Around 12 years since the first designs for the Arvier power plant were put on the table, the eco-power plant in the Aosta Valley finally began operating in June of last year.

The modern high-pressure plant was equipped with two 4-nozzle Pelton turbines from the South Tyrolean hydropower specialist Troyer AG which enable a power output of 9.5 MW. The new Arvier power plant joins an impressive array of small-scale hydropower plants that makes the Aosta Valley a real hydropower hotspot in Italy.

When people think of the Aosta Valley, many of them will associate it with Fontina, the famous cheese, or the excellent wines and skiing. But the fact that the autonomous region of Aosta Valley is very much Italy’s hydropower region is less well known. Throughout the year, the people of this region obtain 99 percent of their power from hydropower plants – this is unique in Italy. In total, the hydropower plants in the Aosta Valley generate almost 3.5 TWh with an installed power output of 950 MW. By contrast, the regional consumption is around 900 GWh, which leaves a positive power balance of around 2,550 GWh per annum. In the winter months, when not much water is avaible, power is produced only from external sources. Italy’s smallest region in terms of both size and population is the hotspot for Italy’s hydropower sector, which is also not yet developed to its full capacity here.  

18 million for new power plant
As one of the flagship projects of recent times, the Arvier power plant was completed in the summer of last year. The plant, which was developed and designed by the Aosta engineer Dr. Ing. Alessandro Mosso, has a previous history stretching back to 2007. “We started to formulate the first plans in 2007, but the planning process and the approval procedure lasted around 10 years, so the construction work did not begin until the end of March 2017,” recalls the designer. The Arvier power plant is a run-of-river power plant in the Verney area in the community of Arvier which utilises the Dora di Valgrisenche – a mountain torrent which originates from the Gliairettaz glacier and feeds into the Dora Baltea. The project is backed by Eaux Valdotaines Srl., a very experienced hydropower company which, with the Arvier power plant, now operates five hydropower plants in the Aosta Valley region. As the Italian business newspaper “Il Sole 24 Ore” reported, Eaux Valdotaines is said to have invested around 18 million euros in the new power plant. “With our other plants, we have previously produced around 18 GWh per year which are fed into the public grid. With the new Arvier power plant, our total electricity production will rise to 38 GWh per year,” said Federico Oriani, President and one of the three founding partners of Eaux Valdotaines Srl, to the media. The electricity will be paid in accordance with the statutory regulations for electricity from renewable sources.

Through the mountain with TBM
Constructing the power plant project would very much present a challenge for all the parties involved. In particular the penstock from the catchment to the powerhouse required special technical solutions. One of these involved constructing a 1.6 km tunnel with a diameter of 3.6 m2. It was created using a tunnel boring machine over a period of around six and a half months. According to the engineer Dr. Alessandro Mosso, this was the most complicated part of the whole construction project. The majority of the around 1.4 km DN1100 penstock, which was manufactured entirely from steel, thus runs in a tunnel.
Another special feature is the sand trap which is installed at the water catchment and was designed with an HSR desander system. The system has acquired an excellent reputation since it was introduced in 2001. This is primarily because it can withstand the extreme wear demands of mountain torrent catchments and ideally satisfies the requirements of today’s operators when it comes to efficient management and minimised maintenance costs.

“Twin heart” with 9.5 MW of power
The fact that hydropower technology from South Tyrol is very popular among the operators of the Aosta Valley was also demonstrated by the choice of machines. Eaux Valdotaines relied on the expertise and the quality of the Sterzing-based hydropower all-rounder Troyer AG, which supplied all the electromechanical equipment for the power plant. Specifically, the “twin heart” of the plant consists of two identical 4-nozzle Pelton turbines with a vertical axis. Each of the two machines is optimally designed to cater for the extraction water quantity of 1,700 l/s and the head of 317 m. Both turbines rotate at 750 rpm and each drive a directly coupled WKV synchronous generator with a nominal output of 6,000 kVA which is cooled via a closed water circuit. Together the pair of machines achieve a bottleneck output of 9.5 MW. The turbine-propelled water is conducted into the river again underground after the electricity has been generated. The produced electricity is transported via an underground 132 kV cable over a distance of 4.8 km to Terna’s transformer station and fed into the grid here.

Proof of water-to-wire expertise
After the first enquiries regarding the electromechanical equipment were made to the turbine construction company in Sterzing back in 2014, the corresponding contract was ultimately signed in February 2017. For Troyer AG, the project was set to provide further proof of its water-to-wire expertise. In addition to the two turbines and the generators, the company also supplied two DN600 P40 ball valves as well as all the hydraulic technology. The package delivered also comprised control and automation, as well as the medium-voltage switchgear and the transformer together with the HS installation. “For the catchment and surge chamber, we also supplied the probes and the sensors, as well as the control for the hydraulic steel construction and the sand trap,” adds the project manager from Troyer AG, Stefan Macrina.

Machine delivery was a challenge
One of the biggest challenges in implementing the project was the transport of the machine to the powerhouse – in particular the final kilometre. To provide access to the construction site, a dedicated, extremely steep road was built down to it, but it was not always drivable. It could not be utilised in the winter months in particular. The absolute acid test for this road was therefore the delivery of the transformer and the generators, which each weighed around 30 tonnes. They had to be brought down using two tractor units. “The two generators were subsequently rotated into an upright position using two mobile cranes so that they could then be transported inside the powerhouse on a carriage which was specially manufactured for this purpose. In the powerhouse itself, it was then necessary to overcome a final height difference with the machines, over 4.50 metres at least,” recalls Stefan Macrina. The control cabinets and the medium-voltage cells were set up on a steel structure located over several levels. The complicated work of laying cables to the machines and control cabinets was accomplished by the Troyer team with the usual neat finish using cable ducts.

Proven cooperation
This was not the first time that the operators from Eaux Valdotaines had relied on the expertise of the hydropower specialists from South Tyrol. On the contrary, as far as the working relationship with Troyer AG is concerned, the company can reflect on a very successful cooperation over many years. They also placed their trust in the specialist from South Tyrol with the previous project, which was constructed less than 15 kilometres away as the crow flies from Arvier power plant in the same valley on the Torrente Mont Fortchat. For Mont Fortchat power plant, which delivers a power output of around 2.5 MW, the company Troyer supplied the horizontal-axis 2-nozzle Pelton turbine which is designed for an expanded flow rate of 630 l/s and a gross head of 449 m. The positive experiences that the operator was able to gain from the previous project would ultimately also be repeated in the subsequent Arvier project, which was implemented to the customer’s complete satisfaction.

Smallest region as hydropower hotspot
Whereas the Mont Fortchat power plant was able to start operating at the end of January 2019, for Arvier power plant it was not until June of last year that the plant was able to be connected to the grid for the first time. The slightly longer implementation time was ultimately attributable in particular to the difficult access to the construction site and the snowy winter of 2017/2018 in Aosta. This caused minor delays to the construction schedule. With a standard capacity of from 2 to 2.5 GWh, today the new power plant makes a valuable contribution to ensuring that the autonomous Aosta Valley region is self-sufficient when it comes to generating power and also to meeting Italy’s climate targets. It is not least thanks to plants such as those from Eaux Valdotaines that Italy’s smallest region can quite rightly be described as the biggest when it comes to hydropower.