In October 2019, the comprehensively modernised Dietikon run-of-the-river hydroelectric plant with its new residual water utility system was tested for the first time under real conditions –

around 18 months after building work commenced. The official opening ceremony for the plant took place on the 6th March this year, with production output having been boosted by approximately 18%. In addition to the modernisation of the electromechanical infrastructure, the project also involved the implementation of several ecological compensatory measures. To ensure fish passage in compliance with ecological standards, two fish ladders were installed – one at the main power plant and one for the residual water basin – both of whose intake infrastructures were fitted with fish-friendly horizontal rakes. All steel weir and channel components were provided by the Bavarian industry specialist Muhr. The horizontal rake for the main power plant is around 33m long and 6.5m high, making it one of the largest of its kind in Central Europe.

Dietikon is situated approximately 12 km west-north-west of the canton’s capital city, Zurich, where there is a long tradition of hydroelectric power usage in the industrial sector. The Grien Insel in the heart of the city first became an island when a diversion channel was excavated parallel to the Limmat between 1857 and 1860. Its hydro-energetic potential was first exploited from 1860 onwards, being used to drive textile factory mechanical transmission infrastructure. The first electric turbines were integrated into the power plant in 1888; and in 1894 the authorities authorised the installation of an additional three turbines. 14 years later, ownership of these power plants passed into the hands of the EKZ (Canton of Zurich Electricity Works). Ultimately, in the early 1930s extreme infrastructural deterioration led to a comprehensive programme of renewal and expansion, including the widening of the banks and underwater channelling, as well as the building of a new dam and machine room – which subsequently accommodated two vertical-shaft Kaplan turbines. This constellation enabled the plant to produce electricity reliably for over eight decades until the implementation of the recent renovation measures.

PROTRACTED LICENSING PROCEDURES
In 1999, just ahead of the new millennium, EKZ began preparations to have the license for the long-serving hydropower station renewed. Tensions between economic and ecological demands are very familiar to hydroelectric power plant operators, and in the case of the Dietikon power station they led to extremely protracted proceedings – before the Canton of Zurich finally authorised construction plans in 2017. Alfredo Scherngell, Chief Hydropower Officer and Central Project Coordinator at EKZ, recounted that consideration of all the licensing issues had been a protracted process due to the multiplicity of parties’ interests involved – including authorities, clubs, associations and local residents. Moreover, following the introduction of the subsidised green electricity tariff in 2009, the EKZ added the construction of a residual water power station at the Wehrsporn part of Grien Island to the overall plans for modernisation, leading to further delays. The relicensing process required an increase in the permitted volume of residual water which could now be used for electricity generation – up to 15m³/s. The average annual power output of the Dietikon station was raised by 18%. Ecologically, the licensing proceedings were contingent upon the implementation of a whole series of compensatory measures. As well as the obligatory up-and-downstream fish ladders, the operators were also required to conduct comprehensive rewilding in riverside forest areas protected by environmental legislation – as is the case for the northern half of the Grien Island.

ALL TURBINES FROM ANDRITZ HYDRO
As before, the main station machinery consists of two identically constructed Kaplan shaft turbines with directly-coupled synchronous generators, and as with the residual water power station, ANDRITZ Hydro also bid successfully to supply the entire range of electromechanical infrastructure. Electricity is used by exploiting the potential of a per-turbine gross head of 4.5m, as well as a maximum flow volume of 47.5m³/s for each turbine. Working at maximum water availability, each of the double-regulated turbines can achieve a stable maximum power output of 1.75MW. In order to keep noise emissions to a minimum the generators, each of which has a nominal output of 1900kVA, were fitted with a water-cooling system. The residual water power station at the southern end of Grien-Insel ensures drops in output at the Dietikion plant can be more than compensated, despite the release of larger volumes of residual water. The new infrastructure allows 10m³/s to be processed by the turbines between October and March, and 15m³/s between April and September, raising overall annual production significantly from the original 17GWh up to 20GWh. The main reason for this jump in productivity is the horizontal-shaft Kaplan bulb turbine. Working with a gross head of 3.5m at maximum water flow volume, the machinery has been set up to deal with a capacity flow volume of 25m³/s for a stable maximum power output of 770kW. Of course, during the structural and electromechanical renewal phase at the Dietikon hydropower station there was a comprehensive update of all the electrical, electronic and control infrastructure. The design and installation of the all-round automation packages for the main and subsidiary plants were conducted by the Swiss specialists in the sector, Rittmeyer of Baar.

