A PLC may only need enough energy to recognize a power loss and execute a controlled response. An industrial PC may need several seconds to save critical information and shut down properly. Remote I/O and industrial networking equipment can also benefit by staying powered through a brief disturbance.

This changes how an engineer might approach UPS sizing. Instead of asking only, “How many minutes of backup do I need?” it may be more useful to ask, “What does my automation system need to accomplish when power is lost?”

Supercapacitors aren't a universal replacement for batteries. If a machine must remain operational for several minutes while a standby generator starts, a battery-based UPS may be the better solution because the application requires significant energy storage. The same applies to critical processes where shutting down isn't an option.

The storage technology should therefore match the failure mode. If the primary concern is a momentary voltage sag or brief interruption, the ability to deliver repeated bursts of power may be more valuable than the ability to provide hours of backup.

Modern UPS systems can also provide information about their operating status, load, stored energy and fault conditions. When connected to the automation controller, the UPS can help the machine recognize a power failure and initiate a controlled shutdown rather than simply allowing the controller to lose power and stop.

The most interesting aspect of supercapacitor-based UPS technology may therefore be its ability to change how engineers think about backup power. A UPS doesn't always have to keep a machine running through an extended outage. Sometimes its most important job is simply to provide the energy and time needed for the automation system to respond correctly.