Published on March 14, 2019

Text | Hauptteil

Although the European Union decided as early as 2015 that the vapour lamps filled with toxic mercury should be phased out, the modernisation of lamps throughout Europe began rather hesitantly – due to the fact that existing luminaires are still operational and will not be replaced for cost reasons until the function failure finally occurs. Pfalzwerke Netz AG manages an area of approx. 6,000 square kilometres as a distribution grid operator. The core business includes the operation of street lighting on behalf of municipalities. In total, almost 450 local communities with around 100,000 streetlights, 70 percent of which are already equipped with modern LED light sources. This ranks Pfalzwerke among the precursors of modernization. Stefan Lang from the Technology & Innovation division at Pfalzwerke explains: "As a utility company, we constantly look for innovative technologies that can help our customers on their way to more sustainability. This particularly includes energy-efficient, sustainable street lighting". Researchers at Light Technology Institute (LTI) of KIT are working on innovative lighting solutions.

Modern street lighting with LEDs

Light-emitting diodes – or LEDs for short – are standard in lighting technology in private households, in the automotive industry and in industry today. What speaks for the technology: LEDs are considered to be particularly flexible in terms of luminosity, light color and spread. Compared to outdated light sources, they are also durable and energy saving. Not least for this reason, modernization activities in municipalities are already strongly linked to refitting to LED technology. "Street lights require a higher luminous power than lights in private areas, for example, in order to light larger areas. High-power LEDs have so far been used for such high-lumen applications. If these are replaced by more efficient, cheaper but weaker LEDs, a large number of mid-power LEDs is required. The operating voltage of the light is proportional to the number of LEDs. As a result, high operating voltages are needed for high-lumen applications," says Michael Heidinger of LTI. The legal framework – safety extra-low voltage, SELV – determines 120 volts as the maximum touch voltage that is not life threatening. With typical LEDs connected in series, the maximum is already reached with 40 LEDs. "So if you want to reach an even higher light output, the LEDs have to be connected in parallel. However, this proves to be difficult, as the failure of a single diode would previously lead to the malfunction of the entire system. A reliable operation of lights is therefore not possible", Heidinger continues.

More than just a nice shine

During his work at LTI, the scientist Heidinger developed a clever circuit topology that ensures even current distribution and enables LEDs to be switched in parallel: "The circuit allows common high-power LEDs to be replaced with mid-power LEDs. LEDs can be switched in parallel in any number of rows on the circuit board, with the LEDs arranged in series within each array". The specific property: A current controller in each parallel circuit ensures that each string switched in parallel is regulated to an average value. The circuit determines its own average value and even allows dimming. This compensation mechanism has several positive effects: Since all LEDs in the system are operated with a homogeneous current, uneven aging is avoided as a result. At the same time, the reliability of the entire module is ensured, as the current division circuit compensates for the breakdown of individual LEDs and the others continue to function.

In addition to the technical advantages, the new technology also results in increased visual comfort and improved lighting properties for LED lights. In the mid-power LED power class, more lumens per watt can be achieved than with high-power LEDs. They illuminate brighter overall without consumption of more power." From a distance, the many small light-emitting diodes seem like a radiating panel, so that the viewer is less dazzled. Another advantage is that the wide-area lighting effect is retained, even if individual LEDs on the circuit board fail," Heidinger describes.

From switching concept to lamp

An important step in the development of the LED circuit of Heidinger was taken by the funded project "Optimiertes Gesamtsystem LED-Leuchte" with GRATZ Luminance GmbH. Managing Director Klaus Müller reports: "The circuit from the KIT is the decisive key to offering high-lumen, long-life lighting solutions with mid-power LEDs. Based on this technology, we have developed an LED lighting module that achieves a significant increase in efficiency and operating service lifetime without having to compromise on illumination". The lighting technology company is pioneering the industrialization of the innovative LED parallel circuit for the first prototype series.

The circuit topology also has a positive influence on manufacturing costs. Mid-power LEDs generally operate at comparatively low voltages and thus have a lower cooling demand, which is already covered by a small cooling element. Compared to high-power LEDs, mid-power LEDs are considerably cheaper relative to their light output. This means that the new light module can be offered at a similar price to conventional modules, even though more LEDs are installed. But much more decisive are service life, energy efficiency and reliability, which also reduce the cost-intensive installation and maintenance work. "Municipalities that change to LED want to save money first. The new modules have a savings potential of up to 20 percent of electricity consumption compared with conventional LEDs. However, new technologies must also be sustainable if they are to penetrate the market. And here we generate even higher added value with our high-quality LED lights and thus differentiate ourselves strongly from consumer lights," emphasizes the entrepreneur Müller. "A technical innovation can only succeed if the direct replacement can be achieved with minimal effort and without compromising quality. Therefore, it was important for us to provide the new modules for our lights as drop-in replacements. This means: old LED module out, new mid-power LED module in", the lighting expert points out. For industrial customers and above all for technicians, the working process should change as little as possible in order to increase acceptance.

With the proven advantages of mid-power LED lights, the partners were able to convince Pfalzwerke to become their third strong partner. "Cooperation with a network operator is important in order to make the technology widely available," stresses Heidinger. Even if the mere replacement of illuminants is a first step in the right direction for many municipalities, the advantages of LEDs can only be fully exploited with the new mid-power LED technology.