First, the existing access tunnel - originally designed for inspection and maintenance - had to be widened to allow the delivery of construction machinery and hydroelectric equipment.
In the Swiss Val Medel, an old cavern is now used for sustainable energy production. The Rasoira tunnel was built more than 60 years ago as part of Switzerland’s large hydropower expansion. Today, with the help of Global Hydro, it has been transformed into a state-of-the-art small hydropower plant that produces clean electricity for around 2,300 households.
The tunnel was constructed in the 1960s to guide water from side valleys into a large reservoir. At that time, Switzerland invested heavily in big reservoirs and long water tunnels to collect water from many side valleys and bring it to large power stations. The Rasoira tunnel was one of these connection tunnels, but it was never used for electricity production itself. For decades, it remained an unused part of the system – until new technology and today’s focus on sustainability created a new opportunity. Instead of building new structures in sensitive alpine nature, the project used what already existed – a good example of infrastructure reuse.
The 15-kilometer Rasoira tunnel, originally constructed as a free-flow connection, is now partly pressurized and supports controlled turbine operation. With a design head of 21 m and a maximum water flow of 22–24 m³/s, the plant generates around 9 GWh of green energy per year – without new ecological interventions in the alpine environment. The cavern was blasted to precise dimensions (22 m long, 11 m wide, 10 m high), carefully aligned with the turbine layout to reduce excavation volume and costs.
Installed capacity
Turbine type
double-regulated
Annual generation
~2,300 households
Tunnel length
Olivone - Malvaglia reservoir
Head range
Maximum flow
To meet the structural and logistical requirements, a modular turbine design was implemented, with the spiral casing manufactured in pre-assembled segments. These were temporarily tack-welded in our Austrian Innovation and Manufacturing Center, then disassembled, transported through the narrow access route, and reassembled and fully welded on site. This approach ensured a seamless fit in the confined installation area and minimized handling complexities.
In terms of mechanical design, one major challenge was the risk of backpressure from the high tailwater level, which could exert substantial upward force on the generator. To address this, robust oil-lubricated guide bearings were chosen for their ability to withstand such loads, ensuring safe and stable operation even during extreme conditions.
Equally forward-thinking was the integration of water-lubricated bearing systems. Unlike conventional oil-based bearings, this solution uses processed water for lubrication, eliminating any risk of oil leakage into the environment. The system not only supports ecological safety by preventing contamination but also reduces maintenance complexity. Key to its reliability is a dual-pump setup: should the main water supply system fail, a redundant battery-powered pump ensures continuous lubrication, even in the event of a grid outage. This innovation reflects Global Hydro's commitment to both technological excellence and environmental responsibility.
Designing a fully functional hydropower plant within a cavern is a challenge that requires precision and extensive experience. In the case of Rasoira, several major civil construction steps were required before the cavern itself could be realized:
A key technical requirement was managing the highly variable head, fluctuating between 11 and 26 meters. This variability, caused by inconsistent inflows and changing reservoir levels, posed a unique challenge to conventional turbine control systems.
To meet these demands, Global Hydro developed a custom hydraulic control system specifically for Kaplan spiral turbines in cavern settings. This system included a specially developed 5-blade Kaplan runner, newly configured spiral casing, and an optimized draft tube adapted to the downstream tunnel profile. The integrated control system HerosControl governs water levels, turbine blade angles, and flow rates in real-time, ensuring efficiency and safety under all operational scenarios.
Notably, a specially developed regulation gate at the end of the main tunnel allows partial pressurization of the otherwise open-channel tunnel. This enables the system to accumulate up to 50,000 m³ of water for controlled turbine operation even during low inflow conditions, allowing maximum utilization of the available water.
Given the unique hydraulic geometry and high variability in water levels, Global Hydro partnered with Jaberg & Partner GmbH to conduct advanced transient simulations. These included pressure surge analyses and dynamic simulations of emergency shutdown scenarios. The goal was to ensure no pressure shock respectively no fast pressure reduction would compromise the integrity of the tunnel system.
In addition to these hydraulic assessments, the critical process of installing the generator - including its rotation from a horizontal to a vertical position within the cavern - was fully simulated to ensure feasibility and safety. Due to limited crane hook height and extremely tight clearances, precise modeling was essential to avoid costly spatial conflicts during installation.
Transporting the 57-ton generator and other key components to the secluded mountain site posed a significant logistical challenge that required precise planning and execution. The 85-kilometer transport route from the valley to the cavern entrance passed through narrow village streets, winding mountain roads with steep gradients, and several old bridges. Moreover, snow and ice clearance was essential during winter months to keep the transport route safe and navigable.
A particular difficulty was the lack of maneuvering space along the final stretch. The access road featured tight curves, abrupt inclines, and minimal clearance zones, necessitating advance coordination with authorities and support by an escort team to manage traffic. Local residents were informed of the delivery windows to minimize disruptions. In one instance, a tunnel wall had to be partially removed to allow the transport vehicle to pass.
Even the equipment design was adapted to logistical realities: components such as auxiliary enclosures or control boxes mounted on the generator were transported separately to reduce transport dimensions. Each detail, from access route analysis to unloading sequences, was meticulously planned to ensure timely and safe delivery.
The Rasoira project is a testament to what is possible when state-of-the-art hydropower expertise meets visionary infrastructure reuse. Despite spatial limitations, fluctuating head conditions, and the complexities of building deep within a mountain, Global Hydro and its partners successfully delivered a high-performance renewable energy solution. The project not only highlights the untapped potential lying dormant in existing systems, but also sets a benchmark for technical precision, environmental responsibility, and innovation in the hydropower industry.
Instead of building new structures in a sensitive alpine valley, the task was to reuse what already existed. This brought specific challenges: Limited space inside an underground cavern, difficult access routes for heavy equipment, and technical risks when adapting new technology to old civil structures. At the same time, sustainability and minimum environmental impact were key priorities.
Global Hydro develops compact, custom-tailored turbine solutions designed specifically for complex site conditions. A compact Kaplan turbine was engineered to fit the cavern dimensions, with modular parts for easy transport and assembly underground. Eco-friendly water-lubricated bearings avoid oil leakage, while advanced simulations and pressure surge analyses guaranteed safe operation within the existing tunnel system. Every element was designed to turn limitations into workable solutions.
By reusing the old tunnel, the operator now produces renewable electricity without new ecological interventions. Costs and approval times were reduced because existing infrastructure was used, while the custom design ensures long-term efficiency and reliability. The result is a sustainable, future-proof asset that shows how unused infrastructure can be turned into clean energy and real value.
Many hydropower assets hide untapped opportunities. Global Hydro is your partner to turn existing structures into sources of sustainable energy. With our experience in custom turbine design and integration of state-of-the-art technology, we help operators maximize output while keeping environmental impact low. Together, we can transform unused capacity into long-term value.
Thierry Burckhart
Director Revitalization
+1 (970) 710 1285
thierry.burckhart@global-hydro.eu Contacto
