FULLY OPERATIONAL BLACK FOREST BENZ II HYDRO- POWER PLANT WITH NEW CROSSFLOW TURBINE
In 2018, Alois Benz GmbH in the Black Forest town of Lahr, a sawmill and timber-planing business under family ownership for almost 200 years, finally took the decision to fundamentally modernise ...
... the company’s own hydroelectric power generation infrastructure. Ultimately, extensive damage and defects had meant the existing infrastructure required far too much care and maintenance, forcing the management to implement a comprehensive programme of technical modernisation. Already at the end of 2017 the inlet was fitted with a fully automatic rake system, and in 2018 the entire machine room technology was modernised. As with the electrification of the plant in 1957, a trusted and reliable Ossberger crossflow turbine solution was chosen for the task. Ossberger, based in Weissenburg in Middle Franconia, has been in this business for a long time and was also able to provide a full range of electrical and structural steel infrastructure.
The hydro-energetic potential of the 55 km Schutter, a Danube tributary in the south-western German state of Baden-Württemberg, has been exploited to generate hydroelectric power in several places for many years. According to the 1925 Baden watercourse plan, at that time there were already 36 power plants along the Schutter, many of which were still using traditional paddlewheel technology. Today, located on a bend in the Schutter in Kuhbach, a part of the Black Forest town of Lahr, the Benz family’s timber processing operation has always placed great emphasis on harnessing the power of water. Johann Benz, a direct ancestor of today’s owners, acquired the sawmill in 1822. The first electricity power plant here was installed in 1957. In the course of taking on the water rights from the neighbouring landowner who had discontinued his mill operations, an Ossberger crossflow turbine had been installed to replace the two original mechanical transmission paddlewheels. The machinery was controlled by a hydraulic actuator and provided all the necessary power to run the sawmill for many years.
Good reasons for modernisation
For economic reasons, when the brothers Martin and Konrad Benz took over the business in the 1970s, they decided to run it as a planing operation. The decision to specialise proved a good one! Over the past few decades Alois Benz GmbH – with Konrad Benz at the helm – has established a solid customer base, serving demand for customised manufacturing and tailored profiling for the building and industrial sectors. Today, the processing and finishing of Douglas fir and spruce wood at the company is primarily executed by the automated machine park. Although in later years the hydroelectric output had only been sufficient to cover part of the company’s demand, Martin Benz identified several reasons in favour of a fundamental overhaul of the aging infrastructure, including the fact that the existing dam and machine room infrastructure – weir, intake channel, rake, machine room and water release channel – could continue to be used without significant adaptation. Furthermore, the plant operators had received confirmation from the Federal Grid Agency (Bundesnetzagentur) that – if a specified increase in the plant’s power output was achieved – a 20-year tariff discount would be granted. Additionally, Deutsche Bahn (DB) announced its willingness to fully subsidise the construction expenses, and the costs required to install a fish ladder in the dam. Martin Benz explained: “This offer allowed DB to meet its statutory obligation to implement ecologically compensatory measures wherever rail network expansion made them necessary – right across the country”, adding that it would have been very difficult for the plant operators to finance the required integration of a fish ladder without this help.
Ossberger delivers the complete package
The complete technical overhaul of the hydroelectric infrastructure commenced in 2017 with the installation of an articulated hydraulic-arm rake at the head of the intake channel. On average, the old rake cleaner had required the physical removal of debris every two hours. The current cleaner is triggered by water level sensors and operates around the clock to ensure free flow at all times. Benz points out that the mere installation of the fully automatic rake cleaner led to a marked improvement in performance. Having shown the new infrastructure could achieve the goals set, the Federal Grid Agency granted the tariff discount pledged at the start of 2018. The summer months then saw reconstruction of the machine room. The original turbine infrastructure was dismantled by the operators themselves, enabling the new turbine to be installed at the end of August. Once again, the managers of this hydropower generator chose a crossflow turbine solution produced by Ossberger, an engineering company with a long history in the field. It is ideally suited to accommodating the seasonal changes in flow volumes carried by the Schutter. As well as providing a crossflow turbine, customised to deal with a maximum throughflow of 1.9 m³/s and a gross head of 3.4 m, the Middle Franconian hydropower specialists also supplied the entire range of electrical infrastructure, switchgear equipment and steel construction components. Reconstruction of the machine room took just weeks to complete with the new machinery already producing electricity in September 2018.
The significant improvement of flow into the water intake channel facilitated by the automated rake cleaner, and by the electromechanical modernisation measures in the machine room, enhanced the constant maximum power output of the turbine quite considerably. The suction pipe on the old turbine had rusted to such a degree that it had no longer performed its original function for around 15 years. Consequently, power output had topped out at 36 kW. However, working with a maximum available head of feed water, the new Ossberger turbine is capable of delivering a stable maximum power output of 51 kW. The structural benefit of the crossflow turbine is most obvious when working below capacity as it guarantees effective production and a constant supply of electricity. This is made possible by a drum-shaped propeller wheel made of two separate cells. Each of the cells is driven independently by the feed water to ensure electricity production is maintained, even if the volume of water driving the turbine is very limited. As regards operational reliability, the gear system link-up between the turbine shaft and the asynchronous generator immensely enhanced performance compared to the previous set-up. Originally power was transmitted between the slow-running turbine and the rapidly spinning generator via two differently sized pulley wheels – and a belt that frequently jumped out of its groove due to leaks in the turbine. These problems are undoubtedly all in the past now. Since reconstruction was completed the 105 rpm turbine and a gear box up to accommodate 1008 rpm harmonise to guarantee uninterrupted operation in all conditions.
Infrastructure in mint condition ahead of A major corporate anniversary
Technically, the comprehensive rebuilding of the Benz II power plant was completed one and a half years ago. In autumn 2019, in order to comply with the highest ecological standards, construction work was commenced on the installation of fish ladders around the dam wall. The technical solution chosen was a vertical-slot fish pass incorporating concrete elements. Martin Benz was delighted with the all-round modernisation of the company power plant which is now capable of delivering for an average annual power consumption of 125,000 kWh. It is estimated the electricity generated is equivalent to the average annual consumption of 30 to 40 households, and easily enables the timber processing operation to achieve power autarchy; unused power being fed directly onto the public power grid. In terms of energy technology, the Benz family is now optimally equipped to celebrate the company’s first 200 years in business.