Direct current instead of alternating current: Benefits for industry

Dr. Jürgen Brandes does not tend to exaggerate. But when the CEO of Schaltbau GmbH talks about the use of direct current in factories, he does so with passionate conviction. In September 2023, the Munich-based manufacturer of electromechanical components such as connectors and contactors for applications with high safety requirements opened a factory in Velden, Lower Bavaria, which runs almost exclusively on direct current.

“This reduces our energy costs by about a third,” explains Jürgen Brandes. The direct current required for this - abbreviated as “DC”, from the English “direct current” - is produced by a photovoltaic system with a capacity of 1.6 megawatts peak (MWp) on the roofs of the production hall and office buildings.

Since the electricity from the system does not first have to be converted from alternating current from the public grid into direct current before it can drive the speed-controlled motors of the machines in the factory hall, Schaltbau saves the energy that is lost in conversion processes. “This increases the energy efficiency of our direct current factory by 15 percent,” reports Jürgen Brandes.

Circular economy in energy supply

But that is not all. “The great potential of direct current supply in production lies in the coupling of all electrical systems of the factory in an intelligent DC network,” explains Dr. Timm Kuhlmann, head of the Industrial Energy Systems department at the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA). Then consumers can exchange excess energy with each other.

In a factory powered by alternating current, this is lost. There, resistors convert braking energy, which arises from the movements of robot arms or the operation of lifting systems in high-bay warehouses, into heat and waste it. In a facility supplied with direct current, this energy is fed back into the factory's grid and supplies other consumers. Through this recuperation alone, a company can reduce its electricity demand by up to five percent.

The central direct voltage management, with which the operation is still connected to the public grid, only needs to draw and convert the electricity it requires to close gaps in the direct current network. However, it can also obtain this energy from storage systems integrated into the factory network. For example, Schaltbau has installed batteries with a storage capacity of one megawatt-hour (MWh) and thermal storage systems that can hold ten MWh of energy at its plant in Velden.

Direct current increases plant availability to up to 98 percent

“This makes it possible to improve the availability and quality of the local power supply and thus increase the reliability of production,” explains Timm Kuhlmann from Fraunhofer IPA. This is because the storage systems reliably supply energy even when the voltage in the public grid fluctuates - for example, because less electricity from renewable energies is fed in due to weather conditions. The load changes that usually occur then often interrupt production processes uncontrollably. This often reduces the quality of the workpieces produced at that moment to such an extent that they end up as rejects.

This does not happen in factories operated with direct current. There, plant availability is up to 98 percent, assures the German Electrical and Electronic Manufacturers' Association ZVEI. This increases the productivity of the entire company. Factories running on direct current require 70 percent less peak load. At the same time, in addition to the overall lower power demand of a factory operated with direct current, integrated storage makes DC systems attractive from the perspective of buyers. This is because they supply power when particularly energy-intensive processes, such as welding, require it at short notice.

“This allows the connection power that factories still need from the public grid despite direct current supply to be reduced by 60 to 70 percent,” explains Schaltbau CEO, Jürgen Brandes. This alone makes a direct current network pay for itself. This is because suppliers charge particularly high prices for the required peak load.

According to its own information, robot manufacturer KUKA was able to reduce the peak power required from the public grid thanks to direct current from 50 kilowatts (kW) to 20 kW. As part of the two research projects “DC-Industrie” and “DC-Industrie 2” funded by the Federal Ministry for Economic Affairs, the connection power of a welding cell with robots in automotive manufacturing could even be reduced from 450 kW to 50 kW.

Direct current networks save 40 percent copper

BMW also installed a direct current-operated test facility in its body shop in Dingolfing as part of the project. This made energy savings of up to 20 percent seem realistic for the car manufacturer. In addition, BMW saved 40 percent copper alongside voltage converters and other components necessary in an alternating current network.

Since DC systems operate at 650 volts and not 400 volts like alternating current networks, the conductor current in their cables is lower. Therefore, they function with smaller cross-sections without overheating. Also, unlike an alternating current system, DC systems require only three conductors instead of five.

“Additionally, up to 25 percent less insulating material is needed,” explain the 33 companies and five research institutes that participated in the “DC-Industrie 2” project in their final report.

Securing direct current networks is complex

In light of the massively increased prices for raw materials and cables since Russia's invasion of Ukraine and the disruptions in global procurement markets due to the corona pandemic, this is a significant argument for buyers. Just as the fact that they no longer need to procure many components required for alternating current operation when their operation is based on direct current.

However, in return, wherever the safety of the DC network is concerned, higher costs and more effort arise in the search for suitable providers and components. This is because, on the one hand, an arc is created when direct current is switched off. "On the other hand, operators in a direct current network must make a switch within milliseconds in the event of a short circuit, without having a zero crossing. So there is no point at which you can switch without current flowing," adds Schaltbau CEO Jürgen Brandes.

Few norms and standards for direct current networks in industry

Conventional switches, plugs, or rails cannot be used everywhere in a DC network. Although suppliers like Vahle, Lapp, Schaltbau, or Phoenix Contact have corresponding components in their catalogues, purchasers must be familiar with direct current and coordinate very precisely with specialist colleagues in the company during procurement.

The effort for them also increases because, although it is known that, for example, insulating materials age differently in a direct current environment than under the influence of alternating current, scientists are still researching how quickly and why exactly.

Therefore, there are not yet norms and standards for all technical questions related to direct current technology, which buyers, plant planners, and engineers could use as a guide. These norms are only just being developed.

With direct current, companies in Germany remain competitive

Nevertheless, it is clear: if companies convert manufacturing facilities to direct current, they benefit from higher plant availability and productivity while significantly reducing electricity bills.

This keeps them competitive in Germany, even if energy costs will never return to the level before the war in Ukraine, as economist Professor Monika Schnitzer assumes.

However, instead of making their operations more energy-efficient, many entrepreneurs are currently considering relocating parts of their production to the USA or Asia. By doing so, they forgo location advantages that they only have in Germany - such as a particularly high level of legal certainty, dual-trained specialists, or proximity to first-class research institutions.

This does not have to be the case, warns Schaltbau CEO Jürgen Brandes soberly: “We demonstrate with our factory in Velden that investments in Germany still pay off when digitalisation and high automation are combined with a progressive, decentralised energy concept based on direct current.”