Managing power demand at Gardena

At Gardena Manufacturing near the ancient town of Ulm in southern Germany, work proceeds at a brisk pace, and in a high-pressure environment - quite literally. Each year, the site’s 93 injection-moulding machines turn 9400 metric tons of plastic into 500 million parts, such as handles for pruning shears and push lawn mowers. Work proceeds seven days per week, from Monday to Friday at full capacity in three shifts, while weekends are a bit slower with fewer machines and workers on duty.

It is a clear pattern, and power demand at the plant follows the same routine. On an average workday, Gardena Manufacturing uses an average of between 2 and 2.3 MW. On Saturdays and Sundays energy demand drops to one or two megawatts, depending on the workload.

This regularly repeated pattern is something the power company has adjusted to. It knows how demand will change over the course of the week and can plan its output accordingly. This pattern is reflected in its supply contract guidelines with Gardena Manufacturing. On the one hand, the company must pay for the electrical energy it uses. On the other hand, the power company guarantees that Gardena can always pull up to 2.68 MW of power.

But exceeding that amount on even a single occasion would be expensive for the plant. “The power company determines our average power use in fifteen-minute intervals,” explains Jürgen Röck, who shares responsibility for automation and load management at Gardena Manufacturing. “If we are over the contractually agreed maximum during even a single interval, that costs us a lot of money, because then the power company raises the guaranteed maximum value for the rest of the year, and of course we have to pay for that.”

A few years ago it happened. When Gardena’s machines were started up after Easter, power demand suddenly rose to 3.2 MW for a short period, because the machines required a particularly large amount of power when starting up in unison. Gardena wanted to prevent that from happening again. As a result, the company decided to use the latest load management technology. In the spring of 2010, engineers installed nine Siemens Sentron PAC 3200 monitoring devices at the plant’s transformers. The devices measure current, voltage, and power. Their measurements are fed into a Simatic S7-400 controller that produces a power demand forecast for the current 15-minute interval. The goal here is clear: average power demand should never exceed the limit of 2.68 megawatts.

Röck, an electrical technician, can look at the plant’s current status at any time. All he has to do is to click the Siemens Simatic WinCC Powerrate app on his desktop computer. “A green area shows me how much electrical energy we’ve already used,” he says. “An orange-coloured line shows the forecast for the remainder of the 15-minute interval. As a result, we see immediately whether we’re in danger of going over the maximum.”

But if the plant gets too close to its limit, the system responds automatically. The controller gradually scales back power use until things are back to the normal range. In 200 kW stages, it reduces the power of the systems that extract heat from the process water that cools off the moulds in the injection-moulding machines. Here, a short-term temperature increase is not a problem. Similarly, the drier for the plastic pellets can also respond flexibly to load management needs. Its power can be reduced in four stages by 28 kW each time.

The control system has proven to be remarkably effective. Since it was installed, power demand has never exceeded the stipulated maximum. “Since we already use a great deal of Siemens equipment, the installation of the load management system didn’t cost very much. For example, we were able to use the existing S7 communication system for data transfer among the Siemens controllers,” says Röck. “Thanks to its reduction in peak loads, the load management system will pay for itself within a year.”

And Gardena Manufacturing plans to further expand its energy monitoring. In the near future, for instance, Röck will be able to use WinCC to keep track of how the compressed air and heating systems are working. Although he won’t be able to influence power demand this way, the readings will provide him with valuable clues regarding possible leaks in pipes or pumps that could be draining energy from the plant.