LARGE-SCALE HORIZONTAL TRASH RACK
In order to maximise environmental protection of river life over such a broad area around the plant, horizontal, fish-friendly trash racks were installed at the main dam station and the residual water unit. All steel engineering components, including trash racks and trash rack cleaning machines, gates, stop logs, hydraulic power units and mitre gates were provided, delivered and installed by Muhr – South-German hydropower steel engineering experts from the Bavarian town of Brannenburg. “At a length of around 33m and a height of 6.5m, the horizontal trash rack at the main power plant is one of the largest of its kind in Central Europe. On top of its sheer size, what really made this project so challenging was the degree of manufacturing precision required to ensure fish-friendly rack bar distancing of just 30mm – and 20mm interior spaces between the bars. The flow volume lost due to the fine-mesh trash rack is compensated by the rack size, by the sophisticated stream­lining of the shape and positioning of the rack bars, and that of the twelve supports. This solution ensures the interests of fish ecology and productive economics are both served harmoniously”, explained Muhr’s engineering editor Florian Kufner, adding that the trash rack – serving approximately 211m² around the intake – was kept free of debris by a HYDRONIC H-8000 V series trash rack cleaning machine (TRCM). The TRCM is equipped with a low-maintenance Omega chain drive with around 30kN of propulsion force to ensure reliable cleaning performance in all conditions. This infrastructure was also augmented with a loading crane and timber grab to remove floating bulky or long timber debris, flotsam and jetsam, from the intake section. Similarly, they facilitate the installation and removal of stop logs for the bypass intake. Kufner stated that the EKZ’s demands on the mitre gate regulating the flow of water to the station’s bypass channel were particularly strict. The floating debris removed by the TRCM is transferred to the mitre gate and ultimately channelled off via the bypass. As drive system powering the mitre gate only one overhead hydraulic cylinder had to be used. Rounding off, Mr. Kufner stated: “At an overall height of 6,6m and a maximum water level difference of 6m this demands an extremely rigid and sturdy construction. In addition, we had to equip the mitre gate with to integrated regulating valves to regulate the bypass feeder and enable fish swimming downstream at various water levels in a species-appropriate manner.” At a length of 26m and a height of 3.2m, the horizontal trash rack on the residual water channel plant is roughly half the size of its counterpart servicing the main dam set-up. It was fitted with the same flow-optimised, fish-friendly bars. A HYDRONIC H-5000 V series TRCM with a comb bar drive and 20kN of thrust is used to clean the trash rack. The overall scope of delivery for Muhr comprised three hydraulic power units, each of which installed in the TRCMs to power the drives and the lifting cylinders of the rakes. At the main dam TRCM, the hydraulic power unit also guarantees sufficient oil pressure for the hydraulic loading crane. Another hydraulic power unit in the main power plant drives the mitre gate cylinder and the flow control flap gate. Kufner summarises that the key challenge this project had posed for Muhr had been in the immense scope of delivery required – and the large range of individual items this entailed. This set tough challenges for the entire project organisation team – in the project planning phase, in construction management, in purchasing, and all the way through to manufacturing, assembly, installation and documentation.

OFFICIAL OPENING CEREMONY THIS MARCH
The official opening ceremony was held in the attendance of several illustrious guests on 6th March 2020. “When the project commenced, just before the beginning of the new millennium, we couldn’t have known that the opening of a power station run on a renewable energy source – water power – would have become so acutely relevant to today’s debate on the issue of climate protection”, explained EKZ CEO Urs Rengel. “Back then, who could have predicted the closure of nuclear power plants – or that we would be backing the use of renewable energies like hydroelectrics?” Martin Neukom, senior civil servant and EKZ advisory board member, stresses the significance of hydropower because, in contrast to solar power, water is available throughout the entire year. From an energy technology point of view, the investment of approximately 39 million Swiss Francs will definitely pay off; particularly since power output is now almost a fifth greater than before, and because now the EKZ can self-dependently produce an energy volume equating to the annual power consumption of around 4500 average households.